The purpose of this study is to monitor <i>in vivo</i> the delivery of <i>trans</i>-retinol into human skin. Delivery to real systems, such as skin, can be extremely difficult to execute and is problematic to confirm and measure. So far, methods for studying the delivery of compounds through the skin are mostly <i>ex vivo</i> and so inherently influence the skin and may not translate directly to the <i>in vivo</i> situation. Raman spectroscopy is uniquely placed to be able to measure biological processes <i>in vivo</i>, and this paper shows that the <i>trans</i>-retinol penetration into the skin can successfully be measured <i>in vivo</i> using this technique. This study measured the volar forearm of volunteers treated with 0.3% <i>trans</i>-retinol in propylene glycol (PG)/ethanol and 0.3% <i>trans</i>-retinol in caprylic/capric acid triglyceride (MYRITOL<sup>®</sup>318), an oil found in skin creams. Solutions were applied and then confocal Raman depth profiles were obtained of the stratum corneum (SC) and into the viable epidermis (VE) up to 10 hours after treatment. Remarkable differences between a penetrating and a nonpenetrating solution can clearly be observed. Treating with <i>trans</i>-retinol in PG/ethanol results in <i>trans</i>-retinol penetrating through the SC and into the VE. Its penetration was also observed to be highly correlated with the depth of penetration of the PG, which is well known as an efficient penetration enhancer. In contrast, while treating with <i>trans</i>-retinol in MYRITOL<sup>®</sup>318, <i>trans</i>-retinol hardly penetrates at all. For the first time, the penetration of <i>trans</i>-retinol has been monitored directly after application of solutions, <i>in vivo</i> without skin excision. Here, the effect of two different solutions on the delivery of <i>trans</i>-retinol into the skin was measured very effectively <i>in vivo</i> by Raman spectroscopy.

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