Effects of Weathering and Simulated Gastric Fluid Exposure on Cellular Responses to Polystyrene Particles

Microplastics pose a growing environmental threat with complex implications for human health. Despite the extensive research on the cytotoxicities of microplastics, gaps remain in understanding cellular responses to the interplay between environmental weathering and physiological processes. This study aims to fill this knowledge gap by evaluating and comparing the in vitro cellular responses to pristine polystyrene particles, particles weathered under UV light in DI water and seawater, and particles with subsequent incubation in simulated gastric fluid (SGF). In this study, Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) were implemented to conduct surface chemistry and morphology characterizations of the particles. The combination of these techniques allowed a comprehensive understanding of surface morphology and chemistry alterations due to the weathering degradation and SGF incubation. Results showed nitrogen and carbonyl groups formed on weathered particles, and seawater-weathered particles showed a more pronounced weathering degree. After SGF incubation, stronger nitrogen and amide groups were detected on the surface of weathered particles, and more organic matter was attached. Two cell lines that are widely used for the evaluation of microplastic cytotoxicity were used, RAW264.7 macrophage and Caco-2 intestine epithelial cells. Results showed weathered and SGF-treated particles enhanced macrophage viability and pro-inflammatory effects compared to pristine particles. Elevated reactive oxygen species (ROS) generation was detected for all particle groups. Weathered particles caused higher cytotoxicity effects on Caco-2 cells and damaged tight junction integrity. The organic matter formation from the SGF incubation protected tight junction integrity and reduced cytotoxicity. These findings highlight the importance of taking both environmental and physiological factors into account for a more comprehensive assessment of microplastic toxicity.