Adhesion Force Measurements on Single Gecko Setae

Geckos are known for their exceptional ability to climb up vertical walls. The adhesion mechanism has been the subject of debate for nearly a century. It is commonly assumed in the biology community that this remarkable wall-climbing ability in geckos is attributed to van der Waals forces [l]. In this theory, the tiny hairlike structures on the bottom of gecko feet, known as setae, form van der Waals bonds with the surface to facilitate adhesion. These forces are on the order of pN per seta. Various attempts have been made to experimentally quantify the adhesive forces on whole animals, but none had been able to measure the forces on single setae. Recent development in MEMS technology has produced numerous highly sensitive devices for small force measurements, including a piezoresistive cantilever capable of simultaneous force detections in two directions [2]. We have successfully performed series of adhesion measurements on single setae using this device to support the van der Waals adhesion hypothesis. This paper describes the setup and procedures used to obtain adhesive force measurements on single gecko setae. Experimental results showed that adhesion increases with increasing contact preload and lateral pulling speed. Our data also revealed some interesting characteristics of setal adhesion. The magnitude of the adhesive forces measured supports the hypothesis that individual setae operate by van der Waals forces and sum to generate sufficient forces to explain macroscopic behavior such as clinging or vertical running.