Wearable PPG Multi-Sensor for Skin Humidity, Temperature, and Contact Pressure Measurement in Weak Magnetic Field Environment: First-Step Experiments

This study describes the developed special prototype of a wearable measuring device based on a photoplethysmography (PPG) sensor. It contains also a humidity sensor and a thermometer to measure skin moisture and temperature, and a force-sensitive (FSR) element to sense a contact pressure between the measuring probe and the skin surface. All parts of the multi-sensor are shielded, to be applicable in a weak magnetic field environment. After the basic sensor's functionality verification inside the magnetic resonance imaging tomograph, a set of experiments was performed. Comparative measurements by an oximeter confirm good correspondence with heart rate values determined from PPG (<i>HR</i>_PPG) and FSR (<i>HR</i>_FSR) signals-the mean absolute error lies below 0.5 min^-1 for both types. The sensing of PPG signals on wrists was realized for Normal, Dry, and Wet skin. In comparison with normal skin conditions, drying decreases the PPG signal range by 7% and the systolic pulse width by 8%, while moistening increases the signal ripple by 3% and decreases the correlation between <i>HR</i>_PPG and <i>HR</i>_FSR values by 5%. The detailed analysis per hand and gender types yields differences between male and female subjects, while the results for left and right hands differ less.