Quantitative AFM friction measurements of human hair

Atomic force microscopy (AFM) collaborative R&D work, sponsored by L'Oréal Research, France. £9,917

The purpose of this fundamental study was to assess whether atomic force microscopy (AFM), and a derivative technique, lateral force microscopy (LFM), could reveal differences in the layer of 18-methyleicosanoic acid (18-MEA) maintained at the outer surfaces of Caucasian, bleached, grey and African hairs. A Caucasian hair which had been treated with hydroxylamine, a gentle treatment thought to remove 18-MEA, was also investigated. AFM force of adhesion measurements, carried out in air with a Si3N4 tip, were acquired on hairs obtained from fraternal twins, one of whom had Maple Syrup Urine Disease (MSUD), an inherited disease in which 18-MEA is replaced largely by straight-chain fatty acids, and the other, whom was normal. These experiments revealed mean adhesion forces for the MSUD sample to be markedly less than those of normal hair; this difference was attributed to the presence of 18-MEA. Using this system as a control, similar AFM adhesion studies revealed that the mild hydroxylamine treatment was not as effective in removing 18-MEA as methanolic KOH. These investigations also suggested the 18-MEA layer on bleached hair and grey hairs to be damaged. LFM studies revealed higher relative friction coefficients (mu-R) for bleached, 18MEA removed and grey hairs with respect to untreated Caucasian hair. African hair was found to possess a mu-R value significantly less than that of Caucasian European brown hair.

To further investigate 18-MEA and other fatty acid (C10-C24) monolayers, similar AFM and LFM studies were performed on fabricated self-assembled monolayers. The18-MEA monolayer was found to possess a surprisingly high water contact angle (and low surface energy) compared to the expected value based upon its chain length position. AFM studies showed significantly higher adhesion between an uncoated Si3N4 tip and 18-MEA compared with straight-chain fatty acids. The 18-MEA monolayer was also found to have a low relative friction coefficient.