Optimizing performance of reflectance-based organic Photoplethysmogram (PPG) sensor

Wearable sensors enable the continuous monitoring of various physiological conditions of individuals without constraints on time and place. Primary vital signs of human body such as; heart rate (HR), Oxygen saturation (SpO2) and respiration rate, can be extracted from the PPG signal. In comparison to conventional inorganic based sensors, the use of organic semiconductor-based devices opens the possibility of devising inexpensive, lightweight, flexible sensors. Reflection-mode PPG sensors overcome the limitations posed by transmission-mode PPG sensor as it can be positioned anywhere on the body. The state of art has not exploited the reflection-mode of PPG sensors extensively, as opposed to transmission-mode. In this work, we have fabricated reflection mode PPG sensor, which comprises of a red (631 nm) organic light emitting diode (OLED) (EQE = 8%) and organic photodetector (OPD) (EQE =47 %) on the same substrate. With motivation to improve the existing PPG sensing technologies, OLED and OPD performances were optimized on a single substrate. Further, we have estimated the best pattern and optimal distance between OLED and OPD in order to maximize signalnoise ratio and lower the power consumption of the device. An analog circuit is designed to read out PPG signals. For realtime pulse monitoring, the signals were sent via a Bluetooth interface to the computer. In summary, a low cost, organic based sensor is developed to detect the heart rate with wireless enabled data monitoring. Our device displayed promising results with 1.5 % error in the heart rate measurement compared to the commercial reference.