Femtosecond laser–assisted printing of hard magnetic microrobots for swimming upstream in subcentimeter-per-second blood flow

Magnetic microrobots have garnered increasing attention in the biomedical field due to their wireless motion control and noninvasive therapeutic potential. However, current microrobots fabricated from soft magnetic materials exhibit critical limitations, including weak magnetic response and uncontrollable locomotion in high-rate blood flow, which hinder their practical applications. Here, we develop a fabrication method for three-dimensional (3D) hard magnetic microrobots (HM-microrobots) containing neodymium-iron-boron particles. HM-microrobots, featuring programmable 3D helical morphologies, are fabricated by integrating two-photon polymerization printing of molds in positive photoresist with subsequent vacuum infusion molding of hard magnetic slurries. The microrobots achieve a maximum swimming velocity of 22.6 body lengths per second (3 millimeters per second) in a weak rotating magnetic field of 2 millitesla, which represents a notable improvement over their soft magnetic counterparts. Furthermore, they demonstrate controllable upstream propulsion in subcentimeter-per-second blood flow, showing great potential for endovascular applications.