Sample of e-paper, demonstrating its flexibility. Reprinted by permission from Macmillan Publishers Ltd: Nature Communications, Volume:3, doi:10.1038/ncomms2175, copyright 2012. Click image to enlarge. 

Since the introduction of e-readers, consumers and manufactures have dreamt of bright, flexible electronic paper without a battery-draining backlight. Now, researchers at the University of Cincinnati have demonstrated a proof-of-concept in a recent issue of Nature Communications  which, according to the principal investigator Professor Jason Heikenfeld, is “a great step toward the holy grail of bright and colorful electronic paper display, eventually even rollable displays.”

The new type of e-paper uses microfluidics to quickly transport ink, generating brighter images than e-papers currently on the market.  The electrofluidic imaging film, as the authors call it, consists of a highly porous polymer membrane covered by a layer of clear oil with an ink well underneath.  The entire device is sandwiched between two thin-layer electrodes.  While, microfluidic display pixels have been shown before, this new approach is highly novel in both the physics of operation and improved performance.

A potential applied to the top electrode facilitates the electromechanical transport of ink to the top of the white porous film to reveal an image, perhaps a word or a picture.  The voltage alters the local surface pressure which drives ink flow through the pores to release pigment while the clear oil fills the empty bottom well.  This microfluidic film transports the ink almost 100 times faster than traditional electrophoresis methods. To erase the image, a second voltage applied to the bottom electrode withdraws the ink behind the white film to remove pigment.  The clear oil returns to the front of the device and the screen looks white again.

To speed the withdrawal of ink from the front of the film, the researchers developed a sophisticated but simple technique to split conglomerated volumes of fluid.  “Normally, once you merge fluid you can’t split it,” Heikenfeld explains, “You have infinite pressure to overcome.”  To get around this problem, raised spacers were fabricated in a random pattern along the top of the film to give fluid splitting what Heikenfeld calls “a head start,” allowing withdrawal though multiple pores.  This method provides pixel switching speeds under 15 ms, which makes video a possibility for future devices.

Splitting the ink in such a way eliminates the need for encapsulating ink within a defined area which requires a physical boundary like the pixel borders used in current devices which can dull the screen leaving the background slightly gray.  Since the spacers are clear and the ink can be completely withdrawn behind the film, the white contrast is much higher than in current devices and approaches magazine contrast quality.  This enhanced brightness allows users to see the display in direct sunlight, a quality that is lacking in backlit tablet-style e-readers.

“All of the complexity is captured in a single membrane,” says Philipp Schmaelzle, a researcher in the large area display effort at Xerox PARC, who was not involved in the work described here.  “They basically gave the membrane all of the heavy lifting, which is a very powerful route.”

In addition to increasing speed and brightness, the device is readily fabricated through non-aligned lamination.  The imaging film can be laminated onto the device electronics instead of being built layer-by-layer, which greatly simplifies the manufacturing process. 

The researchers are currently pursuing commercial applications in electronic labeling and hope to incorporate their technology into the next generation of e-readers.  “New consumer electronics technology is a brutal space to be in,” Heikenfeld says, “but were doing everything we can to make this a reality.”

Read the abstract in Nature Communications  here.