Not everybody agrees with what it is and not everybody thinks it's plausible or even worthwhile. The story of electronic paper (e-paper), so far, has proved to be what could be termed "techno-hyperenthusiasm."

But what we have today in the real world are two interesting technologies, both of which might make possible the precursor of e-paper: flexible, planar displays. Each of them has some of the attributes of what we might think of as e-paper, but neither is really close. Each is near the beginning of a long process of technology evolution, infrastructure development and-more daunting by far-changes in working paradigms for human users, all of which will be necessary to approach the eventual goal of e-paper.

Today, there are two competing technologies for the ink component: one, spun off from the Media Lab at MIT, under the control of a company called eInk. The other, spun off from the Palo Alto Research Center (of Ethernet, mouse and windows fame), is being commercialized by a company called Gyricon.

The two are similar in their fundamental concepts. The basic element in each is a small particle that is electrically polarized: positively charged on one end and negatively charged on the other. The principle of operation is that by placing the particles in a sufficient electric field, you can set their orientation.

The next step is to color the particles with something so that they are white on one end and black on the other (or red and green, or whatever.) Now, if you can apply an electric field and orient the particles, you also establish which color is facing which way in the field.

Finally, we apply the encapsulated goo, with its suspended, painted particles, to a substrate appropriate to whatever application that we have in mind.

There. We have invented electronic paper.

Except for a few details. For our e-paper to work, we must have some way to apply an electric field to it to write it. The material itself must be able to retain the image written on it and to survive the sort of handling that paper ordinarily gets. The image must, in fact, be a reasonable facsimile of what we would see on a real piece of paper. And the completed system must give us some compelling reason to switch from paper to e-paper. It turns out that each of these issues is problematic.

To begin, there's the writing problem. As mentioned, the particles in the goo orient themselves to a sufficiently strong electric field. There is a fundamental trade-off here: The easier it is to get the particles to orient themselves, the easier it will be to disturb them, and the harder it will be to keep them oriented once the field is removed.

After all, paper doesn't have two wires running off to a battery pack. So, at present, we are talking about an electric field created by sandwiching the e-paper between two metal electrodes and applying a voltage to each pixel using an active-matrix-type backplane-much like an LCD.

In fact, at the moment it is very, very much like an LCD. "We have recently announced that eInk, in collaboration with Philips, has produced a planar display using eInk material with a conventional amorphous-silicon-on-glass backplane," said Darren Bischoff, senior marketing manager at eInk. "The result is a high-contrast reflective display with about half the thickness and half the weight of an equivalent LCD."

E-paper as signage uses Gyricon material to show contrast. An electric field created by sandwiching the e-paper between two metal electrodes and applying a voltage to each pixel uses an active-matrix-type backplane-very much like a liquid crystal display.

That is the current reality: The need for a rigid glass plate on which to fabricate the amorphous silicon transistors of the active matrix confines e-paper to flat-panel displays. Since color hasn't really been worked out yet-although eInk has shown color displays in the lab-the reality right now is black-and-white planar displays. A most likely application is reusable or remotely modifiable signage.

"Right now, the industry is looking for something in between low-cost planar displays and real paper," explained PARC area manager for hardware systems Eric Shroder. "On the one hand, there's not really a compelling reason to abandon real paper. It works wonderfully. On the other hand, the quality of desktop displays right now is unimpeachable. They are fragile and not very portable, but they look wonderful. So you have to find something in between those two."

The application people are fastening on now is signage, Shroder said. Signs don't require the flexible backplanes that aren't ready yet, but they benefit from low cost and easy modifiability. Significantly, they usually don't require high resolution. And they can often be attached to a battery.

Flexible and printable conductors are a technology already in hand, according to PARC member of research staff Raj Apte. PARC is even making progress on using off-the-shelf Xerox ink-jet technology for printing amorphous silicon.

Once the backplane is sufficiently flexible and reliable, everyone's favorite idea is the roll-out display. Apte suggested a cell phone shaped like a fat pen, from which a rectangular e-paper display would pull out like a window-blind. "It would allow you to carry the phone in your shirt pocket, but to unroll a display large enough to do serious Web searching," Apte said.

Of course, if the information on the e-paper is going to be modified, there has to be a link to some external data source. In the movies, that link is wireless, built into the flexible circuitry of the paper itself, and presumably powered by a thin, light and equally flexible battery. In our time, the e-paper must be tethered to a computer and a power supply.

Except in some applications, PARC's Shroder is quick to add. If the e-paper material has good enough image retention, it can be written by a device analogous to a printer, and then taken out and used just like a normal sheet of paper. PARC has also developed a clever alternative: a wand, connected by Ethernet to a computer. The wand has wheels and tracking capability, like a mouse, so once you set it on a piece of e-paper, it keeps track of its location. By moving the wand over the paper in a serpentine pattern, you can write the paper with the information provided via the Ethernet link. You just have to be careful not to lift the wheels.

Flexible and printable conductors are a technology already in hand, according to PARC member of research staff Raj Apte. PARC is even using off-the-shelf Xerox ink-jet technology for printing amorphous silicon.

"So what?" you say. Who needs a dollar-a-sheet piece of paper that's slower to write than the real thing? Actually, there are applications, Shroder said.

One Japanese manufacturing giant calculated that it was throwing away so much paper that it could save significant money by using Gyricon's e-paper instead-simply because at the end of a pass, it can be reused. "One near-term application of electronic paper without an attached backplane will be as simple reusable paper," Shroder suggested. "We will see this in areas where the organization is sensitive to the cost of consumable paper."

eInk's Bischoff is more skeptical about applications for e-paper without attached backplanes.

He sees the future of e-paper unrolling as the backplane becomes more flexible. "There's a continuum of improvements as we approach the idea of 'radio paper' on which eInk was founded. Right now, we can do thin displays. Next, we will be able to do what I would call conformable displays-for instance, a display that would mold to your wrist or that could be applied to a curved wall. We have a partner in Korea today doing that for signage.

"Finally, we will achieve continuously flexed displays that can be rolled and unrolled repeatedly. Then you can see the pen cellphone or the display scroll."