Open-air human skin equivalent platform enabling photobiological studies and topical product testing

Human skin equivalents (HSEs) are widely used for photobiological research and cosmetic product testing. However, their use is typically limited to sterile environments, restricting realistic manipulations such as direct sunlight exposure or topical product application. In this study, we propose an open-air culture platform with a structurally integrated barrier architecture that allows melanocyte-containing HSEs to be maintained and evaluated outside of clean environments. The platform exposes only the cornified epidermal surface to air while shielding non-barrier regions using a medium-permeable nylon sheet and surrounding device components that mechanically support the HSE and maintain a stable air-liquid configuration. This configuration enables the natural barrier function of the cornified layer to operate effectively, preserving sterility and supporting tissue viability. Upon UV irradiation, the HSEs exhibit visible pigmentation responses with a moderate increase in pigmentation (~1.5-fold), enabling quantitative assessment of tanning responses. Moreover, application of commercial sunscreens results in dose-dependent suppression of pigmentation, reducing this response to near-baseline levels and validating the platform for sunscreen efficacy testing. Furthermore, the platform maintains both tissue viability and barrier function under natural sunlight and wind, demonstrating its robustness for real-world applications. This platform provides a versatile tool for evaluating the photobiological safety and efficacy of skincare products in realistic environments and establishes a fabrication strategy that bridges conventional in vitro skin models and real-world environmental exposure testing.