Stretchable Photodiodes with Polymer-Engineered Semiconductor Nanowires for Wearable Photoplethysmography

Healthcare systems worldwide have been stressed to providesufficientresources to serve the increasing and aging population in our society.The situation became more challenging at the time of pandemic. Technologyadvancement, especially the adoption of wearable health monitoringdevices, has provided an important supplement to current clinicalequipment. Most health monitoring devices are rigid, however, humantissues are soft. Such a difference has prohibited intimate contactbetween the two and jeopardized wearing comfortableness, which hurdlesmeasurement accuracy especially during longtime usage. Here, we reporta soft and stretchable photodiode that can conformally adhere ontothe human body without any pressure and measure cardiovascular variablesfor an extended period with higher reliability than commercial devices.The photodiode used a composite light absorber consisting of an organicbulk heterojunction embedded into an elastic polymer matrix. It isdiscovered that the elastic polymer matrix not only improves the morphologyof the bulk heterojunction for obtaining the desired mechanical propertiesbut also alters its electronic band structure and improves the electricalproperties that lead to a reduced dark current and enhanced photovoltagein the stretchable photodiode. The work has demonstrated high fidelitymeasurements and longtime monitoring of heat rate variability andoxygen saturation, potentially enabling next-generation wearable photoplethysmographydevices for point-of-care diagnosis of cardiovascular diseases ina more accessible and affordable way.