Mechanics of Robust and Reversible Adhesion in Biology: Bottom-up designed hierarchical structures of gecko
Max Planck Institute for Metals Research
Heisenbergstr. 3, D-70569 Stuttgart
Gecko and many insects have evolved specialized hairy tissues which allow them to manoeuvre on vertical walls and ceilings against gravity. It is interesting to observe that these bio-adhesion systems all involve hierarchical, nonhomogeneous and strongly anisotropic materials design. How does gecko stick robustly to random, unpredictable rough surfaces? How can an adhesion system designed for robust attachment simultaneously allow easy detachment upon animal movement? These questions have motivated our investigation of the mechanisms of robust and releasable adhesion of gecko. For single hair contact, we show that optimal adhesion can be achieved by a combination of size reduction and shape optimization. The smaller the size, the less important the shape and the more robust the adhesion. We show that the nanometer scale plays a key role in allowing geckos to achieve their superior adhesive capabilities. We suggest that the principle of flaw tolerance may have had an overarching influence on the evolution of the adhesive tissue structures of gecko. In particular, the nanoscale sizes allow the spatulae ultrastructure of gecko to achieve optimal adhesion strength by restricting the characteristic dimension of the basic structure components so that crack-like flaws do not propagate to break contact. At the level of single hair contact, releasable adhesion can be achieved via asymmetric design of the seta structure of gecko. For large scale contact with rough surfaces, we use a fractal gecko hair model to illustrate how the principle of flaw tolerance leads to a systematic way of extending the superior properties at the nanostructure level to macroscopic length scales via bottom-up hierarchical design. On the other hand, we show that strong elastic anisotropy allows the adhesion strength to vary strongly with the direction of pulling, resulting in an orientation-controlled switch between attachment and detachment. These findings not only provide a theoretical foundation to understand adhesion systems in biology but also suggest possible strategies to develop novel adhesive materials for engineering applications.
 H. Gao and H. Yao, "Shape insensitive optimal adhesion of nanoscale fibrillar structures," 2004, Proceedings of the National Academy of Sciences of USA, Vol. 101, pp. 7851-7856.
 H. Gao, X. Wang, H. Yao, S. Gorb and E. Arzt, "Mechanics of hierarchical adhesion structure of gecko," 2005, Mechanics of Materials, Vol. 37, pp. 275-285.
 H. Yao and H. Gao, “Mechanics of robust and releasable adhesion in biology: bottom-up designed hierarchical structures of gecko,” 2006, Journal of the Mechanics and Physics of Solids, in the press.