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 common 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” is actually a bit of a grab bag that 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 to making artificial muscles uses materials called shape memory alloys, such as a mixture of nickel and titanium. But these alloys can cost up to $5000 per kilogram. Even more powerful artificial muscles have been made from yarns spun from hollow carbon fibers called single-walled nanotubes (SWNTs). 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 made with SWNT yarns. Along the way, he and his students learned that if they twist their yarn until it coils, they can make a powerful rotating motor. The effect is similar to the way a twisted rubber band turns the propeller on a toy plane. But in this case, Baughman’s team could zap the yarn with an electric current to wind the motor back up.
That success got Baughman and his students thinking. In their yarns, most of the SWNT 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. That same alignment also exists in conventional plastic fibers, such as those made from nylon, where the individual polymer chains in the plastic line up along the length of the fiber. So Baughman and his students decided to explore whether everyday plastic fibers could act as artificial muscles.
They succeeded beyond their wildest hopes. Baughman, along with colleagues in Texas, Australia, and China, twisted plastic fibers and threads into yarns. Then when they applied heat, they found that the yarns contracted by up to 50%, a result they report today in Science. And cooling the plastic muscles returns them to their original length. Natural muscles, by comparison, only contract by 20%. Twisting together a bundle of polyethylene fishing lines, whose total diameter is only about 10 times larger than a human hair, produces a coiled polymer muscle that can lift 7.2 kilograms, the team found. Operated in parallel, an arrangement that increases their power and is similar to the way natural muscles are configured, a hundred of these polymer muscles could lift about 725 kilograms, Baughman says. Producing this force requires only off-the-shelf 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, California. “They’ve taken inexpensive materials and basically turned them into a gold mine.”