Sensores RFID UHF para monitoramento das frequências cardíaca e respiratória

Abstract

In this work, the development of Ultra High Frequency Radio Frequency Identification sensors for monitoring heart and respiratory rates and the evaluation of their feasibility is presented. The RFID tag was used on a flexible polyimide substrate (Kapton®) with a 4 mm expanded polyethylene foam, including a metamaterial-based structure in the reflector layer to improve its performance in proximity to the body, increasing its maximum gain value. A simulated maximum read range of 4.52 m was achieved without DC voltage input with the tag containing a two-ring Complementary Split Ring Resonator cell. Respiratory rate monitoring is based on variations in the Received Signal Strength Indicator by the reader due to respiratory movements. Measurements with the respiratory rate sensor were performed with the tag on the abdomen and chest while the participant was standing and sitting. The maximum measured read range with this sensor was 2.26 m with the tag on the participant's abdomen while sitting. The measured RSSI data were processed using Daubechies wavelets of order 7 and Symlet of order 6, considering three levels of signal decomposition. The obtained respiratory rates, after processing, were validated by comparison with the metronome frequency and with the results of a vital signals monitor, with the exception of the measurement with the tag on the participant's chest while standing, where a difference of one breathing cycle was observed. Heart rate monitoring, on the other hand, was based on the time interval between two successive interruptions in RFID communication, which occur when a heartbeat is detected. For this purpose, a sensor was developed consisting of an Arduino UNO with AD8232 module and three electrodes, non-inverting amplifier, inverting comparator, timer, voltage divider and RFID tag. When the R wave of the electrocardiogram signal is detected, the DC voltage input signal of the tag's RFID chip remains high for 145.20 ms, interrupting communication between the tag and the RFID reader. The acquired heart rate, after identifying the time interval between two consecutive communication interruptions, was also validated by comparison with the results of a vital signals monitor. In this way, it was possible to verify the feasibility of using RFID technology for monitoring heart rate and respiratory rate.