The aerogel sheet, about 20 microns (one micron equals about 40 millionths of an inch) thick when initially produced and about 50 nanometers (one nanometer equals 40 billionths of an inch) thick when made more dense, can expand up to three times its original size when positive voltage is applied (anything more would damage the material), and shrink back down to its original size when the juice is shut off. This expansion comes from the repulsive forces the carbon nanotubes generate (pushing them farther apart) when electricity is applied to the material. The team has "created a material that no one else has created," says Madden, who wrote about the research in a Science article that accompanies the study. "Not only is it light, it's very strong in one direction, but in the other direction it has almost no stiffness. I've never seen anything with as much variation between the directions."
Since the nanotubes diffract light perpendicular to their alignment direction, the ability to change the density of aerogel sheets and then freeze them in this shape can be used to improve "nanotube electrodes used in organic light-emitting displays, solar cells, charge stripping from ion beams, and cold electron field emission," according to the Science report.
Longer term, the artificial muscle's temperature-resistant qualities could prove useful when exploring other planets. "You may need this if you want to change the orientation of solar cells used to power a spacecraft while it's traveling through the low temperatures of space," Baughman says. In addition, a satellite, exploratory rover or spacecraft using artificial muscle made from the aerogel rather than a hydraulic system or motor made from steel would be much lighter and require less energy to launch into space. "For applications where you want to minimize weight," he says, "that's where the aerogel would do well."
The material's low density, however, would be less of an advantage for buildings on Earth, because it would take a lot of aerogel to do the work of a steel beam in a building, for example. "The beam [made from aerogel] might be lighter," Madden says, "but it would have to be a lot bigger."
Baughman's is the latest example of an artificial muscle; several other types have been in the works for years. SRI International and Japan's Hyper Drive Corp. in December tested a jointly developed buoy-mounted, ocean wave–powered generator off the coast of Santa Cruz, Calif., that used an accordionlike device inside, made from an electroactive polymer artificial muscle (EPAM), to create mechanical energy that was converted into electricity. And in 2005 high-school student Panna Felsen (17 at the time) bested three different artificially muscled robotic arms in an arm wrestling competition. A robotic arm manufactured by Environmental Robots, Inc., (ERI) in New Mexico put up the best effort, surviving 26 seconds, whereas arms from Virginia Polytechnic Institute and the Swiss Federal Institute of Technology's Laboratories for Materials Testing and Research lost in less than four seconds each.