Digital hyperextension has no influence on the active self-drying of gecko adhesive subdigital pads

The remarkable properties of the gecko adhesive system have been intensively studied. Although many gecko-inspired synthetic adhesives have been designed and fabricated, few manage to capture the multifunctionality of the natural system. Analogous to previously documented self-cleaning, recent work demonstrated that gecko toe pads dry when geckos take steps on dry substrates (i.e., self-drying). Whether digital hyperextension (DH), the distal to proximal peeling of gecko toe pads, is involved in the self-drying process, had not been determined. Here, the effect of DH on self-drying was isolated by preventing DH from occurring during normal walking locomotion of Gekko gecko after toe pads were wetted. Our initial analysis revealed low statistical power, so we increased our sample size to determine the robustness of our result. We found that neither DH nor the DH-substrate interaction had a significant effect on the maximum shear adhesive force after self-drying. These results suggest that DH is not necessary for self-drying to occur. Interestingly, however, we discovered that shear adhesion is higher on a surface tending hydrophobic compared to a hydrophilic surface, demonstrating that gecko adhesion is sensitive to substrate wettability during the subdigital pad drying process. Furthermore, we also observed frequent damage to the adhesive system during shear adhesion testing post-drying, indicating that water may compromise the structural integrity of the adhesive structures. Our results not only have behavioral and ecological implications for free-ranging geckos but also have the potential to influence the design and fabrication of gecko-inspired synthetic adhesives that can regain adhesion after fouling with water.