Patent Publication Number: US-6982734-B2

Title: Printing on electrically writable media and electrically writable displays

Description:
TECHNICAL FIELD 
   This invention relates to apparatus and methods of printing on an electrically writable medium and to electrically writable displays. 
   BACKGROUND 
   Many companies are developing electronic paper, which is a display system that retains images with little or no power. Images typically are generated on an electronic paper medium by selectively applying an electric field to switchable display elements (e.g., dichroic spheres) in localized regions of the medium. In a typical implementation, an electrically conductive backplane electrode is placed behind the electronic paper medium and a second electrically conductive front plane electrode is placed in front of the electronic paper medium. Applying an electric field of one polarity to the medium switches the display elements to one orientation (e.g., black-side-up), and reversing the polarity of the applied electric field switches the display elements to a second orientation (e.g., white-side-up). A two-dimensional electrode grid with individually addressable cells may be used to provide an electric field in selected areas of the electronic paper medium. Alternatively, a single electrode or multiple electrodes in a head may be scanned across the electronic paper as the paper is advanced by a roller system. In some systems, the position of the electronic paper is fixed and one or more electrodes are scanned across the electronic paper. The electronic paper medium remains in the switched (or “printed”) state after the electric field is removed, until a new electric field is applied to change the orientation of the display elements. 
   One known electrode array printer for printing on rewritable electronic paper includes an array of independently addressable electrodes, each capable of applying a localized field to the rewritable media to rotate dichroic spheres within a given pixel area of a rewritable medium. In another known electrically writable media printing technique, a laser scanner is used to erase a uniform high-voltage charge that was deposited on the surface of a photoconductor drum or belt. The voltage swing between charged and discharged areas of the photoconductor is conventionally on the order of about 500–600 volts. When the rewritable medium is brought in contact with the charge-written photoconductor through a biased back electrode roller, electric fields that are generated between the photoconductor and back electrode cause color rotation of the dichroic spheres to develop a desired print image. 
   SUMMARY 
   In one aspect, the invention features a printer for printing on an electrically writable medium. The printer includes a source of an invisible charge species, and a charge species projector. The charge species projector is operable to project charge species from the source onto the medium to electrically reorient switchable display elements in the medium. 
   In another aspect, the invention features a method of printing on an electrically writable medium, in which invisible charge species from a source are projected onto the medium to electrically reorient switchable display elements in the medium. 
   In another aspect of the invention, a printer for printing on an electrically writable medium includes a source of a charge species, and a charge species projector. The charge species projector includes a mask defining an image and configured to project charge species from the source through the mask and onto the medium to electrically reorient switchable display elements in the medium. 
   In another aspect, the invention features a method of printing on an electrically writable medium in which charge species from a source are projected through a mask and onto the medium to electrically reorient switchable display elements in the medium. 
   In one aspect of the invention, a system includes a memory interface and a print head. The memory interface is operable to write data to a memory of a portable data storage device including an electrically writable medium. The print head is operable to print on the electrically writable medium of the portable data storage device by electrically reorienting switchable display elements in the medium. 
   In another aspect of the invention, a portable storage device that includes a memory and an electrically writable medium is received. Data is written to the memory of the portable data storage device. The electrically writable medium of the portable data storage device is printed on by electrically reorienting switchable display elements in the medium. 
   In another aspect, the invention features an electrically writable display that includes an electrically writable medium containing electrically responsive switchable display elements. The electrically writable medium is incorporated into a continuous web having an outward-facing side and an inward-facing side. The outward-facing side of the continuous web presents a display surface of the electrically writable medium. The electrically writable display further includes a scroll system that is operable to scroll the display surface of the electrically writable medium through a display area. 
   The invention also features display method in accordance with which an electrically writable medium containing electrically responsive switchable display elements is provided. The electrically writable medium is incorporated into a continuous web having an outward-facing side and an inward-facing side. The outward-facing side of the continuous web presents a display surface of the electrically writable medium. The display surface of the electrically writable medium is scrolled through a display area. 
   In another aspect, the invention features an electrically writable display that includes an electrically writable medium and an external print head. The electrically writable medium has a front side and a back side and contains electrically responsive switchable display elements. The front side of the electrically writable medium presents a display surface that includes an optically transparent, electrically conductive layer. The external print head is disposed adjacent to the back side of the electrically writable medium and is operable to apply an electric field sufficient to electrically reorient switchable display elements contained in the electrically writable medium. The applied electric field extends from the back side of the electrically writable medium, through the electrically writable medium, to the front side display surface. 
   Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is diagrammatic cross-sectional side view of an implementation of an electrically writable medium. 
       FIG. 2  is a block diagram of an embodiment of a printer that includes a print head for printing on an electrically writable medium. 
       FIG. 3A  is a diagrammatic view of an implementations of the print head in the printer embodiment of  FIG. 2  that includes a source of an invisible charge species. 
       FIG. 3B  is a diagrammatic view of an implementation of the printer embodiment of  FIG. 2  that includes a source of an invisible molecular species. 
       FIG. 3C  is a diagrammatic view of an implementation of the print head in the printer embodiment of  FIG. 2  that includes a series of lenses guiding charged species from a source through a mask defining at least a portion of an image to be printed on the electrically writable medium. 
       FIG. 4  is a diagrammatic view of an implementation of the printer embodiment of  FIG. 2  that includes a source of an invisible molecular species and a source of a visible molecular species. 
       FIG. 5  is a block diagram of an implementation of the printer embodiment of  FIG. 2  that includes a substrate type detector, an erasing station, and a print head. 
       FIG. 6A  is a block diagram of an embodiment of a system for processing a portable data storage device that includes a memory interface for writing data to a memory of the portable data storage device and a print head for printing on an electrically writable medium carried on the portable data storage device. 
       FIG. 6B  is a block diagram of an embodiment of the processing system of  FIG. 6A  that includes a print head implemented in the form of a two-dimensional addressing array for printing on the electrically writable medium. 
       FIG. 7  is a diagrammatic view of an embodiment of a print head printing on an electrically writable medium carried by a portable data storage disk. 
       FIG. 8  is a diagrammatic perspective view of an embodiment of an electrically writable display. 
       FIG. 9  is a diagrammatic perspective view of a portion of an electrically writable display that includes an external print head and an electrically writable medium with a front side presenting a display surface including an optically transparent, electrically conductive layer. 
   

   DETAILED DESCRIPTION 
   In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
   Multiple embodiments of printers for printing on electrically writable media and multiple embodiments of electrically writable displays are described in detail below. In general, the printer embodiments may print on and the electrically writable displays may include any type of medium that has localized regions with display elements that are electrically switchable to produce visible content (e.g., an image containing one or more of pictures, graphics, and text). Exemplary switchable display elements include bi-stable, dual-color microcapsules, dichroic spheres, and optically anisotropic colorant particles. 
     FIG. 1  shows an embodiment of an electrically writable medium  10  that includes at least one colorant layer  12  that is disposed between a pair of protective layers  14 ,  16 . The colorant layer  12  is formed from a polymer binder and a plurality of switchable display elements that are implemented in the form of bi-stable, dual-color microcapsules  18 . Each microcapsule  18  includes a solid bi-colored sphere  20  housed in a microencapsulating shell  22 . Each microcapsule sphere  20  is coated with a lubricating fluid. Each sphere  20  is colored white on one hemisphere and colored black on the opposing hemisphere. The black colorant may be vapor-deposited, for example, on a solid white sphere that may be made of, for example, a pigmented glass, a polymer, or a ceramic. The vapor deposit contains charge species that give each of the spheres  20  an electric dipole for field alignment. The resulting charge on each bi-colored sphere allows the bi-colored spheres  20  to be oriented in accordance with an applied electric field so that each sphere  20  presents either the white hemisphere face or the black hemisphere face at the top surface of the electrically writable medium. The microcapsules  18  may be supported in a fixed polymer coating layer, while allowing each microcapsule sphere  20  to rotate within the microencapsulating shell  22 . The electrically writable medium  10  preferably contains a sufficient density of microcapsules  18  so that the electrically writable medium  10  appears completely white or completely black when all of the microcapsules  18  are oriented in the same direction. 
   In general, protective layer  14  may be formed of any flexible, fibrous or non-fibrous sheet material. In some embodiments, the protective layer  14  of electrically writable medium  10  has the look and feel of paper, but has far greater durability than most, commonly-used cellulose fiber papers. Such media are known in the art, and commonly consist of polymeric impregnated papers or polymeric fibers woven or assembled into films that have a paper appearance. Examples of such papers include Tyvek® (available from E. I. du Pont de Nemours and Company of Wilmington, Del., U.S.A.) and a series of Master-Flex™ papers (available from Appleton Papers Inc. of Appleton, Wis., U.S.A.). 
   Top protective layer  16  is optional and may be coated over the colorant layer  12  to increase the durability of electrically writable medium  10 . Protective layer  16  may be formed of a transparent polymer, such as PMMA (polymethylmethacrylate), or a blend of polymers. In some embodiments, the polymer binder and microcapsule shells  20  have matching refractive indices to minimize light scattering within the colorant layer  12 , improving image contrast. The gloss of the electrically writable medium  10  may be controlled by the characteristics of the colorant layer  12  or the optional protective layer  16 , or both. In some embodiments, the refractive indices of protective layer  16  and colorant layer  12  may be mismatched to enhance the “white paper” mode by inducing additional light scattering to enhance whiteness. 
     FIG. 2  shows an embodiment of a printer  24  that includes a print head  26  that is operable to print on electrically writable medium  10 . Printer  24  may be implemented in the form of a conventional desktop printer (e.g., a DESKJET® printer available from Hewlett-Packard Company of Palo Alto, Calif., U.S.A.). Print head  26  includes a source  28  of a charge species and a charge species projector  30  that projects charge species  32  from the source onto the medium  10  to electrically reorient switchable display elements in the medium  10  to produce visible content. As used herein, the term “charge species” broadly refers to any type of charged particle (e.g., electrons or ions) or charged molecule. In some implementations, charge species of only a single charge polarity (e.g., positive charge polarity) are deposited onto the electrically writable medium  10  during printing. 
   Electrically writable medium  10  is conveyed through printer  24  along a feed path that includes a section adjacent to print head  26  where charge species are deposited onto the exposed surface of electrically writable medium  10 . The deposited charge species produce localized electric fields that are greater than the threshold electric field needed to reorient the switchable display elements of electrically writable medium  10  in the vicinity of the deposited charge species. In the illustrated embodiment, the feed path includes an electrically conductive support member  34  that forms an electrically conductive backplane (or back electrode), which contributes to the establishment of an electric field from the deposited charge species through electrically writable medium  10 . In some implementations, support member  34  is connected to a fixed electric potential (e.g., ground). In other implementations, the electric potential of support member  34  is allowed to float with respect to the charge of the deposited charge species. 
     FIG. 3A  shows an implementation of print head  26  that includes a housing  40  that encloses an ion generation region including an electrically conductive chamber  42  and an electrode  44 . The electrode  44  is connected to a source of electric potential. The electrically conductive chamber  42  is connected to a reference potential source (e.g., ground). Upon application of a high potential to the electrode  44 , a corona discharge is created around the electrode  44  that creates within chamber  42  ions of a particular polarity (e.g., positively charged ions) that are attracted to the walls of chamber  42 . 
   An inlet port  46  delivers pressurized fluid (e.g., air) into the chamber  42  from a source  48 . The pressurized fluid transports ions out of chamber  42 , through an outlet port  50 , and into an ion modulation region  52 , which is defined by a pair of modulation electrodes  54 ,  56 . In some implementations, chamber  42  is cylindrical and includes multiple sets of outlet ports and associated modulation electrodes that are arranged along a line parallel to the cylindrical axis and are configured to selectively deposit ions in respective localized areas along a linear path on the surface of electrically writable medium  10 . In these implementations, printer  24  is configured to convey electrically writable medium  10  past print head  26  along a feed path that is substantially perpendicular to the cylindrical axis of chamber  42 . 
   One of the modulation electrodes  54 ,  56  is connected to a reference voltage (e.g., ground), and the other modulation electrode is switched selectively between a switching voltage (e.g., on the order of 10–20 volts direct current) in a non-deposition mode and the reference voltage in an ion-deposition mode. In the non-deposition mode, an electric modulation field is established between the modulation electrodes  54 ,  56  in a direction transverse to the direction of ion flow out of outlet port  50 . In one implementation, the electric modulation field drives ions toward the electrode that is connected to the reference potential, where they are neutralized into uncharged (i.e., neutral) particles or molecules. In this mode of operation, no ions are deposited on electrically writable medium  10 . In the ion deposition mode, the ions entrained by the transport fluid freely pass without modulation through the ion modulation region and impinge on a localized region of the surface of electrically writable medium  10  that is directly in the path of the ions. In this way, a pattern of charge species may be formed on electrically writable medium  10  by selectively turning on and off the modulation field between modulation electrodes  54 ,  56 . 
   In some implementations electrically conductive support member  34  is connected to a potential source that has a sign opposite to that of the potential applied to the electrode  44 . In these implementations, the potential applied to support member  34  generates an electric field between electrode  44  and support member  34  that accelerates ions from chamber  42  toward electrically writable medium  10 . 
   The ions that are deposited on electrically writable medium  10  are not themselves visible to the naked eye. A visual image, however, is created in electrically writable medium  10  by the electric fields created by the regions of deposited ions, as explained above in connection with  FIG. 2 . After the visual image is formed, the deposited charge species may be actively neutralized (e.g., by contact with a grounded electrode) or they may be passively neutralized by interaction with free charges in the printer environment. 
     FIG. 3B  shows an implementation of print head  26  that includes a source  62  of a liquid containing an invisible molecular species and a droplet generator  64 . As used herein, the term “invisible molecular species” refers to any type of molecular species that is not visible to the naked eye when deposited on a substrate medium. The type of molecular species supplied from source  62  typically is characterized by properties, such as viscosity, wettability and the ability to retain a charge, that are similar to the properties of typical ink jet inks. In one exemplary implementation, the invisible molecular species includes a conventional invisible ink. 
   In operation, source  62  supplies a liquid stream  65  containing invisible molecular species under pressure to the droplet generator  64 . The droplet generator  64  creates ultrasonic pressure waves in the liquid stream  65  that breaks the liquid stream  65  into separate droplets  66 . The droplets  66  pass through a charging region of a charge electrode  68 , where each droplet  66  is charged electrostatically. The amount of charge carried by each charged droplet  70  depends upon the voltage applied to the charge electrode  68 . The charged droplets  70  then pass through an electrostatic field set up between two high voltage deflector plates  72 ,  74 . The flight direction (or trajectory) a charged droplet  70  passing between deflector plates  72 ,  74  is changed by an amount that depends on the applied electric field strength and the amount of charge carried by the droplet. In some implementations, droplets that are not required for printing either are not charged by charge electrode  68  or are deflected to a gutter location where they are collected for recycling. 
   The charged molecular species that are deposited on electrically writable medium  10  are not themselves visible to the naked eye. A visual image, however, is created in electrically writable medium  10  by the electric fields created by the regions of deposited charged molecular species, as explained above in connection with  FIG. 2 . After the visual image is formed, the deposited molecular species may be actively neutralized (e.g., by contact with a grounded electrode) or they may be passively neutralized by interaction with free charges in the printer environment. 
     FIG. 3C  shows an embodiment of print head  26  that includes a series of lenses  80 ,  82 ,  84  guiding charged species from source  28  through a mask  88  defining at least a portion of an image to be printed on electrically writable medium  10 . The source  28  may supply any type of invisible charge species, including charged particles, such as ions, and charged molecules. Lens  80  is an electrostatic lens (e.g., an Einzel lens), which focuses the charge species onto mask  88 . Mask  88  may be any form of stencil-type mask that selectively transmits charge species in areas corresponding to the regions of electrically writable medium  10  that are to be charged to form a selected image and that blocks charge species corresponds to other areas of electrically writable medium  10 . Lens  82  may be an electric or magnetic lens that projects the aperture of lens  80  onto the aperture of lens  84 . Lens  84  is an electrostatic lens (e.g., an Einzel lens) that focuses the charge species passing through mask  88  onto electrically writable medium  10 . In this way, charge species may be deposited onto localized areas of electrically writable medium  10 , where they produce electric fields that reorient switchable display elements in the medium  10  to form an image as described above in connection with  FIG. 2 . 
     FIG. 4  shows a dual printing mode implementation of printer  24  that includes the invisible molecular species source  62  and the implementation of print head  26  shown in  FIG. 3B , but this implementation of printer  24  further includes a source  90  of a visible molecular species and a manifold  92 . The visible molecular species may be, for example, any form of visible ink jet printer ink. This implementation of printer  24  is operable to print on both electrically writable media and conventional paper-like substrates. In particular, when printing on electrically writable media, manifold  92  is switched to supply the invisible molecular species from source  62  to droplet generator  64 . When printing on non-electrically-writable media (e.g., paper), on the other hand, manifold  92  is switched to supply visible molecular species from source  90  to droplet generator  64 . The deposition of invisible and visible molecular species onto electrically writable medium  10  is controlled in the same way described above in connection with the implementation of  FIG. 3B . 
   Referring to  FIG. 5 , some dual printing mode implementations of printer  24  include, upstream of the print head  26 , a substrate type detector  94  that is operable to detect whether an electrically writable medium or a conventional print medium has been loaded for printing. For example, substrate type detector  94  may include a test electrode  96  that applies a bias to mark (e.g., produce a discernable color change in a localized region) a substrate  98  that is being fed through the printer  24 . A sensor  100  (e.g., a photodetector), which is positioned downstream of the test electrode  96 , detects whether the applied bias produced a test mark on the substrate  98  and produces a signal indicative of the type of substrate that is loaded into the printer for printing. With respect to the exemplary dual mode printer embodiment of  FIG. 4 , if the test mark is detected, the manifold  92  is switched to supply invisible molecular species from source  62  to droplet generator  64 . If the test mark is not detected, manifold  92  is switched to supply visible molecular species from source  90  to droplet generator  64 . 
   As shown in  FIG. 5 , some embodiments may include, upstream of print head  26 , an erasing station  102  that includes, for example, a charged-electrode that is biased to orient all of the switchable display elements of an electrically writable medium in the same direction (e.g., white sides facing up) before visible content is printed on the electrically writable medium. 
   In some dual printing mode implementations, printer  24  is configured to accept user input specifying the type of media that has been loaded for printing. 
     FIG. 6A  shows a system  110  for processing a portable data storage device  112 , which includes a memory  114  and an electrically writable medium  10 . Portable data storage device  112  may be implemented in the form of any self-contained portable memory device form factor, including a card (e.g., a Smart Card or magnetic swipe card), a circular disk (e.g., a DVD or CD disk), or a rectangular disk (e.g., memory card, a 3.5 inch floppy disk, or a ZIP Drive disk). Memory  114  may include any type of non-volatile memory, including, for example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic memory devices, such as removable hard disks; magneto-optical disks; and optical disks, such as DVD-ROM, DVD-RAM, CD-ROM, and CD-RAM. Among the application environments for which processing system  110  may be tailored are the following, automatic teller machines, automatic checkout machines (e.g., for retail purchases and library book borrowing), and automated medical history update machines. 
   Processing system  110  includes a holder  115  (e.g., a slot or a tray) that is configured to receive portable data storage device  112  and hold portable storage device  112  while data is being written to memory  114  and while content is being printed on electrically writable medium  10 . 
   Processing system  110  includes a memory interface  116  that is constructed and arranged to write data to the memory  114  of portable data storage device  112 . The particular implementation of memory interface  116  depends on the particular implementation of memory  114 . For example, in some application environments (e.g., CD or DVD based memory environments), the memory interface  116  includes an electromechanical data head configured to write data to memory  114 . In other application environments (e.g., smart card based memory environments), the memory interface  116  includes a connector that delivers electronic instructions to a mated connector of memory  114 . In some implementations, memory interface  116  is operable to read data from memory  114  of portable data storage device  112 . 
   Processing system  110  also includes a print head  118  that is operable to print on the electrically writable medium  10  that is carried by portable data storage device  112 . The particular implementation of print head  118  depends on the particular implementation of electrically writable medium  10 . In general, print head  118  may be any form of print head that is capable of printing on electrically writable medium  10  by electrically reorienting switchable display elements in the medium. For example, in some implementations, print head  118  may be implemented in accordance with any one of the print head embodiments described above. In some embodiments, print head  118  also is operable to selectively erase regions of electrically writable medium  10 . 
   Processing system  110  also includes a processing unit  120 , which may include one or more processors, each of which may be in the form of any one of various commercially available processors. Processing unit  120  interfaces with memory interface  116  through a read/write controller  122  and interfaces with print head  118  through a print controller  124 . 
   As shown in  FIG. 6B , in one implementation, print head  118  includes a two-dimensional addressing array  126  that includes multiple independently addressable electrodes for simultaneously printing on localized areas of the electrically writable medium  10  by selective application of electric fields to the medium that are sufficient to reorient switchable display elements in the medium. 
     FIG. 7  shows an exemplary embodiment of a print head  118  that includes a linear array of electrodes  130  that are operable to simultaneously print on multiple localized areas along a linear path across the surface of an electrically writable medium  10 , which is carried on a disk-shaped portable data storage device  112 . Portable data storage device  112  may include an optical storage medium  132  (e.g., a CD or DVD based storage medium) on the side of portable data storage device  112  that is opposite the side carrying electrically writable medium  10 . In one implementation, portable data storage device  112  spins within holder  115  of processing system  110  in a direction indicated by arrow  134 . While device  112  spins, print head  118  may print on electrically writable medium  10 . In another implementation, print head  118  is scanned across the surface of electrically writable medium  10  during printing. 
     FIG. 8  shows an embodiment of an electrically writable display  140  that includes an electrically writable medium  10  containing electrically-responsive switchable display elements. The electrically writable medium  10  is incorporated into a continuous web  142  that has an outward-facing side  144  and an inward-facing side  146 . The outward-facing side  144  of the continuous web  142  presents a viewing or display surface of the electrically writable medium  10  (i.e., the surface of electrically writable medium  10  displaying printed content). The continuous web  142  is mounted on a scroll system that includes a pair of rolls  148 ,  150  that scroll the display surface of the electrically writable medium  10  through a display area  152  (shown by the superimposed dashed rectangle in  FIG. 8 ). The display area  152  may correspond, for example, to a window in a housing containing electrically writable display  140 . 
   Electrically writable display  140  further includes an external print head  154  that is operable to print on the electrically writable medium  10 . Print head  154  is located adjacent to the outward-facing side  144  of the continuous web  142 . In some embodiments, print head  154  is located adjacent to the inward-facing side  146  of the continuous web  142 . In the illustrated embodiment, the print head  154  is fixed in position relative to the scroll system. The print head  154  includes a linear array of electrodes  156  oriented in a direction substantially perpendicular to a direction  158  in which the continuous web  142  is scrolled by the scroll system. 
   In general, print head  154  may be any form of print head that is capable of printing on electrically writable medium  10  by electrically reorienting switchable display elements in the medium. For example, in some implementations, print head  154  may be implemented in accordance with any one of the print head embodiments described above. In some embodiments, print head  154  also is operable to selectively erase regions of electrically writable medium  10 . 
     FIG. 9  shows an embodiment of an electrically writable display  160  that includes an electrically writable medium  162 , which has a front side  164  and a back side  166  and contains electrically responsive switchable display elements  167 . The front side  164  presents a viewing or display surface that includes an optically transparent, electrically conductive layer  168 . Transparent, electrically conductive layer  168  may be formed of any type of material (e.g., indium-tin-oxide) that is electrically conductive and substantially transparent to light in the visible wavelength range. In the illustrated embodiment, the display elements  167  correspond to the microcapsules  18 , which are incorporated into the colorant layer  12  described above in connection with the embodiment of  FIG. 1 . Protective layer  14  is disposed on the backside  166  of electrically writable display  160 . 
   Electrically writable display  160  also includes an external print head  170  that is disposed adjacent to the back side  166  of the electrically writable medium  162  and is operable to apply an electric field sufficient to electrically reorient switchable display elements  167  contained in the electrically writable medium  162 . The print head  170  includes a linear array of electrodes that are oriented in a direction substantially perpendicular to directions  174 ,  176  in which the print head  170  may be scanned across the backside  166  of electrically writable medium  10 . 
   The electric fields that are applied by the electrodes of print head  170  extend from the back side  166  of the electrically writable medium  162 , through the electrically writable medium  162 , to the front side display surface  168 . In general, print head  170  may be any form of print head that is capable of printing on electrically writable medium  10  by electrically reorienting switchable display elements in the medium. For example, in some implementations, print head  170  may be implemented in accordance with any one of the print head embodiments described above. In some embodiments, print head  170  also is operable to selectively erase regions of electrically writable medium  10 . 
   Among the application environments into which electrically writable display  160  may be incorporated are the following, roadside billboard displays, flight arrival and departure displays in airport terminals, and other displays providing information content on-demand, such as information-rich content for museum displays. 
   Other embodiments are within the scope of the claims. 
   For example, although the above embodiments are described in connection with one exemplary type of electrically writable medium, these embodiments readily may be used with other types of electrically writable media, including electrically writable media that incorporate optically anisotropic particles having one or more colors in addition to or replacing one or more of the black and white colors, and electrically writable media in which protective layer  14  is electrically conductive and forms an electrically conductive backplane. In some printer embodiments that are designed for use with electrically writable media that have electrically-conductive backplanes, the external surface of support member  34  may be electrically-insulating. 
   In addition, the above embodiments are described in connection with exemplary print head designs. Other embodiments, however, may be used with different print head designs. 
   Although systems and methods have been described herein in the context of a particular computing environment, these systems and methods are not limited to any particular hardware or software configuration, but rather they may be implemented in any computing or processing environment, including in digital electronic circuitry or in computer hardware, firmware or software.