# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 6 Wearable Devices for Heart Rate Monitoring
### PPG-Based Devices  
Owing to its simplicity, reflectance-type PPG is the most commonly used technique for monitoring different cardiovascular bioparameters--HR and blood pressure among others [35; 71] through pulse oximeters embedded in wearable devices [49; 122]. Modern wearable PPG-based options include smartphones, wristbands, watches, scales, shirts, rings, bracelets and eyeglasses (see Figure 19) [6]. Commercialized PPG-based wearable measurement systems include Withings Pulse O2, Fitbit Zip, Garmin vivofit, Moto 360, Huawei wear [123; 124; 125; 126; 127; 128]. Healthcare wearable devices based on PPG are Everion (placed on the arm) [129], Empatica E4 (placed on the wrist) [130], ViSi Mobile (placed on the finger) [131]. Figure 18: The flowchart of JOSS final steps in peak tracking and selection. Wrist-type wearable devices fabricated in a laboratory setting are presented in Table 2, while photos of the manufactured PPG wearable devices are shown in Figure 20. In [72], a heart rate detection algorithm performed via a microcontroller in real time was used with an average computational time of 141 ms per window. In [71], three additional thermoreceptor sensors designed to detect contact pressure between the device and the skin and also skin temperature were attached on the back surface of the wearable device. The purpose of the sensors is to detect the onset of loss of conformity between the wearable device and the skin, flagging the occurrence of the MAs. Table 3 presents commercially available wearable devices, including brands such as Apple, Fitbit, Garmin and others, highlighting their technical details, such as sensors, MCUs, dimensions and measurement capabilities. The table lists the advanced health monitoring features of these devices, e.g., the monitoring of heart rate, sleep tracking and stress detection, among others. However, it should be taken into account that the parameter the device is designed to measure is not always represented as an actual output. \begin{table}
\begin{tabular}{l l} \hline \hline
**Source** & **Specifications** \\ \hline \multirow{7}{*}{[71]} & **PPG sensor—Reflective, Maxim MAX30101 + photodetector + 525 nm LED, sample rate \\  & 125 Hz; \\  & **MIM sensor—STMicroelectronics LSM9 DS1, sample rate 125 Hz; \\  & **Microcontroller—MCU: STMicroelectronics STM32L476;** \\  & **Analog-digital conversion—Analog Devices 7606 CE;** \\  & **Case dimensions—4.5 nm \(\times\) 4.0 mm, 1.5 mm (resin, 3D printed);** \\  & **Circuit system—digital + analog for signal amplification and filtering;** \\  & **Signal transmission—wireless (2.4 GHz, Ashining: AS69-T20)** \\ \hline \multirow{7}{*}{[72]} & **PPG sensor—Reflective, SFH 7070, Osram opto-semiconductors, Regensburg, Germany; IMU sensor—LSM6 DSMR, STMicroelectronics, Geneva, Switzerland;** \\  & **Analog-digital conversion—analog frontend MAX86141 (Maxim integrated, San Jose, CA, USA);** \\  & **Microcontroller—STM32F413 CGU6, STMicroelectronics, Geneva, Swiss, clock rate 25 MHz;** \\  & **User interface--switch, 1.3 inch liquid crystal display (LCD, KWH013 ST03-F01, FORMIKE, Shenzhen, China)** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Wearable PPG devices manufactured in a laboratory setting. Figure 19: Types of PPG wearable devices and the monitored parameters [6]. The output parameters are often not calculated on the device itself, but rather using Android applications that analyze the measured data.