ARTIFICIAL MUSCLE: This scanning electron micrograph shows a carbon nanotube sheet being drawn from a cluster of nanotubes (also referred to as a nanotube forest). An artificial muscle made from a sheet of these carbon nanotubes can operate at extreme low temperatures that would cause other artificial muscle systems to freeze and at very high temperatures that would cause other muscle systems to decompose. Image: © 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.