Human hair: color changes caused by daily care damages on ultra-structure

Abstract

The relation between hair ultra-structure damages and color changes was studied. Virgin dark-brown hair was hand-washed, using lauryl sodium sulfate solution in 40 °C water, rinsed, wet-combed, heat-dried and dry-combed for up to 120 times. Ultra-structure changes were studied by electron microscopy. The treatments damage the cuticle and the cortex. The extraction of soluble material renders cavities, or holes, in the endocuticle. The cavities are 50–200 nm in diameter. There are two kinds of cavities: some filled with lower density material than the remaining endocuticle and some filled with air or water vapor. Displacement, cracking and cleavage of cuticle cells are also observed. Cuticle removal was found to proceed in two ways: via cleavage through the cell membrane complex, and via endocuticle rupture, taking place preferentially in the cavities’ surroundings. In the cortex, cavities develop in the intermacrofibrilar cement, in the cell membrane complex and around the melanin granules. These ultra-structural damages give rise to significant changes on hair color, as shown by diffuse reflectance spectrophotometry. The hair lightness was found to increase after soft washing treatments (5–20 washes), or after keeping it in 40 °C water. Deeper hair degradation turns the hair lightness undistinguishable from the initial value, but changes the color mainly by a yellowing of the hair. A simple model based on light reflection was developed to explain hair reflectance behavior before and after damage; results show a reasonable agreement with the experimental data.