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6/11/2007

10:53 PM

Page 473

Gecko Adhesion:

Structure, Function, and Applications Kellar Autumn

Abstract Geckos attach and detach their adhesive toes in milliseconds while running with reckless abandon on nearly any surface. The adhesive on gecko toes differs dramatically from that of conventional adhesives. Conventional pressure-sensitive adhesives (PSAs) are soft viscoelastic polymers that degrade, foul, self-adhere, and attach accidentally to inappropriate surfaces. In contrast, gecko toes bear angled arrays of branched, hair-like fibers (setae) formed from stiff, hydrophobic keratin that act as a bed of angled springs with an effective stiffness similar to that of PSAs. Setae are selfcleaning and maintain function for months during repeated use in dirty conditions. Setae are an anisotropic “frictional adhesive” in that adhesion requires maintenance of a proximally directed shear load. Thus, gecko setae resist inappropriate bonding and are capable of easy and rapid attachment and detachment. Engineered adhesive nanostructures inspired by geckos may become the glue of the future—and perhaps the screw of the future as well.

Introduction More than two millennia ago, Aristotle commented on the ability of the gecko to “run up and down a tree in any way, even with the head downwards.” The world’s supreme climbers, geckos are capable of attaching and detaching their adhesive toes in milliseconds while running with apparently reckless abandon on vertical and inverted surfaces. More complete reviews of gecko adhesion can be found in recent literature.1–3 A single foot hair, or seta (Figure 1, D), of the tokay gecko (Figure 1, A) is approximately 110 µm in length and 4.2 µm in diameter.4–6 Setae are similarly oriented and uniformly distributed in arrays (Figure 1, C) on approximately 20 leaf-like layers of tissue, called scansors, on each toe (Figure 1, B). Each seta branches to form a nanoarray of hundreds of spatular structures (Figure 1, E) that make intimate contact with the surface. A single spatula consists of a stalk with a thin, roughly triangular plate at the tip, with the apex of the triangle connecting the plate to its stalk. Spatulae are approximately 0.2 µm in length and width at the tip.4,5 Gecko setae are formed primarily of beta keratin, the tough, insoluble protein that makes up

reptile scales,7–9 with some alpha keratin components.10 Whereas the tokay is currently the best studied of any adhesive gecko species, there are more than 1000 species of gecko,11 encompassing an impressive range of morphological variation at the spatula, seta, scansor, and toe levels.4–8,12–22 Setae have even evolved on the tails of some gecko species.23 Remarkably, setae have evolved convergently in iguanian lizards of the genus Anolis,5,13,24 and in the Scincidae family of lizards of the genus Prasinohaema.4,14

van der Waals Adhesion in Gecko Setae Whereas the setal structures of many gecko species are well documented, a complete understanding of what makes them adhere has proved more elusive. At the turn of the 20th c