Design of a Two-Channel Forehead PPG Sensor.

Photo-plethysmography (PPG) is an affordable, noninvasive, and popular solution for real-time monitoring of blood oxygen content. PPG sensor at the forehead can be further integrated with Electroencephalogram (EEG) sensors or functional near-infrared spectroscopy (fNIRS) sensors. Due to the large area and the complex vessels on the forehead, reduced reliance on positioning and the alignment of the sensor is crucial for a consistent PPG sensor design. This paper discusses the relationship between changes in the position of the sensor and the distribution of vessels and the PPG signal. A wearable two-channel PPG sensor is designed and tested with 9 participants at varying sites on the forehead. Two sets of red (660 nm) and near-infrared (NIR, 940 nm) LEDs are implemented. Different positioning of the sensor’s result to the signal is discussed. The experiment have further demonstrated that the averaged R-value from two channels ( 0.0503 ± 0.0172) had a lower standard deviation compared to measurements from individual channels (right: 0.0659 ±0.069, left: 0.0774 ± 0.1016) at each site.