Geckos' hairy grip inspires adhesive research.

Hairs give gecko feet their incredibly strong, dry, reversible adhesion. Credit: © PNAS

Strong, waterproof, re-usable adhesives could be just around the corner, thanks to geckos.

Researchers have created two prototype glues after confirming that geckos owe their amazing ability to scamper across ceilings and cling to polished glass solely to many thousands of tiny, spatula-tipped split hairs on their toes. These bond weakly with the molecules in any surface on which the lizards run1.

The scientists, based at the University of California at Berkeley, cast two sets of imitation gecko toe hairs. Their mould was a microfilter with which biologists usually remove bacteria from solutions. They then tipped the hairs with silicon rubber or polyester.

In the lab, both materials adhered as well to most surfaces as the natural wall-crawlers. Moreover, because gecko adhesive switches on and off mechanically, its stickiness never wears off, Berkeley engineer Ron Fearing explains. "If a gecko runs through a sandbox, it can still stick to a wall - just try that with scotch tape".

Engineers are increasingly looking at biological structures for new approaches to design problems. Lizard locomotion has already inspired the mecho-gecko, a robot with sticky feet and a tail for balance that can scuttle across dangerous terrain.

"This study shows nicely that you can pull the essential principles from biology" to make a simple man-made adhesive, says Bill Kier, a zoologist at the University of North Carolina in Chapel Hill, who works on how the sucker muscles of octopi and squid generate a different type of adhesion. To copy a complex biological structure such as the gecko's footpad piece by piece would give engineers "a terrible headache", Kier adds.

Kellar Autumn of Lewis and Clark College in Portland, Oregon, is studying other animals and insects to build a library of such bioengineering information. "Such a library would be really valuable," he says. "But unfortunately, extinctions are taking books off the shelves faster than we can study them."

Feet first

The soles of Tokay geckos (Gekko gecko) have sticky ridges called lamellae. The ridges of each foot contain half a million hairs, called setae. The end of each hair splits into between 100 and 1,000 tiny spatulas, visible only with an electron microscope.

The ultra-close contact of the split ends with a wall or floor creates billions of weak molecular attractions called van der Waals forces. Robert Full and colleagues at Berkeley now confirm that together these confer an incredibly strong dry adhesion that is also immediately reversible - at a certain angle, the ends automatically detach and roll up like a party favour.

Geckos get a grip using van der Waals forces. Credit: © PNAS

Full's earlier work on a single toe hair had suggested as much2. And experiments carried out more than 30 years ago showed that geckos, like fried eggs, don't stick to Teflon, which does not support van der Waals forces.

To confirm their hypothesis, Full's team placed geckos on two types of surface capable of forming van der Waals forces - one extremely water-loving, the other strongly water-repellent. The animals got great traction on both surfaces.

References 1 Autumn, K. et al. Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences, published online, doi:10.1073/pnas.192252799 (2002). 2 Autumn, K. et al. Adhesive force of a single gecko foot-hair. Nature 405, 681 - 685 (2000). Download references

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