Evidence for van der Waals adhesion in gecko setae

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.I n the 4th century B.C., Aristotle observed the ability of the gecko to ''run up and down a tree in any way, even with the head downwards'' (1). Two millennia later, we are uncovering the secrets of how geckos use millions of tiny foot-hairs to adhere to even molecularly smooth surfaces. We tested the two currently competing hypotheses (2, 3) of adhesion mechanisms in gecko setae: (i) thin-film capillary forces (or other mechanisms relying on hydrophilicity) and (ii) van der Waals forces. First, we tested the capillary and van der Waals hypotheses experimentally. Second, we used our experimentally measured adhesion forces in a mathematical model (4) to generate an independent prediction of the size of a setal tip. We compared the predicted size with the empirical values measured by electron microscopy (5). Third, we fabricated a physical model of gecko setal tips from two different materials. We then compared the adhesive function of the physical model to predicted force values from the mathematical model. Previously, we showed by calculation that our direct force measurements of a single gecko seta (3) were consistent with adhesion by van der Waals forces, but we could not reject the only other untested mechanism-wet, capillary adhesion that relies on the hydrophilic nature of the surface. Capillary forces contribute to adhesion in many insects (6-13), frogs (14-16), and even some mammals (17). Unlike many insects, geckos lack glands on the surfaces of their feet...

Frictional and elastic energy in gecko adhesive detachment

“…We harvested and mounted 17 setal arrays from five live non-moulting tokay geckos (G. gecko) using the methods modified from Autumn et al (2002). Under a microscope, we trimmed excess lamella from isolated arrays, mounted them to scanning electron microscope stubs using Loctite 410 (Henkel Co., CT) and allowed them to set overnight.…”

Fabrication and adhesion of hierarchical micro-seta

“…Therefore, both shear and normal adhesion demonstrate the "slipping adhesion" effect, which is also observed in geckos [2]. Unlike shear adhesion, the adhesions of the seta arrays measured in the normal direction do not vary much.…”

“…A proposed specification for such a robot is that loaded with communication equipment with 5 km working range (10 g), a 0.3 mol L -1 pixel optical sensor (10 g), a power source (40-50 g), the gecko can climb ten floors within 20 s. This climbing ability is derived from the structure of the feet and toes, which have millions of setae with hierarchical branched structures [2][3][4][5]. These micro-scale setae control contact between the toe and the surface, by van der Waals' intermolecular attractive forces.…”

“…A tribometer (CETR, USA) with high sensitivity force sensor was used to measure adhesion. The gecko uses a "slipping adhesion" technique to achieve a high level of adhesion performance [2], where the adhesion of the setae arrays are strongly influenced by direction of motion, angle, and preload [33,34], this slipping adhesion is significantly affected by motion directions and preload. The aim of this paper is to fabricate a gecko-inspired micro-setae array for use in a TDOF robot.…”

Design of gecko-inspired fibrillar surfaces with strong attachment and easy-removal properties: a numerical analysis of peel-zone

“…The earlier-mentioned numerical method does not take E b , E s and the bundling effect into account; thus, the method is not suitable for gecko-inspired surfaces with large aspect ratios. Therefore, anti-self-adhesion conditions should be included to determine the critical aspect ratio [21], and then the numerical method can be applied to design the geometrical parameters.…”