Sticky fingers

Spend time anywhere in southern Europe and it won't be long before you spot a gecko, blithely disregarding gravity, running over walls, ceilings and windows at all angles.

How they do it has been a mystery for centuries, but the secret has now been cracked - and BAE Systems' researchers have made a major advance replicating it.

The answer is in the texture of the gecko's feet. The skin of the underside of each toe isn't sticky or tacky as we understand it, but hairy. Each toe is covered by ridges which are coated in millions of hairs, whose ends are split into a multitude of filaments. Each of these ends in a mushroom-shaped cap called a spatula, less than a micron in diameter. It's these that keep the gecko stuck to the wall, and the force that does it is one of the weakest known to science - van de Waals attraction.

This is an electrostatic interaction caused by the movement of the electron cloud around a molecule, and it's tenuous in the extreme - just about capable of holding together liquid hydrocarbons at very low temperatures and only active at very close range. But the hairy texture of gecko toes ensures that many of the spatulas are in extremely close contact with the surface the lizard is on, no matter how rough or smooth or in whatever orientation.

The attraction from a single spatula is tiny, but several million will always be touching the surface - and that provides ample sticking force to keep the lizard on the surface. And when it wants to unstick a foot, it simply peels it off the surface.

Jeff Sargent and Sajad Haq of BAE Systems' Advanced Technology Centre have mimicked the texture of the lizard's feet by fabricating sheets of polyimide, a speciality polymer whose high strength- to-weight ratio and wide temperature resistance range have previously made it useful as a high-performance insulator. The sheet's surface is coated with a regular array of stalks with splayed ends around a micron across. It looks just like the gecko's toes, and has similar properties.

Testing the material on glass, Sargent and Haq measured that a force of 3,000kg/sq m is needed to pull the material free. This means that a sheet about 60cm square could support the weight of a family car, they claim.

'We recognised that a synthetic material could have tremendous potential, not only in our own aerospace and defence business, but also in other commercial applications,' said Sargent. And BAE has identified several possible uses for the inherently sticky, but not tacky, material.

For example, it could be used as a closure for access panels which wouldn't need additional fasteners; make instant slap-on repair patches for holed aircraft skins or fuel tanks; rapidly attach armour or stealth panels to vehicles; or hold aircraft in position on carrier decks.

The gecko-copying material could be taken a stage further, to give crawler robots extra grip, or to make super-grippy tyres and trainers. And if it covered the palms of a pair of gloves, it could support the weight of an average person. Spider-Man-style wall-crawling would then become a possibility.

However, the technology has some way to go before it hits the market. The material can't match the gecko's sticking ability across a range of surface roughnesses, and the team is evaluating the effect of water on stickability. But Sargent believes that the manufacturing process can be tailored to suit different applications, depending on the kind of adhesion needed; moreover, he said the fabrication method should be easy to scale-up.

The team is also working on further enhancement of the process, which would allow the gripping force to be varied so that bonds can be created - or released - without need to move the surfaces.