# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors
### Other Inorganic Photodiodes
As the patch-type devices can be mounted firmly on the skin, stable and accurate waveforms caused by a cardio-ventricular contraction can be obtained, compared to long-type PPG sensors without motion artifacts (Figure 8d). The patch-type PbS QD-based real-time biomedical electronic devices were successfully demonstrated. Given their harmfulness, the use of heavy metals in the application of skin-compatible sensors is rejected or proper passivation layers preferentially should be  
Figure 7: (a) Illustration of the attachable pulse sensor. The microsized inorganic photodetectors (IPDs; size: 760 \(\times\) 760 \(\upmu\)m) built on a GaAs substrate are fabricated on a polyimide film (PI; \(\sim\)12.5 \(\upmu\)m) with the support of an adhesive layer (SU-8; thickness: \(\sim\)2 \(\upmu\)m). The device was encapsulated with an Ecoflex layer (thickness: \(\sim\)300 \(\upmu\)m). (b) Schematic diagram of the principle of a pulse sensor utilizing reflection mode using IPD and LED. The LED light is reflected by blood and tissue, and the IPD receives the reflected light. (c) The PPG signal was used to measure the heart rate before and after exercise in the forearm. The heart rate during the rest period was \(\sim\)74 ppm and the heart rate after exercise was \(\sim\)126 ppm. Adapted with permission from [33], ACS Publications, 2017.  
developed. In addition, stronger light responsivity and thinner device architectures are required for further development. As a photoresponsive material, graphene offers a fast response owing to efficient optical carrier separation [86], enabling various types of optoelectronic applications. [PERSON] and co-workers [35] reported a transparent and flexible PPG sensor using graphene quantum dot (GQD) PDs. For the fabrication of skin-compatible PPG sensors, CVD-grown graphene on copper was transferred onto PEN substrates, and photoresponsive colloidal QD layers were deposited on the graphene layer (Figure 9a). In particular, GQD PDs have key features, such as broadband (300-2000 nm) wavelength sensitivity. The generated electron-hole pairs in the QD layer are separated and rapidly transferred into each layer by the graphene-QD built-in electric field. Charged carriers (holes) are trapped in the QD layer, and other separated carriers (electrons) migrate to the graphene. This leads to a measurable change in photoconductivity; thus, the responsiveness of these GQD PDs shows a high value of photoconductive gain, approximately \(10^{5}\) A/W. Their broader wavelength sensitivity allows the health patch to be operated in transmission mode even under ambient light owing to the deeper skin penetration of longer-wavelength light. Consequently, the integrated wearable device can continuously detect biometric signals over a long period with very low power consumption (140 uW, 16 uW on standby) (Figure 9b,c). For wireless applications, the GQD PDs were integrated with a printed circuit board (PCB), operating battery-free and with wireless data transmission using a smartphone. The noninvasive and skin-compatible GQD PDs can accurately measure vital signals on fingers, the forehead, feet, and chest with an improved degree of design freedom owing to the wide detection range of light. Figure 8: (**a**) Cross-sectional view of structure of the stretchable PbS QD photodetector excluding the elastomer substrate. (**b**) Optical image of the stretchable QD photodetector from 0% to 40% strain. (**c**) Image of working QD-LED and QD photodiode on the fingertip. (**d**) Records PPG pulses collected in real time under red QD-LED conditions. Adapted with permission from [34], ACS Publications, 2017.