Shear Adhesion Strength of Thermoplastic Gecko-Inspired Synthetic Adhesive Exceeds Material Limits

Here we examine the behavior of high-density polyethylene (HDPE) and polypropylene (PP) microfiber arrays during repeated cycles of engagement on a glass surface, with a normal preload of less than 40 kPa. We find that fiber arrays maintained 54% of the original shear stress of 300 kPa after 10 000 cycles, despite showing a marked plastic deformation of fiber tips. This deformation could be due to shear-induced plastic creep of the fiber tips from high adhesion forces, adhesive wear, or thermal effects. We hypothesize that a fundamental material limit has been reached for these fiber arrays and that future gecko synthetic adhesive designs must take into account the high adhesive forces generated to avoid damage. Although the synthetic material and natural gecko arrays have a similar elastic modulus, the synthetic material does not show the same wear-free dynamic friction as the gecko.

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SEM images of HDPE (left column) and PP (right column) after 0, 150, 300, and 10000 cycles showing the progressive plastic deformation of the fibers over the course of the trials.

Maximum shear force and the corresponding normal force for 300 load–drag–pull steps of (a) HDPE and (b) PP fibers showing no significant change in the peak force over the course of the first 300 trials. For 10000 load–drag–pull steps of (c) HDPE and (d) PP fibers, the shear forces eventually start to decrease.