Ray Baughman and colleagues made the muscles by transforming multiwalled nanotube forests into carbon nanotube aerogels that have a density of just 1.5 mg/cm3, which is not much heavier than air. The researchers began with a material consisting of an array of vertically aligned carbon nanotubes, also known as forests because the nanotubes are lined up like trees. The aligned nanotubes can be transformed into carbon nanotube aerogel sheets at rates of up to 2 m/s.

The finished devices can expand like rubber in the sheet width direction when electrically charged, but are stiffer than steel by weight in the nanotube orientation direction. "This apparently unprecedented degree of anisotropy is akin to having diamond-like behaviour in one direction and rubber-like behaviour in the others," says John Madden of the University of British Columbia, who was not involved in the work.

The devices operate over a range of temperatures that previous artificial muscles could only dream of, and can expand and contract at rates that are around 1000 times higher than those for natural muscle. They also generate a force that is 30 times bigger than that exerted by natural muscle.

All of these novel properties mean that the muscles might be used as actuators in medical and other devices, as well as in electrodes for solar cells, light-emitting diodes and displays. They might also find their way into future robot arms and legs.

Their ability to function over extreme temperature ranges may also make them attractive for aerospace and space applications, say the researchers.

The work was reported in Science.