DITTMORE, A.*; STRATTON, T.; KELLY, T.; SCHRODER, K.; GEISLER, B.; AUTUMN, K.; Lewis & Clark College: Why geckos don’t fall off ceilings
Directional arrays of branched microscopic setae constitute a dry adhesive on the toes of pad-bearing geckos. Geckos are able to easily and rapidly detach their sticky toes as they run on smooth vertical or inverted surfaces. There are two known mechanisms of detachment: 1) on the microscale, the seta detaches when the shaft reaches a critical angle with the substrate, and 2) on the macroscale, geckos digitally hyperextend their toes, apparently peeling like tape. This raises the question of how geckos prevent detachment while inverted on the ceiling, where body weight should cause toes to peel and setal angles to increase. Geckos use opposing feet and toes while inverted, possibly to maintain shear forces that prevent detachment of setae or peeling of toes. If detachment occurs by macroscale peeling of toes, the peel angle should monotonically decrease with applied force. In contrast if adhesive force is limited by microscale detachment of setae at a critical angle, the toe detachment angle should be independent of applied force. We tested the hypothesis that adhesion is increased by shear force in isolated setal arrays and live gecko toes. We also tested the corollary hypotheses that 1) adhesion in toes and arrays is limited as on the microscale by a critical angle, or 2) on the macroscale by adhesive strength as predicted for adhesive tapes. We found that adhesion depended directly on shear force, and was independent of detachment angle. Therefore we reject the hypothesis that gecko toes peel like tape. The linear relation between adhesion and shear force is consistent with a critical angle of release in live gecko toes and isolated setal arrays, and also with our prior observations of single setae. Inverted locomotion of geckos requires opposing shear forces at the level of toes or feet to maintain adhesion and support the weight of the animal. Support: DARPA N66001-03-C-8045, NSF-NIRT 0304730, and J.S. Rogers.
