In vivo demonstration of injectable microstimulators based on charge-balanced rectification of epidermically applied currents

Abstract

It is possible to develop implantable microstimulators whose actuation principle is based on rectification of high frequency (HF) current bursts supplied through skin electrodes. This has been demonstrated previously by means of devices consisting of a single diode. However, previous single diode devices caused DC currents which made them impractical for clinical applications. Here flexible thread-like stimulation implants which perform charge balance are demonstrated in vivo. Approach. The implants weight 40.5 mg and they consist of a 3 cm long tubular silicone body with a diameter of 1 mm, two electrodes at opposite ends and, within the central section of the body, an electronic circuit made up of a diode, two capacitors and a resistor. In the present study, each implant was percutaneously introduced through a 14G catheter into either the gastrocnemius muscle or the cranial tibial muscle of a rabbit hind limb. Then stimulation was performed by delivering HF bursts (amplitude < 60 V, frequency 1 MHz, burst repetition frequency from 10 Hz to 200 Hz, duration = 200 μs) through a pair of textile electrodes strapped around the hind limb and either isometric plantarflexion or dorsiflexion forces were recorded. Stimulation was also assayed 1, 2 and 4 weeks after implantation. Main results. The implants produced bursts of rectified current whose mean value was of a few mA and were capable of causing local neuromuscular stimulation. The implants were well tolerated during the 4 weeks. Significance. Existing power supply methods, and in particular inductive links, comprise stiff and bulky parts. This hinders the development of minimally invasive implantable devices for neuroprostheses based on electrical stimulation. The proposed methodology is intended for relieving such bottleneck. In terms of mass, thinness and flexibility, the demonstrated implants appear to be unprecedented among the intramuscular stimulation implants ever assayed in vertebrates.