Magnetic Vortex Nanodiscs Enable Remote Magnetomechanical Neural Stimulation

Abstract

Magnetic nanomaterials in magnetic fields can serve as versatile transducers for remote interrogation of cell functions. In this study, we leveraged the transition from vortex to in-plane magnetization in iron oxide nanodiscs to modulate the activity of mechanosensory cells. When a vortex configuration of spins is present in magnetic nanomaterials, it enables rapid control over their magnetization direction and magnitude. The vortex configuration manifests in near zero net magnetic moment in the absence of a magnetic field, affording greater colloidal stability of magnetic nanomaterials in suspensions. Together, these properties invite the application of magnetic vortex particles as transducers of externally applied minimally invasive magnetic stimuli in biological systems. Using magnetic modeling and electron holography, we predict and experimentally demonstrate magnetic vortex states in an array of colloidally synthesized magnetite nanodiscs 98-226 nm in diameter. The magnetic nanodiscs applied as transducers of torque for remote control of mechanosensory neurons demonstrated the ability to trigger Ca²⁺ influx in weak (≤28 mT), slowly varying (≤5 Hz) magnetic fields. The extent of cellular response was determined by the magnetic nanodisc volume and magnetic field conditions. Magnetomechanical activation of a mechanosensitive cation channel TRPV4 (transient receptor potential vanilloid family member 4) exogenously expressed in the nonmechanosensitive HEK293 cells corroborated that the stimulation is mediated by mechanosensitive ion channels. With their large magnetic torques and colloidal stability, magnetic vortex particles may facilitate basic studies of mechanoreception and its applications to control electroactive cells with remote magnetic stimuli.

Keywords: cellular signaling; electron holography; magnetic nanoparticles; magnetic vortex; mechanosensitive ion channels; mechanotransduction; neuromodulation.

Figure 1:. The magnetic vortex phase in magnetite nanodiscs.

a-c, Schematic representation of the three possible configurations of magnetic spins in magnetite nanodiscs (MNDs); a, vortex, b, in-plane and c, out-of-plane. d, MND that supports the magnetic vortex state (left) in the absence of applied magnetic field can transition to being magnetized in-plane (right) upon the application of a weak magnetic field. e, MNDs with a range of thicknesses and diameters were each simulated 10 times using OOMMF, and their average final configuration was plotted to create a magnetic domain state phase diagram. For points with an intermediate color, either the equilibrium phase is a hybrid of the three canonical phases or two distinct equilibrium phases can result from different initial conditions in the simulation. White star markers on the phase diagram represent five MND compositions synthesized and used for experiments in this study.

Figure 2:. Synthesis and characterization of magnetite nanodiscs.

a, Crystal units of non-magnetic hematite phase converted to magnetic magnetite after reduction. Nanodiscs synthesized in two-step solvothermal reaction with 6% H₂O (b,g), 7% H₂O (c,h), 8% H₂O (d,i), 9% H₂O (e,j) and 10% H₂O. (f,k). b-f are hematite structures (α-Fe₂O₃) resulting from the first step of the synthesis. g-k are magnetite structures (Fe₃O₄) derived from the reduction of hematite discs. Scale bars = 200 nm. l, Diameter (blue, left axis) and thickness (red, right axis) of nanodiscs before and after reduction as a function of the percentage of H₂O in the synthesis (inset: calculated d/t ratio). m, XRD spectra for MNDs after reduction (hematite reference in red, magnetite reference in black). n, Saturation magnetization for MNDs and reference to saturation magnetization of bulk magnetite shown (dashed line).

Figure 3:. Simulations and experimental study of vortex state in magnetite nanodiscs.

a-c, Magnetization configuration emerging from micromagnetic simulation of MNDs 226 nm, 181 nm, or 98 nm in diameter, respectively. d-i, Electron holography on MNDs: magnetic field lines overlapping the magnetic nanodiscs 226 nm, 181 nm or 98 nm in diameter, respectively. Blue and red lines denote vortices of opposite directions. g-i, Corresponding magnetization of upper images. Scale bars = 100 nm.

Figure 4:. Magnetomechanical stimulation of MND-decorated DRG neurons allows for remote activation of Ca²⁺ influx.

a, DRGs relay sensory information, including mechanosensory information, to the spinal cord. DRG explants incubated with MNDs can be stimulated when slowly varying MFs cause MNDs to transition from vortex to in-plane magnetization, producing forces on mechanosensitive ion channels and resulting in Ca²⁺ influx. b, Scanning electron microscope image of DRG explant culture incubated with MNDs. Scale bar = 200 μm. c, Detail of a region of the DRG. Individual MNDs are visible on the surface. Scale bar = 2 μm. d, Bath temperature monitored during a magnetomechanical stimulation experiment. Shaded area represents standard error. e, False color stills from a representative video of DRG explant culture loaded with the Ca²⁺ indicator Fluo-4 and subject to magnetomechanical stimulation. Scale bar = 50 μm. 5 Hz, 26 mT MF stimulation of DRGs decorated with MNDs 226 nm (f) and 98 nm (g) in diameter. Heat maps (top) represent the fluorescence change of individual cells during the experiment. On the same time axis are plotted the total number of responding cells at any time (bottom). h, Comparison of the efficacy of magnetomechanical stimulation for 226 nm and 98 nm diameter MNDs on DRGs. A two-way ANOVA was conducted on the influence of culture type (DRG or hippocampal neurons) and MND type (226 nm, 98 nm, and none) on the percentage of stimulated cells in calcium imaging. The main effect for MND type yielded an F-test statistic of F_2,30 = 38, p = 5.6×10⁻⁹ (where in F_x,y = z, x is number of degrees of freedom (df) for the factor of interest (e.g., MND type), y is df for the error, and z is the F-test statistic (i.e., the ratio of the mean square error (MSE) between groups to the MSE within groups), and p is the probability, given a chance model in which the means of each group are the same, of observing this F-test statistic). The main effects for culture type and the interaction were also significant (F_1,30 = 33, p = 2.8×10⁻⁶ and F_2,30 = 6.4, p = 4.9×10⁻³, respectively). In all experiments, MF is applied in 3 epochs of 10 s as shown in shaded grey regions. The cell number per condition is 300, and the number of experiments per condition is n = 6. Post-hoc testing was performed for pairwise comparison of means using Tukey’s honest significant difference method; all p-values are listed in Supplementary Table 1. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Figure 5:. Tuning parameters for magnetomechanical stimulation in DRG neurons and demonstration of magnetomechanical stimulation in HEK-293 cells transfected with TRPV4.

a, Fluorescence traces resulting from stimulation of DRG neurons incubated with 226 nm diameter MNDs (left) and 98 nm diameter MNDs with MF frequencies of 5 Hz (top) and 1 Hz (bottom). MF amplitude is sequentially increased from 7 mT to 28 mT (marked by the shaded regions). MF is applied in 4 pulses of 10 s with 30 s wait times between pulses. b, Summary of cell response rate for conditions permuted in (a). Error bars represent standard error of the mean. c, DRGs incubated with 1 μM piezo inhibitor GsMTx4 (left) and with TRPV4 antagonist HC-067047 (right) both show a decrease in activity after the first stimulation sequence. d, Confocal images of HEK-293 cells loaded with the calcium indicator Fluo-4 and transfected with the mechanosensitive TRPV4 channel labeled with mCherry. Scale bars = 100 μm. e, The response of unmodified HEK-293 cells decorated with MNDs to MF application (left), the response of HEK-293 cells expressing TRPV4 decorated with MNDs to MF application (middle), and the blocked response of HEK-293 cells decorated with MNDs, expressing TRPV4, and incubated 1 μM TRPV4 antagonist, HC-067047 (right). In experiments shown in (c) and (e), a 5 Hz, 26 mT MF is applied in 3 sequences of 10 s as shown in the shaded grey regions. The number of cells per condition is 300. Standard error is represented by shaded area in fluorescence traces.