Amazon.com's Kindle has turned a long under performing category of tech gadget -- e-book readers -- into an overnight hit, and in the process has boosted interest in electronic paper display (EPD) technology. The Kindle and its rival, the Sony Reader 505, both boast e-paper displays that look unnervingly like printed pages and consume next to no power. However, today's EPDs -- and today's e-book readers -- are only the beginning.

EPD technology has been a long time coming. The idea of e-paper, a data display that looks and works like a sheet of paper, has been around for decades. In theory, such a screen could be "printed" electronically, would hold its contents without consuming power, could be viewed using reflected light (rather than the backlight required for LCD screens), and could be "erased" and "rewritten" as often as desired.

Current products like the Kindle -- a clever mix of features, including a low-power processor, inexpensive flash memory, built-in EVDO wide-area networking and, of course, an e-paper display that consumes next to no power -- have finally brought the technology into the public eye. "E-book readers have gotten the world excited about e-paper," says Barry Young, an analyst at market research firm DisplaySearch.

But although the technology behind e-paper displays has improved greatly over time, it's still just on the threshold of real success, according to Young and other observers. Displays like the Kindle's are beginning to provide the contrast and resolution of traditional ink on paper, but physical flexibility and full-color display are still around the corner.

A display technology based on electronic 'ink'

The first successful demonstration of e-paper technology was made by Nick Sheridon at Xerox's Palo Alto Research Center (PARC) in the 1970s. His technology, called Gyricon, used tiny rotating spheres of electrically charged plastic, black on one side, white on the other, suspended in bubbles of oil between transparent electrodes.

Gyricon technology never delivered the contrast and resolution that a display screen for personal electronic devices requires, but it was used for signs. (Xerox and PARC are now looking at other ways to combine paper and technology, including papers whose printed images fade away over a short period of time -- like a day.)

The current crop of EPD displays is based on electronic "ink" that the E Ink, a supplier of electronic ink technology, has been developing since 1997. E Ink's electrophoretic technology puts oppositely charged black and white pigments into tiny "microcapsules" filled with a transparent fluid. The capsules are fixed to a substrate and sandwiched between electrodes, and when a current is applied, one pigment is drawn to the positive electrode, one to the negative.

The ink is bistable -- that is, it requires electrical power only to change its state, making it very energy-efficient. Although displays based on this ink are not as high-contrast as backlit computer screens, which can make them hard to read in dim light, their reflective surface allows them to be read in daylight situations that would wash out conventional laptop displays. Most importantly, eliminating the power demands of the backlighting needed by conventional LCD displays means that e-paper displays draw negligible power.

Another advantage is that e-paper displays can now take any shape, according to Sri Peruvemba, E Ink's vice president for marketing. Until recently, they had been built on glass -- particularly the active-matrix displays used by today's e-book readers. But the technology is rapidly moving to plastic substrates that will make e-paper almost as flexible as ... paper.