Flexible and Printed PPG Sensors for Estimation of Drowsiness

We report printed flexible optoelectronic sensors composed of red organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) for detection of various biological signals in a photoplethysmograph (PPG) device. Fabricated flexible OLEDs achieved maximum luminance >1000 cd/m² at 9 V, with peak at 640 nm. Maximum flexible OPD photosensitivity for the poly(3-hexylthiophene-2, 5-diyl) and phenyl-C61-butyric acid methyl ester (PCBM) heterojunction is <inline-formula> <tex-math notation="LaTeX">$ {2} \times {10}^{{2}}$ </tex-math></inline-formula> at 0 V and 1.76 at −1 V, irradiated with 1.2 mW/cm² at 660 nm. The diketopyrrolopyrrole thieno [3,2-b]thiophene blended with PCBM OPDs with poly (3,4-ethylenedioxythiophene):polystyrene sulfonate anode showed photosensitivity = 84 at −1 V bias to almost <inline-formula> <tex-math notation="LaTeX">${6} \times {10}^{{4}}$ </tex-math></inline-formula> at 0 V accompanied by low dark current (<inline-formula> <tex-math notation="LaTeX">${9.5} \times {10}^{-{8}}$ </tex-math></inline-formula> A/cm² at −1 V). PPG signals were successfully detected using the developed flexible PPG sensor and the conventional driving circuit. Human studies were conducted to evaluate the flexible PPG sensor performance in practical applications. Subject drowsiness was estimated from heart rate variability, extracted from the PPG signals, using machine learning algorithms. The flexible PPG sensor achieved 79.2% accuracy and 72.1% area under the receiver (AUC) to predict drowsiness (60-s window), which are meaningful results compared with conventional PPG sensors (83.3% accuracy and 69% AUC). Drowsiness estimation experiments using two PPG signals showed that the flexible PPG sensor achieved similar or better performance compared to conventional PPG sensors.