Bioinspired Functionally Graded Adhesive Materials: Synergetic Interplay of Top Viscous-Elastic Layers with Base Micropillars

Inspired by the amazing adhesion abilities of the toe pads of geckos and tree frogs, we report an experimental study on the integration of a dissipative material (resembling the dissipative and wet nature of the tree frog toe pads) to an elastic fibrillar interface (resembling the dry and fibrillar nature of the gecko foot pads). Accordingly, a new type of functionally graded adhesive is introduced, which is composed of an array of elastic micropillars at the base, a thin elastic intermediate layer and a viscoelastic top layer. A systematic investigation of this bioinspired graded adhesive structure was performed in comparison with three control adhesive materials: a viscoelastic film, a viscoelastic film coated on a soft elastomer, and elastic film-terminated micropillars. The results showed that this graded structure bestows remarkable adhesive properties in terms of pull-off force, work of adhesion, and structural integrity (i.e., inhibited cohesive failure). Moreover, an extraordinary compliance was observed, which is attributed to the polymer slippage at the top layer. Overall, we attribute the improved adhesive properties to the synergetic interplay of top viscous-elastic layers with the base biomimetic micropillars.