Wireless Breath Monitoring With a Microwave Textile Stretchable Belt Sensor Based on Phase Detection

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Abstract
This paper proposes a system for monitoring respiration wirelessly. The system is composed of a stretchable belt that should be placed in the thorax or abdomen of the individual under analysis, a textile microwave sensor integrated on it, and the associated electronics for signal generation, processing, and communication. The sensing module detects the breath rate through the variation in the phase of the reflection coefficient of a transmission line loaded with a movable resonator, caused by the elongation and contraction of the belt. Both elements, the line and the resonator (an electric-LC −ELC− resonator), are implemented in cotton substrate with metallic layers (including the ground plane, the microstrip line, and the ELC resonator) consisting of a conductive textile material tapered to the substrate. Advantageously, the sensor operates at a single frequency, resulting in simple associated electronics, essentially, a voltage-controlled oscillator (VCO), a phase detector, and a power splitter (implemented by means of a directional coupler), necessary to generate a reference signal for the phase detector. Additionally, the system incorporates a microcontroller that manages the VCO, captures the phase of the detector, and transmits it to the receiver (a smartphone) via Bluetooth. The functionality of the system is demonstrated by using it in a volunteer breathing under different conditions.

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