Signal robustness dependency on sensor contact force in a novel photoplethysmography-based multi-purpose sensing device

Abstract

A new photoplethysmography-based device called the Multiphotodiode Array (MPA) was validated to successfully measure the PWV in the vasculature of the distal phalanx of healthy subjects. It comprises an array of photodiodes and an array of opposing LEDs to detect blood volume changes due to the passing pressure pulse wave in the vasculature of the distal phalanx of the index finger. This study aimed to discern how the signal robustness of a novel modular sensor based on the technology of the MPA is dependent on the contact force between the vascularized tissue and the sensor surface. PWV data was collected as the distal phalanx of the left index finger from 26 subjects was placed on the Force Alterable- MPA (FA-MPA). Contact force was altered by placing differing weights (0-300g, 50g increments) on top of the phalanx using a linear stage. Contact force was determined as weight measured by a scale underneath the FA-MPA. PWV and weight data was collected from a total of 182 measurements. Measurements were pooled in 8 weight groups between 0 and 400g at increments of 50g according to the weight that was measured during that measurement. PWV data per weight group was analyzed for three characteristics: 1. Pulse Wave Quality Ratio (PWQR); 2. PWV variance; 3. Realistic and non-negative values for PWV. ANOVA of PWQR resulted in a significant effect between weight groups (p = 6.91E-7). Further qualitative judging of data resulted in the recommendation to thoroughly redesign the FA-MPA for structural and electrical integrity and measurement protocol for elimination of movement and placement artifacts and to reiterate the experiment for the contact force range of 1.96 to 4.43N.