There are now several types of electronic paper. The first development of this type of display was commonly known as Gyricon, and was developed by Xerox in the 1970s. It is a technique where one takes dielectric spheres and coats a white layer on one hemisphere and a black layer on the opposite side. By putting it in a matrix slurry that allows these spheres to rotate under the influence of an electric field, they can re-orient to appear either white or black. The beauty is that you can write a very high contrast image and when the field is removed, the image stays in place. It looks as close to paper as you can find in a display. There had been issues, however, with these spheres moving, agglomerating, or plating out to one surface or another so there was some long-term degradation. E-ink has put considerable investment in this technology and the result is the electrophoretic display. It retains an image indefinitely with very little of the early problems of image retention. This type of display has found wide usage in electronic readers. It does have some drawbacks: its reflectivity ratio is limited, so the addition of a touch screen is impractical. It also requires a high voltage to cause the display to change phase, and this phase change has to have a full negative written followed by the next positive image, to prevent stuck pixels.
Another variant to make electronic paper is cholesteric nematic phase transition (CNPT). This is a liquid crystal phenomena that allows you to go from one liquid crystal texture to another. One texture is known as a grandjean and the other is known as focal cone. The focal cone texture microscopically looks like fans and causes light to be scattered. It goes from a non-scattering to a scattering state and by properly illuminating this type of display, it is possible to achieve very high contrast. Doing this in conjunction with color filters, you can make one of the best looking color flat panels ever seen. The benefit is very wide viewing angles that look like paper. The drawback is that it takes a high energy AC field which consumes a lot of power to erase it. So, if you are writing an occasional image, it works well. If you tried to write a video image it would require power beyond the capability of conventional transistor arrays used today. So you will see this used only for limited applications.
Another technology for e-paper is Iridigm. By using an elastomeric material with a reflector and by applying a field, you can deflect a cantilevered layer such that – because of hysteresis – it stays in a deflected position with a relatively small voltage applied. So one can write an image and retain it with extremely low amounts of power. Used in conjunction with color filters you can make full color displays. To erase the image, you simply take the applied voltage to zero. Although it holds great promise for the future, the investment level remains very small.
