Schematic illustration of Fe3O4/BSA microspheres coated with rod-like SiO2 (Fe3O4/BSA/rSiO2) for cancer therapy. a Synthesis of the Fe3O4/BSA/rSiO2 magnetic microspheres. b Magneto-mechanical force and ROS generated by the microspheres exposed to a VMF for killing cancer cells and inhibiting tumor growth