Biologists unlock secret of gecko sticking power

Geckos, nature’s supreme climbers, can race up a polished glass window at a meter per second. They can even do it upside down. Moreover, they can support their entire body weight from a wall with only a single toe.

In the journal Nature, a team of scientists, led by Kellar Autumn, assistant professor of biology at Lewis & Clark, and Robert Full, head of the PolyPEDAL Laboratory at the University of California at Berkeley, reports the first direct measurement of the adhesive function that makes it possible for the gecko to master those amazing feats.

Engineers are using their new knowledge to design tiny robots that may eventually aid search and rescue missions and to create the world’s first dry, self-cleaning glue that will work in outer space and underwater.

The findings have generated a flurry of media attention. Stories have appeared in virtually every major newspaper and news magazine and on television and radio broadcasts throughout the world.

In addition to Autumn and Full, the interdisciplinary team includes engineer Thomas Kenny of Stanford University and engineer Ronald Fearing of the University of California at Berkeley.

Autumn explains that geckos have millions of setae, microscopic hairs on the bottom of their feet that function as a unique adhesive. These tiny setae are only as long as two diameters of a human hair. That’s 100 millionth of a meter long. Each seta ends with 1,000 even tinier pads at the tip. These tips, called spatulae, are only 200 billionths of a meter wide—below the wavelength of visible light.

Using a new type of microscopic force sensor designed by Kenny and a system using a fine aluminum wire designed by Fearing, the scientists took a single seta from a Tokay gecko (Gekko gecko) and measured the tiny forces of adhesion for the first time.

“We got a nasty surprise when we tried to touch the end of the hair to a surface,” Autumn says. “It wouldn’t stick. But when we manipulated it in a particular way, simulating the movements of a gecko’s foot, we discovered that the seta is 10 times more adhesive than predicted from prior measurements on whole animals. In fact, the adhesive is so strong that a single seta can lift the weight of an ant. A million setae, which could easily fit onto the area of a dime, could lift a 45-pound child. Our discovery explains why the gecko can support its entire body weight with only a single finger.”

The measurements reject two proposed forms of adhesion: suction and chemical bonding. Suction is less effective than the adhesive force of geckos, and geckos can stick to a smooth surface even in a vacuum. Also, geckos don’t have glue glands on their feet and don’t leave glue residue on surfaces.

Moreover, the setae don’t interlock with surfaces as with Velcro, and friction can’t explain the ability of geckos to walk on ceilings. Other researchers had ruled out electrostatic attraction.

“We believe that the seta sticks using only intermolecular forces,” Autumn says. “These are weak electrodynamic forces that operate over very small distances.”

“The intermolecular forces come into play because the gecko foot hairs get so close to the surface,” Full adds. “The hairs allow the billion spatulae to come into intimate contact with the surface, combining to create a strong adhesive force.”

“Our results are consistent with the hypothesis that intermolecular forces, such as van der Waals or fixed dipolar interactions due to water molecules, are responsible for adhesion in gecko setae,” Autumn says. “The precise intermolecular force, or forces involved, remains undetermined. This is a focus of ongoing research.”

Anne Peattie, a Lewis & Clark senior, is pursuing this question for her honors thesis in biology.

The scientists also discovered that geckos have an amazing way of walking that enables them to stick and unstick themselves 15 times a second as they run up walls.

“When the gecko attaches itself to a surface, it uncurls its toes like a party favor that uncurls when you blow into it,” Autumn says. “But,” he adds, “if you are a gecko, getting yourself to stick isn’t really that difficult. It’s getting off the surface that is the major problem.

“We discovered that effective function of the adhesive requires a small push into the wall and then a small rearward displacement,” Autumn explains. “We also discovered that simply increasing the angle of the setal shaft allows the adhesive to pop off the wall.”

Also, the scientists discovered that the gecko adhesive works in a vacuum and underwater, leaves no residue and is self-cleaning.

“Tape gets dirtier and dirtier with repeated use,” Autumn notes. “However, a gecko’s foot gets cleaner and cleaner.”

The scientists clogged the gecko hairs with submicron particles, but five steps later the hairs were clean.

“We don’t know how geckos do that, but it is so amazing that we are working hard to figure it out,” Autumn says.

He foresees countless applications for synthetic gecko adhesive.

“Our discoveries show us how to control the adhesive to rapidly attach and detach the setae,” Autumn says. “Single fabricated setae could be used for pick-and-place applications under clean-room conditions in microchip manufacturing and nanoscale surgery. And astronauts could use gecko tape to stick to satellites.”

Currently, the scientists have filed a patent and are working with Fearing at U.C. Berkeley to design and fabricate synthetic gecko setae.

“We will now see if we can fabricate the gecko hair to produce the first dry, self-cleaning glue,” Autumn says.

Autumn recently received funding from the Defense Advanced Research Projects Agency (DARPA) to continue his research on gecko adhesion. The scientists are working with IS Robotics in Boston to design tiny robots that can climb on walls and can even climb upside down. Because geckos treat an obstacle like any other surface and crawl right over it, gecko robots might be useful in search and rescue missions.

Currently, Autumn is working on sending his geckos into outer space to test gecko adhesion under zero-gravity conditions.

The diverse gecko, with its 850 species, has a worldwide range. Most of the species are nocturnal, —effective climbers and energy efficient and most species live in the tropics, where extinction is rampant. Almost all have yet to be carefully studied.

“We are using evolution as a sort of rapid prototyping,” Autumn says.

“With 850 species, which prototype do we choose? Biodiversity represents a library of potential engineering principles. But extinction is taking the books off the shelf before we get a chance to read them.

“If for no other reason, it makes good sense dollarwise to protect this planet’s tremendous biodiversity,” Autumn says.

The gecko research is supported by the National Science Foundation, IS Robotics, DARPA in the U.S. Department of Defense and the Office of Naval Research.

—by Jean Kempe-Ware