## 4 Importance of Electrode Configurations for FECG Measurement
### Electrode Types  
In FHR monitoring, the hydrogel electrodes improve ion conductivity and provide impedance matching between the skin and electrodes, thus minimizing noises and improving signal quality. However, hydrogel electrodes degrade and collect hair and dirt in addition to the possibility of causing skin irritation whilst being used, so they must be frequently replaced [94; 95; 96; 97]. Moreover, the hydrogel layer on the electrodes tends to dry out over time so the long-term monitoring capability to monitor fetal heart rate will be compromised [98; 99]. In view of the shortcomings of hydrogel electrodes in FHR monitoring, the smart-based electrodes have attracted more attention [99; 100; 101]. Smart textiles exploit the universality of textiles which cover all types of woven, nonwoven, and knitted materials/fabrics. In recent years, the interest in smart textiles as tools to continuously monitor physiological signals, such as ECG, has increased due to the high comfort and portability, unlike traditional monitoring systems using hydrogel electrodes which could create skin irritation [101; 102]. Conventional textile manufacturing methods have been used to fabricate e-textile electrodes. An example shown in Figure 10a is the smart textile electrodes made through sewing conductive yarn to form electrodes. These electrodes are highly stretchable and flexible due to the nature of interlocking stitching patterns. Gold- or silver-coated yarn [103] as well as stainless yarn [104] are commonly used due to their high conductivity and the \begin{table}
\begin{tabular}{c c c} \hline \hline
**No. of Electrodes** & **Discussion** & **Reference** \\ \hline Group 1: range of electrodes & Advantages: For NIFECG monitoring, the electrodes are placed on the abdomen of & \\  & pregnant women. Hence, less are used for fetus monitoring, resulting in a simple & \\  & procedure without any discomfort to pregnant women. & [15; 81; 82; 83; 84; 85; 86; 87; 88; 89] \\  & Disadvantages: The data recorded does not provide detailed information about the & \\  & growth of the fetus. & \\ \hline Group 2, comprises a range & Advantages: It is evidenced that the more electrodes placed on the abdomen of & \\  & pregnant women, the more comprehensive information can be gained about the & \\  & fetus, thus helping in analyzing the well-being of the fetus efficiently and effectively. & \\  & 20 electrodes. & Disadvantages: Discomfort among the pregnant women due to several electrode & \\  & placements on the abdomen of pregnant women. & \\ \hline Group 3, comprises a range & Advantages: Provides the detailed information about the growth of the fetus. & \\  & Disadvantages: The deployment of 20 or more electrodes for FHR monitoring & \\  & involves a complex setup procedure in addition to being expensive. & [91; 92] \\  & Furthermore, results in skin irritation and severe discomfort to pregnant women due & \\  & to gel used in setting up the electrodes to establish effective electrode configuration. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Different groups of electrode placement for NIFECG. fact that they are readily available in the market [39]. However, the drawback is to build up a firm contact between skin and electrode surface without applying too much compression force through a strap. In contrast, printing techniques (e.g., screen printing) have also been adapted to fabricate electrodes on textiles. The carbon-based silicone is commonly used as an interface between skin and conductive layer to enhance the skin impedance matching and to provide a solid/stable contact, thus improving the ECG signal quality and minimizing the noise. An example of screen-printed ECG electrodes is shown in Figure 10b, where carbon-loaded silicone is printed directly onto textile where it is needed to form the electrodes. These printed electrodes provide a higher degree of flexibility in terms of mass production, customization, impedance matching, and noise reduction. [PERSON] et al.