IT HAS inspired scientists and superhero fans alike with its ability to scale walls and zip across ceilings. Now the gecko has been put to shame by a new material made out of carbon nanotubes.
The lizard is able to stick fast to a surface, but then detach itself easily, thanks to countless microscopic setae 鈥� hairs that cover its toes and branch off into even tinier hairs, called spatulae. Each hair bonds weakly with the molecules on the surface it is in contact with, through van der Waals forces. But the total force exerted by the huge number of hairs pressed onto a surface add up to a significant bond, giving geckos their renowned ability.
To reproduce this force, a team led by Liming Dai of Daytona University, Ohio, and Zhong Lin Wang of Georgia Institute of Technology in Atlanta grew carbon-nanotube arrays on a square, 4 by 4-millimetre silicon wafer. While the trunks of these tubes were straight, the team ensured that the ends grew in such a way that they became coiled and tangled. The trunks are the equivalent of the setae, and the curly tangled segments act like the spatulae on a gecko鈥檚 foot, Dai says.
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Pressing the structure onto a surface ensures that more of the tangled tubes makes contact with it. And the greater the surface area involved, the bigger the attractive force. Tested with a range of surfaces, including glass, Teflon and sandpaper, the nanotubes stuck best to glass: a patch of 1 square centimetre could resist a pulling force of up to 100 newtons in the shear direction (see diagram). That鈥檚 almost 10 times as strong a grip as a gecko and three times as strong as the best artificial competitor, the team claims.
However, when the material is moved away from a surface, fewer nanotubes remain in contact with it, reducing the van der Waals forces and allowing easy removal.
A current shortcoming of the new material is the need to press it onto the surface with a very large force 鈥� about 120 newtons, or the equivalent of 12.5 kilograms in weight, for a square centimetre 鈥� to engage the sticking effect, says Kellar Autumn from the Lewis and Clark College, in Portland, Oregon. He was the first to suggest the stickiness of the gecko鈥檚 foot was down to its geometry rather than chemistry.
But he says, 鈥渢he ability of this material to support large shear loads and to detach easily is very encouraging鈥�.
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