# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Organic Photodiodes
Adapted with permission from [35], AAAS, 2019.  
from 99% to 90%, the amplitude of the light-responsive signal from the green PLED was observed to have decreased more than that of the red PLED (Figure 10e), indicating good agreement with previously reported results using organic oximeters on glass and thick plastic substrates [49]. Organic PPG sensors simultaneously targeting mechanical conformability and fast responsivity in the near-infrared (IR) region are a prerequisite for expanding the capabilities for skin-mounted applications. [PERSON] et al. reported an ultra-flexible skin conformal PPG sensor (Figure 11a) [37], utilizing a BHJ active layer of a polyindacenodithiiophene-pyridyl[1; 2; 3]thiadiazole-cyclopentadithiophene (PIPCP) polymer as an acceptor (Figure 11b) and PC61 BM as the donor, respectively. They were built on an ultra-thin (1 \(\upmu\)m-thick) parylene substrate with an inverted structure, and the devices were located in the neutral plane within 3 \(\upmu\)m of the overall device thickness, resulting in high skin-conformability between the ultra-flexible device and the skin surface (Figure 11c). The OPDs exhibit high response speed (>1 kHz) due to the favorable self-organization of the regioregular polymer chains. The implemented PPG sensor exhibited high on/off switching characteristics and excellent skin compatibility characteristics. As the devices were in neutral positions, the near-infrared responsivity was maintained under extremely deformed conditions. The invariant V\({}_{\text{OC}}\), FF, and normalized J\({}_{\text{SC}}\) values under 20-200% tensile strain indicate that the BHJ maintained its charge collection, transporting, and collection behavior under mechanical deformation. Using an OPD with a commercial rigid near-infrared (NIR) LED, they demonstrated that the ultra-thin organic photodetector can detect consistent and repetitive cardiovascular signals under the illumination of near-infrared (800 nm) light, which is comparable to the sensitivity of the glass references (Figure 11d). In addition, Fourier analysis in the frequency domain in Figure 11e indicates a representative heart rate (i.e., 64 BPM) in the acceptable range for an adult. Thus, additional work on the integration of highly flexible near-infrared organic LEDs and OPDs will allow the practical implementation of all organic-based conformal  
Figure 10: (**a**) Image of an ultra-flexible organic device used on the finger. (**b**) External quantum efficiency (EQE) of organic photodetector (OPD) (black line) according to the wavelength of light with polymer light-emitting diode (PLED) blue (blue line), green (green line), and red (red line). (**c**) Current density and EQE correlation characteristics in ultra-flexible PLED. (**d**) Working principle of the reflection mode pulse oximeter on the finger. (**e**) Output signal of OPD obtained using the ratio of oxyhemoglobin in blood. The green and red lines signify signals when the green and red PLEDs work, respectively. The left figure is the signal when SpO2 is 99%, and the right figure is the signal when SpO2 is 90%. Adapted with permission from, [36] AAAS, 2016. PPG sensors. As a proof-of-concept for a portable cardiovascular sensor, fast responsivity and mechanical conformability should be balanced for skin-conformable electronics. In addition to the development of unit device performance measuring the oxygenation level of a single spot in the transmission mode, two-dimensional (2D) mapping of vital conditions based on the reflectance mode enables the spatial monitoring of tissue and organ wounds. [PERSON] et al. [38] reported the reflectance oximeter array (ROA), a flexible organic electronic system realized by printing, and its integration with silicon circuits for oximetry. They fabricated the ROA with eight OPDs and eight OLEDs that irradiate near-infrared (725 nm) and red (612 nm) wavelengths of light (Figure 12a). ROA is made by stacking OPD and OLED arrays made on separate substrates.