The ultimate goal, says Darren Bischoff, senior marketing manager at E Ink, is "having this lightweight, rollable display that has next-generation electronics built into it so you can include the proc-essor, memory -- everything you need -- into this display material."
Building an entire PC on a flexible substrate would be tricky, although Brug thinks it's technically possible. Some of HP's earliest research on flexible electronics was done on memories, he says. "We won't be doing microprocessors anytime soon, but it won't be too long before the pieces start coming together," he predicts. Bob Street, a senior research fellow at Palo Alto Research Center says the issue isn't whether it can be done, but whether such a device can be created cost- effectively. He notes that attempts to print the antennas and transistors for RFID tags on a single sheet have so far been more expensive than using silicon and bonding the antenna to the chip. A flexible PC would be much more complex to engineer. "I'm sure you can do it," Street says. "The question is whether it is cost-effective versus doing it with crystalline silicon."
Bending OLEDs
OLEDs could be a replacement for traditional color LCDs, but most research with OLEDs is focused on rigid displays. U.K.-based Cambridge Display Technology is putting jet-printed OLEDs on polymers, but Chief Technology Officer Jeremy Burroughes says the primary benefit isn't flexibility but the fact that such displays are thinner, lighter and less breakable than traditional LCDs. "If your laptop weighed less and was less likely to break if you dropped it, you would be a happy person to have it," he says, adding that his company has no plans for flexible displays. But don't expect to see OLEDs in laptops any time soon. While the product life span has improved, blue LEDs are still only good for about 12,000 to 15,000 hours, researchers say. That's enough for a PDA or cell phone display but not for laptop screens or desktop monitors.
Universal Display Corp. (UDC), however, is focused exclusively on flexible OLED technology using an active matrix on a thin foil substrate. "Plastics aren't quite ready for use with OLEDs," says Janice Mahon, vice president of technology commercialization. UDC has demonstrated flexible quarter-VGA (320-by-240-pixel) color displays and is working on a rollable display for a universal communication device funded by the U.S. Department of Defense. UDC is also experimenting with wearable displays embedded in clothing, but it is starting with conformable displays. "The next step is to get it so you can bend \[the display\] a thousand times," he says. While others think flexible OLEDs could be 10 years off, Mahon thinks UDC's technology will be viable much sooner. "In a couple of years, there will be a flexible OLED in the marketplace," she predicts.
Hewlett-Packard is working on both flexible bi-stable and OLED displays on polymer. Its first bi-stable display, a 125-color, 128-by-96-pixel passive-matrix LCD developed in its lab in Bristol, England, is nearing production for use in consumer devices such as digital photograph viewers and e-book readers.
HP's "emissive ink" technology is a variation on the OLED that uses nanocrystals called "quantum dots." The crystals, 2 to 10 nanometer in size, are mixed into polymers and emit a color that's more pure than traditional active-matrix LCDs can deliver. "Because they're so small, they can only emit one color. We put these into the polymer, which conducts the electrons and holds the nanocrystals, and they emit a photon," says Jim Brug, imaging materials department manager at HP Laboratories.
"Fabricating good quality transistors on plastics is one major development. The other is to show that quantum dots can emit very high quality light in polymer materials," Brug says.
Beyond flexible, stretchable computing
If flexible displays sound like a stretch, John Rogers' research will sound even more fantastic.
The materials scientist at the University of Illinois Urbana-Champaign has already created electronic transistors and diodes that stretch. Now he is busy working on stretchable circuit boards that could lead to wearable electronics. "The next generation is to go beyond flexible to stretchable," he says.
Products created from stretchable electronics could be used in "any kind of system where you need stretchability," Rogers says. One example: smart surgical gloves with embedded sensors that provide feedback on the patient's condition for any point on the body that the surgeon touches. Another application, he says, would be for a structural health monitor that wraps around an airplane wing to measure stresses and strains.
To achieve stretchability, Rogers combines a relatively inflexible substance for the circuits, silicon, with the ultimate stretchy substrate: rubber. Rogers' process requires first stretching the rubber and then bonding an ultrathin (0.1-micron) silicon circuit to it using a stamping process. When the rubber returns to its unstrained length, the circuit compresses and buckles into an accordion-like pattern, but it doesn't break. Although silicon is normally inflexible, the extreme thinness of the circuit allows it to bend.
"What we've done is achieved bendability in a material that's inherently not bendable. We've made it bendable because we've made it thin," Rogers says. When combined with rubber, the bendability translates to stretchability. The material can be stretched and relaxed over and over again.
Rogers helped found Printed Silicon Technologies Inc. to commercialize the technology. So far, he has demonstrated stretchable transistors and diodes -- the basic building blocks of circuits -- but Rogers doesn't think circuit boards will be much of a stretch.
"There's no fundamental challenge to that, just tricky engineering. We'll have a demo by end of year," he says. But actual products built with stretchable electronics are still at least three to five years away, he says.
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