Controlling the intracellular distribution of magnetic nanostars in barrier cells using oriented magnetic fields

Magnetic fields can enhance cellular uptake of magnetic nanoparticles, yet the impact of field orientation on subcellular distribution remains unclear. In this study, star-shaped, Co_xFe_1-xO magnetic nanostars (MNSs) were synthesized using a hydrothermal method and functionalized as fluorescent probes via fluorescein isothiocyanate (FITC) -dextran ligand exchange. A tight human umbilical vein endothelial cell (HUVEC)-based endothelial barrier was exposed to fluorescent-labelled MNSs while applying an external magnetic field at 0° (parallel), 45° and 90° (vertical) relative to the cell plane. Cell viability remained above 93% under all tested magnetic-field conditions, while no significant changes in transendothelial electrical resistance (TEER) were observed before and after MNS treatment, indicating that the treatment did not substantially compromise cell viability or endothelial barrier integrity. To quantify the intracellular distribution of MNSs, a harmonic partitioning algorithm was used to segment irregular cell boundaries and to extract interval-resolved fluorescence profiles. Both 45° and vertical magnetic fields increased cellular uptake compared with parallel- and no-field conditions. However, the vertical field tended to accumulate MNSs in the perinuclear region, while the 45° field favored enrichment around the harmonic center of the cytoplasm. Our results demonstrate that the direction of the magnetic field is a critical parameter for the intracellular distribution of magnetic nanoparticles.