Development and Optimization of a Nanoemulsion-Based Lip Balm Incorporating Mycosporine-like Amino Acids from Catenella sp. for Enhanced Photoprotection

Background: Solar ultraviolet (UV) radiation is a major contributor to skin damage, driving the demand for safer and more sustainable photoprotective systems. Mycosporine-like amino acids (MAAs) have emerged as promising natural UV filters due to their strong absorption and photostability; however, their application is limited by poor chemical stability in aqueous environments. Objective: The present study aimed to develop and optimize a nanoemulsion-based lip balm incorporating a MAAs-rich extract from Catenella sp., addressing stability limitations while enhancing photoprotective performance. Methods: A MAAs-rich extract was obtained and characterized by UHPLC-DAD, followed by cytotoxicity evaluation in NIH-3T3 fibroblasts and stability assessment under stress conditions. A water-in-oil (w/o) nanoemulsion was rationally developed using pseudo-ternary phase diagrams and optimized through a Box–Behnken experimental design, considering aqueous phase, surfactant mixture, and sonication time as key variables. The optimized nanoemulsion was subsequently incorporated into a lip balm matrix, which was formulated and optimized using a mixture design approach to evaluate thermal, mechanical, and sensory properties. Results: The extract exhibited a high MAAs content (9.53 mg g−1 DW) and low cytotoxicity (IC50 > 100 µg/mL), but pronounced hydrolytic instability, particularly under alkaline conditions, while remaining photostable. The optimized nanoemulsion achieved a droplet size below 200 nm and low polydispersity (PDI < 0.3). Importantly, incorporation of MAAs into nanoemulsion significantly enhanced the in vitro sun protection factor (SPF), increasing from 16.1 (extract) to 35.4, while maintaining broad-spectrum UV coverage (λc = 380 nm). The blank nanoemulsion also contributed to UV attenuation, indicating a synergistic effect of the colloidal system. The optimized lip balm formulation (33% beeswax, 40% nanoemulsion, 22% shea butter, 1% candelilla wax, 4% carnauba wax) demonstrated suitable melting behavior, mechanical resistance, and high sensory acceptance. Conclusions: This study demonstrates that nanoemulsion-based structuring combined with statistical formulation design provides an effective strategy to stabilize MAAs and enhance their photoprotective efficacy, supporting the development of high-performance, natural sunscreen products.