Magnetic stimulation of the sciatic nerve using an implantable high-inductance coil with low-intensity current

Objective. Magnetic stimulation using implantable devices may offer a promising alternative to other stimulation methods such as transcranial magnetic stimulation (TMS) or electric stimulation using implantable devices. This alternative may increase the selectivity of stimulation compared to TMS, and eliminate the need to expose tissue to metals in the body, as is required in electric stimulation using implantable devices. However, previous studies of magnetic stimulation of the sciatic nerve used large coils, with a diameter of several tens of mm, and a current intensity in the order of kA. Approach. Since such large coils and high current intensity are not suitable for implantable devices, we investigated the feasibility of using a smaller implantable coil and lower current to elicit neuronal responses. A coil with a diameter of 3 mm and an inductance of 1 mH was used as the implantable stimulator. Main results. Before in vivo experiments, we used 3D computational models to estimate the minimum stimulus intensity required to elicit neuronal responses, resulting in a threshold current above 3.5 A. In in vivo experiments, we observed successful nerve stimulation via compound muscle action potentials elicited in hind-limb muscles when the applied current was above 3.8 A, a significantly reduced current than that used in conventional magnetic stimulation. Significance. We report the feasibility of magnetic stimulation using an implantable millimeter-sized coil and low current of a few amperes to elicit neural responses in peripheral nerves. The proposed method is expected to be an alternative to TMS, with the merit of improved selectivity in stimulation, and to electrical stimulation based on implantable devices, with the merit of avoiding the exposure of conducting metals to neural tissues.