Viscoelastic response of human hair cortex.

The present study describes a new methodological approach which allows the separate analysis and quantitative determination of the viscous and elastic components of human hair using a computerised experimental system. In a first series of experiments subsequent to the estimation of the cross-sectional surface area of the cuticle, the cortex and the medulla, the viscoelastic response of hair (considered as a homogeneous material) and of its cortex were determined in ten hair specimens derived from the same human subject. Statistically significant differences were found between hair and its cortex with regard to the values of the modulus of elasticity, limit of linearity and post-yield slope. In a second series of experiments the viscoelastic parameters of hair and its cortex were separately determined in 100 hair specimens derived from ten healthy human subjects (ten specimens from each subject). It was found that the dispersion of the values of modulus of elasticity, limit of linearity and post-yield slope of the cortex was significantly reduced compared with that of the hair. Our results indicate that the pattern of the viscoelastic parameters of human hair cortex, which strongly depends on its microstructural features, may be regarded as a more reliable and sensitive diagnostic marker than that of hair in various structural and functional hair disorders.