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Flex Appeal: Researchers Create Carbon Nanotube Muscles

Researchers test a new buff and tough material that expands or contracts when an electrical charge is applied and is able to withstand blazing heat and freezing cold
carbon nanotube, artificial muscle
carbon nanotube, artificial muscle



© MEI ZHANG

Researchers for decades have been developing polymers and other materials they hope to someday use to create artificial muscles that, when given an electrical charge, mimic the real thing more cheaply and effectively than the hydraulic systems and electric motors used today. A group of scientists at the University of Texas at Dallas' Alan G. MacDiarmid NanoTech Institute reports in Science today that they have demonstrated a fundamentally new type of artificial muscle, consisting almost exclusively of carbon nanotubes, which can operate at extreme low temperatures that would cause other artificial muscles systems to freeze and at very high temperatures that would cause other muscle systems to decompose.

Study co-author and institute director Ray Baughman, a chemistry professor, says such a lightweight, low-density artificial muscle able to endure temperatures between liquid nitrogen (-321 degrees Fahrenheight, or -196 degrees Celsius) and the melting point of iron (2,800 degrees F, or 1,538 degrees C) could be used to move joints, arms and other components of structures for space, aerospace, and planetary exploration, where a hostile environment prohibits use of any other type of actuating material.

Although artificial muscles generally operate on the same principal as animal muscles, the carbon nanotube artificial muscle is not likely to be used in prosthetic limbs or to replace tissue. "The high voltages used for actuation eliminate the possibility of tissue replacement," Baughman says, adding that prosthetic limbs do not need the rapid response rate or ability to endure extreme temperatures that the new material possesses. Other types of artificial muscles, particularly those that transform the chemical energy of fuels into mechanical energy are better suited for prosthetics, he adds.

The new artificial muscle is actually a transparent "aerogel" sheet (so called because most of the volume in the sheet is either air or vacuum). The aerogel consists of aligned carbon nanotubes that run through the material: The sheet's "specific strength" (strength divided by density) exceeds that of the strongest steel plate when an attempt is made to stretch it in the same direction that the nanotubes are aligned, Baughman says. However, the material does stretch more easily when pulled in a lateral direction. "This material has these properties whether or not it is charged," he adds.

"The main goal of Ray's group is to look at different materials, see if they can get motion and force out of them, and then see how far they can push them," says John Madden, an associate professor of electrical and computer engineering at the University of British Columbia in Vancouver.

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.

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