Alyssa Y. Stark; Nicholas A. Wucinich; Eva L. Paoloni; Peter H. Niewiarowski; Ali Dhinojwala

“ To our knowledge there exists little evidence of self-drying in the natural world. Insects, including beetles, and tree frogs, use capillary adhesion and thus self-drying would be detrimental. For the dry adhesive system of the gecko however this appears to be imperative. But how can we relate these controlled laboratory tests to how geckos may utilize this unique property in their natural environments? First, we found that actively stepping significantly reduces the time it takes to regain maximum shear adhesion on either of the two substrates used. ”

“ While it is interesting to consider a gecko making behavioral choices about where to walk or run, for how long and in what direction, it is important to be reminded of the complexity of the system as a whole. Specifically, the impressive safety factor that geckos utilize will likely allow for negligible effects on overall adhesion when only one or two toes are wet and all others are dry. ”

“ In self-cleaning models we know that detachment of seta may help to actively and rapidly expel dirt particles from the toe [7] , so perhaps a similar mechanism is occurring in self-drying. When dynamically self-cleaning dirt the toes are peeled distally, the setae separate in a fan-like manner and dirt particles jump off the setae [7] . If water droplets behaved similarly we would expect the toes to dry uniformly, however this is not the case. Likewise, if the fanning of the setae caused air to penetrate deeper into the setal mat for evaporation we would again expect no clear drying pattern. ”

“ Our results here suggest that self-matting can be reduced or even eliminated by standing on the toes, as there is no clear evidence of matting in the toe pads when in contact with a substrate (Figure 4) and that any changes in surface chemistry or modulus of the material is reversible over a relatively short timeframe (∼15 min for adhesive recovery) . Therefore it is most likely that actual removal of water is most critical for regaining adhesion after toes have become wet. ”

“ In this study we tested a new hypothesis: gecko toe pads are self-drying. It is counterintuitive that geckos from tropical environments routinely encounter wet surfaces which make their toes dysfunctional [11] , yet have no way to regain their adhesion quickly. In response to this we found that active self-drying of toe pads occurs in Tokay geckos (Gekko gecko) , and that substrate wettability does not have an effect on time to recovery. ”

“ Although the discovery of the self-drying mechanism and the observations we made have direct relevance for application to synthetics, the biological relevance of such a finding is also noteworthy. It is not entirely clear why adhesion is compromised when the toe pads of a gecko become wet but there are four possible explanations that may not be mutually exclusive. Generally, as expected, water layers between the setae and the substrate may cause the gecko to slip due to inadequate surface contact for van der Waals forces to occur. ”

“ It is not clear why active self-drying (stepping) is substrate insensitive and passive self-drying (non-stepping) is so clearly substrate sensitive, especially since we do not expect there to be a significant difference in surface roughness or amount of water initially held within the toe pad, which could contribute to differences in time to maximum shear adhesion. ”

“ Second, we considered time to maximum shear adhesion to also be instances where damage to the toes occurred, even if this was below the 20 N threshold. When this occurred we also discontinued further timed testing and recorded the time where damage occurred as the maximum force the animal could sustain (to the point of damage) . Finally, in some treatments, specifically the non-stepping glass treatment, neither “maximum” force (∼20 N) nor toe damage occurred after 60 min of testing, making this last time interval (60 min) our final cut-off for repeated timed testing. ”

“ While there may be many reasons for the disparity between the necessary force to support body weight and the actual force available to geckos, wet toe pads may be one such factor. Many gecko species are native to tropical environments that experience high levels of atmospheric humidity and rainfall, likely wetting the surfaces a gecko moves across. ”

“ First, the distance covered by walking is likely much shorter than that by running, in fact we estimate that controlled stepping reduces the estimated running distance by at least half and second, the dynamic process of running may enhance self-drying and further lower the distance needed to regain maximum adhesion. ”

“ Chambers, B & Sullivan, T (2012) , Faster but Not Stickier: Invasive House Geckos Can Out-Sprint Resident Mournful Geckos in Moorea, French Polynesia, vol. 46, pp. 194–197 NiewiarowskiPHStarkAMcClungBChambersBSullivanT 1 2 Autumn, K ”