The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications.
A team of material scientists at the University of Texas at Dallas have just discovered a new way to create powerful artificial muscles—synthetic sinew that forcefully expands and contracts on command—from low-cost, everyday fibers such as fishing line and high-tension sewing thread. In a study published today in the journal Science¸ the researchers described how they're doing it: by twisting the materials into springy and energy-dense coils.
Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.
The researchers take polyethylene or nylon string, the plastics that can make up fishing line, and twist it under high tension over and over again. Once the plastic can't twist any more, it starts to coil up on itself like a curled telephone cord. This tightly bound coil is then temperature treated so that it's locked into place.
When this coil is heated, the plastic tries to untwist. But this causes the entire thing to compress. "At first it seems confusing, but you can think of it kind of like a Chinese finger-trap," Baughman says. "Expanding the volume of the finger-trap, or heating the coil, actually makes the device shorten." And this is compounded by the fact that the molecules in polyethylene and nylon string also naturally contract lengthwise ever-so-slightly when they're heated. Together these effects make the plastic coil contract with incredible power—like a muscle.