Cumulative skin exposure to surfactants can damage the stratum corneum (SC). The free surfactants in rinsed-off product can penetrate the skin or adhere to SC protein after washing step. This potentially alter or strip the intercellular lipid component of SC, resulting in skin barrier disruption and dryness. Additionally, even in short-term exposure, the surfactants can bind to the SC's protein, leading to swelling or denaturation of the cornified envelope protein. The damaged skin barrier allows free surfactants to penetrate deeper skin layers, interact with the skin cells and cause irritation (Ananthapadmanabhan et al., 2004; Lips et al., 2006; Bujak et al., 2020).

Several in vivo human test protocols have been developed to assess the safety profile of rinse-off products, including the modified Frosch-Kligman soap chamber test, human patch test and antecubital flex test (Simion, 2006). Among these, Forearm Controlled Application Test (FCAT) is the most well-adopted technique, capable of distinguishing skin dryness and roughness (Lukacovic et al., 1988; Ertel et al., 1995). However, the efficiency of FCAT is limited by its long testing duration, low throughput and high testing cost. Besides, for ethical and practical reasons, pre-clinical clearance is recommended before any in vivo human tests. Therefore, a quicker, cost-effective pre-clinical test method for rinsed-off products is necessary.

In this proof-of-concept study, we used an integrative approach that combined the biological and chemical in vitro assays to explore the feasibility of establishing a predictive model for skin barrier function and skin moisture capacitance change observed in FCAT in vivo. The biochemical approach utilizes the OECD-validated Reconstructed Human Epidermis (RHE) model, which closely mimics the structure and function of human epidermis, to determine the irritancy and penetration potential of surfactant monomers and their impurities-derived irritants prior to rinsing. On the other hand, the chemical assays of using Zein test and FAME test determine the potential damage to the protein and lipid, leading to skin barrier damage and dryness, respectively.