Wearable PPG multi-sensors for measurement in the weak magnetic field environment of an NMR tomograph

Investigators: Jiří PŘIBIL, Tomáš Dermek

As part of the VEGA project, a new prototype of a multi-channel wearable PPG sensor with integrated contact thermo-chips and a pressure sensor based on force-sensitive resistors (FSR) was developed and tested in practice. The sensor is capable of measuring in the weak magnetic field environment of an NMR tomograph. Furthermore, a calibration device was created for semi-automatic measurement of the characteristics of FSR force sensors based on comparative measurements using calibration weights. For the planned use of FSR sensors in measurement experiments on human subjects, only the lower part of the force range (up to 250 grams) was investigated. For each measured FSR sensor, the conversion characteristics were then linearized using linear regression. The main measurement experiments focused on analyzing the effect of temperature and contact pressure of the measuring probe placed on the fingertips on the quality and properties of the recorded PPG signals.The measurements were performed using different levels of contact pressure (CPF) for men and women separately, due to fundamental differences in finger physiology. The results confirmed changes primarily in the amplitude and degree of ripple of the recorded PPG signal depending on the intensity of the applied pressure. A higher CPF level subsequently leads to an overall deterioration in the quality of the recorded PPG signals, which is also reflected in increased heart rate variability. Conversely, too low an applied CPF can cause an insufficient level of recorded PPG, which further negatively affects the success/accuracy of heart rate calculation. Other PPG signal parameters appear to be less dependent on contact force or are influenced by other physiological factors.

Projects: VEGA 2/0004/23

Fig. 1. Example of an experiment with a PPG multi-sensor equipped with a contact thermometer and a pressure gauge in laboratory conditions (a), photo of a calibration device for FSR sensor conversion characteristics (b).

Publications:

  • PŘIBIL, Jiří – PŘIBILOVÁ, Anna – DERMEK, Tomáš. Wearable PPG optical sensor enabling contact pressure and skin temperature measurement. In Journal of Electrical Engineering, 2025, vol. 76, no. 2, p. 134-146. ISSN 1335-3632. (1.2 – IF2024) Q3
  • PŘIBIL, Jiří – PŘIBILOVÁ, Anna – FROLLO, Ivan. Wearable PPG sensors applicable for measurement in a weak magnetic field environment of an MRI device. In Advances in Signal Processing: Volume 3. Editor: S.Y. Yurish. – Barcelona, Spain: International Frequency Sensor Association Publishing (IFSA), 2025, p. 1-22. ISBN 978-84-09-69173-9
  • PŘIBIL, Jiří – PŘIBILOVÁ, Anna – DERMEK Tomáš. Measurement of resistance/force conversion characteristics of FSR elements for evaluation of contact skin pressure by PPG sensors. In Proceedings of the 15th International Conference on Measurement. – Bratislava : Institute of Measurement Science, SAS, 2025, p. 101-104. ISBN 978-80-69159-01-3.
  • PŘIBIL, Jiří – PŘIBILOVÁ, Anna – DERMEK, Tomáš. A Wearable PPG Multi-Sensor for Measurement of Skin Humidity, Temperature, and Contact Force, in Proceedings of the 12th International Electronic Conference on Sensors and Applications, 12–14 November 2025. Available online: https://sciforum.net/paper/view/26558