Wearable Reflectance PPG Optical Sensor Enabling Contact Pressure and Skin Temperature Measurement

This paper describes the design, realization, and application of a wearable sensor based on the photoplethysmography (PPG) principle supplemented with a force-sensitive resistor and a thermometer for the measurement of contact pressure force and the temperature of the skin at the point where the optical part of the PPG sensor touches the finger. The performed experiments confirmed the essential influence of the applied contact force on the amplitude and ripple of the sensed PPG signal and the stability and precision of heart rate values determined from the PPG wave. Preliminary measurements showed that the response to the applied contact force was principally different for fingers of male and female tested persons, so different scaling and pressure levels were applied in the main experiments. Contrariwise, differences between left and right hands were not significant. The influence of skin temperature changes could be ignored for these measurements due to the short time duration of the PPG signal recording (approx. 1 min).