Hyaluronic acid: elucidating its penetration into, and effect on hair fibers using confocal Raman spectroscopy and biometric techniques

Abstract

Background: Hyaluronic acid (HA) is well known in the cosmetic industry for its hydrating properties linked to its interaction with keratin in the stratum corneum. Hair is also mainly composed of keratin, but no evidence of penetration and interaction of HA in hair had yet been established. Herein, we examined, with appropriate methods, this penetration and interaction. Methods: Confocal Raman Spectroscopy was used to track HA penetration in hair fibers. Human hair was UV-irradiated, or not, before washing with a number of different shampoo formulations. Low-molecular weight hyaluronic acid, high-molecular weight HA, and an optimized blend of low- and high-molecular weight HA, were used. The benefits of HA for cosmetic application were then evaluated at the ex vivo level. Results: Raman analysis of non-irradiated samples revealed that the HA blend penetrated the hair cortex 5.9 times more than the placebo and with irradiated samples, a 1.8-fold increase in penetration was observed. Whatever the irradiation status, penetration of this optimal HA blend was significantly greater than that of the other two active substances. Compared to the placebo, the HA blend significantly decreased spontaneous frizzing by 11%, repaired the hair by increasing the elastic modulus and the break force, and increased the water content in the hair shafts. Conclusion: Confocal Raman Spectroscopy can be considered as a powerful, non-invasive technique to investigate the penetration of active ingredients into the hair.