## 2 EBI Measurements
### Measurement Method and Setup  
The forearm is a classical location of the wristwatch, which is highly suitable for a wearable biomedical signal monitoring device--targeted in this research. The electrode configurations for comparison were selected by the following criterion:  
1. Electrode system: bipolar or tetrapolar;
2. Electrode placement: perpendicular or distal relative to the forearm (radial and ulnar arteries). We reduced the original electrode dimensions by cutting it into two widths, i.e., 10 mm and 17 mm (Figure 2), to evaluate the effect of electrode-skin contact area size and distance between the electrodes. The initial reason for such a modification was to reduce the electrode width by removing its foam side areas (adhesive and nonconducting) to gain higher density on the restricted wrist area. The experiments were performed with two alternative electrode widths (Figure 2):  
1. Original sticky conductive gel area width (17 mm);
2. Width reduced to the diameter of the electrode metal snap (10 mm). Positioning of the electrodes with the 17 mm width on the wrist was performed based on the following criteria:  
1. We marked the radial artery location with a line on the wrist surface, and two electrodes were attached so that the marked line remained in between these electrodes;
2. The third and fourth electrodes were attached similarly regarding the location of the ulnar artery. In the case of 10 mm width electrodes, the locations of both arteries remained in between the electrodes, just with a wider gap. Electrode locations on the forearm in the cases of perpendicular and distal electrodes of both widths are visible in Figure 3a-d)). The distance between the electrodes was 7 mm in the case of narrower electrodes (10 mm) and 1 mm in the case of wider electrodes (17 mm). Simultaneously with EBI measurements, we acquired the PPG from the tip of the index finger. Figure 2: Electrode contact area sizes (10 mm width on the left and 17 mm width on the right). Ten electrode configurations were defined, of which eight were based on bipolar and two on tetrapolar electrode systems (Table 1). For measuring the EBI (\(Z(t)\)), the excitation signal with the frequency of 100 kHz and amplitude of 300 mV was used in all the cases. These values fall into the classical ranges for EBI signal acquisition, shown to provide optimal performance properties [39]. Three volunteers (healthy males in their thirties) were used during the measurements in the case of EBI measurements by using the bipolar electrode system. In the case of the tetrapolar electrode system (included for comparison, involving a single volunteer (a healthy male in their thirties)). The measurement protocol is listed subsequently:  
1. The volunteer sits comfortably on the chair while the left arm is resting on the armrest;
2. The locations of the radial and ulnar artery are determined by palpation;
3. Electrodes are attached to the skin surface in the desired configuration (concurrently to the PPG sensor on the index finger);
4. Acquisition of the signals (EBI and PPG) (three minutes considered for settling time of electrodes)--while three consecutive measurements were performed in each configuration. We applied all the ten electrode configurations in both placements: perpendicular and distal. The initial signal analysis was performed in LabView, firstly by using the Savitzky-Golay filter at side points of 5 and then by determining the \(\Delta Z(t)\) from the signal waveforms. \(\Delta Z(t)\) is an indicative variable, characterizing the modulation caused by the pulsating flow of blood in the measured signal of EBI (Figure 4), expressed as Equation (1) :  
\[Z(t)=Z_{0}(t)+\Delta Z(t),\]