Impact of microcoil shape and the efficacy of soft magnetic material cores in focal micromagnetic neurostimulation

Abstract

Micromagnetic neurostimulation $(\upmu \text{MS})$, despite being in its infancy, has shown promising results in spatially selective activation of neurons. The devices are micrometer-sized coils or microcoils $(\upmu \mathbf{coils})$ which work on the principle of Faraday's Law of electromagnetic induction. Upon applying a time-varying current through these $\upmu \text{coils}$ they generate a time-varying magnetic field which in turn induces an electric field that activates the neurons. These $\upmu \text{coils}$ are spared from biofouling nuances as this induced electric field is not in direct electrochemical contact with the tissues. However, these $\upmu \mathbf{coils}$ have a high power of operation which lead to undesirable thermal effects on neurons. In this work, we have studied the efficacy of soft magnetic material (SMM) cores on these $\upmu \text{coils}$ to solve two existing challenges for $\upmu \text{MS}$. First, to minimize the power consumption for these $\upmu \text{coils}$. Second, to achieve even more precise and focal activation of the neural tissues. We have studied 3 shapes of $\upmu \text{coils}$ with comparable sizes in terms of spatial contour plots of magnetic field and induced electric field. Furthermore, the efficacy of 2 shapes of SMM cores, cone and rod, of varying sizes have been studied to obtain a spatially focal magnetic field and increased magnitude of induced electric field.