Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix–fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

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The contribution of water to gecko and gecko-inspired adhesion remains a debate. Here, by investigating the adhesion performances of gecko-inspired nanopillar arrays in humid environments, the function of water in dry adhesion is discussed. Adhesion increases with the increase of relative humidity for both hydrophilic and hydrophobic nanopillar arrays. The superficial layer of both kinds of nanopillars are softened by water, forming a kind of “soft shell–stiff core” structure. The core–shell structure reduces the stress at contact perimeter, enlarges the cohesive zone, and increases the tolerance to misalignment on contacting surfaces, contributing to the enhancement of normal adhesion. The result suggests a mechanism for the function of water in gecko and gecko-inspired dry adhesions.

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