The effect of Skin Melanin Concentration on Wrist Reflectance Photoplethysmography based on Monte Carlo Simulation

Darkness of skin has a significant effect on the characteristics of reflectance photoplethysmographic (PPG) signals, which further influence the universality of the blood oxygen saturation detection method based on reflectance PPG. In this study, a multi-layer optical model of wrist skin is firstly established. Base on it, the signal characteristics of the wrist reflectance PPG at red (660 nm) and infrared (940 nm) wavelengths under different skin melanin concentrations, which is the main factor determining the darkness of skin, are investigated by using Monte Carlo simulation method. The results show that the baseline (DC) and the pulse amplitude (AC) of both red and infrared PPG signals increase with decreasing melanin concentration, and the lower the melanin concentration the greater the increase of both the DC and AC of PPG signals. In addition, the AC/DC of infrared PPG signal is not significantly affected by melanin concentration, but that of red PPG shows an increasing trend with decreasing melanin concentration, which is more obvious at lower oxygen saturation. Since the AC and DC components of red and infrared PPG signals are essentially important for prediction of blood oxygen saturation, the simulation results provide a basis for analyzing and eliminating the influence of skin darkness on blood oxygen saturation detection, which may help to improve the accuracy of wrist-worn pulse oximetry.