Superstrong Artificial Muscles and More From New Nanotube Material

19 March 2009—A revolutionary new material, light as air yet stronger than steel, could be used to make artificial muscles for robotic explorers operating on the broiling plains of Venus or the ice sheets of Europa, scientists say. The material could also be used for more down-to-earth applications, such as improving solar cells or organic LED displays, powering industrial robots, or reinforcing airplane fuselages.

The material, described in the 20 March issue of Science, is an aerogel—a porous, low-density solid—made from carbon nanotubes, and it has an eye-popping list of special properties. Its density is approximately 1.5 milligrams per cubic centimeter, only slightly denser than air. In one direction (along the axis of the tubes), it’s stiffer than steel. But when a voltage is applied across the aerogel [see video], repulsive forces between the nanotubes rapidly triple the material’s width, causing it to expand at 37 000 percent per second. That’s 10 times as far and 1000 times as fast as natural muscle can move, and the material does so while generating 30 times as much force as a natural muscle.

What’s more, ”[the muscle] can operate at extreme temperatures, where no other type of artificial muscle can operate,” says Ray Baughman, director of the MacDiarmid NanoTech Institute at the University of Texas at Dallas, where the material was created. Baughman says he knows of just one type of actuator material that operates up to about 500 °C. Most work in a range somewhere between about room temperature and 100 °C. By contrast, his artificial muscle can work from about –190 °C, colder than liquid nitrogen, to over 1600 °C, above the melting point of steel, without growing brittle in the cold or decomposing in the heat.

The aerogel is made of multiwalled carbon nanotubes, concentric tubes of carbon atoms with an outer diameter of about 12 nanometers and about nine walls per nanotube. The researchers used chemical vapor deposition to make forests of these nanotubes, then drew them into sheets with the majority of the nanotubes aligned in the same direction [see video].Soaking the sheets in ethanol and allowing them to evaporate caused them to collapse to a state 400 times as dense and about 50 nm thick. The researchers can layer the sheets on top of one another to make a stronger actuator.

Yoseph Bar-Cohen, who heads the group at NASA’s Jet Propulsion Laboratory that’s trying to develop advanced actuators for space applications, calls the material ”very promising and exciting,” though he worries it may prove difficult to build an actuator out of something so light. Acting as a muscle for a robot, it could handle the 460 °C surface of Venus or the –200 °C found on Jupiter’s moon Europa. The material’s low mass is also appealing to an agency always concerned about launch weight. ”It can be very useful for planetary exploration,” Bar-Cohen says.

The Texas scientists also claim a number of other potential applications for the nanotube sheets beyond their use as artificial muscles. Sheets can be stretched by applying a voltage, then locked into place by putting them on a substrate and allowing the natural attraction between molecules to make them stick. This could make the sheets useful for transparent electrodes used in displays or as electron-collecting conductors in solar cells. Placed between two panes of glass, the sheets could be sources of polarized incandescent light [see video]; put them in your window and flick a switch and they’ll light up your room while blocking the view from outside.

The nanotube sheets could be used to reinforce other materials, particularly in aerospace applications that require strength and light weight. Baughman says such uses will require improvements in manufacturing to produce the necessary amount of material [see video].”One ounce could cover an acre,” he says. ”If you want to make an aircraft wing out of this stuff, we’re very far away from that.”