Abstract
Micro- and nano-hierarchical structures (lamellae, setae, branches, and spatulae) on the toe pads of many animals play key roles for generating strong but reversible adhesion for locomotion. The hierarchical structure possesses significantly reduced, effective elastic modulus (Eeff), as compared to the inherent elastic modulus (Einh) of the corresponding biological material (and therefore contributes to a better compliance with the counterpart surface). Learning from nature, three types of hierarchical structures (namely self-similar pillar structure, lamella–pillar hybrid structure, and porous structure) have been developed and investigated.
Highlights
Several groups of animals possess outstanding locomotive abilities on various surfaces in different environments
We summarized the artificial adhesives as belonging to three groups: self‐similar hierarchical pillars, the lamella–pillar hybrid structure, and the porous structure
The self‐similar hierarchical pillar has the highest similarity to the gecko setae; it suffers from the difficulties of Inspired by the structured adhesives in nature, various artificial adhesives have been developed to gain controllable adhesions
Summary
Several groups of animals (gecko, beetle, spider, fly, and tree frog among others) possess outstanding locomotive abilities on various surfaces in different environments. Millions of these contact points per toe pad acquire enough adhesion force to support the body weight of the gecko [14,15] In another example, the material at the tip of adhesive tarsal setae of the ladybird beetle (Coccinella septempunctata) is composed of hard material (resilin) with an Einh ~7.2 GPa in the dehydrated state [16]. The 3D model indicates that the increase in the roughness of the counterpart surface reduces the adhesion performances for the pillar arrays with any hierarchical levels, which is caused by the reduction of contact possibilities. Mimicking the hierarchical adhesive structures, the multilevel spring model was proposed to evaluate the influence of hierarchy on the adhesion performance It suggests that the hierarchical pillar array with more levels, smaller elastic modulus, and larger preload possesses better adaptation to rough surfaces, enhancing adhesion. 2, much structure, which higher than that of the one-level structure [27]
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