Implantable Telemetry System for Strain Measurements

Abstract In this paper we present design aspects of an inductively powered, implantable telemetry system which can be used in monitoring strains in orthopedic implants, in designing and testing of new devices and materials for internal fixation. As strain is usually measured by the electrical resistance of a gauge, attention has been given to telemetry systems which use resistive transducers. The continuous wave oscillator circuit chosen consists of a single field effect transistor (FET) with a feedback resistance in the source lead simulating the strain gauge. As this oscillator will be inductively powered, there will be variations in supply voltage (due to relative movements). Hence the changes in gauge resistance must produce greater changes in the oscillator frequency than those produced by supply voltage variations. The use of semiconductor strain gauges was found desirable.