Chock-full of transistors, the average circuit board is a rigid and delicate thing. Such stiff circuit boards are fine for computers and other large, stationary devices. But engineers are pushing to weave electronics into the objects all around us--including our clothes--and doing that requires flexible circuits. Some circuit boards can bend, but they don't twist or stretch. Now, a Japanese team has produced a rubbery, stretchy conducting material--the first step toward building a flexible circuit.
To do it, Takao Someya, an electronics engineer at the University of Tokyo, and his team mixed tiny tubes of carbon known as nanotubes with a polymer. The nanotubes carry the electricity, and the polymer provides the flexibility. To get the technique to work, the researchers had to overcome several obstacles. For example, the nanotubes attract one another so strongly that it's difficult to keep them from clumping.
So first, Someya and colleagues made the carbon nanotubes much less mutually attractive by mixing them into a substance called an ionic liquid. The treatment turns the nanotubes into a black, pasty concoction the researchers call bucky gel. (The molecular structure of nanotubes resembles the famous geodesic domes designed by Buckminster Fuller.) Next, they mixed the bucky gel with a rubberlike substance called a fluorinated copolymer and poured the mixture onto a glass plate. Last, Someya's team coated the substance with silicone rubber and punched tiny holes all over the matrix to increase its flexibility.
The resulting material looks a bit like a woman's nylon stocking, and Someya says it can be stretched by up to 38% of its original length without loss of conductivity because enough of the nanotubes stay in contact to continue to carry electricity. That's nearly four times more elastic than any other conducting substance, he says, and about 100 times more conductive than any other known elastic material. And that's just the prototype. "We believe there is much room for further improvement in elastic conductors," he says.
It's an important finding, says materials scientist John Rogers of the University of Illinois, Urbana-Champaign. For example, he says, any attempt to integrate electronics with the human body requires flexibility that doesn't hinder movement, and this can't be achieved with conventional devices. "Fully stretchable electronics is the best option for this broad area," Rogers says. Possible applications for the technology include large, stretchable video displays, artificial skin, and electronic books in Braille for the blind.