Contact: Sarah Yang
University of California - Berkeley
Berkeley -- A new anti-sliding adhesive developed by engineers at the University of California, Berkeley, may be the closest man-made material yet to mimic the remarkable gecko toe hairs that allow the tiny lizard to scamper along vertical surfaces and ceilings.
The researchers say that such an adhesive 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 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, UC Berkeley 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 said, however, 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, only "sticking" when it slides along a smooth surface, not when it is pressed down.
"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," said Fearing. "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."
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.
"For a gecko, this seemingly subtle change could mean the difference between life or death," said 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."
The new research is described in a pair of papers published online Jan. 23 in the Journal of the Royal Society Interface. Members of Fearing's UC Berkeley research team are Jongho Lee, a graduate student in mechanical engineering, and Bryan Schubert, a graduate student in electrical engineering and computer sciences. Co-author Carmel Majidi, a former UC Berkeley graduate student in electrical engineering and computer sciences, is now a post-doctoral researcher at Princeton University.
"This is a major milestone in the new field of gecko-inspired adhesives," said Kellar Autumn, associate professor of biology at Lewis & Clark College in Portland, Ore., who is considered one of the nation's leading experts on gecko movement and who was not a co-author on this latest research. "Fearing's adhesive is made from a plastic that is very hard