# Graphene-Based Flexible Electrode for Electrocardiogram Signal Monitoring
## 1 Introduction  
Cardiovascular diseases, including ischemic heart disease and stroke, are the top global causes of death in the world [1]. Although many different factors contribute to cardiovascular diseases, both aging and unbalanced medical resources lower the threshold for the manifestation of cardiovascular diseases [2]. Populations are becoming older in most developed regions and poor medical conditions in some developing regions [3] are contributing to a further increase in the incidence of cardiovascular disease, making it a global, universal problem. In the face of one of humanity's leading health threats and people's increasing concern for cardiovascular health, long-term electrocardiogram (ECG) signal monitoring is particularly important for early diagnosis and prevention of cardiovascular diseases [4]. With this comes the need for flexible electrodes with high performance and suitable for long-term ECG monitoring. The broadly used commercial Ag/AgCl gel electrode plays an important role in long-term ECG monitoring as it is low-cost, has achieved large-scale manufacturing, andconforms well to most people's skin [5]. However, its shortcomings have also been exposed over time [6,7]. For example, the gel in Ag/AgCl electrode is airtight and contains ions, which leads to the accumulation of sweat and might cause irritation during long-term ECG monitoring. Besides, dehydration of the gel during long-term wearing reduces its flexibility and increases electrode-skin contact impedance, which reduces comfort and affects the quality of ECG monitoring. The rapid development of wearable electronics [8, 9, 10] has advanced ECG electrodes while creating the development conditions for both high-quality and long-term monitoring of ECG signals [11, 12]. For high-quality monitoring, the electrode should have low skin-electrode contact impedance [13] and a high signal-to-noise ratio [14]. For long-term monitoring, the electrode should be flexible [15], biocompatible [16], stable [17], and comfortable to wear [18]. As a two-dimensional material with excellent properties, graphene has many physical and chemical advantages, including high mobility up to 10,000 cm\({}^{-2}\cdot\)s\({}^{-1}\) at room temperature, high thermal conductivity over 5000 W\(\cdot\)mK\({}^{-1}\) for a single-layer sheet, good intrinsic mechanical properties with 42 N\(\cdot\)m\({}^{-1}\) breaking strength and 1.0 TPa Young's modulus (making it one of the strongest materials ever tested), and good chemical inertness due to its covalent bond structure [19, 20, 21]. These excellent properties enable graphene to have a bright performance in biomedical applications in recent years, especially in wearable biomedical electronics [22, 23]. As one of the most prominent biomedical applications of graphene, when used as flexible electrode material graphene can contact human skin with high conformability [24, 25], which is beneficial to reduce contact impedance between the electrode and skin [26], improve the signal-to-noise ratio [27], and ensure high-quality signal acquisition [28]. Besides, graphene shows stability and biocompatibility for on-skin applications [29]. Combining the excellent properties of graphene, it is an ideal material for flexible ECG electrodes for long-term, high-quality ECG signal monitoring. When used for ECG signal monitoring, graphene electrodes can obtain ECG signals in different manners, including skin surface contact, implantable contact, and indirect contact [8]. These monitoring methods all obtain the ECG signals by recording the small voltage changes produced by the depolarization and repolarization of the human heart [11]. Besides, in the case of graphene electrodes, the process of monitoring ECG signals involves converting the ionic current generated by the heart into an electric current in the electrode. Graphene ECG electrodes act as a transducer during ECG monitoring [12].