For years artificial muscles have promised to deliver a more flexible, more durable alternative to electric motors and hydraulic systems. These lab-made actuators are usually created by putting an electrical charge into a piece of polymer or into an aerogel sheet made from carbon nanotubes, causing those materials to expand and contract. This motion could someday be used to power turbines, animate robots or move prosthetic limbs.

In a new study published in the October 14 issue of Science, an international team of researchers describes how they're taking carbon nanotube artificial muscles in a new direction. Scientists at the University of Texas at Dallas' Alan G. MacDiarmid NanoTech Institute have already demonstrated in the lab the ability of these muscle-like carbon nanotubes to flex even in extreme temperatures that would freeze or, on the other end of the spectrum, decompose electroactive polymer-based artificial muscles.

Their latest work—with scientists from the University of Wollongong in Australia, the University of British Columbia in Canada, and Hanyang University in Korea—indicates that these carbon nanotubes can be spun into yarns (see inset to the left) and, when dipped into an electrolyte solution, move in a twisting direction that mimics the stiffness, extension, bending and torsion of natural muscles found in an elephant's trunk and squid tentacles. The researchers found that the yarns can even untwist (moving with the opposite rotation) when the applied voltage is changed.

If this research progresses it would multiply the potential uses for artificial muscles, in particular those made from carbon nanotubes. "Actuator materials producing rotation are rare and demonstrated rotations are small, though rotary systems like electric motors, pumps, turbines and compressors are widely needed and utilized," according to the researchers. Because the carbon nanotube yarns being studied are so slim—thinner than a human hair—there is also the potential to miniaturize such rotary systems.

In one experiment to test the twisting action, the researchers attached a tiny paddle to a carbon nanotube yarn (see images). Using a low-voltage battery as the power source, the electrochemical charge and discharge of the yarn provided torsional rotation in either direction. This allowed the paddle to mix the liquid in which it was submerged, something that might be useful as part of a microfluidic device used for chemical analysis. Such devices contain a microscopic matrix of tunnels, valves and chambers through which chemical solutions can be routed.

Images courtesy of the University of Texas at Dallas