ROBOTS may one day do their work using muscles made from sheets of tangled
nanotubes. Geoff Spinks at the University of Wollongong in Australia has
discovered that sheets of the tiny carbon tubes can be made to contract and
relax by applying a voltage to them.
Spinks鈥檚 team was investigating the properties of nanotube mats鈥攚hich
are made by straining a solution of nanotubes through a fine filter. Filtering
deposits the nanotubes in tangled layers, which then stay stuck together due to
intermolecular van der Waals forces.
Spinks was checking an effect whereby nanotube mats immersed in water appear
to grow slightly bigger when a voltage is applied to them. At low voltages,
charge builds up on the mats, causing bonds between adjacent carbon atoms to
lengthen, making the material expand. But as Spinks turned the voltage up, the
mats stopped expanding. 鈥淲e were pushing them to their limit, when we suddenly
noticed they were contracting,鈥� says Spinks. It wasn鈥檛 much鈥攋ust a 2 per
cent contraction鈥攂ut it was enough to prick their interest.
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Close inspection of the mats showed that blisters of gas鈥攆rom
electrolysis of the water鈥攈ad formed between adjacent nanotube layers,
pushing the layers apart and making the material thicker. If a nanotube mat is
made highly positive, pockets of oxygen form inside it. Reversing the voltage
produces blisters of hydrogen, which flushes out the oxygen and makes the mat
shrink. Spinks says the shrinking mats, which were about 20 micrometres thick,
produced stresses of 1 megapascal鈥攎ore than three times as much as the
stresses in human skeletal muscle. So he predicts they could one day be used to
make artificial muscles for robots.