Study of three complementary techniques for measuring cutaneous hydration in vivo in human subjects: NMR spectroscopy, transient thermal transfer andcorneometry - application to xerotic skin and cosmetics

Background/aims: The aim of this study was to determine the capability and the analytical quality of three different in vivo, noninvasive, quantitativ e methods for measuring skin hydration: two innovative methods that have be en used for more than eight years - nuclear magnetic resonance spectroscopy (NMR-S) and transient thermal transfer (TTT) - and the more widely used an d conventional comeometry. Methods: The work presented evaluated the capability and precision, as well as cutaneous exploration depths, of the three methods. Experiments were ca rried out in vivo following the hydration, in kinetic terms, induced by top ic application of reference moisturizing products. Spatio-temporal efficacy of a lipolotion was also studied by the TTT method. Cases of xerotic skin were studied with TTT and comeometry. Results: The results obtained showed better repeatability and reproducibili ty with the TTT and NMR-S methods than with comeometry. NMR-S is one of the only direct hydration measurement methods. It measures skin hydration down to the outer dermis with high precision. It is indicated for products havi ng an action down to the deep cutaneous layers. By changing thermal power p arameters, the TTT method can determine hydration to the outer, middle or d eep epidermal layers. It is, therefore, possible to track the penetration o f products in various layers of the epidermis. The small size of the probe enables the hydration measurement of skin sites (lips, eyelids) that were n ot, up to now, measurable with the two other methods. Comeometric investiga tions are restricted to the surface of the horny layer; measurements are ea sy and rapid but influenced by the composition of products applied to the s kin and their phases: aqueous, oily or ionic. The xerotic skin study highli ghts the importance of exploration in different layers of the epidermis, as dehydration concerns not only the upper layers of the epidermis but also t he medial and deep layers. With the TTT method, it has been possible to hig hlight the penetration dynamics of a lipolotion with, initially, an increas e in the hydration in the outer epidermis, followed 3 h later by a transfer from the outer to the middle epidermis. Conclusion: NMR-S, TTT and comeometry represent three possible ways to asse ss skin hydration. Because they explore different cutaneous depths, they ar e more complementary than competitive. Transient thermal transfer, although a semi-direct method, is a precise, informative, and innovative solution t o evaluate skin hydration at different epidermal depths and sites.