Direct evidence of acid-base interactions in gecko adhesion

Abstract

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 headgroups 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.

Evidence that gecko setae are coated with an ordered nanometre-thin lipid film

“…However, thickness and orientation estimates are very close to what Singla et al . have proposed based on sum frequency generation experiments of setae attached to sapphire [30].…”

“…Singla et al . recently proposed that lipids could support adhesion to sapphire surfaces by an acid–base mechanism [30]. The lipid layer may also be involved in keeping the proteins of setae and spatulae hydrated by enclosing the proteinaceous tissue and thereby reducing water evaporation.…”

“…Lipids are known to reside in the epidermis of reptiles in a ‘brick and mortar’ pattern [28,31]. Lipids have also been observed in gecko footprints [29], within setae [32] and at the surface of setae tissue [21,28,30]. This led Jain et al .…”

“…More recent models of gecko adhesion now indicate that chemistry cannot be ignored and that chemical details matter for gecko surface adhesion. Vibrational surface spectroscopy and nuclear magnetic resonance (NMR) indicate an important role of lipids and acid-base interactions for gecko adhesion [28][29][30]. As in any adhesion phenomenon, the outermost molecular structure of the setae defines the mechanical and chemical contact during adhesion.…”

Wetting of the tarsal adhesive fluid determines underwater adhesion in ladybird beetles

“…Geckos are thought to rely on van der Waals forces via dry contact with the substrate ( Autumn et al, 2002 ), although observations of phospholipid footprints left behind by walking geckos ( Hsu et al, 2012 ) could change that picture. A recent study has in fact presented evidence for the importance of polar interactions in gecko adhesion mediated by this phospholipid layer ( Singla et al, 2021 ). This calls for a reinterpretation of previously reported gecko adhesion data by considering the influence of the phospholipid layer.…”