Stretchy circuits promise elastic gadgets

[Blín D'ñero]

The first elastic, foldable, integrated silicon circuits could take previously brittle electronics to new locations, including the surface of the human brain.

John Rogers of the University of Illinois at Urbana-Champaign, US, and colleagues made flexible silicon and plastic circuits that are just 1.5 micrometres thick.

"Making it thin makes it bendable, just as a piece of paper is bendable whereas a piece of wood is not," Rogers says. The silicon in the circuits is just one crystal thick.
Until now integrated circuits had been limited by a dependence on much thicker, brittle silicon wafers.

As well as being thin, the circuits are designed so that the plastic, and not the silicon, absorbs most of the stress when the chips are bent. The final product is a chip so flexible it can be folded around the edges of a US one cent coin, Rogers says.

Accordion bellows

To make the elastic circuits, the team binds the silicon wiring to a thin sheet of rubbery plastic that has been stretched out to be approximately 15% wider and longer than it was before.

Once the two materials are bound together, the researchers release all tension from the rubber. "It snaps back to its pre-stretched state and buckles with the attached circuit like an accordion bellows," Rogers says. A video (see right), shows this process happening under a microscope.

Although the deformations look violent at the scale shown in the video, the buckling is actually a gentle wave formation, says Rogers. The performance of the circuits is unaffected and they can be stretched by up to 15 percent without fracturing.

"People have talked about foldable and stretchable integrated circuits for decades but nobody has been able to do this," says Zhenqiang Ma, of the University of Wisconsin in Madison, US, who wasn't involved in the study but is working on developing similar circuits. "This will not completely transform the integrated circuited world, but it will fill an important gap, where regular semiconductors can not go."

Seizure prediction

Flexible circuitry could help efforts to create clothing containing wearable gadgets. Rogers says his circuits could also be useful inside the human skull and is collaborating with Brian Litt and colleagues at the University of Pennsylvania, Philadelphia, US, to take the elastic circuits there.

"We'd like to have an electric circuit that could wrap around part of the brain and detect signal patterns to predict the onset of seizure before it happens," Rogers says.

Flexible electronics made without silicon from semi-conducting plastics are also in development. But Rogers says flexibility alone is not enough to wrap circuits around rounded objects like the brain.

"You can't take a sheet of plastic and wrap a brain, you really need stretchability," he says. Silicon-based circuits also benefit from using an already established material, Rogers adds.

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