32 SUN CARE
Microscopic revelations in sunscreen formulation
Afshan Nayyir-Mazhir, Helene Hine, Aline Souza - Croda Europe
Sunscreen formulations are complex and multi-component systems that are based on either organic UV filters, inorganic UV filters, or both. Formulations are most commonly created in emulsion form. Emulsions, as inherently thermodynamically unstable systems by nature, pose a challenge in maintaining stability over time. The overarching objective in the formulation process is to achieve and preserve stability throughout the entire duration of the product’s shelf life. Formulators work to address the inherent
instability of emulsions, implementing various techniques and strategies to ensure that the sunscreen maintains its efficacy and quality from production through its intended period of use. The pursuit of stability involves a nuanced understanding of the chemical and physical interactions at play, allowing for the development of formulations that withstand environmental factors and the passage of time, ultimately ensuring a reliable and effective sunscreen product for consumers. One key element of predicting stability is using microscopy. Delving into various facets of microscopy,
this article covers critical elements such as timeframes for analysis, slide preparation, incompatibilities, droplet size, and the distinctive appearance of titanium dioxide (TiO2
) and zinc oxide (ZnO). With a keen focus
on agglomeration challenges posed by these inorganic UV filters, and similarly, crystallisation of organic UV filters, the article also offers guidance to overcome these challenges. As formulators navigate the complexities of sunscreen development, this article provides detailed insights into how microscopy can enhance the understanding of formulation microstructure, offering valuable information beyond what is visible to the naked eye.
Timeframes for microscopic evaluation Incorporating microscopy as an analytical step 24 hours post-formulation is a highly beneficial practice, as it can reveal incompatibilities or instabilities that may remain imperceptible to the naked eye until subsequent stages of stability testing. Twenty-four hours allows any waxes within the formulation to form the necessary gel networks and achieve optimum thickening. While the ideal approach involves assessing microscopy at every stability test time point, the minimum recommendation is to examine at both the initial and final stages of stability testing.
Slide preparation process The first step in microscopy is sample preparation and researchers are advised
to employ glass slides and cover slips. The procedure involves applying a minute quantity of undiluted formulation onto the glass slide, utilizing either a pipette or a spatula. Subsequently, a glass cover slip is delicately placed over the sample, with special attention given to the edges and corners. A gentle yet intentional press on the cover slip is recommended to facilitate the compression and spreading of the sample into a thin layer (Figure 1). It is crucial, however, to exercise caution during this step to avoid exerting pressure directly on the sample. Pressing down on the sample itself carries the risk of disrupting the micelle structure, potentially compromising the integrity of the sample. Such disruption can have implications for the accuracy of conclusions drawn from subsequent microscopy images. Therefore, researchers are advised to handle the cover slip with care, ensuring that the compression is applied evenly without undue pressure, thereby preserving the microstructure for both a comprehensive and reliable analysis.
Figure 1: Example of a microscope slide prepared with a sunscreen emulsion PERSONAL CARE February 2024
Formulation insights, an overview Microscopy can help to assess the following: ■ Formulation stability ■ Identify agglomeration of inorganic UV filters ■ Identify crystallisation of organic UV filters ■ Detect undissolved salts or presence of crystalline waxes
www.personalcaremagazine.com
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