A Battery-Free Tag for Wireless Monitoring of Heart Sounds

We have developed a wearable, battery-free tag that monitors heart sounds. The tag powers up by harvesting ambient RF energy, and contains a low-power integrated circuit, an antenna and up to four microphones. The chip, which consumes only 1.0 uW of power, generates digital events when the outputs of any of the microphones exceeds a programmable threshold voltage, combines such events together by using a programmable logic array, and transmits them to a base station by using backscatter modulation. The chip can also be programmed to trade-off microphone sensitivity for power consumption. In this paper, we demonstrate that the tag, when attached to the chest, can reliably measure heart rate at distances up to 7 m from an FCC-compliant RF power source. We also suggest how delays between signals measured by microphones at the wrist and neck can be used to provide information about relative blood-pressure variations.