Wireless Breathing Sensor Based on Wearable Modulated Frequency Selective Surface

This paper presents a wireless apnoea detector using a passive breathing sensor. The device is based on the measurement of the airflow temperature changes that are produced during breathing. The sensor is a negative temperature coefficient resistor, which is to be placed close to the nose. The functional principle is based on the change of resistance, since the body increases the temperature of the airflow, which the device receives during breathing. The wireless communication is performed using the backscattered field technique, and is based on the modulation of the response from a transponder. This backscattering technique saves a lot of energy in comparison to its alternatives and guarantees a long lifetime for the device. The transponder is composed of an array of dipoles, loaded with varactor diodes, which implement a frequency selective surface. NTC measured temperature controls the oscillation frequency of a low-frequency, two-inverter oscillator. The output of the latter modulates the varactor diodes which, in turn, modulate the backscattered response of the transponder. An algorithm, which is based on the detection of the peaks on the breathing signal in order to calculate real-time respiration and apnoea intervals, has been implemented. The reader used is also presented and experimental results are shown. The sensor can be used to perform continuous real-time measurements for long period on time due to its low-power consumption.