Nano meets beetles from wing to tiptoe: Versatile tools for smart and reversible adhesions

Nanoscale observation of beetle's attachment systems has revealed various exquisite multiscale architectures for essential functions such as wing fixation, crawling, mating, and protection from predators. Some of these adhesion systems are mediated by liquid secretion (capillary force), whereas some are purely operated by direct interlocking of high-density microfibers or contact of mushroom-like hairy structures (van der Waals force). In this review, we present an overview of recent advances in beetle-inspired, artificial dry and wet adhesives in the context of nanofabrication and material properties. For convenience, the beetle's adhesions from wing to tiptoe are classified into four types: hair interlocking, mushroom-shaped pads, oil-assisted spatula-shaped pads, and claws. After introducing the structural features and functions of these systems, we describe how the current nanofabrication methods can be applied to mimic or exploit the systems. Furthermore, relevant beetle-inspired structural materials, devices (fastener, medical tape, electric connector, etc.) and microrobots are briefly overviewed, which would shed light on future smart, directional and reversible adhesion systems.