Science / AAAS

The newest material used to make artificial

muscle can lift more than 100 times more than the meaty stuff you have stuck to your skeleton. It forms a sinewy band that's flexible, dynamic, and can be reused millions of times. No, we're not talking about carbon nanotubes, shape-memory metal, or some hyper-strong liquid alloy. This super-material is ordinary fishing line.

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.

"These muscles are something Popular Mechanics readers could easily make in their living rooms," says Ray Baughman, a materials scientist with the team. "The coiling process is actually quite trivial. We're getting high school students to do it—you just have to pay attention to how much tension and weight you apply to the thread you're twisting. I'm not sure why no one else has ever discovered this before."

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.

Despite the simplicity of their design, these muscles are anything but trivial, says Richard Vaia, a materials scientist at the Air Force Research Laboratory who was also not involved the study, "This definitely gives the entire field an entirely new perspective on how to create artificial muscles," Vaia says, "it's a really unique insight into how to combine existing materials with novel architectures."

The University of Texas at Dallas team discovered that their twisting process can be used with a huge number of different kinds threads. By braiding and twisting different threads together and coiling them in different ways, can create an enormous level of variation in the qualities of the muscle, such as how far and at what temperature the muscles contract and expand. Also, by blending in conductive wire or wrapping the muscle with a light-absorbing coating, the researchers can control the muscles' movements with electricity and light instead of direct heat.

The team has already made muscles with rather astonishing properties. "The power density of the muscles is impressive," says Junqiao Wu, a nano-engineer at the University of California, Berkeley, who was also not involved in the study. The twisting architecture is so fundamentally strong that, in the same amount of space, the muscles are more energy dense "than commercial electric motors and even the jet engines of airplanes," Wu says.

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But by far the most impressive aspect of the muscles is their low cost. Because Baughman and his colleagues are using cheaply available resources, pound for pound their muscles are hundreds to thousands of times cheaper than those made from promising but expensive materials like shape-memory alloy or carbon nanotube yarn. This is one reason Baughman expects almost immediate commercialization of this new finding. Although he rattled off dozens of various applications he could imagine for these new muscles, including building facial muscles for humanoid robots and crafting exoskeleton suits—he admits the muscles could find their first practical use in devices like automatically heat-regulated window shutters and ventilation systems, or blended into clothing that reacts to temperature by opening and closing pores in the textile.

One caveat with the new muscles is their relative inefficiency, which the researchers are working on now. But Wu and Vaia say that's no surprise. "Every artificial muscle has its own advantages and limitation," Wu says, "The metrics for artificial muscles are multiple, including things like size, density, speed, power and efficiency. But so far there is no single artificial muscle developed for general utilization in all areas."

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