Electronic ink, or e ink, is a special type of electronic display. Somewhat similar to regular ink, e ink is unique in three primary ways: it is made up of many tiny capsules, these capsules are filled with ink, and the ink contains white particles that have a negative charge. These three properties allow an electronic ink display to be changed on demand.

E ink can be used on many of the same surfaces as regular ink. For example, e ink might be applied to a piece of thin plastic containing many small cells. The cells are connected to microelectronics in the sheet, and the ink covers the entire surface of the sheet. The microelectronics create a positive or negative charge in each of the capsules. A negative charge beneath a capsule causes the particles in the ink to go to the top of the capsule, covering its top surface and making it appear white. A positive charge attracts the particles to the bottom of the capsule, allowing the dark ink to show and making the capsule take on the color of the ink.
Another type of e ink is one in which the capsules are filled with tiny balls that are dark on one half and light on the other half. Electrical charges make the balls rotate so that either the dark side or the light side of the balls face up. The capsules in either type are very small at only 100 microns across, so approximately 100,000 of these capsules will fit into a square inch. Developing e ink displays is very difficult, because it is necessary to use extremely small electronic components in order to keep the display close to the thickness of regular paper while maintaining durability, flexibility, and strength.

One obvious use of this technology is digital books. Digital books could be displayed with e ink, allowing readers to store many books on a single device while creating a reading experience that is very similar to reading regular ink printed on regular paper. E ink could also be used for newspaper subscriptions, displaying records and files in offices, or for a variety of other uses. An advantage of e ink displays over the backlit displays of cell phones, PDAs, and similar other electronic devices is that they consume much less energy because they require power only when the display is changed. In addition, they are easier on the human eye because backlighting strains the reader's eyes.

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