Visualization of zinc pyrithione particles deposited on the scalp from a shampoo by tape-strip sampling and scanning electron microscopy/energy dispersive X-ray spectroscopy measurement.

Abstract

Zinc pyrithione (ZnPT) is widely used as an anti-fungal active in commercial anti-dandruff (AD) shampoos. The AD efficacy of ZnPT is highly dependent on the deposition of ZnPT particles onto the scalp during the process of shampoo application and rinse-off. Since ZnPT materials with different particle sizes and morphologies have different deposition behaviours, the measurement of the actual ZnPT deposition is critical to understand the AD performance delivered by different ZnPT shampoos. The aim of this study is to develop a robust and reliable method for visualizing the particle size and morphology of ZnPT deposited on the scalp from AD shampoos. Hair was washed with a commercially available AD shampoo containing ZnPT and zinc carbonate (ZnCO3 ). Tape strips were applied to collect the deposited particles from the scalp after AD shampoo application and rinse-off. The scalp tape strip samples were subjected to scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) measurement. The morphology of the ZnPT particles was visualized by SEM imaging and identification of ZnPT particles was confirmed by EDX analysis. For the commercial shampoo studied it was observed that two zinc-containing particulates with different morphologies and composition remained on the scalp after shampoo application and rinse-off. As indicated by the EDX spectra, the ZnPT particles deposited onto the scalp surface had polygonal crystal structures. ZnCO3 was also deposited onto the scalp surface. This material was mainly present as aggregated particulates. An exvivo method that combines tape strip sampling and SEM/EDX has been developed for measuring and visualizing the particle size, morphology and composition of ZnPT deposited on the scalp from AD shampoos. This exvivo measurement method provides higher imaging resolution and more chemical specificity than reflectance confocal microscopy (RCM). To the best of our knowledge, this is the first time that ZnPT particles were distinguishable from other zinc particles on the scalp. Moreover, the new method allows the microstructures of both ZnPT and other zinc particles on the scalp to be imaged.