Applied Cosmetic Science and Technology Applied Cosmetic Science and Technology

Water in the stratum corneum (SC) is essential for maintaining healthy skin, making accurate measurement of the SC moisture content vital for developing effective skincare formulations. However, accurately evaluating the moisture content of the SC when coated with various formulations has been challenging. To address this, we previously introduced a terahertz attenuated total reflectance (THz-ATR) system that could determine the water content of the SC, even when a formulation was applied. Nevertheless, the consistency of the THz-ATR system with conventional skin moisture measurements has not been clarified, and the system cannot overcome the thick formulation interposition because it decreases the interacting field of the electromagnetic wave with the SC. In this study, we compare the THz-ATR measurements with skin moisture meters that use conductance or capacitance as indicators and investigate methods to mitigate the influence of thick formulations. This study demonstrates the consistency of our system with conventional skin moisture meters and mitigates the influence of formulation thickness by using the ratio of ATR signals at two different frequencies as a moisture index. Thus, the corrected ATR signal is useful for evaluating the moisture content of SCs coated with cosmetics. Therefore, it could potentially improve the accuracy of skincare product evaluation.