Environmental Effects on the Mechanical Properties and Adhesion of Gecko Setae
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Geckos have an amazing ability to climb walls and cling to inverted surfaces. While other terrestrial animals, such as tree frogs also process such abilities, geckos are unique given their reasonably large body mass. Geckos owe their remarkable adhesiveness to millions of dry setae on their toes, and the mechanism of adhesion in gecko setae has been the topic of scientific scrutiny for over two hundred years. Previously, it was demonstrated that van der Waals forces alone are sufficient for strong adhesion and friction in gecko setae. However, more recent studies have demonstrated that adhesion increases with relative humidity (RH) and proposed that surface hydration and capillary water bridge formation is important or even necessary. In this study, we confirmed a considerable effect of RH on gecko adhesion; but we reject the capillary adhesion premise. While the contact forces of isolated Tokay gecko setal arrays increased with humidity, the increase was similar on both hydrophobic and hydrophilic surfaces which are inconsistent with a capillary adhesion mechanism. Additionally, contact forces increased with RH even at high shear rates, where capillary bridge formation is too slow to contribute to adhesion. How can a humidity-related increase in adhesive and frictional force be explained? The effect of relative humidity on the mechanical properties of setal â-keratin has escaped consideration. We discovered that an increase in RH softens setae and increases viscoelastic damping, which increases adhesion. We measured the effect of RH on the tensile deformation properties, fracture, and dynamic mechanical response of an isolated tokay gecko setae and strips of the smooth lamellar epidermal layer. The mechanical properties of gecko setae were affected strongly by RH. The changes in mechanical properties of setal keratin were consistent with previously reported increases in contact forces, supporting the hypothesis that an increase in RH softens setal keratin. Changes in setal materials properties, not capillary forces, fully explain humidity-enhanced adhesion, and van der Waals forces remain the only empirically supported mechanism of adhesion in geckos.
- Electrical engineering