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A New Twist on Artificial Muscles

Carbon nanotube yarns produce as much torque as an electric motor.

Researchers have created artificial muscles that can twist 1,000 times more than any suitable material made in the past—a development that could prove useful in robots and prosthetic limbs.

Thin fiber: A scanning electron microscope shows a 3.8-micrometer-diameter carbon nanotube yarn that twists when connected to an electrode and immersed in conductive liquid.

Artificial muscles are typically made from polymers and metals that change size and shape. But to be truly useful, these materials need to twist or rotate when an electric current is applied, and very few such materials created so far can do this.

The new muscles—carbon nanotube fibers spun into a yarn—can produce as much torque, or twisting force, as commercial electric motors.

“This is remarkable,” says James Tour, a professor of chemistry and computer science at Rice University, who was not involved with the work. “To have such torsion in a fiber is fascinating and likely to lead to applications in mechanics that have hitherto been unattainable with any other material. [They] really knocked the ball out of the park on this one.”

The twisty nanotube yarn could open up novel uses. It might help miniaturize electric motors, compressors, and turbines. Tiny pumps based on the rotating actuator could be integrated into lab-on-a-chip devices, which currently use large external pumps. “This is a fascinating new way to provide torsion,” says Ray Baughman, director of the Nanotech Institute at the University of Texas at Dallas. Baughman led the work.

In a paper published on the website of the journal Science today, the researchers show that the new yarn can spin a paddle 1,800 times heavier than itself at 590 revolutions per minute. They demonstrated how a simple device based on this concept could be used to mix two liquids on a microfluidics chip; in a fluid mixer, a 15-micrometer-wide yarn rotated a paddle that was 200 times wider and 80 times heavier than itself at up to one rotation per second.

Mix it up: A 15-micrometer-thick carbon nanotube yarn spins a paddle back and forth to mix blue- and yellow-dyed water.

Baughman and his colleagues have already made carbon-nanotube-based muscles that are 100 times stronger than natural muscle and more flexible than rubber. The researchers created the yarn by first growing forests of carbon nanotubes, each about 400 micrometers tall and 12 nanometers wide. They then spun them together into long bundles that are less than 10 micrometers thick.

To create the twisting motion, the yarn is connected to an electrode and immersed in an electrolyte. Ions from the electrolyte enter the yarn, which causes it to first swell and then contract and rotate along its length.

Boris Yakobson, a professor of chemistry, materials science, and mechanical engineering at Rice University, says he is curious about the energy efficiency of the system, and would have liked to see some measurements or even estimates in the paper. He says with any such motor, some important characteristics are “how much electrical energy is put in, and how much mechanical work can be produced in rotation. In any case, this is fascinating nanoengineering.”

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