A Sticky Situation: Dry and Wet Adhesion

Biological adhesion systems represent some of the most structurally and functionally versatile interfacial solutions in nature, enabling organisms to attach, locomote, and manipulate surfaces across environments ranging from dry rock faces to turbulent marine substrates. This review surveys the structural and mechanistic diversity of biological adhesion, organizing known systems into two principal categories: dry and wet adhesion. Dry adhesion, exemplified by geckos, spiders, and beetles, relies primarily on physical contact forces, most notably van der Waals interactions, arising from hierarchical micro- and nanoscale surface architectures that maximize real contact area without the use of adhesive secretions. Wet adhesion, as demonstrated by marine mussels, octopuses, and tree frogs, operates through biochemical crosslinking, negative-pressure suction, and fluid-displacement mechanisms that act specifically at aqueous or fluid-saturated interfaces. The engineering relevance of these systems is substantial: bioinspired adhesives derived from gecko-like hierarchical fibrillar surfaces have found application in climbing robots, transfer printing, and reversible electronic attachment; mussel-inspired catechol chemistry has enabled a generation of underwater sealants and surgical tissue adhesives; and octopus- and remora-inspired suction technologies inform the design of soft robotic grippers and marine sensor platforms. Despite significant advances, challenges remain in scalable fabrication, contamination resistance, and translating organism-level performance to engineered systems at commercially viable scales. This manuscript reviews each biological model with respect to its structural form, functional mechanism, adhesion classification, documented engineering applications, and inherent limitations, concluding with a synthesis of future research directions critical to advancing the field of bioinspired adhesive materials and systems.