Tiny muscles pull a big punch

Coated carbon nanotubes form the basis of smart new material

Beefing up some muscles doesn’t take steroids or exercise — paraffin wax will do. Incorporating wax into artificial muscles spun from carbon nanotubes gives them superior flexing power, a discovery that could lead to smart materials such as fabrics that respond to environmental changes.

KNIT ME A MUSCLE Coating a yarn spun from carbon nanotubes with wax (scanning electron microscope image shown) or other materials that expand upon exposure to heat or a different stimulus creates tiny, lightweight, exceedingly strong muscles. Courtesy of Science/AAAS

The new artificial muscles, about twice the diameter of a human hair, can lift 175,000 times their weight, outperforming mammalian muscles and all previously made artificial muscles (SN: 12/4/10, p. 20), a team reports in the Nov.16 Science.

“This is a new kind of smart material, different from all the rest,” says Mark Schulz, a mechanical engineer at the University of Cincinnati who was not involved in the research.

An international team of scientists grew slender carbon nanotubes — each about 9 nanometers across, or nine billionths of a meter — and drew them out into a sheet, which they slathered with paraffin wax. Then, the researchers spun the material into a yarn. Anchoring the ends of a length of yarn and twisting it into a tight coil made the muscles ready to spring, akin to the twisted rubber band that powers a balsa wood model airplane.

Next the scientists used a battery or a flash of light to apply heat to the muscles, slightly melting the wax, which swells. The yarn expands in diameter while contracting in length, causing a twisting motion. Experiments with an attached paddle found the torque created was better than a good-sized electric motor, says Ray Baughman of the University of Texas at Dallas, who led the research.

The yarns can also be embedded with other materials that expand in response to factors other than heat. Nanotubes coated with palladium, for example, flex when they absorb hydrogen gas.

Like regular yarn and regular muscles, the new material has many potential uses, says Baughman. The artificial muscles might help operate cameras, toys, or small robots. They might also prove useful as environmental sensors or be woven into clothing that seals up or becomes more porous on command.

“These muscles are extraordinarily simple,” he says, “but they have extraordinarily high performance.”

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