# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 7 Discussion and Concluding Remarks  
This review has highlighted the state-of-the-art and challenges associated with wearable devices for heart rate (HR) estimation using photoplethysmography (PPG) and electrical bioimpedance (EBI) technologies. The key findings of the review are as follows:  
* **PPG vs EBI. ** PPG-based devices are a comparatively mature technology with some commercial brands, while very few attempts have been made to design a wearable device for HR estimation based on EBI. However, compared to the PPG technology, devices based on EBI have the potential to provide a more accurate HR estimation due to the fact that skin tone affects properties of light absorption and reflection diminish \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Source** & **Measurement Mode** & **Accuracy** & **Range (\(\Omega\))** & **Power Active** & **Power Sleep** & **Size (cm)** & **Sense Time (s)** \\ \hline
[13] & MF & 1.5 \% & 100-1000 & 53 mW & 15.7 \(\mu\)W & 3 \(\times\) 1.8 \(\times\) 0.6 & 0.05 \\
[165] & MF & 0.5 \(\Omega\) & R \(<\) 54.0 \(\times\) X\({}_{c}\)\(<\) 5 & 14.4 mW & 0.9 mW & 4.8 \(\times\) 3 \(\times\) 2 & - \\
[166] & SF & 0.1 \% & 0–120 & 750 \(\mu\)A at 2.8 V & - & - \\
[155] & MF & 4.5 \% & 0–10,000 & 3 mA at 1.8 V & 8 \(\mu\)A at 1.8 V & 4 \(\times\) 4.6 \(\times\) 1.3 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of EBI wearable systems [13]. SF—single frequency, MF—multi-frequency. Figure 25: Designed EBI wearable sensor node showing its system architecture, front-end, hardware and the finished product [13]. ing the reliability of PPG-based devices. On the other hand, wearable devices based on EBI face challenges related to power consumption and signal noise originating from a variety of sources including electromagnetic interference from electric circuitry, electrode size and material, levels of skin hydration and quality of contact between skin and surface of electrodes. Advances in flexible, self-adhesive materials have shown potential in enhancing sensor-skin contact, thereby improving signal quality. However, the integration of flexible substrates with rigid electronic components poses significant challenges, often leading to connection failures. Therefore, ongoing research into material science and engineering is vital to overcoming these hurdles. All in all, EBI-based devices have the potential to become mainstream once the electrode properties are carefully tailored to the general user. * **Design of wearable devices. ** The development of low-power consumption strategies, such as those employing multi-frequency EBI measurements and modifications in hardware design, for example, demodulation and current driver blocks, have been essential in extending the operational life of wearable devices. Despite these advancements, achieving a balance between power efficiency and high signal-to-noise ratio remains a critical area for further research and development. Until a breakthrough in power source development is achieved, the key question concerning wearable device progress is energy efficiency. The energy demand is primarily related to the computation complexity--a trade-off with edge computing, i.e., the speed of data representation and battery duration has to be considered. Solutions exist, in the form of analog edge computing-based machine learning [167], or relying on an analog circuit design [168]. * **Motion artifact removal. ** Accurate HR estimation can easily be sabotaged by motion artifacts (MA). Various noise filtering and MA removal techniques have been employed to mitigate these issues, including band-pass filters and advanced algorithms that utilize supplementary signals for more precise correction. The algorithms most commonly employed in the literature include the ones based on signal decomposition--wavelet transform, empirical mode decomposition and Fourier decomposition. Then the user discards the components that are not related to the signal itself.