Exploring Micromagnetic Neurostimulation as an Alternative to Electrode Neurostimulation

Abstract

Neuromodulation is defined to be alteration – or modulation – of nerve activity by delivering an external stimulus directly to a target area. Micromagnetic stimulation (μMS), although still in its infancy, is one such neuromodulation technique where the devices are micrometer-sized coils or microcoils (μcoils) as neurostimulation implants. The µcoils when driven by an alternating current, generate a time varying magnetic field. As per Faraday’s law of electromagnetic induction, this magnetic field induces an electric field that activates the neurons. Spatially selective and galvanic contact-free neural activation coupled with magnetic resonance imaging (MRI) safe implant performance are some of the features of µMS that make this neuromodulation technique so unique. The only drawback of this technology is that the µcoils need to drive a high amplitude current waveform for successful neural activation. This thesis reports the design, fabrication and testing of two micromagnetic implants – the Magnetic Pen (MagPen), a solenoid-shaped single µcoil prototype and the Magnetic Patch (MagPatch), a rectangular helix shaped planar µcoil array prototype. The efficacy of micromagnetic activation using MagPen has been tested over the following rodent models: on the rat hippocampal CA3-CA1 synaptic pathway in vitro; on the medial forebrain bundle (MFB) of rodents for the study of striatal dopamine release in vivo; on the rat sciatic nerve to demonstrate the dose-response relationship for µMS in vivo; and, on the vagus nerve to demonstrate fiber-specific activation of the nerve in vivo. These experimental tests added several unique features for this neuromodulation technique. Of them, the experimental demonstration of fiber-specific activation of a nerve by µMS and portraying the importance of the directionality of induced electric field on successful neural activation deserve special mention. For the convenience of driving these µcoils in a clinical setting, the design and development of a portable µcoil driver has also been reported. The MagPen prototype had its own caveat in terms of mm-size, lack of multidimensional spatial control and activation at the cellular-level. To bridge this research gap, the MagPatch array was designed and fabricated with the goal to study μMS at the single cell resolution. Furthermore, a study on the effect of the directionality of induced electric fields from each of the µcoils in the array for successful neuron activation was made. In summary, this thesis explores the efficacy of µMS in several experimental settings, both in vitro and in vivo, augmenting the unique features of µMS. In addition, techniques using soft magnetic material (SMM) cores at the center of these µcoils to reduce their high-power thresholds in µMS have been proposed, simulated and reported in this thesis.