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Recent progress in printed optoelectronics and their integration in wearable sensors have created new avenues for research in reflectance photoplethysmography (PPG) and oximetry. The reflectionmode sensor, which consists of light emitters and detectors, is a vital component of reflectance oximeters. Here, we report a systematic study of the reflectance oximeter sensor design in terms of component geometry, light emitter and detector spacing, and the use of an optical barrier between the emitter and the detector to maximize sensor performance. Printed red and near-infrared (NIR) organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) are used to design three sensor geometries: (1) Rectangular geometry, where square OLEDs are placed at each side of the OPD; (2) Bracket geometry, where the OLEDs are shaped as brackets and placed around the square OPD; (3) Circular geometry, where the OLEDs are shaped as block arcs and placed around the circular OPD. Utilizing the bracket geometry, we observe 39.7% and 18.2% improvement in PPG signal magnitude in the red and NIR channels compared to the rectangular geometry, respectively. Using the circular geometry, we observe 48.6% and 9.2% improvements in the red and NIR channels compared to the rectangular geometry. Furthermore, a wearable two-channel PPG sensor is utilized to add redundancy to the measurement. Finally, inverse-variance weighting and template matching algorithms are implemented to improve the detection of heart rate from the multi-channel PPG signals.INDEX TERMS Reflection photoplethysmography sensor, organic optoelectronics, pulse oximetry, wearable sensors, printed electronics, flexible electronics.

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We report a very high performance lOOnm gate length CMOS transistor structure operating at 1.2-1 SV. These transistors are incorporated in a 180nm logic technology generation. Various process enhancements are incorporated to significantly improve transistor current drive capability relative to the results published in [I]. Unique transistor features responsible for achieving high performance are described. NMOS and PMOS devices demonstrate drive current of 1.04 mA/pm and 0.46 mA/pm respectively at 1.5V and 3nA/pm IOFF. These are the best drive currents reported to date at fixed IOFF. They represents 10% drive current improvement for both NMOS and PMOS devices relative to the results published in Ref.[l] without any change in gate-oxide thickness. High performance is demonstrated down to 1.2V. Inverter delay of less than 10 psec is reported at 1.5V at very moderate IOFF values.