Electrofluidic Technology Enters E-paper Arena

Innovations in the electronic paper arena seem to be coming fast and furious these days. The latest news concerns researchers at the Novel Devices Lab at the University of Cincinnati, who have recently demonstrated a new display technology, termed “electrofluidic,”  that claims higher brightness than other technologies currently available on the market.  The electrofluidic nomenclature is chosen because the mechanism involves charge-induced movement of liquids through microfluidic cavities.

While most electronic paper devices typically have a 40% (E Ink) to 50% (electrowetting) reflectance rate, the new device is said to reflect 55% of ambient light. For the future, that number could be improved to approach 85%, which is the reflectance ratio of paper printed with traditional inks. This could be an important breakthrough, because one of the stumbling blocks to wider acceptance of e-paper is that it still doesn’t approach the look of conventional ink on paper.

How It Works

Images of electrofluidic displays and pixels. a, Bright-field image of a 170 DPI direct-drive demonstrator with _30,000 pixels (left) and pixel dark-field images (right). b, Time-lapse images of 500-mm-square pixels. c, Hexagonal pixel with two separate ducts, one for the reservoir and one for the pixel border. d, Hexagonal pixels in which the reservoir comprises only 5% of the viewable area. (Image courtesy of Nature Photonics)

Rather than attempting to improve on existing technologies to achieve the desired brightness, the researchers took a new approach which leverages the use of high-performance pigments that are used in the traditional printing process. (One of the participants in the research was Sun Chemical Corporation, the largest global supplier of printing inks and pigments.) In this respect, the process differs from electrowetting, which uses colored oils.

A foundation layer of polymer has reservoirs which contain aqueous pigment dispersions (in the black-and-white display, carbon black was used for optical density) and surface channels.  According to Jason Heikenfeld, the inventor of the technology, if the chemical dispersion is right, any pigment can be used. This would be an important advantage for future projects, for example, for military camouflage projects, which use special pigments.  These reservoirs represent 5-10% of the visible area; the channels 90-95% of the visible area.  All of the features can be inexpensively formed in a single photolithographic or micro-replication step. An aluminum film, then an indium tin oxide (ITO) transparent electrode layer is deposited respectively. The aluminum layer is designed to reflect light.

At rest, the pigment dispersions are hidden from view. When voltage is applied to the device, electromechanical pressure pulls the dispersions out of the well into the channels, so that nearly the entire device area will exhibit the coloration of the pigment. Heikenfeld notes that the pigments are right beneath the viewing glass and the brightness difference compared to other display technologies is visually obvious. Because the pixels are so small, the device has a resolution of 300 dpi. This is actually a higher resolution than many e-readers currently available. When power is cut, the dispersions again become hidden.

Color To Come

Images of pigment droplets and an overlay of three CMY prototypes. (Image courtesy of Nature Photonics)

The present architecture of the demonstration model is black and white and does not provide grey scale. However, the researchers are aware of and addressing the challenges to moving the technology to color.

The goal is to develop a prototype for electrofluidic displays that can achieve performance equal to the brilliance of pigments printed on paper.  One approach would be to use conventional RGBW (red, green, blue, white) colored filters. This would reduce the reflectance of the display by about half, but it would still be equal to the 40% of current electrophoretic e-readers. According Heikenfeld, two electronic layers would be used for color displays. This would allow a CMY subtractive approach similar to printing ink on paper. While this would need double the power of a single plate, it would still be sufficient to be attractive to consumers. An additional advantage is the fast switching speed. By tweaking variables such as geometry, surface tension and viscosity, Heikenfeld says the speed could be maximized to a sub-millisecond. The displays can be manufactured using existing processes for producing LCDs.

Gamma Dynamics

A start-up called Gamma Dynamics has been established to commercialize the technology. Right now, it consists of John Rudolph as President and Heikenfeld as Principal Scientist. A third team member, the CTO, will probably be formally announced this summer. The company has the support of strategic partners Sun Chemical (see above) and Polymer Vision, to expedite manufacture of commercial products. “Why reinvent the wheel,” said Heikenfeld. “Our partners already have much of the expertise needed for such a project.”

While commercialization may still be a few years down the road, the technology has significant potential for developing a full-color flexible display which approaches or equals the reflectance and color saturation of traditional printed paper. Moreover, switching speeds are more than capable of handling video. The development of electrofluidic technology is an important breakthrough in the electronic paper area and one that definitely bears watching for further progress.

Here are some videos that demonstrate what the little ink pores look like in action on a micro scale.

By Linda M. Casatelli

Electrofluidic, electrowetting