Adhesion and Friction Force Coupling of Gecko Setal Arrays: Implications for Structured Adhesive Surfaces ² Boxin Zhao, ‡ Noshir Pesika, ‡ Kenny Rosenberg, ‡ Yu Tian, ‡,§ Hongbo Zeng, ‡ Patricia McGuiggan, ‡ Kellar Autumn, | and Jacob Israelachvili* ,‡ Department of Chemical Engineering and Materials Research Laboratory, UniVersity of California at Santa Barbara, Santa Barbara, California 93106, State Key Lab of Tribology, Department of Precision Instruments, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Biology, Lewis & Clark College, Portland, Oregon 97219 ReceiVed July 16, 2007. In Final Form: October 12, 2007 The extraordinary climbing ability of geckos is partially attributed to the fine structure of their toe pads, which contain arrays consisting of thousands of micrometer-sized stalks (setae) that are in turn terminated by millions of fingerlike pads (spatulae) having nanoscale dimensions. Using a surface forces apparatus (SFA), we have investigated the dynamic sliding characteristics of setal arrays subjected to various loading, unloading, and shearing conditions at different angles. Setal arrays were glued onto silica substrates and, once installed into the SFA, brought toward a polymeric substrate surface and then sheared. Lateral shearing of the arrays was initiated along both the “gripping” and “releasing” directions of the setae on the foot pads. We find that the anisotropic microstructure of the setal arrays gives rise to quite different adhesive and tribological properties when sliding along these two directions, depending also on the angle that the setae subtend with respect to the surface. Thus, dragging the setal arrays along the gripping direction leads to strong adhesion and friction forces (as required during contact and attachment), whereas when shearing along the releasing direction, both forces fall to almost zero (as desired during rapid detachment). The results and analysis provide new insights into the biomechanics of adhesion and friction forces in animals, the coupling between these two forces, and the specialized structures that allow them to optimize these forces along different directions during movement. Our results also have practical implications and criteria for designing reversible and responsive adhesives and articulated robotic mechanisms. Introduction The extraordinary climbing ability of geckos has stimulated extensive research because of interest in its own right and also because of technological implications in designing dry, responsive adhesive systems and robots. Much effort has been focused on understanding the high adhesion of geckos, which includes experimentally imaging and characterizing the fine structures of gecko foot pads 1-4 and measuring the adhesive forces of single gecko foot hairs and even single spatulae 5,6 and theoretically modeling the adhesion by applying continuum mechanics-based theories such as the Hertz and JKR theories. 7-9 The superadhesive ability of geckos to stick to almost any surface, whether hydrophilic or hydrophobic, rough or smooth, has been attributed to the fine structure of its toe pads, which contain arrays consisting of thousands of micrometer-sized stalks (setae) that are terminated by millions of fingerlike pads (spatulae) having nanoscale dimensions. These millions of fingerlike pads allow for a large “real” contact area to form so that the gecko foot pads can adhere to almost any surface via the weak but universal van der Waals force 10,11 and other particular types of noncovalent forces such as capillary forces. 12,13 The hierarchy of biological nano/micro/macrostructures and functions has been applied technologically in the design of man- made gecko mimics 14-16 and the development of new method- ologies to tune adhesion by fabricating fine microstructured surfaces. 17,18 Thus, gecko foot pad analogues or “structured adhesives” are often straight pillars or fibers arrayed perpendicular to the substrate 14 or inclined to the substrate, 16,19 and are made of either stiff silicon materials 15 or compliant polymers. 14,16,20 Both experimental and theoretical studies 9,19,21,22 have revealed that these fibrillar surfaces or structures are more deformable and have larger fracture zones so as to have stronger adhesion ² Part of the Molecular and Surface Forces special issue. * To whom correspondence should be addressed. 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