A Wearable PPG Multi-Sensor for Measurement of Skin Humidity, Temperature, and Contact Pressure

The aim of our work was to analyze the influence of changes in humidity and temperature on temporal features of sensed photoplethysmography (PPG) waves. This paper describes a special prototype of a wearable PPG multi-sensor with an integrated I2C humidity sensor and a thermometer to carry out measurements at three skin moisture levels. This sensor is supplemented with a force-sensitive resistor for the measurement of the physical contact pressure between the measuring probe and the skin surface, which can be used to sense heart pulsation in the wrist radial artery. The experiments conducted show that the performed skin manipulation (skin drying, moistening) was always detectable; the PPG signal range is mainly affected, while changes in signal ripple and heart rate variance are smaller. The detailed analysis per hand and gender type yielded differences between male and female subjects, and the results for left and right hands differed less.