Light-as-Air, Heatproof Nanotube Muscles

New nanotube sheets combine unique properties.

Carbon-nanotube ribbons developed by researchers at the University of Texas at Dallas are stronger than steel, as stretchy as rubber, and as light as air. The ribbons, which are made of long, entangled 11-nanometer-thick nanotubes, can stretch to more than three times their normal width but are stiffer and stronger than steel or Mylar lengthways. They can expand and contract thousands of times and withstand temperatures ranging from -190 to over 1,600 °C. What’s more, they are almost as light as air, and are transparent, conductive, and flexible.

Ballooning tubes: This illustration shows the inside of a two-millimeter-wide strip of an airy carbon-nanotube material that expands to more than three times its width when a five-kilovolt voltage is applied to it.

The material, presented in the journal Science this week, was developed by Ray Baughman, director of the Nanotech Institute at UT Dallas, who is developing various kinds of carbon-nanotube-based “artificial muscles” for prosthetics and robotics. These materials change shape and size in response to electrical or chemical signals; some expand by up to 1 percent and exert 100 times more force than natural human muscle over the same area.

The new actuators, on the other hand, expand by up to 200 percent but generate small forces per unit area, making them less than ideal for many applications, including robotics. However, their novel properties, especially their temperature range, could open up exciting new applications. “No other actuator technology can provide actuation at these extreme temperatures,” Baughman says. “And these actuation rates are giant.”

Qibing Pei, a materials-science and engineering professor at the University of California, Los Angeles, believes that the material could be a good candidate for shape-changing aircraft wings. Pei has developed polymer actuators that expand by up to 400 percent and work between -40 and 200 °C.

Since the nanotube ribbons are ultralight and can handle extreme temperatures, they could perhaps also be useful for making shape-shifting spacecraft parts, says Yoseph Bar-Cohen, a senior research scientist at NASA’s Jet Propulsion Laboratory, in Pasadena, CA. “It’s exciting that the material behaves this way over a wide temperature range,” he says. “On one side we have Mars, and on the other side we have Venus. Their temperatures are within the performance range of this material.”

But for now, Baughman and his colleagues are focusing on optical applications for the material. Because carbon nanotubes are highly conductive, the flexible sheets could perhaps be used to make electrodes for solar cells and organic light-emitting diodes with controllable transparency and conductivity. “For that application, you want to tune the density of carbon nanotubes per unit area,” Baughman says. “That determines how much transparency the sheet has.” In the Science paper, the researchers show that the ribbons can be deposited on a silicon substrate in their expanded, more transparent state. The ribbons also diffract light so that they could perhaps prove useful in optical communications. Changing their dimensions sends different wavelengths of light in different directions.

The researchers make the material by growing entangled carbon nanotubes and then pulling intertwined nanotube bundles into ribbons. When a voltage is applied to the strips, the nanotubes become charged and push each other away, making the material expand. It normally returns to its original state when the voltage is removed.

The ribbons will probably still need to generate more force before they are practical for many applications. Right now, they generate 32 times as much force per unit area as heart muscles, which is a lot for their nanoscale dimensions, says Ian Hunter, a professor of mechanical engineering at MIT. However, electroactive polymers generate up to eight times as much force per unit area as the nanotube sheets. “For artificial muscle, you need a large change in force coupled with a large change in length,” Hunter says.

Polymer actuators also need just a few volts to contract. The ribbons, in contrast, require three to five kilovolts, which Hunter says is too high for use in humans and higher than ideal for robotics. However, he adds that “the nanotube ribbons will find many important applications because they change dimensions much faster than existing polymer actuators.”

The ultralow density of the sheets could be the reason why they do not generate large forces. John Madden, an electrical- and computer-engineering professor at the University of British Columbia in Vancouver, Canada, suggests that one way to increase the force that they supply could be to make the sheets denser and increase the degree of interlocking between the nanotubes.

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