Abstract
Recent progress in printed optoelectronics and their integration in wearable sensors have created new avenues for research in reflectance photoplethysmography (PPG) and oximetry. The reflection-mode 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.
Highlights
In the human body, cardiac rhythm changes the blood volume passing through the arteries, which generates a pulsatile signal that can be optically measured using a light source and a detector; this optical sensing technique is known as photoplethysmography (PPG)
The sensor is interfaced to multiplexers that switch between the pixels and connects to an analog front end (AFE)
We explore three different sensor geometries as shown in Fig. 2a-c: (1) Rectangular geometry (R), where the organic light-emitting diodes (OLEDs) are placed at either side of the organic photodiodes (OPDs); (2) Bracket geometry (B), where the OLEDs are shaped as brackets and placed around the square OPD; (3) Circular geometry (C), where the OLEDs are shaped as block arcs and placed around the circular OPD
Summary
Cardiac rhythm changes the blood volume passing through the arteries, which generates a pulsatile signal that can be optically measured using a light source and a detector; this optical sensing technique is known as photoplethysmography (PPG). Composed of solid-state light-emitting diodes (LEDs) and photodiodes (PDs) are used to measure SpO2 at the extremities of the body where light can penetrate thin regions of tissue, such as the earlobes and the fingertips This method of measuring SpO2 presents a few limitations - (i) Transmission-mode oximetry has limited sensing locations [2], and (ii) Solid-state LEDs and PDs do not conform well to the skin, reduce the signal-tonoise ratio (SNR) [3]. Wearable reflection-mode PPG sensors and oximeters are prone to different kinds of noises, such as motion artifacts (MAs), thermal noise, and electromagnetic interference [22]. We utilize a printed, flexible, and two-channel reflectance oximeter to collect PPG signals using red and near-infrared (NIR) organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs). We report sensor design, optimization, and implementation of a two-channel organic optoelectronic sensor which is promising for wearable smartwatches and wristbands
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