Unraveling the contact force impact on the complex multi-wavelength PPG signal morphology at different vascular depths.

The contact force is an important parameter for the quality assessment of the photoplethysmography (PPG) signals since it markedly alters the PPG waveform. Despite the well-known fact that the force levels can induce changes on the PPG amplitude, little is known about the influence of contact force on the contour of the PPG signal, especially for multi-wavelength PPG systems. In this work, the influence of the contact force on the contour of the PPG signal at different vascular depths is evaluated. A multi-wavelength PPG signal was recorded on the fingertip of both hands of eleven healthy volunteers while the level of contact force exerted by the PPG probe was gradually modified. The morphological characteristics, the amplitude changes, the AD/DC ratio and the normalized pulse area of the AC and DC components of the five recorded PPG wavelengths were analyzed, as well as the AC and DC component of the contact force signal. Our results suggest that the contact force has a marked influence on the contour of the multi-wavelength PPG signal. Moreover, the degree of deformation obtained also depends on the viscoelastic characteristics of the tissue, as shown by the morphological differences among the five wavelengths at different force levels. The use of these results for the improvement of pulse detection, the diagnosis based on signal morphology and the reduction of movement artifacts in clinical and wearable devices is discussed. We conclude that the contact force is a crucial parameter for those applications where the morphological characteristics of the PPG signal influence the accuracy of the results.