Mechanical stress-induced microplastic release from bottle caps: Insights on material properties and their impact on...

The release of microplastics (MPs) from plastic drinking water bottles has raised global attention; however, the impact of material quality on MPs release and their toxicity in cellular systems have not been studied thoroughly. Scraped-off MPs up to 1000 µm were observed; scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and mechanical property analysis revealed brand-dependent differences in polymer behaviour. These MPs increased cytotoxicity in a dose and time dependent manner in HEK293 cells, with AB-MPs showing greater toxicity (IC₅₀ = 6.24 mg/mL). Mitochondrial swelling, perinuclear aggregation of endoplasmic reticulum, and micronuclei formation were observed in the cells after 48 h. ROS content increased at 24 h, but normalized to its physiological level after 48 h. qRT-PCR analysis revealed that both AB-MP and BB-MP (6 mg/mL) induced marked upregulation of inflammatory genes NF-κB (9.34-fold), TNF-α (2.58-fold), and oxidative stress response gene SOD2 (3.56-fold), and CAT (5.7-fold) at 24 h of exposure with higher induction upon AB-MP treatment. However, apoptotic gene expression revealed that Bcl-2 levels were initially elevated (4.68-fold) at 24 h but declined at 48 h, accompanied by a corresponding increase in Bax (0.44-fold). Therefore, MPs generated from plastic bottles affected cellular systems by inducing reactive oxygen species, inflammation, apoptosis and genotoxicity, while altering cell-organelle dynamics. These findings highlight the risk of microplastics released from bottle caps and necks which can be ingested during mouth-to-bottle contact while drinking.