# Study on the Anti-Interference Performance of Substrate-Free PEDOT:PSS ECG Electrodes
## 3 Results and Discussion  
### Effect of Static Factors on Impedance and ECG  
#### 3.1.1 Influence of the Contact State  
In the conformal contact state, where the electrode fully conforms to the skin, the electrode-skin interface exhibits stable impedance that does not change when subjected to interference. At the conformal state, the equivalent capacitance (\(C_{\text{es}}\)) in the equivalent circuit diagram, as described by Equation (2), is maximized due to the full effective contact area (S) and small spacing (d) between the conductive surfaces.  
In contrast to the conformal state, the skin-electrode interface in a partial contact state experiences a reduced contact area and increased gap distance. Consequently, according to Equation (2), the equivalent capacitance \(C_{\text{es}}\) decreases as the capacitive area S decreases and the spacing d increases compared to the conformal contact conditions. Simultaneously, the decrease in the contact area leads to an increase in the corresponding electron transfer resistance \(R_{\text{es}}\). Since impedance comprises both capacitive and resistive components, the reduction in capacitance and increase in resistance result in a higher overall impedance at the skin-electrode interface in the partial contact state.  
\[C_{\text{es}}=\varepsilon\frac{S}{d} \tag{2}\]  
Figure 5 shows the impedance and ECG results of PEDOT:PSS electrodes at different contact states. As shown in Figure 5a, the skin-electrode impedance was higher for partial contact than conformal contact for the full frequency range. As the frequency increased, the impedance difference between partial contact and conformal contact decreased. At a frequency of 1 Hz, the impedance difference between partial contact and conformal contact was 427.120 k\(\Omega\), and at a frequency of 1000 Hz, the impedance difference between partial contact and conformal contact was 1.340 k\(\Omega\), indicating that the contact status has more influence on low-frequency impedance, mainly owing to the decrease in \(C_{\text{es}}\). From Figure 5c, it could be seen that the partial contact electrode was more susceptible to interference, and the interference was mainly baseline drift  
in Figure 5c. As the frequency increased, the amplitude gradually decreased, but partial contact still exhibited a higher amplitude, with notable peaks at specific frequency points.  
The SNR of the partial contact electrode averaged at 1.27675 \(\pm\) 4.0299 dB, indicating a high variability and lower overall signal quality. This high standard error suggested inconsistent contact quality, likely resulting in significant fluctuations in the recorded signals. On the other hand, the conformal contact electrode exhibited a more stable and higher average SNR of 7.26365 \(\pm\) 1.4897 dB, reflecting better and more consistent electrode-skin contact, leading to improved signal quality and reliability.  
The contact status result shows that the ECG characteristics of the partial contact electrode were much less pronounced and contained more ECG interference, while the conformal contact electrode was insensitive to interference. It was found that the full conformal contact substrate-free PEDOT:PSS electrode could measure the ECG signal more stably.