Contact measurement of skin temperature using a wearable two-channel PPG optical sensor supplemented by thermometers

This paper describes realization, basic properties, testing, and experiments with a special prototype of a wearable two-channel photoplethysmography (PPG) sensor supplemented by contact thermometers that maps the skin temperature at the place where the optical part of the sensor touches a measured hand part (typically a wrist and fingers). Preliminary measurement confirms that proposed I2C thermometers have proper stability and precision, so can be successfully used in the developed PPG sensor. The performed main experiments show that the significant increase of temperature was always observed at the place of the worn PPG sensors during the whole measurement. This rise of temperature affects mainly the PPG signal range, thus it has also influence on the quality of the sensed PPG wave. The conclusion formulates the final recommendation about the necessity of at least 5-minute time delay between the PPG sensor placement on the hand and the start of the PPG signal acquisition. While the current measurements were realized in normal laboratory conditions, the whole wearable PPG sensor consists of non-ferromagnetic materials and all parts are fully shielded by aluminum boxes to enable measurement in a low magnetic field environment which, is our final long term research aim.