THE DESIGN OF AN IMPLANTABLE TELEMETRIC
DEVICE FOR THE USE IN NEURAL PROSTHESES

INTRODUCTION Recent research efforts in neural technology aim at interfacing to the nervous system of living beings. In the case of auditory prostheses, which are already commercially available, this has been implemented successfully. Currently, efforts are directed towards interfacing to the peripheral nervous system for the use in functional nervous stimulation systems (INTER). Concerning the telemetry part of such a device, the challenge lies in the high data rate which is needed for closed loop control multichannel stimulators.

METHODS Here, the design of a telemetric link is described, which is intended for the use in neural functional stimulation. An implanted, programmable stimulator is to be powered transcutaneously and provided with a bi-directional data link. For this task, an inductive link has been developed, as this is the only choice for a safe energy transmission across the skin and tissue barrier. In contrast to many other neuro-stimulators found in literature, more complicated stimulation pulses than ordinary biphasic rectangular shaped pulses are to be transmitted, giving more flexibility to functional stimulation. This requires higher data rates, which is achieved by using a carrier in the 10MHz range and by implementing data reduction. ASK is installed as a modulation scheme. The transmission distance for this application has been fixed to 10mm. The total size of the implant is determined to be less than 10mm in diameter. The implant coil, which operates without a magnetic core, is therefore restricted to a diameter of 8mm. For the reverse transmission of data, an active transmitter operating at 30MHz with BPSK is implemented. A class-E amplifier powers the external coil.

RESULTS The telemetric link described here manages a full duplex transmission with 250kbits per second towards the implant and 500kbits per second in the opposite direction. The overall power efficiency of the link is approximately 10% at the specified distance.

DISCUSSION A functional 10MHz range miniaturized telemetric device for transcutaneous nerve signal transmission has been implemented and tested. Further efforts will focus on enhancing link efficiency and on the final design of a micro implant using a minimum of custom designed chips.