Biological microtribology: anisotropy in frictional forces of orthopteran attachment pads reflects the ultrastructure of a highly deformable material

Evolutionarily optimized frictional devices of insects are usually adapted to attach to a variety of natural surfaces. Orthopteran attachment pads are composed of hexagonal outgrowths with smooth flexible surfaces. The pads a re designed to balance the weight of the insect in different positions and on different materials. In a scanning electron microscopy study followed by freezing-substitution experiments, the ultrastructural architecture of the pad material was visualized. In friction experiments! the interaction was measured between the attachment pad and a polished silicon surface. The inn er structure of this material contains distally directed rods, branching cl ose to the surface, and spaces filled with fluid. The specific design of th e pad material provides a higher frictional force in the distal direction. Frictional anisotropy is more enhanced at higher normal forces and lower sl iding velocities. II is concluded that optimal mechanical functionality of biosystems is the result of a combination of surface structuring and materi al design.