Although we now have thousands of studies focused on the nano-, micro-, and whole-animal mechanics of gecko adhesion on clean, dry substrates, we know relatively little about the effects of water on gecko adhesion. For many gecko species, however, rainfall frequently wets the natural surfaces they navigate. In an effort to begin closing this gap, we tested the adhesion of geckos on submerged substrates that vary in their wettability. When tested on a wet hydrophilic surface, geckos produced a significantly lower shear adhesive force (5.4 ± 1.33 N) compared with a dry hydrophilic surface (17.1 ± 3.93 N). In tests on an intermediate wetting surface and a hydrophobic surface, we found no difference in shear adhesion between dry and wet contact. Finally, in tests on polytetrafluoroethylene (PTFE), we found that geckos clung significantly better to wet PTFE (8.0 ± 1.09 N) than dry PTFE (1.6 ± 0.66 N). To help explain our results, we developed models based on thermodynamic theory of adhesion for contacting surfaces in different media and found that we can predict the ratio of shear adhesion in water to that in air. Our findings provide insight into how geckos may function in wet environments and also have significant implications for the development of a synthetic gecko mimic that retains adhesion in water.contact angle | superhydrophobicity | van der Waals | friction | bioinspired adhesive O ver the past decade, researchers have made extraordinary progress in understanding how the gecko adhesive system works (1-8). Indeed, many laboratories have tested hundreds of synthetic mimics for potential use in robotics, medicine, space, and everyday life (9-21). Although the range and performance of synthetic "gecko-tapes" are impressive, important gaps remain in our knowledge of the system and its capabilities in natural environments. Geckos are extremely diverse, constituting more than 1,400 species worldwide (22, 23). However, knowledge of the natural substrates and conditions geckos use is very limited. For example, it is likely that many species move across leaves and other plant structures that are not perfectly smooth and have variable surface chemistries (24,25). In principle, the interaction of gecko feet with such surfaces may have a significant effect on adhesion, yet gecko research has only just begun to tackle such questions (26-28). Additionally, natural surfaces are likely to become wet (especially in the tropics) and dirty, potentially reducing adhesion. Although research on the ability of geckos to remove dirt from their toes has received some attention (29,30), studies on wetting and the effect of water are limited, despite the well-known antiwetting properties of the toes, which are superhydrophobic and have a low-contact-angle hysteresis (31, 32).Somewhat surprisingly, geckos cannot stick to hydrophilic glass when it is covered with a layer of water (33). Anecdotally, this effect has been long and widely appreciated; nevertheless, the effect of water on gecko adhesion is complex. For example, a thin water ...

When the adhesive toe pads of geckos become wet, they become ineffective in enabling geckos to stick to substrates. This result is puzzling given that many species of gecko are endemic to tropical environments where water covered surfaces are ubiquitous. We hypothesized that geckos can recover adhesive capabilities following exposure of their toe pads to water by walking on a dry surface, similar to the active self-cleaning of dirt particles. We measured the time it took to recover maximum shear adhesion after toe pads had become wet in two groups, those that were allowed to actively walk and those that were not. Keeping in mind the importance of substrate wettability to adhesion on wet surfaces, we also tested geckos on hydrophilic glass and an intermediately wetting substrate (polymethylmethacrylate; PMMA). We found that time to maximum shear adhesion recovery did not differ in the walking groups based on substrate wettability (22.7±5.1 min on glass and 15.4±0.3 min on PMMA) but did have a significant effect in the non-walking groups (54.3±3.9 min on glass and 27.8±2.5 min on PMMA). Overall, we found that by actively walking, geckos were able to self-dry their wet toe pads and regain maximum shear adhesion significantly faster than those that did not walk. Our results highlight a unexpected property of the gecko adhesive system, the ability to actively self-dry and recover adhesive performance after being rendered dysfunctional by water.