Copyright 2000 Plain Dealer Publishing
The Plain Dealer
June 11, 2000 Sunday, FINAL / ALL
SECTION: NATIONAL; Pg. 23A
LENGTH: 571 words
HEADLINE: GECKOS HAVE FEET THAT STICK LIKE TAPE, BIOLOGISTS DISCOVER
BYLINE: By JEFF NESMITH; COX NEWS SERVICE
Geckos, the small lizards that chirp through tropical nights, appear to invoke an obscure molecular force when they scurry along vertical surfaces or even upside down in seeming defiance of the law of gravity.
The tens of millions of microscopic hairs on geckos' toes are so fine that they are able to interact with the molecules of the surfaces over which they are moving, said biologist Robert Full of the University of California, Berkeley. He and collaborators from Stanford University and Lewis and Clark College described their gecko studies in this week's issue of the journal Nature.
Geckos can race over a piece of polished glass and crawl upside down on the ceiling of a room. A gecko can even hang from the ceiling by a single toe.
Yet their hand-like hairy feet do not have suction cups or glands that produce any known form of glue.
"Geckos have developed an amazing way of walking that rolls these hairs onto the surface and then peels them off again, just like tape," Full said in a telephone interview. "But it's better than tape."
If the observations and calculations hold up, a century-old mystery is solved and a possible new principle has been found for creating an adhesive that is both dry and extremely strong.
Using high-speed video cameras to record the mechanics of gecko locomotion, the scientists discovered that the creatures attach their toes to surfaces by "unrolling" them - somewhat like a "Happy New Year" party toy unrolls when someone blows through it.
Then they lift the toes by peeling them from the surface, like peeling off adhesive tape, Full said.
Both actions, happening 15 times a second, are necessary for the tips of the hairs to invoke the molecular force and withdraw from it, Full said.
The force that Full and his collaborators believe the lizards are using is called the "van der Waals force."
Named for the Dutch physicist who discovered it, the force causes molecules to cling together, like magnets. But it works only at extremely small distances. As the space between two molecules increases by, say, the diameter of an atom, the attraction between them weakens at an exponential rate.
Full and his collaborators believe that when the gecko plants his hairy little toe on the surface of a mirror, the extremely fine toe hair gets close enough to the molecules of the mirror to establish a van der Waals force attraction.
In fact, the contact between the hairs and the molecules of the mirror becomes so intimate that, for a split second, the hairs are actually "part of" the mirror, Full said. And that's about as sticky as anything gets.
The hairs on the bottom of a gecko foot are known as setae, pronounced "SEE-tee." A gecko has around 500,000 of these hairs, each about one-tenth the diameter of a human hair, on each foot. The tip of each of the setae is further divided into several hundred to 1,000 even tinier hairs called spatulae, the scientists said.
"These billion spatulae, which look like broccoli on the tips of the hairs, are outstanding adhesives," said Full, a professor of biology at UC Berkeley.
In fact, all the tiny hairs on the four feet of a single gecko could theoretically produce enough holding power to lift nearly 100 pounds, the scientists calculated.
When the angle at which the hairs are pushed into the surface is changed, they appear to lose touch with the molecular van der Waals force and easily disengage.