A Simple Passive System for Telemetry of Physiological Data

Abstract

The system described uses a passive implant circuit powered from a low power 1 mHz external transmitter. Data are returned by frequency changes in an audio tone. The use of audio frequencies precludes frequency shift problems in the presence of conductive body fluids. It was designed particularly for intracranial pressure monitoring but has other uses. 

The circuit used for implanting is of very simple design and depends on changes in resistance. It may therefore respond to pressure, temperature, light, humidity, or any physical action which can be converted to a resistance change. It does not contain tuned circuits, and is accurate in the presence of wide changes in input power.

The external circuits also are very elementary - a one-transistor transmitter, a low pass filter system, and a high-gain audio amplifier. Readout may be either a frequency counter or a simple demodulator/meter circuit. Total equipment cost is low, and the transducer may be made very small. However, when the transmitting and receiving antennae are small, the range is limited to about 2 cm. This is adequate for intracranial pressure.