A new deformation model of hard α-keratin fibers at the nanometer scale: Implications for hard α-keratin intermediate filament mechanical properties

The mechanical behavior of human hair fibers is determined by the interactions between keratin proteins structured into microfibrils (hard α-keratin intermediate filaments), a protein sulfur-rich matrix (intermediate filaments associated proteins), and water molecules. The structure of the microfibril-matrix assembly has already been fully characterized using electron microscopy and small-angle x-ray scattering on unstressed fibers. However, these results give only a static image of this assembly. To observe and characterize the deformation of the microfibrils and of the matrix, we have carried out time-resolved small-angle x-ray microdiffraction experiments on human hair fibers stretched at 45% relative humidity and in water. Three structural parameters were monitored and quantified: the 6.7-nm meridian arc, which is related to an axial separation between groups of molecules along the microfibrils, the microfibril's radius, and the packing distance between microfibrils. Using a surface lattice model of the microfibril, we have described its deformation as a combination of a sliding process and a molecular stretching process. The radial contraction of the matrix is also emphasized, reinforcing the hydrophilic gel nature hypothesis.