Frictional adhesion: a new angle on gecko attachment
Author:
Autumn K.1, Dittmore A.1, Santos D.2, Spenko M.2, Cutkosky M.2
Affiliation:
1. Department of Biology, Lewis & Clark College, 0615 SW Palatine Hill Road, Portland, OR 97219, USA and 2. Department of Mechanical Engineering, Stanford University, Building 530,440 Panama Mall, Stanford, CA 94305-3030, USA
Abstract
SUMMARY
Directional arrays of branched microscopic setae constitute a dry adhesive on the toes of pad-bearing geckos, nature's supreme climbers. Geckos are easily and rapidly able to detach their toes as they climb. 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 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. We introduced a new model,frictional adhesion, for gecko pad attachment and compared it to existing models of adhesive contacts. In an analysis of clinging stability of a gecko on an inclined plane each adhesive model predicted a different force control strategy. The frictional adhesion model provides an explanation for the very low detachment forces observed in climbing geckos that does not depend on toe peeling.
Publisher
The Company of Biologists
Subject
Insect Science,Molecular Biology,Animal Science and Zoology,Aquatic Science,Physiology,Ecology, Evolution, Behavior and Systematics
Reference38 articles.
1. Autumn, K. (2006). Properties, principles, and parameters of the gecko adhesive system. In Biological Adhesives (ed. A. Smith and J. Callow), pp.225-255. Berlin Heidelberg: Springer Verlag. 2. Autumn, K. and Hansen, W. (2006). Ultrahydrophobicity indicates a nonadhesive default state in gecko setae. J. Comp. Physiol. A doi:10.1007/s00359-006-0149-y. 3. Autumn, K. and Peattie, A. (2002). Mechanisms of adhesion in geckos. Integr. Comp. Biol.42,1081-1090. 4. Autumn, K., Liang, Y. A., Hsieh, S. T., Zesch, W., Chan, W.-P.,Kenny, W. T., Fearing, R. and Full, R. J. (2000). Adhesive force of a single gecko foot-hair. Nature405,681-685. 5. Autumn, K., Sitti, M., Peattie, A., Hansen, W., Sponberg, S.,Liang, Y. A., Kenny, T., Fearing, R., Israelachvili, J. and Full, R. J.(2002). Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA99,12252-12256.
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