  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.
