Making waves.

Flexible silicon is no longer an oxymoron. Scientists have created thin, wavy silicon ribbons that stretch along with their rubber backing. The technique could lead to comfortable, sensor-filled uniforms that monitor a soldier's vital signs or to electric devices that can wrap around complex shapes such as aircraft wings.

Fashioning a rigid material such as silicon into a thin film can make it bendable but not stretchable, says materials scientist John A. Rogers of the University of Illinois, at Urbana-Champaign. Rogers' team uncovered silicon's flexible nature by accident, he says.

A lab member inadvertently stretched the rubber stamp used to apply thin silicon strips to a plastic backing. When the rubber snapped back, the silicon ribbons buckled along their lengths into a rippled shape. These strips turned out to be 10 to 20 times as stretchy as rigid silicon is.

The ribbons expand and compress much as an accordion bellow might, notes Rogers. His team makes them in thicknesses ranging from 20 to 200 nanometers, widths of a few micrometers, and lengths up to an inch.

To make devices such as transistors and diodes out of the flexible material, the researchers added components such as conductors to thin strips of silicon on a wafer. Then, they transferred the device onto a uniformly stretched rubber backing and released the rubber's strain to introduce waves. The device's electrical properties withstood 100 cycles of stretching and compression, the group reports in the Jan. 13 Science.

Rogers says that the team is now working on squares of silicon that give in two directions, and the researchers are investigating how they might increase silicon's stretchiness by an additional factor of 10.--A.C.