Coarse-Grained Molecular Simulation Model for Gecko Feet Keratin.

The mechanical properties of gecko setae and spatulae are investigated with a coarse-grained model having two bead types. The two-bead model is based on both experimental information of the internal structure of setae and the chemical amino acid composition. Because the seta is composed of a stiff fibril region and a soft matrix region, we model each of the regions separately. Our model is parameterized in a bottom-up way, and it successfully predicts essential mechanical properties without optimization against the macroscopic properties of keratin. Young's modulus of the fibril area (13.2 ± 0.02 GPa) is 6 times stiffer than that of the pure matrix (2.13 ± 0.059 GPa). Because the volume fraction of the matrix decreases toward the top of a seta, its distal area should be stiffer than its proximal area. The anisotropy of the fibrils is clearly confirmed by the comparison between Young's modulus and the shear modulus of the fibril region. Our model also lends some support to the notion that water uptake selectively weakens the axial cohesion of the fibrils, which leads to the experimentally observed plateau in the stress-strain curves beyond 2% strain for setae under high relative humidity.