In the movie Spider-Man, Peter Parker looks down at the palms of his hands to find that they have sprouted thick, black hairs, giving him a firm grip on walls and ceilings. A growing body of evidence indicates that gecko lizards, too, cling to surfaces with the help of hairlike projections. The gecko hairs are so tiny, however, that they operate not by catching on substrate irregularities, but by facilitating the formation of molecular bonds that create electrodynamic attraction between the gecko's feet and the surface upon which it is walking. As a result, the charismatic creatures can crawl upside down even on polished glass.
Previous research, conducted by Kellar Autumn of Lewis and Clark College and his colleagues, had suggested that the gecko's foot-hairs, or setae, stick to surfaces by virtue of these so-called van der Waals forces. But the team had been unable to reject a competing hypothesis, which holds that the adhesion arises from water-based forces. The new work, detailed in a report published online this week by the Proceedings of the National Academy of Sciences, disproves that theory.
The researchers reasoned that if water-based forces, such as capillary adhesion, were the secret to gecko grip, then the animal's toes--each of which bears hundreds of thousands of setae--should not stick to hydrophobic ("water-fearing") surfaces. In subsequent experiments, however, the gecko toes clung equally well to hydrophobic and hydrophilic ("water-loving") substrates. Single, isolated setae were likewise effective on both types of surfaces.
Autumn and his collaborators further determined that the size of the setal tips--the hundreds of spatulae that branch from each hair, increasing surface density--is remarkably close to what one would expect if van der Waals forces are the principle mechanism underlying the gecko's sticking power. That implied that it is the size of the setal tip, not the nature of the setal material, that gives the lizard its toehold. Verification of this idea came when the researchers fabricated setal tips from two different materials and found that both adhered to surfaces as predicted.
According to the investigators, the finding not only provides insight into the function of setal structures in geckos and other creatures, it hints at how synthetic dry adhesives could be improved: subdividing their surfaces into small, setal tip-like protrusions, thus increasing surface density, might enhance stickiness.