# Study on the Anti-Interference Performance of Substrate-Free PEDOT:PSS ECG Electrodes
## 1 Introduction  
Bioelectrical monitoring often utilizes various types of electrodes, each with distinct characteristics and applications. Wet electrodes, which require the use of electrolyte gels, are commonly used due to their low impedance and reliable signal quality. However, the long-term use of wet electrodes can lead to skin irritation and performance degradation. Dry electrodes, which do not include gels, include several subtypes such as metal dryelectrodes, microneedle array dry electrodes, and substrate-free dry electrodes [1]. Each type offers unique advantages and challenges in terms of comfort, stability, and signal quality. Specifically, substrate-free dry electrodes stand out due to their ultra-thinness, lightweight, and skin compatibility, showing great potential in bioelectrical monitoring, especially for long-term applications such as electrocardiogram (ECG) monitoring [2; 3; 4; 5; 6]. Compared to traditional wet electrodes, substrate-free dry electrodes do not require electrolyte gels, which helps prevent skin irritation and long-term property degradation, which makes them more suitable for extended ECG monitoring sessions. Substrate-free poly(3,4-ethylenedioxythiophene)/poly(styrene-sulfonate) (PEDOT:PSS) electrodes can achieve long-term conformal contact with the skin, ensuring comfortability and a stable skin-electrode interface [7]. Several recent studies have demonstrated the superior performance of substrate-free electrodes in applications of heart rate/ECG monitoring [8; 9; 10; 11]. ECG monitoring encompasses interferences including baseline wander, power line interference, electromyographic (EMG) artifacts, and motion artifacts, among others [12; 13; 14]. Traditionally, low and stable skin-electrode impedance is considered to be crucial for obtaining high-quality ECG signals [15; 16; 17]. [PERSON] [18] discovered that skin-electrode impedance can directly reflect changes at the skin-electrode interface and ECG interference. [PERSON] [19] found that ECG interference during wet-electrode ECG monitoring primarily originates from the electrolyte-skin interface. Theoretically, a larger electrode area corresponds to a lower electrode impedance, suggesting that a larger electrode area can enhance signal quality. [PERSON] et al. [20] corroborated this finding in their study involving ECG monitoring with fabric electrodes. However, some studies have found that it is the stability of the impedance, rather than the absolute value, that affects ECG quality. [PERSON] [21] found that the stability of the impedance was more critical to obtaining a high-quality ECG signal than the absolute value of the impedance. [PERSON] et al. [22] found that the signal quality of biological surface potentials measured by microneedle array dry electrodes was independent of electrode size but related to the skin-electrode interface. Substrate-free PEDOT:PSS dry electrodes offer superior breathability and comfort, but their working principle for ECG monitoring is not entirely the same as that of wet electrodes and metal dry electrodes [7; 23]. Studies on the response of substrate-free electrodes to various disturbances during ECG monitoring and the relationship between impedance changes and ECG signal quality would be beneficial for further understanding the substrate-free dry electrode and the corresponding performance improvement. In this paper, the anti-interference performance of substrate-free PEDOT:PSS ECG electrodes was systematically studied. The influencing factors for ECG detection were classified into static influencing factors (including the contact state of skin-electrode interface, electrode size, and electrode shape) and dynamic influencing factors (including breathing frequency and the friction between electrode and cloth). The relevant skin-electrode impedance measurements and ECG detection experiments were designed for each of these factors, and the results were analyzed by using impedance, waveforms, spectrograms, and SNRs.