Development of a multi-source, multi-detector, finger photoplethysmography system

Abstract

Finger-based photoplethysmography (PPG) is traditionally acquired using a finger clip form factor on the fingertip. This form factor is not conducive for long-term measurements during regular everyday activities due to its inability to be easily worn while moving outside a clinical setting. However, advancements in finger-based PPG sensors over the last several decades include ring designs at the proximal phalanx of the finger, which are more conducive to being worn during everyday living conditions. The anatomy of this location is more complex relative to the tip as there is a smaller density of blood vessels. Also, the finger's proximal phalanx is comprised of more opaque tissue and bone, which affects the path of the light. In this study, a ring design that included 8 LEDs/PDs each at 45 degrees that included two wavelengths (940 nm: infrared and 655 nm: red) on the proximal phalanx of the finger was studied in terms of their relative placement to each other for detection. Specifically, the system was used to investigate how the pulsatile and DC component amplitudes of the photodiodes varied around the finger in both transmission and reflectance modes as a function of the 8 LEDs/PDs pairs at 45 degrees on the ring. All 64 combinations of LEDs/PDs located at the proximal phalanx of the finger were investigated. The results showed the locations that yielded the strongest amplitude PPG signals.