Magnetoelectric Nanodiscs Enable Wireless Transgene-Free Neuromodulation

Abstract

Deep-brain stimulation (DBS) with implanted electrodes revolutionized treatment of movement disorders and empowered neuroscience studies. Identifying less invasive alternatives to DBS may further extend its clinical and research applications. Nanomaterial-mediated transduction of magnetic fields into electric potentials offers an alternative to invasive DBS. Here, we synthesize magnetoelectric nanodiscs (MENDs) with a core-double shell Fe₃O₄-CoFe₂O₄-BaTiO₃ architecture with efficient magnetoelectric coupling. We find robust responses to magnetic field stimulation in neurons decorated with MENDs at a density of 1 μg/mm² despite individual-particle potentials below the neuronal excitation threshold. We propose a model for repetitive subthreshold depolarization, which combined with cable theory, corroborates our findings in vitro and informs magnetoelectric stimulation in vivo. MENDs injected into the ventral tegmental area of genetically intact mice at concentrations of 1 mg/mL enable remote control of reward behavior, setting the stage for mechanistic optimization of magnetoelectric neuromodulation and inspiring its future applications in fundamental and translational neuroscience.

Fig. 1|. Magnetoelectric nanodiscs for neuromodulation.

a, An illustration of neuromodulation mediated by magnetoelectric nanodiscs (MEND). b,c, Transmission electron microscopy (TEM) images of b, Fe₃O₄ magnetic nanodiscs (MNDs), which form core of MEND and c, core-shell Fe₃O₄-CoFe₂O₄ nanodiscs (CFONDs). Scale bars are 100 nm. Insets show selected area electron diffraction (SAED) patterns revealing epitaxial growth. Scale bars are 10 nm⁻¹. d, Scanning electron microscopy (SEM) image of core-double shell Fe₃O₄-CoFe₂O₄-BaTiO₃ MENDs. Scale bar = 100 nm. e, Illustration of the electrochemical measurement apparatus that employs surface charge variation of MENDs in response to applied magnetic field (MF) to determine the magnetoelectric coefficient (α_ME). f, Simulated magnetostriction constant (bars) and magnetostrictive displacement maps (framed insets) for hexagonal CFONDs (pink), hexagonal MNDs (blue), and Fe₃O₄-CoFe₂O₄ core-shell spherical nanoparticles (green). The volume of the particles was fixed across conditions. Inset: Color index for direction in magnetostrictive displacement maps. g, Electric polarization generated in BaTiO₃ shells deposited onto CFONDs upon exposure to an offset magnetic field (OMF) of 220 mT and an alternating magnetic field (AMF) with an amplitude of 10 mT. h, α_ME at an AMF with a frequency ƒ_AMF =150 Hz and amplitude H_AMF = 10 mT measured at varying magnitudes of OMF for MENDs (red), isotropic magnetoelectric nanoparticles (MENPs, black), and CFONDs (grey). i, α_ME for MENDs as a function of AMF frequency at H_AMF = 10 mT and OMF magnitude 220 mT. j, α_ME for MENDs as a function of AMF amplitude for ƒ_AMF =150 Hz and OMF magnitude 220 mT.

Fig. 2. MEND-mediated neuronal stimulation in vitro.

a, Relative GCaMP6s fluorescence change (ΔF/F₀) in hippocampal neurons decorated with MENDs before (left) and after (right) magnetic field application. Scale bars: 150 μm. b, Individual (top) and average (bottom) of the relative GCaMP6s fluorescence change (ΔF/F₀) of the hippocampal neurons in response to 10 mT AMF with 1 kHz frequency and 220 mT OFM. c, Individual and d, average traces of GCaMP6s ΔF/F₀ in 300 hippocampal neurons decorated with MENDs in response to 10 mT AMF with frequencies 100, 150, 250, 500, and 1000 Hz (OMF magnitude 220 mT). The grey and magenta dashed lines indicate the beginning and end of MF stimulation, respectively. e, Individual cell (top) and mean (bottom) GCaMP6s fluorescence changes in 300 neurons in response to 2 s MF epochs applied at varying intervals (OMF 220 mT; AMF 150 Hz, 10 mT). f, Number of GCaMP6s fluorescence peaks as a function of stimulation epoch length for rest intervals of 10, 30, 60, 90, and 120 s (OMF 220 mT; AMF 150 Hz, 10 mT). g, Individual cell (top) and mean (bottom) GCaMP6s fluorescence changes in response to 2 s MF epochs at 40 s intervals for 0.75 μg mm⁻² MEND density. h, i, Fluorescent image of a Live-Dead assay in neurons before and after 3 cycles of MF at h, 1 μg mm⁻² and i, 0.75 μg mm⁻² MEND densities. j, The change in live cell ratio (normalized to the total number of the cells counted with Hoest staining) following 3 cycles of MF for neurons decorated with different MEND densities. Statistical significance was tested via one-way ANOVA and Tukey’s multiple comparison tests (n = 5 plates per condition, P=3.79×10⁻⁷ for 1 μg mm⁻²; P=0.79 for 0.794 μg mm⁻²; P=0.998 for 0.75 μg mm⁻²; ****P≤0.0001, n.s. P>0.05).

Fig. 3|. Mechanistic study of MEND-mediated neuromodulation.

a, Illustration of stimulation mechanism combining cable model and repeated excitation of neural activity, where d is spacing between MEND particles, a is cell radius, ΔV is the change in membrane potential per half-period of an AMF, and V_# is the voltage generated by a single MEND. b, Calculated membrane potential change ΔV for every half-period of an AMF, as a function of V₀ and d. c, Simulated membrane potential V(t) after AMF onset to activate MENDs with V₀ = 0.03 mV for varying a values. The threshold potential for action potential firing, −55 mV, is indicated with the dashed line. d, Simulated membrane potential at a time t = 2 s after onset of magnetic field as a function of d, for varying V₀ values. V₀=0.03 mV is the measured value for MEND particles in this study, highlighted in red. e, Time to reach threshold membrane potential (−55 mV) from the resting potential (−75 mV) as a function of AMF frequency ƒ_AMF for varying V₀ values. f, (i-iii) SEM images showing MENDs decorating cultured hippocampal neurons. (ii) A higher magnification image of the area marked by a box in panel (i). (iii) MENDs on the neuron surface shaded in blue as identified by the presence of iron, titanium, and barium in energy dispersive X-ray spectroscopy. Scale bars: 20 μm (i), 5 μm (ii), 100 nm (iii). g, h, GCaMP6s fluorescence changes in neurons decorated with MEND following 2 s stimulations (OMF 220 mT; AMF 150 Hz, 10 mT, marked by vertical grey bars) in the presence of g, tetrodotoxin (TTX, 1 μM, green) or h, a cocktail of (2R)-amino-5-phosphonovaleric acid (AP5, 100 μM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 μM) (blue).

Fig. 4|. MEND-mediated neuronal stimulation in mice.

a, Schematic of MEND-mediated neuromodulation. MEND particles were injected into the mouse ventral tegmental area (VTA), and the mice were placed inside a permanent magnet field providing OMF of 220 mT and a surrounding solenoid providing AMF with an amplitude 10 mT and a frequency 150 Hz. b, Confocal images of c-Fos-expressing neurons among DAPI-marked cells in the VTA. Top left: MENDs (1.5 mg/ml) with (+) magnetic stimulation (ON); top right: MEND particles (1.5 mg/ml) without (−) magnetic stimulation (OFF); bottom left: control MNDs (1.5 mg/ml) + magnetic stimulation; bottom right: MENDs (0.5 mg/ml) + magnetic stimulation. Scale bars are 25 μm. c, Quantification of c-Fos positive neurons for the conditions shown in (B) as well as the subjects injected with PBS and exposed to magnetic field. d, Confocal images and e, quantification of c-Fos positive neurons in the medial prefrontal cortex (mPFC) for the same conditions shown in (B) and (C). f, Confocal images and g, quantification of c-Fos positive neurons in the nucleus accumbens (NAc) for the same conditions shown in b and c. d, f, Scale bars are 100 μm. c, e, g, Statistical significance was tested via one-way ANOVA and Tukey’s multiple comparison tests (n = 6 mice per condition, VTA F_3,20=93.2 P=1.21×10⁻¹²; mPFC F_3,20=60.81 P=1.21×10⁻¹¹; VTA F_3,20=62.03 P=1.29×10⁻¹²; ****P≤0.0001). h, Representative confocal images showing expression of GCaMP6s in the VTA. Scale bar: 50 μm. i Fiber photometry recordings of relative GCaMP6s fluorescence change (ΔF/F₀) in the VTA of anaesthetized mice injected with MENDs in the same region. (Top) Individual trial ΔF/F₀. (Bottom) Average ΔF/F₀ across trials shown above. Solid line represents mean and shaded areas mark s.e.m. (n=4 mice, 10–15 trials per mouse). The grey square box indicates magnetic field epochs (2s, OMF 220 mT, AMF 10 mT, 150 Hz). j Schematic of the place preference behavioral apparatus (top) and experimental timeline (bottom). k, Time spent in the stimulation chamber out of a total test time of 600 s, for pre- (Day 1, open markers) and post- (Day 5, solid markers) learning. Paired t-test was performed for MEND (n=11) and MND (n=7) groups, and Wilcoxon signed-rank test was performed for PBS (n=7) group because the data did not follow normal distribution. ***P ≤ 0.001, **P ≤ 0.01, *P ≤ 0.05, n.s. P>0.05). l, The change in time spent in stimulation chamber between Day 1 and Day 5. (One-way ANOVA with Tukey’s post-hoc comparison test, ***P ≤ 0.001, **P ≤ 0.01, *P ≤ 0.05, n.s. P>0.05.)