SPONBERG, S.; HANSEN, W.; PEATTIE, A.; AUTUMN, K.: Dynamics of Isolated Gecko Setal Arrays

Geckos have the extraordinary ability to run up walls, and to adhere to even molecularly smooth surfaces. The ventral surface of gecko toe pads form arrays of millions of ~5 micron diameter setae, each branched into hundreds of 200 nm spatular tips that form intermolecular bonds with most surfaces. In prior studies, we determined the mechanical requirements of adhesion of isolated gecko setae, and rejected the role of thin capillary films in setal adhesion. Our work instead showed that van der Waals forces are responsible for geckos� ability to climb smooth vertical surfaces. However, the simultaneous function of millions of setae in an array, and the dynamics of setal adhesion have yet to be investigated. Theory predicts that dry, van der Waals surfaces should exhibit no positive force dependence on velocity when pulled parallel to each other. We harvested intact gecko scansors and created isolated arrays of gecko setae. We used a servomanipulation system and a piezoresistive force sensor to test the effect of velocity on parallel force during adhesion of setal arrays. Surprisingly, we discovered that force increased dramatically with velocity. Furthermore, when pulled rapidly (6 mm/s) setal arrays produced forces more than five times that predicted from previous whole animal experiments. Thus, while van der Waals dry adhesion occurs at the level of individual setae, the integration of thousands of setae in an array can yield complex -even fluid-like- dynamics at the macroscopic scale. The dynamic response of arrays of setae may enable a gecko to maintain adhesion during large, rapid perturbations such as with jumping or falling.