In vitro dosimetry of microplastics: the importance of understanding the delivered dose in toxicity testing

Research into the potential health hazards of microplastics (MPs) has intensified in recent years, with growing efforts to compare and rank their toxicity based on polymer type, size, and shape. A critical but often overlooked aspect of this screening is particle dosimetry and, particularly, the distinction between administered and delivered dose. This study compares the sedimentation behavior of MPs as estimated by two computational models (RiskGone and Stokes’ Law) to experimentally generated data to better understand their accuracy and applicability in predicting MPs’ deposition. Aminated fluorescent polystyrene microspheres (PS) of 1 and 10 μm were used to assess the settling rates of MPs in an in vitro cell-free system over time in a dose-response relationship through fluorescence intensity measurements. Based on the in vitro system’s parameters (medium density, viscosity, etc.) and particle characteristics (size distribution, density, etc.), the modelling with RiskGONE and Stokes’ Law was also performed. In parallel, THP-1 cells were exposed to the same MPs under the same conditions (medium composition, plate setup and exposure volume) to investigate differences and potential incorrect conclusions when using the administered dose instead of the delivered dose when calculating MPs’ potency. Computational models and experimental data revealed that within 24 h, neither of the particle sizes will reach 100% deposition. Sedimentation behavior of smaller MPs was accurately predicted by the RiskGONE model, while the sedimentation of bigger MPs aligned closely with Stokes’ Law. Viability measurements in the THP-1 cells revealed that relying solely on the administered dose can lead to an underestimation of the MPs’ hazard potential of about a factor of 2.