article: These colourful lizards can scamper rapidly up walls, scuttle along ceilings and even hang upside down on polished glass. Yet the secret of their amazing climbing ability remained a mystery until relatively recently. The underside of a gecko's foot looks like a tyre tread and is covered in millions of microscopic hairs. Each hair splits into hundreds of tips just 200 billionths of a metre wide. The secret of this lizard's adhesion turns out to be the relatively weak intermolecular forces that draw materials together any time they get close. These van der Waals forces nevertheless explain how a gecko can support its own body weight on just one finger, and a single gecko hair can lift the weight of an ant. To un-stick, the gecko pulls its foot away at a different angle. "What we're talking about is something that is about as sticky as sticky tape - it's not crazy glue," says Prof Kellar Autumn, who became intrigued by gecko adhesion after being funded to develop climbing robots by the US military. Prof Autumn, from Lewis & Clark College in Oregon, added: "What is special about it is that it is controllable. And its controllability is based on geometry and physics, not chemistry." The effort to uncover the mechanisms behind gecko climbing has already yielded synthetic material that sticks in the same way. Stick, peel and re-stick a piece of existing adhesive tape several times and it quickly loses its clingy properties. Prof Autumn and Mark Cutkosky, from Stanford University, compared natural and polymer-based synthetic gecko hairs using a machine that simulated gecko climbing. This showed both versions could be re-used some 30,000 times without losing their stickiness. Synthetic adhesives could yield transformative applications in robotics, industry, medicine, sports and clothing. But one possible use always comes up in any popular discussion: could they allow humans to scale walls like Spider-Man? In 2007, physicist and engineer Nicola Pugno, from Turin Polytechnic in Italy, calculated that a person wearing gloves and boots made of carbon nanotubes and structured to mimic gecko feet could indeed cling safely to a wall or a ceiling. Challenges would include wear and tear and the propensity for dirt particles to collect and inhibit stickiness. The suit would have to work on every kind of surface and for long periods of time. But Prof Pugno says: "We are not very far, in my opinion, from a kind of Spider-Man suit." Prof Metin Sitti, from Carnegie Mellon University, says the idea is "not impossible". Selecting a lightweight person and applying the adhesive to many parts of the suit (not just the feet and hands) would improve the chances of success, he explains. But, he comments: "We get questions like: 'Can you carry a one-tonne weight'. And we say: 'Maybe, but that's not our biggest advantage'." A. The toes of geckos are covered in ridge-like lamellae, producing a tyre tread pattern B. Millions of microscopic hairs, or setae, cover each toe. These are only as long as two diameters of a human hair C. Each seta ends in up
summary: Geckos are among the superheroes of the animal world.