Depth Analysis of Pigment Nanoparticles in Cosmetic Contact Lenses Using Electron Microscopy

Abstract

Pigment exposure and surface roughness have long been concerns regarding health hazards of cosmetic contact lenses. However, apart from Z-stack analysis, no effective method for the depth measurement of pigments exists. In this study, the development of a quantitative method to measure pigment depth at high resolution and the utility of electron microscopy are examined. The samples were prepared according to conventional methods for transmission electron microscopy observation of biological materials. As there is almost no difference in electron density between the resin that constitutes lenses and embedding resin, labeling methods were examined using scanning electron microscopy (SEM) to visualize the surfaces of lenses in the embedding resin. The boundaries were clearly visualized using platinum/palladium (Pt/Pd) sputter coating after freeze-drying. A clear correlation was observed between the results measured by scanning transmission electron microscopy (STEM) and Z-stack analysis. The results indicate that no morphological changes corresponding to the resolution of Z-stack analysis occurred as a result of our preparation method. Pigment depths were analyzed using STEM and the surface-to-pigment distances ranged from 36.9 nm to 18.7 μm for the specimens used in this study. The use of STEM facilitates the measurement of pigment distributions below 100 nm, which cannot be analyzed using Z-stack analysis; this method was able to measure lengths ranging from sub-mm to sub-nm scales. This method has the potential to be a powerful tool to assess the causal relationship between the product quality of cosmetic contact lenses and related health hazards.