The hybrid structure on a tree frog’s toe pad alters the stress distribution at contact interface (red represents highest stress and blue represents lowest stress), enhancing the attaching ability of the frog.
The hybrid structure on a tree frog’s toe pad alters the stress distribution at contact interface (red represents highest stress and blue represents lowest stress), enhancing the attaching ability of the frog.

Why don’t tree frogs slip off wet leaves? The answer lies with their sticky toe pads, which, like many natural composites, are made up of a mixture of hard and soft materials. The combination of soft skin cells, which enable close contact with a surface, reinforced by hard, densely packed nanoscale fibers made from keratin to provide mechanical stability, create a tight grip.

It has proven difficult to fabricate artificial materials with a similar combination of properties but now researchers have mimicked the tree frog’s toe pad with a composite made from polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars [Xue et al., ACS Nano (2017), doi: 10.1021/acsnano.7b04994].

The team from Wuhan University in China, Max-Planck-Institut für Polymerforschung, INM-Leibniz Institute for New Materials, Saarland University, Universität Osnabrück, and Karlsruhe Institute of Technology in Germany, Instituto de Ciencia y Technología de Polímeros in Spain, and the University of Pennsylvania have devised a fabrication method that not only produces a tree-frog-like PDMS/PS composite, but could also be applied to other material combinations and surface pattern designs.

The process starts with the fabrication of PS nanopillars using an aluminum oxide membrane as a template. The reinforcing nanopillars are then treated with vinyl groups that link covalently to a liquid PDMS precursor, which completely fills the gaps between the pillars. A nickel mold is then used to form a hexagonal pattern, before the PDMS is cured. When the nickel mold is removed, the resulting structure consists of hexagonal PDMS pillars reinforced with aligned but rootless PS nanopillars separated by thin channels.

“We found that the inner nanostructure changes the stress distribution at the contact interface, and this results in an enhancement of normal adhesion forces,” explains Longjian Xue,first author of the study.

The nanocomposite structure shifts the maximum stress to the central part of the contact area, suppressing the initiation of cracks from the edges during detachment and enhancing adhesion.

“This is a general design principle, applicable to many different materials,” says Xue. “Tree-frog inspired structured adhesives show the same or even better performance than gecko-inspired structures. In fact, they will work in wet environments, where gecko-inspired adhesives fail.”

The researchers believe that their approach provides useful insights for the design of bioinspired materials possessing both strong adhesion and frictional properties.

“So far, bio-inspired structured adhesive materials have usually been produced using homogeneous, soft polymeric materials,” comments Lars Heepe of the Zoological Institute at Kiel University. “In this sense, the hybrid adhesive material produced by Xue and co-workers presents significant progress in the development of next generation bio-inspired adhesives.”

He believes that it would be interesting to test the researchers’ approach with other surface microstructure geometries, which are known to provide even higher adhesion forces than hexagonal micropillars.

The researchers are planning to do just this, says Xue, by investigating the various design parameters of the nanocomposite, which they believe are most likely to influence adhesion performance. Different,more efficient fabrication approaches, including 3D printing, will also be explored for creating new nanocomposites.

This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.10.003.