Magnetically-induced electrical stimulation with sub-millimeter coils

Transcranial magnetic stimulation (TMS) is a well-established method of modulating neuronal activity in the brain. The TMS coil usually spans several centimeters and produces a lot of heating, making it unsuitable for intracranial implantation. The study, led by Dr. Fried at the Massachusetts General Hospital, aimed to develop a sub-millimeter sized coil that can be implanted into deep brain targets, such as the basal ganglia. Their prototype micro-coil was based on a commercial multilayer copper coil (ELJ-RFR10JFB, Panasonic) that was coated with a dielectric varnish, placed on a tip of a needle, and manually positioned above the freshly dissected rabbit retina. With the photoreceptor side down and the ganglion cell layer on top, the patch electrode was positioned on a ganglion cell to record the light-stimulated activity of individual retinal neurons. The micro-coil was oriented in two positions, either parallel or perpendicular to the retinal surface, and the DC voltage (0.5-10 V) was applied for 20 μs to induce a circulating electric field (E-field) in the retinal tissue (see the figure).  The parallel orientation was considerably more effective in inducing the neuronal activation that the perpendicular one. The train of action potentials was readily induced in the parallel orientation as far as 1.1 mm from the retinal surface.  Even more intriguingly, essentially the same amount of DC voltage (6V) was required at the micro-coil to induce the neuronal response at different distance from the retina, ranging from 0.3 to 1.1 mm. The finite element method (FEM) modeling of the electric field distribution around the micro-coil indicated that the magnetically-induced E-field was ~1 V/m at a radial distance of 1 mm from the coil core and was only slightly decreasing with distance. In contrast, the E-field decreased rather rapidly in the vertical dimension form the coil, being only 0.1 V/m at a 1 mm distance. This study provides an initial ex vivo proof of the principle and opens up a possibility of developing novel implantable neuroprosthetic devices with several features that are superior to the electrical stimulation. Among such desirable features are: 1) less steep E-field gradient, resulting in more uniform stimulation of neurons over a larger volume, and 2) absence of electrochemical reactions at the electrode-tissue interface, resulting in longer electrode lifetime and healthier tissue. Other, perhaps unexpected, benefits of the magnetically-induced electrical stimulation may become evident once the in vivo evaluation study is completed.