1 MHZ RF LINKS FOR TWO TYPES OF IMPLANTABLE NEUROSTIMULATORS

DI Stefan Sauermann, Dr. Hermann Lanmüller
Department for Biomedical Engineering and Physics, University of Vienna
1090 Vienna, Waehringerguertel 18-20, Austria

Introduction High frequency inductive links were developed for two implantable stimulation devices: a telemetrically supplied muscle stimulator and a battery powered miniaturized single channel neurostimulator. This paper describes design considerations and performance of both links.

Methods Both links use ampitude modulation and passive telemetry to achieve bidirectional data transmission. For the muscle stimulator power efficiency was the main goal. Miniaturization was not of that importance, so the diameter of the receiver coil could be increased appropriately. The final coil diameter was 7 cm. To improve the performance of the passive telemetry the transmitter frequency is crystal-stabilised to 1MHz, although a self-oscillating transmitter would be more power-efficient. Coupling turned out to be high enough for passive telemetry by impedance modulation in the implant. A phase comparator was used to detect the data from the implant.

Passive telemetry in the miniaturized implant was more demanding. The receiver coil has a diameter of 14 mm, inductive coupling to the transmitter is accordingly lower. The amplifiers and filters had to be improved for data detection.

Results The link for the muscle stimulator has a maximum working distance of 6 cm, 20 mA of current at +/-15Volts can be supplied for the implant circuitry. HF output power can be adjusted by pulse-width modulation. The bidirectional data link works with a standard RS232 protocol at 19600 Baud. The link to the battery powered implant works up to a distance of 4 cm, with 600 Baud. Baud rate for this link can optionally be raised, but a clock rate of 72 kHz was chosen for the implanted microprocessor to minimize its supply current. This restricts the transmission rate to the above value.

Discussion According to in vitro tests both links have proved to serve their purpose. They are to be tested in animal experiments in the near future. However the transmitter for the muscle stimulator consumes 1 Watt of energy, so warming of the transmitter could become a problem in most unfavourable situations. Further research will be done into power-efficient transmitter design using class E amplifiers and different coil configurations.