Adhesion of multi-level hierarchical attachment systems in gecko feet

Several creatures including insects, spiders and lizards have developed a unique clinging ability that utilizes dry adhesion. Geckos, in particular, have developed the most complex adhesive structures capable of smart adhesion - the ability to cling to different smooth and rough surfaces as well as detach at will. These animals make use of about three million microscale hairs (setae) (about 14000/mm(2)) that branch off into hundreds of nanoscale spatulae (about a billion spatulae). Van der Waals forces are the primary mechanism utilized to adhere to surfaces, and capillary forces are a secondary effect that can further increase adhesion force. The hierarchical surface construction gives the gecko the adaptability to create a large real area of contact with surfaces. Modeling of the gecko attachment system as a hierarchical spring model has provided insight into adhesion enhancement generated by this system. It is shown that a multi-level hierarchical structure produces adhesion enhancement, and this enhancement increases with an increase in applied load and a decrease in the stiffness of springs. A significant adhesion enhancement occurs when the maximum spring deformation is greater than two to three times the root mean square amplitude of surface roughness. Although a gecko is capable of producing on the order of 20 N of adhesion force, it retains the ability to remove its feet from an attachment surface at will. Adhesion design databases for biomimetic attachment systems are presented. These adhesion design databases are useful for understanding biological systems and for guiding the fabrication of biomimetic attachment systems. A man-made fibrillar structure capable of replicating gecko adhesion has the potential for use in dry, superadhesive tapes that would be of use in a wide range of applications. These adhesives could be created using micro/nanofabrication techniques or self-assembly.