Developing adhesives for climbing robots requires the ability to support large shear forces, and adhesive structures capable of generating large friction forces are therefore required. Weak, attractive van der Waals forces exist between every pair of materials in intimate contact in air/vacuum. However, except for certain molecularly smooth surfaces, roughness due to asperities prevents close contact from being achieved over areas large enough for practical purposes. Several species of gecko are able to achieve such close contact over a wide variety of rough surfaces in nature and can thus exploit the universal van der Wals force for climbing by using a vast array of multi-scale hierarchical fibers. These fibers, by conforming to both micro- and nano-scale asperities, achieve a large true area of contact for high adhesion and friction. Further, the fibers on the gecko’s feet are angled so as to naturally exhibit anisotropy in their adhesion and friction forces when engaged and displaced along opposing directions.
Synthetic adhesives are being developed that make use of large arrays of micrometer and submicron hierarchical polymer fibers, batch-fabricated by massively parallel microelectromechanical systems (MEMS) fabrication techniques. Adhesion controllability is a key property for repeated sticking and peeling off during climbing. The pre-angled molded hierarchical polymer structures currently being developed combine high adhesion and friction with on-off controllability.