How Geckos Get a Grip

Since the time of Aristotle, scientists have tried, and failed, to understand the mysterious ability of geckos to climb walls, cling to ceilings and scamper across slippery panes of glass.

Is it glue? Friction? Suction cups? A type of biological Velcro? Electrostatic attraction? One by one, such clever explanations have been ruled out.

Now, a group of biologists and engineers has combined extensive knowledge of the little tropical lizards’ anatomy and behavior with mini-machines to measure the smallest forces exerted by gecko feet. And they say they have the answer.

That answer, they predict, may one day lead to creation of tiny, climbing robots and a revolutionary type of reusable and self-cleaning glue.

“Here’s a study of an unusual animal that you wouldn’t expect to have any practical application--and it does,” said Robert J. Full, a biology professor at UC Berkeley who led the research and specializes in studying the locomotion of crabs and cockroaches to inspire the creation of many-legged robots.

The gecko’s secret, the group says, lies in a multitude of tiny hairs, or setae, bristling from the surface of its feet. Each of the gecko’s toes contains 100,000 hairs. Less than a tenth as thick as a human hair, each is split at the end into hundreds of even tinier, spatula-shaped tips.

The structures allow a gecko to make contact with walls in, literally, a billion places. Each tip gets so close to whatever the gecko is crawling on that it generates the same type of force that holds molecules together. The force of an individual tip doesn’t amount to much, but collectively the hairs generate an impressive amount of force. As many visitors to the tropics know, geckos are extremely difficult to pull from walls.

“They’re so effective. A gecko can hang on by one toe,” said Kellar Autumn, an assistant professor at Lewis and Clark College in Portland, Ore., and the lead author of a paper titled “Adhesive Force of a Single Gecko Foot-Hair” published in the June 8 issue of the journal Nature. “They’re these nocturnal acrobats. They run and jump in the forest canopy on surfaces that other animals don’t have access to.”

By Autumn’s estimation, the hairs on just two gecko feet are strong enough to hold a 45-pound child. Four feet could nearly hold Calista Flockhart or Kate Moss.

The feet even stick underwater.

Gecko feet are not all alike. Amid the world’s 800 species of geckos, there are desert geckos with “sand shoes,” geckos with fringed feet and geckos with “bulldozer” feet that can move pebbles in their paths, said Anthony P. Russell, a professor of zoology at the University of Calgary. Russell has studied geckos since 1969 and is the world’s authority on the evolution and anatomy of their feet.

“There’s a lot of weird stuff going on,” he said. “Geckos seem to be hooked on this pattern of exploiting their skin to stick to surfaces.”

For the current experiment, the group used the Tokay gecko, a foot-long, barking, red-spotted lizard from Southeast Asia known for two things: its propensity to bite and its ability to cling beyond all expectation. “I would call it the Ferrari of the gecko world,” said Russell. “High performance and terrific engineering.”

Unlike other geckos, Tokays have long hairs with multiple branches that cover a large portion of their toes, he said.

The experiment showed that for hairs to cling--and more important, to release--they have to be oriented at the proper angle and slightly weighted or un-weighted. The geckos apply these pressures and angles to the hairs by moving their toes.

The finding fits with the observation that geckos unfurl their toes like inflatable party favors and then peel them off again once they are ready to unstick.

“If you know about geckos, you say, ‘Yes, that’s exactly what you expect,’ ” said Russell, who was not involved with the research but applauded the finding.

“Measuring them was the hardest thing,” said Full. “These hairs are really, really tiny. You can barely see them.”

The measurements were possible, he said, only because of diminutive sensors and wires created by two engineers on the team, Thomas Kenny of Stanford and Ron Fearing of UC Berkeley.

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The findings about intermolecular forces help prove what was one of the last possibilities to explain the adhesiveness of gecko feet. It can’t be glue because gecko feet have no glands to produce such a substance, and they leave no footprints.

It’s not suction because geckos can stick in a vacuum, where forces that create suction do not apply. It’s not friction because the animals stick on surfaces as smooth as mica, and it’s not electrostatic attraction because geckos shot with antistatic guns keep sticking.

The group has already applied for a provisional patent to cover a new kind of gecko “glue” that could be as revolutionary as the Post-It note. Researchers envision it as a kind of tape made from simulated, nearly microscopic hairs modeled after those on gecko feet. Like the animals, it would stick, then unstick and stick again.

“It would be as good as Velcro without needing the other side,” said Full. The glue might prove useful in the manufacture of silicon chips and in surgery, Full said, and could be widely used by consumers to stick objects to almost any surface.

It would also probably be reusable because gecko feet are almost instantly self-cleaning. “Nature is dirty, but [gecko] feet always look clean when we pick them up,” said Autumn.

Despite the potential of such a glue, the researchers are most excited about funneling their knowledge of geckos into the creation of robots that can climb. Climbing ability has eluded roboticists for years, said S. Venkat Shastri, who develops robots at SRI International, a nonprofit research organization in Menlo Park, Calif.

Plans for “geckobots” are part of a growing Defense Department push to understand and harness the enviable powers of various animals. Leaders of the Defense Advance Research Projects Agency hope to create mechanical reproductions of dog noses to detect land mines, of flies for micro-air surveillance and of beetles that can sense forest fires from 40 miles away.

The team also plans to look at other lizards--anoles and skinks--that also stick but may use simpler structures to do so.

Sticking with hairs appears to be a passive process; even dead geckos can stick to walls. But being able to release toes--something that happens about 15 times a second when a gecko is running--may be harder to duplicate because it may require a complex feedback loop, Russell said.

The research has both inspired and humbled engineers, who now see that the biology of creatures like geckos is far more complex than the robots they hope to construct. “We don’t want to build a gecko. Geckos are really complicated,” said Autumn. “Why copy nature when we can just take the parts we need?”

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Usha Lee McFarling can be reached at [email protected]