# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 6 Wearable Devices for Heart Rate Monitoring
### PPG-Based Devices
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**Device** & **Sensors** & **MCU** & **Dimensions (mm)** & **Apps** \\ \hline Apple Watch series 9 [132] & Electric Heat; PPG; Temp; Compass; Altimeter; ACC; Gyro; Ambient light; & S9-SiP-64-bit & 45 \(\times\) 38 \(\times\) 10.7 & ECG; Cycle Tracking with retrospective ovulation estimates; Heart Rate; High and low HR notifications; Irregular rhythm notifications; Medications; Mindfulness; Noise; Sleep, including sleep stages \\ \hline Fitbit charge 5 [133] & 3-axis ACC; PPG; vibration motor;relative SpO2; ECG; EDA & Arm 32-bit Cortex M-4 & 36.7 \(\times\) 22.7 \(\times\) 11.2 & ECG; HR Monitoring: Continuously tracks HR; including irregular heart rhythm notifications and high and low HR notifications; Stress Detection: EDA-measure stress; Sleep Tracking; SpO2; Skin Temp; Health Metrics Dashboard; Exercise Tracking \\ \hline Garmin fenix 6 pro [134] & PPG; BAROMETRIC ADTIMETER; GPS; COMPASS; Gyro; ACC; Term & NXP Kinetis & 47 \(\times\) 47 \(\times\) 14.7 & Wrist-based HR and Pulse Ox; Advanced Sleep Monitoring; Stress Tracking; Body Battery Energy Monitoring; Respiration Tracking; Hydration Tracking; Women’s Health Tracking; Performance Metrics; \\ \hline Oura Ring gen3 [135] & IR, Green, Red LEDs; ACCs; NTC; Gyro & PSoC 6 BLE & - & Nighttime resting HR; HR variability; skin Temp; respiratory rate; Detailed sleep analysis; 24/7 HR tracking (Daytime, Nighttime, Workout); Integration with third-party health and wellness apps like Strava; Advanced Temp monitoring; SpO2; Cycle Insights; Pregnancy Insights; Daytime Stress; Resilience; Dynamic activity goals with Automatic Activity Detection; Weekly, monthly, quarterly, yearly, and anniversary reports \\ \hline Whoop Band 4.0 [136] & ACC and Gyro; PPG; SpO2; Temp & Nordic nRF52840 SoC & 43 \(\times\) 28 \(\times\) 10 & Live HR; skin Temp; blood oxygen saturation; resting heart rate; HR variability; respiratory rate \\ \hline Polar H10 Strip [137] & ECG; ACC & BLE & 34 \(\times\) 65 \(\times\) 10 & Full ECG chart; HR (HR), Average HR, Lowest HR, Peak HR; R - R; Calories \\ \hline Firstbeat Sport Sensor [138] & ECG; ACC & BLE & 54 \(\times\) 38 \(\times\) 7.7 & HR (HR), Average HR, Lowest HR, Peak HR; Time in Highest Zone; Excess Post-Exercise Oxygen Consumption (EPOC); Training Impulse (TRIMP), TRIMP/min; Training Effect (aerobic, anaerobic) \\ \hline Zephyr Bioharness 3 [139] & ECG; ACC; Temp; BLE & BLE & 28 (Diam) \(\times\) 7 & HR in beats per minute (BPM); Breathing Rate; Posture; Activity Level; Skin Temp; Peak Acceleration; Breathing Wave Amplitude; ECG Amplitude and Noise; HR RR Values \\ \hline \end{tabular}
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Table 3: Commercially available wearable devices, highlighting some technical details, e.g., the sensors, microcontroller units (MCU), dimensions, and key health monitoring features. The abbreviations used in the table can be found in the abbreviations section. Even though the implementation of clinical wearable devices mentioned above was conducted in a controlled environment, thereby creating intrinsic biases in the results when compared to the real world implementation [34], the major limitation of the PPG-based systems is poor signal quality due to the shallow penetration of light [140] for people with varying skin-tones [141; 142; 143], body-mass index [144] and skin temperature [145]. The effect of skin tones on the measured blood volume changes using a PPG-based system is shown in Figure 21. As the skin becomes darker, scatter of beat-to-beat amplitude values increases, rendering the PPG system inaccurate. In comparison, the effect of skin color on EBI systems is minimal. Lastly, the manufactured PPG wearable devices have been tested on a sample size exceeding one hundred only in a handful of studies [40; 146]. Hence, until the sample size is increased, the reliability of these studies will remain questionable [7].