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First published online August 30, 2006
Journal of Experimental Biology 209, 3558-3568 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02469

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Effective elastic modulus of isolated gecko setal arrays

K. Autumn^1,*, C. Majidi², R. E. Groff^2,, A. Dittmore¹ and R. Fearing²

¹ Department of Biology, Lewis & Clark College, Portland, OR 97219, USA
² Department of Electrical Engineering and Computer Science, University of California, Berkeley, CA 94720, USA

^* Author for correspondence (e-mail: autumn{at}lclark.edu)

Accepted 5 June 2006

Conventional pressure sensitive adhesives (PSAs) are fabricated from soft viscoelastic materials that satisfy Dahlquist's criterion for tack with a Young's modulus (E) of 100 kPa or less at room temperature and 1 Hz. In contrast, the adhesive on the toes of geckos is made of ß-keratin, a stiff material with E at least four orders of magnitude greater than the upper limit of Dahlquist's criterion. Therefore, one would not expect a ß-keratin structure to function as a PSA by deforming readily to make intimate molecular contact with a variety of surface profiles. However, since the gecko adhesive is a microstructure in the form of an array of millions of high aspect ratio shafts (setae), the effective elastic modulus (E_eff) is much lower than E of bulk ß-keratin. In the first test of the E_eff of a gecko setal adhesive, we measured the forces resulting from deformation of isolated arrays of tokay gecko (Gekko gecko) setae during vertical compression, and during tangential compression at angles of +45° and -45°. We tested the hypothesis that E_eff of gecko setae falls within Dahlquist's criterion for tack, and evaluated the validity of a model of setae as cantilever beams. Highly linear forces of deformation under all compression conditions support the cantilever model. E_eff of setal arrays during vertical and +45° compression (along the natural path of drag of the setae) were 83±4.0 kPa and 86±4.4 kPa (means ± s.e.m.), respectively. Consistent with the predictions of the cantilever model, setae became significantly stiffer when compressed against the natural path of drag: E_eff during -45° compression was 110±4.7 kPa. Unlike synthetic PSAs, setal arrays act as Hookean elastic solids; setal arrays function as a bed of springs with a directional stiffness, assisting alignment of the adhesive spatular tips with the contact surface during shear loading.

Key words: gecko, adhesion, pressure sensitive adhesive, material science, contact mechanics, biomechanics

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