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Abstract
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Grand canonical ensemble molecular dynamics simulations are done to
calculate the water content of gecko β-keratin as a function of relative humid-
ity (RH). For comparison, we experimentally measured the water uptake of
scales of the skin of cobra Naja nigricollis. The calculated sigmoidal sorption
isotherm is in good agreement with experiment. To examine the softening
effect of water on gecko keratin, we have calculated the mechanical properties
of dry and wet keratin samples, and we have established relations between the
mechanical properties and the RH. We found that a higher RH causes a
decrease in the Young’s modulus, the yield stress, the yield strain, the stress
at failure and an increase in the strain at failure of the gecko keratin. At low
RHs (less than 80%), the change in the mechanical properties is small, with
most of the changes occurring at higher RHs. The changes in the macroscopic
properties of the keratin are explained by the action of sorbed water on the
molecular scale. It causes keratin to swell, thereby increasing the distances
between amino acids. This has a weakening effect on amino acid interactions
and softens the keratin material. The effect is more pronounced at higher RHs.
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