Rational QbD-based Assessment of Formulation Ingredients' Impact on Sustainable Sunscreen Physical Stability

Traditionally, cosmetic emulsions have been developed using trial-and-error approaches, which can be ineffective and time-consuming. This study employed a Quality by Design approach to determine how formulation ingredients impact the physical stability of a sustainable sunscreen. A 2³ full factorial design assessed the effects of the concentrations of polyglyceryl-2 stearate, glyceryl stearate, stearyl alcohol (self-emulsifier), brassica glycerides (co-emulsifier), and xanthan gum (gelling agent) on the apparent viscosity and instability index of the emulsified vehicle. Another 2³ full factorial design evaluated the effects of the concentration of zinc oxide, titanium dioxide, and phytic acid on the instability index of the sustainable sunscreen formulation. The instability index of the emulsified vehicle was influenced by self-emulsifier and co-emulsifier concentrations. In contrast, its apparent viscosity was influenced by self-emulsifier and gelling agent concentrations, and by interactions among self-emulsifier, co-emulsifier, and gelling agent. The instability index of the sunscreen formulation was mainly influenced by zinc oxide and phytic acid concentrations. Robust mathematical models were developed and used to predict the optimal compositions of the emulsified vehicle and sunscreen formulation within the experimental domains. The optimal compositions predicted by the mathematical models developed were 8% self-emulsifier, 3% co-emulsifier, and 0.3% gelling agent for the emulsified vehicle, and 6.3% zinc oxide, 2% titanium dioxide, and 1% phytic acid for the sunscreen formulation. These optimized formulations met the Quality Target Product Profiles, ensuring appropriate physical stability, rheological performance, antioxidant properties, and the desired SPF, water resistance, and UVA-PF characteristics.