Abstract
Hair keratin has a complex hierarchical structure, and in order to understand the correlation of the structure and the mechanical properties of hair fiber, not only must the relationships between structure and properties in the respective layers be grasped but the properties must also be grasped as being expressed by interactions among the respective layers. This paper consists of three sections, which contain (1) the architectural structure of intermediate filament (IF) protein in the hair cortex, (2) disulfide (SS) cross-linked network structures of IF and matrix protein (KAP), and (3) the effect of the time of washing after reduction on the concentration of mixdisulfide groups in permanent waved hair prepared by reduction with thioglycolic acid. In the first section, the structure models presented for IF protein in the cortex, which controls mechanical properties of hair are categorized into the tetramer antiparallel molecular model and the tetramer parallel chain model. The latter has been presented recently on the basis of the piezoelectric effect arising from the parallel chain configuration of peptide dipoles of helix molecules. Mechanical properties are also deeply related not only to the architecture of IF, but to the network structures of IF and KAP components in the cortex. In the second section, a mechanical method for analyzing the number, type, and location of SS cross-links in the keratin cortex and the results obtained to date are explained in detail with some illustrations. In the last section, a SS cross-linked network structure for permanent waved hair is proposed. The structure was determined by applying a rubber elasticity theory to the corresponding swollen hair fiber. The reformation mechanism of SS cross-links during washing after reduction is exhuastively discussed on the basis of the values of structural parameters obtained for SS bonding in the cortex. An important conclusion is derived that the reproduction of SS cross-links followed by simultaneous loss of mixdisulfide groups is due to the reverse reaction of reduction induced by removing reductant molecules from an equilibrium system by those diffusions.
