An in vitro testing system for wrist-worn PPG devices

Cardiovascular disease is the leading cause of death in the United States. Thus, much work is being done to develop monitoring technologies to lessen its morbidity and mortality. A common cardiovascular sensing modality is photoplethysmography (PPG), which relates local blood volume to changes in the intensity of light reaching a photodetector after traveling through tissue. Sufficient PPG signal quality from individuals with certain physiologies/anatomies can be difficult to obtain: one such example is the absorbing effect of melanin, causing poor PPG signal from individuals with a dark skin tone. Using optical phantoms, in vitro testing systems can enable device developers to quickly explore device performance under different conditions to help ensure strong performance. Here, we propose a phantom testing system for the in vitro evaluation of PPG. A pump system flows blood phantom through a 3-layer polydimethylsiloxane (PDMS) tissue phantom (mimicking epidermis, dermis, subcutis) of the volar wrist with an inset vessel representing the radial artery. The pump changes pressure to represent pressure throughout the cardiac cycle, and a PPG sensor (660nm) is placed on the tissue phantom. The effect of skin tone is analyzed by using epidermal phantoms with different optical properties representing Fitzpatrick Skin Tone I (3% volume fraction melanosome) and Fitzpatrick Skin Tone III (15% volume fraction melanosome). It was found that the PPG sensor was able to successfully and accurately register the synthetic PPG waveform, and the PPG AC/DC ratio decreased 17.7% when comparing the 3% volume fraction melanosome phantom to the 15% volume fraction melanosome phantom.