Curl formation of hair: A result of subtle, biomechanical engineering by the follicle

Human and all other mammal hairs are alpha-keratinous, biological composite materials. Depending on their specific biological function, they come in macroscopic forms that can vary between straight, curled, and crimped. The mechanism by which a specific macroscopic type of the mature hair shaft is formed is generally considered to be not well understood. It is established that alpha-keratinous fibres contain various groups of cortical cells, which show more or less distinct differences with respect to the structural arrangement of their major components, namely, the intermediate filaments (IFs) and the matrix of IF-associated proteins. So-called ortho-cortical cells show a whorl-like, essentially helical arrangement of IFs, while para-cortical cells exhibit a more parallel alignment of IFs with respect to the fibre axis. One or possibly two further cell-groups (meso-cortical cell groups) show intermediate behaviour. The different cell-groups appear to be present in virtually all keratinous fibres, though in various amounts. From a biomechanical point of view, the various hypotheses on hair curliness can be reconciled if it is assumed that the follicle generates a gel-like, pre-mature hair, where the different cell groups are either formed in a random (straight hair) or with a more or less pronounced tendency towards a non-random, spatially asymmetrical, bilateral arrangement (curly hair). Even for a laterally asymmetrical cell arrangement, the pre-mature hair remains essentially straight in the follicle but is mechanically only meta-stable. Once hair maturation starts by cross-linking of IF-associated proteins and loss of water, the straight geometry becomes unstable due to differences in the mechanical properties of the cell groups and the mature fibre collapses into its individual, stable, curled state. The extent of individual fibre crimp is enhanced if the pre-mature hair has already a non-round, elliptical cross-section.