# Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors
## 4 Human Skin-Compatible PPG Sensors  
### Silicon Photodiodes  
Silicon photodiodes are widely available, cost-effective [79], and nontoxic [80] electronic devices, with light-responsive properties from the UV range (190 nm) to the infrared spectral region (1100 nm) [81]. In addition to their miniature size and high response speeds, they are suitable for both civilian and defense-related applications [82]. In this section, the basic considerations in the design of skin-compatible PPG sensors using Si-PDs are discussed. Additionally, their relative advantages and limitations based on device architectures are highlighted. [PERSON] et al. reported a battery-free epidermal oximeter with near-field communication (NFC) wireless communication [30], as shown in Figure 4a. An active device is composed of a silicon PIN photodetector with red and infrared LEDs connected for amplifying circuits and a coil for transmitting power and data by magnetic induction. The components are monolithically integrated on elastomeric substrates using serpentine-shaped interconnects with widths of 60-90 \(\upmu\)m and a thickness of 5 \(\upmu\)m. The integrated devices are located at the neutral plane by encapsulation with physical and electrical isolation. It was confirmed that these devices can maintain operational stability even after stretched condition and can be synchronized with a smartphone at 2 cm, owing to the low flexural rigidity of the metal interconnects. For oximeter applications, the devices are laminated on black textiles coated with a silicone elastomer. The systolic peak and dicrotic notch were observed with a sampling rate resolution of 25 Hz. To confirm whether the developed oximeter can be applied in diagnostic applications, an assessment for peripheral vascular diseases was conducted by placing the devices on a bicep (Figure 4b). Similar variations in oxygenated conditions before and after venous occlusion were obtained using a commercial oximeter and a wireless epidermal device, as shown in Figure 4c. In addition to oximeter applications, they also demonstrated UV dosimetry and a skin spectrometer containing UV-responsive dyes sandwiched between PDs and LEDs in four different colors (i.e., IR, red, orange, and yellow). Although they successfully demonstrated that an expanded functional device can detect UV-dose dynamics and jaundice, further improvements, such as device miniaturization and flexibility, are required for practical application in human-compatible biomedical electronic devices. For more precise control and accurate data acquisition with a miniaturized device architecture, they improved the inductance and electronic system by using a millimeter-scale stacked loop Cu antenna with a stable microcontroller instead of multivibrator circuits (Figure 5a) [31]. The integrated devices, including the NFC chip, were fabricated in a system with a diameter of approximately 10 mm, thickness of approximately 0.9 mm, and bending radius of 3 mm. The total weight and area size are 90% less than the previously reported device architecture. This device can transmit data over a 10 cm distance using  
Figure 4: **(a)** Image of the device that includes near-field communication (NFC), amplification, a stable oscillator, two LEDs, and a photodiode on a flexible substrate (black space). **(b)** Photograph of the device used on the forearm. The inset shows the image of the diode receiving light in the reflective mode. **(c)** Measurements obtained by an epidermal device recorded from adjacent regions of the forearm. The gray area shows changes in oxyhemoglobin and deoxyhemoglobin concentrations when vein occlusion for 120 s (\(\Delta\)HHb = \(\Delta\) O\({}_{2}\)Hb + \(\Delta\) HHb). Adapted with permission from [30] AAAS, 2019.  
a high-power, large-transmission antenna. Owing to their miniaturized form factor, the fabricated devices can be mounted on a fingernail or earlobe (Figure 5b).