Abstract:
A display material for use in an electric paper system includes two opposing outer surfaces, between which is disposed a plurality of optically and electrically anisotropic elements suspended in a substance capable of being liquified. A rotatable disposition of each element is achievable while the element is suspended in the substance and the substance is liquified.

Description:
INCORPORATIONS BY REFERENCE 
     This application is based on a Provisional Patent Application No. 60/339,789 filed Dec. 17, 2001. 
     The following patents are hereby incorporated by reference into this application: U.S. Pat. No. 4,143,103 by Sheridon titled “Method of Making a Twisting Ball Panel Display”; U.S. Pat. No. 5,825,529 by Crowley titled “Gyricon Display with No Elastomer Substrate”; U.S. Pat. No. 5,894,367 by Sheridon titled “Twisting Cylinder Display Using Multiple Chromatic Values”; U.S. Pat. No. 6,110,538 by Sheridon titled “Method of Making a Gyricon Display Using magnetic Latching”; U.S. Pat. No. 6,222,513 by Howard et al. titled “Charge Retention Islands for Electric Paper and Applications Thereof”; and U.S. Pat. No. 6,262,707 by Sheridon titled “Gyricon Displays Utilizing Magnetic Addressing and Latching Mechanism”. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to field activated display sheets and more particularly concerns a field activated display sheet which utilizes both an electric field and a thermal field to activate twisting-ball displays, such as gyricon displays and the like. 
     Typically, a display device, in sheet form, comprises a thin sheet, which has many attributes of a paper document. It looks like paper, has ambient light valve behavior like paper (i.e. the brighter the ambient light, the more easily it may be seen), is flexible like paper, can be carried around like paper, can be written on like paper, can be copied like paper, and has nearly the archival memory of paper. 
     There have been different approaches to making a field induced display sheet such as U.S. Pat. No. 4,143,103 titled “Method of Making a Twisting Ball Panel Display”, in which the display sheet utilizes an elastomer substrate containing an array of addressable elements sandwiched between two electrode layers. The conductors of the first electrode layer are oriented orthogonally relative to the conductors of the second electrode layer. An addressable element is rotated upon application of an electric field between opposing conductors. 
     An alternate approach was disclosed in U.S. Pat. No. 5,717,515 titled “Canted Electric Fields for Addressing a Twisting Ball Display”, having an array of addressable elements, with each array element including at least one spheroidal rotational element. A preferred direction of orientation is selected for rotational elements of a selected array element, with the direction of orientation forming an angle with a vector normal to a planar portion of the substrate surface in the vicinity of the selected array element. Rotational elements of the selected array element are aligned with the preferred direction of orientation by applying an electric field in the vicinity of the selected array element. The electric field has an electric field vector oriented parallel to the selected preferred direction, thereby causing rotational elements of the selected array element to rotate so as to align with the preferred direction of orientation. 
     Multithreshold addressing was disclosed by U.S. Pat. No. 5,739,801 titled “Multithreshold Addressing of a Twisting Ball Display”, in which electrically and optically anisotropic spheroidal rotational elements have at least two different rotation thresholds. The spheroidal rotational elements are disposed in an elastomer substrate together with an addressing electrode assembly. The addressing electrode assembly allows a preferred region of the substrate to be selected in which at least one rotational element of the first set and at least one rotational element of the second set are disposed. A first and second electric field are applied to the selected region, with each of the first and second electric fields extending throughout the region. The first field caused rotation of rotational elements of both the first and second sets of rotational elements in the region. The second electric field facilitates rotation of rotational elements of the second set, without causing further rotation of any rotational element of the first set. 
     Although these approaches, utilizing a standard vertical electric field, are useful, it is desirable to improve on their performance. Accordingly, it is an object of this invention to provide a means for more effectively moving material within electric paper pixels than is possible with a standard vertical electric field, while improving bistability. Bistability is defined herein as maintenance of an image formed by an electric field for a suitable period of time after removal of the electric field. 
     SUMMARY OF THE INVENTION 
     Briefly stated, and in accordance with one aspect of the present invention, there is disclosed a display material for use in an electric paper system. The material includes two opposing outer surfaces, between which is disposed a plurality of optically and electrically anisotropic elements suspended in a substance capable of being liquified. A rotatable disposition of each element is achievable while the element is suspended in the substance and the substance is liquified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the instant invention will be apparent and easily understood from a further reading of the specification, claims and by reference to the accompanying drawings in which: 
     FIGS. 1A-1C illustrate a technique for fabricating display sheets in the prior art; 
     FIG. 2 illustrates a side view of a gyricon display in an embodiment wherein the gyricon rotational elements are arrayed in a close-packed monolayer; 
     FIG. 3 shows an exploded view of one embodiment of the gyricon display sheet, wherein the monolayer of FIG. 2 is shown with an examplary addressing scheme; 
     FIG. 4 is a top view of the gyricon display sheet of FIG. 3; 
     FIG. 5 shows an exploded view of another embodiment of the gyricon display sheet, wherein the gyricon rotational elements are arrayed in multiple layers; 
     FIG. 6 is a portion of the top view of another embodiment of the addressing scheme shown in FIG. 3; and 
     FIG. 7 illustrates an alternative embodiment of the gyricon display sheet in which a xlose-packed monolayer of gyricon rotational elements is placed in a fluid directly between transparent layers, without an elastomer substrate medium. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 a , which shows an example of a prior art device disclosed in U.S. Pat. No. 4,143,103, cited above. Character  2  designates a display which has a display panel  4  sandwiched between substrates  6  and  8 . Intermediate the display panel  4  and substrate  6  is a first grid  10  of parallel electrical conductors  10 ′. A second grid  12 , having parallel electrical conductors  12 ′ oriented orthogonally relative to the conductors  10 ′ of the first grid  10 , is provided between the substrate  8  and the display panel  4 . At least one of the substrates  6  and  8  and at least the conductors of the grid adjacent that substrate are optically transparent so that ambient light can impinge upon the display panel  4  and so that the display provided by panel  4  can be viewed. As shown in FIG. 1 b , substrate  6  and conductors  10 ′ are of optically transparent materials so that the ambient light incident upon the display will provide a visible image at I. 
     The display panel  4  includes a distribution of minute particles  14  which are optically anisotropic. The particles  14  are surrounded by a transparent dielectric fluid which, due to the optical anisotropy of the particles  14  and the difference in Zeta potential due to the coatings used to achieve that optical anisotropy, causes the particles  14  to have an electrical anisotropy. In addition to the particles  14  and the dielectric liquid which surrounds those particles, the panel  4  includes a solid, optically transparent support material  15  which permits the particles  14  to have the desired rotational freedom without having translational freedom. 
     Due to the difference in Zeta potential between the hemispheres  14   a  and  14   b  and the immersion of each of the spheres  14  in the dielectric liquid  18 , the spheres  14  acquire an electrical charge, as shown symbolically in FIG. 1 c , where hemispheres  14   a  are more positive than hemispheres  14   b . When a power source  19  is coupled across one of the electrodes  10 ′ of the grid  10  and one of the electrodes  12 ′ of the grid  12 , as shown in FIG. 1 b , the positively charged hemisphere  14   a  will be attracted to the more negative electrode  12 ′ and the spheres  14  within the field developed by the energized electrodes  10 ′ and  12 ′ will rotate, but without substantial translation, such that the light reflecting hemispheres  14   b  are oriented toward I. Thus, a light spot on a dark background is provided. By reversing the polarity of source  19 , a black spot on a light background can be provided. By sequentially coupling the source  19  to selected of the crossover points of electrodes  10 ′ and  12 ′, as is done in conventional matrix addressing, an image is provided and viewable at I. 
     However, for the prior art device of FIG. 1 to be used as a flexible and re-imageable document, it must have bistability. When the image is formed with an electric field, the field must be maintained or the image deteriorates when the field is removed. For the prior art example, the image deteriorates slowly when the electric field is removed. To extend this device to become a re-imageable paper-like device, a method for freezing or fixing the image is necessary. 
     FIG. 2 provides a more detailed side view of a gyricon display  200  in a specific embodiment. In display  200 , gyricon rotational elements  201  are placed in a monolayer in elastomer substrate  202 . Substrate  202  is permeated by a dielectric fluid, for example Isopar®, to which has been added about 0.5 to about 2 weight percent of a gelating agent, for example Trans-4-t-butyl-1-phenyl-cyclohexanol, or any other suitable gelating agent. Substrate  202  is swelled by the dielectric fluid, thus creating cavities  203  in which the gyricon rotational elements may rotate. Gyricon rotational elements are electrically dipolar in the presence of the dielectric fluid and so are subject to rotation upon application of an electric field. 
     FIG. 3 illustrates an exploded view of one embodiment of the display sheet, with one embodiment of an addressing method. In this embodiment, gyricon display sheet  300  has elastomer layer  310  with a single layer of gyricon rotational elements  315 . Elastomer layer  310  is permeated by a dielectric fluid, for example Isopar®, to which has been added about 0.5 to about 2 weight percent of a gelating agent, for example Trans-4-t-butyl-1-phenyl-cyclohexanol, or any other suitable gelating agent. Each gyricon rotational element  215  resides in its own liquid-filled cavity  318  within layer  310 . Above layer  310  is electrode sheet  320  having parallel conductor strips  325 , which can generate electric fields in or parallel to the plane of layer  310 . Disposed on the opposite side of layer  310  is layer  320  having parallel heater strips  335  oriented orthogonally to the conductor strips  325  of layer  320 . Parallel heater strips  335  may comprise resistance heaters or any other known heater means. An optional low-resistance ground plane electrode  360  may be disposed on the opposite side of heater layer  330  from sheet  310 . Thin dielectric separator layer  350  separates heater layer  330  from ground plane  360 . Layer  350  can be, for example, a deposited polymer or a plastic sheet. Surrounding the electrode configuration are two substrate layers  340  and  370 . At least one face of gyricon sheet  300  is optically transparent. For example, if an observer at I is to view gyricon sheet  300 , then substrate layer  340  and electrode layer  320  preferably should all be transparent. 
     It should be noted that in all the drawings of this specification for the purpose of clarity where there is a plurality of each element, only a few are numbered. However, it should be understood that all the elements that have the same shape as the numbered elements are the same as the numbered elements. 
     Referring to FIG. 4, there is shown a top view of sheet  300 , wherein parts corresponding to like parts of FIG. 3 have the same reference numerals. Conductive strips  325  are placed on layer  420  to form lines parallel to the edge  310  of layer  420 . Also, heater elements  335  are placed on layer  330 , not shown, to form parallel lines along the length  420  of layer  330 . When a given electrode strip  325  and a given heater strip  335  are activated, an electric field is created between the activated electrode and the ground plane, thereby causing the gyricons to rotate, but only where a pulse of power is applied to the activated heater strip. At this crossing point, the fluid in the cavities surrounding the affected gyricons is heated to a desired temperature, for example, about 80° C. At this temperature, the dielectric fluid containing the gelating agent becomes a low viscosity liquid, thus allowing the affected gyricons to rotate. Since only one column of gyricons is activated by the electric field and simultaneously heated, only the gyricon rotational elements in that column are rotated. When the thermal field is removed, the liquid in the cavities surrounding the affected gyricons cools to below the gel point of the liquid in the cavities, thereby fixing the image by the gelation of the liquid phase. This procedure is repeated to develop an image on sheet  300  as seen by an observer at I. To minimize the effects of thermal diffusion, the dimensions of the paper thickness, heater strip widths, and the addition of an optional backside heat sink, shown in FIG. 5, are selected so that there is only local heating to enable one column to switch at a time. The display sheet  300  is capable of activating or deactivating the gyricons by a passive matrix addressing using a plurality of voltage sources and a plurality of heater drivers contacting the display sheet  300  at the sides of the sheet. 
     Referring now to FIG. 5, there is shown an exploded view of an alternate embodiment of an addressing method for the gyricon display sheet, wherein parts corresponding to like parts of FIG. 3 have the same reference numerals. A gyricon display sheet  500  has elastomer layer  310  with multiple layers of gyricon rotational elements  315 . Elastomer layer  310  is permeated by a dielectric fluid, for example Isopar, to which has been added about 0.5 to about 2 weight percent of a gelating agent, for example Trans-4-t-butyl-1-phenyl-cyclohexanol. Each gyricon rotational element  315  resides in its own liquid-filled cavity  318  within layer  310 . Above layer  310  is electrode sheet  320  having parallel conductor strips  325 , which can generate electric fields in or parallel to the plane of layer  310 . Disposed on the opposite side of layer  310  is layer  330  having parallel heater strips  335  oriented orthogonally to the conductor strips  325  of layer  320 . An optional low-resistance ground plane electrode  360  may be disposed on the opposite side of heater layer  330  from layer  310 . Thin dielectric separator layer  350  separates heater layer  330  from ground plane  360 . Layer  350  can be, for example, a deposited polymer or a plastic sheet. Surrounding the electrode configuration are two substrate layers  340  and  370 . Heat sink  380  is disposed on the opposite side of substrate layer  370  from layer  310 . At least one face of gyricon sheet  500  is optically transparent. For example, if an observer at I is to view gyricon sheet  500 , then substrate layer  340  and electrode layer  320  preferably should all be transparent. 
     Referring once more to FIG. 4, there is shown a top view of sheet  300 , wherein parts corresponding to like parts of FIG. 3 have the same reference numerals. Conductive strips  325  are placed on layer  420  to form lines parallel to the edge  310  of layer  420 . Also, heater elements  335  are placed on layer  330  to form parallel lines along the length  320  of layer  330 . When a given electrode strip  325  and a given heater strip  335  are activated, an electric field is created between the activated electrode and the ground plane, thereby causing the gyricons to rotate, but only where a pulse of power is applied to the activated heater strip. At this crossing point, the fluid in the cavities surrounding the affected gyricons is heated to a desirable temperature, for example, about 80° C. At this temperature, the dielectric fluid containing the gelating agent becomes a low viscosity liquid, thus allowing the affected gyricons to rotate. Since only one column of gyricons is activated by the electric field and simultaneously heated, only the gyricon rotational elements in that column are rotated. When the thermal field is removed, the liquid in the cavities surrounding the affected gyricons cools to below the gel point of the liquid in the cavities, thereby fixing the image by the gelation of the liquid phase. This procedure is repeated to develop an image on sheet  300  as seen by an observer at I. To minimize the effects of thermal diffusion, the dimensions of the paper thickness, heater strip widths, and the addition of an optional backside heat sink, shown in FIG. 5, are selected so that there is only local heating to enable one column to switch at a time. 
     Referring now to FIG. 6, there is shown another embodiment of an addressing method for the gyricon display sheet. In this embodiment electrically conductive strips  625  are arranged in such a manner that they form parallel lines which are diagonal with respect to the heater strips  635 . Again, when a given conductive strip  625  and a given heater strip  635  are activated, a field is created at the crossing point which causes the corresponding gyricons to rotate as above and become fixed in the rotated position upon removal of the electric and thermal fields. 
     As will be apparent to one of skill in the art, in any of the above-described embodiments the location of electrode layer  320  and heater layer  330  could be reversed. For example, referring again to FIG. 3, layers  320  and  3230  could be interchanged such that layer  330  is located above layer  310  and layer  320  is disposed on the opposite side of layer  310 . As above, when a given conductive strip  325  and a given heater strip  335  are activated, a field is created at the crossing point which causes the corresponding gyricons to rotate as above and become fixed in the rotated position upon removal of the electric and thermal fields. 
     Although the embodiments discussed hereinabove include layers of electrode and heater strips, various alternate addressing methods are also contemplated. For example, the electrodes may form charge-retaining islands, as taught by Howard et al. in U.S. Pat. No. 6,222,513 and incorporated by reference in its entirety, and the heaters form heat-retaining islands or be in the form of a single continuous layer. Various mechanical arrangements are envisioned for external charge transfer to the gyricon display sheet. One such arrangement is a single element stylus, which could be utilized like a pen or pencil. A soft, conductive tip is used to make contact with the electrode layer. The tip is connected by a conductive core to a power supply. The conductive core is surrounded by insulating material that allows a user to handle the stylus without threat of electric shock. A single element stylus is also possible. In this embodiment, the heaters form a single continuous layer, which is heated while the stylus moves across the sheet, thus allowing the rotational elements to rotate. When the stylus is lifted, heat is no longer applied by the heater layer, thus locking the rotated elements into position. 
     Alternatively, a one-dimensional array of charge transfer elements could also be built and used like a print head or wand. The contact charging wand may be comprised of alternating conductive charge transfer elements and insulating elements, as described in U.S. Pat. No. 6,222,513. The charge transfer elements must make reliable contact to the electrode layer while moving with respect to the gyricon display sheet during image generation. In this embodiment the heaters may form strips disposed in the same direction as the print head or wand. 
     A two-dimensional array of addressing elements is also envisioned that addresses entire gyricon display sheets. In such a device, a charge transfer platen, a gyricon display sheet is temporarily positioned inside the device, which includes a supporting base and a two-dimensional matrix addressing array of addressing elements. The matrix addressing array can be positioned in contact with or in proximity to the gyricon display sheet. The matrix array may be rotated about a hinge through an arc into position above the gyricon display sheet. Charge is transferred to the electrode contact points of the gyricon display sheet simultaneously, an image is created, and the gyricon display sheet can be removed. In this embodiment, the heaters are in the form of a continuous sheet, which is heated when the matrix array contacts the display sheet. Hybrid plate-wand configurations are also conceivable, which might provide a compromise between cost and performance provided by the two approaches. 
     It should be noted that although the above embodiments of this invention have been described as utilizing spherical gyricons, cylindrical elements such as those described in U.S. Pat. No. 5,894,367 titled “Twisting Cylinder Display Using Multiple Chromatic Values”, cited above, could be used instead of the spherical gyricons. Additionally, the display elements, whether they are spherical or cylindrical, may be utilized in black and white, highlight, or color display sheets according to this invention. 
     As described above, the gyricon display sheet is made with swelled elastomer, with each gyricon rotational element situated in a cavity. However, the display sheet may also be constructed without elastomer and without cavities. In such a display sheet, the gyricon rotational elements are placed directly in a dielectric fluid, for example Isopar®, to which has been added about 0.5 to about 2 weight percent of a gelating agent, for example Trans-4-t-butyl-1-phenyl-cyclohexanol, or any other suitable gelating agent. The rotational elements and the dielectric fluid with gelating agent are then sandwiched between two retaining members (e.g., between addressing electrodes and heaters). There is no elastomer substrate. 
     FIG. 7 illustrates a side view of a no-cavities gyricon display sheet. In sheet  700 , a monolayer of gyricon rotational elements  701  of uniform diameter is situated in dielectric fluid  709  between retaining members  704   a  and  704   b . The gyricon rotational elements are electrically dipolar in the presence of dielectric fluid  709  and so are subject to rotation upon application of an electric field. Upper surface  705  is preferably transparent. This alternate embodiment of the display sheet contemplates use of all of the addressing methods described hereinabove. 
     The above embodiments of this invention utilize an electric field and a thermal field to move the gyricons within their respective cavities. However, it should be noted that any external field, which can cause the rotation of gyricons within their cavities, can replace the electric field of this invention. For example, if the gyricons are fabricated with magnetizable pigments, as described in U.S. Pat. No. 6,110,538 titled “Method of making a Gyricon Display Using Magnetic Latching”, cited above, the gyricons may be rotated through application of a magnetic field in combination with the thermal field. 
     The advantage of the display disclosed in this invention over a conventional display is that this invention provides bistability. Images formed through conventional electric field structures applied to elements rotating in a hydrocarbon solvent such as Isopar® without a gelating agent will begin to deteriorate when the electric field is removed. However, images formed through the combination of an electric field and a thermal field acting on elements rotating in a hydrocarbon solvent to which a gelating agent has been added will not deteriorate with the removal of the applied fields. As the temperature of the gelating agent drops with the removal of the fields, the device cools to a temperature below the gel point of the gelating agent, thereby fixing the image until a thermal field is once again applied. 
     EXAMPLE 1 
     An electric paper device was fabricated having two indium tin oxide coated substrates sandwiching a siloxane elastomer containing gyricon rotational elements, swelled with a hydrocarbon solvent such as Isopar with about 0.5 weight percent of gelating agent, Trans-4-t-butyl-1-phenyl-cyclohexanol. The electric paper device was heated to a temperature of about 15° C. above the gel point of the Isopar/TBPC mixture, and an electric field was applied and an image formed. Heat was then removed from the device, allowing it to cool below the gel point, at which time the electric field was removed. Once cooled to below 40° C. or to room temperature, the solvent phase gels to result in a fixed image. At 0.5 weight percent gelating agent, it is necessary to cool the image to below room temperature to freeze the image, as shown in Table 1. 
     EXAMPLE 2 
     An electric paper device was fabricated having two indium tin oxide coated substrates sandwiching a siloxane elastomer containing gyricon rotational elements, swelled with a hydrocarbon solvent such as Isopar with about 0.5 weight percent of gelating agent, Trans-4-t-butyl-1-phenyl-cyclohexanol. The electric paper device was heated to a temperature of about 15° C. above the gel point of the Isopar/TBPC mixture, and an electric field was applied and an image formed. Heat was then removed from the device, allowing it to cool below the gel point, at which time the electric field was removed. Once cooled to below 40° C. or to room temperature, the solvent phase gels to result in a fixed image. At 1.0 weight percent gelating agent, the image gels within 2 minutes on cooling to room temperature, as shown in Table 1. 
     EXAMPLE 3 
     An electric paper device was fabricated having two indium tin oxide coated substrates sandwiching a siloxane elastomer containing gyricon rotational elements, swelled with a hydrocarbon solvent such as Isopar with about 0.5 weight percent of gelating agent, Trans-4-t-butyl-1-phenyl-cyclohexanol. The electric paper device was heated to a temperature of about 15° C. above the gel point of the Isopar/TBPC mixture, and an electric field was applied and an image formed. Heat was then removed from the device, allowing it to cool below the gel point, at which time the electric field was removed. Once cooled to below 40° C. or to room temperature, the solvent phase gels to result in a fixed image. At 2.0 weight percent gelating agent, the image gels within 30 seconds on cooling to room temperature, as shown in Table 1. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 
                 Agent 
                 Imaging Temp. 
                 Gelation Temp. 
               
               
                   
                   
               
             
             
               
                   
                 Example 1 
                 0.5% 
                 30° C. 
                   14° C. 
               
               
                   
                 Example 2 
                 1.0% 
                 50° C. 
                   39° C. 
               
               
                   
                 Example 3 
                 2.0% 
                 70° C. 
                 43.5° C. 
               
               
                   
                   
               
             
          
         
       
     
     The above devices are presented by means of example only. It will be appreciated that the percentage of gelating agent to Isopar could beneficially range to as much as 3% to 4%. 
     While the present invention has been illustrated and described with reference to specific embodiments, further modification and improvements will occur to those skilled in the art. For example, numerous changes in details of construction and the combination and arrangement of elements and materials may be resorted to without departing from the true spirit and scope of the invention. It is to be understood, therefore, that this invention is not limited to the particular forms illustrated and that it is intended in the appended claims to embrace all alternatives, modifications, and variations which do not depart from the spirit and scope of this invention.