Geckos are the envy of rock climbers. Without glue, suction, or claws, these lizards scamper up walls and hang from ceilings.
Scientists finally have pinned down the molecular basis of this seeming magic. Gecko feet are covered by billions of tiny hair tips, or spatulae, that hug surfaces. Temporary shifting of the electrons in the molecules of the spatulae and of opposing rocks, walls, or ceilings creates adhesive van der Waals forces, according to a study in the Aug. 27 Proceedings of the National Academy of Sciences. The collective action of these subtle intermolecular interactions contributes to countless properties, including a liquid’s boiling point and a polymer’s strength.
Previous research had shown that gecko adhesion relies on intermolecular forces (SN: 7/15/00, p. 47), but scientists weren’t sure whether van der Waals bonding or water adsorption was at work. In water adsorption, a thin layer of the liquid acts like glue, but only on surfaces that readily bond water. The new study, however, shows that geckos cling equally well to water-attracting and water-repelling surfaces. Using mathematical models, the authors report that the width of each spatula is just what would be expected if van der Waals forces were operating.
The small size and high density of the spatulae, rather than their chemical composition, enable geckos to stick to the world so well, report Kellar Autumn of Lewis and Clark College in Portland, Ore., and his colleagues.
Gecko spatulae are made of keratin, the protein in human hair. However, when the scientists made spatulae mock-ups out of either silicon rubber or polyester, each material adhered to many surfaces as well as the real spatulae did.
“Just by splitting a surface into multiple small tips, we can get dry adhesion,” Autumn says. Such structures might serve as a new type of adhesive that doesn’t require messy, smelly liquids.
The work shows that strong adhesion can arise from what are thought to be relatively weak forces, comments Matthew Tirrell of the University of California, Santa Barbara. The highly divided gecko foot is also minutely adaptable to bumpy surfaces and is easy to reposition, he says.
A good adhesive has to both stick and release easily, adds Anthony Russell of the University of Calgary in Alberta. “Getting something to stick is not that hard,” he notes. “Getting it off and being able to use it again, that is one of the neat things that geckos have been able to do.”
