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vanderwaals F of setae - Evidence for van der Waals...

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Evidence for van der Waals adhesion in gecko setae Kellar Autumn †‡ , Metin Sitti § , Yiching A. Liang , Anne M. Peattie , Wendy R. Hansen , Simon Sponberg , Thomas W. Kenny , Ronald Fearing § , Jacob N. Israelachvili**, and Robert J. Full †† Department of Biology, Lewis & Clark College, Portland, OR 97219; Departments of § Electrical Engineering and Computer Science, and †† Integrative Biology, University of California, Berkeley, CA 94720; Department of Mechanical Engineering, Stanford University, Stanford, CA 94305; and **Department of Chemical Engineering, University of California, Santa Barbara, CA 93105 Edited by Thomas Eisner, Cornell University, Ithaca, NY, and approved July 9, 2002 (received for review April 29, 2002) Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydro- phobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydro- phobic and hydrophilic surfaces of a microelectro-mechanical sys- tems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures. I n the 4th century B.C., Aristotle observed the ability of the gecko to ‘‘run up and down a tree in any way, even with the head downwards’’ (1). Two millennia later, we are uncovering the secrets of how geckos use millions of tiny foot-hairs to adhere to even molecularly smooth surfaces. We tested the two currently competing hypotheses (2, 3) of adhesion mechanisms in gecko setae: ( i ) thin-film capillary forces (or other mechanisms relying on hydrophilicity) and ( ii ) van der Waals forces. First, we tested the capillary and van der Waals hypotheses experimentally.
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