Ultrastrong Carbon-Nanotube Muscles

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.

However, much work remains to be done in developing the materials. For one thing, as greater loads are applied to actuators, they can start to exhibit “creep”–that is, they do not completely return to their original state with successive cycles. Baughman says that before these actuators can be useful, creep must be eliminated. “Under load, the cycle is not reversible–you’ve got a little creep. In most actuator applications, you don’t want any creep.”

Another key issue is scaling up from thin individual threads. Although the carbon-nanotube muscles can outperform natural muscles on a per-area basis, exerting 100 times the force, natural muscles are much larger, making them stronger. This scale-up issue has been a challenge for artificial muscles in general, which is why they still can’t beat human muscles in such functions as arm wrestling, Bar-Cohen says.

Despite the challenges, Baughman’s work so far represents important advances for carbon nanotube-based artificial muscles. “[Baughman] has really taken these very far in terms of processing,” says Elizabeth Smela, professor of mechanical engineering at the University of Maryland. “The fact that he can form transparent conducting sheets, yarns, and other materials out of these carbon nanotubes is attractive. Processing is very important. You can have a promising material, but if you can’t figure out how to process it to make things out of it, it doesn’t do you much good.”