Mass Balance Tracing of <i>In Vivo</i> Biodistribution, Relocation, and Excretion of Europium-Doped Micro/Nanoplastics in Rats

The discovery of micro/nanoplastics (MNPs) in the human circulatory system raises great health concerns. It is generally believed the in vivo transport and fate of MNPs are size dependent regulated by various physiological barriers, but recently, increasing reports of MNP accumulation in the brain or excreted through urine challenges this knowledge. Herein, the biodistribution, relocation, and excretion of intravenously administered MNPs were quantitatively traced in female rats for 3 months based on mass balance assessment using europium-doped polystyrene MNPs. With an overall recovery ∼90%, MNPs were mainly found in liver and spleen but seldom in brain and heart, indicating in vivo MNP transport follows classical size-dependent rules. Muscles with low concentrations of MNPs become the second largest long-term sink after the liver due to their high overall mass within the body. Dose-dependent accumulation was observed in RES (reticuloendothelial system) organs, while similar concentrations in low- and high-dose treatments imply a low maximum accumulation for MNP in the heart and brain. Although only minor toxicity effects were observed, just ∼80% of 77 nm and ∼15% of 300 and 2000 nm MNPs were exclusively excreted through feces, mostly in the first 3 weeks following a growth-curve pattern, thereby demonstrating long-term accumulation and potential health concerns.