# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 6 Wearable Devices for Heart Rate Monitoring
### Bioimpedance-Based Devices
In this device, EBI sensing is combined with a nine-axial inertial measurement unit (IMU) sensor. At its core, the device features the nRF5340 System-on-Chip (SoC), which is notable for its dual-core ARM Cortex-M33 architecture, offering robust performance for complex computational tasks. It also incorporates the MAX30001 bioimpedance chip for precise physiological parameter monitoring and the ICM-20994 nine-axial IMU chip, which includes accelerometer, gyroscope, and magnetometer sensors for detailed motion tracking. Power flexibility is ensured through a Li-ion battery, which can be recharged via USB, providing versatility in usage scenarios. For user interaction andfeedback, the device is equipped with LEDs, enhancing the user experience through visual signals and tactile feedback. Additional on-board storage is made possible with external flash memory and a microSD card, allowing for extensive data collection and retention. Lastly, a user button facilitates easy interaction with the device, making it user-friendly and accessible for a wide range of applications. Signals measured with the EBI technique are affected by three main factors:  
1. Electrode-skin interface (see Section 5.1.1). 2. Characteristics of the alternating current of excitation. The important parameters are the frequency [156] and amplitude of excitation current. 3. The measured bioimpedance signal is affected by the type of tissues and the physiological and physiochemical variations that occur within the tissues [156]. The main challenges related to validity of using EBI-based wearable solutions for cardiac parameter monitoring and impeding the translation of this powerful technology to practice can be divided in two main categories:  
1. The difficulty of acquiring a signal of high enough quality due to: 1.1. Patient-related issues--spontaneous movements of the patient, micro-movements that are not controllable, disorders of heart rhythm [156]. Movement with an amplitude above a certain threshold may cause MAs severely degrading reliability of the estimated bioparameters. 1.2. Inherently low signal-to-noise ratio of the technique [157] thereby establishing a reliable, long-lasting and firm contact interface between the electrodes and  
Figure 23: An in-house wrist-worn bioimpedance wearable device designed in EDI. (**a**) Custom 3D-printed body; (**b**) PCB from the same device. Figure 22: Block diagram of a typical bioimpedance device—(**a**) using analog demodulators [14]; (**b**) new architecture proposing hardware solutions for demodulation and current driver [14]. \(\Delta\)Z and \(Z_{0}\) denote time-dependent and static EBI components, respectively. the skin is of prime importance [18]. On a hardware/software level, signal pre-processing steps involving noise filtration with filters and algorithms is necessary. One solution of obtaining a signal with strength above a background noise is performing measurements of the systolic and diastolic impedance variations close to the ascending thoracic aorta [157]. This approach, however, is limited by the empirical nature and sensitivity to electrode positioning and calibration inherent in the method [158]. 2. Changes in the baseline impedance levels when direct comparisons to a healthy state are no longer valid due to physiologic and pathophysiologic conditions, such as pregnancy, obesity, gas or fluid pleural effusion, chronic congestive heart failure with pulmonary edema, or when there is severe aortic valve disease or modified mechanical properties of the arterial tree [157]. Another crucial factor to consider when designing a wearable device is choosing the power source. This is a major challenge in enabling a large-scale implementation of wearable EBI-based devices as these systems have higher power consumption requirements when compared to other wearable systems [13]. At present, most of these devices are powered with batteries which not only make the wearable design bulky but also necessitates regular battery replacement or periodic charging [159; 160].