What makes geckos stick?

PORTLAND, Ore. -- The mystery of what makes geckos stick to just about anything -- a question that has puzzled scientific minds since Aristotle -- finally has been solved, according to a new study.

The answer involves the geometry, not the biochemistry, of the lizard's feet, meaning scientists may be able to duplicate the same geometric principles to create things such as robots that can walk on any surface in any direction, the researchers say. Another possibility is something as simple as Band-Aids that hold tight but don't stick when they're peeled off.

"I just saw the movie 'Spiderman' and I realized that some day we'll be able to do even better than he does sticking to things," said Kellar Autumn, a Lewis & Clark College biologist in Portland and lead author of the study published recently in the Proceedings of the National Academy of Sciences.

Researchers found that the tips of the hairs on the bottom of gecko feet are tiny enough to take advantage of a weak attraction between individual molecules called van der Waals forces.

Peels toes like tape

Geckos have millions of microscopic hairs on the bottoms of their feet that are narrower than human hairs, and each splits off into 1,000 tips that are so small they cannot be seen with a conventional microscope and can be detected only with an electron microscope.

The shape of the hairtips also is critical, allowing the small lizard to scamper up walls and across ceilings by sticking its toes to nearly any smooth surface in less than one eight-thousandth of a second and unsticking them in half that time.

"What we discovered was the angle the little shaft of the hair makes with the surface is the critical variable," Autumn said.

"The gecko has this really unique way of taking its feet off the wall -- it peels its toes like tape," he said. "When that angle reaches 30 degrees, the hair pops off."

Aristotle noted the climbing abilities of geckos in the fourth century B.C. and scientists have been actively trying to solve the mystery for roughly the past century, ever since Dutch physicist Johann van der Waals won the Nobel Prize in physics 1910 for his research on the gaseous and liquid states of matter, Autumn said.

Earlier studies of gecko feet had reduced the explanation to either the capillary effect of tiny amounts of water that create suction or something that worked whether there was any water or not, in this case, the van der Waals attraction between molecules.

Autumn and a team of researchers at the University of California at Berkeley, the University of California at Santa Barbara, and Stanford University fabricated a synthetic version of the gecko hairs and tried them on two different kinds of highly polished, extremely smooth semiconductor chip materials.

Both chips allow van der Waals forces to work, but a silicon-based chip also allows the capillary action of water to work while a gallium arsenide chip prevents any effect by water.