Low Reynolds Number Locomotion of Erythrocyte-based Magnetic Micromotors

We report on the dynamics of soft magnetic achiral micromotors propelled in low Reynolds number environments. Micromotors are fabricated by the self-assembly of biotinylated red blood cells (RBCs) with streptavidin-coated RBCs and magnetic particles, forming one-, two-, or three-cell configurations. Under a uniform rotating magnetic field, micromotors exhibit either surface rolling or bulk swimming depending on their position relative to a boundary. Rolling is driven by hydrodynamic asymmetry near the substrate, where unequal flow resistance above and below the micromotor breaks time-reversal symmetry, generating net torque and translational motion. In bulk fluid, swimming is enabled by structural asymmetry and deformation of the RBCs, which produce a non-zero mobility tensor allowing effective propulsion. Propulsion experiments conducted in phosphate buffered saline, blood serum, and methylcellulose demonstrate that three-cell swimmers exhibit the highest propulsion efficiency due to increased geometrical complexity, while single-cell rollers achieve the highest efficient surface locomotion, due to less hydrodynamic drag between their body underlying surfaces. These findings advance our understanding of soft micromotor locomotion at low Reynolds number and presents a promising strategy for actuating biohybrid systems in complex physiological environments.