# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Organic Photodiodes
The OPD array consists of eight OPD pixels, rows 1 and 3 contain the four red OLED pixels, and rows 2 and 4 contain the four NIR OLED pixels. Using a flexible flat cable, the printed ROA was connected to the control electronic board. With an additional circuit of the analog front end (AFE), OPDs can read the PPG signals corresponding to sequentially operated red and NIR OLEDs (Figure 12b). The fabricated ROA was tested to monitor oxygen saturation in the foreheads (Figure 12c) and forearms (Figure 12d) of human volunteers under varying oxygen concentrations. It was revealed that a mean error rate of 1.1% was confirmed compared to a commercial finger probe. After collecting data from all nine pixels, 2D contour maps of PPG signals from red and NIR OLEDs and changes in oxygen saturation level are created (Figure 12e). Through the 2D-mapping function, the ROA can provide the spatial oxygen saturation level and can be adopted to monitor the oxygen supply in tissue, wounds, and newly transplanted organs. Their proposed ROA device can be placed in various locations beyond those used for conventional sensing and is expected to be used in medical detection applications, such as real-time chronic health monitoring and post-operative recovery management. Figure 11: (**a**) Schematic device structure of the near-IR photoresponsive ultra-flexible organic photodetector. The devices are fabricated on polymeric substrates. After fabrication, the devices are passivated with a 1 μm-thick parylene layer. (**b**) Chemical structure of PIPCP. The regioregularity of the PIPCP polymer is highlighted with a red arrow, indicating the orientation of asymmetric pyridyl-[1-3]thiazolzole (PT) unit. (c) (i, left top image) Photograph of fingerprint-conformal near-IR photodetectors and inset indicates the position of the skin-conformal near-IR photodetector on the finger (ii, right top image) enlarged image of (i), and (iii, right bottom image) microscope image of top silver electrode and skin surface, indicating high skin-conformability. (bottom) 3D microscope image of ultra-flexible near-IR organic detectors on the surface of a human index fingertip. (**d**) Comparison of the output signal between the glass reference (black) and the skin-conformal near-IR photoplethysmogram sensor (purple) wrapped over a fingertip with a 99% blood oxygenation level. (**e**) Beats per minute (BPM) analyzed from measured output signals using fast Fourier transform (FFT) signal processing (64 BPM is the representative heart rate in the acceptable range for adults aged 15–30 years). Adapted with permission from [37], [PERSON], 2018. For accurate vital signal measurement, a high signal-to-noise (SNR) ratio should be determined. The deliberate design of PPG devices could encourage the development of highly sensitive skin-compatible PPG sensors. [PERSON] and coworkers [39] studied wearable PPG sensors in reflection mode by careful consideration of sensor geometry, such as shape, spacing, and optical barriers. They explored three sensor geometries, namely, a rectangular shape, a bracket shape, and a circular geometric shape (Figure 13a-d) with optical barriers to minimize light scattering (Figure 13e,f). As a reference device, the rectangular shape exhibited a low SNR value, as the OLED does not surround the OPD. Therefore, new bracket and circular geometry designs with reduced distances between the light-emitting and responsive devices were proposed to minimize the loss of reflected light. The bracket and circular designs enhance the magnitude of the PPG signal, resulting in the improvement of SNR values. The proposed geometry improved by as much as 48.6% in the red channel and 18.2% in the NIR channel, compared to the rectangular geometry. In addition, optical barriers were used to minimize light scattering between the OLED and OPD (Figure 13f). Based on the results of the geometry study, the developed PPG sensors with the new architecture were applied on the wrist at different locations (Figure 13g).