Synopsis

Background:
Higher SPF claims in sunscreens are typically achieved by increasing the concentration of UV filters. This approach assumes UV filter performance is both additive and independent of formulation design. This is despite well-documented variation in SPF performance observed between formulations containing identical UV filters and levels.

Objectives:
In this work, we demonstrate that chassis design – including emulsifiers, waxes and film-forming agents – plays a critical and previously under-exploited role in determining photoprotection performance.

Methods:
A systematic formulation screening programme was conducted to evaluate the performance impact of diversified emulsifier architectures across a broad formulation space. More than 100 sunscreen emulsions were developed using fixed base compositions with deliberate variation of emulsifier systems (oil‑in‑water and water‑in‑oil), wax chemistries and complementary film formers to isolate structure–function relationships. Formulations were comprehensively characterised using an integrated analytical approach, including:
• Accelerated stability testing
• Rheology
• Microscopy
• Cryo SEM
• Raman Spectroscopy
• Both in-vitro (ISO 23675) and in-vivo (ISO 24444) SPF assessments
• Both in-vitro and in-vivo sensory assessments

Results:
Microstructural analysis under polarised light identified both spherical and needle shaped crystal morphologies formed via wax-emulsifier co-crystallization. Additionally, multivariate analysis revealed a strong correlation between high-shear viscosity and SPF efficacy, indicating that formulations resistant to deformation enable better UV-filter distribution and, as a result, deliver higher SPF. Advanced imaging demonstrated that film formers enhance UV-filter distribution within sunscreen films, delivering SPF increases of up to 16 units while preserving UVA:UVB balance for international compliance

Conclusion:
These insights allow the creation of completely optimised chassis systems. Combining emulsifiers, waxes, and film formers to deliver robust emulsion frameworks that facilitate the confident delivery of high SPF performance with reduced UV-filter loading. This work demonstrates how formulation science – integrating rheology, microstructure, and ingredient synergy – can transform sunscreen design, unlocking elegant, high-performance sun care solutions that meet consumer expectations for protection and sensory appeal.