Functionally Graded Gecko Setae and the Biomimics with Robust Adhesion and Durability

Geckos have the extraordinary ability to adhere and move across varied surfaces, while keeping their tiny high-aspect-ratio foot-hairs intact for thousands of attachment-detachment cycles. Inspired by the dry adhesive structure of gecko sole, various gecko-inspired artificial mimics have been developed, but many of them suffer from premature failures and short fatigue life. Herein, we discover that individual gecko seta is a functionally graded material. Its Young's modulus gradually decreases from base to tip, with up to 20 times of difference in magnitude. Finite element analysis indicates that this gradient design is the key to make the natural setal stalks more flexible (critical for producing large frictional adhesion on rough surfaces) and less stressed (critical for achieving high fatigue resistance) during each attachment. Inspired by these findings, we have fabricated poly(dimethylsiloxane) (PDMS)-based artificial gecko foot-hairs with a gradient distribution of magnetic nanoparticles as the reinforcements, achieving similar varying modulus/stiffness. The biomimetic hairs/pillars show enhanced fatigue resistance compared to the uniform counterparts. This work opens a door in designing dry adhesives with both high adhesive strength and long fatigue life.