Spinning yarn into artifical muscles

The latest high-tech gizmos usually spring from advances made with exotic — and expensive — materials. Not this time.

An international team of researchers report that they've spun plastic fishing line and sewing thread into the most powerful artificial muscles ever created.

Synthetic muscles are already being explored for use in prosthetic limbs, RoboCop-like exoskeletons for soldiers, and humanoid robots. So a sharp drop in price could propel progress in all of these technologies and many others.

The term "artificial muscles" refers to materials that contract, expand or rotate when heated, zapped with electricity, or hit with some other stimulus. The materials return to their original shape when the stimulus is reversed.

One common approach uses materials called shape memory alloys, such as a mixture of nickel and titanium. But these alloys can cost as much as $5,000 per kilogram. Even more powerful artificial muscles have been made from yarns spun from hollow carbon fibers called single-walled nanotubes. But because there is no cheap way to make these nanotubes on an industrial scale, their cost is off the charts.

Ray Baughman, a chemist at the University of Texas, Dallas, has spent years pioneering work on artificial muscles with nanotube fibers. He and his students learned that if they twist their yarn until it coils, they can make a powerful rotating motor.

That success got them thinking. In their yarns, most of the nanotube fibers were aligned along the length of the yarn. That consistent orientation is critical, because it ensures that when an electrical stimulus changes the length of one nanotube fiber, all of its neighbors change in the same way, causing the yarn to contract or expand. So Baughman decided to explore whether everyday plastic fibers could act as artificial muscles.

Twisting polyethylene fishing lines, whose total diameter is only about 10 times larger than a human hair, produces a polymer muscle that can lift 7.2 kilograms, they found. Operated in parallel, a hundred of them could lift about 725 kilograms. Producing this force requires materials that cost about $5 per kilogram.

"These are really exciting results," says Yoseph Bar-Cohen, a physicist and artificial muscle expert at the Jet Propulsion Laboratory in Pasadena, Calif. "They've taken inexpensive materials and basically turned them into a gold mine."

Science/AAAS