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

The motivation of our work was to analyze 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 integrated I2C humidity and thermo meters to carry out measurements in three skin moisture levels. This sensor is supplemented with a force-sensitive resistor for measurement of the physical contact-pressure between the measuring probe and the skin surface next usable to sense heart pulsation on the wrist radial artery. The realized experiments 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 a hand and gender type yielded differences between male and female subjects, results of left and right hands differ less.