In-Ear PPG for Vital Signs

Earables are now pervasive, and their established purpose, ergonomy, and noninvasive interaction uncover exciting opportunities for sensing and healthcare research. However, it is critical to understand and characterize sensory measurements’ accuracy in earables impacting healthcare decisions. We report a systematic characterization of in-ear photoplethysmography (PPG) in measuring vital signs: heart rate (HR), heart rate variability (HRV), blood oxygen saturation (SpO$_2$2), and respiration rate (RR). We explore in-ear PPG inaccuracies stemming from different sensor placements and motion-induced artifacts. We observe statistically significant differences across sensor placements and between artifact types, with ITC placement showing the lowest intersubject variability. However, our study shows the absolute error climbs up to 29.84, 24.09, 3.28, and 30.80%, respectively, for HR, HRV, SpO$_2$2, and RR during motion activities. Our preliminary results suggest that in-ear PPG is reasonably accurate in detecting vital signs but demands careful mechanical design and signal processing treatment.

Earables are becoming a permanent augmentation of the human ear, offering meaningful and engaging hearing experiences and enhancements. Recently, sensory research in and around the ear has sharply risen as earables can transform how we perceive and experience sound, i.e., spatially aware, distraction-free, and personalized. However, earables can also revolutionize personal health and clinical research by enabling noninvasive, continuous, and accurate health monitoring. Due to its proximity to brain and eyes, earables can be used to monitor a plethora of biomarkers like eye-movements, cerebral activity (EEG), or heart-related photoplethysmography (PPG) signals. Indeed, in the pervasive computing literature, we have already seen ear-worn devices applied for continuous monitoring of cardiovascular functions, heart rate and stress, sleep, measurement of oxygen consumption and blood flow, and tracking dietary and swallowing activities.