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.