Tropical evenings are not complete without a gecko scurrying up walls and across a ceiling. The stickiness of the lizards' feet is legendary, giving them a unique and mysterious way of defying gravity. Scientists say they have now answered the mystery that has occupied thinkers since at least Aristotle. They are trying to copy the answer for practical uses.
Researchers have long known that the toes of a gecko are densely packed with half a million fine hairs. The end of each hair splits into up to 1,000 filaments, each just 200 billionths of a meter wide. This gives the gecko foot 500 million tiny branches making contact with a surface.
But how do they hold? By glue? By suction? By some electrostatic force? Suction has been ruled out, because the lizards can stick in a vacuum. Friction is also not the method because they can run across polished glass. The answer comes in the Proceedings of the U.S. National Academy of Sciences.
"We discovered that the tiny hairs stick far better than we ever imagined before," says Biologist Robert Full of the University of California at Berkeley.
Mr. Full and his colleagues say atomic bonding is responsible for the gecko's sticky feet. This is the attraction between molecules at the ends of the gecko's toe hairs and on the contact surface.
"We made the first direct measurement of how big the forces are for an individual hair, and showed they can actually be explained by intermolecular forces," he said. "The force is a result of the molecules being essentially so close together that the unbalanced electrical charges around the molecules can attract one another. That can provide a net attraction between the foot of the gecko and the wall."
Mr. Full's laboratory showed that a single gecko toe hair exerts a strong force, powerful enough to bend an aluminum wire or lift an ant. The combined force of all the hairs is more than enough to bind the lizard to the ceiling.
"A small patch the size of a coin could possibly hold a small child of, let's say, 45 pounds or 20 kilograms," Mr. Full said.
The research might someday lead to the development of powerful dry adhesives. How about a sticky bandage that can be removed without pain, or a reusable tape that can adhere underwater?
These are apt analogies because the researchers found that a gecko peels its toes off surfaces like tape. The angle of the toes is critical to the release of the hairs. They must be 30 degrees off the surface.
Using these insights, engineers on the Berkeley, California campus have created synthetic hair tips that stick almost as well as the geckos' do to highly polished semiconductor chips. Each filament adheres with a force that equals the weight of a single blood cell. They have found a way to make one-centimeter-square groupings containing 10,000 rubber bumps.
Mr. Full's laboratory has also shared its research with a Cambridge, Massachusetts, company called iRobot, which is developing wall-climbing robots.
The president of iRobot, Helen Greiner, envisions a multiude of tasks for them, including surveillance and rescue work. "When we first decided we wanted to build robots that can climb walls, I did not know that it could be done, but we could look to nature and say, hey, these guys do it, whether it is an ant or spider or gecko," she said. "It can be done. Nature does it. Now, can we duplicate it mechanically?" If the research succeeds, then pity the poor gecko. It might have to start competing with roving machines for wall space!