28 April 2009—A new technology that uses ambient light and pigments used in commercial printing promises to make thin electronic displays that are as bright and vibrant as the pages of a glossy magazine, according to research reported this week in Nature Photonics.
Researchers at the University of Cincinnati’s Novel Devices Laboratory have developed what they call electrofluidic display technology over the past two years in collaboration with color experts from ink and pigments manufacturer Sun Chemical Corp. Sun Chemical also funded the work and has applied for a patent on the technology with the university.
An electrofluidic display is built from two sheets of plastic. Onto one sheet, mesa-like polymer structures are printed to form pixels. For each pixel, a hole taking up 5 to 10 percent of the pixel area (about 50 micrometers) is formed in the polymer and filled with a droplet of pigmented fluid. Surrounding the pixel is a trench cut into the polymer that contains air or oil. The pixels are topped by another sheet of plastic—this one containing a transparent electrode—leaving a 3-µm gap between it and the polymer pixel.
When there is no voltage between the plastic sheets, the pigment will stay inside the hole, essentially invisible to the naked eye. But when a voltage is applied, the pigment is pulled out of the hole and spread out along the glass, revealing its rich color to the viewer. The air or oil surrounding the pixel prevents the pigment in one pixel from spilling into another. Switching off the power lets the pigment recoil back into the hole.
If they meet their potential, electrofluidic displays ”would be the best technology there is,” says Russell J. Schwartz, vice president of color technology at Sun Chemical. ”It’s got durability, it has brightness of color, it has video speed, it has very low power consumption. So what am I missing?”
Michael Sinclair, a principal researcher working on displays at Microsoft who was not involved in the research, says electrofluidic displays are a novel idea. ”The fact that it is a reflective display is a big plus,” says Sinclair. ”Competing against the less than 10 percent efficiency of today’s LCD and their backlights, this technique ought to be a tremendous power-efficiency improvement.”
But Sinclair points out that researchers still need to work on a gray scale. And, he notes, if this new technology is to compete with the likes of the E Ink display in the Kindle, the Ohio engineers will have to find a way for the display to hold an image even when the power is off—a property called bistability.
Response time is another challenge. The current prototype has an average response time of just over 30 milliseconds, barely fast enough to display video. But the researchers say they have identified ways to improve the design that would theoretically decrease the response time to less than 1 ms.
Jason Heikenfeld, who led the research and is director of the Novel Devices Laboratory, says electronic paper would be only one of many possible applications. There is also potential for rollable displays, adaptive camouflage, and even cellphone cases that can change color on the fly, he says.
Heikenfeld says the reason this design works so well is that there is nothing between the viewer and the pigments except a pane of glass. ”You basically get to see the pigment without any losses, any polarizing filters. You actually get to look straight at the pigment,” he says.
Heikenfeld and some partners have formed a start-up company, Gamma-Dynamics, in Cincinnati, to develop electrofluidic technology. Prototypes may roll out in about three years, he says, followed by the commercialization of some of the simpler applications.