Development and Evaluation of Sunscreen Formulations Containing Thai Cinnamon (Cinnamomum bejolghota) Bark Extract and a Cinnamaldehyde-Rich Essential Oil: Sun Protection Potential and Synergistic Effects with Commercial UV Filters

Cinnamon bark extract, rich in cinnamaldehyde, possesses antioxidant, anti-tyrosinase, anti-inflammatory, and UVB protection properties. However, its potential as a natural UV filter or its synergy with commercial UV filters remains unexamined. This study aimed to develop and evaluate sunscreen formulations incorporating Cinnamomum bejolghota bark extract for protection against UVB, UVA, and visible light. Two extract types were prepared: crude ethanolic extract (CBE) and water-distilled essential oil (CBO). The cinnamaldehyde content was analyzed using UV-Vis spectrophotometry. CBE showed a higher extraction yield (19.14 ± 0.05%) but a lower cinnamaldehyde content (6.38 ± 0.21%) compared to CBO (0.36 ± 0.04% yield; 79.44 ± 1.32% cinnamaldehyde). Due to the extremely low yield of CBO, further evaluations were conducted using a commercial cinnamaldehyde-rich essential oil with a comparable cinnamaldehyde content (79.01 ± 0.99%), referred to as CBO*. Sunscreen formulations containing 1%–10% CBE or CBO* were assessed for appearance, pH, stability, and sun protection efficacy, expressed as sun protection factor (SPF), ultraviolet A protection factor (UVAPF), and blue light protection. CBE-based formulations exhibited lower SPF values (1.06–1.61) compared to those containing CBO* (4.75–8.72). A 5% CBO* formulation was selected for synergy testing with commercial UV filters: 5% ethylhexyl triazone (UVB filter) and 5% diethylamino hydroxybenzoyl hexyl benzoate (UVA filter). The resulting formulations demonstrated homogeneous textures, pH 6.0–6.7, good stability, and enhanced sunscreen efficacy, with synergistic effects most pronounced when both UV filters were combined. Future studies on photostability and long-term stability are recommended.