Key result
Increasing contact pressure force on the PPG sensor decreased the relative signal range and increased the relative ripple, with optimal signal quality achieved at middle pressure forces.
The study demonstrates that contact pressure force significantly affects PPG signal amplitude and pulse shape, highlighting a critical factor for the design and calibration of wearable optical sensors.
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Technical feasibility shown for multi-parameter finger sensor; leaves open prospective validation before cardiovascular research or clinical use.
The article describes the design, realization, and testing of a new special prototype of a wearable sensor working on photoplethysmography (PPG) principle with a thermometer and a force-sensitive resistor (FSR) component. This sensor is primarily designated for measurement of skin temperature and applied contact pressure force (CPF) at the point where the optical part of the sensor touches a finger. At first, the sensor’s functionality including stability and quality of wireless connection was tested inside the running magnetic resonance tomograph. Next, the catalogue FSR’s resistance/force conversion characteristic was verified by comparative measurements by a set of calibration weights and its sufficient linearization was proposed. Reactions of applied different CPFs on finger(s) of males and females were mapped to find a proper methodology for the main experiment. The main measurements were realized in the normal laboratory conditions using five levels of CPF for male and female subjects separately. The obtained results confirm an essential influence of applied CPF on the PPG signal amplitude and ripple. Increasing of CPF causes also changes in the systolic pulse shape, while other PPG wave features seem to be less dependent on the CPF level. The skin temperature changes observed during the measurement were also minimal.
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Přibil et al. (2025) studied Healthy volunteers (n=6). Different levels of contact pressure force (CPF) vs. Different force levels was evaluated on PPG signal amplitude (relative signal range) and ripple. Increasing contact pressure force on the PPG sensor decreased the relative signal range and increased the relative ripple, with optimal signal quality achieved at middle pressure forces.
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