# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 5 Conclusions  
The skin-compatible PPG sensors have enabled significant advances in biomedical and health self-care platforms. In this paper, various types of skin-compatible PPG sensors were systematically studied. Different types of semiconducting materials, device architectures, designs, and strategies for securing skin compatibility with high SNR values  
Figure 16: (**a**) Schematic of the device structure fOPD and the chemical structures of the materials used. (**b**) Photographs of fOPD integrated into a medical bandage with red and green LEDs for PPG sensing. (**c**) PPG signals measured from the fOPD in reflection and transmission modes. Adapted with permission from [42], ACS Publications, 2020.  
Figure 15: (**a**) Sectional view of green OLED, OPD, and red OLED in a complete OPO sensor. (**b**) Image of the proposed OPO sensor with an enlarged cross-sectional view to depict the device arrangement and light-receiving process through the skin. The picture of an OPO sensor in operation is also shown. Note that the Figure 8-shaped OPD wraps around red and green OLEDs in operation. (**c**) PPG signals, SpO2 values, and heart rate obtained from various body parts. (**d**) Comparison of the power consumption of R and G light sources of oximetry sensors: \(\uparrow\), discrete optical elements integrated on a PCB substrate with the edge-to-edge distance between elements being 2 mm (=PO1); \(\updownarrow\), a commercially available reflective pulse oximetry head (SFH7050, Osram Sylvania Inc.) (=PO2); \(\updownarrow\)\(\updownarrow\), OPO sensor when rectangular OLEDs and OPDs are arranged side by side with an edge-to-edge distance of 2 mm (=PO3); \(\updownarrow\), the OPO sensor proposed in this work (=PO0). The layer configurations of OLEDs and OPD in the OPO sensor used in \(\updownarrow\) are identical to those used in this work. Adapted with permission from [41], AAAS, 2018.  
were highlighted. However, challenges for future practical applications remain. From the device performance perspective, accurate and stable PPG signals without motion artifacts should be obtained. Therefore, the mechanical durability and skin conformability of skin-compatible detectors are necessary without compromising electrical performance. In addition to the accurate signal acquisition, the introduction of array structures with a systematic algorithm for the raw signal calculation can help in the exact evaluation of human body conditions. For further studies, the detection of specific information with significant hemoglobin derivatives can provide early notice of exposure to specific drugs or environmental toxins with a suitable photodetector measuring at various wavelengths. We believe that real-time healthcare monitoring systems that are compatible with human skin will be helpful in research and development through innovative ideas in current wearable PPG sensors.  
[PERSON], [PERSON], and [PERSON] conceived the review; [PERSON], [PERSON], [PERSON], and [PERSON] wrote the manuscript; [PERSON] and [PERSON] revised the manuscript. All authors have read and agreed to the published version of the manuscript.  
National Research Foundation of Korea: NRF-2020R1F1A1073564; Ministry of Food and Drug Safety: Program No. 21153-431; National Research Foundation of Korea: 2009-0082580.  
Not applicable.  
The authors declare no conflict of interest.