Broadly speaking, nano-drug delivery systems can be divided into: (1) passive targeting, transported to target sites through regular physiological processes with carriers including emulsions, liposomes, microcapsules, or microspheres; (2) active targeting carriers, the surfaces of which have been modified with specific ligands to enable binding to receptors on the target cells or organs; (3) physical and/or chemical targeting carriers directed by external forces such as temperature, pH, or magnetic field.