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.