Body-worn fully-passive wireless analog sensors for physiological signal capture through load modulation using resistive transducers

Fully-passive wireless body-sensors pose viable solutions for unobtrusive monitoring of physiological signals at natural settings. While fully-passive capacitive analog passive wireless sensors has been reported, we present an alternative solution with resistive based transducers. The passive sensor is composed of a loop antenna, a tuning capacitor, and a resistive transducer suitable for the type of physiological signals to be measured. The scanner transmits carrier RF signal at 13.75MHz whose amplitude is modulated based on the resistive loading by the transducer. The load modulation is captured with the signal analyzer. The system was characterized for various resistive loads of 1.2Ω to 82KΩ and open at 5, 10, 20, and 40 mm co-axial distances between the transmitter and the receiver antennas. We demonstrate the practicality of the system by measuring several physiological signals like heart rate, temperature, and pulse oximetry. The wireless power used for remote sensing is very low (-20dBm, except pulse oximetry requires 0dBm). The results show the potential of developing a new set of body-worn fully-passive sensors for physiological signal monitoring.