Numerical and experimental characterization of through-the-body UHF-RFID links for passive tags implanted into human limbs

Radio frequency identification (RFID) in the UHF band has been recently proposed as enabling technology to develop implanted radio-sensors to be integrated into orthopedic prosthesis because of the power autonomy and standardized communication protocols. This paper investigates the feasibility of direct and forward links for UHF-RFID (860-960 MHz) tags implanted into human limbs, that are interrogated by a noncontacting reader's antenna, with the purpose to label and, in a near future, to collect data about the health status of an implanted orthopedic prosthesis. Performance gain indicators of the through-the-body RFID channel are estimated by electromagnetic simulations over an anthropomorphic phantom as well as by means of experimentation with a real RFID communication link involving a simplified in vitro setup. The achieved results suggest that, by exploiting the current potentialities of RFID technology, and for the specific tag (loop antenna) and reader antenna (SPIFA) herein considered, a stable communication link with tags implanted inside limbs might be already feasible up to 10-35 cm from the body in full compliance with the constrains over electromagnetic exposure. In the particular case of implanted tag into an elbow, the estimated power margin in the direct and inverse links could be even suitable to set up sensing-oriented systems based onto turn-on and backscattered power modulation.