Physicists have found a new way to make "smart materials" that can behave like muscle tissue in humans. A team of researchers from the United States, Australia, Italy and Germany, led by Ray Baughman of AlliedSignal in New Jersey, placed a single layer of carbon nanotubes - long, cylindrical carbon molecules - on either side of double-sided scotch tape and placed the tape in beaker of salt water. By applying an electrical charge to one side of the tape, they found that the nanotubes on that side of the tape contracted, while those on the other side increased in length, causing the tape to bend (Science 284 1340). A similar effect is seen in body tissue - an electrical signal from the brain causes muscle fibres to contract, flexing the muscle.
The new technique works because the salt water conducts electricity. As the voltage on the tape changes, electrical charge carriers are injected into the nanotubes. These form electrolyte ions near the surface of the tape. On the cathode side of the tape, electrons are attracted to the nanotubes and cause them to expand. On the anode side, electron “holes” cause the nanotubes to contract.
According to Baughman and his colleagues the nanotube ‘muscle’ could generate and survive higher stresses than any other materials, including real muscle tissue and ferroelectric materials. Moreover, the voltages needed to bend the nanotubes are much lower than those needed to change the shape of ferroelectric materials. This suggests the nanotube sheets could have a wide range of practical applications including optical fibre switches and microscopic pumps.
