Wireless respiration monitoring using a flexible sensor and bistable circuit

Passive wireless sensors based on resonant circuits can provide battery-free monitoring of physiological signals, but their use is limited by the sensitivity of wireless readout. Here we propose a wireless physiological sensing scheme using a flexible on-body sensor and a bistable circuit based on nonlinear parity-time symmetry. When operating in the bistable region, our system converts a periodic physiological signal into a sequence of bistable mode transitions, which exhibits greater sensitivity and ease of readout than conventional readout approaches. We demonstrate a wireless monitoring of human respiration rate and show a SNR enhancement over 10 $\boldsymbol{d}\boldsymbol{B}$ compared to the standard readout. Our method may pave the way for noise-robust wireless biosensors for health monitoring during daily life.