Wearable PPG optical sensor enabling contact pressure and skin temperature measurement

Abstract The article describes the design, realization, and testing of a new special prototype of a wearable sensor working on photoplethysmography (PPG) principle with a thermometer and a force-sensitive resistor (FSR) component. This sensor is primarily designated for measurement of skin temperature and applied contact pressure force (CPF) at the point where the optical part of the sensor touches a finger. At first, the sensor’s functionality including stability and quality of wireless connection was tested inside the running magnetic resonance tomograph. Next, the catalogue FSR’s resistance/force conversion characteristic was verified by comparative measurements by a set of calibration weights and its sufficient linearization was proposed. Reactions of applied different CPFs on finger(s) of males and females were mapped to find a proper methodology for the main experiment. The main measurements were realized in the normal laboratory conditions using five levels of CPF for male and female subjects separately. The obtained results confirm an essential influence of applied CPF on the PPG signal amplitude and ripple. Increasing of CPF causes also changes in the systolic pulse shape, while other PPG wave features seem to be less dependent on the CPF level. The skin temperature changes observed during the measurement were also minimal.