In-Ear PPG for Vital Signs

Earables are now pervasive, and their established purpose, ergonomy, and non-invasive 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 (SpO2), 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 in-the-canal placement showing the lowest inter-subject variability. However, our study shows the absolute error climbs up to 29.84%, 24.09%, 3.28%, and 30.80% respectively for HR, HRV, SpO2, 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.