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Mechanisms of Gecko Adhesion and Their Application in Gecko-Like Synthetic Adhesives (GSAs)
Geckos use millions of hierarchical adhesive nanostructures on their toes to climb at speeds of over 1 m/s. No conventional adhesive meets the challenges of climbing like a gecko. Imagine the difficulties a gecko would encounter if it employed a conventional pressure sensitive adhesive (PSA) on its toes. PSAs are soft viscoelastic polymers that degrade, foul, self-adhere, and attach accidentally to inappropriate surfaces. By contrast, gecko toes bear angled arrays of branched, hair-like setae formed from stiff, hydrophobic keratin that act as a bed of angled springs with similar effective stiffness to that of PSAs. Setae are self-cleaning and maintain function for months during repeated use in dirty conditions. Setae are an anisotropic “frictional adhesive” in that adhesion is initiated and controlled by a proximally directed shear load, enabling either a tough bond or spontaneous detachment. In 2002 (Autumn+2002, PNAS), we showed that van der Waals forces were a sufficient mechanism for adhesion in gecko setae, and proposed that capillary forces were not required. Recently, other workers demonstrated that adhesion increases with relative humidity (RH) and proposed that capillary adhesion is important or even necessary. In a subsequent study, we confirmed a significant effect of RH, but rejected capillary adhesion. While adhesion of isolated tokay gecko setal arrays increased with RH, the increase was similar on hydrophobic and hydrophilic surfaces, inconsistent with a capillary mechanism. Contact forces increased with RH even at high shear rates when there was insufficient time for capillary bridges to form. How then can a humidity-related increase in adhesion and friction be explained ? We discovered that an increase in RH softens setae and increases viscoelastic damping, increasing adhesion. Changes in materials properties not capillary forces explain the effect of humidity-enhanced adhesion. Van der Waals force remains the only empirically supported intermolecular mechanism of adhesion in geckos. The smart material properties of adhesive nanostructures may enable rigid, inert, recyclable materials to replace glues, screws, and other attachment devices in the future.