# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
## 6 Wearable Devices for Heart Rate Monitoring
### Bioimpedance-Based Devices  
Sensing arterial blood flow via the bioimpedance is provided when electrodes are firmly attached to the skin and aligned with the underlying arteries [12]. On the other hand, other setups exist. For example, focused impedance measurements [147]. Compared to the bipolar electrode configuration, it is possible to capture and thus remove electrical impedance caused by the contact between the skin and the electrodes using the tetrapolar configuration [148]. Even though there are existing EBI wearable devices for measuring respiratory rate that are put on the chest, such as Sensium [149] and Equivital [150], the technology for wearable heart rate monitoring has not been developed. Despite the attempts of Jawbone to introduce a new activity tracker with a heart rate sensor [151], this project was discontinued presumably due to a unreliable performance caused by MAs. Nevertheless, some research efforts to design wearable EBI devices for bioparameter monitoring can be found. The main components of a wearable EBI measurement system are demodulation to extract the real and imaginary part of EBI modulus and phase components, analog/digital filtering to separate the static and time-dependent components of the EBI. These components are depicted in Figure 22a. A demodulation block can be implemented either with standard demodulators (as in Figure 22a) (cannot be used for a continuous monitoring), synchronous sampling [152] (guarantees very low power consumption at a high cost [14]) or gain and phase detection [153], but all these approaches require at least two channels, resulting in an increase in consumption, cost and size [14]. Another key component is instrumentation amplifier (IA). Its bandwidth has to be large, since typically the working frequency for EBI systems is in a range from tens of kilo-hertz to mega-hertz. High bandwidth implies an increase in power consumption and also in cost [14]. The same concept applies also to the current driver. In order to obtain small measurement errors, the output impedance has to be as high as possible--the higher the output impedance, the lower the dependence of the current amplitude on the load. Since the load is constituted by electrodes to tissue impedance, it can vary a lot with frequency, electrode size and type, contact quality, and so on. If the operational amplifier can be approximated as ideal, high output impedance of voltage-to-current converters (VI converters) can be achieved. However, this output impedance quickly decreases with the increase in frequency, because of the limited gain-bandwidth product (GBP) of the operational amplifier. Therefore, a very high GBP is needed also for a 50 kHz working frequency [154], resulting in high current consumption. To relax the requirements for the current driver output impedance, current sensing is sometimes used at the cost of two instrumentation amplifiers [14]. The authors of [14] proposed a new hardware architecture for EBI measurements as shown in Figure 22b. Here, a 50 kHz driving current with a square waveform is filtered into a sinusoidal waveform using a second-order low-pass filter. A VI converter is used to drive the current in the body. The demodulator is moved before the IA enabling IA to work on the baseband demodulated signal. This, in turn, dramatically reduces the bandwidth requirements reducing power consumption and cost. Analog filtering is used to separate and amplify the AC coupled component \(\Delta\)Z of the EBI before the analog-to-digital conversion (ADC). For relatively low working frequencies (\(f_{max}\leq\)100 kHz) a commercial integrated circuit chip AD5933 is one of the preferred options for impedance measurement [60]. An in-house EBI measurement device designed in the Institute of Electronics and Computer Science in Latvia is a wrist-worn wearable device [155] with a functionality to evaluate physiological parameters such as body composition, hydration levels, heart rate, and breathing (see in Figure 23).