Flexible substrates harness carbon

PORTLAND, Ore.— Switching to a new formulation of cabon nanotube ink can enable the fabrication of smart devices that are flexible, fast, cheap and extremely durable, according to new results at the U.S. Department of Energy's Lawrence Berkeley National Laboratory.

Berkeley Lab said the new material could be used for displays as well as for sensors that monitor cracks in bridges, buildings and planes, for medical bandages that actively treat infections, for disposable food packaging that detects spoilage, and to turn textiles into solar panels.

Flexible electronics deposited with ink-jet printers on plastic substrates can result in smart devices that can be rolled, folded or even disposed of after a single use due to their potentially ultra low cost. Unfortunately the organic inks currently used to print flexible electronics are slower and biodegradable in the presence of air.

To demonstrate the prowess of its new semiconductor-enriched carbon-nanotube ink, Berkeley Lab demonstrated and artificial electronic skin (e-skin) installed on a smart baseball that senses the throwers grip for potential sports applications.

"We have produced mechanically flexible and stretchable active-matrix backplanes, based on fully passivated and highly uniform arrays of thin film transistors made from single walled carbon nanotubes," said Ali Javey, a faculty scientist in Berkeley Lab's Materials Sciences division and a professor of electrical engineering at the University of California-Berkeley. He performed the work with researchers Kuniharu Takei and Toshitake Takahashi.

The new ink enables lithography-free thin-film transistors to be fabricated that are much faster than the organic transistors gracing other flexible electronics implementations, according to Javey. Made possible by a slurry of 99 percent semiconducting nanotubes used as ink, Berkely Lab's process allowed the prototype baseball to sense the pressure being applied to its surface by the fingers holding a grip.

Fabricated on a flexible polymide substrate, the e-skin used a honyeycomb of laser-cut hexogonal holes to make the substrate stretchable as well as flexible. A total of 96 TFT sensors were fabricated on a 24 square centimeter substrate stretched over the baseball.


Optical image of flexible and stretchable thin film transistor array covering a baseball shows the mechanical robustness of this backplane material for future plastic electronic devices.

Flexible substrates harness carbon