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.