An Overview of Physical (Particulate) Sunscreens

Skin is constantly assaulted by sun exposure and terrestrial solar ultraviolet radiation (UVR) is a major factor deleterious to our health. The most reliable approach to sun protection is to cover up the skin by using a thin film of a topical sunscreen formulation containing active chemical ingredients that absorb, scatter or reflect the incident UVR to reduce the direct penetration and effect of UV exposure. Physical (particulate) actives are very stable and benign. From a regulatory perspective, zinc oxide and titanium dioxide are the only ones universally allowed as sunscreen actives. The development of sunscreen formulations containing ZnO and TiO2 requires knowledge of their optical properties as these dictate both the resulting aesthetics and the efficacy of the final sunscreen product and, thereby, acceptance by the user; these factors critically impact the economics of commercial sunscreen manufacturing and sales. The optical properties are uniquely related to the particle size (distribution) and particle morphology (e.g., surface area) of the two oxides. Precise measurement of these parameters is therefore a critical metric in formulating sunscreen products. X-ray disc centrifugation, acoustic attenuation spectroscopy and nuclear magnetic resonance relaxation are useful techniques that can be used to characterize fundamental properties of particulate suspensions at concentrations that are commercially relevant.