Strength-frequency curve for micromagnetic neurostimulation through excitatory postsynaptic potentials (EPSPs) on rat hippocampal neurons and numerical modeling of magnetic microcoil (μcoil)

Impact of anesthesia on micromagnetic stimulation (μMS) of the vagus nerve

To treat diseases associated with vagal nerve control of peripheral organs, it is necessary to selectively activate efferent and afferent fibers in the vagus. As a result of the nerve's complex anatomy, fiber-specific activation proves challenging. Spatially selective neuromodulation using micromagnetic stimulation(μMS) is showing incredible promise. This neuromodulation technique uses microcoils(μcoils) to generate magnetic fields by powering them with a time-varying current. Following the principles of Faraday's law of induction, a highly directional electric field is induced in the nerve from the magnetic field. In this study on rodent cervical vagus, a solenoidalμcoil was oriented at an angle to left and right branches of the nerve. The aim of this study was to measure changes in the mean arterial pressure (MAP) and heart rate (HR) followingμMS of the vagus. Theμcoils were powered by a single-cycle sinusoidal current varying in pulse widths(PW = 100, 500, and 1000μsec) at a frequency of 20 Hz. Under the influence of isoflurane,μMS of the left vagus at 1000μsec PW led to an average drop in MAP of 16.75 mmHg(n = 7). In contrast,μMS of the right vagus under isoflurane resulted in an average drop of 11.93 mmHg in the MAP(n = 7). Surprisingly, there were no changes in HR to either right or left vagalμMS suggesting the drop in MAP associated with vagusμMS was the result of stimulation of afferent, but not efferent fibers. In urethane anesthetized rats, no changes in either MAP or HR were observed uponμMS of the right or left vagus(n = 3). These findings suggest the choice of anesthesia plays a key role in determining the efficacy ofμMS on the vagal nerve. Absence of HR modulation uponμMS could offer alternative treatment options using VNS with fewer heart-related side-effects.

Improving focality and consistency in micromagnetic stimulation

The novel micromagnetic stimulation (μMS) technology aims to provide high resolution on neuronal targets. However, consistency of neural activation could be compromised by a lack of surgical accuracy, biological variation, and human errors in operation. We have recently modeled the activation of an unmyelinated axon by a circular micro-coil. Although the coil could activate the axon, its performance sometimes lacked focality and consistency. The site of axonal activation could shift by several experimental factors, including the reversal of the coil current, displacement of the coil, and changes in the intensity of the stimulation. Current clinical practice with transcranial magnetic stimulation (TMS) has suggested that figure-eight coils could provide better performance in magnetic stimulation than circular coils. Here, we estimate the performance of μMS by a figure-eight micro-coil, by exploring the impact of the same experimental factors on its focality and consistency in axonal activation. We derived the analytical expression of the electric field and activating function generated by the figure-eight micro-coil, and estimated the location of axonal activation. Using NEURON modeling of an unmyelinated axon, we found two different types (A and B) of axon activation by the figure-eight micro-coil, mediated by coil currents of reversed direction. Type A activation is triggered by membrane hyperpolarization followed by depolarization; Type B activation is triggered by direct membrane depolarization. Consequently, the two types of stimulation are governed by distinct ion channel mechanisms. In comparison to the circular micro-coil, the figure-eight micro-coil requires significantly less current for axonal activation. Under figure-eight micro-coil stimulation, the site of axonal activation does not change with the reversal of the coil current, displacement of the coil, or changes in the intensity of the stimulation. Ultimately, the figure-eight micro-coil provides a more efficient and consistent site of activation than the circular micro-coil in μMS.

Development and Characterization of a Micromagnetic Alternative to Cochlear Implant Electrode Arrays

To stimulate the auditory nerve, cochlear implants directly inject electrical current into surrounding tissue via an implanted electrode array. While many cochlear implant users achieve strong speech perception scores, there remains significant variability. Since cochlear implant electrode arrays are surrounded by a conductive fluid, perilymph, a spread of excitation occurs. The functionality of the cochlea is spatially dependent, and a wider area of excitation negatively affects the hearing of the user. Importantly, magnetic fields are unaffected by the material properties of biological components. To utilize the electromagnetic properties of the human ear, a microcoil array was developed. The microcoils are 4-turn solenoids with a 250-μm turn radius and a 31.75-μm wire radius, coated with Parylene-C. The efficient design was implemented to accelerate testing. The obtained results describe stimulation capabilities. Functionality was validated using a frequency response analyzer to measure how the generated electromagnetic power radiates in space. 99.8% power loss was observed over a 100-μm separation between a pair of identical microcoils. Obtained through finite-element modeling, the microcoils can be driven by a 60 mA, 5 kHz, sinusoidal input for 10 minutes before predicted inflammation. Rattay's activating function was calculated to evaluate the magnetic stimulation effect of external fields on target neurons. Combined with the frequency response analysis, magnitude and spatial effects of the generated potential is established. As a result, each microcoil requires a 400-μm-wide area for each independent stimulation channel, which is 84% narrower than a commercial cochlear array channel, thereby suggesting greater spatial selectivity.

Finding the Location of Axonal Activation by a Miniature Magnetic Coil

Magnetic stimulation for neural activation is widely used in clinical and lab research. In comparison to electric stimulation using an implanted electrode, stimulation with a large magnetic coil is associated with poor spatial specificity and incapability to stimulate deep brain structures. Recent developments in micromagnetic stimulation (μMS) technology mitigates some of these shortcomings. The sub-millimeter coils can be covered with soft, biocompatible material, and chronically implanted. They can provide highly specific neural stimulation in the deep neural structure. Although the μMS technology is expected to provide a precise location of neural stimulation, the exact site of neural activation is difficult to determine. Furthermore, factors that could cause the shifting of the activation site during μMS have not been fully investigated. To estimate the location of axon activation in μMS, we first derived an analytical expression of the activating function, which predicts the location of membrane depolarization in an unmyelinated axon. Then, we developed a multi-compartment, Hodgkin-Huxley (H-H) type of NEURON model of an unmyelinated axon to test the impact of several important coil parameters on the location of axonal activation. The location of axonal activation was dependent on both the parameters of the stimulus and the biophysics properties of the targeted axon during μMS. The activating function analysis predicted that the location of membrane depolarization and activation could shift due to the reversal of the coil current and the change in the coil-axon distance. The NEURON modeling confirmed these predictions. Interestingly, the NEURON simulation further revealed that the intensity of stimulation played a significant role in the activation location. Moderate or strong coil currents activated the axon at different locations, mediated by two distinct ion channel mechanisms. This study reports several experimental factors that could cause a potential shift in the location of neural activation during μMS, which is essential for further development of this novel technology.