Influence of external contact pressure in reflection mode photoplethysmography: a multidimensional manufacturing approach

Optical wearable sensors provide crucial information on cardiovascular biomarkers used to estimate heart rate variability, blood oxygen saturation, and arterial blood pressure. However, environmental factors, including external contact pressure, can significantly affect the quality of the acquired signal but have been poorly studied. Objective: In this work, we investigate the influence of external contact pressure on the photoplethysmography (PPG) signal in reflection mode. By systematic application of external contact pressure to the fingertips of volunteers, we aim to examine how such pressure affects the morphological features of a representative cardiac cycle. Methods: We designed, and 3D printed a mounting system to apply controlled pressure to the fingertips of volunteers. This system generated a controllable force using a spring mechanism coupled with a rotating screw. First, we quantified the spring constant and the pressure it applies per revolution. Then, using a PPG sensor operating at a wavelength of 660 ± 20nm, we recorded raw photocurrent signals from three healthy adults. Using our proposed signal processing algorithm, we created ensemble-averaged representative cardiac cycles. Results: We calculated the spring's constant within the mounting system as 339.3N/m. Using this system, we applied external contact pressure values from 20 to 180mmHg. Our results show that the amplitude of systolic peak, dicrotic notch, and diastolic peak for this external contact pressure range continuously rise with a factor of 2.5, 5, and 2, respectively. Conclusion: Our 3D printed mounting system provided a reliable means of applying controlled and reproducible external contact pressure to the fingertips of adult volunteers. We conclude that such contact pressure substantially influences the amplitude of the obtained PPG photocurrent, being a crucial factor in optical wearable sensor design. Significance: Our findings pave the way for determining the optimal level of external contact pressure, as an environmental factor. Such optimal pressure should balance user comfort with the quality of the measured PPG signal, thereby supporting the reliable estimation of cardiovascular parameters.