# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Organic Photodiodes
The error bars are represented by data obtained after three repeated experiments. (**g**) Figure showing the three locations where the sensor is placed: on top of the wrist, ulnar artery, and radial artery. (**h**) The magnitude of the PPG signal measured in the wrist, ulnar, and radial arteries. The error bars are represented by data obtained after three repeated experiments. Adapted with permission from ([PERSON] et al., 2018), IEEE, 2019. (hPPG) sensor composed of an OPT and LED by embedding in 300 \(\upmu\)m-thick Polydimethylsiloxane (PDMS) with a thin (\(\approx\)25 \(\upmu\)m) elastic acrylic adhesive to the skin (Figure 14d). It was observed that the mean values of systolic blood pressure (SBP) and diastolic blood pressure (DBP) were consistent with those obtained from the conventional rigid type PPG sensor (cPPG)/ECG setup. This study suggests that low-power hybrid PPG sensors have a benefit in the diagnosis and prognosis of cardiovascular diseases for real-time and portable physiological monitoring systems. [PERSON] et al. [41] proposed a patch-type organic pulse oximeter (OPO) sensor with very low power consumption based on flexible OLEDs and OPDs. The device structure and materials of the green and red OLEDs with OPDs for the proposed OPO sensor are presented in Figure 15a. The peripheral oxygen saturation (SpO2) measurements require two spectral light sources (i.e., green and red). Therefore, OPDs are composed of 4,4\({}^{\prime}\)-cyclohexylidenebis [N, N-bis (4-methylphenyl)] benzenaminel (TAPC) and C70 of the mixed layer as an active component. The light absorption of OPDs occurs in all directions with a surrounding structure resembling the number 8, indicating reduced light wastage (Figure 15b). In a reflective measurement mode, the completed OPO sensors can be applied to various body parts, such as the finger, wrist, neck, and nose, and operate properly (Figure 15c). The power consumption of the green and red OLEDs was as low as 31 and 17 \(\upmu\)W, respectively. It is noted that the developed device has the lowest power usage, including OLED- and LED-based sensors with the same duty ratio, compared to previous reports (Figure 15d). The study demonstrates the potential of a low-power health-monitoring system that could be operated throughout the day with form factor benefits from using organic materials. As growing attention for wearable sensor platforms, textile-based sensors are of great significance to skin-compatible electronics. [PERSON] et al. [42] fabricated highly efficient OPDs on fiber (fOPD) using spirally-wrapped carbon nanotubes (CNT) as microelectrodes (Figure 16a). The feasibility of bandage-type fOPD was tested by incorporating red (638 nm) and green (525 nm) LEDs (Figure 16b). The repetitive systolic and diastolic peaks were observed in reflection and transmission modes with the same period (Figure 16c). The fab  
Figure 14: (**a**) Schematic of the device structure of the flexible NIR organic phototransistor (OPT). (**b**) Process diagram of fabricating a flexible hPPG sensor and applying to the finger. (**c**) The image of the flexible sensor in the peel-off state during the process. (**d**) Image of a finger attached with the epidermal NIR hybrid PPG (hPPG) sensor (scale bar, 5 mm). Adapted with permission from [40], [PERSON], 2017.  
ricated fOPD can maintain 80% of the electrical performance when it is bent with a bending radius of 1.75 mm, which shows the prospect of the fiber-based optoelectronic devices.