The soldier who loses a limb in Iraq and the DOD weapons developers who want to create a completely robotic warrior share this problem: To function for any length of time, prosthetic muscles must be plugged into a wall socket and recharged.

Prosthetic muscles already face one huge challenge in needing to bend and exert force like the real thing, but the power supply problems are equally daunting. For the disabled veteran, being dependent on the plug could mean living with a leg that runs out of juice in the middle of a stroll on the beach. For the robotic soldier on the front line, the idea of muscles that suddenly cease functioning is a non-starter.

Ray Baughman, director of the NanoTech Institute at the University of Texas at Dallas, is working to solve the problems of these nascent devices. For more than two decades, he has made great advances in his quest to create artificial muscles that can function just like - or better than - natural ones. In January, he accepted the Kapitza Medal, a lifetime achievement award from the Russian Academy of Natural Sciences, for conceiving new types of artificial muscle that goes a long way toward solving the power source problem.

At the core of many of these achievements is materials made of artificially grown carbon nanotubes - tubes that measure a few nanometers in diameter and can be used to conduct electricity and store hydrogen. To lengthen the typical 30-minute charge capacity of electrically powered prosthetics, Baughman shifted his focus from polymers, the material traditionally used in prosthetics, to sheets made of carbon nanotubes. Because the carbon nanotubes are able to hold hydrogen, they become the perfect catalyst for fuel-cell-powered artificial muscles. The carbon nanotube materials also possess a larger surface area than that of polymers, allowing a higher electrochemical charge to be injected into the material. By using fuel cells - like hydrogen and methanol - and working with a larger (molecular) surface area, Baughman was able to create an artificial muscle whose charge lasts 30 times longer than a traditional electrically powered muscle made from polymers.

A longer charge is important, but the strength and flexibility of human muscles must also be captured before carbon-nanotube materials can be applied as a cure-all to the artificial muscle dilemma. To address these shortfalls, Baughman and his team have most recently developed a carbon nanotube yarn.

To make the yarns, a forest of carbon nanotubes are grown, harvested, and spun like wool. Because the threadlike structure can be made at competitively greater lengths - about 3 feet - they can support loads up to 150 times greater than nanotube sheets and are 100 times stronger than natural muscle (force generated per area). But before yarns can be put to work in artificial muscles, researchers must reduce “slippage” - deformation of the muscles that gradually occurs over the span of many cycles of use. Baughman is also exploring how to increase the range of movement in the yarn so it can properly mimic the flexing and contracting of a natural muscle. Currently, the yarn’s movement memory performs at just 10 percent of a natural muscle’s.

What’s more, those studying any area of carbon nanotube application face a major hurdle in bringing their developments to market. Yes, individual tubes have mechanical strength up to 10 times greater than commercially available fibers, and they have the ability to carry and transfer heat as well as carry 1,000 times more current than copper wire, but many of these qualities diminish once the individual tubes are assembled.

“They’re like minute bits of string. How do you assemble trillions of them to make useful objects?” asks Baughman. In the meantime, Baughman says fuel-powered muscles coated with platinum nanoparticles, which have been developed to exhibit true shape memory and hold a long electrochemical charge, can be commercialized for use in as little as three years. Potential applications include creating robots to defuse land mines and companion robots to ensure the safety of the elderly. - Erika Stalder