Directional adhesion for climbing: theoretical and practical considerations

Using the gecko as inspiration, important principles are revealed for reliable maneuvering on vertical surfaces. Foremost among these is the directional behavior of the gecko adhesive system, which permits control of adhesion via control of the tangential forces at the feet. Multiple hierarchical levels of compliance are also important for conforming intimately to surfaces with varying degrees of roughness and different length scales. In light of these requirements, most previously developed synthetic adhesives are not well suited for application on a climbing robot. We describe a synthetic fibrillar adhesive, termed Directional Polymer Stalks, made from relatively soft polyurethane (modulus of elasticity ≈ 300 kPa). The fibrils are angled 20° with respect to vertical and are approximately 1 mm long and 380 μm in diameter. Rather than having a flat top, they have angled faces at 45°. The directional nature of these angled stalks is shown, achieving a maximum adhesion of approximately 1 N for a 3.9 cm2 patch when pulled in the direction in which the stalks are angled. When pulled in the non-adhesive direction, the adhesion forces are negligible. The application to a climbing robot is presented and limitations of the current design are discussed along with ongoing efforts to address them.