Toward an Energy-Autonomous Wearable System for Human Breath Detection

This work presents a research about a 5.8 GHz system for vital signals monitoring, specifically human breath. The system consists of two main components: a Self-Injection Locked Oscillator (SILO), whose input and output ports are aperture-coupled to a dual-polarized patch antenna, and a passive receiver, coupled to the SILO output port, consisting of the cascade connection of a peak detector and a full-wave RF-to-DC rectifier. The SILO generates the carrier, that is frequency-modulated by chest displacements and backscattered to the SILO itself. The latter is loosely coupled to the passive receiving sub-system. In this way, the SILO output signal is simultaneously demodulated by the detector, and DC-converted to provide energy for the wireless communication of the received vital signals, e.g. by means of an IoT (Internet of Things) low-power node. The system is designed for being fully wearable; it can be mounted inside a plastic case and worn by the user under test at chest-level position. Critical breath rates can be detected and sent to a caregiver, thus enabling monitoring of chronic diseases, such as bradypnea or tachypnea, while performing a normal life.