## 3 Results and Discussion  
### System Verification Experiment  
The work of impedance measurement is based on the arrangement of bipolar electrodes. We input a small interference signal to the test system. The signal is transmitted from one electrode to another through the medium between the two electrodes, and finally, the signal is read through the electrochemical workstation. Figure 1a shows the physical diagram of the flexible interdigitated electrode array obtained by the photolithography process, Figure 1b shows the physical diagram of the experimental test device, Figure 1c,d show the partial dimensions of the interdigital electrode. Prior to performing the immunoassay experiment, as shown in the electrode schematic diagram of Figure 1b, three test areas were selected from the \(3\times 3\) interdigital electrode array test area. Three test areas were evaluated using 20 different PBS solutions. Figure 2a,b show the Bode plots (the relationship between impedance and phase and frequency) recorded in 5 PBS solutions of different concentrations. It can be seen that in the high frequency region (10 kHz-100 kHz), the impedance exhibits a pure resistance characteristic, which is mainly related to the difference in ion concentration in PBS solution. The impedance value decreases with increasing concentration. This relationship can be expressed by the following equation:  
\[\rho=\text{K}*1/\text{n}. \tag{1}\]  
In the above formula, \(\rho\) is the resistivity of the liquid, \(n\) is the concentration of the liquid (usually proportional to the ion concentration), and \(K\) is constant when the temperature of the liquid is constant. The recorded EIS measurement data can be fitted by an equivalent circuit. The fitted line is represented by the solid line in Figure 2a,b. Here, the fitted curve and the measured data remain highly coincident. The fitting circuit is shown in the sub-graph of Figure 2a. This is a typical equivalent circuit [14], where the capacitor \(\text{C}_{1}\) is the solution capacitor, the resistor Rs is the test resistor, and the capacitor \(\text{C}_{2}\) is the polarization capacitor, and there is a constant phase element (CPE). By fitting all the data, the test resistance Rs is extracted. Figure 2c shows the trend of \(\text{R}_{\text{s}}\) with solution samples of 20 different concentrations. The data plotted are the average of 20 measurements in different wells, and standard deviations (STD) are plotted as error bars. The data shows that the sensor exhibits high uniformity between different wells in the experiment of solution concentration sensing. When the PBS concentration is in the range of 0.01x-0.0001x, the relationship between \(\text{R}_{\text{s}}\) and concentration is highly linear, and the error bars of the concentration fitting data \(\text{R}_{\text{s}}\) are very small thus can be almost ignored, so the linear detection interval of the sensor for the solution concentration is 0.01x-0.0001x (0.1 mM-0.001 mM). Figure 2d shows the comparison between this PBS concentration measurement experiment and the previous work. It can be seen, in the figure, that the oblique line of this test is almost equivalent to shifting to the upper left compared to the previous time, which indicates that the sensitivity is almost unchanged. In this case, the sensor can maintain the test sensitivity at lower concentrations. The sensitivity difference between this experiment and the previous work is 4% [10]. The reason for the above phenomenon is that the miniature interdigital electrode used in this electrode has an electrode gap of 20 microns. The electrode gap and the contact area between the electrode and the liquid are different. The above proves that this kind of electrode made with flexible PEN has good uniformity and sensitivity. In order to further prove that the electrodes remain of good sensing function after bending, we conducted a verification experiment of the detection function under different bending angles. See supplementary documents Figures S3 and S4.