BioEd Online

A film of upright carbon nanotubes can be compressed like a concertina, say researchers. They believe that the material could make ideal padding for tiny objects, or form components for microscopic mechanical devices.

Unlike standard compressible foams, whose low density makes them less robust, the nanotube film is both strong and squeezable. The material can be squished to just 15% of its normal height and rebound perfectly, thousand and thousands of times, without showing any wear or losing springiness. The film is also resistant to chemical attack and high temperatures.

The scientists discovered the unusual behaviour by chance when they were trying to squeeze the nanotubes into a thin sheet. "The films did not flatten permanently, but kept springing back to their original shape," says Anyuan Cao, a materials scientist from the University of Hawaii at Manoa, and part of the team that reports its discovery in this week's Science¹.

"Interesting applications might result from this fascinating discovery," says Ray Baughman, a materials scientist at the University of Texas, Dallas. "It is truly remarkable that a nanotube forest behaves like a supercompressible foam," he adds.

Bend over backwards

Carbon nanotubes have long been known to have some flexibility. A single nanotube, if compressed, would be expected to bend like a bow. But for some unknown reason, when a lot of nanotubes are lined up together, they bend in a more complex series of folds. Cao thinks that the supporting effect of neighbouring nanotubes has something to do with this ordered buckling.

The team grew their nanotubes at about 800 ºC from carbon atoms in the chemical xylene, using an iron-based catalyst. Each nanotube is made of several cylinders of carbon atoms, nested inside each other like Russian matryoshka dolls.

Baughman compares the nanotubes to a bamboo forest. Except that, if it were scaled up to that size, with each stalk 2.5 centimetres wide, then it would be more than a kilometre tall. The compression would be like a giant's foot squashing the forest to less than 200 metres in height, he says, with some 60 to 70 bends in each stalk.

Smaller buckles

The team has found that the size of the buckles in the tubes depends on how much stress is applied, and on the radius of the tubes. This may provide a useful way of tuning the material's properties. "Single-walled nanotubes or smaller multiwalled nanotubes could be expected to have smaller buckles," says Pulickel Ajayan, a materials scientist at Rensselaer Polytechnic Institute in Troy, New York, and part of the team.

Thanks to the relatively high price of nanotubes, they are unlikely to find use as a protective packing material any time soon. "Certainly it is still far away to make bulk products that can compete with plastic foams in the market," admits Cao. But he adds that the springy molecules could become excellent cushions for tiny mechanical devices.

Because carbon nanotubes conduct electricity, the springy material could make an excellent flexible electrical connection. It may also be possible to use a current to make the tubes flex, powering microscopic machinery.