# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Organic Photodiodes  
PPG sensors using organic materials have attracted much attention owing to their multiple advantages, such as high flexibility, easy processing, low fabrication cost, and large fabrication volume. [PERSON] and coworkers reported a flexible PPG sensor consisting of organic absorbers responsive to green (532 nm) and red (626 nm) light [49]. The key technology uses an organic layer responsive to green wavelengths, instead of near-infrared wavelengths (800-1000 nm), as the difference in the light absorption properties of oxygenated and deoxygenated hemoglobin is similar in both wavelength regions. The bulk-heterojunction (BHJ) is composed of poly([4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl]) (PTB7) and PC71 BM as the donor: acceptor system. The fabricated organic photodetectors (OPDs) exhibit EQE of 38% and 47%, respectively, at 532 and 626 nm with a leakage current of approximately 1 nA cm\({}^{-2}\) at 2 V applied with reverse bias. Figure 9c shows the acquired PPG signals using all-organic optoelectronic sensors composed of rigid OLEDs and flexible OPDs. Although the magnitude of signals is reduced compared to conventional inorganic devices, it was sufficient to resolve the PPG waveforms and calculate blood oxygenation levels in arterial blood with an error of 1% for heart rate and 2% for oxygen saturation. It is suggested that flexible wearable devices using organic materials have potential applications as medical devices owing to their large area scalability and flexibility. For more skin conformability, the rigidity of devices should be further reduced by controlling the overall device thickness and by designing compatible device architectures. [PERSON] and coworkers [36] developed ultra-flexible three-color (i.e., red, green, and blue) polymer light-emitting diodes (PLEDs) with the integration of organic photodetectors (OPDs) in the application of PPG sensors (Figure 10a). For a skin-compatible design, the devices were fabricated on an ultra-thin substrate composed of 1 um-thick parylene as the supporting layer and 500 nm-thick polyimide as the planarization layer on the glass substrate. After being peeled from the glass substrate, the total thickness of the PLEDs and OPDs is less than 3 um, which is thinner than the epidermal layer of human skin. Therefore, the device could be flexible with a bending radius of 100 um owing to the thin form factor and low stiffness, while the device's operation under mechanical deformation is negligibly affected as the active component is located at the neutral plane. As shown in Figure 10b,c, each PLED exhibits remarkable color expression with high EQE and luminance as high as 13.9% and 4900 cd/m\({}^{2}\) (for green PLED, \(\lambda_{\text{peak}}=517\) nm) and operational stability under repetitive mechanical deformation. In addition to PLEDs, OPDs with an active layer of poly (3-hexylthiophene)(P3 HT):(6,6)-phenyl-C61-butyric acid methyl ester (PCBM) exhibit prominent spectral responsivity corresponding to the electroluminescence spectra of the fabricated PLEDs. The PLEDs and OPD were integrated to create a skin-compatible pulse oximeter. The devices were wrapped over a finger to measure pulse and blood oxygen levels in reflective mode, as shown in Figure 10d. By changing the oxygen saturation  
Figure 9: **(a)** Photograph of the flexible and transparent GQD PD attached to a bracelet-shaped wrist that can monitor heart rate. (b) Schematic illustration of the assembly of graphene and QDs on a flexible substrate. (c) Normalized PPG signal when the PPG sensor operates in transmission and reflection modes. With the high sensitivity and mechanical flexibility of the GQD PD, the health patch can operate accurately for long periods in transmission and reflection modes.