# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 6 Wearable Devices for Heart Rate Monitoring
### Bioimpedance-Based Devices
This leads to an interruption in normal usage, loss of potentially critical data and environmental pollution due to battery disposal [161]. Hence, EBI devices are not optimized for autonomous long-term operation. One of the reasons is high power consumption of the circuitry in active mode. A potential remedy is to design the front-end of the EBI sensors specifically for low power consumption, as well as to implement power management strategies that are able to increase the energy efficiency of the whole system, resulting in longer battery lifetime [13]. The reduction of power consumption can be obtained by reducing the power duty-cycle [162] (the time it takes to perform a single measurement set) of the measurements, or through application specific integrated circuit (ASIC) design [163]. The authors of [13] achieved a battery life (250 mAh Li-ion) of almost 1 year when measuring EBI only in multi-frequency mode (sense time: 50 ms) and of almost 4 months when measuring EBI in both multi-frequency mode and also skin-electrode impedance (sense time: 250 ms), respectively (see Figure 24). The authors concluded that battery life is mainly limited by the battery's discharge model. Several attempts of optimization of power consumption are reported in the literature. In [164], power reduction and stability improvement had been achieved by decreasing the excitation current tenfold compared to the normally used value. However, this led to a decrease in the SNR affecting the accuracy of impedance calculation. The authors of [165] reported 30 h of EBI measuring monitor operation with rechargeable Li-ion battery and  
Figure 24: Design estimated and measured battery life using a 250 mAh h Li-ion battery for different sensing times with (**a**) only multi-frequency EBI measurements; (**b**) multi-frequency EBI measurements and skin–electrode (SE) contact impedance measurements [13]. a form factor of 4.8 cm \(\times\) 3 cm \(\times\) 2 cm. Power consumption was optimized by designing a narrow bandwidth low-power VI converter. In [166], an EBI system performing only single-frequency (50 kHz) measurements over loads of 0-120 \(\Omega\) achieved a total current consumption of about 750 \(\mu\)A at 2.8 V. However, the sustainability of this device including the power for processing, storing and streaming of the data was not reported; hence, the total operational life of the system is unknown. Table 4 lists the state-of-the-art in EBI sensing wearable devices. The downside of these devices is that they need battery replacement or frequent periodic charging. However, in [13], the authors claimed that their system can operate several months without charging and without compromising sensor performance (see Figure 24). The architecture of their sensor node is shown in Figure 25. It consists of a system on chip (SoC) that includes an ARM Cortex-M4F microcontroller and Bluetooth 5.0 RF front-end, bio-impedance sensor node, accelerometers for motion detection and a power management system supplied by a Li-ion battery (3.7 V). The voltage supply is \(\pm\)2.7 V [13].