Janus-faced micro and nanopillars for geometry and surface chemistry controllable bioinspired dry adhesives

Janus-faced pillar structures are a new class of bioinspired dry adhesives based on the exploitation of unique physical and chemical properties when two different materials are placed on two opposite faces. Using Janus-faced high aspect ratio (HAR) pillars, it is possible to control the geometry and surface chemistry that are key factors in designing gecko-inspired artificial dry adhesives with high performance. In this review, first, we revisit the conventional fabrication techniques for homogeneous, angled HAR structures. Second, we present the fabrication methods for Janus-faced micro and nanopillars via physical and chemical modification of one of the faces. Third, we discuss the geometric features of Janus-faced structures with bending mechanisms of the pillars and the selection of contacting faces. Finally, we cover the theoretical and practical viewpoints of adhesion behavior with a particular focus on the role of work of adhesion, tilt angle, and adhesion hysteresis. We emphasize that Janus-faced structures potentially provide powerful systems to control anisotropic adhesion behavior by taking advantage of both physical and chemical aspects of the angled structures.