KR20250172740A - Piezoelectric electrophoretic film and display and method for manufacturing the same - Google Patents

Abstract

Low-voltage piezoelectric electrophoretic films and display films, including low-profile piezoelectric electrophoretic films and displays. In some embodiments, the piezoelectric material of the piezoelectric electrophoretic films can be selectively patterned using a high-voltage electric field during or after the manufacture of the piezoelectric electrophoretic films. These films have a high contrast ratio and are useful as security markers, authentication films, or sensors. The films are generally flexible. Some films have a thickness of less than 100 μm. Some films have a thickness of less than 50 μm. Displays formed from the films do not require an external power source.

Description

Piezoelectric electrophoretic film and display and method for manufacturing the same

Cross-reference to related applications

This application claims the benefit of U.S. Provisional Patent Application No. 63/579,375, filed August 29, 2023, and U.S. Provisional Patent Application No. 63/579,377, filed August 29, 2023.

The entire disclosures of the aforementioned applications and all other applications or publications referred to below are incorporated herein by reference in their entirety.

Technology field

The invention disclosed herein relates to electrophoretic displays, and more particularly, to thin piezo-electrophoretic displays with improved contrast ratios, and methods for manufacturing the same. The invention disclosed herein also relates to low-profile piezo-electrophoretic displays that can be activated or driven without being connected to a power source, and methods for manufacturing the same.

An electrophoretic display ("EPD") is a non-emissive device based on the electrophoresis of charged pigment particles dispersed in a solvent or solvent mixture. The display typically includes two opposing electrodes that provide an electric field to drive the movement of the charged pigment particles. One of the electrodes is usually transparent. When a voltage difference is applied between the two electrodes, the pigment particle(s) move to one side or the other, making the color of the pigment particle(s) or the color of the solvent (if colored) visible from the viewing side.

Many electrophoretic displays comprise an electrophoretic fluid comprising a nonpolar solvent and one or more sets of charged pigment particles. The particles may have different optical properties (color), different charges (positive or negative), different charge magnitudes (zeta potentials), and/or different absorption characteristics (broad light absorption or broad light reflectivity, or selective absorption or selective reflection). When multiple sets of particles with opposite charge polarities are present, the application of an electric field can cause one set of pigment particles to appear on a viewing surface while causing other sets of pigment particles to repel away from the viewing surface.

Many electrophoretic displays are bistable, meaning that their optical state persists even after the activating electric field is removed. Bistability is primarily a result of the induced dipole charge layer that forms around the charged pigment, resulting from complex interactions among the pigment, charge control agent, and free polymer dispersed in the solvent. Bistable displays can remain in their last addressed optical state for years before being switched back upon application of a new driving electric field.

Driving an electrophoretic display requires a power source, such as a battery, to power the display and/or its driving circuitry. For example, a battery may be used to power a driver IC, which in turn generates an electric field to energize the electrodes of the display. The power source may also be a power supply that receives power from, for example, a photovoltaic cell, a fuel cell, or a wall outlet. The power source may also be a piezoelectric element that generates a charge through physical movement or thermal expansion, as described in U.S. Patent No. 5,930,026, which is incorporated by reference in its entirety.

In all these examples, some type of driving circuitry is required to provide an electrical path between the power source and the electrodes. Typically, the circuitry also includes control elements (e.g., switches, transistors, etc.) and a number of individual components (e.g., resistors, capacitors, etc.).

In most cases, the circuitry used in conventional displays is complex, yet well-known to those skilled in display technology. However, integrating this circuitry can limit the display's durability against mechanical stresses such as bending and/or twisting. Furthermore, the presence of additional components typically inevitably increases the overall physical dimensions of the fully assembled display.

Due to the physical limitations imposed on the display by the addition of power and drive circuitry, such displays may not be suitable for a growing number of applications where reducing the overall thickness of the display is desirable. Therefore, in an effort to reduce display thickness, some electrophoretic displays utilize low-profile piezoelectric elements that generate a charge in response to mechanical strain or thermal cycling. However, the thickness of the piezoelectric material layer typically has a direct correlation to the amplitude of the voltage that the piezoelectric material can generate in response to mechanical stress. That is, decreasing the thickness of the piezoelectric material also reduces the magnitude of the voltage that the piezoelectric material generates under stress (and vice versa). Accordingly, in order to generate a voltage potential large enough to move the charged pigment particles by an amount sufficient to achieve an acceptable contrast ratio, conventional piezoelectric electrophoretic displays have typically incorporated layers of piezoelectric material that are substantially inconspicuous when incorporated into thin, low-profile end products, such as paper or banknotes, and are too thick for use in applications requiring durability.

Therefore, there is a need for a piezoelectric electrophoretic display that is simple to configure, flexible, durable, and thin for applications such as security markers, sensors, and indicators. There is also a need for a piezoelectric electrophoretic display that is thin and durable enough for use in applications that require a low-profile end product and high contrast ratio.

According to one aspect of the invention disclosed herein, an electro-optical display may include a layer of electrophoretic material; a first conductive layer; and a piezoelectric material positioned between the layer of electrophoretic material and the first conductive layer, wherein the piezoelectric material overlaps a portion of the layer of electrophoretic material, and a portion of the first conductive layer overlaps a remainder of the electrophoretic material.

In a first aspect, the present invention comprises an electrophoretic display film having a thickness (from top to bottom) of less than 100 μm, the electrophoretic display film comprising a first adhesive layer, an electrophoretic medium layer, a patterned piezoelectric layer including zones of differential polarization, and a flexible, light-transmissive electrode layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bound to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the nonpolar fluid and charged pigment particles are sealed within the microcells by a sealing layer. In some embodiments, the film has a thickness of less than 50 μm. In some embodiments, the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is poled to create the differential polarization zones. In some embodiments, the flexible optically transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible optically transmissive electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the present invention includes an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film as described above, the release sheet bonded to the first adhesive layer. In some embodiments, the assembly further includes a second adhesive layer bonded to the flexible light-transmitting electrode layer, and a second release sheet bonded to the second adhesive layer.

In a second aspect, the present invention comprises a method for manufacturing an electrophoretic display film. The method comprises the steps of: bonding a film of polyvinylidene fluoride (PVDF) to a polymer film comprising an acrylate, vinyl ether, or epoxide to form a piezoelectric microcell precursor film; bonding the piezoelectric microcell precursor film to a flexible light-transmitting electrode layer; bonding the light-transmitting electrode layer to a first release film using a first adhesive layer; embossing the piezoelectric microcell precursor film to form an array of microcells, the microcells having a bottom, walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; and sealing the top openings of the filled microcells with a water-soluble polymer to form an electrophoretic medium layer. In some embodiments, the method further comprises applying a primer to the polymer film, wherein the primer comprises an acrylate, a vinyl ether, or an epoxide, prior to bonding the polymer film to the film of polyvinylidene fluoride (PVDF). In some embodiments, the method further comprises bonding the water-soluble polymer to a second release film using a second adhesive layer. In some embodiments, the method further comprises removing the first release film to produce an electrophoretic display film having a thickness of less than 100 μm. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and wherein the nonpolar fluid and charged pigment particles are sealed within the microcells with a sealing layer. In some embodiments, the PVDF is polarized to produce differential polarization zones. In some embodiments, the flexible light-transmitting electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible light-transmitting electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the polyvinylidene fluoride film is patterned using an electric field to create regions of different polarization. In some embodiments, the method further comprises patterning the completed electrophoretic display film using an electric field to create regions of different polarization in the polyvinylidene fluoride film.

In a third aspect, the present invention comprises a method for manufacturing an electrophoretic display film. The method comprises the steps of: dispersing a polyvinylidene fluoride (PVDF) solution on a first release to form a PVDF film having a thickness of less than 10 μm; bonding the PVDF film to a second release using a conductive adhesive; removing the first release; bonding a polymer film comprising an acrylate, vinyl ether, or epoxide to form a piezoelectric microcell precursor film; bonding the piezoelectric microcell precursor film to a flexible light-transmitting electrode layer; bonding the light-transmitting electrode layer to the first release film using a first adhesive layer; embossing the polymer film comprising an acrylate, vinyl ether, or epoxide to form an array of microcells, the microcells having a bottom, walls, and a top opening; and filling the microcells with an electrophoretic medium through the top opening. and sealing the top openings of the filled microcells with a water-soluble polymer. In some embodiments, the method further comprises applying a primer to the polymer film, wherein the primer comprises an acrylate, a vinyl ether, or an epoxide, prior to bonding the polymer film to the PVDF film. In some embodiments, the method further comprises bonding the water-soluble polymer to a second release film using the second adhesive layer. In some embodiments, the method further comprises removing the first release film to produce an electrophoretic display film having a thickness of less than 100 μm. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and wherein the nonpolar fluid and charged pigment particles are sealed within the microcells with a sealing layer. In some embodiments, the PVDF is polarized to produce differential polarization zones. In some embodiments, the flexible light-transmitting electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible light-transmitting electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the PVDF film is patterned using an electric field to create regions of differential polarization. In some embodiments, the method further comprises patterning the completed electrophoretic display film using an electric field to create regions of differential polarization in the PVDF film.

In a fourth aspect, the present invention comprises an electrophoretic display film having a thickness (from top to bottom) of less than 100 μm, the film comprising a first adhesive layer, a patterned piezoelectric layer including differentially polarized zones, an electrophoretic medium layer, and a flexible light-transmitting electrode layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the nonpolar fluid and charged pigment particles are sealed within the microcells with a sealing layer. In some embodiments, the sealing layer is conductive. In some embodiments, the film has a thickness of less than 50 μm. In some embodiments, the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is polarized to create differential polarization zones. In some embodiments, the flexible optically transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible optically transmissive electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT). In some embodiments, the present invention comprises an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film as described above, the release sheet bonded to the first adhesive layer. In some embodiments, the electrophoretic display film further comprises a second adhesive layer bonded to the flexible optically transmissive electrode layer, and a second release sheet bonded to the second adhesive layer.

In a fifth aspect, the present invention comprises a method of patterning a piezo-electrophoretic medium film. The method comprises the steps of bonding a film of polyvinylidene fluoride (PVDF) to an electrophoretic medium layer to form a piezo-electrophoretic medium film, and patterning the piezo-electrophoretic medium film using an electric field. In some embodiments, the electric field is provided by a corona discharge. In some embodiments, the method further comprises the step of disposing a conductive mask adjacent to the piezo-electrophoretic medium film prior to patterning the piezo-electrophoretic medium film with the corona discharge. In some embodiments, the electric field is provided by a high voltage recording head. In some embodiments, the patterning comprises forming regions of different polarities within the PVDF. In some embodiments, the patterning creates a security marker. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the nonpolar fluid and charged pigment particles are sealed within the microcells with a sealing layer.

In a sixth aspect, the present invention comprises an electrophoretic display film having a thickness (from top to bottom) of less than 100 μm, the film comprising an adhesive layer, an electrophoretic medium layer, a patterned piezoelectric layer including regions of differential polarization, and a conductive adhesive layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to each other with a polymer binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and wherein the nonpolar fluid and charged pigment particles are sealed within the microcells with a sealing layer. In some embodiments, the sealing layer is conductive. In some embodiments, the film has a thickness of less than 50 μm. In some embodiments, the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is polarized to create differential polarization zones. In some embodiments, the present invention comprises an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film as described above, the release sheet being bonded to the first adhesive layer. In some embodiments, the present invention comprises an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film comprising a conductive adhesive layer, the release sheet being bonded to the conductive adhesive layer.

In a seventh aspect, the present invention comprises an electrophoretic display film having a thickness (from top to bottom) of less than 100 μm, comprising an adhesive layer, a patterned piezoelectric layer including differential polarization zones, an electrophoretic medium layer, and a conductive adhesive layer.

In a seventh aspect, the present invention comprises a method of manufacturing a piezoelectric electrophoretic display. The method comprises depositing a first electrically conductive adhesive on a first substrate, and depositing a piezoelectric material comprising a polyvinylidene fluoride (PVDF) solution on the first electrically conductive adhesive to form a piezoelectric layer having a thickness of less than 5 μm. The method also comprises applying a mask to the piezoelectric layer, the mask comprising a plurality of masking portions that shield a first plurality of regions of the piezoelectric layer and a plurality of unmasking portions that leave a second plurality of regions of the piezoelectric layer unshielded. The method also comprises polarizing the piezoelectric layer to form a plurality of polarized portions of the piezoelectric material corresponding to the second plurality of regions of the piezoelectric layer and a plurality of unpolarized portions of the piezoelectric material corresponding to the first plurality of regions of the piezoelectric layer. The method also includes the steps of removing the mask from the piezoelectric layer, and bonding the piezoelectric layer to a microcell precursor material. The method also includes the step of embossing the microcell precursor material to produce a layer of microcells, wherein the microcells have a bottom, walls, and a top opening. The method also includes the steps of filling the microcells with an electrophoretic medium through the top opening, and sealing the top openings of the filled microcells with a water-soluble polymer to produce a sealing layer. The method also includes the steps of depositing a second electrically conductive adhesive on a second substrate, and bonding the sealing layer to the second electrically conductive adhesive.

In some embodiments, the method further comprises forming the microcell precursor material by bonding a polymer film comprising an acrylate, vinyl ether, or epoxide. In some embodiments, the method further comprises applying a primer to the microcell precursor material prior to bonding the piezoelectric layer to the microcell precursor material. In some embodiments, the primer comprises a thermoplastic or thermosetting material or a precursor thereof, such as a polyurethane, a multifunctional acrylate or methacrylate, vinylbenzene, vinyl ether, epoxide, or an oligomer or polymer thereof.

In some embodiments, the method further comprises activating the microcells by a vapor phase plasma treatment prior to filling the microcells with the electrophoretic medium. In some embodiments, the electrophoretic medium layer comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric layer when the piezoelectric layer is subjected to mechanical stress, and the non-polar fluid and charged pigment particles are sealed within the microcells by the sealing layer. In some embodiments, the piezoelectric layer is polarized using an electric field. In some embodiments, the electric field is provided by a corona discharge.

In some embodiments, the first substrate and the second substrate are release films.

In some embodiments, the method further comprises the steps of peeling the second substrate from the second electrically conductive adhesive, and bonding the second electrically conductive adhesive to a target object. In some embodiments, the step of bonding the second electrically conductive adhesive to the target object comprises the step of hot stamping the second electrically conductive adhesive onto the target object.

In some embodiments, the method further comprises the steps of peeling the first substrate from the first electrically conductive adhesive, and applying a protective coating over the remaining layers of the piezoelectric electrophoretic display and the object. In some embodiments, the protective coating comprises a lacquer. In some embodiments, the object comprises one of paper, banknotes, and currency.

In a ninth aspect, the present invention comprises a method of manufacturing a piezoelectric electrophoretic display. The method comprises depositing a piezoelectric material comprising a polyvinylidene fluoride (PVDF) solution on a temporary substrate to form a piezoelectric layer having a thickness of less than 5 μm. The method further comprises bonding the piezoelectric layer to a first substrate using a first electrically conductive adhesive, wherein the temporary substrate is removed from the piezoelectric layer during the bonding process. The method further comprises applying a mask to the piezoelectric layer, the mask comprising a plurality of masking portions that shield a first plurality of regions of the piezoelectric layer and a plurality of unmasking portions that leave a second plurality of regions of the piezoelectric layer unshielded. The method further comprises polarizing the piezoelectric layer to form a plurality of polarized portions of the piezoelectric material corresponding to the second plurality of regions of the piezoelectric layer and a plurality of unpolarized portions of the piezoelectric material corresponding to the first plurality of regions of the piezoelectric layer. The method also includes the steps of removing the mask from the piezoelectric layer, and depositing a second electrically conductive adhesive on a second substrate. The method also includes the steps of bonding the second electrically conductive adhesive to a microcell precursor material, and embossing the microcell precursor material to produce a layer of microcells, wherein the microcells have a bottom, walls, and a top opening. The method also includes the steps of filling the microcells with an electrophoretic medium through the top opening, and sealing the top openings of the filled microcells with a water-soluble polymer to produce a sealing layer. The method also includes the step of bonding the sealing layer to the piezoelectric layer.

In some embodiments, the method further comprises forming the microcell precursor material by bonding a polymer film comprising an acrylate, vinyl ether, or epoxide. In some embodiments, the method further comprises applying a primer to the microcell precursor material prior to bonding the second electrically conductive adhesive to the microcell precursor material. In some embodiments, the method further comprises activating the microcells by a vapor phase plasma treatment prior to filling the microcells with the electrophoretic medium.

In some embodiments, the electrophoretic medium layer comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is subjected to mechanical stress, and the non-polar fluid and charged pigment particles are sealed within the microcell by the sealing layer. In some embodiments, the piezoelectric layer is polarized using an electric field. In some embodiments, the electric field is provided by a corona discharge.

In some embodiments, the first substrate and the second substrate are release films.

In some embodiments, the method further comprises the steps of peeling the second substrate from the second electrically conductive adhesive, and bonding the second electrically conductive adhesive to a target object. In some embodiments, the step of bonding the second electrically conductive adhesive to the target object comprises the step of hot stamping the second electrically conductive adhesive onto the target object.

In some embodiments, the method further comprises: peeling the first substrate from the first electrically conductive adhesive; and applying a protective coating over the remaining layers of the piezoelectric electrophoretic display and the object. In some embodiments, the protective coating comprises a lacquer. In some embodiments, the object comprises one of paper, banknotes, and currency.

According to a tenth aspect of the invention disclosed herein, an electro-optic display may include a layer of electrophoretic material; a first conductive layer; and a piezoelectric material positioned between the layer of electrophoretic material and the first conductive layer, wherein the piezoelectric material overlaps a portion of the layer of electrophoretic material, and a portion of the first conductive layer overlaps a remainder of the electrophoretic material.

In an eleventh aspect, the present invention features a method for manufacturing a piezoelectric electrophoretic display. The method comprises the steps of depositing a first electrically conductive material on a first substrate to form a first electrode, and bonding the first electrode to a first surface of a layer of electrophoretic material. The method also comprises the step of depositing a piezoelectric material on a second surface of the layer of electrophoretic material, wherein the piezoelectric material overlaps a first surface area of the second surface of the layer of electrophoretic material. The method also comprises the step of depositing a second electrically conductive material to form a second electrode, wherein the second electrode is formed to overlap the entirety of the piezoelectric material and a second surface area of the second surface of the layer of electrophoretic material.

In some embodiments, the electrophoretic material layer includes a first electrophoretic material portion overlapping the first surface area and a second electrophoretic material portion overlapping the second surface area. In some embodiments, the first electrophoretic material portion has a first electrical resistance, and the second electrophoretic material portion has a second electrical resistance.

In some embodiments, the electrophoretic material layer includes a first electrophoretic material portion having a first electrical resistance corresponding to a first volume of electrophoretic material overlapping the first surface area, and a second electrophoretic material portion having a second electrical resistance corresponding to a second volume of electrophoretic material overlapping the second surface area. In some embodiments, the value of the first electrical resistance and the value of the second electrical resistance are based on a ratio of the first surface area to the second surface area.

In some embodiments, applying a mechanical stress to the piezoelectric material generates a first voltage across the first electrophoretic material portion and a second voltage across the second electrophoretic material portion, wherein the first voltage and the second voltage have opposite polarities.

In some embodiments, the bonding step comprises: coating the first electrode with a microcell precursor material; embossing the microcell precursor material to create a layer of microcells, the microcells having a bottom, a plurality of walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; and sealing the top openings of the filled microcells with a water-soluble polymer to create a sealing layer.

In some embodiments, the method further comprises applying a primer to the microcell precursor material prior to embossing the microcell precursor material. In some embodiments, the method further comprises activating the microcells with a vapor phase plasma treatment prior to filling the microcells with the electrophoretic medium. In some embodiments, the electrophoretic medium comprises a nonpolar fluid and charged pigment particles, wherein the charged pigment particles migrate toward or away from the piezoelectric material when the piezoelectric material is subjected to mechanical stress, and wherein the nonpolar fluid and charged pigment particles are sealed within the microcells by the sealing layer.

In some embodiments, the method further comprises applying an adhesive material layer between the piezoelectric material and the first surface area of the second surface of the electrophoretic material layer, wherein the adhesive material layer has a resistivity of from 10 ² ohm*cm to 10 ¹² ohm*cm. In some embodiments, the method further comprises applying an adhesive material layer between the piezoelectric material and the first surface area of the second surface of the electrophoretic material layer, wherein the adhesive material layer has a resistivity that is at least one order of magnitude greater than that of the first and second electrodes.

In some embodiments, the method further comprises depositing a dielectric layer prior to depositing the second electrically conductive material, wherein the dielectric layer is formed to overlap the entire piezoelectric material and the second surface area of the second surface of the electrophoretic material layer, and wherein the second electrode is formed to overlap the entire dielectric layer. In some embodiments, the dielectric layer has a resistivity of from 10 ² ohm*cm to 10 ¹² ohm*cm. In some embodiments, the dielectric layer has a resistivity at least one order of magnitude greater than that of the first and second electrodes.

In some embodiments, the method further comprises printing one or more images on at least one of the first electrode and the second electrode. In some embodiments, the method further comprises attaching the piezoelectric display to a target object selected from the group consisting of paper, banknotes, and currency.

Figure 1a illustrates a side view of a piezoelectric electrophoretic display film of the present invention, which includes star-shaped regions of differential polarization. Three exemplary positions, namely convex, neutral, and concave, are shown from the side. The total thickness of the piezoelectric electrophoretic display film may be less than 100 μm, such as less than 50 μm, or even less than 25 μm.
Figure 1b illustrates a top view of a piezoelectric electrophoretic display film of the present invention, which includes star-shaped regions of differential polarization. Three exemplary positions, namely convex, neutral, and concave, are shown from above. When the piezoelectric electrophoretic display film is bent, the regions of differential polarization result in oppositely charged particles appearing on the viewing surface.
Figure 2a illustrates an exemplary thin layer of piezoelectric material on a substrate.
Figure 2b illustrates a method for generating differential polarization regions in a thin layer of piezoelectric material using the strong electric field of a corona discharge. By moving the piezoelectric material closer and further from the discharge, the amount of polarization can be spatially controlled.
Figure 2c illustrates a method for creating differential polarization regions in a thin layer of piezoelectric material by using a strong electric field of a corona discharge. A conductive mask is used to pattern the piezoelectric material to create the differential polarization regions.
Figure 2d illustrates a polarization (poling) pattern that can be achieved by the methods of Figures 2b and 2c.
Figure 3a illustrates a side view of a piezoelectric film poled in the A direction.
Figure 3b illustrates a top view of a piezoelectric film poled in the A direction.
Figure 3c illustrates a side view of a piezoelectric film poled in the G direction using a conductive mask.
Figure 3d illustrates a top view of a piezoelectric film poled in the G direction using a conductive mask.
Figure 4a illustrates an exemplary thin layer of a piezoelectric microcell precursor film on a substrate.
Figure 4b illustrates a method for generating differential polarization regions in a thin layer of piezoelectric material of a piezoelectric microcell precursor film by using the strong electric field of a corona discharge. By moving the piezoelectric microcell precursor film closer and further away from the discharge, the amount of polarization can be spatially controlled.
Figure 4c illustrates a method for creating differential polarization regions in a thin layer of piezoelectric material of a piezoelectric microcell precursor film by using a strong electric field of a corona discharge. A conductive mask is used to pattern the piezoelectric material of the piezoelectric microcell precursor film to create the differential polarization regions.
Figure 4d illustrates a polarization (poling) pattern in a piezoelectric microcell precursor film that can be achieved by the methods of Figures 3b and 3c.
Figure 5a is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 5b is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 5c is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 5d is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 6a is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic display.
Figure 6b is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic display.
Figure 7 details a method for producing a piezoelectric electrophoretic film or (optionally) a display.
Figure 8a is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 8b is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 9a is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 9b is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic film.
Figure 10a is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic display.
Figure 10b is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic display.
Figure 10b is a schematic cross-sectional view of an embodiment of a piezoelectric electrophoretic display.
Figure 11 details a method for producing a low-profile piezoelectric electrophoretic film.
Fig. 12a is a schematic cross-sectional view of a piezoelectric electrophoretic film produced by the method illustrated in Fig. 11.
Figure 12b is a schematic cross-sectional view of a piezoelectric electrophoretic display produced by the method illustrated in Figure 11.
FIG. 13a is a schematic cross-sectional view of an alternative piezoelectric electrophoretic film produced by the method illustrated in FIG. 11.
FIG. 13b is a schematic cross-sectional view of an alternative piezoelectric electrophoretic display produced by the method illustrated in FIG. 11.
Figure 14 is a flow chart detailing the steps of a method for producing a high contrast piezoelectric electrophoretic film and display.
FIG. 15a is a schematic cross-sectional view of a piezoelectric electrophoretic film at step 1440 of the method illustrated in FIG. 14.
FIG. 15b is a schematic cross-sectional view of the piezoelectric electrophoretic film after completion of step 1440 of the method illustrated in FIG. 14.
FIG. 15c is a schematic cross-sectional view of the piezoelectric electrophoretic film after completion of step 1450 of the method illustrated in FIG. 14.
FIG. 15d is a schematic cross-sectional view of the piezoelectric electrophoretic film after completion of step 1470 of the method illustrated in FIG. 14.
FIG. 15e is a cross-sectional view of a piezoelectric electrophoretic film bonded to a target object after completion of step 1480 of the method illustrated in FIG. 14.
FIG. 15f is a cross-sectional view of a piezoelectric electrophoretic film bonded to a target object and coated with a protective coating after completion of step 1480 of the method illustrated in FIG. 14.
Figure 16 shows an enlarged view of a partial cross-section of a piezoelectric electrophoretic display.
Figure 17 illustrates an exemplary equivalent circuit of the enlarged cross-section shown in Figure 16.
Figure 18 is a flow chart detailing the steps of a method for producing a high-contrast piezoelectric electrophoretic film and display.
FIG. 19a is a schematic cross-sectional view of a piezoelectric electrophoretic film at step 1810 of the method illustrated in FIG. 18.
FIG. 19b is a schematic cross-sectional view of the piezoelectric electrophoretic film at step 1830 of the method illustrated in FIG. 18.
FIG. 19c is a schematic cross-sectional view of the piezoelectric electrophoretic film after completion of step 1840 of the method illustrated in FIG. 18.
FIG. 19d is a schematic cross-sectional view of the piezoelectric electrophoretic film after completion of steps 1850 and 1860 of the method illustrated in FIG. 18.
FIG. 19e is a schematic cross-sectional view of the piezoelectric electrophoretic film after completion of step 1870 of the method illustrated in FIG. 18.
FIG. 19f is a cross-sectional view of a piezoelectric electrophoretic film bonded to a target object according to the method illustrated in FIG. 18.
FIG. 19g is a cross-sectional view of a piezoelectric electrophoretic film bonded to a target object and coated with a protective coating after completion of step 1880 of the method illustrated in FIG. 18.
FIG. 20 is a schematic cross-sectional view of an exemplary piezoelectric electrophoretic display according to the invention disclosed herein.
FIG. 21a is a schematic cross-sectional view illustrating additional characteristics of a piezoelectric electrophoretic display according to the invention disclosed herein.
FIG. 21b is a perspective view illustrating additional characteristics of a piezoelectric electrophoretic display according to the invention disclosed herein.
Fig. 22 illustrates an exemplary equivalent circuit of a piezoelectric electrophoretic display according to the invention disclosed herein.
FIG. 23 is a schematic cross-sectional view of an exemplary piezoelectric electrophoretic display according to the invention disclosed herein.

Disclosed herein are low-profile piezoelectric electrophoretic films and displays, and methods for manufacturing such films and displays. In some embodiments, the piezoelectric material of the piezoelectric electrophoretic film can be patterned using a high-voltage electric field after the piezoelectric electrophoretic film is manufactured. This feature allows the end user to process the piezoelectric material at the point of production, such as with a corona discharge, which can include, for example, a barcode or serial number that is only visible when the piezoelectric electrophoretic film is manipulated. The low-profile films and displays described herein also achieve a high contrast ratio. The films and displays described herein are generally flexible and useful as security markers, authentication films, or sensors. Some films have a thickness of less than 100 μm. In some embodiments, the piezoelectric electrophoretic film is less than 50 μm and is foldable without breakage. Displays formed according to the invention disclosed herein do not require an external power source.

The term "electro-optic" as applied to a material or display is used herein in its conventional sense in the imaging arts to refer to a material having first and second display states that differ in at least one optical property, wherein the material changes from its first display state to its second display state upon application of an electric field to the material. The optical property is typically a color perceptible to the human eye, but may also be another optical property such as optical transmission, reflectance, luminescence, or, in the case of machine-readable displays, pseudo-color in the sense of a change in reflectance for electromagnetic wavelengths outside the visible light range.

The terms "bistable" and "bistability" are used herein in their customary meaning in the art to refer to a display comprising display elements having first and second display states having at least one optical characteristic that differs, such that after any given element is actuated, by an addressing pulse of finite duration, it will assume its first or second display state, and after the addressing pulse is terminated, that state will persist for at least a number of times, for example at least four times, which is the minimum duration of the addressing pulse required to change the state of the display element. U.S. Patent No. 7,170,670 discloses that some particle-based electrophoretic displays capable of grayscale are stable not only in their extreme black and white states but also in intermediate gray states, as are some other types of electro-optical displays. This type of display is more appropriately called "multi-stable" rather than bistable, but for convenience the term "bistable" may be used herein to encompass both bistable and multi-stable displays.

The term "gray state" is used herein in its conventional sense in the imaging arts to refer to an intermediate state between two extreme optical states of a pixel, and does not necessarily imply a black-and-white transition between these two extreme states. For example, several of the E Ink patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and deep blue, such that the intermediate "gray state" would actually be light blue. In fact, as previously noted, a change in optical state may not be a color change at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of a display, and should usually be understood to include extreme optical states that are not strictly black and white, such as the white and deep blue states described above. The term "monochrome" may be used hereinafter to denote a display or drive scheme that drives pixels only in their two extreme optical states, without an intermediate gray state.

The term "pixel" is used herein in its conventional sense in the display technology field to mean the smallest unit of a display capable of producing all the colors that the display itself can display. In a full-color display, each pixel typically consists of a plurality of sub-pixels, each of which can display fewer colors than all the colors that the display itself can display. For example, in most conventional full-color displays, each pixel consists of a red sub-pixel, a green sub-pixel, a blue sub-pixel, and optionally a white sub-pixel, each of which is capable of displaying a range of colors from black to the brightest version of that designated color.

Several types of electro-optical displays are known. One type of electro-optical display uses an electrochromic medium, for example, in the form of a nanochromic film comprising an electrode formed at least partially of a semiconducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, e.g., O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038, 6,870,657, and 6,950,220. This type of medium is also typically bistable.

Another type of electro-optical display is the electrowetting display developed by Philips and described in Hayes, R.A., et al., “Video-Speed Electronic Paper Based on Electrowetting,” Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 shows that such electrowetting displays can be made bistable.

One type of electro-optical display that has been the subject of intensive research and development for many years is the particle-based electrophoretic display, in which multiple charged particles move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can offer excellent brightness and contrast, wide viewing angles, state bistability, and low power consumption.

Electrophoretic displays typically include a layer of electrophoretic material and at least two other layers disposed on either side of the electrophoretic material, one of which is an electrode layer. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define a pixel of the display. For example, one electrode layer may be patterned into long rows of electrodes, and the other electrode layer may be patterned into long columns of electrodes running perpendicular to the row electrodes, with the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more typically, one electrode layer may be patterned into a single continuous electrode, and the other electrode layer may be patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electrophoretic display intended for use with a stylus, print head, or similar movable electrode separate from the display, only one of the layers adjacent to the electrophoretic layer contains electrodes, and the layer opposite the electrophoretic layer is typically a protective layer to prevent the movable electrode from damaging the electrophoretic layer.

Numerous patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies for encapsulated electrophoretic and other electro-optical media. These encapsulated media comprise numerous small capsules, each of which comprises an internal phase containing electrophoretically mobile particles in a fluid medium and a capsule wall surrounding the internal phase. Typically, the capsules themselves are immobilized within a polymeric binder to form a coherent layer positioned between two electrodes. Technologies described in these patents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814;

(b) Capsules, binders and encapsulation processes; see, e.g., U.S. Patent Nos. 6,922,276 and 7,411,719;

(c) films and subassemblies comprising electro-optical materials; see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564;

(d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624;

(e) color formation and color adjustment; see, e.g., U.S. Patent Nos. 7,075,502 and 7,839,564;

(f) Methods of driving displays; see, for example, U.S. Patent Nos. 7,012,600 and 7,453,445;

(g) Applications of displays; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348;

(h) non-electrophoretic displays, as described in U.S. Patent Nos. 6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent Application Publication No. 2012/0293858;

(i) microcell structures, wall materials and methods of forming microcells; see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906; and

(j) Methods of filling and sealing microcells; see, e.g., U.S. Patent Nos. 7,144,942 and 7,715,088.

Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus creating a so-called polymer-dispersed electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, wherein the discrete droplets of electrophoretic fluid within such polymer-dispersed electrophoretic displays can be considered capsules or microcapsules even though a discrete capsule membrane is not associated with each individual droplet; see, e.g., U.S. Pat. No. 6,866,760 as cited above. Accordingly, for the purposes of the present application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

A related type of electrophoretic display is the so-called "microcell electrophoretic display," also known as a MICROCUP®. In a microcell electrophoretic display, charged particles and fluid are not encapsulated within microcapsules, but instead are held within multiple cavities formed within a carrier medium, typically a polymeric film. See, e.g., U.S. Patent Nos. 6,672,921 and 6,788,449, both of which are incorporated by reference in their entirety.

Although electrophoretic media are often opaque (e.g., because the particles in many electrophoretic media substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be configured to operate in a so-called "shutter mode," where one display state is substantially opaque and one is light-transmitting. See, e.g., U.S. Patent Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Electrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can also operate in a similar mode; see, e.g., U.S. Patent No. 4,418,346. Other types of electro-optical displays can also operate in a shutter mode. Electro-optical media operating in shutter mode may be useful in multilayer structures for full-color displays, in which at least one layer adjacent to the viewing surface of the display operates in shutter mode to expose or conceal a second layer further from the viewing surface.

Encapsulated electrophoretic displays typically do not suffer from the clustering and mating failure modes of traditional electrophoretic devices, and offer additional advantages such as the ability to print or coat the display on a variety of flexible and rigid substrates. (The use of the word "printing" is intended to encompass all forms of printing and coating, including but not limited to, pre-metered coating such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife-over-roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition (see U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Furthermore, because the display medium can be printed, the display itself can be made inexpensively using a variety of methods.

The aforementioned U.S. Patent No. 6,982,178 describes a method for assembling a solid-state electro-optic display (including an encapsulated electrophoretic display) suitable for mass production. Essentially, the patent describes a so-called "front plane laminate" ("FPL") comprising, in sequence: an optically transmissive electrically conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically conductive layer; an adhesive layer; and a release sheet. Typically, the optically transmissive electrically conductive layer will be carried on a light-transmissive substrate, preferably flexible, in that the substrate can be manually rolled around a 10-inch (254 mm) diameter drum without permanent deformation. The term "light-transmissive" is used in this patent and herein to mean that the designated layer transmits sufficient light to allow an observer looking through the layer to observe display state changes in the electro-optic medium that would normally be visible through the electrically conductive layer and the adjacent substrate (if present); When the electro-optic medium displays a change in reflectance at non-visible wavelengths, the term "light transmitting" should of course be interpreted as referring to transmission of the relevant non-visible wavelengths. The substrate will typically be a polymer film, typically having a thickness in the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). The electrically conductive layer can conveniently be a thin metal or thin metal oxide layer, for example, aluminum or ITO, or a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are commercially available, for example, as "aluminized Mylar" ("Mylar" is a registered trademark) from E.I. du Pont de Nemours & Company, Wilmington, DE, and these commercial materials can be used in the front laminate with good results.

Assembly of an electro-optic display using such a front laminate can be accomplished by removing a release sheet from the front laminate, contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, and thereby securing the adhesive layer, the layer of electro-optic medium, and the electrically conductive layer to the backplane. This process is suitable for mass production because the front laminate is typically mass-produced using roll-to-roll coating techniques and can then be cut into pieces of any size required for use with a particular backplane.

U.S. Patent No. 7,561,324 describes a so-called "double release sheet," which is essentially a simplified version of the front laminate of the aforementioned U.S. Patent No. 6,982,178. One type of double release sheet comprises a layer of a solid electro-optic medium sandwiched between two layers of adhesive, one or both of which are covered by a release sheet. Another type of double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both types of double release films are intended for use in a process generally similar to the process for assembling an electro-optic display from the front laminates previously described, but involving two separate laminations. Typically, in a first lamination, a double release sheet is laminated to the front electrode to form a front sub-assembly, and then in a second lamination, the front sub-assembly is laminated to the back electrode to form the final display, although the order of these two laminations may be reversed if desired.

The invention disclosed herein relates to a structural design and manufacturing process for a piezoelectric electrophoretic film and display that does not require a power supply (e.g., a battery, a wired power supply, a solar power source, etc.) to operate. Therefore, the assembly process is simplified, and the thickness of such a display is significantly smaller than that of conventional piezoelectric electrophoretic displays.

Piezoelectricity is the accumulation of an electrical charge in a solid material in response to applied mechanical stress. Materials suitable for the invention disclosed herein may include polyvinylidene fluoride (PVDF), quartz (SiO ₂ ), berlinite (AlPO ₄ ), gallium orthophosphate (GaPO ₄ ), tourmaline, barium titanate (BaTiO ₃ ), lead zirconate titanate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithium tantalite, lanthanum gallium silicate, sodium potassium tartrate, and any other known piezoelectric material.

The piezoelectric electrophoretic film and piezoelectric electrophoretic display described herein utilize the piezoelectric effect to drive charged pigment particles in an electrophoretic medium toward one of the display electrodes. Therefore, by manipulating or physically deforming the piezoelectric material when bonded to an electrophoretic medium layer, the color of the electrophoretic material on the viewing surface can be changed. For example, by bending a piece of piezoelectric material or applying other mechanical stress to it, a voltage can be generated across the electrophoretic medium, which can be used to cause movement of the colored pigment particles in the electrophoretic medium. When only a portion of the electrophoretic medium layer is bonded to the piezoelectric material, or when differentially polarized regions are created in the piezoelectric material, an electrophoretic medium having two types of oppositely charged pigments can be used to create patterns with high contrast, as illustrated in FIGS. 1A and 1B . As used herein, the term "contrast ratio" or "CR" for an electro-optical display (e.g., an electrophoretic display) is defined as the ratio of the luminance of the brightest color (white) to the luminance of the darkest color (black) that the display can produce. A high contrast ratio is generally a desirable aspect of an electro-optical display.

Figures 1A and 1B illustrate side and top views of an exemplary piezoelectric electrophoretic display (100) according to the present invention. In this embodiment, a piezoelectric material is laminated to an electrophoretic medium layer (described below) and one or more electrodes are included to provide a suitable electric field to cause electrophoretic particles to move toward (or away from) a viewing surface. In the embodiment illustrated in Figures 1A and 1B, a second region (120) of the piezoelectric material of the piezoelectric electrophoretic display (100) is polarized in the opposite direction to the first region (110), such that when the piezoelectric electrophoretic display (100) is operated from a neutral state (position 2) to a first (position 1) or second (position 3) optical state, the first and second regions (110, 120) will achieve different colors in the two regions. In the case of an electrophoretic medium having sets of oppositely charged particles, black and white, a high-contrast image will be formed, as illustrated in FIG. 1B . Because the first and second regions (110, 120) of the piezoelectric material can be polarized with good resolution (as described below), various images/information can be encoded to "appear" when the piezoelectric electrophoretic display (100) is manipulated. For example, a security ribbon can be created that exists as a gray strip in a neutral state, but when the security ribbon is bent, the ribbon will display a security seal, such as the star shape illustrated in FIG. 1B . Of course, the security seal could alternatively include a barcode, numbers, words, phone numbers, Internet addresses, QR codes, photographs, halftone images, or logos.

In principle, a piezoelectric material (optionally adjacent to an electrophoretic material) can be polarized by a localized strong electric field, as illustrated in Figures 2a through 3d. It is known that piezoelectric materials (especially films) can be stimulated to shift between polarization states by various external stresses, such as mechanical stretching, heat, electromagnetic fields, and applied forces. The piezoelectric effect is closely related to the generation of electric dipole moments in solids. The dipole density, or polarization (P), corresponds to the dipole moment per volume of a crystallographic unit cell and is typically measured in C/m ² . The resulting dipole density (P) is a vector field specific to a specific region of the material (i.e., differential polarization). Similar to a magnet, dipoles close to each other tend to align in domains (Weiss domains). When initially formed, the domains are typically randomly oriented. However, using various multi-step processes, the domains can be aligned to create localized regions of differential polarization. The process of aligning these areas is known as polling.

Although many piezoelectric materials are crystalline, a number of flexible piezoelectrically active polymers are known, such as polyvinylidene fluoride (PVDF) and its copolymers, polyamides, and parylene-C. Amorphous polymers, such as polyimide and polyvinylidene chloride (PVDC), are amorphous bulk polymers. The standard procedure for producing piezoelectrically active films, such as polyvinylidene fluoride (PVDF), is to create a polymer film and stretch it to generate stress and align the dipoles. Stretching converts the non-polarized alpha phase regions of PVDF into the polarized beta phase. Subsequent stimulation is then applied to the beta phase polar regions, for example, using a strong electric field. Other methods for aligning the beta phase, such as laser irradiation and strong magnetic fields, are described in the literature. See, e.g., U.S. Patent No. 9,831,417. If the stimulation can be performed with sufficiently high resolution, the poles can be used to create visible patterns, as illustrated in FIGS. 1A and 1B . In some embodiments, the electric field is applied at an elevated temperature, although this is not always necessary. In particular, for very thin piezoelectric films, such as less than 20 μm, for example less than 10 μm, or less than 5 μm, it is possible to pole the film without an elevated temperature if the electric field is sufficiently strong. In the case of PVDF, an additional advantage is that such films are also optically transparent, allowing them to be bonded to the electrophoretic medium between the viewing surface and the electrophoretic medium, or the electrophoretic medium can be layered between the piezoelectric film and the viewing surface.

An exemplary method for poling a thin film of a piezoelectric material is illustrated in FIGS. 2A through 2D. A thin film of a piezoelectric material (210), such as PVDF, may be melted and spin-coated onto a substrate (220) to form a thin film. The thin film may optionally be thermally conditioned or stretched prior to poling. Suitable bulk PVDF is available, for example, as a bulk powder or as a film from Sigma-Aldrich. Pre-stretched piezoelectrically active PVDF films are also available, for example, from PolyK Technologies (State College, PA, USA). Such films may also be supplied with a metallized electrode coating on one side, which may be used in piezoelectric films and displays, but poling the piezoelectric material with a backing metal layer using an electric field is difficult. Copolymers of PVDF, such as polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), are also available from both Sigma-Aldrich and PolyK. In some embodiments, thin films of PVDF and PVDF copolymers can be produced by preparing a concentrated solution of bulk PVDF in a compatible volatile solvent, such as dimethylformamide (DMF), and slot-coating the concentrated solution onto a suitable transfer substrate or release film, for example, using a roll-to-roll process. The PVDF-coated substrate is then heated to remove the DMF, producing a thin film of PVDF (e.g., less than 20 μm, e.g., less than 10 μm, less than 5 μm). By carefully controlling the thermal cycle, the resulting film can be preconditioned to have a greater number of beta phase domains suitable for poling.

As illustrated in FIGS. 2B and 2C, a thin film of piezoelectric material (210) can be poled with a high-voltage corona discharge (230) having a spatial focus. Suitable corona discharge equipment is available, for example, from Simco-Ion (Alameda, CA, USA). Such a device can generate a localized 10 to 50 kV field, for example, a 30 kV field, for example, a 20 kV field, which can be introduced within a few μm of the piezoelectric material to be poled. The spatial focus can be achieved by steering the electric field and/or gas flow that focuses/steers the flow of ions from the corona discharge. As illustrated in FIG. 2B, the high-voltage corona discharge (230) can be moved in three dimensions to create areas of differential polarization, i.e., to pattern the piezoelectric material (210). Alternatively, the piezoelectric material (210) may be mounted on an XYZ stage that allows the film workpiece to be accessed in a controlled manner by the high voltage corona discharge (230). In an alternative embodiment, a conductive mask (240) may be used to protect areas of the piezoelectric material (210) from the high voltage corona discharge (230), as illustrated in FIG. 2C. The conductive mask may be made of, for example, conductive stainless steel or other conductive material that can withstand proximity to the corona discharge. Alternative masks made of charge-absorbing or charge-blocking materials, such as glass, plastic, or rubber, may also be effective. When the high voltage corona discharge (230) is moved over the thin film of the piezoelectric material (210), the thin film of the piezoelectric material (210) is polled only in areas where the conductive mask (240) does not cover the thin film of the piezoelectric material (210). Additionally, the polarity of the high-voltage corona discharge (230) may be reversed such that some regions may be polarized in the first direction, some regions may be polarized in the second direction, and some regions may be randomly polarized or not polarized. See also FIGS. 3A to 3D.

Using the technique illustrated in FIGS. 2b and 2c, it is straightforward to form a thin film of piezoelectric material (210) having regions of differential polarization (P1 and P2), as illustrated at 260 and 270 in FIG. 2d. The regions of differential polarization (260 and 270) need not necessarily have opposite polarities of equal magnitude, but such an arrangement typically provides better contrast ratio when a two-particle electrophoretic medium is used with the thin film of piezoelectric material (210). For example, as illustrated in FIG. 2d, the first region (260) may be polarized toward the viewer, while the second region (270) may be polarized away from the viewer. This technique is further illustrated in FIGS. 3A through 3D , which show how a single region (360) of a thin film of piezoelectric material deposited on a substrate (320) can be polarized to have a polarization vector emanating from the page, as illustrated in FIG. 3B . Thus, when the thin film of piezoelectric material is manipulated (bent), it will preferentially drive one polarity of the electrophoretic particles toward the viewing surface. As illustrated in FIG. 3C , a second region (370) of the thin film of piezoelectric material can be polarized in a different direction, with or without the addition of a conductive mask (340), resulting in some patterned combination of polarities and sizes as required for the application. As illustrated in FIG. 3D , some portion of the region (370) is polarized toward the viewing surface, but has a shadow created by the conductive mask (340). Therefore, when the piezoelectric material is manipulated (bent), one polarity of the electrophoretic particles will be preferentially driven towards the viewing surface, except for the polarization-masked region, which will remain in a neutral color stage, thereby creating a pattern, e.g., a security seal.

Figures 2a through 3d illustrate various techniques that may be used to create regions of differential polarization in a thin film of piezoelectric material (210). As illustrated in Figures 4a through 4d, these same techniques may also be used to form regions of differential polarization in a thin piezoelectric electrophoretic medium film (405). As illustrated in Figure 4a, a thin film of piezoelectric material (410) may be bonded to a layer of electrophoretic microcells (420) to create the piezoelectric electrophoretic medium film (405). The thin film of piezoelectric material (410) may be bonded to the layer of electrophoretic microcells (420) with an adhesive layer (not shown), or the thin film of piezoelectric material (410) may be spin coated directly onto the layer of electrophoretic microcells (420) (i.e., as described above with respect to Figure 2a). The electrophoretic microcell (420) is typically formed from a polymer such as an acrylate, vinyl ether, or epoxide, as described in detail in, for example, U.S. Patent Nos. 6,930,818, 7,052,571, 7,616,374, 8,361,356, and 8,830,561, all of which are incorporated by reference in their entireties. In some embodiments, the layer of the electrophoretic microcell (420) may be filled with an electrophoretic medium (425) comprising two or more electrophoretic particles (423 and 427) typically having different electrophoretic mobilities and optical properties. The electrophoretic medium (425) may be sealed with a sealing layer (430), preferably a water-soluble sealing layer as described in U.S. Patent Nos. 7,560,004, 7,572,491, 9,759,978, or 10,087,344, all of which are incorporated by reference in their entireties. In some embodiments, a layer of electrophoretic microcells (420) is formed on a release, filled with electrophoretic medium (425), and sealed with a sealing layer (430), and then the filled and sealed electrophoretic microcells (420) are used as a substrate for the formation of a thin film of piezoelectric material (410). The resulting structure is a thin piezoelectric electrophoretic medium film (405). In another embodiment, a thin film of piezoelectric material (410) is laminated to an acrylate, vinyl ether, or epoxide film that is a precursor to a layer of electrophoretic microcells (420). The combined thin film of piezoelectric material (410) and precursor material is then embossed on the precursor side (described below), and thereafter filled with electrophoretic medium (425) and sealed with a sealing layer (430) to create a thin piezoelectric electrophoretic medium film (405). In yet another embodiment (not shown in FIGS. 4A-4D ), a complete microcell front laminate of the type described in U.S. Pat. No. 7,158,282 and commercially available from E Ink Corporation can be used as a substrate for the thin film of piezoelectric material (410), which can be laminated as described below. In particular, when a front laminate material is used, the final structure typically additionally includes a conductive layer that is optically transparent. The front laminate may be oriented so that the light-transmitting electrode layer contacts the thin film of piezoelectric material (410), or the front laminate may be flipped so that the sealing layer contacts the thin film of piezoelectric material (410).

Once the thin piezoelectric electrophoretic medium film (405) is created, the thin film of piezoelectric material (410) can be addressed as described above with respect to FIGS. 2A through 3D. That is, the thin film of piezoelectric material (410) can be poled with a spatially focused high voltage corona discharge (230), for example, by mounting the thin piezoelectric electrophoretic medium film (405) on an XYZ stage so that the film workpiece can be accessed by the high voltage corona discharge (230) in a controlled manner, as illustrated in FIG. 4B. In an alternative embodiment, a conductive mask (240) can be used to protect areas of the thin piezoelectric electrophoretic medium film (405) from the high voltage corona discharge (230), as illustrated in FIG. 4C. As described with respect to FIGS. 2A through 3D, the polarity of the high voltage corona discharge (230) can be reversed such that some regions can be polarized in a first direction, some regions can be polarized in a second direction, and some regions can be randomly polarized or not polarized. As shown in FIG. 2D above, poling a thin film of piezoelectric material (410) on a thin piezoelectric electrophoretic medium film (405) results in the formation of differential polarization regions (P1 and P2) as illustrated at 460 and 470 in FIG. 4D. Importantly, because the thin piezoelectric electrophoretic medium film (405) can be fabricated prior to poling, the end user can control the final step of creating the desired poling design on the thin piezoelectric electrophoretic medium film (405). Thus, if the final product includes a security seal or serial number, the security seal or serial number can be placed after the final product is completed and verified, etc. For example, a U.S. $100 bill can have a serial number printed in metallic ink by the U.S. Treasury Department, while a security ribbon containing a thin piezoelectric electrophoretic medium film (405) is simultaneously polled to generate a verification code corresponding to the serial number. This feature eliminates many logistical issues and associated costs, as it eliminates the need to match pre-manufactured security markers to specific products further down the supply chain.

The techniques described above can be used to achieve a wide variety of thin piezoelectric electrophoretic films as illustrated in the following drawings.

As illustrated in FIGS. 5A-6B, 8A-10C, and 12A-13B, the piezoelectric electrophoretic film or piezoelectric electrophoretic display comprises a layered stack of several components, some of which comprises a thin piezoelectric film and a layer of electrophoretic medium. The piezoelectric material can be any of the materials listed above, but polymers such as PVDF and its copolymers are preferred because they can be fabricated into very thin films. The electrophoretic medium typically comprises one or more sets of charged particles that migrate through a non-polar solvent in the presence of an electric field. The electrophoretic medium is typically contained in microcapsules, microcells, or dispersed droplets. The electrophoretic medium may also be contained in open troughs or wells that are sealed within a larger flexible container. The piezoelectric electrophoretic films and piezoelectric electrophoretic displays exemplified herein can be manufactured very thinly, for example, no more than 100 μm thick, for example, no more than 70 μm thick, for example, no more than 50 μm thick, for example, no more than 35 μm thick, for example, no more than 20 μm thick, for example, no more than 10 μm thick. Such thin materials can be bent without breakage or leakage, and are unnoticeable even when incorporated into final products such as paper or banknotes. Furthermore, most piezoelectric electrophoretic films or piezoelectric electrophoretic displays are entirely optically transparent and/or include sufficiently thin layers to be optically transparent so that the piezoelectric electrophoretic reaction can be viewed from above and below. In such piezoelectric electrophoretic films or piezoelectric electrophoretic displays, when a first image is viewed from the upper surface, for example, position 1 in FIG. 1B , the lower surface will typically exhibit a negative, for example, position 3 in FIG. 1B . However, when incorporating an electrophoretic medium having more than two types of particles, the upper and lower surfaces may not show inverted images due to mixed particle states on one of the two surfaces.

A piezoelectric electrophoretic film or piezoelectric electrophoretic display will often include at least one electrode layer, which may be optically transparent and flexible. Suitable materials include commercially available ITO-coated PET, which can be used as a substrate for fabrication. In some other embodiments, flexible and transparent conductive coatings may be used, including other transparent conducting oxides (TCOs), such as zinc oxide, zinc tin oxide, indium zinc oxide, aluminum zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, or fluorinated variants of these oxides, such as fluorine-doped tin oxide. In many of the embodiments described herein, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is used because of its excellent bending properties and optical transparency. While its overall conductivity is not as high as, for example, PET/ITO, PEDOT:PSS is sufficient to provide the electric field necessary to drive electrophoretic particles in the electrophoretic medium. Other materials include polymers, typically light-transmitting polymers, doped with conductive materials such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. In some cases, the electrode layer is a metal film, such as a copper, silver, gold, or aluminum film or foil. Metal-coated polymer films may also be suitable for use as the electrode layer. The resistivity of the electrode layer may be less than or equal to 500 Ohm-m, such as less than or equal to 100 Ohm-m, such as less than or equal to 1 Ohm-m, such as less than or equal to 0.1 Ohm-m, such as less than or equal to 0.01 Ohm-m. (For comparison, electrophoretic medium layers typically have a resistivity of approximately 10 ⁷ to 10 ⁸ Ohm-m, and piezoelectric materials have a resistivity of 10 ¹¹ to 10 ¹⁴ Ohm-m.)

A piezoelectric electrophoretic film or piezoelectric electrophoretic display will often include at least one adhesive layer formed from a polymer such as an acrylic or polyurethane. Polyurethanes, polyureas, polycarbonates, polyamides, polyesters, polycaprolactones, polyvinyl alcohols, polyethers, polyvinyl acetate derivatives such as poly(ethylene-co-vinylacetate), polyvinyl fluoride, polyvinylidene fluoride, polyvinyl butyral, polyvinylpyrrolidone, poly(2-ethyl-2-oxazoline), acrylic or methacrylic copolymers, maleic anhydride copolymers, vinyl ether copolymers, styrene copolymers, diene copolymers, siloxane copolymers, cellulose derivatives, gum arabic, alginates, lecithins, polymers derived from amino acids, and the like. The adhesive may further comprise one or more low-k polymers or oligomers, ionic liquids, or conductive fillers such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. An adhesive comprising such charged and/or conductive materials is a conductive adhesive. The polymers and oligomers used in the adhesive layer may have functional groups for chain extension or crosslinking during or after lamination. The adhesive layer may have a resistivity value of approximately 10 ⁶ Ohm*cm to 10 ⁸ Ohm*cm, preferably less than 10 ¹² Ohm*cm.

Among the polymers and oligomers mentioned above, polyurethanes, polyureas, polycarbonates, polyesters and polyamides, especially those containing functional groups, are particularly preferred due to their excellent adhesive and optical properties and high environmental resistance. Examples of functional groups include, but are not limited to, -OH, -SH, -NCO, -NCS, -NHR, -NRCONHR, -NRCSNHR, vinyl or epoxides and their derivatives, including cyclic derivatives. In the functional groups mentioned above, "R" may be hydrogen or an alkyl, aryl, alkylaryl or arylalkyl of 20 or fewer carbon atoms, wherein the alkyl, aryl, alkylaryl or arylalkyl may be optionally substituted or interrupted by N, S, O or halogen. "R" is preferably hydrogen, methyl, ethyl, phenyl, hydroxymethyl, hydroxyethyl, hydroxybutyl and the like. Particularly preferred are functionalized polyurethanes, such as hydroxyl-terminated polyester polyurethanes or polyether polyurethanes, isocyanate-terminated polyester polyurethanes or polyether polyurethanes or acrylate-terminated polyester polyurethanes or polyether polyurethanes.

In many embodiments, the piezoelectric electrophoretic film or piezoelectric electrophoretic display will often include a release sheet. The release sheet may be used temporarily to facilitate processing of the piezoelectric electrophoretic film or piezoelectric electrophoretic display, such as during embossing, filling, cutting, etc. In other embodiments, the release sheet may be used to transfer the final piezoelectric electrophoretic film or piezoelectric electrophoretic display to be bonded to the final product. In some cases, the release sheet will protect a functional adhesive layer that will be used to manipulate the piezoelectric electrophoretic film or piezoelectric electrophoretic display before it is placed into the final product. The release sheet may be formed of a material selected from the group consisting of polyethylene terephthalate (PET), polycarbonate, polyethylene (PE), polypropylene (PP), paper, and laminated or cladded films thereof. The release sheet may also be metallized to facilitate quality control measurements and/or to control static electricity during handling, shipping, and downstream integration into the product. In some embodiments, a silicone release coating may be applied over the release to improve release properties.

Although not shown in FIGS. 5A-6B, 8A-10C, and 12A-13B, the piezoelectric electrophoretic film or piezoelectric electrophoretic display may also include additional edge seals and/or barrier materials to allow the piezoelectric electrophoretic film or piezoelectric electrophoretic display to maintain a desired humidity level and prevent leakage of, for example, non-polar solvents or adhesives, and to prevent ingress of water, dirt, or gases. The barrier material may be any flexible material, typically a polymer having a low or negligible water vapor transmission rate (WVTR). Suitable materials include polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, cyclic olefin, and combinations thereof. If the piezoelectric electrophoretic film or piezoelectric electrophoretic display will be exposed to particularly harsh conditions, a flexible glass, such as WILLOW® glass (Corning, Inc.), may be used in the barrier layer. The edge seal may be a metallized foil or other barrier foil attached to the edge of the piezoelectric electrophoretic film or piezoelectric electrophoretic display. The edge seal may also be formed from a crosslinkable, dispensable sealant (thermally, chemically, and/or radiation cured), polyisobutylene, or an acrylate-based sealant. In some embodiments, the edge seal may be a sputtered ceramic such as alumina or indium tin oxide, or an advanced ceramic available from Vitex Systems, Inc. (San Jose, CA, USA).

In general, the layers (501-504) of the piezoelectric electrophoretic film can be arranged/stacked in a sequence that produces the best performance for the final application. For example, as illustrated in FIG. 5A, the piezoelectric electrophoretic film (501) can be prepared by placing a microcell precursor material on a release (510) including a release adhesive (520). The microcell precursor can then be embossed or photolithographed to produce a microcell array (530). The microcells (530) can be thermally cured or cured with electromagnetic radiation, such as UV light. The microcells (530) can then be filled with an electrophoretic medium and sealed with a sealing layer (540), as described above with respect to FIG. 4A. (Although the electrophoretic medium is not depicted in the drawings below, it will be appreciated that the microcells (530) adjacent to the sealing layer (540) are filled with an electrophoretic medium comprising charged particles in a non-polar solvent.) A piezoelectric layer (560) may be laminated to the sealing layer (540) using an adhesive (550), which will typically be an optically transparent adhesive formed from one of the materials listed above. Finally, a flexible electrode (580) will be bonded to the piezoelectric film using a conductive adhesive (570). This piezoelectric film (501) may be subsequently manipulated by processing the release (510) until the stack, minus the release (510), is attached to the final product. In the piezoelectric film (501), the piezoelectric layer (560) is typically polarized to create a differential polarization region before the flexible electrode (580) is bonded to the piezoelectric film. In some embodiments, the flexible electrode (580) and conductive adhesive (570) may be replaced with a thin layer of a transparent conductive oxide, such as ITO. The ITO may be sputtered directly onto the piezoelectric layer (560).

A closely related, but alternative, stack is illustrated in FIGS. 5B-5D . In FIG. 5B , a piezoelectric film (502) is produced, wherein a piezoelectric layer (560) is prepared prior to fabrication on a separate release (510). For example, the piezoelectric layer (560) may be a pre-stretched PVDF film that has already been poled to create a security pattern. The piezoelectric layer (560) is then bonded to a sealed microcell layer (530) that is bonded to a flexible electrode (580). In particular, in the piezoelectric film (502), the apertures in the microcell layer (530) face away from the piezoelectric layer (560), which may facilitate good bonding between the microcell layer (530) and the piezoelectric layer (560). This bonding can be improved by introducing a primer (535) to improve adhesion of the piezoelectric layer (560) to the microcellular material, typically a polymer comprising an acrylate, vinyl ether, or epoxide. The primer (535) may be a polar oligomeric or polymeric material, such as a polyhydroxy-functionalized polyester acrylate (e.g., BOMAR® BDE 1025, Dymax) or an alkoxylated acrylate, such as an ethoxylated nonyl phenol acrylate (e.g., SR504, Sartomer), an ethoxylated trimethylolpropane triacrylate (e.g., SR9035, Sartomer), or an ethoxylated pentaerythritol tetraacrylate (e.g., SR494, Sartomer). Examples of suitable polar polymers for use as the primer (535) include solvent urethane polymers, such as Irostic® polymers.

Of course, it is also possible to build the stack such that the openings of the microcell layer (530) face the piezoelectric layer (560), as in the piezoelectric electrophoretic film (504) illustrated in FIG. 5d. As a further alternative illustrated in FIG. 5c, the piezoelectric electrophoretic film (503) is arranged such that the openings of the microcell layer (530) face away from the piezoelectric layer (560), but the piezoelectric layer (560) is directly bonded to the flexible electrode (580).

The piezoelectric electrophoretic films (501, 502, 503, 504) illustrated in FIGS. 5A-5D can be converted into piezoelectric electrophoretic displays (601, 602) by adding a second flexible electrode (680) in place of the release layer of FIGS. 5A-5D. The piezoelectric electrophoretic displays (601, 602) will typically also include a second conductive adhesive (670), although it should be noted that in some cases the conductive adhesive (670) alone may be sufficient to provide the electric field necessary to switch the electrophoretic material. Additionally, it is possible to directly coat the underside of the microcell layer (530) (FIG. 6A) or the sealing layer (540) (FIG. 6B) with a thin layer of a transparent conductive oxide to create the second electrode. Additionally, if it is not necessary to view through both the top and bottom of the piezoelectric electrophoretic display (601, 602), a conductive metal foil may be used as the second flexible electrode (680). As shown in FIGS. 6A and 6B , it is common to add a release (510) to the completed piezoelectric electrophoretic display (601, 602) to improve handling, and to provide a ready-to-use adhesive to attach the piezoelectric electrophoretic display (601, 602). In some embodiments, the piezoelectric electrophoretic display (601) may be formed by simply bonding the piezoelectric layer (560) to a commercial front laminate comprising the second flexible electrode (680) and a sealed microcell layer (530) comprising an electrophoretic medium. In such a case, the piezoelectric layer (560) is typically polarized to create a differential polarization region before the front laminate is bonded to the piezoelectric layer (560). Although the piezoelectric electrophoretic display (601, 602) of FIGS. 6A and 6B is illustrated as having a piezoelectric layer (560) above a sealed microcell layer (530), it will be appreciated that the piezoelectric layer (560) may also be positioned below the sealed microcell layer (530) to create a piezoelectric electrophoretic display similar to FIGS. 5B and 5D.

Prototype Performance

A series of piezoelectric electrophoretic films of the type illustrated in FIG. 5A were produced using PEDOT:PSS films as flexible electrodes (580). The piezoelectric layer (560) was varied (composition and thickness) as shown in Table 1. The piezoelectric films were sourced from TE Connectivity (Norwood, MA, USA), Fishman (Andover, MA, USA), or cast and cured in-house using PVDF powder from Sigma-Aldrich. Patterns were generated by varying the polarization direction using the described poling technique. The electrophoretic medium comprised low-voltage formulations of black and white particles, black and red particles, or red and black particles designed to switch color states at +/- 3 V. As shown in Table 1, all variations provided suitable switching.

Experiment number Rear collector piezoelectric element Thickness (um) Polarization direction in contact with EPD polarization source Performance Evaluation 1 PEDOT TE PVDF extrusion, drawing, and polling 10 G Pre-fab Good 2 PEDOT TE PVDF extrusion, drawing, and polling 10 G Pre-fab Good 3 PEDOT TE PVDF extrusion, drawing, and polling 10 A Pre-fab Good 4 PEDOT TE PVDF extrusion, drawing, and polling 10 A Pre-fab Good 5 PEDOT Fishman cast copolymer 80/20 5 A Pre-fab Good 6 PEDOT Fishman cast copolymer 80/20 5 A Pre-fab Good 7 PEDOT In-house cast copolymer 80/20 3 A Polled by E field Good 8 PEDOT TE PVDF extrusion, drawing, and polling 10 G and A all together Corona discharge Good

Table 1: Prototype piezoelectric electrophoretic films

Table 1 suggests that many types of electrophoretic media will respond appropriately to the small electric fields generated by bending a thin piezoelectric film. In particular, spin-coated polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) films less than 3 μm were found to have sufficient charge injection to switch the DV electrophoretic media. See Experiment No. 7. Such a piezoelectric electrophoretic film (801) (see FIG. 8a) can be formed using the method described in FIG. 7. First, a thin film of piezoelectric material (940) is formed by casting (slot die coating) a concentrated PVDF/DMF solution onto a suitable substrate and heating to remove the solvent, as in step 710 of FIG. 7. In step 720, the piezoelectric film (960) is removed from the substrate. The cast piezoelectric film (960) may have a thickness of 10 μm or less, for example, 5 μm or less, for example, 3 μm or less. The piezoelectric film (960) may also be stretched and/or poled with a suitable electric field to increase the number of beta phase domains as described above. In step 730, a release (910) is provided along with an adhesive (920), and in step 740, the release (910) and adhesive (920) are subsequently laminated to the cast piezoelectric film (960). Then, in step 750, the piezoelectric film (960) is coated/bonded with an electrophoretic layer. As illustrated in FIGS. 9A and 9B, the electrophoretic layer may be a sealed microcell layer comprising filled microcells (930) and a sealing layer (940), or alternatively, the electrophoretic layer may comprise an electrophoretic medium (990) encapsulated within a polymer binder (995). Bonding of the piezoelectric film (960) to the electrophoretic layer can be facilitated by an intervening primer layer (935), for example, using one of the primer materials described above. If the electrophoretic layer is a sealed microcell layer, the microcell (930) can be positioned such that the sealing layer (940) is adjacent to the piezoelectric film (960), as in FIG. 8A, or the microcell (930) can be positioned such that the sealing layer (940) is positioned on the opposite side of the piezoelectric film (960), as in FIG. 8B. As a final step 760, an electrode layer (980) is created and bonded/deposited to one of the microcells (930), as in FIG. 8A, or bonded/deposited to the sealing layer (940), as in FIG. As described above, the electrode layer (980) may comprise a flexible conductive material such as PEDOT:PSS, or may comprise a directly deposited (e.g., sputtered or vapor-deposited) transparent conductive oxide (TCO). In some embodiments, the electrode (980) may comprise a prefabricated film of ITO on a polymer substrate such as PET. The piezoelectric electrophoretic film (801) comprising the directly deposited TCO electrode layer (980), the thin piezoelectric layer (960), and the thin layer of microcells (930) (approximately 10 μm thick) is excessively thin (i.e., less than 25 μm thick, excluding the release (910)), which allows the piezoelectric electrophoretic film (801) to be flexed without failure and is unnoticeable when attached to an object such as a banknote. The corresponding piezoelectric electrophoretic film (901) comprising microcapsules may also be fabricated with a total thickness of less than 25 μm. Of course, alternative configurations using a thin piezoelectric film (960) are also possible, such as positioning the piezoelectric film (960) between the electrode (980) and the electrophoretic layer, i.e., the layer of microcapsules (990), as illustrated in FIG. 9B. Alternatively, the electrode (980) of FIGS. 8A-9B may be replaced with a conductive adhesive (not illustrated) or with a conductive adhesive together with an additional release layer (not illustrated).

Similar to FIGS. 6A and 6B, the piezoelectric electrophoretic films of FIGS. 8A-9B can include a second electrode layer to form corresponding displays (1001, 1002, 1003) as illustrated in FIGS. 10A-10C. Both the electrode layer (980) and the second electrode layer (1080) can include a flexible conductive material such as PEDOT:PSS, or both the electrode layer (980) and the second electrode layer (1080) can include a directly deposited (e.g., sputtered or vapor deposited) transparent conducting oxide (TCO) or some combination thereof. Again, when both the electrode layer (980) and the second electrode layer (1080) use thin TCO films, the resulting piezoelectric electrophoretic displays (1001, 1002, 1003) can be manufactured very thinly, i.e., less than 25 μm thick, excluding the release (910). In some embodiments, the electrode layer (980) is produced by bonding/depositing onto a microcell (930), as in FIG. 10A. In other embodiments, the electrode layer (980) is bonded/deposited onto a sealing layer (940), as in FIG. 10B. The assembly of the piezoelectric electrophoretic displays (1001 and 1002) can also be used with microcapsules (990) containing an electrophoretic medium held together with a binder (995), thus producing a piezoelectric electrophoretic display (1003), as illustrated in FIG. 10C. Alternatively, the electrodes (980/1080) of FIGS. 10A-10C may be replaced with a conductive adhesive (not shown) or with a conductive adhesive together with an additional release layer (not shown).

An alternative method for constructing a piezoelectric electrophoretic film and a piezoelectric electrophoretic display is described in connection with the flow chart of FIG. 11 . A piezoelectric film (1260) is prepared, which may be a commercial film or a cast film as described above. The piezoelectric film (1260) is laminated to a microcell precursor material in step 1110 . The piezoelectric film (1260) may be stretched and/or poled prior to lamination. The precursor material is typically an acrylate polymer, but any suitable embossable material, such as a vinyl ether polymer or an epoxide polymer film, may be used. Typically, the precursor film is less than or equal to 30 μm thick, such as less than or equal to 20 μm thick. The precursor film may be treated with a primer (1235) prior to laminating step 1110 . Once the piezoelectric film (1260) and the microcell precursor material are combined, the side of the piezoelectric film (1260) opposite the microcell precursor material is coated with a transparent conductive material, typically indium tin oxide, selected from those described above. (Alternatively, depending on the application, the side of the piezoelectric film (1260) opposite the microcell precursor material may be coated with a conductive adhesive that can be carried by the release layer.) This coating step creates the electrodes (1280) depicted in the piezoelectric electrophoretic film (1201) and the piezoelectric electrophoretic display (1202) illustrated in FIGS. 12A and 12B, respectively. (Although not shown in FIG. 11, an alternative configuration is to obtain a piezoelectric film (1260) that is pre-coated with a transparent conductive material, and then laminate the pre-coated piezoelectric film (1260) and the microcell precursor material together, including optional use of a primer (1235). After the stack of electrodes (1280), piezoelectric film (1260), and microcell precursor is created, the stack is laminated to a carrier substrate (1255) using an adhesive layer (1250), as shown in step 1130. The carrier substrate (1255) can be any of the materials described above for use as a release, and the adhesive (1250) can be any of the adhesives described above. In practice, the carrier substrate (1255) is typically PET, as PET sheets are easy to handle during the embossing step (1140). In step 1140, the stack comprising the carrier substrate (1255), the adhesive (1250), the piezoelectric film (1260), and the microcell precursor is microembossed using the technique described above in connection with U.S. Patent Nos. 6,930,818, 7,052,571, 7,616,374, 8,361,356, and 8,830,561. When this process is completed with the thin piezoelectric film and the thin microcell precursor, the final stack thickness (excluding the carrier substrate) may be less than 30 μm thick, such as less than 20 μm thick. This results in an open microcell structure that is subsequently filled with a desired electrophoretic medium in step 1150 and sealed with a water-soluble sealing layer (1240). The sealing layer (1240) may be made conductive by including a conductive species. The sealing layer (1240) is typically light-transmitting or transparent. The open microcells may be cleaned/activated by a vapor plasma treatment (1145) before the microcells are filled with the desired electrophoretic medium. Finally, in step 1160, a release sheet (1210) is bonded to the sealing layer (1240) with an adhesive (1220), facilitating transport of the piezoelectric electrophoretic film (1201) and placement of the piezoelectric electrophoretic film (1201) on the final product. The adhesive (1220) may also be conductive. The resulting structure is illustrated in FIG. 12A. Importantly, by completing the steps of FIG. 11 without poling the piezoelectric electrophoretic film (1260), for example, by creating regions of differential polarity by corona discharge as described above, it is possible to enable the end customer to pattern the piezoelectric electrophoretic film (1201) at the location of the final assembly.

As illustrated in FIG. 12B, the method of FIG. 11 can be extended to create a piezoelectric electrophoretic display (1202) with the addition of a second electrode (1285). The second electrode (1285) may also comprise a transparent conductive material that is added directly to the sealing layer (1240) instead of the release (1210) and adhesive (1220). However, in another embodiment, the release (1210) would be eliminated, and the second electrode (1285) would be laminated to the sealing layer (1240) with the adhesive (1220). If the piezoelectric electrophoretic display (1202) does not require the electrophoretic medium to be visible from both sides, the second electrode (1285) may be a metal film. Alternatively, the second electrode (1285) may be a conductive polymer, such as PEDOT:PSS. In some other embodiments, the adhesive (1220) may be a conductive adhesive that provides sufficient conductivity to act as the second electrode (1285).

Finally, it should be noted that the electrodes need not be bonded to the piezoelectric film (1260) prior to embossing the stack comprising the piezoelectric film (1260) and the microcell precursor material. Rather, a stack comprising the release (1210), the adhesive (1220), the piezoelectric film (1260), and the microcell precursor material may be prepared, and then the microcell precursor may be embossed, filled, and sealed as described above. Alternatively, as illustrated in FIG. 13B , a stack comprising the release (1210), the adhesive (1220), the electrode (1285), the piezoelectric film (1260), and the microcell precursor may also be prepared, and then the microcell precursor may be embossed, filled, and sealed as described above. The resulting piezoelectric electrophoretic film (1301) and piezoelectric electrophoretic display (1302) are illustrated in FIGS. 13A and 13B , respectively. The piezoelectric electrophoretic film (1301) and the piezoelectric electrophoretic display (1302) may be advantageous in applications where it is desirable to have the piezoelectric electrophoretic film (1260) as close as possible to the attachment surface on the final product, i.e., where the piezoelectric electrophoretic film (1301) is used as a strain sensor and it is important that the intervening electrophoretic medium layer does not dissipate force from the surface.

Figure 14 is a flowchart detailing the steps of a method (1400) for producing a high-contrast piezoelectric electrophoretic film and a piezoelectric electrophoretic display. The method (1400) is optimized to produce piezoelectric electrophoretic films using a roll-to-roll manufacturing process.

A method (1400) is described with reference to FIGS. 15A-15F. The method (1400) begins at step 1410, where a first electrode, i.e., electrode (1550), is formed on a first substrate, i.e., substrate (1555), by depositing an electrically conductive material, e.g., selected from those described above, onto the substrate. For example, a thin layer of the electrically conductive material may be deposited (e.g., sputtered, vapor-deposited) directly onto a suitable substrate, such as a polymer substrate (e.g., PET). In some embodiments, the substrate (1555) may be any of the materials described above for use as a temporary release sheet to facilitate the fabrication of the piezoelectric electrophoretic film. In some embodiments, the electrically conductive material used to form the electrode (1550) is an adhesive or tie layer comprising a transparent conductive material (e.g., a first electrically conductive adhesive) comprising a conductive metal oxide, a conductive polymer, and/or other suitable conductive agent, which is coated on the substrate (1555). In some embodiments, to form the electrode (1550), an adhesive or tie layer is deposited on the substrate (1555), and a conductive polymer, such as PEDOT, is deposited over the tie layer. In some cases, the electrode (1550) is a metal film, such as a copper, silver, gold, or aluminum film or foil, which is bonded to a flexible, optically transparent substrate, such as a polymer film. In some embodiments, the electrode (1550) has a thickness of less than 5 μm. In some embodiments, the electrode (1550) has a thickness of 1 to 3 μm.

Next, in step 1420, a piezoelectric layer (1560) is formed on the electrode (1550) by depositing a piezoelectric material on the electrically conductive material (e.g., the electrode (1550)). For example, the electrode (1550) can be coated with a thin film of a piezoelectric material selected from those described above, such as PVDF, using a spin coating process as described above or casting (e.g., slot die coating). In some embodiments, a film deposition process, such as printing, spraying, or gravure coating, is used to form the piezoelectric layer (1560) on the electrode (1550). In some embodiments, the resulting piezoelectric layer (1560) has a thickness of less than 10 μm. In some embodiments, the resulting piezoelectric layer (1560) has a thickness of about 3 μm.

After the piezoelectric layer (1560) is formed on the electrode (1550), a mask (1540) is applied to the piezoelectric material of the piezoelectric layer (1560), as illustrated in step 1430. As in the embodiments described above, the mask (1540) may be used to shield or insulate a first plurality of regions of the piezoelectric layer (1560) from the high voltage corona discharge (1533) used to poll the piezoelectric layer (1560). This allows the piezoelectric layer (1560) to be patterned with images, text, and other information (e.g., machine readable code). The mask (1540) may be made of a material having a dielectric strength sufficient to withstand at least a localized 5 kV field. In some embodiments, the mask (1540) is formed of a disposable material that can be applied in a roll-to-roll process. For example, the mask (1540) may be formed of paper, such as electrically insulating paper, having a pressure-sensitive adhesive applied to one surface for bonding to the piezoelectric layer (1560). Alternatively, the mask (1540) may be a reusable fixture made of a charge-absorbing or charge-blocking material that is integrated into a screen printing process or a roll-to-roll translational stage similar to a rotating roller.

FIG. 15A is a schematic cross-sectional view (1501) of a piezoelectric electrophoretic film in step 1440 of the method illustrated in FIG. 14. As illustrated in FIG. 15A, the mask (1540) includes a masking portion (1542) that shields or insulates a corresponding region of the piezoelectric layer (1560) from a high voltage corona discharge (1533), and a non-masking portion (1544) that allows a corresponding region of the piezoelectric layer (1560) to be polarized in an area where the mask (1540) does not cover the piezoelectric layer (1560). The piezoelectric material is then polarized in step 1440. For example, a high voltage corona discharge (e.g., high voltage corona discharge (1533)) or another suitable electric field described above may be used to polarize the unmasked portion of the piezoelectric layer (1560) in the first direction while leaving the masked portion unpolarized. If a disposable material such as paper was used for the mask, the mask is removed from the piezoelectric layer (1560).

In some embodiments, the patterning process of the piezoelectric material may begin with a sheet or roll of a rolled PVDF film. The film may be patterned by laser cutting, laser ablation (e.g., laser photoablation), die cutting, or other cutting methods, and then laminated to an electrode layer, or to a front-side laminate or FPL based on microcapsules or microcells as described above.

FIG. 15B is a schematic cross-sectional view (1502) of the piezoelectric electrophoretic film after completion of step 1440 of the method illustrated in FIG. 14. As illustrated in FIG. 15B, the piezoelectric layer (1560) now includes a non-polarized portion (1562) corresponding to the position of the masked portion (1542) of the mask (1540) and a polarized portion (1564) corresponding to the position of the non-masked portion (1544) of the mask (1540).

Next, in step 1450, the piezoelectric material is bonded to the electrophoretic material. For example, the piezoelectric layer (1560) may be coated with an electrophoretic medium layer comprising a plurality of microcapsules (not shown in FIG. 15B) containing a non-polar fluid and charged pigment particles. Alternatively, an electrophoretic medium layer comprising microcells (1530) may be formed on the piezoelectric layer (1560) using a process similar to that shown in the flowchart of FIG. 11. For example, an embossable microcell precursor material may be laminated onto the piezoelectric layer (1560). Prior to lamination, the precursor material may be treated or coated with a microcell primer, such as using one of the primer materials described above. In some examples, the microcell primer comprises a thermoplastic or thermosetting material or a precursor thereof, such as a polyurethane, a multifunctional acrylate or methacrylate, such as lauryl methacrylate, vinylbenzene, vinyl ether, epoxide, or an oligomer or polymer thereof.

The microcell precursor is microembossed using the technique described above to form an open microcell structure, which is then filled with a desired electrophoretic medium and sealed with a sealing layer (1535) as illustrated in FIG. 15C, which is a schematic cross-sectional view (1503) of the piezoelectric electrophoretic film after completion of step 1450 of the method illustrated in FIG. 14. The open microcell may optionally be cleaned/activated with a vapor phase plasma treatment (1145) before the microcell (1530) is filled with the desired electrophoretic medium.

In some embodiments, the layer of microcells (1530) has a thickness of 8 to 20 μm and the sealing layer (1535) has a thickness of 3 to 10 μm. In some embodiments, the layer of microcells (1530) has a thickness of about 10 μm and the sealing layer (1535) has a thickness of about 5 μm.

In an alternative embodiment, instead of forming microcells on the piezoelectric layer (1560), the piezoelectric layer (1560) as illustrated in FIG. 15b is bonded or laminated to a microcapsule or microcell based front laminate or FPL as described above.

In step 1460, a second electrode, i.e., electrode (1585), is formed on a second substrate, i.e., substrate (1586). Electrode (1585) may be formed on substrate (1586) using one of the processes described above with respect to electrode (1550) and substrate (1555). As an example, substrate (1586) may be a release sheet temporarily used to facilitate the manufacture of a piezoelectric electrophoretic film, and electrode (1585) may be formed from an adhesive or tie layer comprising a transparent conductive material that is deposited on substrate (1586). In some embodiments, electrode (1585) has a thickness of less than 5 μm. In some embodiments, electrode (1585) has a thickness of 1 to 3 μm.

Next, in step 1470, the electrically conductive material deposited on the second substrate is bonded to the electrophoretic material. For example, the substrate (1586) and the electrode (1585) may be laminated to the sealing layer (1535) of the microcell (1530) to form the structure illustrated in FIG. 15D, which is a schematic cross-sectional view (1504) of the piezoelectric electrophoretic film after completion of step 1470 of the method illustrated in FIG. 14. By adding the electrode (1585) (and the substrate (1586)) to the piezoelectric electrophoretic film, a piezoelectric electrophoretic display that can be bonded to a target object is formed. For example, the structure illustrated in FIG. 15D may be a piezoelectric electrophoretic display sandwiched between two release sheets (e.g., substrates (1550) and (1586)).

In an alternative embodiment not shown in FIG. 14, steps 1460 to 1470 are replaced by the following process. The sealing layer (1535) of the microcell (1530) may be coated with one of the materials described above or an electrically conductive material such as PEDOT to form the electrode (1585). Subsequently, the electrode (1585) may be coated with an adhesive material (e.g., a heat seal adhesive (“HSA”) or one of the materials described above) to bond the piezoelectric electrophoretic display to the target object in step 1480 described below.

Returning to FIG. 14, at step 1480, the piezoelectric electrophoretic display can be bonded to a target object. For example, a piezoelectric electrophoretic display such as that illustrated in FIG. 15d can be processed and attached to a target object (1588) such as paper or banknotes, as illustrated in FIG. 15e, which is a cross-sectional view (1505) of a piezoelectric electrophoretic film bonded to a target object after completion of step 1480 of the method illustrated in FIG. 14.

In some embodiments, the substrate (1586) is a release sheet that is peeled or removed from the electrode (1585), and the electrode (1585) is bonded to the surface of the target object (1588) using a hot stamping process. For example, heat and pressure may be applied to the piezoelectric electrophoretic display and/or the target object (1588) in a roll-to-roll hot stamping process that presses the piezoelectric electrophoretic display against the target object (1588). After peeling the substrate (1586), an adhesive (not shown) left on the electrode (1585) is activated by the heat and pressure, bonding the piezoelectric electrophoretic display to the target object (1588).

In some embodiments, the electrodes (1585) are formed from an adhesive or tie layer that is activated during the bonding process to bond the piezoelectric electrophoretic display to the target object (1588). In some embodiments, a roll-to-roll lamination process is used to bond the piezoelectric electrophoretic display to the target object (1588).

In some embodiments, the substrate (1555) and the substrate (1586) are release sheets, and the force required to remove each release sheet is individually adjusted to ensure that the substrate (1586) is removed before the substrate (1555) during the bonding process. For example, the adhesive used to temporarily bond the substrate (1555) and the substrate (1586) to the electrode (1550) and the electrode (1585), respectively, can be formulated or selected such that the force required to peel the substrate (1586) from the piezoelectric electrophoretic display is less than the force required to peel the substrate (1555) from the piezoelectric electrophoretic display.

FIG. 15F is a cross-sectional view (1506) of a piezoelectric electrophoretic film bonded to a target object and coated with a protective coating after completion of step 1480 of the method illustrated in FIG. 14. As illustrated in FIG. 15F, after bonding the piezoelectric electrophoretic display to the target object (1588), the substrate (1555) may be peeled off from the electrodes (1550), and a protective coating (1589) may be applied to the surface of the piezoelectric electrophoretic display and the target object (1588) to which the piezoelectric electrophoretic display is bonded. In some embodiments, the protective coating (1589) is a lacquer layer applied to the surface of the piezoelectric electrophoretic display and the target object (1588) using a printing process. Suitable materials for the protective coating (1589) may include UV curable polyester acrylates, polyurethane acrylates, UV curable epoxides, and thermally curable epoxides, or any material sufficient to seal the piezoelectric electrophoretic display and object (1588) to block dust and avoid excessive absorption of moisture.

Accordingly, the process described in connection with FIG. 14 can be used to create a piezoelectric electrophoretic display having a much smaller thickness than conventional displays, thereby significantly increasing the overall thickness or allowing it to be bonded to an object, such as a banknote or currency, with a substantially inconspicuous appearance. In some embodiments, the total thickness of the piezoelectric electrophoretic display can be from 50 μm to 100 μm. In some embodiments, the total thickness of the piezoelectric electrophoretic display can be from 25 μm to 50 μm. In some embodiments, the total thickness of the piezoelectric electrophoretic display can be less than 25 μm.

FIG. 16 illustrates an enlarged view (1600) of a partial cross-section of a piezoelectric electrophoretic display (1601) fabricated in accordance with the invention disclosed herein. For the purpose of illustrating the operation of the display, only a subset of the layers of the piezoelectric electrophoretic display (1601) are shown: an electrode (1550), a piezoelectric layer (1560) comprising a non-polarized portion (1562) and a polarized portion (1564), a microcell (1530), a sealing layer (1535), and a second electrode (1585). In the enlarged view (1600), the microcell (1530) and the sealing layer (1535) are represented as the electrophoretic layer (1631).

The enlarged view (1600) includes one of the non-polarized portions (1562) and one of the polarized portions (1564). As indicated by the dotted line (1602), a first portion (1632) of the electrophoretic layer (1631) is positioned over the non-polarized portion (1562), and a second portion (1634) is positioned over the polarized portion (1564). The first portion (1632) and the second portion (1634) each have an electrical resistance based on the volume of the electrophoretic layer (1631) they contain. Additionally, the non-polarized portion (1562) also has an electrical resistance based on the volume of the piezoelectric layer (1560) that the non-polarized portion (1562) contains. As indicated by the "+" and "-" signs, a voltage was generated at the polarized portion (1564) of the piezoelectric layer (1560) in response to, for example, bending or mechanical stress on the piezoelectric material.

Fig. 17 illustrates an exemplary equivalent circuit (1700) of the enlarged cross-section shown in Fig. 16. Three nodes or points shown in Fig. 16, namely 'A' at the polarized portion (1564), 'B' at the electrode (1585), and 'C' at the electrode (1550), correspond to the same points shown in the equivalent circuit (1700) of Fig. 17. Resistance R ₁ corresponds to the sum of the electrical resistances of the first portion (1632) of the electrophoretic layer (1631) and the non-polarized portion (1562) of the piezoelectric layer (1560). Resistance R ₂ corresponds to the electrical resistance of the second portion (1634) of the electrophoretic layer (1631).

The polarized portion (1564) of the piezoelectric layer (1560) is represented as a battery, and the voltage V _PZ is the voltage generated by the polarized portion (1564) across points A and C. Resistors R ₁ and R ₂ are represented in series because the presence of a voltage source beneath part of the electrophoretic layer (1631) effectively divides the layer into distinct sections (as indicated by the dashed line (1602)) with different electrical properties. For example, when voltage V _PZ is generated, the voltage potential at point A is higher than at points B or C. Using a conventional current flow paradigm, current (1701) flows from point A through resistor R ₁ to point B, and from point B through resistor R ₂ to point C. Accordingly, the voltage generated across resistor R ₁ is opposite in polarity to the voltage generated across resistor R ₂ . In effect, two opposing voltages are generated in series across separate portions of the electrophoretic layer.

Thus, fabricating a piezoelectric electrophoretic display as described in connection with method (1400) offers advantages over conventional piezoelectric electrophoretic displays. For example, selectively poling the piezoelectric layer advantageously provides an improved means for driving oppositely charged pigment particles within the electrophoretic medium in opposite directions, even in the absence of an individually addressable pixel electrode matrix. Thus, a piezoelectric electrophoretic display produced according to method (1400) can be made thin enough for use in applications that require durability and being substantially unobtrusive when integrated into a thin, low-profile end product, such as paper or banknotes, while still providing a high contrast ratio between polarized and non-polarized regions due to the effects described above. Furthermore, in this embodiment, only a single poling operation is required for the piezoelectric layer.

Figure 18 is a flowchart detailing the steps of a method (1800) for producing a high-contrast piezoelectric electrophoretic film and a piezoelectric electrophoretic display. The method (1800) is optimized to produce piezoelectric electrophoretic films using a roll-to-roll manufacturing process.

The method (1800) is described with reference to FIGS. 19A through 19G. To facilitate understanding, the same or similar reference numerals and designations have been used in FIGS. 19A through 19G to designate elements that correspond to or are functionally similar to those illustrated in FIGS. 15A through 15F, where possible. However, those skilled in the art will appreciate that the elements of the embodiments described herein are not necessarily identical in composition and structure, and that elements disclosed with reference to one embodiment may be advantageously utilized in other embodiments without specific citation.

The method (1800) begins at step 1810, where a piezoelectric layer (1960) is formed on a temporary substrate (1965) by depositing a piezoelectric material on the temporary substrate (1965). For example, the temporary substrate (1965) can be coated with a thin film of a piezoelectric material selected from those described above, such as PVDF, using a spin coating process or casting (e.g., slot die coating) as described above. The temporary substrate (1965) can be a release sheet temporarily used to facilitate the fabrication of the piezoelectric electrophoretic film. In some embodiments, the temporary substrate (1965) is a release sheet formed of a material selected from the group consisting of polyethylene terephthalate (PET), polycarbonate, polyethylene (PE), polypropylene (PP), paper, and laminated or cladded films thereof. A silicone release coating can also be applied on the temporary substrate (1965) to improve release properties. FIG. 19a is a schematic cross-sectional view (1901) of a piezoelectric electrophoretic film in step 1810 of the method (1800).

In some embodiments, a film deposition process, such as printing, spraying, or gravure coating, is used to form the piezoelectric layer (1960) on the temporary substrate (1965). In some embodiments, the resulting piezoelectric layer (1960) has a thickness of less than 10 μm. In some embodiments, the resulting piezoelectric layer (1960) has a thickness of about 3 μm.

In step 1820, the piezoelectric material is bonded to an electrically conductive material coated on a first substrate. For example, a temporary substrate (1965) may be peeled from the piezoelectric layer (1960), and the piezoelectric layer (1960) may be laminated on a first electrode, i.e., electrode (1950), formed on the first substrate, i.e., substrate (1955). The electrode (1950) may be formed on the substrate (1955) using one of the processes described above with respect to the electrode (1550) and the substrate (1555). As an example, the substrate (1955) may be a release sheet temporarily used to facilitate fabrication of the piezoelectric electrophoretic film, and the electrode (1950) may be formed from an adhesive or tie layer comprising a transparent conductive material that is deposited on the substrate (1955). In some embodiments, the electrode (1950) has a thickness of less than 5 μm. In some embodiments, the electrode (1950) has a thickness of 1 to 3 μm.

In an alternative embodiment not shown in FIG. 18, steps 1810 and 1820 are replaced by the following process. Instead of using a temporary substrate (1965), the substrate (1955) can be coated with an electrically conductive material (e.g., one of the materials described above) to form the electrode (1950). In some embodiments, to form the electrode (1950), an adhesive or tie layer is deposited on the substrate (1955), and a conductive polymer, such as PEDOT, is deposited over the tie layer. After forming the electrode (1950), a piezoelectric material is bonded to the electrode (1950) to form a piezoelectric layer (1960). For example, the piezoelectric material can be coated on the electrode (1950) as described above.

Returning to FIG. 18, after the piezoelectric layer (1960) is bonded to the electrode (1950), a mask (1940) is applied to the piezoelectric layer (1960), as shown in step 1830. As in the embodiments described above, the mask (1940) may be used to shield or insulate regions of the piezoelectric layer (1960) from the high voltage corona discharge (1933) used to poll the piezoelectric layer (1960). This allows the piezoelectric layer (1960) to be patterned with images, text, and other information (e.g., machine readable code). The mask (1940) may be fabricated from a material having sufficient dielectric strength to withstand at least a localized 5 kV field. In some embodiments, the mask (1940) is formed from a disposable material that can be applied in a roll-to-roll process. For example, the mask (1940) may be formed of paper, such as electrically insulating paper, having a pressure-sensitive adhesive applied to one surface for bonding to the piezoelectric layer (1960). Alternatively, the mask (1940) may be a reusable fixture made of a charge-absorbing or charge-blocking material that is integrated into a screen printing process or a roll-to-roll translational stage similar to a rotating roller.

FIG. 19B is a schematic cross-sectional view (1902) of the piezoelectric electrophoretic film at step 1830 of the method illustrated in FIG. 18. As illustrated in FIG. 19B, the mask (1940) includes a masking portion (1942) that shields or insulates a corresponding region of the piezoelectric layer (1960) from a high voltage corona discharge (1933), and a non-masking portion (1944) that allows a corresponding region of the piezoelectric layer (1960) to be polled in an area where the mask (1940) does not cover the piezoelectric layer (1960).

Next, in step 1840, the piezoelectric material is polarized or polarized. For example, a high voltage corona discharge (e.g., high voltage corona discharge (1933)) as described above or another suitable electric field may be used to polarize the unmasked portion of the piezoelectric layer (1960) in the first direction while leaving the masked portion unpolarized. If a disposable material, such as paper, was used for the mask, the mask is removed from the piezoelectric layer (1960).

FIG. 19C is a schematic cross-sectional view (1903) of the piezoelectric electrophoretic film after completion of step 1840 of the method illustrated in FIG. 18. As illustrated in FIG. 19C, the piezoelectric layer (1960) now includes a non-polarized portion (1962) corresponding to the position of the masked portion (1942) of the mask (1940) and a polarized portion (1964) corresponding to the position of the non-masked portion (1944) of the mask (1940).

In step 1850, a second electrode, i.e., electrode (1985), is formed on a second substrate, i.e., substrate (1986), by depositing an electrically conductive material, such as those described above, on the substrate. Electrode (1985) may be formed on substrate (1986) using any of the processes described above with respect to electrode (1550) and substrate (1555). In one example, to form electrode (1985), an adhesive or tie layer is deposited on substrate (1986), and a conductive polymer, such as PEDOT, is deposited on the tie layer. In some embodiments, electrode (1985) has a thickness of less than 5 μm. In some embodiments, electrode (1985) has a thickness of 1 to 3 μm.

Next, in step 1860, the piezoelectric material is bonded to the electrically conductive material. For example, the electrode (1985) may be coated with an electrophoretic medium layer comprising a plurality of microcapsules (not shown in FIG. 19D) containing a non-polar fluid and charged pigment particles. Alternatively, an electrophoretic medium layer comprising microcells (1930) may be formed on the electrode (1985) using a process similar to that shown in the flow chart of FIG. 11. For example, an embossable microcell precursor material may be laminated to the electrode (1985). Prior to lamination, the precursor material may be treated with a primer, such as using one of the primer materials described above. A microcell precursor is microembossed using the technique described above to form an open microcell structure, which is then filled with a desired electrophoretic medium and sealed with a sealing layer (1935) as illustrated in FIG. 19D, which is a schematic cross-sectional view (1904) of the piezoelectric electrophoretic film after completion of steps 1850 and 1860 of the method illustrated in FIG. 18. The open microcell may optionally be cleaned/activated by vapor phase plasma treatment prior to the microcell (1930) being filled with the desired electrophoretic medium.

In some embodiments, the layer of microcells (1930) has a thickness of 8 to 20 μm and the sealing layer (1935) has a thickness of 3 to 10 μm. In some embodiments, the layer of microcells (1930) has a thickness of about 10 μm and the sealing layer (1935) has a thickness of about 5 μm.

In an alternative embodiment, instead of forming microcells on the electrode (1985), the electrode (1985) as shown in FIG. 19d is bonded or laminated to a microcapsule or microcell based front layer or "FPL" as described above.

Next, in step 1870, the piezoelectric material deposited on the first substrate is bonded to the electrophoretic material. For example, the piezoelectric layer (1960) may be laminated to the sealing layer (1935) of the microcell (1930) to form the structure illustrated in FIG. 19E, which is a schematic cross-sectional view (1905) of the piezoelectric electrophoretic film after completion of step 1870 of the method illustrated in FIG. 18. In some embodiments, the piezoelectric layer (1960) may be bonded to the sealing layer (1935) using an adhesive layer (not illustrated in FIG. 19E).

A piezoelectric layer (1960) having electrodes (1550) (and a substrate (1555)) is added to a piezoelectric electrophoretic film to form a piezoelectric electrophoretic display that can be bonded to a target object. For example, the structure illustrated in FIG. 19E may be a piezoelectric electrophoretic display sandwiched between two release sheets (e.g., substrates (1950 and 1986)).

At step 1880, the piezoelectric electrophoretic display may be bonded to a target object. For example, a piezoelectric electrophoretic display such as that illustrated in FIG. 19e may be processed and attached to a target object (1988), such as paper or banknotes, as illustrated in FIG. 19f, which is a cross-sectional view (1906) of a piezoelectric electrophoretic film bonded to a target object according to the method illustrated in FIG. 18.

In some embodiments, the substrate (1986) is a release sheet that is peeled or removed from the electrode (1985), and the electrode (1985) is bonded to the surface of the target object (1988) using a hot stamping process as described above.

In some embodiments, the electrodes (1985) are formed from an adhesive or tie layer that is activated during the bonding process to bond the piezoelectric electrophoretic display to the target object (1988). In some embodiments, a roll-to-roll lamination process is used to bond the piezoelectric electrophoretic display to the target object (1988).

In some embodiments, the substrate (1955) and the substrate (1986) are release sheets, and the force required to remove each release sheet is individually adjusted to ensure that the substrate (1986) is removed before the substrate (1955) during the bonding process. For example, the adhesive used to temporarily bond the substrate (1955) and the substrate (1986) to the electrode (1950) and the electrode (1985), respectively, can be formulated or selected such that the force required to peel the substrate (1986) from the piezoelectric electrophoretic display is less than the force required to peel the substrate (1955) from the piezoelectric electrophoretic display.

FIG. 19g is a cross-sectional view (1907) of a piezoelectric electrophoretic film bonded to a target object and coated with a protective coating after completion of step 1880 of the method illustrated in FIG. 18. As illustrated in FIG. 19g, after bonding the piezoelectric electrophoretic display to the target object (1988), the substrate (1955) can be peeled off from the electrodes (1950), and a protective coating (1989) can be applied to the surface of the piezoelectric electrophoretic display and the target object (1988) to which the piezoelectric electrophoretic display is bonded, as described above.

Accordingly, the process described in connection with FIG. 18 can be used to create a piezoelectric electrophoretic display having a much smaller thickness than conventional displays, thereby significantly increasing the overall thickness or allowing it to be bonded to an object, such as a banknote or currency, without being noticeable. In some embodiments, the total thickness of the piezoelectric electrophoretic display can be from 50 μm to 100 μm. In some embodiments, the total thickness of the piezoelectric electrophoretic display can be from 25 μm to 50 μm. In some embodiments, the total thickness of the piezoelectric electrophoretic display can be less than 25 μm. Therefore, the resulting piezoelectric electrophoretic display created using the method (1800) provides substantially similar advantages and benefits to the piezoelectric electrophoretic display created using the method (1400) described above.

Those skilled in the art will appreciate that the steps of method (1400) and method (1800) need not be performed in the exact order in which they are presented herein. For example, steps 1810 to 1840 of method (1800) need not necessarily occur before steps 1850 and 1860.

It should be noted that the piezoelectric electrophoretic films and piezoelectric electrophoretic displays described herein can be combined with other known techniques for creating security markers or authentication labels. For example, the piezoelectric electrophoretic films or piezoelectric electrophoretic displays may further include a translucent overlay whose optical properties do not change when the piezoelectric electrophoretic film is manipulated. For example, a smiley face overlay may include eyes formed from the piezoelectric electrophoretic display, such that the eyes appear to blink when the layered material is flexed. In some embodiments, an image or shape may be printed or laminated onto a monochrome (e.g., white) background, and the pre-arranged pattern must be viewed through the piezoelectric electrophoretic film. Thus, when the device is not in use, the user will only see the monochrome, i.e., the printed image or shape will be hidden. However, when the device is manipulated, the printed image or shape will be displayed. Additionally, it is also possible to attach a piezoelectric film or piezoelectric display to a separate light-transmitting polymer film included in the target product (e.g., banknote), so that the pattern within the piezoelectric layer becomes visible only when the target product is illuminated by a light source and manipulated.

FIG. 20 illustrates a schematic cross-sectional view of another exemplary piezoelectric electrophoretic display (2000) in accordance with the present disclosure. The display (2000) uses a piezoelectric material (2002) to generate a voltage potential sufficient to drive charged pigment particles within a layer (2004) of the electrophoretic material. The display (2000) includes a first electrode, i.e., electrode (2006), overlapping or covering a first surface of the layer of electrophoretic material (2004). The display (2000) further includes a piezoelectric material (2002) overlapping or covering a first portion of a second surface of the layer of electrophoretic material (2004), as indicated in FIG. 20 as a surface area (2020). The second electrode, i.e., electrode (2008), overlaps the entire piezoelectric material (2002) and a second portion of the second surface of the electrophoretic material layer (2004), as indicated by surface area (2021) in FIG. 20.

The piezoelectric material (2002) may be a piezoelectric film that is bonded to the surface area (2020) of the electrophoretic material layer (2004) using a lamination process. In some embodiments, the piezoelectric material (2002) is formed by depositing the piezoelectric material on the electrophoretic material layer (2004). For example, the surface area (2020) of the electrophoretic material layer (2004) may be coated with a thin film of the piezoelectric material, such as PVDF, using a spin coating process or casting (e.g., slot die coating). In some embodiments, a film deposition process, such as printing, spraying, or gravure coating, is used to form the piezoelectric material (2002) on the electrophoretic material layer (2004). In some embodiments, the resulting piezoelectric material (2002) has a thickness of less than 10 μm. In some embodiments, the resulting piezoelectric material (2002) has a thickness of about 3 μm.

The electrode (2008) overlaps or covers the surface area (2021) of the piezoelectric material (2002) and the electrophoretic material layer (2004). The electrode (2008) may be a conductive adhesive material (e.g., copper tape) applied over the surface area (2021) of the piezoelectric material (2002) and the electrophoretic material layer (2004). In some embodiments, the electrode (2008) is a metal film, such as a copper, silver, gold, or aluminum film or foil, which is bonded to a flexible, optically transparent substrate (not shown), such as a polymer film. In some embodiments, the electrode (2008) is an adhesive or tie layer comprising a transparent conductive material (e.g., a first electrically conductive adhesive) comprising a conductive metal oxide, a conductive polymer, and/or other suitable conductive agents, which is coated on the substrate. For example, a thin layer of electrically conductive material may be directly deposited (e.g., sputtered, vapor-deposited) onto a suitable substrate, such as a polymer substrate (e.g., PET). In some embodiments, the electrode (2008) has a thickness of less than 5 μm. In some embodiments, the electrode (2008) has a thickness of 1 to 3 μm. In some embodiments, the electrode (2008) has a thickness of less than 1 μm.

A first electrode, i.e., electrode (2006), is bonded to an electrophoretic material layer (2004) on a surface opposite to the piezoelectric material (2002) and electrode (2008). For example, the electrode (2006) may be laminated to the electrophoretic material layer (2004) to form a microcapsule or microcell-based front-side laminate or FPL as described above in connection with U.S. Patent No. 6,982,178.

Electrode (2006) may be pre-formed on a substrate (not shown) using one of the processes described above with respect to electrode (2008). In some embodiments, electrode (2006) may be formed from an adhesive or tie layer comprising a transparent electrically conductive material deposited on the substrate. The substrate may be a release sheet temporarily used to facilitate fabrication of the piezoelectric electrophoretic film. In some embodiments, electrode (2006) has a thickness of less than 5 μm. In some embodiments, electrode (2006) has a thickness of 1 to 3 μm.

In some embodiments, the electrophoretic material layer (2004) is fabricated on the electrode (2006) prior to bonding with the piezoelectric material (2002) and the electrode (2008). For example, the electrode (2006) may be coated with an electrophoretic medium layer comprising a plurality of microcapsules (not shown in FIG. 20) containing a non-polar fluid and charged pigment particles. Alternatively, an electrophoretic medium layer comprising a plurality of microcell structures may be formed on the electrode (2006). For example, an embossable microcell precursor material may be laminated to the electrode (2006). Prior to lamination, the precursor material may be treated or coated with a microcell primer, such as one comprising an acrylate, a vinyl ether, or an epoxide, as described in detail in, for example, U.S. Patent Nos. 6,930,818, 7,052,571, 7,616,374, 8,361,356, and 8,830,561, all of which are incorporated by reference in their entireties. The microcell precursor may be microembossed or photolithographed into an open microcell structure, which is then filled with the desired electrophoretic medium and sealed with a sealing layer. The open microcell may optionally be cleaned/activated by a vapor phase plasma treatment prior to the microcell being filled with the desired electrophoretic medium.

In some embodiments, the electrophoretic material layer (2004) has a thickness of 10 to 30 μm. In some embodiments, the electrophoretic material layer (2004) has a thickness of about 15 μm.

In some embodiments, the electrode (2006) may be segmented (not shown). As a result, the gray tone changes caused by the movement of charged pigment particles within the electrophoretic material layer (2004) will also appear segmented. Alternatively, the electrode (2006) may comprise a single continuous sheet or film of conductive material, and the gray tone changes will appear continuous. It should be understood that all layers of the display (2000) (e.g., layers 2002, 2004, 2006, and 2008) may be fabricated to be transparent, such that the display (2000) is viewable from any orientation or direction.

In practice, the CR of the piezoelectric electrophoretic display (2000) may vary depending on the ratio of the surface area (2020) (i.e., the surface area of the electrophoretic material layer (2004) overlapped or covered by the piezoelectric material (2002)) to the surface area (2021) (i.e., the surface area of the electrophoretic material layer (2004) overlapped or covered by the electrode (2008). The experimental results of CR are shown in Table 2 below.

Surface area (2020) to surface area (2021) ratio Display response to deformation changes 0 Contrast ratio: 1.7 1:2 Contrast ratio: 2 1:1 Contrast ratio: 5 2:1 Contrast ratio: 7

Table 2. Display CR for surface area ratio

As shown in Table 2, increasing the ratio of the surface area (2020) to the surface area (2021) can improve the CR of the display. For example, the display CR improves from a value of 2 when the ratio of the surface area (2020) to the surface area (2021) is 1:2, to a value of 7 when the ratio is 2:1.

In some embodiments, there is an adhesive layer (not shown) between the piezoelectric material (2002) and the electrophoretic material layer (2004). In some embodiments, the adhesive layer has a resistivity of 10 ² ohm*cm to 10 ⁸ ohm*cm, preferably less than 10 ¹² ohm*cm. In some embodiments, the adhesive layer has a resistivity at least one order of magnitude greater than the resistivity of the electrode. Thus, the adhesive layer may have the resistivity characteristics of a semiconducting material or a high-resistivity insulating material. In this configuration, the adhesive layer may function as a type of dielectric to prevent rapid dissipation of locally generated charges by the piezoelectric material (2002), thereby improving the CR of the display. Furthermore, it has been found that reducing the width of the electrode (2006) or the electrode (2008) and applying a physical stress perpendicular to the longer side of the electrode (2008) can further improve the CR of the display.

FIG. 21A is a schematic cross-sectional view illustrating additional characteristics of the piezoelectric electrophoretic display (2000) illustrated in FIG. 20 according to the invention disclosed herein. As indicated by the dashed line (2122), a first portion (2132) of the electrophoretic material layer (2004) overlaps or is positioned adjacent to the piezoelectric material (2002), and a second portion (2134) of the electrophoretic material layer (2004) overlaps or is positioned adjacent to the electrode (2008). The first portion (2132) and the second portion (2134) each have an electrical resistance based on the volume of electrophoretic material they contain. As indicated by the "+" and "-" signs, a voltage is generated by charge separation occurring within the piezoelectric material (2002), for example, in response to bending or mechanical stress on the piezoelectric material (2002).

FIG. 21B is a perspective view illustrating additional features of the piezoelectric electrophoretic display (2000) illustrated in FIG. 20 according to the invention disclosed herein. For ease of review, the electrode (2006) is not shown in FIG. 21B. As can be seen in FIG. 21B, a first portion (2132) of the electrophoretic material layer (2004) overlaps with the piezoelectric material (2002) in or adjacent to a first surface area (2020) (indicated by a dotted line), and a second portion (2134) of the electrophoretic material layer (2004) overlaps with a second electrode (2008) in or adjacent to a second surface area (2021) (indicated by a dotted line).

FIG. 22 illustrates an exemplary equivalent circuit (2200) of the piezoelectric electrophoretic display illustrated in FIGS. 21A and 21B according to the invention disclosed herein. Three nodes or points illustrated in FIG. 21A, namely point 'A' near the piezoelectric material (2002) and portion (2132), point 'B' at electrode 1 (2106), and point 'C' at electrode 2 (2108), correspond to the same three points A, B, and C illustrated in the equivalent circuit (2200) of FIG. 22. Resistance R ₁ corresponds to the electrical resistance of the first portion (2132) of the electrophoretic material layer (2004), and resistance R ₂ corresponds to the electrical resistance of the second portion (2134) of the electrophoretic material layer (2004).

The layer of piezoelectric material (2002) is represented as a battery in FIG. 22, and the voltage V _PZ is the voltage generated by the piezoelectric material across points A and C. Resistors R ₁ and R ₂ are represented in series because the presence of the voltage source only beneath a portion of the layer of electrophoretic material (2004) effectively divides the layer into distinct sections (as indicated by the dashed line (2122)) with different electrical properties. For example, when voltage V _PZ is generated, the voltage potential at point A is higher than at points B or C. Using a conventional current flow paradigm, current (2201) flows from point A through resistor R ₁ to point B, and from point B through resistor R ₂ to point C. Accordingly, the voltage generated across resistor R ₁ is opposite in polarity to the voltage generated across resistor R ₂ . In effect, two opposite voltages are generated in series across separate portions of the electrophoretic material layer (204).

FIG. 23 is a schematic cross-sectional view of an exemplary piezoelectric electrophoretic display (2300) according to the present invention. The configuration of the display (2300) is similar to the configuration of the display (2000) illustrated in FIGS. 20, 21A, and 21B. For example, the display (2300) includes a piezoelectric material (2302) overlapping or covering a first portion of a surface area of an electrophoretic material layer (2304), as indicated by surface area (2320) in FIG. 23. However, the display (2300) includes a dielectric layer (2330) overlapping the entire piezoelectric material (2302) and a second portion of the surface area of the electrophoretic material layer (2304), as indicated by surface area (2321) in FIG. 23. The electrode (2308) of the display (2300) overlaps the entire dielectric layer (2330).

The dielectric layer (2330) may be similar to the adhesive layer described with respect to the display (2000) of FIG. 20. For example, the dielectric layer (2330) may be formed of a material having resistive properties of a semiconducting material or a high-resistance insulating material. In some embodiments, the dielectric layer (2330) has a resistivity of from 10 ² ohm*cm to 10 ⁸ ohm*cm, preferably less than 10 ¹² ohm*cm.

The dielectric layer (2330) functions to prevent charges generated by the piezoelectric material (2320) from dissipating quickly as would occur if the piezoelectric material (2320) were in direct contact with the electrode (2308). This allows these charges to be applied more effectively and efficiently across the electrophoretic material layer (2304), thereby maximizing the movement of the charged pigment particles and ultimately improving the display CR.

A comparison of the CR achieved among various display designs is illustrated in Table 3 below. The first display, in which the piezoelectric material was fabricated to at least partially overlap or contact both electrodes, achieved a CR of 1.7. As mentioned above, the display (2000) achieved a CR of 7 when the ratio of surface area (2020) to surface area (2021) was 2:1. Among the various configurations, the display (2300) illustrated in FIG. 23 exhibited the best CR performance of 18 when the resistivity value of the dielectric layer (2330) was approximately 10 ⁸ ohm*cm.

composition Display response to deformation changes Piezoelectric material in contact with both electrodes Contrast ratio: 1.7 Fig. 20 Contrast ratio: 7 Fig. 23 Contrast ratio: 18

Table 3. CR comparison between display configurations

It should be understood that all layers of the display (2300) can be fabricated to be transparent, thereby allowing the display (2300) to be viewed in any orientation or direction. Referring to the display configurations illustrated in FIGS. 20 through 23, it should also be noted that conductive paths are completed between the electrode, piezoelectric material, and electrophoretic material layers, eliminating the need for any other conductors or contacts to enable the display to operate. This advantageously reduces the overall thickness of the final piezoelectric electrophoretic display device while also improving the display's CR ratio.

Latent Images

Displays fabricated according to the invention herein can be used to display hidden or so-called "latent" images. In particular, an image (e.g., a shape, text, a barcode, etc.) can be laminated or printed on one electrode of the display, such that the image becomes visible only when charged pigment particles move in response to a voltage generated by bending or otherwise applying mechanical stress to the piezoelectric material.

In some embodiments, an image is printed or laminated onto one of the electrodes over a white background, and the display is viewed from the opposite electrode. When the display is displaying a white color (e.g., the white pigment particles are positioned closest to the electrode on which the image is not printed), the printed image is obscured or hidden. However, when the positions of the pigment particles shift in response to mechanical stress on the piezoelectric material, the white pigment particles move away from the viewing surface, while the pigment particles of the other color (typically a darker color) move toward the viewing surface, thereby allowing the image to be displayed.

In another embodiment, a dark-colored image is printed or laminated onto one of the electrodes without a background color, and the display is then viewed from the opposite electrode. In this embodiment, when the display is placed in front of a dark or black background, the image remains substantially obscured or hidden regardless of whether the display is displaying white or another color. However, when the display is placed in front of a light or white background, the image becomes visible. In this embodiment, the image is visible when the display is displaying white, but appears more clearly when the display is displaying a darker color.

The piezoelectric electrophoretic display created as described above can be attached to low-profile objects, such as currency or banknotes. Thus, an image can be integrated into the banknote, allowing a user to easily distinguish real banknotes from counterfeit ones based on how the display's optical state changes (or remains unchanged) when the banknote is bent or flexed.

The display configuration described herein enables the fabrication of a fully functional piezoelectrically driven display device with a thickness of less than 50 μm. Furthermore, the display structure described herein is significantly simplified, making the resulting display more responsive to smaller applied physical stresses.

Therefore, fabricating a piezoelectric electrophoretic display having the structure described herein offers advantages over conventional piezoelectric electrophoretic displays. For example, the piezoelectric electrophoretic display described herein provides an improved means for driving oppositely charged pigment particles within an electrophoretic medium in different directions without requiring an individually addressable pixel electrode matrix. Thus, the piezoelectric electrophoretic display produced as described herein can be made sufficiently thin for use in applications that require durability and being substantially unobtrusive when integrated into a thin, low-profile end product, such as paper or banknotes, while still providing a high contrast ratio between different portions of the electrophoretic material layer due to the effects described above.

It will be apparent to those skilled in the art that numerous changes and modifications can be made to the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the entire foregoing description should be construed in an illustrative rather than a restrictive sense.

The present disclosure provides aspects and embodiments set forth in the following clauses:

Article 1: An electrophoretic display film having a thickness of less than 100 μm (from top to bottom), the electrophoretic display film sequentially comprising: a first adhesive layer; an electrophoretic medium layer; a patterned piezoelectric layer including differential polarization zones; and a flexible light-transmitting electrode layer.

Clause 2: An electrophoretic display film according to clause 1, wherein the electrophoretic medium layer comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the patterned piezoelectric layer when the patterned piezoelectric layer is bent, and the microcapsules are bonded to each other with a polymer binder.

Clause 3: An electrophoretic display film according to Clause 1, wherein the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the patterned piezoelectric layer when the patterned piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells by a sealing layer.

Clause 4: An electrophoretic display film according to any one of clauses 1 to 3, wherein the electrophoretic display film has a thickness of less than 50 μm.

Clause 5: An electrophoretic display film according to any one of clauses 1 to 4, wherein the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF).

Clause 6: An electrophoretic display film according to clause 5, wherein the PVDF is polarized to create the differential polarization regions.

Article 7: An electrophoretic display film according to any one of Articles 1 to 6, wherein the flexible light-transmitting electrode layer comprises a metal oxide including tin or zinc.

Article 8: An electrophoretic display film according to any one of Articles 1 to 6, wherein the flexible light-transmitting electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT).

Clause 9: An electrophoretic display film assembly comprising a release sheet bonded to any one of the electrophoretic display films of Clauses 1 to 8, wherein the release sheet is bonded to the first adhesive layer.

Clause 10: An electrophoretic display film assembly according to Clause 9, further comprising a second adhesive layer bonded to the flexible light-transmitting electrode layer, and a second release sheet bonded to the second adhesive layer.

Article 11: A method for manufacturing an electrophoretic display film, comprising: bonding a film of polyvinylidene fluoride (PVDF) to a polymer film comprising acrylate, vinyl ether, or epoxide to form a piezoelectric microcell precursor film; bonding the piezoelectric microcell precursor film to a flexible light-transmitting electrode layer; bonding the light-transmitting electrode layer to a first release film using a first adhesive layer; embossing the piezoelectric microcell precursor film to form an array of microcells, the microcells having a bottom, walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; and sealing the top openings of the filled microcells with a water-soluble polymer to form an electrophoretic medium layer.

Clause 12: A method according to clause 11, further comprising the step of applying a primer to the polymer film, the primer comprising an acrylate, vinyl ether, or epoxide, prior to bonding the polymer film to the film of polyvinylidene fluoride (PVDF).

Clause 13: A method according to clause 11 or 12, further comprising the step of bonding the water-soluble polymer to a second release film using a second adhesive layer.

Clause 14: A method according to any one of clauses 11 to 13, further comprising the step of removing the first release film to produce an electrophoretic display film having a thickness of less than 100 μm.

Clause 15: A method according to any one of clauses 11 to 14, wherein the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer of the piezoelectric microcell precursor film when the piezoelectric layer is bent.

Clause 16: A method according to any one of clauses 11 to 15, wherein the PVDF is poled to create differential polarization regions.

Clause 17: A method according to any one of Clauses 11 to 16, wherein the flexible light-transmitting electrode layer comprises a metal oxide including tin or zinc.

Clause 18: A method according to any one of clauses 11 to 16, wherein the flexible light-transmitting electrode layer comprises poly(3,4-ethylenedioxythiophene) (PEDOT).

Clause 19: A method according to any one of clauses 11 to 18, wherein the film of polyvinylidene fluoride is patterned using an electric field to create regions of different polarization.

Clause 20: A method according to any one of clauses 11 to 18, further comprising the step of patterning the completed electrophoretic display film using an electric field to create regions of different polarization in the film of polyvinylidene fluoride.

Article 21: A method of manufacturing a piezoelectric electrophoretic display, comprising: depositing a first electrically conductive adhesive on a first substrate; depositing a piezoelectric material comprising a polyvinylidene fluoride (PVDF) solution on the first electrically conductive adhesive to form a piezoelectric layer having a thickness of less than 5 μm; applying a mask to the piezoelectric layer, the mask including a plurality of masking portions that shield a first plurality of regions of the piezoelectric layer and a plurality of unmasking portions that leave a second plurality of regions of the piezoelectric layer unshielded; polarizing the piezoelectric layer to form a plurality of piezoelectric material polarized portions corresponding to the second plurality of regions of the piezoelectric layer and a plurality of piezoelectric material unpolarized portions corresponding to the first plurality of regions of the piezoelectric layer; removing the mask from the piezoelectric layer; bonding the piezoelectric layer with a microcell precursor material; A method comprising: embossing the microcell precursor material to produce a layer of microcells, the microcells having a bottom, walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; sealing the top openings of the filled microcells with a water-soluble polymer to produce a sealing layer; depositing a second electrically conductive adhesive on a second substrate; and bonding the sealing layer to the second electrically conductive adhesive.

Clause 22: A method according to clause 21, further comprising the step of forming the microcell precursor material by combining a polymer film comprising an acrylate, vinyl ether, or epoxide.

Clause 23: A method according to clause 21 or 22, further comprising the step of applying a primer to the microcell precursor material prior to bonding the piezoelectric layer to the microcell precursor material.

Clause 24: A method according to any one of clauses 21 to 23, further comprising the step of activating the microcells by a gas phase plasma treatment prior to filling the microcells with the electrophoretic medium.

Clause 25: A method according to any one of clauses 21 to 24, wherein the electrophoretic medium layer comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is subjected to mechanical stress, and wherein the non-polar fluid and charged pigment particles are sealed within the microcell by the sealing layer.

Clause 26: A method according to any one of clauses 21 to 25, wherein the piezoelectric layer is polarized using an electric field.

Clause 27: A method according to clause 26, wherein the electric field is provided by a corona discharge.

Clause 28: A method according to any one of Clauses 21 to 27, wherein the first substrate and the second substrate are release films.

Clause 29: A method according to Clause 28, further comprising the steps of: peeling the second substrate from the second electrically conductive adhesive; and bonding the second electrically conductive adhesive to a target object.

Clause 30: A method according to Clause 29, wherein the step of bonding the second electrically conductive adhesive to the target object comprises the step of hot stamping the second electrically conductive adhesive onto the target object.

Clause 31: A method according to clause 29, further comprising the steps of: peeling the first substrate from the first electrically conductive adhesive; and applying a protective coating over the remaining layers of the piezoelectric electrophoretic display and the target object.

Clause 32: A method according to clause 31, wherein the protective coating comprises a lacquer.

Clause 33: A method according to any one of Clauses 29 to 32, wherein the target object comprises one of paper, banknotes, and currency.

Article 34: A method of manufacturing a piezoelectric electrophoretic display, comprising: depositing a piezoelectric material comprising a polyvinylidene fluoride (PVDF) solution on a temporary substrate to form a piezoelectric layer having a thickness of less than 5 μm; bonding the piezoelectric layer to a first substrate using a first electrically conductive adhesive, wherein the temporary substrate is removed from the piezoelectric layer during the bonding process; applying a mask to the piezoelectric layer, the mask including a plurality of masking portions that shield a first plurality of regions of the piezoelectric layer and a plurality of non-masking portions that leave a second plurality of regions of the piezoelectric layer unshielded; polarizing the piezoelectric layer to form a plurality of polarized portions of the piezoelectric material corresponding to the second plurality of regions of the piezoelectric layer and a plurality of non-polarized portions of the piezoelectric material corresponding to the first plurality of regions of the piezoelectric layer; removing the mask from the piezoelectric layer; A method comprising: depositing a second electrically conductive adhesive on a second substrate; bonding the second electrically conductive adhesive to a microcell precursor material; embossing the microcell precursor material to produce a layer of microcells, the microcells having a bottom, walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; sealing the top openings of the filled microcells with a water-soluble polymer to produce a sealing layer; and bonding the sealing layer to the piezoelectric layer.

Clause 35: A method according to Clause 34, further comprising the step of forming the microcell precursor material by combining a polymer film comprising an acrylate, vinyl ether, or epoxide.

Clause 36: A method according to Clause 35, further comprising the step of applying a primer to the microcell precursor material prior to bonding the second electrically conductive adhesive to the microcell precursor material.

Clause 37: A method according to any one of Clauses 34 to 36, further comprising the step of activating the microcells by a gas phase plasma treatment prior to filling the microcells with the electrophoretic medium.

Clause 38: A method according to any one of clauses 34 to 37, wherein the electrophoretic medium layer comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is subjected to mechanical stress, and wherein the non-polar fluid and charged pigment particles are sealed within the microcell by the sealing layer.

Clause 39: A method according to any one of clauses 34 to 38, wherein the piezoelectric layer is polarized using an electric field.

Clause 40: A method according to clause 39, wherein the electric field is provided by a corona discharge.

Clause 41: A method according to any one of clauses 34 to 40, wherein the first substrate and the second substrate are release films.

Clause 42: A method according to Clause 41, further comprising the steps of: peeling the second substrate from the second electrically conductive adhesive; and bonding the second electrically conductive adhesive to a target object.

Clause 43: A method according to Clause 42, wherein the step of bonding the second electrically conductive adhesive to the target object comprises the step of hot stamping the second electrically conductive adhesive onto the target object.

Clause 44: A method according to clause 42, further comprising the steps of: peeling the first substrate from the first electrically conductive adhesive; and applying a protective coating over the remaining layers of the piezoelectric electrophoretic display and the target object.

Clause 45: A method according to clause 44, wherein the protective coating comprises a lacquer.

Clause 46: A method according to any one of Clauses 42 to 45, wherein the object comprises one of paper, banknotes, and currency.

Article 47: A method for manufacturing a piezoelectric electrophoretic display, comprising: depositing a first electrically conductive material on a first substrate to form a first electrode; bonding the first electrode to a first surface of an electrophoretic material layer; depositing a piezoelectric material on a second surface of the electrophoretic material layer, wherein the piezoelectric material overlaps a first surface area of the second surface of the electrophoretic material layer; and depositing a second electrically conductive material to form a second electrode, wherein the second electrode is formed to overlap the entire piezoelectric material and a second surface area of the second surface of the electrophoretic material layer.

Clause 48: A method according to Clause 47, wherein the electrophoretic material layer comprises a first electrophoretic material portion overlapping the first surface area; and a second electrophoretic material portion overlapping the second surface area.

Clause 49: A method according to Clause 48, wherein the first electrophoretic material portion comprises a first electrical resistance, and the second electrophoretic material portion comprises a second electrical resistance.

Clause 50: A method according to Clause 49, wherein the value of the first electrical resistance and the value of the second electrical resistance are based on a ratio of the first surface area to the second surface area.

Clause 51: A method according to clause 49 or 50, wherein applying a mechanical stress to the piezoelectric material generates a first voltage across the first electrophoretic material portion and a second voltage across the second electrophoretic material portion, wherein the first voltage and the second voltage have opposite polarities.

Clause 52: A method according to Clause 47, wherein the electrophoretic material layer comprises a first electrophoretic material portion having a first electrical resistance corresponding to a first volume of the electrophoretic material overlapping the first surface area; and a second electrophoretic material portion having a second electrical resistance corresponding to a second volume of the electrophoretic material overlapping the second surface area.

Clause 53: A method according to Clause 52, wherein the value of the first electrical resistance and the value of the second electrical resistance are based on a ratio of the first surface area to the second surface area.

Clause 54: A method according to clause 52 or 53, wherein applying a mechanical stress to the piezoelectric material generates a first voltage across the first electrophoretic material portion and a second voltage across the second electrophoretic material portion, wherein the first voltage and the second voltage have opposite polarities.

Clause 55: A method according to any one of clauses 52 to 54, wherein the bonding step comprises: coating the first electrode with a microcell precursor material; embossing the microcell precursor material to form a layer of microcells, the microcells having a bottom, a plurality of walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; and sealing the top openings of the filled microcells with a water-soluble polymer to form a sealing layer.

Clause 56: A method according to clause 55, further comprising the step of applying a primer to the microcell precursor material prior to embossing the microcell precursor material.

Clause 57: A method according to clause 56, further comprising the step of activating the microcells by a gaseous plasma treatment prior to filling the microcells with the electrophoretic medium.

Clause 58: A method according to any one of clauses 55 to 57, wherein the electrophoretic medium comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric material when the piezoelectric material is subjected to mechanical stress, and wherein the non-polar fluid and charged pigment particles are sealed within the microcell by the sealing layer.

Clause 59: A method according to any one of Clauses 55 to 58, further comprising the step of applying an adhesive material layer between the first surface area of the second surface of the piezoelectric material and the electrophoretic material layer, wherein the adhesive material layer has a resistivity of 10 ² ohm*cm to 10 ¹² ohm*cm.

Clause 60: A method according to any one of Clauses 55 to 59, further comprising the step of applying an adhesive material layer between the first surface area of the second surface of the piezoelectric material and the electrophoretic material layer, wherein the adhesive material layer has a resistivity at least one order of magnitude greater than that of the first and second electrodes.

Clause 61: A method according to any one of Clauses 55 to 60, further comprising the step of depositing a dielectric layer before depositing the second electrically conductive material, wherein the dielectric layer is formed to overlap the entire piezoelectric material and the second surface area of the second surface of the electrophoretic material layer, and the second electrode is formed to overlap the entire dielectric layer.

Clause 62: A method according to clause 61, wherein the dielectric layer has a resistivity of 10 ² ohm*cm to 10 ¹² ohm*cm.

Clause 63: A method according to clause 61, wherein the dielectric layer has a resistivity at least one order of magnitude greater than that of the first and second electrodes.

Clause 64: A method according to any one of clauses 55 to 63, further comprising the step of printing one or more images on at least one of the first electrode and the second electrode.

Clause 65: A method according to any one of clauses 55 to 64, further comprising the step of attaching the piezoelectric display to a target object selected from the group consisting of paper, banknotes, and currency.

Claims (26)

A method for manufacturing a piezo-electrophoretic display,
A step of depositing a first electrically conductive adhesive on a first substrate;
A step of depositing a piezoelectric material comprising a polyvinylidene fluoride (PVDF) solution on the first electrically conductive adhesive to produce a piezoelectric layer having a thickness of less than 5 μm;
A step of applying a mask to the piezoelectric layer, wherein the mask comprises a plurality of masking portions shielding a first plurality of regions of the piezoelectric layer and a plurality of non-masking portions leaving a second plurality of regions of the piezoelectric layer unshielded;
A step of polarizing the piezoelectric layer to generate a plurality of piezoelectric material polarized portions corresponding to the second plurality of regions of the piezoelectric layer and a plurality of piezoelectric material non-polarized portions corresponding to the first plurality of regions of the piezoelectric layer;
A step of removing the mask from the piezoelectric layer;
A step of bonding the above piezoelectric layer with a microcell precursor material;
A step of embossing the above microcell precursor material to produce a layer of microcells, wherein the microcells have a bottom, a wall and a top opening;
A step of filling the microcells with an electrophoretic medium through the upper opening;
A step of sealing the upper openings of the filled microcells with a water-soluble polymer to create a sealing layer;
a step of depositing a second electrically conductive adhesive on a second substrate; and
A step of bonding the above sealing layer to the second electrically conductive adhesive.
A method for manufacturing a piezoelectric electrophoretic display, comprising: In claim 1,
A method for manufacturing a piezoelectric electrophoretic display, further comprising the step of coupling a polymer film comprising an acrylate, vinyl ether, or epoxide to produce the microcell precursor material. In claim 2,
A method for manufacturing a piezoelectric electrophoretic display, further comprising the step of applying a primer to the microcell precursor material before bonding the piezoelectric layer to the microcell precursor material. In claim 1,
A method for manufacturing a piezoelectric electrophoretic display, further comprising the step of activating the microcells by gaseous plasma treatment before filling the microcells with the electrophoretic medium. In claim 1,
A method for manufacturing a piezoelectric electrophoretic display, wherein the electrophoretic medium layer comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is subjected to mechanical stress, and the non-polar fluid and charged pigment particles are sealed within the microcell by the sealing layer. In claim 1,
A method for manufacturing a piezoelectric electrophoretic display, wherein the piezoelectric layer is polarized using an electric field. In claim 6,
A method for manufacturing a piezoelectric electrophoretic display, wherein the electric field is provided by a corona discharge. In claim 1,
A method for manufacturing a piezoelectric electrophoretic display, wherein the first substrate and the second substrate are release films. In claim 8,
a step of peeling the second substrate from the second electrically conductive adhesive; and
A step of bonding the second electrically conductive adhesive to a target object
A method for manufacturing a piezoelectric electrophoretic display, further comprising: In claim 9,
A method for manufacturing a piezoelectric electrophoretic display, wherein the step of bonding the second electrically conductive adhesive to the target object includes the step of hot stamping the second electrically conductive adhesive onto the target object. In claim 9,
A step of peeling the first substrate from the first electrically conductive adhesive; and
A step of applying a protective coating over the remaining layers of the piezoelectric electrophoretic display and the target object.
A method for manufacturing a piezoelectric electrophoretic display, further comprising: In claim 11,
A method for manufacturing a piezoelectric electrophoretic display, wherein the protective coating comprises a lacquer. In claim 9,
A method for manufacturing a piezoelectric electrophoretic display, wherein the target object comprises one of paper, banknotes, and currency. In a method for manufacturing a piezoelectric electrophoretic display,
A step of depositing a piezoelectric material containing a polyvinylidene fluoride (PVDF) solution on a temporary substrate to create a piezoelectric layer having a thickness of less than 5 μm;
A step of bonding the piezoelectric layer using a first electrically conductive adhesive on a first substrate, wherein the temporary substrate is removed from the piezoelectric layer during the bonding process;
A step of applying a mask to the piezoelectric layer, wherein the mask comprises a plurality of masking portions shielding a first plurality of regions of the piezoelectric layer and a plurality of non-masking portions leaving a second plurality of regions of the piezoelectric layer unshielded;
A step of polarizing the piezoelectric layer to generate a plurality of piezoelectric material polarized portions corresponding to the second plurality of regions of the piezoelectric layer and a plurality of piezoelectric material non-polarized portions corresponding to the first plurality of regions of the piezoelectric layer;
A step of removing the mask from the piezoelectric layer;
A step of depositing a second electrically conductive adhesive on a second substrate;
A step of bonding the second electrically conductive adhesive to a microcell precursor material;
A step of embossing the above microcell precursor material to produce a layer of microcells, wherein the microcells have a bottom, a wall and a top opening;
A step of filling the microcells with an electrophoretic medium through the upper opening;
A step of sealing the upper openings of the filled microcells with a water-soluble polymer to create a sealing layer; and
A step of bonding the above sealing layer to the above piezoelectric layer.
A method for manufacturing a piezoelectric electrophoretic display, comprising: In claim 14,
A method of manufacturing a piezoelectric electrophoretic display, further comprising the step of forming the microcell precursor material by combining a polymer film comprising an acrylate, vinyl ether, or epoxide. In claim 15,
A method for manufacturing a piezoelectric electrophoretic display, further comprising the step of applying a primer to the microcell precursor material before bonding the second electrically conductive adhesive to the microcell precursor material. In claim 14,
A method for manufacturing a piezoelectric electrophoretic display, further comprising the step of activating the microcells by gaseous plasma treatment before filling the microcells with the electrophoretic medium. In claim 14,
A method for manufacturing a piezoelectric electrophoretic display, wherein the electrophoretic medium layer comprises a non-polar fluid and charged pigment particles, wherein the charged pigment particles move toward or away from the piezoelectric layer when the piezoelectric layer is subjected to mechanical stress, and the non-polar fluid and charged pigment particles are sealed within the microcell by the sealing layer. In claim 14,
A method for manufacturing a piezoelectric electrophoretic display, wherein the piezoelectric layer is polarized using an electric field. In claim 19,
A method for manufacturing a piezoelectric electrophoretic display, wherein the electric field is provided by a corona discharge. In claim 14,
A method for manufacturing a piezoelectric electrophoretic display, wherein the first substrate and the second substrate are release films. In claim 21,
a step of peeling the second substrate from the second electrically conductive adhesive; and
A step of bonding the second electrically conductive adhesive to a target object
A method for manufacturing a piezoelectric electrophoretic display, further comprising: In claim 22,
A method for manufacturing a piezoelectric electrophoretic display, wherein the step of bonding the second electrically conductive adhesive to the target object includes the step of hot stamping the second electrically conductive adhesive onto the target object. In claim 22,
A step of peeling the first substrate from the first electrically conductive adhesive; and
A step of applying a protective coating over the remaining layers of the piezoelectric electrophoretic display and the target object.
A method for manufacturing a piezoelectric electrophoretic display, further comprising: In claim 24,
A method for manufacturing a piezoelectric electrophoretic display, wherein the protective coating comprises a lacquer. In claim 22,
A method for manufacturing a piezoelectric electrophoretic display, wherein the target object comprises one of paper, banknotes, and currency.