Short-pulsed micro-magnetic stimulation of the vagus nerve

Vagus nerve stimulation (VNS) is commonly used to treat drug-resistant epilepsy and depression. The therapeutic effect of VNS depends on stimulating the afferent vagal fibers. However, the vagus is a mixed nerve containing afferent and efferent fibers, and the stimulation of cardiac efferent fibers during VNS may produce a rare but severe risk of bradyarrhythmia. This side effect is challenging to mitigate since VNS, via electrical stimulation technology used in clinical practice, requires unique electrode design and pulse optimization for selective stimulation of only the afferent fibers. Here we describe a method of VNS using micro-magnetic stimulation (µMS), which may be an alternative technique to induce a focal stimulation, enabling a selective fiber stimulation. Micro-coils were implanted into the cervical vagus nerve in adult male Wistar rats. For comparison, the physiological responses were recorded continuously before, during, and after stimulation with arterial blood pressure (ABP), respiration rate (RR), and heart rate (HR). The electrical VNS caused a decrease in ABP, RR, and HR, whereas µM-VNS only caused a transient reduction in RR. The absence of an HR modulation indicated that µM-VNS might provide an alternative technology to VNS with fewer heart-related side effects, such as bradyarrhythmia. Numerical electromagnetic simulations helped estimate the optimal coil orientation with respect to the nerve to provide information on the electric field’s spatial distribution and strength. Furthermore, a transmission emission microscope provided very high-resolution images of the cervical vagus nerve in rats, which identified two different populations of nerve fibers categorized as large and small myelinated fibers.


S1 Appendix.
The manuscript contains additional information on the segmentation process pipeline used to generate FIGURE 10.

S2 Appendix.
This section contains additional information on the RMS estimated |E|-field in the x-, y-, and zdirections on a nerve block 5m under the µM-VNS coil with three spatial partial derivatives. The highest RMS |E|-field was 12.70 V/m, and the maximum gradient RMS |E|-field was 166,767 V/m 2 , both located underneath the coil (Supplemental Fig. 2).

Supplemental Fig. S-2: Top view of the |E|-field generation on the nerve surface with a) |E|RMS, b) |Ex|RMS, c) |Ey|RMS, d) |Ez|RMS, e) the strength of the real part of the E-field, and i) the strength of the imaginary part of the E-field; Top view of the spatial gradient of the |Ex|RMS-field with f)
x-, g) y-, and h) z-direction; the spatial gradient of the |Ey|RMS-field with j) x-, k) y-, and l) zdirection, and the spatial gradient of the |Ez|RMS-field with n) x-,o) y-, and p) z-direction; m) gradient of the |E|RMS.

S3 Appendix.
This section contains additional information on the experimental set-up and results of the sciatic nerve stimulation. The sciatic nerve stimulation experiments were conducted to check the efficacy of the magnetic nerve stimulation. The same experimental parameters were used for the micromagnetic VNS (µM-VNS) and electrical VNS (eVNS) experiments (see the main manuscript). Adult male Wistar rats (350-420 g; Charles River Laboratories, Wilmington, MA) were anesthetized using isoflurane. The sciatic nerve was isolated, and a µM-coil was placed around the sciatic nerve in the hindlimb was immersed in a physiological solution. Electromyography (EMG) recordings were performed using needle electrodes (ADInstruments, USA) implanted into the gastrocnemius muscle in the left hindlimb of the rats in response to the ipsilateral sciatic nerve stimulation during electrical and magnetic stimulation. Data were acquired using BioAmp (ADInstruments, USA) connected to a PowerLab 8/35 (ADInstruments, Colorado Springs, CO, USA). LabChart software was used to analyze the data (acquisition rate 2000 samples per second using a mains filter). In the electrical and magnetic stimulation cases, the EMG traces showed clear responses during the stimulation period. Although stimulus duration for eVNS and µM-VNS were similar (i.e., ∆t=11ms for the eVNS, and 9 ms for the µM-VNS), M-waves were different for the two cases could be due to the different fiber recruitment levels.

S4 Appendix.
This paragraph contains additional information on the fusing of the planar spiral coil (PN LQP15MN 33nH SMD inductor, Murata Manufacturing Co., Japan) and the benchtop measurement results of the voltage, current, and time-varying magnetic field (dB/dt) using the short exponential pulse (pulse width= 10µs). c)

Supplemental
The maximum input voltage and fusing current of the µM-VNS coil (PN LQP15MN 33nH SMD inductor, Murata Manufacturing Co., Japan) are shown in Supplemental Fig. S-6. The fusing test was done using a single exponential pulse at 20 Hz (i.e., 20 pulses per second) with various decaying pulse widths between 10 and 400 µs using the two signal generators from Tektronix (AFG1062, Tektronix, Beaverton, OR). The voltage was measured using a mixed-domain oscilloscope (MDO3024, Tektronix, Beaverton, OR). The time-varying magnetic field (i.e., dB/dt) was measured at a 1 mm distance from the coil using a magnetic near-field probe (PN PR262, B&K Precision, Yorba Linda, CA, USA) connected to an oscilloscope. The input current was estimated using a hall effect (Huber et al., 2015) sensor (

Parameter Threshold
Fusing voltage > 35 V

S7 Appendix.
This paragraph contains additional information on the k-means clustered on large fiber groups of three rats. (see FIGURE 11 in the manuscript for the non-overlaid result).