Friday, December 08, 2006

Ultrastrong Carbon-Nanotube Muscles

Artificial muscles made from carbon nanotubes are 100 times stronger than human muscles.

By Kevin Bullis

Carbon-nanotube yarns such as this one can be used as powerful actuators. Credit: Science

By spinning carbon nanotubes into yarn a fraction of the width of a human hair, researchers have developed artificial muscles that exert 100 times the force, per area, of natural muscle. This is according to Ray Baughman, director of the Nanotech Institute at the University of Texas at Dallas, who presented the research in Boston last week at the Materials Research Society conference.

Artificial muscles--actuators based on such materials as certain types of metals and polymers that shrink, grow, or change shape--are useful for prosthetic limbs, microscale machines, and robots. "Our biggest problem right now [in developing artificial muscles] is [that] the level of force being generated is not high," says Yoseph Bar-Cohen, senior research scientist at NASA's Jet Propulsion Laboratory, in Pasadena, CA. "Carbon nanotubes potentially can create enormous force."

Baughman has previously developed carbon-nanotube actuators that convert energy in hydrogen into mechanical force. He uses a configuration similar to a fuel cell in which catalyst-coated carbon-nanotube electrodes also act as actuators, changing size in response to electric charge. Unfortunately, sheets of the carbon nanotubes employed in these experiments do not make good use of the carbon nanotubes' strength. Indeed, finding a carbon-nanotube material that utilizes the extraordinary strength of individual nanotube molecules has been a research challenge.

In Baughman's latest work, done in collaboration with John Madden at the University of British Columbia, the researchers made actuators out of carbon-nanotube yarns. The yarns are created by first growing densely packed nanotubes, each about 100 micrometers long. The carbon nanotubes are then gathered from a portion of this field and spun together into long, thin threads. The nanotube yarn can be just 2 percent of the width of a hair--not even visible--but upwards of a meter long. According to Baughman, spinning these threads was "like hauling in a fish with an invisible line." In his conference presentation, he described yarns that could support loads 150 times greater than nanotube papers could.