Evaluating contact pressure and locational effects on a multiwavelength photoplethysmographic ring

Traditional cardiopulmonary monitoring systems such as pulse oximeters or electrocardiograms typically lack the ability to provide continuous, noninvasive, and unobstructive monitoring of critical vital sign data, including heart rate, respiration rate, and blood oxygenation (SpO2) in a wearable form factor. Wearable devices provide the opportunity to combat the shortcomings of these traditional devices and improve patient outcomes by providing a remote platform for accessing an individuals’ cardiovascular health in ambulatory settings. Such devices, however, typically lack accuracy and robustness. Within this study, we designed and fabricated an optical-based ring system to address the current need for accurate remote monitoring. However, unlike other wearable devices (i.e., wrist, arm, or chest systems), ring devices cannot be actively fit adjusted to ensure appropriate skin contact with the photoplethysmography (PPG) sensor, and because of this, it is crucial to ensure the proper fit of ring systems. To investigate the effects of ring fit, or the amount of contact pressure applied by a ring device, relative to AC amplitude and waveform morphology, the ring system developed here was equipped with multiwavelength PPG (i.e., red and infrared) and contact pressure sensors. One dorsal location and three palmar locations on the finger were studied while varying the fit of the ring, from loose to partial arterial vasculature occlusion. Depending on location, contact pressures ranged from 10-100 mmHg, resulting in distinct applied pressure driven DC offset shifts. Also, an increase in contact pressure generally resulted in an increase in AC amplitude for signals acquired from the palmar side of the finger resulting in a mean absolute of error in heart rate of 2.7 beats per minute when the contact pressure was between 45-60 mmHg. Lastly, SpO2 approximations best corresponded to the gold standard device at a PPG source to detector separation of 8.5 mm. These results suggest the contact pressure and source to detector separation does influence the outcome of the cardiovascular measurement and may provide information for future ring designs.