Skin hydration, which is defined by the hydration index and the spatial distribution of water, represents one of the key parameters reflecting the functional state of the skin. Although various optical methods have been proposed to study the hydration process, these techniques are often limited by a shallow probing depth, insufficient depth resolution, or low spatial and temporal resolution, which precludes a comprehensive assessment of the skin response to topical pharmaceutical agents. Here, we present an approach for the quantitative assessment and visualization of skin dehydration and rehydration processes with high depth and temporal resolutions. The proposed method is based on reconstructing the scattering coefficient from the optical coherence tomography data. We used this approach to quantify the effective skin dehydration depth and rehydration time in rats ex vivo following the topical application of 70% ethanol and an ethanolic solution of the betamethasone dipropionate glucocorticosteroid, both in the presence and absence of ultrasound exposure (1 MHz, 0.5 W/cm², 2 min). As a result, ethanol was shown to induce reversible dehydration of the upper skin layers (168±88 µm) with the recovery occurring within 21±3 min. Ultrasound treatment of the ethanol application area resulted in the enhancement of its penetration depth up to 262±63 µm and reduced the rehydration time to 14±3 min. The addition of betamethasone dipropionate to the ethanol solution further increased the dehydration depth and delayed rehydration process. Visualization of the obtained data as heatmaps enabled a quantitative evaluation of the spatiotemporal changes in the skin. The proposed approach proved to be effective for investigating water diffusion processes in the skin and would facilitate the analysis of dynamic changes in both healthy and pathological tissues, as well as the assessment of the effects of various topical chemical and physical treatments. This work holds significant implications for both basic and practical research at the interface between optics and biomedicine.
Surkov et al. (Tue,) studied this question.