Adhesive force of a single gecko foot-hair

Geckos are exceptional in their ability to climb rapidly up smooth vertical surfaces(1-3). Microscopy has shown that a gecko's foot has nearly five hu ndred thousand keratinous hairs or setae. Each 30-130 mu m long seta is onl y one-tenth the diameter of a human hair and contains hundreds of projectio ns terminating in 0.2-0.5 mu m spatula-shaped structures(2,4). After nearly a century of anatomical description(2,4-6), here we report the first direc t measurements of single setal force by using a two-dimensional micro-elect romechanical systems force sensor(7) and a wire as a force gauge. Measureme nts revealed that a seta is ten times more effective at adhesion than predi cted from maximal estimates on whole animals. Adhesive force values support the hypothesis that individual seta operate by van der Waals forces(8,9). The gecko's peculiar behaviour of toe uncurling and peeling(2) led us to di scover two aspects of setal function which increase their effectiveness. A unique macroscopic orientation and preloading of the seta increased attachm ent force 600-fold above that of frictional measurements of the material. S uitably orientated setae reduced the forces necessary to peel the toe by si mply detaching above a critical angle with the substratum.