Evidence for van der Waals adhesion in gecko setae

TLDR

Geckos possess highly versatile dry adhesion via millions of setae, a phenomenon studied for over a century. The study aims to provide direct experimental evidence that gecko setae adhere via van der Waals forces, rejecting surface polarity mechanisms. A van der Waals mechanism explains that adhesive properties depend on tip size and shape, not surface chemistry, and this was verified using nanofabricated setal tip models. Live Tokay geckos and isolated setae adhered equally to hydrophobic and hydrophilic surfaces, and theoretical predictions matched measured forces, indicating size‑driven adhesion and guiding design of dry adhesive microstructures.

Abstract

Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydrophobic and hydrophilic surfaces of a microelectro-mechanical systems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures.