In Vitro Assessment of Sunscreen Efficacy Using Fourier Transform Infrared (FTIR) Spectroscopy on Synthetic Skin

Although there are several methods for assessing the sun protection factor (SPF) of sunscreen products, there is no standard and reliable in vitro method. Each test entails limitations and drawbacks. Therefore, this study aimed to assess the employability of FTIR as an alternative and quick method to evaluate the efficacy of various sunscreen formulations, their concentrations, and the timing of their application. Infrared radiation has longer wavelengths than ultraviolet, penetrates deeply into the skin, and hence enables the understanding of sunscreens' ability to block the transmission of radiation. The FTIR transmission using synthetic skin to study the effect of sunscreen agents (oxybenzone, octyl methoxycinnamate, titanium dioxide (TiO₂), and zinc oxide (ZnO)) was conducted in the range 450-4000cm^-1. Comparison studies were made at the peak of 805cm^-1. After 2 h of sunscreen application, using the maximum concentrations, the FTIR peak at wavenumber 805cm^-1 demonstrated a significant reduction of transmission from 96.55 to 60.09%, 57.59%, 32.02%, and 37.1% for oxybenzone, octyl methoxycinnamate, TiO₂, and ZnO respectively (P<0.05). A significant reduction in transmission was observed (P<0.05) with increasing sunscreen concentrations after 2 h of application. Nevertheless, the upper limit of concentration showed no appreciable change from the middle level of concentration, and hence it is cost-effective to employ the middle concentration. Inorganic sunscreens showed a higher protection level than organic. Fixed-dose combinations of sunscreens showed an enhanced effect yet were not synergistic. In conclusion, the use of FTIR spectroscopy with synthetic skin is a quick and user-friendly technique that enables the assessment of the efficacy of sunscreen formulations.