Study of three complementary techniques for measuring cutaneous hydration in vivo in human subjects: NMR spectroscopy, transient thermal transfer and corneometry – 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, non‐invasive, quantitative methods for measuring skin hydration: two innovative methods that have been used for more than eight years – nuclear magnetic resonance spectroscopy (NMR‐S) and transient thermal transfer (TTT) – and the more widely used and conventional corneometry. Methods: The work presented evaluated the capability and precision, as well as cutaneous exploration depths, of the three methods. Experiments were carried out in vivo following the hydration, in kinetic terms, induced by topic 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 corneometry. Results: The results obtained showed better repeatability and reproducibility with the TTT and NMR‐S methods than with corneometry. 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 having an action down to the deep cutaneous layers. By changing thermal power parameters, the TTT method can determine hydration to the outer, middle or deep epidermal layers. It is, therefore, possible to track the penetration of products in various layers of the epidermis. The small size of the probe enables the hydration measurement of skin sites (lips, eyelids) that were not, up to now, measurable with the two other methods. Corneometric investigations are restricted to the surface of the horny layer; measurements are easy and rapid but influenced by the composition of products applied to the skin and their phases: aqueous, oily or ionic. The xerotic skin study highlights the importance of exploration in different layers of the epidermis, as dehydration concerns not only the upper layers of the epidermis but also the medial and deep layers. With the TTT method, it has been possible to highlight the penetration dynamics of a lipolotion with, initially, an increase 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 corneometry represent three possible ways to assess skin hydration. Because they explore different cutaneous depths, they are more complementary than competitive. Transient thermal transfer, although a semi‐direct method, is a precise, informative, and innovative solution to evaluate skin hydration at different epidermal depths and sites.