Template-free fabrication of reconfigurable magnetic micropillars and filaments through controlled Nanoflower assembly and actuation

Magnetic nanoflowers (MNFs), which exhibit large intrinsic magnetic losses and high specific absorption rates under clinically relevant alternating magnetic fields, highlight strong potential as efficient mediators for magnetic hyperthermia. In this work, we provide a versatile platform for creating dynamic, field-responsive microstructures based on MNFs through a flexible, low-cost, and template-free self-assembly strategy driven by tunable interparticle interactions, external magnetic fields, and spatial confinement. By controlling ionic strength, particle coverage, surface charge, particle concentration, and confinement, MNFs spontaneously assemble in aqueous solution into magnetic micropillars and microfilaments without predefined scaffolds, with low ionic strength favoring reversible assemblies and intermediate salt concentrations yielding stable, irreversible structures. The size, geometry, and dynamic response of these architectures can be precisely tuned, enabling field-induced behaviors such as cilia-like rotations, oscillations, and torque-driven detachment of micropillars into free-standing, swarming microfilaments. L-dopamine (L-DOPA) was used for surface modification in this work, as it is a biocompatible ligand offering catechol, amine, and carboxylate groups. Resulting MNFs@L-DOPA have negative surface charge and show assembly behavior qualitatively similar to the uncoated system under magnetic actuation. Together, these results establish practical guidelines for the template-free design of biomimetic, functional magnetic and elongated microarchitectures, highlighting their potential for microfluidic manipulation and bio-microrobotic applications.