Multiscale modelling of human hair

We present a multiscale modelling approach to the mechanics of human hair fibres. On the microscale, a coiled coil of filament proteins was mechanically unfolded in a molecular-dynamics simulation. The force at unfolding was found to be ca. 1 nN, which we estimated to be an order of magnitude above the reversible force. Using the concept of folded/unfolded states, we developed a statistical mechanical model, which predicts a linear decrease of the yield stress with temperature. This was confirmed experimentally by stretching human hair fibres into the yield region at elevated temperatures. The role of correlation between unfolding units has been studied in more detail predicting an energy of ca. 12 kJ mol(-1) for the interface. The composite structure of hair at the nanometre scale was addressed using a particle-based model for a macrofibril. Mesoscale particles representing coiled coils of keratin proteins were assembled to filaments and embedded in a matrix of soft particles cross-linked to a network. The macrofibril was extended in a non-equilibrium computer simulation, while monitoring the tensile force. Thermal properties of the macrofibril in the yield region are in correspondence with the two-state model.