Perhaps one of the most astounding characteristics of the gecko adhesive system is its versatility. Geckos can locomote across complex substrates in a variety of conditions with apparent ease. In contrast, many of our synthetic pressure sensitive adhesives fail on substrates that are dirty, wet or rough. Although many studies have investigated the effect of environmental challenges on performance, the interaction of multiple, potentially compromising variables is studied less often. Here we focus on substrate structure and surface water, both of which are highly relevant to the biological system and to synthetic design. To do this we utilized a highly controlled, patterned substrate (Sharklet®, by Sharklet® Technologies Inc.). This allowed us to test independently and jointly the effects of reduced surface area substrates, with a defined pattern, on adhesion in both air and water. Our results show that adhesion is not significantly impaired in air, whereas surface area and pattern significantly affect adhesion in water. These findings highlight the need to study multiple parameters that are relevant to the gecko adhesive system to further improve our understanding of the biological system and to design better, more versatile synthetics.

Many teleost fishes, such as gobies, have fused their paired pelvic fins into an adhesive disc. Gobies can use their pelvic suckers to generate passive adhesive forces (as in engineered suction cups) and different species exhibit a range of adhesive performance, with some even able to climb waterfalls. Previous studies have documented that, in the Hawaiian Islands, species capable of climbing higher waterfalls produce the highest passive pull-off forces, and species found at higher elevation sites are likely to have more rounded suction discs than those found in the lowest stream segments. Morphology of the pelvic girdle also varies between species, with more robust skeletons in taxa with superior passive adhesion. To investigate what factors impact the passive adhesive performance of waterfall climbing gobies, we tested biomimetic suction cups designed with a range of shapes and embedded bioinspired “skeletons” based on micro-CT scans of goby pelvic girdles. We found that while the presence of an internal skeleton may provide some support against failure, the performance of suction cups may be more strongly affected by their external shape. Nonetheless, factors besides external shape and skeletal morphology may still have a stronger influence on sucker tenacity. Our results suggest that the relationship between suction disc morphology and adhesive performance may be influenced by a variety of physical factors, and live animal performance likely is further complicated by muscle activation and climbing behavior. These results have implications for the evolution of suction disc shape in adhesive fishes and for improving the design of biomimetic suction cups.

The roughness and wettability of surfaces exploited by free-ranging geckos can be highly variable and attachment to these substrates is context dependent (e.g., presence or absence of surface water). Although previous studies focus on the effect of these variables on attachment independently, geckos encounter a variety of conditions in their natural environment simultaneously. Here, we measured maximum shear load of geckos in air and when their toes were submerged underwater on substrates that varied in both surface roughness and wettability. Gecko attachment was greater in water than in air on smooth and rough hydrophobic substrates, and attachment to rough hydrophilic substrates did not differ when tested in air or water. Attachment varied considerably with surface roughness and characterization revealed that routine measurements of root mean square height can misrepresent the complexity of roughness, especially when measured with single instruments. We used surface roughness power spectra to characterize substrate surface roughness and examined the relationship between gecko attachment performance across the power spectra. This comparison suggests that roughness wavelengths less than 70 nm predominantly dictate gecko attachment. This study highlights the complexity of attachment in natural conditions and the need for comprehensive surface characterization when studying biological adhesive system performance.

The pelvic sucker of Hawaiian waterfall climbing gobies allows these fishes to attach to substrates while climbing waterfalls tens to hundreds of meters tall. Climbing ability varies by species and may be further modulated by the physical characteristics of the waterfall substrate. In this study, we investigated the influence of surface wettability (hydrophobic versus hydrophilic surface charges) and substrate roughness on the passive adhesive system of four species of gobies with different climbing abilities. Overall, passive adhesive performance varied by species and substrate, with the strongest climbers showing the highest shear pull-off forces, particularly on rough surfaces. Thus, differences in passive adhesive performance may help to explain the ability of some species to migrate further upstream than others and contribute to their ability to invade new habitats.