# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Other Inorganic Photodiodes  
The GaAs is among the most frequently used III-V semiconductor materials. [PERSON] et al. [33] reported an attachable and flexible pulse sensor composed of four inorganic GaAs photodetectors (IPDs) and two red light-emitting diodes (LEDs). To fabricate a skin-attachable pulse sensor, a mesa-etched [83] GaAs active layer on a sacrificial layer was transfer-printed onto a 12.5 \(\upmu\)m-thick PI film substrate with 2 \(\upmu\)m-thick SU-8 encapsulation. The devices were fully encapsulated with a polydimethylsiloxane (PDMS, 50 \(\upmu\)m thick) layer to maintain operational stability under mechanical strain, and an additional layer of sticky silicone elastomer (Eoflex, 20 \(\upmu\)m thick) covered the bottom and top of the sensor for skin adhesion, as shown in Figure 7a. The completed pulse sensor can be attached to the skin with 0.3 N/cm\({}^{2}\) shear adhesion force, indicating a strong adhesion between the skin and inorganic pulse sensors with motion artifacts being minimized during device operation on the skin. The LEDs were placed next to the IPD in the same plane to measure the PPG signal in reflection mode (Figure 7b). Therefore, it could be located on the skin and/or body surfaces, such as a fingernail, fingertip, middle finger, and even a thick forearm. Although the amplitudes of the systolic and diastolic PPG signals were different depending on the device location (fingertip: 36.0 mVp-p, forearm: 2.61 mVp-p) due to the depth of the arterial vessel beneath the skin surface, it was confirmed that the standard and a clear waveform were obtained compared to those waveforms from a conventional rigid PPG sensor. Owing to the strong adhesion to the skin surface, the developed sensors could be adjusted to test HRV for running subjects (Figure 7c). After the experiments, the change in heart rates from resting to post-running was well captured, but the devices exhibited distorted waveforms after exercise due to motion artifacts caused by the external wiring. Figure 6: **(a)** Image of the PPG device based on GaAs and silicone material, including two LEDs and a photodetector. Inset, the left light is red (620 nm) and the right light is infrared (850 nm). **(b)** The image of device attached to the wrist and **(c)** The inset shows the stable operation of the device on fingertip. **(d)** The measured PPG signal while the subject’s forearm moves slowly is applicable for SpO\({}_{2}\) calculation. Adapted with permission from [32], [PERSON], 2017. In addition to wireless data acquisition platforms, the thickness (200 \(\upmu\)m) of LEDs should be minimized for further flexibility improvements. Quantum dots are an emerging material suitable for optoelectronic biomedical applications, including optogenetics, blood oximetry, and drug delivery [84; 85]. The tunability of optical and electronic performance is shown by the nanoscale control of particle sizes. [PERSON] et al. [34] suggested a stretchable PbS quantum dot (QD)-based photoelectric sensor with a buckling structure. A colloidal QD was embedded as an active layer on an elastic substrate, and the full device structure of the PbS QD photodetector was displayed, as shown in Figure 8a. For elasticity, the devices are initially fabricated on PEN substrates and peeled away to be transferred onto the elastomeric substrates. The PbS-based PDs are sensitive in the range of the visible and IR spectra (400-1100 nm) with light responsivity of external quantum efficiency (EQE) of 12% at a wavelength of 618 nm. Owing to the sticky and stretchable characteristics of the devices (Figure 8b), they could adhere tightly to the skin and nail surface. Not only PDs but also stretchable QD-LEDs are demonstrated in the same manner. The operational stability in the stretching test was secured under a mechanical strain of 40% for PDs and 70% for LEDs. Finally, skin-mounted PPG sensors composed of QD-LEDs and QD-PDs were demonstrated by wrapping them around the tip of a forefinger, as shown in Figure 8c.