Vesna Kropar Vančina, Jana Ž. Vujić, Kristian Krog


Electronic paper, also called e-paper, is a display technology designed to mimic the appearance of ordinary ink on paper. Unlike a conventional flat panel display, which uses a backlight to illuminate its pixels, electronic paper reflects light like ordinary paper and is capable of holding text and images indefinitely without drawing electricity, while allowing the image to be changed later.
There are several different technologies to build e-paper, some of which can use plastic substrate and electronics, so that the display is bendable. Due to the stable image, which does not need to be constantly refreshed, the contrast ratio similar to that of a newspaper, and a very large viewing angle, it is more confortable to read than a conventional flat display. It is lightweight and durable, but still lacks good colors.
Applications include e-book readers capable of displaying digital versions of books and e-paper magazines, electronic pricing labels in retail shops, time tables at bus stations, and electronic billboards.
Electronic paper should not be confused with digital paper, which is a pad to create handwritten digital documents with a digital pen.

Electronic paper was first developed in the 1970s by Nick Sheridon at Xerox's Palo AlTO Research Center. The first electronic paper, called Gyricon, consisted of polyethylene spheres between 20 and 100 micrometres across. Each sphere is composed of negatively charged black plastic on one side and positively charged white plastic on the other (each bead is thus a dipole. The spheres are embedded in a transparent silicone sheet, with each sphere suspended in a bubble of oil so that they can rotate freely. The polarity of the voltage applied to each pair of electrodes then determines whether the white or black side is face-up, thus giving the pixel a white or black appearance.


Schema of an Electrophoretic Display

Schema of an Electrophoretic Display Using Color Filters

An electrophoretic display is an information display that forms visible images by rearranging charged pigment particles using an applied electric field.
In the simplest implementation of an electrophoretic display, titanium dioxide particles approximately one micrometre in diameter are dispersed in a hydrocarbon oil. A dark-colored dye is also added to the oil, along with surfactants and charging agents that cause the particles to take on an electric charge. This mixture is placed between two parallel, conductive plates separated by a gap of 10 to 100 micrometres. When a voltage is applied across the two plates, the particles will migrate electrophoretically to the plate bearing the opposite charge from that on the particles. When the particles are located at the front (viewing) side of the display, it appears white, because light is scattered back to the viewer by the high-index titania particles. When the particles are located at the rear side of the display, it appears dark, because the incident light is absorbed by the colored dye. If the rear electrode is divided into a number of small picture elements (pixels), then an image can be formed by applying the appropriate voltage to each region of the display to create a pattern of reflecting and absorbing regions.
Electrophoretic displays are considered prime examples of the electronic paper category, because of their paper-like appearance and low power consumption.
Examples of commercial electrophoretic displays include the high-resolution active matrix displays used in the Amazon Kindle, Sony Librie, Sony Reader, and iRex iLiad e-readers. These displays are constructed from an electrophoretic imaging film manufactured by E Ink Corporation. The Motorola Motofone is the first mobile phone which uses the technology to help eliminate glare from direct sunlight during outdoor use.
Another producer of electrophoretic displays is the California based company SiPix. Sipix, along with manufacturing partner SmartDisplayer, received a 1996 Society for Information Display Gold Award for an IC smart card with an integrated electrophoretic display.
Electrophoretic displays can be manufactured using the Electronics on Plastic by Laser Release (EPLar) process developed by Philips Reasarch to enable existing AM-LCD manufacturing plants to create flexible plastic displays.
In the 1990s another type of electronic paper was invented by Joseph Jacobson, who later co-founded the E Ink Corporation which formed a partnership with Philips Components two years later to develop and market the technology. In 2005, Philips sold the electronic paper business as well as its related patents to Prime View International. This used tiny microcapsules filled with electrically charged white particles suspended in a colored oil. In early versions, the underlying circuitry controls whether the white particles were at the top of the capsule (so it looked white to the viewer) or at the bottom of the capsule (so the viewer saw the color of the oil). This was essentially a reintroduction of the well-known electrophoretic display technology, but the use of microcapsules allowed the display to be used on flexible plastic sheets instead of glass.
One early version of electronic paper consists of a sheet of very small transparent capsules, each about 40 micrometres across. Each capsule contains an oily solution containing black dye (the electronic ink), with numerous white titanium dioxide particles suspended within. The particles a slightly negatively charged, and each one is naturally white.
The microcapsules are held in a layer of liquid polymer, sandwiched between two arrays of electrodes, the upper of which is made from indium tin oxide, a transparent conducting material. The two arrays are aligned so that the sheet is divided into pixels, which each pixel corresponding to a pair of electrodes situated either side of the sheet. The sheet is laminated with transparent plastic for protection, resulting in an overall thickness of 80 micrometres, or twice that of ordinary paper.

The network of electrodes is connected to display circuitry, which turns the electronic ink 'on' and 'off' at specific pixels by applying a voltage to specific pairs of electrodes. Applying a negative charge to the surface electrode repels the particles to the bottom of local capsules, forcing the black dye to the surface and giving the pixel a black appearance. Reversing the voltage has the opposite effect - the particles are forced from the surface, giving the pixel a white appearance. A more recent incarnation of this concept requires only one layer of electrodes beneath the microcapsules.

Bistable LCD
Some companies also produce epaper displays based on bistable LCD technology. The french company Nemoptic commercializes bistable nematic epaper displays (B&W and color) based on a unique principle called “surface anchoring breaking”. The technology used, called BiNem®, has two stable states, the Uniform (U) state and the Twisted (T) state, which are selected by applying simple pulses. Once either state is selected, it stays like it is forever without consuming any additional power. An electrical pulse drives from one state to the other one. This pulse first lifts the molecules on the surface with the weak anchoring layer up to the point where the anchoring is broken. Then, depending on the shape of the falling edge of the pulse, the molecules organize either in U or T state. Bistable LCD diplays offer high reflectivity, resolution up to 200 ppi and a quite neutral white point.
Other technologies
Electronic paper has also been produced using technologies such as cholesteric LCD (Ch-LC). Other research efforts into e-paper have involved using organic transistors embedded into flexible substrates, including attempts to build them into conventional paper. Simple color e-paper consists of a thin colored optical filter added to the monochrome technology described above. The array of pixels is divided into triads, typically consisting of the standard cyan, magenta and yellow, in the same way as CRT monitors (although using subtractive primary colors as opposed to additive primary colors). For commercial releases of e-paper in the forms of newspapers etc, it will most likely be in the 'CMYK' format, for clarity of writing. The display is then controlled like any other electronic color display.
Several companies are simultaneously developing electronic paper and ink. While the technologies used by each company provide many of the same features, each has its own distinct technological advantages. All electronic paper technologies face the following general challenges:
A method for encapsulation
An ink or active material to fill the encapsulation
Electronics to activate the ink
Electronic ink can be applied to both flexible and rigid materials. In the case of flexible displays, the base requires a thin, flexible material tough enough to withstand considerable wear, such as extremely thin plastic. The method of how the inks are encapsulated and then applied to the substrate is what distinguishes each company from each other. These processes are complex and are carefully guarded industry secrets. The manufacture of electronic paper promises to be less complicated and less costly than traditional LCD manufacture.
There are many approaches to electronic paper, with many companies developing technology in this area. Other technologies being applied to electronic paper include modifications of liquid crystal displays, electrochromic displays, and the electronic equivalent of an Etch A Sketch at Kyushu University. Advantages of electronic paper includes low power usage, flexibility and better readability than most displays. Electronic ink can be printed on any surface, including walls, billboards, product labels and T-shirts. The ink's flexibility would also make it possible to develop rollable displays for electronic devices. The ideal electronic paper product is a digital book that can typeset itself and could be read as if it were made of regular paper, yet programmed to download and display the text from any book. Another possible use is in the distribution of an electronic version of a daily paper.
Commercial applications
Education: digital schoolbooks
• In January 2007, the Dutch specialist in e-Paper edupaper.nl started a pilot on a secondary school in Maastricht, using e-Paper as digital schoolbooks. To make it possible for schoolchildren not to have to lug many kilograms of books in their backpacks each day and to reduce prices (no printing, no logistics). On their website you can find more information about this project.
• In November 2006 the iRex iLiad was ready for the consumer market. Consumers could initially read e-Books in PDF and HTML formats, and in July 2007 support for the popular Mobipocket PRC format was added. But price was still a problem. With the introduction of the competing Cybook prices have decreased almost 50%.
• In late 2007 Amazon began producing and marketing the Amazon Kindle, an e-book with an e-paper display.
• In September 2007, the French daily Les Echs announced the official launch of an electronic version of the paper on a subscription basis. Two offers are available, combining a one year subscription and a reading device. One interesting point of the offer is the choice of a light (176g) reading device (adapted for Les Echos by Ganaxa) or the iRex iLiad. Two different processing platforms are used to deliver readable information of the daily, one based on the newly developed GPP electronic ink platform from Ganaxa, and the other one developed internally by Les Echos.
• In February 2006, the Flemish daily De Tijd distributed an electronic version of the paper to select subscribers in a limited marketing study, using a pre-release version of the iRex iLiad. This was the first recorded application of electronic ink to newspaper publishing.
Displays embedded in smart cards
• Flexible display cards enable financial payment cardholders to generate a one-time password to reduce online banking and transaction fraud. Electronic paper could offer a flat and thin alternative to existing key fob tokens for data security. The world’s first ISO compliant smart card with an embedded display was developed by Smartdisplayer using SiPix Imaging’s electronic paper.
Cell phone displays
• Motorola's low-cost mobile phone, the Motorola F3, also uses a monochrome electronic paper screen.