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Stanford Report, June 28, 2000

Tiny foot hairs key to geckos' sticky feats

BY DAWN LEVY

Geckos can climb glass walls and hang from the ceiling by one foot. Just don't shave their feet. For the secret of their stickiness, researchers wrote in the June 8 issue of the journal Nature, is weak forces created by tiny foot hairs.

Scientists at the University of California-Berkeley, Lewis and Clark College and Stanford have been working together to study the adhesive properties of these lizards' supersticky feet. Understanding how geckos attach to surfaces may lead to novel self-cleaning adhesives, scratch-free manufacturing of computer chips and robots that can crawl over rough terrain without toppling.

Graduate student Yiching Liang and assistant professor of Mechanical Engineering Tom Kenny are part of a team that has been studying the adhesive
qualities of geckos' feet and their possible applications to computer chips and robots. (Photo by L.A. Cicero)

"The gecko is a macroscopic creature using a microscopic force to do something interesting," says Stanford's Tom Kenny, an assistant professor of mechanical engineering. For the Nature study, he and graduate student Yiching Liang used an extremely sensitive instrument to measure the force of a single gecko foot hair.

The project is part of a Multi University Research Initiative sponsored by the Office of Naval Research. Its purpose is to create robots that mimic the behavior of animals. Engineers study the locomotion of crabs, cockroaches and other creatures to borrow from nature's designs. They hope to build robots so agile and economical that they can search disaster sites for survivors, find landmines, explore Mars and aid humans in other automated ways.

"The gecko is a possible model for a robot because it does things that we haven't been able to make robots do," says Kenny, who notes that machines often fall over on bumpy surfaces or get stuck due to their limited range of movement. The project is an important demonstration that mechanical engineering can play a big role in building the understanding of biology, he says.

Researchers who looked at macroscopic properties of geckos for the Nature paper included Kellar Autumn at Lewis and Clark and S. Tonia Hsieh, Wolfgang Zesch, Wai Pang Chan and Robert Full of Berkeley. When they needed to shift their focus from macroscopic to microscopic to study the mechanism of adhesion, they turned to Kenny's group at Stanford.

To measure attachment forces parallel and perpendicular to a surface, Liang used a micromachined silicon cantilever connected to a silicon chip. It converts mechanical movement to an electrical signal and is capable of measuring a force more than a million times smaller than that exerted by a drop of water.

courtesy Kellar Autumn

Liang affixed a single gecko foot hair to an insect pin with epoxy and pressed it into contact with the end of the cantilever. By analyzing the results, the researchers were able to determine the adhesive forces of individual hairs and study the variations in the force as the hairs are pressed into or tugged away from the cantilever.

They were able to rule out several suspects in gecko stickiness -- suction, friction, electrostatic charge interactions and glue.

Each lizard's foot sports half a million hairs, which are one-tenth the thickness of a human hair. Their ends are multiply split. At the frayed ends are spatula-shaped structures -- about a billion per gecko. Whereas the pointed tip of a typical shaft of hair can make contact with only a small surface area, a tip shaped like the flipper portion of a spatula can flatten over bumpy surfaces and increase the area with which the hair can make contact.

Big surface area is important because the individual forces responsible for gecko adhesion are weak. Called van der Waals forces, they occur between close objects. Objects have random charges on their surfaces. When two objects near each other, positive charges on one object start to align with negative charges on the other, producing a weak attractive force. But many weak forces taken together can produce an impressive adhesive force.

When geckos run, their feet attach and detach 15 times per second, according to Autumn. How do they stick and unstick so quickly?

Gecko toes curl backward, Liang says. When geckos attach to surfaces, their toes uncurl in a process that resembles inflation of a party favor. The process places the spatula-shaped tips in contact with the surface. To detach, geckos peel their toes away from a surface the way tape is removed, lifting first from one end.

Geckos will stick to metal, plastics or glass, in air or under water, Kenny says. Engineers predicted the force of a single gecko foot hair by measuring the adhesive force of a whole gecko and dividing that by the number of foot hairs per animal. They were surprised when their measurements revealed a single hair to be 10 times more forceful at adhesion than they had estimated.

It turns out a gecko foot exerts a force of 10 Newtons (about a kilogram, or 2.2 pounds). A million hairs could fit onto a dime-sized patch capable of supporting about 20 kilograms (about 44 pounds).

If you want a gecko to really stick, press it gently toward the surface and drag it backward to align its foot hairs and maximize the van der Waals forces. Priming a gecko like this makes its adhesive force 600 times greater than the frictional force of its foot hair.

Attachment is easier than detachment. But to unstick a gecko, Kenny says, it helps to detach the hairs one at a time, which explains why geckos peel their toes away from surfaces. Coating surfaces with talcum powder, graphite shavings or thick oils can prevent the attachment from forming. Fortunately for geckos, these materials are rarely found in nature. SR