Direct evidence of acid-base interactions in gecko adhesion

While it is generally accepted that van der Waals (vdW) forces govern gecko adhesion, several studies indicate contributions from non-vdW forces and highlight the importance of understanding the adhesive contact interface. Previous work hypothesized that the surface of gecko setae is hydrophobic, with nonpolar lipid tails exposed on the surface. However, direct experimental evidence supporting this hypothesis and its implications on the adhesion mechanism is lacking. Here, we investigate the sapphire-setae contact interface using interface-sensitive spectroscopy and provide direct evidence of the involvement of acid-base interactions between polar lipid head-groups exposed on the setal surface and sapphire. During detachment, a layer of unbound lipids is left as a footprint due to cohesive failure within the lipid layer, which, in turn, reduces wear to setae during high stress sliding. The absence of this lipid layer enhances adhesion, despite a small setal-substrate contact area. Our results show that gecko adhesion is not exclusively a vdW-based, residue-free system.

Automated summary

This study identified the involvement of acid-base interactions between polar lipid head-groups on gecko setal surfaces and sapphire, and demonstrated the role of cohesive failure within the lipid layer in reducing setal wear during high stress sliding. Absence of the lipid layer was found to enhance adhesion, despite a small setal-substrate contact area. These results challenge the traditional view that gecko adhesion is solely based on vdW forces and is residue-free.