A Multi-Site Measurement System with Photoplethysmographic and Microwave Sensors for Synchronous and Continuous Cardiovascular Signal Acquisition

The dynamic development of non-invasive medical diagnostics and continuous health monitoring using biomedical signals in recent years has been one of the main research directions in biomedical electronics. This paper presents a developed multimodal, multi-site, and synchronous measurement system designed for continuous cardiovascular signal acquisition. The system integrates reflective and transmissive photoplethysmographic (PPG) sensors and active microwave (MW) sensors operating at a frequency of 4.9 GHz. One of the reflective PPG sensors enables multispectral (MWPPG) recording for six different light wavelengths, ranging from blue (465 nm) to infrared (940 nm). The system was evaluated in a trial involving a group of 32 volunteers. During our research, the PPG sensors were placed on the fingers of the left hand, wrist, and forehead, while the MW sensors were placed on the thumb, wrist, biceps, and chest. In addition, ECG signals and accelerometer signals are recorded synchronously. The averaged pulses of the individual sensors were appointed to compare their shape and quality depending on the technology and location of the measurement. The PPG signals exhibited higher signal quality, with mean SNR values ranging from 8.66 ± 2.17 to 12.61 ± 3.98 dB, compared with 5.91 ± 0.98 to 7.95 ± 1.26 dB for the MW signals. Similarly, the pulse-detection F1-score ranged from 0.34 ± 0.22 to 0.74 ± 0.17 for PPG and from 0.33 ± 0.07 to 0.43 ± 0.15 for MW sensors. Significant differences in the shape of the pulses from the MW sensors were also demonstrated between individual participants. Furthermore, the paper presents examples of recorded signals and discusses the advantages and limitations of various sensor technologies and their locations, while also indicating potential directions for further research.