Remote electrical stimulation by means of implanted rectifiers

Abstract

Miniaturization of active implantable medical devices is currently compromised by the available means for electrically powering them. Most common energy supply techniques for implants--batteries and inductive couplers--comprise bulky parts which, in most cases, are significantly larger than the circuitry they feed. Here, for overcoming such miniaturization bottleneck in the case of implants for electrical stimulation, it is proposed to make those implants act as rectifiers of high frequency bursts supplied by remote electrodes. In this way, low frequency currents will be generated locally around the implant and these low frequency currents will perform stimulation of excitable tissues whereas the high frequency currents will cause only innocuous heating. The present study numerically demonstrates that low frequency currents capable of stimulation can be produced by a miniature device behaving as a diode when high frequency currents, neither capable of thermal damage nor of stimulation, flow through the tissue where the device is implanted. Moreover, experimental evidence is provided by an in vivo proof of concept model consisting of an anesthetized earthworm in which a commercial diode was implanted. With currently available microelectronic techniques, very thin stimulation capsules (diameter <500 µm) deliverable by injection are easily conceivable.

Figure 1. Illustration of a hypothetical use of the method proposed here for a FES application.

The capsules, consisting of a very thin diode and two electrodes in its simplest version, would be implanted by injection within the muscles that require stimulation, close to their motor points. Bursts of AC current would be forced to flow through tissues by means of external electrodes so that locally, at the vicinity of the capsules, low frequency current densities capable of stimulation would be produced by rectification.

Figure 2. Geometry of the model employed for the FEM simulation.

It is solved with axial symmetry around the axis of the implant (drawing not to scale).

Figure 3. Proof of concept model.

a) Picture of the rectifier implant developed for proof of concept; two wires acting as electrodes connected to a Schottky diode. b) The implant is barely noticeable once inserted in the earthworm (lying on an array of horizontal needles acting as electrodes). c) Schematic representation of the experimental setup (First, the AC bursts are applied between two electrodes that do not flank the region in which the implant is located and no movement is observable).

Figure 4. FEM simulation results.

Close up of the implant region when a single period of a 1 MHz sinusoidal signal with amplitude 500 V is applied. a) Average (i.e. DC) voltage for the whole period. b) Average (i.e. DC) current density. White arrows indicate current density direction, not magnitude. c) Current density amplitude.