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Scientists at UC Berkeley have developed a nanoscale polymer material that mimics the directional adhesion of a gecko's feet

Known not only for being the charismatic spokes-animation for popular car insurance companies, gecko lizards are also renown for their almost magical ability to scale smooth vertical surfaces and even run along ceilings with gravity defying ease.

Much work has gone into developing materials that would replicate the adhesive properties of gecko toe hairs, which allow them to perform these feats, and building on past work, Ron Fearing and his team of researchers at the University of California, Berkeley have developed such a material that is as close as any have come.

While work by many groups has produced sticky materials that have even more adhesive power than a gecko's feet, few have managed to do so while preserving the delicate balance achieved by nature. Rather than a simple suction type adhesion, a gecko's microscopic toe hairs actually depend on what is known as van der Waals force. Van der Waals force is the momentary attraction between stable molecules and individual atoms due to the random polarization of the particles. Since the effect is much weaker than covalent or ionic bonding, it is also more easily broken.

The new material is an array of microscopic polypropylene fibers, each fiber rounding out at about 0.6 microns in diameter, about one-hundredth the diameter of a human hair, and 20 microns long or one-fifth as thick as a sheet of paper. There are approximately 42 million of the stiff microfibers in a square centimeter of the material. On a clean and smooth surface, two square centimeters of the adhesive will hold 400 grams (0.88 pounds) of weight, but lifts off with ease and without residue.

Like the gecko's feet, the material only sticks when it is moved or forced, as by weight in the previous paragraph, along a smooth surface. Perpendicular forces do not contribute to the adhesiveness in other than the more pressure is applied in a diagonal vector, the more the fibers will make contact with the surface and strengthen the adhesion.

"This difference is critical because if you're climbing up vertical surfaces, you can't afford to use a lot of energy pressing down into the surface to stick," explained Fearing, who is a professor of electrical engineering and computer sciences and UC Berkeley. "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."

While the microfiber material works perfectly on smooth, clean surfaces, and thanks to the stiff polypropylene fibers, is less prone to collecting foreign particles than other materials, Fearing's team's next step is to create a similar material that will also adhere to rough surfaces while being self-cleaning.




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