Microactuators, defined as nanoscale to microscale structures capable of actuation, hold significant potential for executing tasks such as serving as micro-sensors, micro-valves, and components in microrobotic systems. Among actuation mechanisms of microactuators, magnetic actuation stands out due to its wireless operation, precise directional control, and stable response. However, current magnetic microactuators predominantly rely on soft magnetic materials (e.g., Fe₃O₄ for doping or Ni for coating), which exhibit limited magnetic strength, hindering effective motion in low-Reynolds-number fluids. Hard magnetic materials, such as sintered NdFeB powders (5 μm average diameter, high saturation magnetization, and coercivity), offer a promising solution but face fabrication challenges in achieving microscale and three-dimensional (3D) geometries. Existing methods, for example, femtosecond laser direct writing struggles to integrate opaque and large NdFeB particles into high-resolution 3D structures, as particle sizes exceed the resolution limits of two-photon polymerization (TPP). To address this, the study here introduces a femtosecond laser mold-assisted fabrication strategy to overcome these limitations. By combining TPP-based template fabrication with NdFeB-polymer slurry infusion, we demonstrate the successful fabrication of hard magnetic microactuators with complex 3D architectures. The resulting microactuators exhibit superior motion performance compared to a soft magnetic counterpart, particularly in high-viscosity environments, which is attributed to the high remanence and coercivity of NdFeB. This approach bridges the gap between high-resolution 3D microfabrication and hard magnetic material integration, enabling the development of robust, functionally advanced microactuators or microrobots for biomedical and microfluidic applications.

The fabrication process begins with femtosecond laser patterning of templates in positive photoresist. A Ti∶sapphire femtosecond laser oscillator generates pulsed laser beams (central wavelength of 800 nm, pulse duration of 75 fs). The laser beams are expanded and directed through galvanometric scanners before being focused by a 60× oil-immersion. 3D scanning is achieved through coordinated control of x-y galvanometer mirrors and a z-axis precision stage. Approximately 0.1 mL positive photoresist is dispensed onto a coverslip and spin-coated at 1000 r/min for 10 s. The substrate undergoes pre-baking at 110 ℃ for 1 h to eliminate bubbles and enhance structural stability. Subsequent femtosecond laser exposure creates the microactuator architecture in the photoresist layer [Fig. 4(a)]. The developed template is obtained after 30 min immersion in developer [Fig. 4(b)], followed by thorough cleaning in deionized (DI) water. A slurry containing NdFeB particles and epoxy precursor is prepared at a mass ratio of 1∶1 and deposited onto the template surface [Fig. 4(c)]. Vacuum degassing at ambient temperature effectively removes entrapped air bubbles. Excess slurry is removed using lint-free wipes prior to thermal curing at 80 ℃ for 2 h [Fig. 4(d)]. Magnetization is performed using a pair of permanent magnets with a 1.2 T magnetic field intensity [Fig. 4(e)]. Final release of microactuators is achieved through template dissolution in ethanol, with subsequent transfer to DI water for further tests [Fig. 4(f)].

The developed femtosecond laser template-assisted method enables robust fabrication of hard magnetic microactuators with complex 3D geometries and tunable NdFeB content [Figs. 5(a)?(c)]. The microactuators retain structural integrity, with NdFeB particles uniformly encapsulated within epoxy resin [Fig. 5(b)]. Magnetic torque generation is directly related to the NdFeB mass fraction, enabling programmable actuation performance. Under rotating magnetic fields [Fig. 6(a)], the hard magnetic microactuators exhibit distinct rotational modes [Fig. 6(b)], with step-out frequencies increasing proportionally with applied field strength [Fig. 6(c)]. Crucially, the high remanence of NdFeB confers exceptional performance in viscous solutions. In 60 cP glycerol solution, the hard magnetic actuator outperforms a soft magnetic counterpart that cannot rotate under identical conditions [Fig. 6(f)]. The results highlight that hard magnetic materials overcome the torque limitations of soft magnetic materials. This advance expands the operational scope of magnetic microactuators for biomedical applications requiring robust fluidic manipulation.

This study presents a method for fabricating 3D microscale structures containing hard magnetic NdFeB materials through femtosecond laser two-photon polymerization combined with a template infusion approach, successfully realizing the preparation of hard magnetic microactuators. A systematic investigation of this technique demonstrates its capability to process diverse geometries, employ multiple materials, and achieve controlled magnetic content within specific ranges. The fabricated hard magnetic microactuators exhibit excellent structural integrity and leverage the advantages of hard magnetic materials, demonstrating superior locomotion capabilities in liquid environments. Comparative actuation experiments in high-viscosity fluids reveal that the fabricated hard magnetic microactuators have significant performance advantages over conventional soft magnetic microactuators. This methodology holds potential for broader applications in manufacturing hard magnetic microstructures, actuators, and microrobots, potentially enabling more demanding biomedical applications in the future.