New approach for hair keratin characterization: Use of the confocal Raman spectroscopy to assess the effect of thermal stress on human hair fibre

OBJECTIVE: The objective of our research was to investigate the heat‐protecting effect of a product ex vivo and in vivo on human hair fibres. METHODS: A preparatory study was carried out in order to determine an optimal threshold of thermal stress. For this, the structure of cross‐sections of the hair fibre was observed by optical microscopy. Then, Scanning Electron Microscopy (SEM) and Confocal Raman Spectroscopy (CRS) were applied to analyse ex vivo and in vivo morphological and molecular damage in hair structure after heat stress. Finally, in vivo tests were used to collect consumer perception. RESULTS: The preparatory study enabled us to determine an optimal stress threshold of 10 heating cycle for SEM and 5 heating cycle for CRS. Based on spectral hierarchical classification using Ward's clustering algorithm, the ex vivo Raman results show that the spectral signature of the hair treated and heated is very close to the negative control. This shows that the product preserves the keratin structure after thermal stress. These results were also confirmed by an in vivo Raman analysis performed on hair samples from 5 donors. In concordance with Raman results, SEM shows that treated hair presents lesser “bubbles” and “crackling” on the hair surface. Finally, the in vivo studies proved that hair was more protected from heat. CONCLUSION: The authors concluded that the product shows protective properties with respect to morphological and molecular heat damage. We also demonstrate that the product promotes the α‐helix keratin conformation and preserves the S‐S disulfide bands.