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Engineers create new adhesive that mimics a gecko's super-sticky toe hairs
It's directional, relieving the lizards - and, perhaps someday, even robots - of the need to apply pressure to adhere to a surface

| 06 February 2008

A new anti-sliding adhesive developed by Berkeley engineers may be the closest man-made material yet to mimic the remarkable toe hairs that allow a gecko to scamper along vertical surfaces and ceilings. Such an adhesive, they say, could one day be used to outfit a small robot that could climb up walls.

Taking a cue from the millions of hairs covering a gecko's toes, researchers squeezed 42 million hard-plastic microfibers onto each square centimeter of backing material and loaded it with various weights. They found that on a smooth, clean, vertical surface, two square centimeters of the synthetic adhesive could hold 400 grams (0.88 pounds). At the same time, the adhesive easily lifts off with minimal force and no residue.

Scientists have long marveled at the gravity-defying feats of the gecko, and a number of research teams across the world are working on duplicating the lizard's adhesive forces. Ron Fearing, professor of electrical engineering and computer sciences and head of the research team developing the new material, notes that previous research on gecko-like adhesives has focused on the strength of the adhesion. He says that the ease of attachment and detachment are equally important when developing a material that can practically be used for scaling vertical walls and ceilings.

What sets this new gecko-inspired adhesive apart from the others created thus far is that it is directional, "sticking" only when it slides along a smooth surface, not when it is pressed down.

"This difference is critical," says Fearing. "If you're climbing up vertical surfaces, you can't afford to use a lot of energy pressing down into the surface to stick. Furthermore, using force to attach also requires force to detach. A gecko running uphill may be attaching and detaching its feet 20 times a second, so it'd get very tired if it had to work hard to pull its feet off at every step."

No force required

The microfibers, made of polypropylene, are 20 microns long, or one-fifth the thickness of a sheet of paper, with a diameter of 0.6 microns, or one-hundredth the diameter of a human hair. The structure is similar to a microfiber array developed by the same group in 2006. That material relied upon friction to work, however, requiring the application of force to make it stick. Changes made to the plastic backing enabled the directional adhesion reported in this new material to work on truly vertical surfaces.

Changes made to the adhesive's plastic backing enabled the directional adhesion reported in this new material to work on truly vertical surfaces. "For a gecko, this seemingly subtle change could mean the difference between life or death," says Fearing. "With friction only, a gecko would fall from a wall or ceiling. With directional adhesion, a gecko can stop itself from falling because the mechanism works without the need for force that is perpendicular to the surface."

This new material is also an example of a "smart adhesive," becoming stronger with use, says Fearing, who is also part of the campus Center for Information Technology Research in the Interest of Society (CITRIS). "More microfibers bend and engage automatically as the weight increases. When the load is removed, the microfibers disengage. This allows for controlled attachment and detachment and is a critical innovation for clean release and reuse. It's the first time this has been demonstrated with a hard-microfiber array."

Another benefit to using hard polymers is that the fibers are less prone than softer plastic to collecting dirt after repeated use.

So far, the new adhesive works only on smooth, clean surfaces. The next step, says Fearing, is to develop a material that can also adhere to rough surfaces and that is self-cleaning.

Fearing and his colleagues are part of a Nanoscale Interdisciplinary Research Team supported by the National Science Foundation and specifically tasked in 2003 with developing synthetic adhesives that perform like gecko hairs. The new research is described in a pair of papers published online Jan. 23 in the Journal of the Royal Society Interface.