BioEd Online

As light as air, yet stronger than steel and bendier than rubber. A new material made from bundles of carbon nanotubes combines all of these characteristics in a substance that twitches like a bionic man's biceps when a voltage is applied.

The 'artificial muscle' is an aerogel — a lightweight, sponge-like material consisting mostly of air — drawn into a long ribbon.

Applying a voltage across the width of the ribbon electrically charges the nanotubes that thread through the material. This makes them repel one another, and the ribbon can expand sideways by up to three times its original width in an instant. "These muscles are remarkably fast," says Ray Baughman at the University of Dallas, Texas, who led the research.

The artificial muscle can expand about 4,000 times faster than human muscle does, says Baugman, and can be switched on and off up to 1,000 times a second with no deterioration (see video).

Applying a voltage along the length of the ribbon has a very different effect. It triggers the nanotube structure to contract, making the material more dense and very stiff (see animation). This means that the 'muscles' could pack a mighty punch — along the length of the ribbon, the nanotube aerogel is, weight for weight, stronger than steel. The work is published today in Science¹.

"What really impresses me is that the ribbons show an incredibly large difference in stiffness depending on which direction is probed," says John Madden, a materials scientist from the University of British Columbia in Vancouver, Canada. "They are perhaps a million times stiffer in one direction than in the other two. Imagine feeling a material that is like diamond in one direction and rubber in the other two."

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Baughman is excited about another property of these muscles: their ability to withstand extreme temperatures. They keep their properties down to 80 K (-193ºC) and up to 1900 K (1627ºC), and Baughman sees no reason why these temperatures need be the limits — the reported temperature range was restricted only by their ability to make measurements in those conditions, he says.

This means that the muscles could easily be used in harsh environments such as the cold of space, or the heat of a combustion chamber, says Madden. "There are no artificial muscles that I know of that can operate over the wide range of temperatures that these nanotube devices can," he adds.

The stretched out muscles can be frozen in position by placing them on to a substrate. This may allow them to be used as a transparent electrode that could sit atop a solar cell, with the nanoscale pores in the material helping to trap or channel the electrons generated, says Baughman. And a tiny amount of the material would go a long way. "One ounce of this material would cover an acre," he says.