Implantable intracranial pressure telemeter using custom integrated circuits

The design, fabrication, and evaluation of an implantable intracranial pressure (ICP) telemetry transmitter is described. Data transmission is accomplished by means of a radio frequency (RF) link. A silicon piezoresistive pressure transducer is used. Electrostatic bonding of this transducer onto a tubular glass support provides long-term stability, stress isolation and a hermetic package. Pressure baseline stability of better than 2 mm Hg/month has been achieved. Pulse code modulation is employed in the pressure channel to assure accurate data transmission. Low duty cycle pulse powering technique lowers the power consumption of the telemeter to only 0.3 mW. The complexity of the electronics is reduced by the use of a custom integrated circuit (PM2) which is a 40-pin bipolar device measuring 150 mil x 150 mil. It contains 75 NPN transistors, 26 PNP transistors, 45 base resistors (480 K total) and 16 pinch resistors (560 K total). The operating characteristics of the telemeter are described.