Mechanical principles of robust and releasable adhesion of gecko

Robust attachment to rough surfaces and controllable detachment are two features that must coexist for gecko to locomote on walls and ceilings. While robust adhesion ensures that the animal can stick reliably in the presence of random surface roughness, the same adhesion must be readily releasable upon its movement. What are the mechanical principles of such robust and releasable adhesion in nature? In this paper, we review some of the most essential results obtained in our previous studies on this question and also add a number of new results aimed to facilitate a more complete understanding. We begin with addressing the shape and size effects in adhesion between single asperities, and then extend the discussion to contact between rough surfaces, focusing on strategies to achieve robust (flaw-tolerant) and releasable adhesion across multiple length scales. Based on the fundamental principles in fracture and adhesion mechanics, we show that flaw tolerant adhesion is controlled by a dimensionless parameter which suggests a combination of strategies including size confinement, hierarchical energy dissipation, graded elasticity and strength degradation over multiple size scales for robust attachment to rough surfaces. At the same time, we show that strong elastic anisotropy allows adhesion to be controllable depending on the direction of pulling. These results and their future extensions may lead to a theoretical basis to understand robust and releasable adhesion in a variety of biological systems.