Abstract:
An electronic paper device includes a ground plane, a charge receiving layer, and a porous stand off layer disposed over the charge receiving layer. An active layer is interposed between the ground plane and the charge receiving layer, the active layer including a plurality of microcapsules containing charged pigments. A system for writing information to electronic paper is also provided.

Description:
BACKGROUND 
     Electronic paper (“e-paper”) is a display technology designed to recreate the appearance of ink on ordinary paper. E-paper reflects light like ordinary paper and may be capable of displaying text and images indefinitely without using electricity to refresh the image, while allowing the image to be changed later. E-paper can also be implemented as a flexible, thin sheet, like paper. By contrast, a typical flat panel display does not exhibit the same flexibility, typically uses a backlight to illuminate pixels, and constantly uses power during the display. Typical e-paper implementations, such as electronic books (“e-books”), include an e-paper display and electronics for rendering and displaying digital media on the e-paper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate various examples of the principles described herein and are a part of the specification. The illustrated examples are merely examples and do not limit the scope of the claims. 
         FIG. 1A  is a plan view of an illustrative piece of electronic paper, according to one example of principles described herein. 
         FIG. 1B  is a cross sectional view of a portion of the electronic paper, according to one example of principles described herein. 
         FIGS. 2A and 2B  are examples of illustrative e-paper applications, according to one example of principles described herein. 
         FIG. 3  is cross sectional diagram of an illustrative e-paper printing system, according to one example of principles described herein. 
         FIG. 4  is a cross sectional view of an illustrative e-paper structure which includes a porous standoff layer, according to one example of principles described herein. 
         FIGS. 5A and 5B  are a cross sectional view and a plan view, respectively, of an e-paper test coupon with a porous standoff layer, according to one example of principles described herein. 
     
    
    
     Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. 
     DETAILED DESCRIPTION 
     E-paper is used in a variety of display applications such as signage, e-books, tablets, cards, posters, and pricing labels. E-paper has several paper-like features. For example, e-paper is a reflective display that uses ambient light as an illumination source. The ambient light strikes the surface and is reflected to the viewer. The usage of pigments similar to those which are used in printing allows the e-paper to be read at a wide range of angles and lighting conditions, including full sunlight. The use of ambient light also eliminates the need for illumination produced by the device. This minimizes the energy used by the e-paper. Additionally, the e-paper does not use energy to maintain the image. Once the image is written, the image remains on the e-paper for an extended period of time or until the e-paper is rewritten. Thus, a typical e-paper primarily uses energy for changes of state. 
     E-paper is typically written by generating a charge on a surface in proximity to a layer of microcapsules that contain charged pigment particles. The charge on the surface attracts or repels the charged pigment particles in the microcapsules to create the desired image. The pigment particles are stable within the microcapsules after they are moved into position. However, a wide variety of methods can be used to alter the image or text on the e-paper after it has been written. This can restrict the use of e-paper to applications that do not require the images or text to be secure against alteration. However, the principles described below illustrate a porous standoff layer that prevents alteration of e-paper using common techniques such as an electrified stylus or corona discharge mechanisms. By preventing alteration of the e-paper using easily accessible technology, the security of the e-paper improves and the e-paper can be used a wider variety of applications. 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present apparatus, systems and methods may be practiced without these specific details. Reference in the specification to “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that one example, but not necessarily in other examples. 
       FIG. 1A  shows a plan view of an illustrative piece of e-paper  102  and includes an enlargement  104  of a small portion of the e-paper  102 . The enlargement  104  shows that this e-paper implementation includes an array of embedded, spherical-shaped microcapsules  106 . The line  118  is created by selectively applying a charge to the e-paper  102 . The charge moves the particles within the microcapsules  106 . In this example, a charge has been applied that moved dark particles to the front of the microcapsules  114  to form the line  118 . 
       FIG. 1B  is a cross-sectional view of a portion of the e-paper  102  shown in  FIG. 1A . The cross-sectional view shows an illustrative multilayer structure of the e-paper  102 , including an active layer  109  with microcapsules  106 ,  114  sandwiched between a transparent charge receiving layer  108  and a conductive ground layer  110 . As shown in  FIG. 1B , the conductive ground layer  110  is disposed on a substrate  112 . 
     In this example, each of the microcapsules  106 ,  114  contains both white particles  120  and black particles  124  suspended in a fluid medium. Ambient light is transmitted through the charge receiving layer  108 , strikes the pigment, and reflected back to the viewer  122 . When white particles  120  of a microcapsule are located near the transparent charge receiving layer  108 , the microcapsule appears white to a viewer  122 , and when the black particles  124  of a microcapsule are located near the charge receiving layer  108  the microcapsule appears black to the viewer  122 . The particles can be of opposite charges. For example, the black particles  124  can be positively charged particles and the white particles  120  can be negatively charged particles. Various shades of gray can be created by varying the arrangement of alternating microcapsules with white and black particles located near the charge receiving layer  108  to produce halftoning. 
     The microcapsules  106 ,  114  are designed to exhibit image stability using chemical adhesion between particles and/or between the particles and the microcapsule surface. For example, the black and white microcapsules  106 ,  114  ideally can hold text and images indefinitely without drawing electricity, while allowing the text or images to be changed later. 
     The structure, materials, and dimensions of the various layers and components shown in  FIG. 1B  can be adapted to specific design criteria. In one implementation, the transparent charge receiving layer  108  can be composed of a transparent polymer and can range in thickness from approximately 100 nm to approximately 14 μm. The transparent charge receiving layer  108  can also be composed of a material that holds charges or is porous or semi-porous to charges and/or ions. The transparent charge receiving layer  108  can also be composed of a first insulating layer and second patterned conductive layer. 
     The microcapsules  106 , described in greater detail below, can have a diameter of approximately 50 μm but may also range in diameter from approximately 20 μm to approximately 100 μm. The conductive ground layer  110  can be composed of a transparent conductive material, such as indium tin oxide, or an opaque conductive material and can have a thickness ranging from approximately 5 nm to approximately 1 mm. In one example, the layers  108 ,  109 , and  110  have a total thickness of approximately 100 μm. The substrate  112  can be composed of an opaque or transparent material and can range in thickness from approximately 20 μm to approximately 1 mm, or the thickness can be much larger depending on the how the e-paper is used. The substrate  112  can be composed of polyester, plastic, or transparent Mylar. In some implementations, the substrate  112  can be omitted and the layers  108 ,  109 , and  110  can be mounted on a wall or a product chassis. In this case, the transparent charge receiving layer  108  serves as a wear protection layer for the layer of microcapsules  109  and normalizes the e-paper surface, eliminating surface topography and blocking surface conduction paths on the microcapsule surfaces. 
     A variety of other configurations may be used. For example, the microcapsule  106  may include black particles suspended in a white colored fluid. The black particles can be positively charged particles or negatively charged particles. One or more microcapsules form a pixel of black and white images displayed on the e-paper  102 . The black and white images are created by placing black particles near or away from the charge receiving layer  108 . For example, the microcapsules  106  with black particles located away from the transparent charge receiving layer  108  reflect white light, corresponding to a white portion of an image displayed on the e-paper. By contrast, the microcapsules with black particles located near the charge receiving layer  108 , such as microcapsule  114 , appear black to the viewer  122 , corresponding to a black portion of the image displayed on the e-paper  102 . Various shades of gray can be created using halftoning to vary the arrangement of alternating microcapsules with black particles located near or away from the charge receiving layer  108 . 
     Where the microcapsules include black particles suspended in a white colored fluid, the charge receiving layer  108  may be tinted with alternating blue, red, and green regions. Adjacent blue, red, and green regions form color pixels. Color images are created by placing different combinations of white or black particles near the charge receiving layer  108 . For example, the microcapsules of color pixel with white particles located near the red and green regions of the transparent charge receiving layer  108  reflect red and green light from the e-paper. The viewer  122  will perceive this combination as a yellow pixel. When the black particles in the microcapsules are located near the transparent charge receiving layer  108 , that color pixel will appear black to the viewer  122 . Additionally or alternatively, the black particles  124  of each microcapsule are replaced by either blue, red, or green positively, or negatively, charged particles. Particles could be used alone or in combination with a tinted charge receiving layer  108  to create the desired color image. 
       FIGS. 2A and 2B  show two illustrative cards  200 ,  205  that use a strip of e-paper  204  across the width of the card to display information. As discussed above, it may be desirable to secure the information displayed by the e-paper against alteration.  FIG. 2A  is a gift card  200  used in a retail setting. The card  200  displays text  214  that communicates the amount remaining on the card  200 . Additional text  208  and an image  202  describing a featured product are also included on the card  200 . If the text  214  has not been secured against alteration, it cannot be relied on to accurately communicate the balance of the card. Consequently, other techniques such as a magnetic strip or embedded radio frequency circuitry may be included in the card to communicate the balance of the card. 
       FIG. 2B  is a security card  205  that grants the user access to specific buildings for a predetermined period of time. The user&#39;s name  206  and access permissions  210  are printed on the e-paper  204 . The use of e-paper  204  allows the user and others to visually identify the information that is associated with the card. However, if the e-paper  204  has not been secured against alteration, the text  206 ,  210  cannot be relied upon and alternative techniques are employed to communicate the identity of the card, the name of the card bearer and the access privileges of the card bearer. 
       FIG. 3  describes writing to illustrative unsecured e-paper  102  with a writing system  300 . The writing system  300  includes a writing module  302 , writing unit  304 , and an erasing unit  306 . The writing unit  304  and erasing unit  306  are connected to the same side of the writing module  302  that faces the outer surface  308  of the charge receiving layer  108 , with the writing unit  304  suspended above the surface  308 . In the example of  FIG. 3 , the writing unit  304  is an ion head and the erasing unit  306  can be an electrode that comes into close contact with, or can be dragged along, the surface  308  in front of the ion head  304 . The writing module  302  can be moved in the direction indicated by the arrow and the e-paper  102  can be held stationary; or the e-paper  102  can be moved in the opposite direction and the writing module  302  held stationary; or the writing module  302  and e-paper  102  can be moved simultaneously. In the example shown in  FIG. 3 , the black particles  124  and the white particles  120  of the microcapsules are positively charged and negatively charged, respectively. The erasing unit  306  erases any information stored in the microcapsules prior to writing information with the ion head  304 . In the example shown in  FIG. 3 , as the e-paper  102  passes under the writing module  302 , the positively charged erasing unit  306  can remove negatively charge ions attached to the surface  308 . The positively charge erasing unit  306  also creates electrostatic forces that drive positively charged black particles  124  away from the charge receiving layer  108  and attract negatively charged white particles  120  toward the charge receiving layer  108 . By passing the erasing unit  306  over the charge receiving layer  108 , the information written to the e-paper  102  is erased by positioning the negatively charged white particles  120  near the top of the microcapsules and pushing the positively charged black particles  124  to the bottom of the microcapsules  106 . Additionally or alternatively, a corona source or the ion head  304  could be used to erase prior images present on the e-paper. 
       FIG. 3  also shows an illustrative writing operation performed by the ion head  304 . The ion head  304  is designed and operated to selectively eject ions  314  (shown as black bars) toward the charge receiving layer  108 , when a region of the e-paper  102  located beneath the ion head  304  is to be changed from white to black. The ions  314  reach the surface  308  and remain on the surface to create negatively charged areas  316 . The negatively charged white particles  120  are repelled and driven away from the negatively charged areas  316  on the charge receiving layer  108 , while the positively charged black particles  124  are attracted to the negatively charged area  316  and driven toward the charge receiving layer  108 . For example, as shown in  FIG. 3 , as the ion head  304  passes over a portion of microcapsule  106  while ejecting electrons/ions  314 , the negatively charged white particles  120  are repelled away from the charge receiving layer  108  and the positively charged black particles  124  are driven toward the charge receiving layer  108 . Thus, to a viewer  122 , the positively charged areas of the charge receiving layer  308  will appear white and the negatively charged areas  316  will appear black. 
     In addition to ion heads, a number of alternative writing devices can be used to write on the e-paper or alter the contents of the e-paper. One of the simplest writing devices is a charged stylus that is manually brought into proximity with the charge receiving surface. The tip of the charged stylus creates an electromagnetic field which can influence the position of the charged pigments in the microcapsules  106 . 
     In contrast to this relatively simple stylus, the use of an ion head  304  to write to e-paper is much more involved. The construction of the ion head  304  is exacting and requires specialized equipment. The operation of the ion head  304  includes computerized control and data transfer. The construction or use of an ion head  304  to forge or alter e-paper is a significant hurdle that many forgers may be unable or unwilling to implement. 
     Securing e-paper  102  against alteration by a charged stylus or other field writing device while allowing alteration by an ion head  304  can result in e-paper  102  being significantly more secure. Consequently, the visual information conveyed by the e-paper  102  could be relied on to a greater extent. This may reduce the need for alternative technology to be incorporated into the card. Further, the information conveyed by secured e-paper  102  could be used to visually verify the information conveyed by a magnetic strip, embedded microchip or other technology. 
       FIG. 4  is a cross sectional diagram of an illustrative secure e-paper  402 . The secure e-paper  402  includes a substrate  112 , a ground plane  110 , an active layer  410  that includes microcapsules  106 , and a standoff layer  408  that imposes an electrical standoff distance while being permeable to free ions. This standoff layer  408  may be implemented in a variety of ways. In one implementation, the standoff layer  408  is formed from material that is porous at the appropriate length scale. For example, screen meshes that have porosity levels of approximately 60% and openings in the range of 0.0059″ showed good imaging capabilities with a 300 dot/inch ion head while preventing image modification with a stylus at a 600 Volt potential. 
       FIG. 4  shows an ion head  304  positioned over the e-paper  402  and directing a stream of ions  314  toward the e-paper  402 . A stylus  404  is brought into contact with the standoff layer  408  and equipotential lines  406  are shown emanating from the stylus  404 , through the standoff layer  408  and into the active layer  410 . 
     There are several differences between writing an image with an ion beam device  304  and a stylus  404 . An ion beam  314  may be kept focused over long distances with a relatively small field (&lt;1 v/μm to keep a 50 μm beam focused over lengths of 250-500 μm). However, the electrical potential generated by a stylus  404  rapidly becomes larger and weaker with distance. For example, the equipotential lines  406  generated by the stylus  404  are nominally spherical, so for a stylus of radius R kept at an offset d from the imaging plane (i.e. the imaging plane is where the dot will be formed), the spot diameter will be roughly ˜2x(R+d). If the radius R of the stylus  404  is 50 μm and the thickness d of the standoff layer  408  is 50 μm, the effective spot diameter at the active layer  410  is roughly ˜200 μm. 
     This is shown schematically in  FIG. 4 . In practice, the effect of the offset is slightly worse than the calculation given above or the illustrated equipotential lines shown in  FIG. 4 . This is because the proximity of the ground plane causes the equipotential lines to become flat at the ground plane  110 . This further increases the spot size generated by the stylus  404 . 
     To allow ion printing through the standoff layer  408 , the standoff layer is porous. This allows the ion beam  314  to permeate the standoff layer  408  and directly influence the position of the charged pigments in the active layer  410 . However, the physical offset will cause a much greater increase in the image spot from the stylus/styli  404 . Further, the physical offset severely reduces the voltage potential created by the stylus  404 . Consequently, using a stylus  404  in an attempt to alter an image in the secure e-paper results in vague, low resolution markings. In many instances, the stylus  404  will have no visible impact on the image at all. 
     The amount of porosity in the standoff layer  408  can be selected using a number of factors. Less porous surfaces have a tendency to accumulate charges from the ion head and cause an increase in spot size. However, a less porous standoff layers may be more robust and less prone to absorb contaminants. For example, the less porous layer may have fewer or smaller pores. A more porous standoff layer may permit the ion beam to pass more efficiently to the active layer. 
     The standoff layer  408  can be formed from a range of materials and have a variety of pore structures. In one implementation the porosity may be simple grain boundaries or micron scale pores, such as those encountered in anodized aluminum layers. In another implementation, open cell micro-foam of a suitable dielectric material could be formed over the active layer  410 . Alternatively, a variety of printing and lithographic processes could be used to form a mesh structure over the active layer  410 . For example, an impression die could be pressed into a dielectric coating in an uncured state. Another manufacturing method would be to etch a porous structure onto a solid film/coating. This porous coating would still provide a tough mechanical protection to the e-paper  402 . 
     In addition to porosity, the resistivity of the standoff layer  408  can be important. For example, the resistivity of the standoff layer  408  can be between 10 8  to 10 14  ohm centimeters. This resistivity provides a layer time constant of no more than 10 seconds to eliminate reverse charging during handling after imaging and no less than 0.1 seconds to prevent lateral charge flow (blooming) during the e-ink switching time. In one example, the resistivity of the standoff layer  408  is between 10 11  to 10 13  ohm centimeters. 
     In other examples, the standoff layer  408  may exhibit macro-level porosity such as encountered in commercially available meshes with sizes between 60-325 openings per linear inch. In one implementation, mesh with between 100 to 180 openings per inch can be used. Mesh with 180 openings per inch has a filament spacing of approximately 140 μm and a filament diameter of about 70 μm. This creates a stylus spot size of approximately 140 μm plus the stylus diameter. To provide a high level of permittivity to the ion beam created by the ion head  304 , it can be desirable to employ meshes having a high percentage of open area. Open areas of 60 percent are readily available and perform well. For example, nylon mesh may be used as the standoff layer  408 . Nylon meshes have an electrical resistivity that ranges from between approximately 10 9  to 10 12  ohm centimeters depending on the processing conditions, additives and moisture content. When using these meshes as offset layers, writing to the e-paper  402  with a stylus  404  is ineffective and while writing to the e-paper  402  with an ion head  304  is relatively unimpeded. 
     The standoff layer  408  could be formed from a variety of materials. For example, the standoff layer  408  could be formed from a hydrophobic material or have a hydrophobic coating. This would protect the exposed surface of the standoff layer  408  from accumulating a potentially harmful layer of a liquid electrolyte such as sweat or atmospheric moisture. A number of additional layers may also be included in the e-paper  402 . For example, a thin coating layer may provide a bond between the porous layer and the e-ink. 
       FIGS. 5A and 5B  are illustrations of a 15 mm by 30 mm test coupon that was constructed and tested according to the principles described above.  FIG. 5A  is a cross-sectional view of the test coupon and  FIG. 5B  is a plan view of the viewing side of the test coupon. 
     As shown in  FIG. 5A , the test coupon includes a ground layer  508 , an active layer  506  that contains the microcapsules, and a charge receiving layer  504 . The left hand portion of the charge receiving layer  504  has been covered by a nylon mesh standoff layer  502 . Two electrodes  510  at either end of the test coupon  500  provide electrical contact with the ground plane  508 . 
     To form the test coupon  500 , microcapsules were deposited on the ground plane  508  to form the active layer  506 . The charge receiving layer  504  was deposited over the active layer  506 . In this example, the charge receiving layer  504  is formed using a white alkyd enamel paint deposited using a draw down bar. The coating gap was 62 microns and the dry coating thickness 75 microns. In some implementations, the charge receiving layer  504  may have semiconducting properties. For example, the charge receiving layer  504  may have an electrical resistance between 10 3  to 10 12  ohm-centimeters. 
     After a short drying period to set the paint, a 104 mesh (per inch) nylon screen was pressed into the coating  504  to form the standoff layer  502 . The dry thickness in the screen area was measured to be 150 microns. The screen filament diameter is 0.0037 inch and the open gap 0.0059 inches on a side. 
     An ion print head  304  passed over both the exposed charge receiving layer  504  and the mesh standoff layer  502 . The ion print head  304  deposited charges onto the charge receiving layer  504  as a row of dots. The ion print head  304  made two passes over the test coupon  500  resulting in two rows of dots. A first row of dots were formed using a pulse length of 50 microseconds. 
     A stylus  404  was also passed over both the exposed charge receiving layer  504  and the mesh standoff layer  502 . In this example, the stylus  404  has a 0.5 mm diameter. During the first pass, 200 volts was applied to the stylus  404 . During the second pass 400 volts was applied to the stylus  404  and on the third pass 600 volts was applied to the stylus  404 . The marks made by the deposited charges are viewed from the opposite side as illustrated by the viewer  122 . 
       FIG. 5B  is a plan view of the viewing side of the test coupon  500 . In this example, the viewing side looks through ground plane  508 ,  FIG. 5A .  FIG. 5B  shows dot images  518 ,  519  made using the ion print head  304 ,  FIG. 5A  and lines made using the stylus  404 ,  FIG. 5A . The mesh standoff layer  502  underlies the left side of the test coupon  500 , while the right side of the test coupon  500  includes only white alkyd paint. Thus, the left side of the test coupon  500  is protected from alteration by a stylus, while the right side of the test coupon  500  is not. 
     The images  512 ,  514 , and  516  formed by the stylus  404  in  FIG. 5A  are very distinct on the right side of the test coupon  500 . The line  512  made by the stylus  404 ,  FIG. 5A  with an applied voltage of 200 volts is relatively thin. The line  514  made by the stylus with an applied voltage of 400 volts is thicker and the line  516  made by the stylus with an applied voltage of 600 volts is the thickest line. However, as the lines pass onto the left side of the test coupon  500 , the lines disappear or become faint smudges  520 ,  524 . There is no trace of the thinnest line  512  on the left side of the test coupon  500 . There are slight smudges  520  left by the stylus at 400 volts on the left side of the test coupon. The marks made by the stylus at 600 volts on the left side of the test coupon  500  are wider and somewhat better defined but still indistinct when compared to those made on the unprotected right hand side of the test coupon. 
     A first row of small dots  518  were formed by the ion head  304  using pulse lengths of 50 microseconds and have a diameter between 150 and 200 microns. The second row of larger dots  519  were formed using pulse lengths of 150 microseconds and have a diameter of approximately 250 microns. In contrast to the lines  512 ,  514 ,  516 , the dot images  518 ,  519  are clear and distinct on both the right and left hand portions of the test coupon  500 . This clearly demonstrates that the mesh standoff layer  502  is effective in preventing useful marks from being made with the stylus  404 ,  FIG. 5A  while allowing the ion head  304 ,  FIG. 5A  to print well defined marks. 
     In conclusion, the incorporation of a standoff layer provides security against undesirable rewriting of electronic paper. This allows the electronic paper to be used in a variety of more secure applications. The implementation of the secure coating is a low cost solution that is readily scalable to large scale production. Further, the standoff layer may also make images on the e-paper more durable and resistant to handling. 
     The preceding description has been presented only to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.