# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Organic Photodiodes
For  
Figure 12: (**a**) Image of the reflectance oximeter array (ROA) applied to the human forearm. (**b**) Schematic of the reflectance oximeter system. (**c**) Setup for changing the human subject’s oxygen saturation. The subject breathes through a face mask, and the oxygen content is changed using an altitude simulator. SpO2 was measured with a conventional probe on the finger and a reflectance oximeter on the forehead. (**d**) Setup to measure oxygen saturation changes in the forearm using ROA. The pressure cuff controls the blood supply to the forearm. The ROA provides 3 \(\times\) 3 oximeter pixels in a 4 \(\times\) 4 device. (**e**) Two-dimensional contour maps showing red, NIR, and ASO2; normal condition (t = 0 s), ischemia condition (t = 60, 120 s), and after releasing the pressure cuff (t = 180, 240, 300 s). Adapted with permission from [38], AAAS, 2016.  
multichannel data acquisition, motion artifacts (MA) should be reduced to evaluate the heart rate (HR) and pulse oximetry results. By implementing two algorithms, template matching (TM) and inverse variance weight (IVW) with an ideal PPG signal, the HR and oximetry data were accurately evaluated. The pulsating PPG signal above the wrist was the weakest, while the ulnar and radial artery provided the clearest PPG signal, as shown in Figure 13h. The study suggested that the proper design of the effective area, the spacing between the light emission and detection devices, and proper adjustment of optical barriers to minimize light scattering could improve the SNR values. The use of inverse-variance weighting and a template matching algorithm could also improve the accurate detection of vital signals. Its high-power consumption makes it difficult to use as a standalone continuous monitoring system that can be integrated easily into everyday life. A low-power-consuming health monitoring system that can be worn all day in real-time would help in the diagnosis and prognosis of cardiovascular diseases. [PERSON] et al. (2018) reported on epidermal and flexible near-infrared (NIR) PPG sensors by combining low-power, high-sensitivity organic phototransistors (OPTs), and high-efficiency inorganic light-emitting diodes. NIR has the advantage of having deeper penetration into the skin due to low light attenuation. Therefore, larger pulsatile PPG signals can be collected from arterioles in the deeper layers of the skin. In this study, they demonstrated skin-compatible NIR-responsive OPTs (Figure 14a-c). For device fabrication, a 1.8 \(\upmu\)m-thick polyimide (PI) layer is formed on the Si wafer by spin-coating and acts as a polymeric thin substrate after delamination from the silicon wafer. It is noted that a bilayer of SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) was utilized to achieve a low voltage (<3 V) device operation. SiO\({}_{2}\) is sputtered over the Al\({}_{2}\)O\({}_{3}\) layer to form a better interface profile on the surface. The devices were finally encapsulated with 200 nm-thick polytetrafluoroethylene (PTFE) and 1.6 \(\upmu\)m-thick parylene C as a sufficient barrier to the diffusion of oxygen and water molecules in direct contact with human skin. In conclusion, they designed a bandage-shaped, NIR responsive epidermal Hybrid PPG  
Figure 13: (**a**) Two OLEDs are arranged on both sides of the OPD in a rectangular geometry, (**b**) in the geometry of brackets, and (**c**) in a circular geometry. (**d**) PPG signal magnitudes depending on OLED and OPD shape conditions. In R2 (rectangular), B2 (brackets), and C2 (circular), 2 implies the distance between the emitter and detector is 2 mm. Red bars represent the data for red OLEDs and peach-colored bars represent data for NIR OLEDs. The error bars are represented by data obtained after three repeated experiments. (**e**) Illustration of an oximeter sensor in reflection mode with an optical barrier (black space) in the space between the emitter and detector. (**f**) The PPG signal magnitudes with or without an optical barrier in the red and NIR channels.