Exploring the Local Properties of the Cosmetic Substrate (hair and Skin) by Scanning Nanoprobes

We will present a review of our knowledge of the complex structure of skin and hair, emphasizing those aspects of special interest for its lasting and protection. At the surface, we have been most interested in the friction (nano-tribology) of hair fibers based on the properties of covalently linked fatty acids (i.e. 18-methyleicosanoic acid) that are part of its original natural structure, as well as the subtle modifications induced by water and other standard cosmetic treatments. By using AFM in combination with Nanoindentation our current understanding on the nano-mechanics (modulus, hardness) of substrates is also improved. Of special interest is to localize the contribution of hair and skin constituents to the overall macroscopic structure. We measured the mechanical properties of hair and stratum corneum at different humidity. In hair a comparison with results obtained on surface transversal cuts was also done. Both hardness values and Young’s modulus changes clearly show the complex structure of the hair. Cuticle behavior is different from cortex behavior. A gradient of mechanical properties (hardness and Young’s modulus) depending on indentation plastic depth is observed. Changes as a function of humidity have been measured on the loss factor evolution.