Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry Adhesion

For biologically inspired dry adhesives, the fibrillar structure of the surface requires sufficient flexibility to form contacts and meanwhile high rigidity to maintain stability. This fundamental conflict has greatly hindered the advance of synthetic adhesives toward mass-scale and practical applications, where adhesion is desired to be simultaneously strong, durable, directional, and roughness-adaptive. In this work, we overcome such a long-term challenge by developing fibrillar structures that combine both slanted geometry and gradient material of micropillars. The termed slanted functional gradient pillars (s-FGPs), fabricated by a magnetically assisted mold replication technique, exhibit flexible tips for contacts, gradually stiffened stalks for reinforcement, slanted structure to give rise to anisotropy, and high aspect ratio (AR) to facilitate surface adaptation. We demonstrate that the material and structure of the s-FGPs complement each other, synergetic effects of which result in a multifunctional combination of adhesion properties including high strength (∼9 N/cm2 in shear), ultradurability (over 200 cycles of attachment/detachment without adhesion degradation), super anisotropy (anisotropic ratio of ∼7), and good adaptability to rough surfaces. The s-FGPs not only step forward the bioinspired adhesion toward optimized designs and performances for practical applications but may also open up other concepts for various high-AR and structurally stable fibrillar surfaces with emerging functionalities and applications in the fields of self-cleaning, superhydrophobicity, biosensors, energy harvesting, etc.