Physical characterization of sensor-skin coupling for wrist photoplethysmography signal acquisition

This is the authors' abstract. We don't add key points for this paper.

Photoplethysmography (PPG) is central to wrist-worn physiological monitoring, but its signal acquisition is strongly influenced by mechanical and optical coupling at the sensor-skin interface. This study presents an integrated physical characterization of contact force, illumination wavelength, source-detector separation, skin-surface properties, and motion-artifact patterns using a controlled wristwatch platform. In the two tested participants, contact force produced nonmonotonic and metric-dependent changes in signal-to-noise ratio (SNR), perfusion index (PI), and waveform morphology, and the most favorable tested force condition differed between the two cases. Across three illumination wavelengths, increasing the source-detector separation from 4 to 8 mm produced distinct wavelength-dependent attenuation of the alternating-current (AC) area, revealing the coupled influence of optical wavelength and sensor geometry on detected pulsatile signals. In the tested participant, hair removal was associated with improved PPG signal characteristics under the tested condition. In ten participants, localized tanning caused pronounced wavelength-dependent reductions in the AC and direct-current (DC) components and harmonic amplitudes, whereas SNR, PI, and relative spectral morphology remained comparatively stable. These results indicate that changes associated with localized tanning should be evaluated jointly using detected signal levels, ratio-based quality metrics, and waveform characteristics rather than any single indicator. Motion artifacts were further examined under two controlled wearing conditions intended to emphasize ambient-light intrusion or coupling-related deformation, and corresponding differential-processing and detection-excision-reconstruction strategies improved signal recovery under the tested conditions. Overall, this work provides an integrated physical framework for mechanical fitting, optical configuration, evaluation of tanning-associated signal changes, and mechanism-matched artifact handling in wrist-worn PPG systems.