BEST known as the ultra-strong material that might one day form the cables of a "space elevator" capable of raising people into Earth orbit, carbon nanotubes also have a springy side. The discovery that nanotubes keep bouncing back after being compressed repeatedly means this exotic form of carbon may be just the thing to give artificial muscles some extra strength.

Engineers want to build artificial muscles - actuators that change length in response to a stimulus - because they create a smoother, more human-like motion than jerky electric motors or pneumatic devices. Such muscles would be used to power robots, prosthetic limbs and artificial tissue for implantation.

Today's most promising artificial muscles are based on electroactive polymers (EAPs) - plastics that change shape when activated electrically or with chemicals. But they lack mechanical robustness and as a result soon succumb to fatigue and fail. Now engineers led by Victor ...