Biomedical Engineering Reference
In-Depth Information
Chapter 10
Mechanics of Self-Similar Hierarchical
Adhesive Structures Inspired by Gecko Feet
Haimin Yao and Huajian Gao
Geckos and many insects have evolved specialized adhesive tissues with hierarchical
structures that allow them to maneuver on vertical walls and ceilings. The adhesion
mechanisms of gecko must be robust enough to function on unknown rough surfaces
and easily releasable upon animal movement. How does nature design such robust and
releasable adhesion devices? How can an adhesion system designed for robust
attachment simultaneously allow easy detachment? These questions are discussed in
this chapter from the point of view of contact mechanics and fracture mechanics. On
the question of robust adhesion, a fractal gecko hairs model shows that structural
hierarchy plays a key role in robust adhesion: it allows the work of adhesion to be
exponentially enhanced with each added level of hierarchy. Barring fiber fracture, the
fractal gecko hairs can be designed to achieve flaw tolerant adhesion at any length
scale. However, consideration of crack-like flaws in the hairs themselves results in an
upper size limit for flaw tolerant design. On the question of releasable adhesion, the
asymmetrically aligned seta hairs of gecko form a strongly anisotropic material with
adhesion strength that can significantly vary with the direction of pulling. It is shown
that a strongly anisotropic elastic solid indeed exhibits a strongly anisotropic adhesion
strength when sticking on a rough surface. Therefore, the switch between attachment
and detachment can be achieved through direction control. These findings not only
provide a theoretical foundation to understand adhesion mechanisms in biology, but
H. Yao
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom,
Kowloon, Hong Kong
H. Gao ( * )
School of Engineering, Brown University, Providence, RI, USA
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