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Machine Design

Sticky lizard toes may be the key to a self-cleaning adhesive

A tiny lizardlike creature might hold the key to duct tape that never loses its stick and bandages that come off without sticky residue.

Gecko feet may hold the key to the development of synthetic self-cleaning adhesives, according to a biologist from Lewis & Clark College. "How geckos manage to keep their feet clean while walking about with sticky feet has remained a puzzle until now," said Kellar Autumn, Lewis & Clark associate professor of biology. "Geckos don't groom their feet, and the adhesive on their toes is much too sticky for dirt to be shaken off. Conventional adhesives like tape just get dirtier and dirtier, but we discovered that gecko feet actually become cleaner with repeated use."

This research found that the microscopic adhesive hairs, or setae, that create the gecko's adhesive qualities are also the first known selfcleaning adhesive. According to Autumn, gecko setae isolated from the gecko become cleaner by themselves.

"Our mathematical models suggest that self-cleaning in gecko setae is a result of geometry not chemistry," said Autumn. "This means that synthetic self-cleaning adhesives could be fabricated from a wide variety of materials. The possibilities for future applications of a dry, self-cleaning adhesive are enormous. We envision uses for our discovery ranging from nanosurgery to aerospace applications." Who knows? Maybe a gecko-inspired robot with sticky, self-cleaning feet will walk on the dusty surface of Mars someday.

An interdisciplinary team of researchers confirmed speculation that the gecko's amazing climbing ability depends on weak molecular van der Waals forces, electrodynamic forces that operate over very small distances. The research team rejected a 30-yearold model based on the adhesion chemistry of water molecules.

The microscopic hairs on the bottom of gecko's feet are only as long as two diameters of a human hair. That's 100 millionths of a meter long. Each hair 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.

In 2002, Ronald Fearing, a researcher at the University of California at Berkeley, was able to produce two artificial hair tips, while Autumn and colleagues concluded that "both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures." Fearing's group later made the first array of synthetic gecko hairs with long stalks (6-µm stalk) and relatively large diameters (6-µm diameter).

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