Converting a Wireless Biotelemetry System to an Implantable System Through Antenna Redesign

The two major challenges associated with the conversion of a wireless system operating in air to an implantable version, antenna detuning and biocompatibility, are addressed in a coherent way. An RF identification (RFID)-based biomedical telemetry system designed for free-space operation was chosen as the starting reference. A new pin-compatible space-saving antenna with a ground plane was designed, fabricated, and tested to replace the original “free-space” antenna in the active RFID tag without making any other changes to the tag circuit, such that the tag would function well when it is placed under rat skin and fat. Biocompatibility and potential antenna detuning due to rat tissue variations were addressed in the design process, without significantly increasing the tag physical height, by applying a thin coating of biocompatible material directly over the antenna. The operation of the medical telemetry system was successfully demonstrated, with the tag placed under rat skin and fat, and its range of 60-72 cm was found to be sufficient to support medical research experiments conducted with rats in cages. Due to the biocompatible coating over the antenna, antenna matching is very insensitive to changes in tissue dielectric constants and thickness. The footprint of the new antenna is 33% less than that of the original antenna, its measured 10-dB return-loss bandwidth is 100 MHz or 11%, and overall efficiency is 0.82% at 920 MHz.