Implantable Planar Micro-Coils for Ultra-Focal, Orientation-Specific Stimulation of Neurons

Neuromodulation with a high spatial and directional specificity is highly desired but very challenging to achieve using electrode-based electrical stimulation. Microscopic magnetic stimulation, eliciting neural elements via induced eddy current, is a promising alternative as it is able to modulate neurons with unprecedented focality and directionality. Moreover, thanks to advanced microfabrication technology, it is possible to fabricate a micrometer-sized coil (i.e., micro-coil) array on a compliant substrate as an implant for long-term neuromodulation. Here we present our design and fabrication of an implantable planar micro-coil chip, consisting of an array of ten spiral-shaped micro-coils in a figure-8 configuration. The fabricated micro-coil array showed high consistency and reproducibility in impedance measurement, with a resistance range of about 10-20 Ω, and inductance of about 30 nH. We demonstrated the feasibility of micro-magnetic stimulation in rodent experiments, by using the fabricated chip on the sciatic nerve of an anesthetized rat, while measuring the elicited neuromuscular (EMG) responses which suggests the high focality of the micromagnetic stimulation system.

I. INTRODUCTION

Technologies that can stimulate neurons at specific orientations are highly desired in therapeutic applications (e.g., to mitigate potential side effects of unintentional neuronal modulation) and neuroscience research (e.g., to elucidate neural circuitry with increased resolution) [1]. As an emergent neuronal stimulation strategy, microscopic magnetic stimulation (μMS) is gaining increasing attraction for its advantages in allowing selective and directional neuronal activation compared to electrode-based electrical stimulation (EES) [2]. Moreover, μMS stands out with distinct merits. Firstly, unlike EES, it does not require charge balance for input stimuli; instead, it utilizes a time-varying magnetic field to induce the stimulating current, thus avoiding charge buildup issues that may lead to undesired stimulation or electroporation in EES [3]. Secondly, through configuration design, μMS stimulation can be confined to an ultra-focal region on neural elements along a specific direction, which, on the contrary, is very challenging for EES. Another key benefit of μMS relates to its contactless stimulation mode that, when packaged with a biocompatible material, may mitigate inflammation responses for chronic stimulation [4]. Contactless stimulation may also enhance compatibility with medical imaging by minimizing heat accumulated in tissues.