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

Abstract

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.

Figures (7)

• Figure 1: Aqueous suspension of uncoated MNFs at pH = 2.0 with varying NaCl concentrations under a constant external magnetic field, applied horizontally, parallel to the glass substrate and perpendicular to the microscope's optical axis. Top row: Colloidal suspension prior to magnetic field application. Middle row: Suspension under a magnetic field ($B_x = 2.0$ mT) for 300 s. Bottom row: Suspension following field removal (Movie 1). The scheme on the right represents the field-induced process in 3D at [NaCl] = 50 mM.
• Figure 2: Suspension of uncoated MNFs at pH = 2.0 with different NaCl concentrations under a constant external magnetic field oriented vertically, perpendicular to the substrate plane. Top row: Colloidal suspension prior to magnetic field application. Middle row: Suspension under a magnetic field ($B_z = 0.9$ mT) for 300 s. Bottom row: Suspension following field removal. Red-framed images highlight conditions in which the filaments formed under the action of the field remain visible for seconds after the field is turned off (Movie 2). The scheme on the right represents the field-induced process in 3D at [NaCl] = 50 mM.
• Figure 3: A) Columnar formations of MNFs under the application of a constant vertical magnetic field ($B_z = 0.9$ mT) at pH = 2.0 with two different NaCl concentrations. Top row: Structures observed under the applied field for 300 s. Bottom row: Same regions observed after field removal (Movie 2). Framed insets show the time evolution of a representative filament formed at [NaCl] = 60 mM. B) Logarithm of the projected area by the filaments, normalized to its value just after the field along the z-axis was switched off, $A/A_0$, plotted as a function of time. The averaged values and associated errors result from the analysis of three different micropillars monitored under the same conditions. The inset shows the linear relationship between the logarithm of the characteristic absorption time, $t_c$, and [NaCl].
• Figure 4: A) Schematic of the transport of magnetic filaments formed under a rotating magnetic field applied perpendicular to the substrate plane. The gray spot marks one end of the filament for easy identification. B) Sequence of images showing the motion of magnetic filaments under a rotating field oriented perpendicular to the substrate ($B_z = 13.5$ mT, $B_x = 3.3$ mT, $f = 1.0$ Hz, $\beta = -0.9$) and at a salt concentration of [NaCl] = 70.0 mM. The time of each frame is indicated. C) The image below shows how some filaments change their transport direction under the action of the rotating field ($B_x = B_z = 3.3$ mT, $f = 1.0$ Hz, $\beta = 0$, Movie 9). The graph aside shows the trajectory of a filament along x. D) Dimensionless velocity as a function of filament length for different field frequencies, under a circular rotating field applied perpendicular to the horizontal substrate ($B_z = B_x = 3.3$ mT, $\beta = 0$). The averaged values and associated errors result from the analysis of at least three different filaments of similar length monitored under the same conditions.
• Figure 5: A) (top) The scheme illustrates the strategy for extracting filaments from the fabrication chamber by exploiting their coupled rotation and translation under a rotating magnetic field applied in the plane perpendicular to the substrate. (bottom) Images showing the driven transport of magnetic filaments under a rotating field ($B_z = 0.9$ mT, $B_x = 2.5$ mT, $f = 1.0$ Hz) and at a salt concentration of [NaCl] = 70.0 mM. Black arrows highlight the directed motion of the filaments as they move out through the open edge of the chamber, covering approximately 20 µm in 1.2 seconds. The thick lines running vertically through the center of the images mark the edge of the glass coverslip that bounds the chamber. B) SEM images of the magnetic filaments at different magnifications after extraction from the fabrication chamber and subsequent drying. C) Length, $L$, and diameter, $a$, distributions of the filaments collected using the procedure described.