SUMMARYGeckos owe their remarkable stickiness 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 centuries. Previously, we demonstrated that van der Waals forces are sufficient for strong adhesion and friction in gecko setae, and that water-based capillary adhesion is not required. However, recent studies 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 significant effect of RH on gecko adhesion, but rejected the capillary adhesion hypothesis. While contact forces of isolated tokay gecko setal arrays increased with humidity, the increase was similar on hydrophobic and hydrophilic surfaces, inconsistent with a capillary mechanism. Contact forces increased with RH even at high shear rates, where capillary bridge formation is too slow to affect adhesion. How then can a humidity-related increase in adhesion and friction be explained? The effect of RH on the mechanical properties of setal -keratin has escaped consideration until now. We discovered that an increase in RH softens setae and increases viscoelastic damping, which increases adhesion. 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.

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Different types of biological adhesion can be categorized according to the length scales, structures, and materials involved. The setal adhesion system of the gekkonid lizards occupies a hierarchy of scales from the toes (~ 1 cm) to the terminal spatular pads on the setal branches (~ 100 nm). This unique combination of scale and foot-hair morphology allow the animal robust, controllable, and near-universal adhesion via van der Waals attraction, but it is also apparent that the mechanical behavior of the β-keratin plays an important role in an animal’s climbing ability. Experimental results show a four-fold increase in the viscoelastic loss tangent of β-keratin, alongside a substantial increase in adhesion of setal arrays, over a range of relative humidity from 10 to 80%. A model of single-spatular deformation predicts that the elastic energy stored in the setal branches, energy which is not completely recovered on detachment, is strongly influenced by these properties changes. The enhanced dissipation characteristics of the system explain the effects of environmental humidity on the clinging ability of geckos.