*3.1. Mechanics of PPG*

Photoplethysmography (PPG) is emerging as an alternative to ECG for cardiovascular monitoring due to its small size and ability to capture many different physiological parameters. The LED operates at red and near-infrared (NIR) frequencies, and the light intensity reaching the photodiode changes depending on the volumetric changes in the veins and arteries [46]. PPG sensors consist of two basic components: a light-emitting diode (LED) and a photodiode. The PPG sensor can function in two main modes: (1) transmission, where the LED and photodiode are placed on opposite sides of the medium; or (2) reflection, where the LED and photodiode are placed on the same side of the medium [47]. Due to the many factors affecting blood flow, including cardiac, neural, and respiratory factors, it is possible to look at many physiological parameters. PPG is currently used to measure several different aspects of heart health, including blood oxygen saturation (SpO2), blood pressure, heart rate, and respiratory rate [48–52].

Because PPG sensors only require two simple components, an LED and photodiode, they are commonly implemented in already-existing devices, such as watches and smartphones, at low cost. However, the fundamental mismatch between the shape and rigidity

of these devices with the skin makes them prone to heavy motion-artifact noise. Thus, flexible materials are essential for increasing the accuracy of optoelectronic sensors.
