Thermoresponsive alginate/PNIPAM/Fe3O4 microbots under magnetic surface locomotion for ocular therapies

Magnetically actuated microbots capable of controlled locomotion and material adaptability are of growing interest for minimally invasive biomedical applications, particularly in confined and soft biological environments. This work presents the design, fabrication, and magnetic actuation of thermoresponsive alginate/poly(N-isopropylacrylamide) (PNIPAM) hydrogel microbots for future ocular therapies. The composite hydrogel exhibits a reversible lower critical solution temperature (LCST) transition between 28–40 °C, producing temperature-dependent changes in viscosity and optical appearance. Spherical microbots (average diameter ~194 µm) were fabricated using a centrifugation-assisted extrusion method and uniformly loaded with iron oxide nanoparticles to enable untethered magnetic actuation. Under a 10 mT rotating magnetic field, the microbots demonstrated stable rolling locomotion with a linear velocity–frequency relationship between 0.25–2 Hz, achieving slip ratios near unity (1.03 ± 0.09). Open-loop, closed-loop, and coordinated group actuation yielded consistent translational velocities ranging from 112–169 µm/s. Group-level actuation yielded average heading errors of ~12.4° and lateral errors of ~17 µm, while closed-loop control enabled convergence to sequential targets with steady-state errors of ~45 µm (~23% of the microbot diameter). As a proof of concept for non-planar environments, microbots successfully climbed curved, ocular-inspired PDMS surfaces up to ~50°. These results establish alginate/PNIPAM hydrogels as a viable material platform for responsive microrobotic systems and provide the first demonstration of controlled magnetic locomotion in a hydrogel microbot capable of temperature-dependent material property switching. The combined actuation and thermoresponsive behavior highlight potential utility in minimally invasive biomedical applications, including targeted therapy, ocular microsurgery, and particularly, future use in military-relevant injuries such as vitreous hemorrhage recovery.