Geckos are able to scurry up walls and across ceilings thanks to two million microscopic hairs on their toes that glom onto surfaces in a way that has given engineers an idea for a novel synthetic adhesive that is both dry and self-cleaning.
In a paper in this week's issue of Nature, University of California, Berkeley, biologist Robert J. Full, Lewis and Clark College biologist Kellar Autumn and their colleagues report the first measurement of the forces that these hairs, or setae (see' tee), exert on a surface.
Working with the "Cadillac" of the gecko world, a Tokay gecko (Gekko gecko) native to Southeast Asia, the team of biologists and engineers showed that the combined adhesive force of all the tiny hairs lining the gecko's toes is 10 times greater than the maximum force reportedly needed to pull a live gecko off the wall. Geckos apparently use only a fraction of the hairs at one time, though they have been known to hang from the ceiling by one toe.
The key seems to be the hundreds to thousands of tiny pads at the tip of each hair. These pads, called spatulae, measure only about ten millionths of an inch across. Yet, they get so close to the surface that weak interactions between molecules in the pad and molecules in the surface become significant. The combined attraction of a billion pads is a thousand times more than the gecko needs to hang on the wall.
"These billion spatulae, which look like broccoli on the tips of the hairs, are outstanding adhesives," said Full, a professor of integrative biology and head of the Poly-PEDAL (Performance, Energetics, Dynamics, Animal Locomotion) Laboratory at UC Berkeley. "Geckos have developed an amazing way of walking that rolls these hairs onto the surface, and then peels them off again, just like tape. But it's better than tape."
"Getting yourself to stick isn't really that difficult, it's getting off that is the problem," noted Autumn, a former postdoctoral student in Full's laboratory who now is an assistant professor of biology at Lewis and Clark College in Portland. "When a gecko runs it has to attach and detach its feet 15 times a second. We think we know how it does that so rapidly."
In order to measure the extremely tiny forces involved when one hair sticks to a surface, Full and Autumn teamed up with two engineers, Ron Fearing at UC Berkeley and Thomas Kenny of Stanford University.
Kenny micro-machined a minuscule device to measure the forces involved in attaching to a surface, while Fearing employed a fine aluminum wire to measure the forces when detaching.
With Kenny's MEMS (microelectromechanical system) tool, Full, Autumn and their colleagues showed how touching the end of a hair to a surface is not good enough - the hair slips right off. If the gecko instead engages the surface by pushing in and pulling slightly downward, it can achieve 600 times greater sticking power than friction alone could account for. Full suspects that the uncurling motion as the gecko attaches its foot to a surface does this automatically.
Using Fearing's device, they also showed that pulling away is not enough to disengage. The strength of attachment is so strong that a single gecko hair, only one-tenth the diameter of a human hair, could bend the aluminum wire. In fact, Autumn said, a single hair could lift an ant, while a million hairs covering an area the size of a dime could lift a small child of about 45 pounds.
If the hair is levered upward at a 30 degree angle, however, the spatulae at the end of the hair easily detach. The gecko does this simply by peeling its toes off the surface.
"It's quite challenging to make these measurements," Full said. "The integration of interdisciplinary ideas really made this project work."
To explain the ability of geckos to rapidly run up a vertical surface and even stick to the ceiling, Full, Autumn and their colleagues considered and ruled out the most common ways animals use to stick to a surface.
They calculated that suction is much less effective than the measured sticking force of geckos. Plus, geckos can stick to the wall in a vacuum. Also, there is no evidence that geckos use glue: there are no glue glands on the foot, nor is glue residue left on the surface. The hairs do not interlock with the surface, as with Velcro, nor is friction likely: friction could not explain their ability to walk on the ceiling. Electrostatic attraction was ruled out by other researchers.
The most likely explanation, Full said, is intermolecular forces so weak that they are normally swamped by the many stronger forces in nature. Intermolecular forces come into play, though, because the gecko foot hairs get so close to the surface.
"The hairs allow the billion spatulae to come into intimate contact with the surface, combining to create a strong adhesive force," Full said. "Our calculations show that van der Waals forces could explain the adhesion, though we can't rule out water adsorption or some other types of water interaction."
Van der Waals forces are among several types of intermolecular forces that are weak until surfaces get very close. When a large area is in contact, though, they can add up to a strong attraction. Van der Waals forces are responsible for the attraction between layers of graphite, for example, and the attraction between enzymes and their substrate.
Van der Waals forces arise when unbalanced electrical charges around molecules attract one another. Though the charges are always fluctuating and even reversing direction, the net effect is to draw two molecules together, such as molecules in a gecko foot and molecules in a smooth wall.
Though the setae work extremely well in adhering to a smooth surface such as glass, Autumn said that in the natural world, waxy coatings on leaves may hinder adhesion by resisting the intermolecular interactions. Therefore, while in the laboratory geckos may not need more than 10 percent of their setae to stick to glass, they may need to use more to walk around on vegetation.
Since the hairs and spatulae work so well, Fearing and Kenny have launched an effort to make artificial hairs that use the same sticking technique and could make a strong yet dry adhesive. In a yet-to-be-published paper, Autumn and Full report, too, that the gecko hairs are self-cleaning, unlike any other known adhesive.
"We clogged their hairs with microspheres, and five steps later they were clean," Full said. "We don't know why, but it's amazing."
The next step, Full and Autumn said, is to find a way to study individual spatulae and measure their attractive force.
They also want to study other creatures that use the same technique, but with somewhat different equipment. Lizards like the anole and the skink use foot hairs of different shapes to walk on vertical surfaces, while even the blood-sucking insect known as the kissing bug has apparently evolved foot hairs with a single spatula at the tip.
Full and Autumn also have an association with IS Robotics, Inc., that has resulted in a mechanical gecko that adopts the peeling action of geckos to walk on vertical surfaces. At the moment, the small robot uses an adhesive glue to stick to the wall. But Full hopes that an artificial dry adhesive will yield a much better and longer lasting robot.
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