Mechanics and bio-mimicking of wet adhesion in nature

Achieving strong and reliable adhesion in wet environments remains a persistent challenge for biomedical, marine, and robotic applications. Many aquatic organisms have evolved specialized strategies to address this limitation, such as the collagen rich thread-plaque systems of marine mussels, the suction cups of octopuses, the capillary-driven toe pads of tree frogs, and the friction-enhancing attachment organ of Pomphorhynchus laevis. Inspired by these systems, researchers have developed bio-mimetic adhesives that combine chemical adhesion with mechanical design principles to perform effectively underwater or in wet conditions. This review surveys recent advances in experimental approaches for quantifying adhesion, with particular emphasis on traction force microscopy (TFM), and highlights the crucial role of substrate materials and tracking features in determining force resolution. TFM-compatible substrates, including polyacrylamide (PAA) hydrogels, polyethylene glycol (PEG) hydrogels, and polydimethylsiloxane (PDMS), are evaluated with respect to their tunable mechanics, fabrication methods, and application-specific suitability. Theoretical models, covering peel, tension, and shear for chemical adhesion, as well as interlocking and friction, suction, and capillarity mechanisms, are critically assessed to reveal their progress, limitations, and key parameters for predicting adhesion in complex environments. Bio-inspired applications are examined across biomedical adhesives, wearable devices, underwater grippers, and energy-harvesting platforms, emphasizing the multi-functionality enabled by hierarchical and adaptive designs. Finally, emerging opportunities in advanced force measurement techniques and the translation of biological principles into multifunctional technologies are outlined as pathways toward scalable, adaptive wet adhesives with broad technological impact. This work provides an integrative framework linking natural wet adhesion with engineered solutions and identifies future directions for materials innovation, experimental development, and real-world implementation.