AFM study of 18-MEA on human hair

Atomic force microscopy (AFM) collaborative R&D with L'Oréal Research, France. £8,950

The aim of the project was to investigate differences between untreated and various bleached Caucasian hairs and untreated African hairs using atomic force microscopy (AFM) techniques. Both cross-sections and outer surfaces were investigated. The samples were: untreated Caucasian hair (D499), Decolores Plat 'heavily' bleached hair (D499, DP12, commercial product, bleaching past, applied directly with a paint-brush onto the hair), 'lightly' bleached hair (D499, classical bleaching solution, H2O2 and NH3), untreated African hair (CC002), and methanolic KOH treated Caucasian hair (D499) to remove 18-MEA.

The outer surfaces of all the hair samples were imaged. There did not appear to be any obvious differences between these hairs and typical surface architectural features, such as striations and ghost edges, could be seen on both 'lightly' and 'heavily' bleached Caucasian hairs and untreated African hairs (Swift, 1991). The cross-sections of the untreated Caucasian and lightly bleached hair samples showed a thickness of four cuticle layers although no unusual features were observed. The heavily bleached hair, however, showed gross delamination of the innermost cuticle to the cortex. In some areas on these hairs, the effect was not observed, although the clear distinction between the endocuticle and exocuticle was not apparent. In addition, small (ca. 70 nm dia.) 'granular' deposits appeared in the four cuticle layers, at higher concentrations near the surface and decreasing towards and into the first 1 um of the cortex. The nature of these features is uncertain, but may correspond with oxidized protein. These features were not present in the untreated African hair, the cuticle layers of which appeared very similar to Caucasian hair.

A humidity chamber for the AFM was installed and tested. The range could be altered from 5% to 60% relative humidity (RH). Adhesion values recorded in air on Caucasian hair using an unmodified Si-N tip showed no change over the available RH range. All subsequent measurement were recorded under ambient conditions and the RH noted (typically ca. 54%).

Gold-coated silicon nitride tips derivatised with CH3- and COOH-terminated alkanethiols were used to measure adhesion to all the hair types in air using AFM force-curves. Adhesion values for both types of tip were not statistically different (p