# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 3 Photoplethysmography (PPG) Sensor and Its Application  
### Mechanism of PPG Sensor  
A PPG sensor is an optically obtained plethysmogram to monitor blood volume changes in the microvascular system. Conventional PPG sensors comprise two main components: light-emitting diodes (LEDs) and photodetectors (PDs). In the device architecture, red and near-infrared light from LEDs has generally been used as the long wavelengths are suitable for measuring deep-tissue blood flow [43]. The PDs detect volumetric changes in blood from cardiac pressure by absorbing light illumination from LEDs through the skin. The noninvasive and optical measurement system enables a PPG to monitor breathing, hypovolemia, and other circulatory conditions [44]. The PPG signal can be acquired in reflection or transmission mode [45]. In transmission mode, the light transmitted through the medium (tissue, bone, and/or blood vessels) is detected, while backscattered or reflected light is detected in the reflection mode. In the PPG sensors' transmission mode, the sensing locations are limited owing to confined transillumination [46]. Therefore, the location of PDs in transmission mode is limited by the thinness of the subject, such as on the fingers, earlobes, and neonatal feet. Conversely, in the reflection mode, the LEDs and PDs are integrated into the same plane, possibly located on various spots, such as the forehead, forearm, abdomen, and legs (Figure 2a). The PDs can detect changes in light intensity from transmitted and/or reflected light in response to volumetric changes in veins and capillaries. As shown in Figure 2b, the typical PPG waveform consists of direct current (DC) and alternating current (AC) components. The DC component is caused by the absorption of nonpulsating arterial blood and scattering in all tissues, while the AC component is attributed to the volumetric change of arterial blood between systolic and diastolic phases in cardiac cycles [47; 48]. The height of the AC signal fluctuates over time due to arterial blood pulsed by the heartbeat. Figure 1: PRISMA flowchart of the identification and selection of studies. Pulse oximetry is a revolutionary method for evaluating oxygen saturation levels (SpO\({}_{2}\)) using PPG devices. In a theoretical estimation, the oxygen saturation of the arterial blood can be expressed as the amount of oxygenated hemoglobin against the full quantity of hemoglobin:  
\[\%\;\text{SpO}_{2}=\frac{\text{HbO}_{2}}{\text{HbO}_{2}+\text{Hb}}\times 100\;\% \tag{1}\]  
where HbO\({}_{2}\) and Hb are the concentrations of oxy and deoxygenated hemoglobin, respectively. Typically, a pulse oximeter utilizes two different wavelengths of light. One is red and the other is infrared with wavelengths of 660 and 940 nm, respectively. The absorption intensity of light differs significantly depending on the oxygenated condition of hemoglobin. The iron-containing hemoglobin in the red blood cells can bind and transport oxygen during internal respiration. As shown in Figure 2c, the absorption spectra of hemoglobin varied under oxy/deoxygenated conditions. Significantly lower absorption in red (660 nm) light and slightly higher absorption in infrared (940 nm) are observed for oxyhemoglobin compared to deoxyhemoglobin [49]. The red/infrared modulation ratio (\(R\)), double-ratio of pulsatile (AC), and nonpulsatile (DC) components in red and infrared light can be used to calibrate the SpO\({}_{2}\) level as follows:  
\[R=\frac{\ln\frac{\frac{\text{I}_{\text{max,LC}}(\text{A}_{\text{red}})}{\text {I}_{\text{max,DC}}(\text{A}_{\text{red}})}}{\ln\frac{\text{I}_{\text{max,LC}}( \text{A}_{\text{IR}})}{\text{I}_{\text{max,DC}}(\text{A}_{\text{IR}})}}} \tag{2}\]  
where A is absorbance. The calculated values of the modulation ratio R correspond to the SpO\({}_{2}\) level.