PEATTIE, A M; FEARING, R S; FULL, R J; Univ. of Calif. Berkeley; Univ. of Calif. Berkeley; Univ. of Calif. Berkeley: Using a simple beam model to predict morphological variation in adhesive gecko hairs

Geckos possess some of the strongest, most versatile adhesives in the animal kingdom. These ‘dry’ adhesives – arrays of microscopic, epidermally-derived hairs (setae) ending in flattened tips called spatulae – have evolved independently in multiple gekkonid lineages as well as in anoles, skinks, and spiders. Setae take both branched and unbranched forms while ranging in length from 1-140μm. To investigate the consequences of this variation, we have developed a simple cantilever beam model of setae that allows predictions of relative adhesive performance based on known or measurable parameters (setal length, width, and density; elastic modulus). Using this model, we predict that (1) longer hairs adhere better to rough surfaces and (2) stiffer hairs can be packed more densely without self-adhering. Using scanning electron microscopy, we recorded setal morphometric data for over 30 species. The model does not predict the geometric scaling of setal morphology with body size, and the data reflect no correlation between body size (SVL) and setal length or width. Setal length and width were highly correlated with each other, possibly resulting in maintenance of stiffness to avoid self-adhesion. The longest setae measured belong to species that routinely climb rough rock surfaces (79.7μm � 18SE), longer on average than animals living on (relatively smooth) plant surfaces (66.7μm � 5.5 SE). We propose to test the beam model�s predictions in the future by direct force measurements of adhesive force in both natural setae as well as manufactured physical models. DARPA N66001-01-C-8072.