Use of opposite frictional forces by animals to increase their attachment reliability during movement

Many animals have the natural ability to move on various surfaces, such as those having different roughness and slope substrates, or even vertical walls and ceilings. Legged animals primarily attach to surfaces using claws, soft and hairy pads, or combinations of them. Recent studies have indicated that the frictional forces generated by these structures not only control the movement of animals but also significantly increase the reliability of their attachment. Moreover, the frictional forces of various animals have opposite characteristics and hierarchical properties from toe-to-toe and leg-to-leg. These opposite frictional forces allow animals to attach securely and stably during movement. The coordination of several attachment (adhesion) modes not only helps animals adhere, which would be impossible in single mode, but also increases the overall stability of the attachment (adhesion) system. These findings can help the design of highly adaptable feet for bionic robots in the near future.