One of the constant refrains in discussions of the various e-book readers now (or soon to be) on the market is the request for color. For a large population of users, the current greyscale screens that provide the extremely low-power displays for devices like the Kindle simply aren't good enough, and there are some markets—science textbooks spring to mind—where color would really add value to the product. It's not clear what technology will power the first generation of color devices, but researchers are looking to steal a page from traditional print media by making colored pigments part of the future of e-books.

One of these techniques was the subject of a recent publication in Nature Photonics, written by researchers at the University of Cincinnati and Sun Chemicals. Their technique, which they termed an electrofluidic display, relies on a combination of a small reservoir hooked up to a narrow space that sits above a reflective surface. Under normal circumstances, an aqueous solution will retract into the reservoir, driven by what's called Young-Laplace pressure. Creating a voltage difference across the narrow space, however, will create an electromechanical force that draws the liquid out from the reservoir and into the space. Simply put a pigment in the aqueous solution, and you've got a stable, voltage-switched color pixel.

The key feature of these devices is that the reservoir takes up only five to 10 percent of the dimensions of the pixel it can fill with its contents. Combined with the current reflective capabilities of the materials used in the device, this allows a contrast difference between the on and off states of about 55 percent, but the authors say that there's nothing in theory that should prevent improvements from reaching over 80 percent. Switching between on and off states can also be accomplished in a matter of milliseconds, and the authors suggest a number of optimizations, any of which could drop the refresh rates down to the point where video displays would be possible, provided the battery had the power to handle the frequent refreshes.

The one structural limitation with the device is that you don't want to stack too many pigments in a single pixel, lest the five-to-10-percent figure for the reservoir starts adding up. So, the authors actually intend for each individual device to form a sub-pixel, much as is already in use on many computer displays. The authors suggest using two reservoirs per subpixel, with a total of four subpixels. These would work in two possible combinations. One would be an RGB-blank combination, where the three colors and an empty reservoir were combined with a black pigment that controlled reflectance. Alternately, a more complicated CMYK display is possible.

The materials involved—water-soluble pigments, reflective aluminum, and a transparent indium-tin-oxide—are all fairly standard, and producing these devices involves standard photolithography techniques. If the above optimizations pan out, the authors claim that the resulting device "can hide the pigment or reveal the pigment with a visual brilliance that is similar to pigment printed on paper."

Meanwhile, researchers at Philips are looking at a system that would allow manufacturers to avoid using a composite pixel of this sort. In this device, a single reservoir holds two different pigments (the researchers are targeting a CMYK display) with different charge properties. By carefully manipulating the voltage on the device, different amounts of each pigment will flow out of the reservoir, providing careful control over the color of the pixel. Philips hasn't been very generous with the details on how that control is provided, however.

In any case, the striking aspect of both techniques is how they're a bit of a return to the past. Pigment-based colors have a long history in print, as they provide vibrant and stable color when paired with a highly reflective white paper. Given their properties, it's no real surprise that they may make a return in the electronic era.

Nature Photonics, 2009. DOI: 10.1038/NPHOTON.2009.68

Journal of the Society for Information Display, 2009. DOI: 10.1889/JSID17.4.383