Piezophoretic film containing patterned piezoelectric polars for generating images via an electrophoretic medium - Patent Application 20070123633

Abstract

Thin piezoelectric films and display films comprising the thin piezoelectric films. In some embodiments, the piezoelectric material of the piezoelectric films can be patterned with a high-voltage electric field after fabrication of the piezoelectric films. Such films are useful as security markers, authentication films, or sensors. The films are generally flexible. Some films are less than 100 μm thick. Displays formed from the films do not require an external power source.

Description

[Background technology]

[0001] (Reference to Related Application) This application claims priority to U.S. Patent Application No. 63 / 314,584, filed February 28, 2022. All patents and publications disclosed herein are incorporated by reference in their entirety.

[0002] Electrophoretic displays (EPDs) are non-emissive devices based on electrophoresis of charged pigment particles dispersed in a solvent or solvent mixture. The displays typically comprise two electrodes placed opposite each other, which 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 particles migrate to one side or the other, causing either the color of the pigment particles or the color of the solvent (if colored) to be seen from the viewing side. Electrophoretic fluids typically include a non-polar solvent and one or more sets of charged particles. The particles may have different optical properties (colors), different charges (positive or negative), different charge magnitudes (zeta potentials), and / or different absorptive properties (broadly, light absorbing, broadly, light reflecting, or selectively absorbing or reflecting). In cases where there are multiple sets of particles with opposite charge polarity, application of an electric field can cause one set of particles to appear at the viewing surface while other particles are driven away from the viewing surface.

[0003] Many electrophoretic displays are bistable, meaning that their optical state persists after the activating electric field is removed. Bistability is primarily due to an induced dipole charge layer that develops around the charged pigment due to complex interactions between the pigment, charge control agent, and free polymer dispersed in a solvent. Bistable displays can persist for years in the last addressed optical state before being switched again with the application of a new driving field.

[0004] Driving an electrophoretic display requires a power source to provide an electric field between the electrodes. The power source is typically a battery, which provides power to the electrodes via drive circuitry. One or more electrodes may be incorporated into an active matrix backplane. The power source may be, for example, a photovoltaic cell, a fuel cell, or a power source that operates from wall currents. The power source may also be a piezoelectric element, which generates charge through physical motion or thermal expansion, as described in U.S. Pat. No. 5,930,026 (incorporated by reference in its entirety). In all of these examples, some type of drive circuitry is required to provide an electrical path between the power source and the electrodes, and typically the circuitry includes control elements such as switches, transistors, etc. In most cases the circuitry is fairly routine, however it typically adds bulk and structural limitations to the final display (i.e., it is not flexible or twistable). There is a need for very simple, flexible, durable and thin electrophoretic displays for applications such as security markers, sensors and indicators. Summary of the Invention [Means for solving the problem]

[0005] According to one aspect of the subject matter 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, where the piezoelectric material overlaps a portion of the layer of electrophoretic material and where a portion of the first conductive layer overlaps a remainder of the electrophoretic material.

[0006] In a first aspect, the invention includes an electrophoretic display film less than 100 μm thick (top to bottom), comprising a first adhesive layer, an electrophoretic medium layer, a patterned piezoelectric layer with zones of differential polarization, and a flexible light-transmitting electrode layer. In some embodiments, the electrophoretic medium layer comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, the microcapsules being bonded together using a polymeric binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, the non-polar fluid and charged pigment particles being sealed within the microcells using a sealing layer. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is polarized to produce zones of polarization that exhibit a difference. In some embodiments, the flexible light-transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible light-transmissive electrode layer comprises poly(3,4-ethyleneoxythiophene) (PEDOT). In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film as described above, the release sheet being bonded to a first adhesive layer. In some embodiments, a second adhesive layer is bonded to the flexible light-transmissive electrode layer, and the second release sheet is bonded to the second adhesive layer.

[0007] In a second aspect, the invention includes a method of making an electrophoretic display film. The method includes bonding a film of polyvinylidene fluoride (PVDF) to a polymer film comprising an acrylate, vinyl ether, or epoxide to produce 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 with a first adhesive layer, embossing the piezoelectric microcell precursor film to produce an array of microcells, the microcells having a bottom, a wall, 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. In some embodiments, the method further includes applying a primer to the polymer film comprising an acrylate, vinyl ether, or epoxide before bonding the polymer film to the film of polyvinylidene fluoride (PVDF). In some embodiments, the method further includes bonding the water-soluble polymer to the second release film with a second adhesive layer. In some embodiments, the method further includes removing the first release film, resulting in an electrophoretic display film that is less than 100 μm thick. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells with a sealing layer. In some embodiments, the PVDF is polarized to create zones that exhibit polarization differences. 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-ethyleneoxythiophene) (PEDOT). In some embodiments, a film of polyvinylidene fluoride is patterned with an electric field to create areas of different polarization.In some embodiments, the method further comprises patterning the completed electrophoretic display film with an electric field to create areas of different polarization within the polyvinylidene fluoride film.

[0008] In a third aspect, the invention includes a method of making an electrophoretic display film, the method including dispersing a polyvinylidene fluoride (PVDF) solution onto a first release material to produce a PVDF film of less than 10 μm thickness, bonding the PVDF film to a second release material with a conductive adhesive, removing the first release material, bonding a polymer film comprising an acrylate, vinyl ether, or epoxide to produce 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 with a first adhesive layer, embossing the polymer film comprising an acrylate, vinyl ether, or epoxide to produce 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. In some embodiments, the method further comprises applying a primer to the polymer film comprising an acrylate, vinyl ether, or 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 with a second adhesive layer. In some embodiments, the method further comprises removing the first release film to produce an electrophoretic display film that is less than 100 μm thick. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and the charged pigment particles are sealed within the microcells with a sealing layer. In some embodiments, the PVDF is polarized to produce zones of polarization that exhibit a difference. 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-ethyleneoxythiophene) (PEDOT).In some embodiments, the PVDF film is patterned with an electric field to create areas of differential polarization. In some embodiments, the method further comprises patterning the completed electrophoretic display film with an electric field to create areas of differential polarization in the PVDF film.

[0009] In a fourth aspect, an electrophoretic display film of thickness (top to bottom) less than 100 μm comprises a first adhesive layer, a patterned piezoelectric layer with zones of differential polarization, 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 non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to each other using a polymeric binder. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells using a sealing layer. In some embodiments, the sealing layer is conductive. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is polarized to produce zones that exhibit polarization differences. In some embodiments, the flexible light-transmissive electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible light-transmissive electrode layer comprises poly(3,4-ethyleneoxythiophene) (PEDOT). In some embodiments, the invention includes 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 electrophoretic display film additionally comprises a second adhesive layer bonded to the flexible light-transmissive electrode layer and a second release sheet bonded to the second adhesive layer.

[0010] In a fifth aspect, the invention includes a method of patterning a piezoelectric medium film. The method includes bonding a film of polyvinylidene fluoride (PVDF) to a layer of electrophoretic medium to produce a piezoelectric medium film, and patterning the piezoelectric medium film with an electric field. In some embodiments, the electric field is provided by corona discharge. In some embodiments, the method additionally includes placing a conductive mask adjacent to the piezoelectric medium film before patterning the piezoelectric medium film with corona discharge. In some embodiments, the electric field is provided by a high voltage write head. In some embodiments, the patterning includes forming regions of different polarity in the PVDF. In some embodiments, the patterning produces a security marker. In some embodiments, the layer of electrophoretic medium comprises a plurality of microcapsules containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to one another using a polymeric binder. In some embodiments, the layer of electrophoretic medium comprises a plurality of microcells containing a non-polar fluid and charged pigment particles that move toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells using a sealing layer.

[0011] In a sixth aspect, the invention includes an electrophoretic display film less than 100 μm thick (top to bottom) that includes an adhesive layer, an electrophoretic medium layer, a patterned piezoelectric layer with zones of differential polarization, and a conductive adhesive layer. In some embodiments, the electrophoretic medium layer includes a plurality of microcapsules containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to each other using a polymeric binder. In some embodiments, the electrophoretic medium layer includes a plurality of microcells containing a non-polar fluid and charged pigment particles, the charged pigment particles moving toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells using a sealing layer. In some embodiments, the sealing layer is conductive. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezoelectric layer includes polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is polarized to produce zones of polarization that exhibit a difference. In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film as described above, the release sheet being bonded to a first adhesive layer. In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film that includes a conductive adhesive layer, the release sheet being bonded to the conductive adhesive layer.

[0012] In a seventh aspect, the invention comprises an electrophoretic display film of less than 100 μm thickness (top to bottom) comprising an adhesive layer, a patterned piezoelectric layer with zones of differential polarization, an electrophoretic medium layer, and a conductive adhesive layer. [Brief description of the drawings]

[0013] [Figure 1A]1A shows a side view of a piezoelectric display film of the present invention, which includes star-shaped areas of differential polarization. Three exemplary positions are shown from the side: convex, neutral, and concave. The total thickness of the piezoelectric display film can be less than 100 μm, such as less than 50 μm, such as less than 25 μm.

[0014] [Figure 1B] 1B shows a top view of a piezoelectric display film of the present invention, which includes star-shaped areas of differential polarization. Three exemplary positions are shown from above: convex, neutral, and concave. When the piezoelectric display film is bent, the areas of differential polarization result in oppositely charged particles appearing at the viewing surface.

[0015] [Figure 2A] FIG. 2A shows an exemplary thin layer of piezoelectric material on a substrate.

[0016] [Figure 2B] 2B illustrates how the strong electric field of a corona discharge can be used to create areas of differential polarization in a thin layer of piezoelectric material. By moving the piezoelectric material closer and farther from the discharge, the amount of polarization can be spatially controlled.

[0017] [Figure 2C] 2C illustrates how areas of differential polarization can be created in a thin layer of piezoelectric material by using the strong electric field of a corona discharge. A conductive mask is used to pattern the piezoelectric material to create areas of differential polarization.

[0018] [Figure 2D] FIG. 2D illustrates a polarization pattern that can be achieved using the methods of FIGS. 2B and 2C.

[0019] [Figure 3A]FIG. 3A illustrates a side view of a piezoelectric film poled in the A direction.

[0020] [Figure 3B] FIG. 3B illustrates a top view of a piezoelectric film poled in the A direction.

[0021] [Figure 3C] FIG. 3C illustrates a side view of a piezoelectric film poled in the G direction using a conductive mask.

[0022] [Figure 3D] FIG. 3D illustrates a top view of a piezoelectric film poled in the G direction using a conductive mask.

[0023] [Figure 4A] FIG. 4A shows an exemplary thin layer of piezoelectric microcell precursor film on a substrate.

[0024] [Figure 4B] 4B illustrates how the strong electric field of a corona discharge can be used to create areas of differential polarization within a thin layer of piezoelectric material of a piezoelectric microcellular precursor film. By moving the piezoelectric microcellular precursor film closer and farther from the discharge, the amount of polarization can be spatially controlled.

[0025] [Figure 4C] 4C illustrates a method of creating areas of differential polarization in a thin layer of piezoelectric material of a piezoelectric microcellular precursor film by using the strong electric field of a corona discharge. A conductive mask is used to pattern the piezoelectric material of the piezoelectric microcellular precursor film to create areas of differential polarization.

[0026] [Figure 4D] FIG. 4D illustrates a poling pattern in a piezoelectric microcellular precursor film that can be achieved using the methods of FIGS. 3B and 3C.

[0027] [Figure 5A] FIG. 5A is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0028] [Figure 5B] FIG. 5B is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0029] [Figure 5C] FIG. 5C is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0030] [Figure 5D] FIG. 5D is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0031] [Figure 6A] FIG. 6A is a schematic cross-sectional view of an embodiment of a piezoelectric phoretic display.

[0032] [Figure 6B] FIG. 6B is a schematic cross-sectional view of an embodiment of a piezoelectric phoretic display.

[0033] [Figure 7] FIG. 7 details a method for producing a piezophoretic film or (optionally) a display.

[0034] [Figure 8A] FIG. 8A is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0035] [Figure 8B] FIG. 8B is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0036] [Figure 9A] FIG. 9A is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0037] [Figure 9B]FIG. 9B is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0038] [Figure 10A] FIG. 10A is a schematic cross-sectional view of an embodiment of a piezoelectric phoretic display.

[0039] [Figure 10B] FIG. 10B is a schematic cross-sectional view of an embodiment of a piezoelectric phoretic display.

[0040] [Figure 10C] FIG. 10C is a schematic cross-sectional view of an embodiment of a piezoelectric phoretic display.

[0041] [Figure 11] FIG. 11 details a method for producing thin piezoelectric films.

[0042] [Figure 12A] FIG. 12A is a schematic cross-sectional view of a piezoelectric film produced using the method shown in FIG.

[0043] [Figure 12B] FIG. 12B is a schematic cross-sectional view of a piezoelectric phoretic display produced using the method shown in FIG.

[0044] [Figure 13A] FIG. 13A is a schematic cross-sectional view of an alternative piezoelectric film produced using the method shown in FIG.

[0045] [Figure 13B] FIG. 13B is a schematic cross-sectional view of an alternative piezoelectric phoretic display produced using the method shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Disclosed herein are thin piezoelectric films and display films that include thin piezoelectric films. In some embodiments, the piezoelectric material of the piezoelectric film can be patterned using a high voltage electric field after the fabrication of the piezoelectric film. This feature allows the end user to address the piezoelectric material, for example, using corona discharge during production, which may include, for example, a bar code or serial number that is only visible when the piezoelectric film is manipulated. Such films are useful as security markers, authentication films, indicators, or sensors. The films are generally flexible. Some films are less than 100 μm thick. In some embodiments, the piezoelectric film is less than 50 μm thick and can be folded without breaking. Displays formed using the films do not require an external power source.

[0047] The term "electro-optical" as applied to a material or display is used herein to refer to a material having first and second display states that differ in at least one optical property in its conventional sense in the imaging arts, and that is changed from its first display state to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property such as optical transmittance, reflectance, luminescence, or, in the case of displays intended for machine reading, pseudocolor in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

[0048] The terms "bistable" and "bi-stable" are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states that differ in at least one optical property, such that after any given element is driven with an address pulse of finite duration to exhibit either a first or second display state, that state will persist after the address pulse is terminated for at least several times, e.g., at least four times, the minimum duration of the address pulse required to change the state of the display element. In U.S. Pat. No. 7,170,670, it is shown that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states, but also in their intermediate gray states, and the same is true for some other types of electro-optic displays. Displays of this type are appropriately referred to as "multistable" rather than bistable, but for convenience the term "bistable" may be used herein to cover both bistable and multistable displays.

[0049] The term "gray state" is used herein in its conventional sense in the imaging arts to refer to a state intermediate between two extreme pixel optical states, and does not necessarily imply a black-white transition between these two extreme states. For example, some of the E INK patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, such that the intermediate "gray state" is actually light blue. In fact, as already described, the change in optical state may not be a change in color at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of the display, and should generally be understood to include extreme optical states that are not strictly black and white, such as the white and dark blue states mentioned above. The term "monochrome" may be used hereinafter to refer to a display or driving scheme that drives pixels to only their two extreme optical states, with no intervening gray states.

[0050] The term "pixel" is used herein in its conventional sense in the display art to mean the smallest unit of a display capable of generating all the colors the display itself can show. In a full-color display, each pixel typically consists of a number of sub-pixels, each of which can display less than all the colors the display itself can show. 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 can display a range of colors from black to the brightest version of its defined color.

[0051] 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 consisting of electrodes formed at least in part from a semiconducting metal oxide and a plurality of dye molecules attached to the electrodes that can reverse the color change. See, for example, 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. Patent Nos. 6,301,038, 6,870,657, and 6,950,220. This type of medium is also typically bistable.

[0052] Another type of electro-optic display is the electrowetting display developed by Philips and described in Hayes, RA, et al, "Video-Speed ​​Electronic Paper Based on Electrowetting", Nature, 425, 383-385 (2003). It is shown in U.S. Patent No. 7,420,549 that such electrowetting displays can be bistable.

[0053] Another type of electro-optic display that has been the subject of intensive research and development for many years is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared to liquid crystal displays.

[0054] An electro-optical device typically comprises a layer of electrophoretic material and at least two other layers arranged on opposite sides of the electrophoretic material, one of these two layers being an electrode layer. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongated row electrodes and the other into elongated column electrodes extending at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode, and the other electrode layer is 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 display and a separate stylus, print head, or similar movable electrode, only one of the layers adjacent to the electrophoretic layer comprises an electrode; the layer opposite the electrophoretic layer is typically a protective layer intended to prevent the movable electrode from damaging the electrophoretic layer.

[0055] Numerous patents and applications assigned to or in the name of Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in encapsulating electrophoretic and other electro-optic media. Such encapsulated media comprises a number of small capsules, each of which itself comprises an internal phase that contains electrophoretically movable particles in a fluid medium, and a capsule wall that surrounds the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer that is positioned between two electrodes. Techniques described in these patents and applications include: (a) electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814); (b) capsules, binders, and encapsulation processes (see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719); (c) films and subassemblies containing electro-optical materials (see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564); (d) backplanes, adhesive layers, and other auxiliary layers and methods used in displays (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624); (e) color formation and color control (see, e.g., U.S. Pat. Nos. 7,075,502 and 7,839,564); (f) methods of driving displays (see, e.g., U.S. Patent Nos. 7,012,600 and 7,453,445); (g) display applications (see, e.g., U.S. Pat. Nos. 7,312,784 and 8,009,348); (h) non-electrophoretic displays (see U.S. Pat. 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 the microcells (see, e.g., U.S. Pat. Nos. 7,072,095 and 9,279,906); (j) Methods for filling and sealing microcells (see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088).

[0056] Many of the aforementioned patents and applications recognize that the walls surrounding the separate microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus giving rise to so-called "polymer-dispersed electrophoretic displays," in which the electrophoretic medium consists of a plurality of separate droplets of electrophoretic fluid and a continuous phase of polymeric material, and in which the separate droplets of electrophoretic fluid in such polymer-dispersed electrophoretic displays may be considered capsules or microcapsules even though no separate capsule membrane is associated with each individual droplet. See, for example, the aforementioned U.S. Patent No. 6,866,760. Thus, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0057] A related type of electrophoretic display is the microcell electrophoretic display, also known as MICROCUP®. In a microcell electrophoretic display, the charged particles and fluid are not encapsulated in microcapsules, but instead are held within a plurality of cavities formed in 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.

[0058] Although electrophoretic media are often opaque (e.g., in many electrophoretic media, because the particles substantially block the transmission of visible light through the display) and may operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "obstructed mode" in which one display state is substantially opaque and one is light-transmitting. See, for example, U.S. Pat. Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode. See U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in the obscured mode. Electro-optic media operating in the shielding mode may be useful in multi-layer structures for full-color displays, in which at least one layer adjacent to the viewing surface of the display operates in the shielding mode to expose or conceal a second layer that is more remote from the viewing surface.

[0059] Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices, and offer additional advantages such as the ability to print or coat displays on a wide variety of flexible and rigid substrates. (The use of the word "printing" is intended to include 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 using a variety of methods, the display itself can be made inexpensively.

[0060] The aforementioned U.S. Patent No. 6,982,178 describes a method for fabricating solid electro-optic displays (including particle-based electrophoretic displays) that is well adapted for mass production. Essentially, this patent describes a so-called "front plane laminate" ("FPL"), which comprises, in order, a light-transmitting conductive layer, a layer of a solid electro-optic medium in electrical contact with the conductive layer, an adhesive layer, and a release sheet. Typically, the light-transmitting conductive layer may be supported on a light-transmitting substrate, which is preferably flexible in the sense that the substrate may be manually rolled around a 10 inch (254 mm) diameter drum (for example) without permanent deformation. The term "optically transparent" is used in this patent and specification to mean that the layer so designated transmits sufficient light to enable an observer looking through the layer to observe a change in the display state of the electro-optic medium, usually as viewed through the conductive layer and adjacent substrate (if present); if the electro-optic medium exhibits a change in reflectance at non-visible wavelengths, the term "optically transparent" should of course be interpreted to refer to the transmission of the relevant non-visible wavelengths. The substrate will typically be a polymeric film, usually having a thickness within 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 conductive layer may conveniently be a thin metal or metal oxide layer, for example, aluminum or ITO, or may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are commercially available, for example as "aluminized Mylar" from EI du Pont de Nemours & Company (Wilmington Del.) ("Mylar" is a registered trademark), and such commercial materials can be used with good results in front plane laminations.

[0061] Assembly of an electro-optic display using such a front plane laminate can be accomplished by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium, and conductive layer to the backplane. This process is well suited to mass production, as the front plane laminate can typically be mass produced using roll-to-roll coating techniques and then cut into pieces of any size required for use with a particular backplane.

[0062] No. 7,561,324 describes a so-called "dual release sheet," which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Pat. No. 6,982,178. One form of the dual release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the dual release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the dual release film are intended for use in a process generally similar to the process for assembling an electro-optic display from the front plane laminate already described, but involving two separate laminations, typically in a first lamination the dual release sheet is laminated to a front plane electrode to form a front plane assembly, and then in a second lamination the front plane assembly is laminated to a backplane to form the final display, although the order of these two laminations may be reversed, if desired.

[0063] The subject matter presented herein particularly relates to a piezoelectric display structural design, which does not require a power source (e.g., a battery or wired power supply, a photovoltaic source, etc.) for the electrophoretic display to operate. The assembly of such an electrophoretic display is therefore simplified. In some embodiments, the piezoelectric material and the electrophoretic medium are directly laminated together. The electrophoretic medium may be contained within a microcell, a microcapsule, as described above, or the electrophoretic medium may be dispersed in a polymer matrix. In some embodiments, the piezoelectric material is poled (i.e., written) using a high-voltage electric field after the piezoelectric film or piezoelectric display is produced, as discussed below.

[0064] Piezoelectricity is the accumulation of electrical charge in solid materials in response to applied mechanical stress. Suitable materials for the subject matter disclosed herein may include polyvinylidene fluoride (PVDF), quartz (SiO2), berlinite (AlPO4), gallium phosphate (GaPO4), tourmaline, barium titanate (BaTiO3), lead zirconate titanate (PZT), zinc oxide (ZNO), aluminum nitride (AlN), lithium tantalate, lanthanum gallium silicate, sodium potassium tartrate, and any other known piezoelectric material. In piezoelectric materials:

[0065] The piezoelectric films and displays described herein use piezoelectricity to drive charged pigments of an electrophoretic medium. Thus, when the piezoelectric material bound to the electrophoretic medium layer is manipulated, the color of the electrophoretic material at the viewing surface changes. For example, by bending or introducing stress into a piece of piezoelectric material, a voltage can be generated, which can be utilized to cause the color pigments of the electrophoretic material to move. If segments of piezoelectric material with different polarizations are used, or if areas of differential polarization are created within the piezoelectric film, an electrophoretic medium with two types of oppositely charged pigments can be used to create patterns with high contrast ratios, as shown in Figures 1A and 1B. As used herein, the term "contrast ratio" (CR) for electro-optic displays (e.g., electrophoretic displays) is defined as the ratio of brightness of the brightest color (white) to the darkest color (black) that the display can produce. Typically, a high contrast ratio, or CR, is a desired aspect of a display.

[0066] 1A and 1B illustrate side and top views of an exemplary piezoelectric phoretic display 100 according to the subject matter disclosed herein. In this embodiment, a piezoelectric material is laminated to an electrophoretic medium layer (discussed below), and one or more electrodes are included to provide a suitable electric field to drive the electrophoretic particles toward (or away from) a viewing surface. In the embodiment shown in FIGS. 1A and 1B, a second area 120 of the piezoelectric material of the piezoelectric phoretic display 100 is polarized in an opposite direction to the first area 110, so that when the piezoelectric phoretic display 100 is manipulated from a neutral state (position 2) to either a first (position 1) or second (position 3) optical state, the first and second areas (110, 120) will achieve different colors within the two areas. In the case of an electrophoretic medium with a set of black and white oppositely charged particles, a high contrast image will be formed, for example, as shown in FIG. 1B. Because the first and second areas of piezoelectric material (110, 120) can be polarized with good resolution (as discussed below), various images / information can be encoded to "appear" when the piezoelectric phoretic display 100 is manipulated. For example, a security ribbon can be generated to exist in a neutral state as a gray strip, but when the security ribbon is bent, the ribbon will display a security seal, such as the star shape shown in FIG. 1B. Of course, the security seal could alternatively include a barcode, a number, a word, a phone number, and an internet address, a QR code, a photograph, a halftone image, or a logo.

[0067] In principle, piezoelectric materials (and optionally adjacent electrophoretic materials) can be polarized using strong localized electric fields, as shown in Figures 2A-3D. It is known that piezoelectric materials (particularly films) can be stimulated to move between polarization states using a variety of external stresses, such as mechanical stretching, heat, electromagnetic fields, and applied forces. The piezoelectric effect is closely related to the occurrence of electric dipole moments in solids. The dipole density or polarization (P) corresponds to the dipole moment / volume of the crystallographic unit cell and is typically expressed as C / m 2The density of dipoles is measured in terms of the density of dipoles, P. The resulting dipole density P is the characteristic vector field (i.e., differential polarization) for a particular region of the material. Similar to magnets, dipoles in the vicinity of each other tend to align in regions (Weiss domains). When first created, the domains are usually randomly oriented. However, using various multi-step processes, the domains can be made to align to produce localized areas of differential polarization. The process of these regions becoming aligned is known as polarization.

[0068] Although many piezoelectric materials are crystalline, several flexible piezoelectric active polymers are known, such as polyvinylidene fluoride (PVDF) and its copolymers, polyamides, and parylene-C. Non-crystalline polymers, such as polyimides and polyvinylidene chloride (PVDC), are classified as amorphous bulk polymers. The standard procedure for making piezoelectric active films, such as polyvinylidene fluoride (PVDF), is to produce a polymer film and stretch it to generate stress and align the dipoles. The stretching converts the non-polarized alpha phase regions of PVDF to a polarized beta phase. A subsequent stimulus is added to the polar regions of the beta phase, for example, using a strong electric field. Other methods of aligning the beta phase, such as laser irradiation and focused magnetic fields, are also described in the literature. See, for example, U.S. Pat. No. 9,831,417. If the stimulus can be performed with a high enough resolution, the poles can be used to generate visible patterns, for example, as illustrated in Figures 1A and 1B. In some embodiments, the electric field is applied at high temperature, however, this is not always necessary. Particularly for very thin piezoelectric films, e.g., less than 20 μm, e.g., less than 10 μm, less than 5 μm, it is feasible to pole the film without high temperature, provided that the electric field is strong enough. In the case of PVDF, an additional benefit is that such films are also optically transparent, so they can be bonded to the electrophoretic medium between the viewing surface and the electrophoretic medium, or the electrophoretic medium is layered between the piezoelectric film and the viewing surface.

[0069] Exemplary methods for poling thin films of piezoelectric material are illustrated in Figures 2A-2D. A thin film of piezoelectric material 210, such as PVDF, can be melted and spin-coated onto a substrate 220 to form a thin film. The thin film can optionally be thermally conditioned or stretched prior to poling. Suitable bulk PVDF is available, for example, from Sigma-Aldrich, as a bulk powder or as a film. Pre-stretched piezoelectric active PVDF films are also available, for example, from PolyK Technologies (State College, PA). Such films can also be fabricated with metallized electrode coatings on one side, which can also be used for piezoelectric films and displays, however, piezoelectrics with a backing metal layer are difficult to pole using an electric field. 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 transport substrate or release material, for example, using a roll-to-roll process. The PVDF-coated substrate is then heated and the DMF is stripped off, resulting in 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 larger number of beta-phase domains suitable for poling.

[0070] As shown in Figures 2B and 2C, a thin film of piezoelectric material 210 can be polarized using a high voltage corona discharge 230 with spatial focusing. Suitable corona discharge equipment is available, for example, from Simco-Ion (Alameda, CA). Such devices can generate a localized 10-50 kV field, e.g., a 30 kV field, e.g., a 20 kV field, that can be brought within a few microns of the piezoelectric material to be polarized. Spatial focusing can be achieved using a steered electric field and / or gas flow, which focuses / steers the flux of ions emanating from the corona discharge. As shown in Figure 2B, the high voltage corona discharge 230 can be moved in three dimensions to generate areas of differential polarization, i.e., pattern the piezoelectric material 210. Alternatively, the piezoelectric material 210 can be mounted on an XYZ stage, which allows the film workpiece to approach the high voltage corona discharge 230 in a controlled manner. In an alternative embodiment, a conductive mask 240 can be used to protect areas of the piezoelectric material 210 from the high voltage corona discharge 230, as shown in FIG. 2C. The conductive mask can be made, for example, of conductive stainless steel or another conductive material that can withstand proximity to a corona discharge. Alternative masks made of charge absorbing or charge blocking materials such as glass, plastic, or rubber would also work. When the high voltage corona discharge 230 is moved over the thin film of piezoelectric material 210, the thin film of piezoelectric material 210 is polarized only in areas where the conductive mask 240 does not cover the thin film of piezoelectric material 210. In addition, the polarity of the high voltage corona discharge 230 can be reversed so that some areas are polarized in a first direction, some areas are polarized in a second direction, and some areas can be randomly polarized or unpolarized. See also FIGS. 3A-3D.

[0071] Using the technique shown in Figures 2B and 2C, it is straightforward to produce a thin film of piezoelectric material 210 with areas of differential polarization P1 and P2, shown as 260 and 270 in Figure 2D. The areas of differential polarization 260 and 270 do not necessarily have equal magnitude and opposite polarity, however, such an arrangement generally provides a better contrast ratio when a two-particle electrophoretic medium is used with the thin film of piezoelectric material 210. For example, as shown in 2D, a first area 260 may be polarized toward the viewer, while a second area 270 may be polarized away from the viewer. This technique is further illustrated in Figures 3A-3D, which show how a single area 360 of a thin film of piezoelectric material deposited on a substrate 320 may be polarized to have a polarization vector pointing out of the page, as shown in Figure 3B. Thus, when the thin film of piezoelectric material is manipulated (bent), it will preferentially drive one polarity of the electrophoretic particles towards the viewing surface. As shown in FIG. 3C, a second area 370 of the thin film of piezoelectric material can be polarized in different directions, with or without the addition of a conductive mask 340, resulting in several patterned combinations of polarity and magnitude, as needed for the application. As shown in FIG. 3D, some parts of 370 are polarized towards the viewing surface, but shading is created by the conductive mask 340. Thus, when the piezoelectric material is manipulated (bent), it will preferentially drive one polarity of the electrophoretic particles towards the viewing surface, except in the areas where the polarization is masked, which will remain in a neutral color stage, thereby giving rise to a certain pattern, for example, a security seal.

[0072] 2A-3D illustrate various techniques that may be used to create areas of differential polarization in a thin film of piezoelectric material 210. As illustrated in FIGS. 4A-4D, these same techniques can also be used to create areas of differential polarization in a thin piezoelectric medium film 405. As shown in FIG. 4A, a thin film of piezoelectric material 410 can be bonded to a layer of electrophoretic microcell 420 to create a piezoelectric medium film 405. The thin film of piezoelectric material 410 can be bonded to the layer of electrophoretic microcell 420 using an adhesive layer (not shown) or the thin film of piezoelectric material 410 can be directly, i.e., spin-coated onto the layer of electrophoretic microcell 420 as discussed above with respect to FIG. 2A. 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, a layer of electrophoretic microcell 420 may be filled with an electrophoretic medium 425 that typically includes two or more electrophoretic particles 423 and 427 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 entirety. In some embodiments, a layer of electrophoretic microcells 420 is produced on a release material, filled with electrophoretic medium 425, and sealed with a sealing layer 430, and the filled and sealed electrophoretic microcells 420 are then used as a substrate for the production of a thin film of piezoelectric material 410. The resulting structure is a thin piezoelectric medium film 405. In other embodiments, the thin film of piezoelectric material 410 is laminated to an acrylate, vinyl ether, or epoxide film that is a precursor to the layer of electrophoretic microcells 420.The combined thin film of piezoelectric material 410 and precursor material is then embossed on the precursor side (discussed below) and subsequently filled with electrophoretic medium 425 and sealed with sealing layer 430 to produce thin piezoelectric medium film 405. In yet another embodiment (not shown in FIGS. 4A-4D ), a complete microcellular front plane 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 poled as described below. Notably, when a front plane laminate material is used, the final structure additionally includes a conductive layer, which is typically optically transparent. The front plane laminate can be oriented so that the optically transparent electrode layer is in contact with the thin film of piezoelectric material 410, or the front plane laminate can be flipped over so that the sealing layer is in contact with the thin film of piezoelectric material 410.

[0073] Once the thin piezoelectric medium film 405 has been produced, the thin film of piezoelectric material 410 can be addressed as described with respect to Figures 2A-3D above. That is, the thin film of piezoelectric material 410 can be polarized using a high-voltage corona discharge 230 with spatial focus, as shown in Figure 4B, for example, by mounting the thin piezoelectric medium film 405 on an XYZ stage that allows the film workpiece to approach the high-voltage corona discharge 230 in a controlled manner. In an alternative embodiment, a conductive mask 240 can be used to protect areas of the thin piezoelectric medium film 405 from the high-voltage corona discharge 230, as shown in Figure 4C. As discussed with respect to Figures 2A-3D, the polarity of the high-voltage corona discharge 230 can be reversed, so that some areas are polarized in a first direction, some areas are polarized in a second direction, and some areas can be randomly polarized or unpolarized. As in FIG. 2D above, poling the thin film of piezoelectric material 410 in the thin piezoelectric medium film 405 results in areas of polarization P1 and P2 exhibiting differences shown as 460 and 470 in FIG. 4D. Importantly, because the thin piezoelectric medium film 405 can be fabricated prior to poling, it is feasible for the end customer to control the final step of creating the desired polarization design in the thin piezoelectric medium film 405. Thus, if the end product will include a security seal or serial number, the security seal or serial number can be installed after the end product is completed and verified, etc. For example, a U.S. $100 bill is printed in metallic ink at the U.S. Treasury along with a serial number, while at the same time the security ribbon comprising the thin piezoelectric medium film 405 can be polarized to create a verification code corresponding to the serial number. This feature eliminates many logistical issues and associated costs, for example, since there is no need to match prefabricated security markers to specific products further down the supply chain.

[0074] The techniques described above can be used to achieve a wide variety of thin piezophoretic films, as illustrated in the following figures.

[0075] As shown in Figures 5A-6B, 8A-10C, and 12A-13B, a piezoelectric film or display includes a layered stack of a number of components, including a thin piezoelectric film and a layer of electrophoretic medium. The piezoelectric material can be any of the materials listed above, however, polymers such as PVDF and its copolymers are also preferred because they can be fabricated into very thin films. The electrophoretic medium typically includes one or more sets of charged particles that move through a non-polar solvent in the presence of an electric field. The electrophoretic medium is typically contained within, i.e., microcapsules, microcells, or dispersed droplets. The electrophoretic medium can also be contained within an open valley or well sealed within a larger flexible container. The piezoelectric films and displays exemplified herein can be made very thin, e.g., 100 μm or less thick, e.g., 70 μm or less thick, e.g., 50 μm or less thick, e.g., 35 μm or less thick, e.g., 20 μm or less thick, e.g., 10 μm or less thick. Such thin materials can bend without breaking or leaking, and are unnoticeable when incorporated into a final product such as paper or banknotes. In addition, many of the piezoelectric films or displays include layers, all of which are optically transparent and / or thin enough to be optically transparent, thus allowing the piezoelectric response to be viewed from above and below. In such piezoelectric films or displays, when a first image is visible from the top surface, e.g., position 1 in FIG. 1B, the bottom surface will typically show an inverted image, e.g., position 3 in FIG. 1B. However, when incorporating more than two types of particles in the electrophoretic medium, the top and bottom may not show the inverse image due to mixed particle conditions at one of the two surfaces.

[0076] Piezophoretic films or displays often include at least one electrode layer, which may be optically transparent and which may be flexible. Suitable materials include commercial ITO-coated PET, which may be used as a substrate for fabrication. In some other embodiments, flexible and transparent conductive coatings including other transparent conductive oxides (TCOs) may be used, 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 versions of these oxides, such as fluorine-doped tin oxide. In many of the embodiments described herein, poly(3,4-ethyleneoxythiophene) polystyrene sulfonate (PEDOT:PSS) is used because it has excellent bending properties and is optically transparent. Although the overall conductivity is not as high as, for example, PET / ITO, PEDOT:PSS is sufficient to provide the electric field required 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. Polymer films coated with metals may also be suitable for use as electrode layers. The resistance of the electrode layer may be 500 ohm-meters or less, e.g., 100 ohm-meters or less, e.g., 1 ohm-meter or less, e.g., 0.1 ohm-meters or less, e.g., 0.01 ohm-meters or less. (For comparison, electrophoretic medium layers typically have a resistance of about 10 7 ~10 8 The piezoelectric material has a resistance of 10 Ohm meters. 11 ~10 14 It has a resistance in ohms.)

[0077] A piezophoretic film or piezophoretic display will often include at least one adhesive layer formed from a polymer such as acrylic or polyurethane, polyurethane, polyurea, polycarbonate, polyamide, polyester, polycaprolactone, polyvinyl alcohol, polyether, polyvinyl acetate derivatives such as poly(ethylene-co-vinyl acetate), polyvinyl fluoride, polyvinylidene fluoride, polyvinyl butyral, polyvinyl pyrrolidone, 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, alginic acid, lecithin, polymers derived from amino acids, etc. The adhesive may additionally include one or more low dielectric polymers or oligomers, ionic liquids, or conductive fillers such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. Adhesives containing such charged and / or conductive materials are conductive adhesives. 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 be approximately 10 6 ohm*cm~10 8 Ohm*cm, preferably 10 12 It may have a resistivity value of less than ohm*cm.

[0078] Of the above mentioned polymers and oligomers, polyurethanes, polyureas, polycarbonates, polyesters and polyamides, especially those equipped with functional groups, are preferred due to their better adhesive and optical properties and high environmental resistance. Examples of functional groups may include, but are not limited to, -OH, -SH, -NCO, -NCS, -NHR, -NRCONHR, -NRCSNHR, vinyl or epoxides, and derivatives thereof, including cyclic derivatives. "R" in the above mentioned functional groups may be hydrogen or alkyl, aryl, alkylaryl or arylalkyl of up to 20 carbon atoms, which 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, etc. 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, are particularly preferred.

[0079] In many embodiments, the piezoelectric film or display will often include a release sheet. The release material may be used temporarily to facilitate processing of the piezoelectric film or display, for example, when embossing, filling, cutting, etc. In other embodiments, the release material may be used to deliver the final piezoelectric film or display that will be adhered to the final product. In some cases, the release material will protect a functional adhesive layer that will be used to manipulate the piezoelectric film or display prior to the piezoelectric film or display being placed in the final product. The release material may be formed from a material selected from the group consisting of polyethylene terephthalate (PET), polycarbonate, polyethylene (PE), polypropylene (PP), paper, and lamination or cladding films thereof. The release material may be metallized to facilitate quality control measurements and / or to control static electricity during handling, shipping, and downstream incorporation into products. In some embodiments, a silicone release coating may be applied over the release material to improve release properties.

[0080] Although not shown in Figures 5A-6B, 8A-10C, and 12A-13B, the piezoelectric film or display may include additional edge seals and / or barrier materials that allow the piezoelectric film or display to maintain a desired humidity level, for example, to prevent leakage of non-polar solvents or adhesives, and to prevent intrusion of water, dust, or gas. The barrier material can be any flexible material, typically a polymer with low to negligible WVTR (water vapor transmission rate). Suitable materials include polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, cyclic olefin, and combinations thereof. If the piezoelectric film or display will be exposed to particularly harsh conditions, flexible glass such as WILLOW® glass (Corning, Inc.) may be used for the barrier layer. The edge seal can be a metallized foil or other barrier foil that is coated and adhered over the edge of the piezoelectric film or display. The edge seal can also be formed from a dispensed sealant (heat, chemical, and / or radiation cured), polyisobutylene, or an acrylate-based sealant, which can be crosslinked. In some embodiments, the edge seal can be a sputtered ceramic, such as alumina or indium tin oxide, or an advanced ceramic, such as those available from Vitex Systems, Inc. (San Jose, Calif.).

[0081] In general, the layers of the piezoelectric film 501-504 can be arranged / stacked in the order that produces the best performance for the end application. For example, as shown in FIG. 5A, the piezoelectric film 501 can be prepared by placing microcell precursor material on a release material 510 that includes a release material adhesive 520. The microcell precursor can then be embossed or photolithographically processed to produce an array of microcells 530. The microcells 530 can be thermally cured or cured using 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 discussed above with respect to FIG. 4A. (It should be understood that the microcells 530 adjacent to the sealing layer 540 are filled with an electrophoretic medium comprising charged particles in a non-polar solvent, although the electrophoretic medium is not shown in subsequent figures.) The piezoelectric layer 560 can 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. Such a piezoelectric film 501 can then be manipulated by handling the release material 510 until such time that the stack without the release material 510 is attached to the final product. In the piezoelectric film 501, the piezoelectric layer 560 is typically polarized to produce areas of polarization that exhibit a difference before the flexible electrode 580 is bonded to the piezoelectric film. In some embodiments, the flexible electrode 580 and the conductive adhesive 570 can be replaced with a thin layer of transparent conductive oxide, such as ITO. The ITO can be sputtered directly onto the piezoelectric layer 560 .

[0082] A closely related, but alternative stack is shown in Figures 5B-5D. In Figure 5B, a piezoelectric film 502 is produced in which a piezoelectric layer 560 is prepared prior to fabrication on a separate release material 510. For example, the piezoelectric layer 560 can be a pre-stretched PVDF film that has already been poled to produce a security pattern. The piezoelectric layer 560 is then bonded to a sealed microcell layer 530, which is already bonded to a flexible electrode 580. Notably, in the piezoelectric film 502, the openings of the microcell layer 530 face away from the piezoelectric layer 560, which can promote good bonding between the microcell layer 530 and the piezoelectric layer 560. This bonding can be improved with the introduction of a primer 535 to improve adhesion of the piezoelectric layer 560 to the microcell material, typically a polymer comprising an acrylate, vinyl ether, or epoxide. Primer 535 can be a polar oligomeric or polymeric material, such as a polyhydroxy-functionalized polyester acrylate (e.g., BOMAR® BDE1025 from Dymax) or an alkoxylated acrylate, such as ethoxylated nonylphenol acrylate (e.g., SR504 from Sartomer), ethoxylated trimethylolpropane triacrylate (e.g., SR9035 from Sartomer), or ethoxylated pentaerythritol tetraacrylate (e.g., SR494 from Sartomer). Examples of polar polymers that are suitable for use as primer 535 include solvent urethane polymers, such as Irostic® polymers.

[0083] Of course, it is also possible to construct the stack such that the openings of the microcell layer 530 face towards the piezoelectric layer 560, as in the piezoelectric film 504 illustrated in Figure 5D. As a further alternative, shown in Figure 5C, the piezoelectric film 503 is positioned such that the openings of the microcell layer 530 face away from the piezoelectric layer 560, however the piezoelectric layer 560 is directly bonded to the flexible electrode 580.

[0084] The piezoelectric films (501, 502, 503, 504) shown in Figures 5A-5D can be converted into piezoelectric displays (601, 602) with the addition of a second flexible electrode 680 instead of the release layer in Figures 5A-5D. The piezoelectric displays (601, 602) will typically also include a second conductive adhesive 670, however, it should be noted that in some cases, the conductive adhesive 670 alone may be sufficient to provide the electric field required to switch the electrophoretic material. In addition, it is also possible to directly coat the bottom of the microcell layer 530 (Figure 6A) or the sealing layer 540 (Figure 6B) with a thin layer of transparent conductive oxide to create the second electrode. Furthermore, a conductive metal foil can be used as the second flexible electrode 680 if it is not necessary to be visible through both the top and bottom of the piezoelectric displays (601, 602). As shown in Figures 6A and 6B, it is typical to add a release material 510 to the completed piezoelectric display (601, 602) to improve handling and provide an easy to use adhesive for attaching the piezoelectric displays (601, 602). In some embodiments, the piezoelectric display 601 can be formed by simply bonding the piezoelectric layer 560 to a commercial front plane laminate, which includes a second flexible electrode 680 and a sealed microcell layer 530 containing the electrophoretic medium. In such cases, the piezoelectric layer 560 is typically polarized to create areas of differential polarization before the front plane laminate is bonded to the piezoelectric layer 560. Although the piezoelectric display (601, 602) of Figures 6A and 6B is shown with a piezoelectric layer 560 above the sealed microcell layer 530, it should be understood that the piezoelectric layer 560 can also be placed below the sealed microcell layer 530 to produce a piezoelectric display similar to Figures 5B and 5D.

[0085] (Prototype performance) A series of piezoelectric films of the type illustrated in FIG. 5A were produced using PEDOT:PSS films as flexible electrodes 580. The piezoelectric layer 560 was varied as shown in Table 1 (composition and thickness). Piezoelectric films were obtained from TE Connectivity (Norwood, MA), Fishman (and over, MA) or cast and cured in-house using PVDF powder from Sigma-Aldrich. Using the poling techniques described, the poling direction was altered to produce patterns. The electrophoretic medium included low voltage formulations of black and white particles, or black and red particles, or red and black particles designed to switch color states using + / - 3V. As shown in Table 1, all of the variations provided suitable switching. [Table 1]

[0086] Table 1 suggests that several types of electrophoretic media will respond favorably to the small electric field generated by bending a thin piezoelectric film. In particular, it was found that a spin-coated polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) film less than 3 μm had sufficient charge injection to switch the DV electrophoretic medium. See experiment number 7. Such a piezoelectric film 801 (see FIG. 8A) can be formed using the method described in FIG. 7. First, a thin film of piezoelectric material 940 is produced by casting (slot die coating) a concentrated PVDF / DMF solution onto a suitable substrate, heating, and driving off 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 can be 10 μm thick or less, for example, 5 μm thick or less, for example, 3 μm thick or less. Additionally, the piezoelectric film 960 may be stretched and / or poled with a suitable electric field to increase the number of beta phase domains, as discussed above. In step 730, a release material 910 is provided along with an adhesive 920, which is subsequently laminated to the cast piezoelectric film 960 in step 740. The piezoelectric film 960 is then coated with / bonded to an electrophoretic layer in step 750. The electrophoretic layer can be a sealed microcell layer including filled microcells 930 and a sealing layer 940, as shown in FIGS. 9A and 9B, or alternatively, the electrophoretic layer can include an encapsulated electrophoretic medium 990 within a polymer binder 995. Bonding of the piezoelectric film 960 to the electrophoretic layer may be facilitated with an intervening primer layer 935, for example, using one of the primer materials discussed above. If the electrophoretic layer is a sealed microcell layer, the microcell 930 can be positioned so that the sealing layer 940 is adjacent to the piezoelectric film 960 (as in FIG. 8A), or the microcell 930 can be positioned so that the sealing layer 940 is positioned on the opposite side from the piezoelectric film 960 (i.e., as in FIG. 8B).As a final step 760, an electrode layer 980 is produced, either bonded / deposited on the microcells 930, as in FIG. 8A, or bonded / deposited on the seal layer 940, as in FIG. 8B. As described above, the electrode layer 980 can include a flexible conductive material such as PEDOT:PSS, or can include a directly deposited (e.g., sputtered or evaporated) transparent conductive oxide (TCO). In some embodiments, the electrode 980 can include a prefabricated film of ITO on a polymer substrate such as PET. The piezoelectric film 801, including the directly deposited TCO electrode layer 980, the thin piezoelectric layer 960, and the thin layer of microcells 930 (approximately 10 μm thick), is remarkably thin (i.e., less than 25 μm thick, excluding the release material 910), which allows the piezoelectric film 801 to bend without breaking and is unnoticeable when attached to an object such as a banknote. The corresponding piezoelectric film 901 including the microcapsules can also be fabricated with a total thickness of less than 25 μm. Of course, alternative structures 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 shown in Figure 9B. Alternatively, the electrode 980 in Figures 8A-9B can be replaced with a conductive adhesive (not shown) or a conductive adhesive together with an additional release layer (not shown).

[0087] Similar to Figures 6A and 6B, the piezoelectric films of Figures 8A-9B can include a second electrode layer to form a corresponding display (1001, 1002, 1003), as shown in Figures 10A-10C. Both the electrode layer 980 and the second electrode layer 1080 can comprise a flexible conductive material such as PEDOT:PSS, or both the electrode layer 980 and the second electrode layer 1080 can comprise a directly deposited (e.g., sputtered or evaporated) transparent conductive oxide (TCO), or some combination thereof. Again, in the case where both the electrode layer 980 and the second electrode layer 1080 use a thin TCO film, the resulting piezoelectric display (1001, 1002, 1003) can be made very thin, i.e., less than 25 μm thick, excluding the release material 910. In some embodiments, the electrode layer 980 is produced bonded to / deposited on the microcell 930, as in Fig. 10A. In other embodiments, the electrode layer 980 is produced bonded to / deposited on the sealing layer 940, as in Fig. 10B. The assembly of piezoelectric displays 1001 and 1002 can also be used with microcapsules 990 containing an electrophoretic medium held together with a binder 995, thus producing piezoelectric display 1003, as shown in Fig. 10C. Alternatively, the electrodes 980 / 1080 in Figs. 10A-10C can be replaced with a conductive adhesive (not shown) or a conductive adhesive, along with an additional release layer (not shown).

[0088] An alternative method of constructing a piezoelectric film and a piezoelectric display is described with respect to the flow chart of FIG. 11. A piezoelectric film 1260 is procured, 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, however, any suitable embossable material such as a vinyl ether polymer or an epoxide polymer film may be used. Typically, the precursor film is 30 μm thick or less, for example 20 μm thick or less. The precursor film may be treated with a primer 1235 prior to the lamination step 1110. Once the piezoelectric film 1260 and the microcell precursor material are bonded, 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, for example. (Alternatively, depending on the application, the side of the piezoelectric film 1260 opposite the microcell precursor material can be coated with a conductive adhesive, which can be supported by a release layer.) This coating step produces the electrodes 1280 shown in the piezoelectric phoretic film 1201 and piezoelectric display 1202 shown in Figures 12A and 12B, respectively. (Although not shown in FIG. 11 , an alternative construction is to take a piezoelectric film 1260 that has been pre-coated with a transparent conductive material, and then laminate the pre-coated piezoelectric film 1260 and the microcell precursor material together (including the optional use of primer 1235).) After the stack of electrodes 1280, piezoelectric film 1260, and microcell precursor is produced, 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 material, and the adhesive 1250 can be any of the adhesives described above.In practice, the carrier substrate 1255 is typically PET, since PET sheets are easy to handle during the embossing step 1140. In step 1140, a stack comprising the carrier substrate 1255, adhesive 1250, piezoelectric film 1260, and microcell precursors is microembossed using the techniques described above with respect to U.S. Patent Nos. 6,930,818, 7,052,571, 7,616,374, 8,361,356, and 8,830,561. When this procedure is completed with a thin piezoelectric film and a thin microcell precursor, the final stack thickness (not including the carrier substrate) can be 30 μm thick or less, for example 20 μm thick or less. This results in an open microcell structure, which is subsequently filled with the desired electrophoretic medium and sealed with a water-soluble sealing layer 1240 in step 1150. The sealing layer 1240 can be made conductive by including conductive species. The sealing layer 1240 is typically light-transmitting or transparent. The open microcells can be cleaned / activated using a water vapor plasma treatment 1145 before the microcells are filled with the desired electrophoretic medium. Finally, the release sheet 1210 is bonded to the sealing layer 1240 using an adhesive 1220 in step 1160 to make the transfer of the piezoelectric film 1201 easier and to facilitate the placement of the electrophoretic film 1201 on the final product. The adhesive 1220 can also be conductive. The resulting structure is shown in FIG. 12A. Importantly, it is possible to complete the steps of FIG. 11 without poling the piezoelectric film 1260, thereby allowing the end customer to pattern the piezoelectric film 1201 at the site of final assembly, for example by using corona discharge to create areas of differential polarity as described above.

[0089] As shown in FIG. 12B, the method of FIG. 11 can be extended to produce a piezoelectric display 1202 with the addition of a second electrode 1285. The second electrode 1285 can also include a transparent conductive material added directly to the sealing layer 1240 instead of the release material 1210 and adhesive 1220. However, in other embodiments, the release material 1210 will be removed and the second electrode 1285 will be laminated to the sealing layer 1240 with the adhesive 1220. If the piezoelectric display 1202 does not require the electrophoretic medium to be visible from both sides, the second electrode 1285 can be a metal film. Alternatively, the second electrode 1285 can be a conductive polymer such as PEDOT:PSS. In some other embodiments, the adhesive 1220 can be a conductive adhesive that provides sufficient conductivity to act as the second electrode 1285.

[0090] Finally, it should be appreciated that the electrodes do not need to 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 including the release material 1210, the adhesive 1220, the piezoelectric film 1260, and the microcell precursor can be prepared, and the microcell precursor can then be embossed, filled, and sealed as described above. Alternatively, as shown in FIG. 13B, a stack including the release material 1210, the adhesive 1220, the electrode 1285, the piezoelectric film 1260, and the microcell precursor can also be prepared, and the microcell precursor can then be embossed, filled, and sealed as described above. The resulting piezoelectric film 1301 and piezoelectric display 1302 are shown in FIGS. 13A and 13B, respectively. Piezoelectric film 1301 and piezoelectric display 1302 may be preferred for applications where it is desirable to have the piezoelectric film 1260 as close as possible to the mounting surface on the final product, i.e., when the piezoelectric film 1301 is used as a strain sensor and it is important that the intervening electrophoretic medium layer does not dissipate forces away from the surface.

[0091] It should be understood that the piezoelectric film and piezoelectric display described herein can be combined with other known techniques for generating security markers or authenticity indicators. For example, the piezoelectric film or piezoelectric display may additionally include a translucent top layer that does not change optical properties when the piezoelectric film is manipulated. For example, a smiley face top layer may include eyes constructed from a piezoelectric display such that the eyes appear to blink when the layered material is bent. In some embodiments, an image or shape may be printed or laminated onto a monochromatic (e.g., white) background, and one must view through the piezoelectric film to see the pre-arranged pattern. Thus, when not in use, the viewer will see only a monochromatic color, i.e., the printed image or shape will be hidden. However, the printed image or shape will be displayed when the device is manipulated. It is also feasible to adhere the piezoelectric film or piezoelectric display to a separate light-transmitting polymer film contained within the target product (e.g., a banknote), so that the pattern in the piezoelectric layer is only visible when the target product is held up and manipulated relative to a light source.

[0092] It will be apparent to those skilled in the art that numerous changes and modifications may be made in the specific embodiments of the invention described above without departing from the scope of the invention. Therefore, the whole of the foregoing description is to be interpreted in an illustrative sense, and not in a restrictive sense.

Claims

1. An electrophoretic display film having a thickness of less than 100 μm (from top to bottom), wherein the electrophoretic display film is, in order, A first adhesive layer (520) and Electrophoretic media layer (405, 530), A piezoelectric layer (410, 560, 960), wherein the piezoelectric layer (410, 560, 960) includes a plurality of differential polarization areas, each area having a different polarity from the adjacent area, and the piezoelectric layer (410, 560, 960) comprises Flexible light-transmitting electrode layer (580) and Equipped with, The electrophoretic medium layer and the piezoelectric layer together form a single layer that is a piezoelectric electrophoretic medium film. The piezoelectric electrophoretic medium film comprises a polyvinylidene fluoride (PVDF) layer bonded to the electrophoretic medium layer, wherein the PVDF layer has a polarization pattern including multiple areas of differential polarization, thereby forming an electrophoretic display film.

2. The electrophoretic media layer (405, 530) comprises a plurality of microcapsules (990) containing a nonpolar fluid (425) and charged pigment particles (423, 427), wherein the charged pigment particles (423, 427) move toward or away from the piezoelectric layer (410, 560, 960) when the piezoelectric layer (410, 560, 960) is bent, and the plurality of microcapsules (990) are bound together with a polymer binder (995), or The electrophoretic display film according to claim 1, wherein the electrophoretic medium layer (405, 530) comprises a plurality of microcells (420, 530) containing a nonpolar fluid (425) and charged pigment particles (423, 427), the charged pigment particles (423, 427) move toward or away from the piezoelectric layer (410, 560, 960) when the piezoelectric layer (410, 560, 960) is bent, and the nonpolar fluid (425) and the charged pigment particles (423, 427) are sealed within the plurality of microcells (420, 530) using a sealing layer (430).

3. The electrophoretic display film according to claim 1, wherein the electrophoretic display film has a thickness less than 50 μm.

4. The electrophoretic display film according to claim 1, wherein the flexible light-transmitting electrode layer (580) comprises a metal oxide containing tin or zinc, or poly(3,4-ethyleneoxythiophene) (PEDOT).

5. An electrophoretic display film assembly comprising a release sheet (510) bonded to the electrophoretic display film described in claim 1, wherein the release sheet (510) is bonded to the first adhesive layer (520).

6. The electrophoretic display film assembly according to claim 5, further comprising a second adhesive layer bonded to the flexible light-transmitting electrode layer (580) and a second release sheet bonded to the second adhesive layer.

7. A method for producing an electrophoretic display film, wherein the method is: A piezoelectric microcell precursor film is produced by bonding a polyvinylidene fluoride (PVDF) film (1260) to a polymer film (1230) containing acrylate, vinyl ether, or epoxide, and The piezoelectric microcell precursor film is bonded to a flexible light-transmitting electrode layer (1280), The flexible light-transmitting electrode layer (1280) is bonded to the first release film (1255) using the first adhesive layer (1250), The piezoelectric microcell precursor film is embossed to generate an array of multiple microcells (1230), wherein each of the multiple microcells has a bottom, a wall, and an upper opening. The plurality of microcells are filled with electrophoretic media (405, 530) through the upper opening, By sealing the upper openings of the multiple filled microcells with a water-soluble polymer (1240), an electrophoresis medium layer (405, 530) is prepared. Methods that include...

8. The method according to claim 7, further comprising applying a primer (1235) to the polymer film (1230) containing an acrylate, vinyl ether, or epoxide before bonding the polymer film (1230) to the polyvinylidene fluoride (PVDF) film (1260).

9. The method according to claim 7, further comprising bonding the water-soluble polymer (1240) to the second release film (1210) using a second adhesive layer (1220).

10. The method according to claim 7, further comprising removing the first release film (1255) to produce an electrophoretic display film having a thickness of less than 100 μm.

11. The method according to claim 7, wherein the electrophoretic medium layer (405, 530) comprises a nonpolar fluid (425) and charged pigment particles (423, 427), and the charged pigment particles (423, 427) move toward or away from the polyvinylidene fluoride (PVDF) film (1260) when the polyvinylidene fluoride (PVDF) film (1260) is bent.

12. The method according to claim 7, wherein the polyvinylidene fluoride (PVDF) film (1260) is polarized to generate a plurality of differential polarization areas (460, 470).

13. The method according to claim 7, wherein the flexible light-transmitting electrode layer (1280) comprises a metal oxide containing tin or zinc, or poly(3,4-ethyleneoxythiophene) (PEDOT).

14. The method according to claim 7, wherein the polyvinylidene fluoride (PVDF) film (1260) is patterned using an electric field to generate a plurality of differential polarization areas (460, 470).

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