Graphene-based electrodes for ECG signal monitoring: Fabrication methodologies, challenges and future directions

Abstract

Electrocardiogram (ECG) is the most common and simple technique to diagnose cardiovascular diseases. Cardiovascular diseases can be detected effectively if ECG signals are monitored for a long time, producing innovative clinical outcomes to diagnose and treat cardiovascular diseases. Due to skin irritation and degradation of signal quality with time, traditional wet electrodes are unsuitable for long-term ECG monitoring. Researchers are trying to fabricate flexible, wearable, highly conductive and lightweight ECG sensors, which can be applied for long-term monitoring of ECG signals and the detection of several cardiovascular diseases. Graphene is used for fabricating dry ECG electrodes because it exhibits robust mechanical flexibility, good environmental stability and excellent carrier mobility. This review paper presents the progress of various fabrication methods to make graphene-based ECG electrodes and provides the researcher’s clarification on recent advancements and direction in this domain. This paper focuses on a systematic review and comparative study of various fabrication methods of graphene-based ECG electrodes, such as screen printing, dip coating, drop casting, wet transfer, electrospinning, wet transfer and dry patterning, spin coating, spray coating, ink-jet printing etc.

Keywords:

• Electrocardiogram (ECG)
• dry electrodes
• graphene
• dip coating
• screen printing
• wet transfer

1. Introduction

There is a growing emphasis on personalized healthcare instead of the traditional hospital-centric approach in the contemporary healthcare landscape. Individuals place significant importance on self-monitoring their health conditions, and wearable devices have emerged as a prominent technology (Bauer et al., 2014). These devices have also found considerable utility in sports applications. Cardiovascular diseases, including cardiac arrhythmia and coronary heart disease (CHD), remain a major global health concern, contributing significantly to human mortality (Deaton et al., 2011). In 2019, these diseases accounted for approximately 17.9 million deaths, comprising 32% of all global fatalities. One challenging aspect of these conditions is their often asymptomatic nature, making regular follow-up evaluations insufficient for early detection (Dupre et al., 2009). To address this issue, electrocardiography (ECG) has proven to be the most common and straightforward technique for recording the heart’s electrical activity (Arquilla et al., 2020). It is a crucial tool for diagnosing cardiovascular diseases (Bong et al., 2020; Liu et al., 2016; Verweij et al., 2020). Prolonged monitoring of ECG signals (Hong et al., 2019; Vuorinen et al., 2019) has shown promise in effectively detecting cardiovascular diseases, offering innovative clinical outcomes that facilitate accurate diagnosis and treatment (Pullano et al., 2022).

Wearable ECG monitoring devices are receiving much importance as emerging research areas. These devices can capture bioelectric signals from a user’s body, which can be transmitted anywhere globally. These devices are reliable, friendlier to the skin, flexible, portable, and wearable (Kim et al., 2018; Lou et al., 2016; Yang et al., 2016). Research is still being continued to make them miniaturised. These real-time wearable ECG monitoring systems are available in the market in various forms, such as t-shirts, smart vests, smart watches, etc. Integrating wearable ECG devices with IoT systems (Brezulianu et al., 2019; Nurdin et al., 2016), these systems can be made remote and can be used to monitor ECG signals in the long term (Khan et al., 2014; Raykar & Shet, 2020). Portable and light weighted materials are used as sensors in these devices. Integrating functional materials with normal textiles, devices such as energy harvesters (e.g., piezoelectric materials (Lee et al., 2014) or photovoltaic cells (Salvado et al., 2012)), sensors, and antennas (Gorgutsa et al., 2014) can be developed, which are known as “smart textiles” or electronic textile(e-textile). Materials with thermal, mechanical, and electrical properties are embedded into normal clothing to achieve targeted functionalities in a particular application. If wireless transmission modules are integrated into the textiles, several physiological parameters can be transferred to the cloud (Lee & Chung, 2009) or any remote areas, ensuring its application to IoT-based technologies. These textiles have several advantages, such as permeability to moisture and air, easy assimilation to normal clothing and flexibility.

2. Electrodes for ECG detection

The electrodes are the major component of the ECG monitoring system. Flexible electrodes are highly preferable for long-term monitoring of ECG signals. They have received huge attention for their high performance and superior compatibility with skin surfaces for long-term ECG monitoring (Celik et al., 2016). Three categories of electrodes for measuring bipotential are wet, capacitive, and dry. The most widely used commercially available wet electrode for ECG signal monitoring is silver/silver chloride (Ag/AgCl) electrode (Noh et al., 2016; Xiao et al., 2017; Yoo et al., 2009). Since these electrodes use conductive gel for signal transduction, they are known as wet electrodes (Cui et al., 2022). The conductive gel also behaves like an ECG electrode’s electrolyte leading to high sensitivity (Deaton et al., 2011). The gel also establishes a stable contact between the skin and the electrodes, which reduces contact impedance and increases signal-to-noise ratio (Deaton et al., 2011). But there are a few disadvantages to using wet electrodes. First, if the electrodes are used for a long time, the electrolyte gel slowly suffers from dehydration, increasing contact impedance between the skin and the electrodes. High contact impedance degrades the quality of the ECG signal. Hence, they are not suitable for monitoring ECG signals for long periods. Additionally, the conductive gel is in contact with the skin for a long time, leading to skin irritation, discomfort, and even allergic reactions (Lee et al., 2014). Capacitive electrodes are more comfortable and reliable than other types since they do not come in direct contact with the skin. In this case, displacement currents only pass signals to the signal acquisition circuit, not charge transfer. Earlier researchers have tried to fabricate dry active capacitive electrodes (Fu et al., 2020). But, these types of electrodes face high instability at the interface between the skin and the electrode, making the acquired ECG signal prone to motion artifacts and noise (Van Lam et al., 2016). Hence, researchers are trying to develop dry electrodes without using conductive gels for wearable applications and long-term ECG monitoring (Meng et al., 2016; Terada et al., 2021). To achieve high-quality ECG signals during long-term monitoring, the electrode should possess properties such as a high signal-to-noise ratio (Nigusse et al., 2021), low electrode-skin impedance (Yeo et al., 2013), biocompatibility (Li et al., 2021), flexibility (Qiao et al., 2022), comfortability while wearing (Dong et al., 2019) and stability (Scalise & Cosoli, 2018).

Carbonaceous materials such as activated carbon (Noh et al., 2016), carbon black (CB) (Beach et al., 2018), graphene (Bansal & Gandhi, 2019), carbon nanotube (CNT) (Chung & Gray, 2019); intrinsically conducting polymer (Gualandi et al., 2018; Zhou et al., 2014), and metallic nanoparticles (Cho et al., 2007; Zhu et al., 2010), are used as sensing materials in flexible electrodes.

2.1. Graphene based electrodes

Carbon is known as the materia prima of all living bodies. Carbon exhibits many complex, unusual behaviours. As a result of their bonding flexibility, carbon-based materials can be used to construct a wide variety of structures with different physical properties. Different dimensions of the structure are also responsible for different physical properties. Graphite and diamond are the well-known 3-dimensional structures of carbon. Other than diamond and graphite, there are other allotropes of carbon, such as graphene (2-dimensional), carbon nanotubes (1-dimensional) and fullerenes (0-dimensional). Graphene is theoretically the most studied 2-dimensional allotrope of carbon. In graphene, carbon atoms are organised in a planar hexagonal honeycomb-like structure, and it is considered the initial step for all calculations on fullerenes, carbon nanotubes and graphite.

2.1.1. Properties and synthesis of graphene

Graphene has a multifunctional 2D-atomic crystal structure, showing many unique chemical and physical properties. Graphene has a large surface area of 2630 m²/g (Novoselov et al., 2005). It exhibits high electron mobility up to 2,50,000 cm²/Vs (Balandin et al., 2008), thermal conductivity in the order of 5000 W/Mk (Lee et al., 2008) and mobility at room temperature as high as 10,000 cm⁻².s⁻¹. Graphene also has excellent intrinsic mechanical properties. It is considered the strongest material ever tested since it has Young’s modulus of 1.0 TPa (Yan et al., 2012) and breaking strength of 42 N.m⁻¹. Graphene is a good conductor of electricity and is chemically inert since all the carbon atoms are covalently bonded with each other (Huang et al., 2011).

Temperature and synthesis conditions vary depending on the methods of synthesis of graphene. Mechanical exfoliation of graphene is a low-temperature process. However, while peeling, the tape temperature increases slightly due to friction. Chemical vapour deposition (CVD) is another commonly used technique for synthesising high-quality graphene on metal substrates. It is usually carried on at high temperatures (800°C to 1100°C). Applying a very high temperature from 1100°C to 1600°C in a controlled environment, epitaxial graphene growth can be seen growing on silicon carbide substrate. Another process of procuring graphene is the reduction of graphene oxide, which can be carried out at moderate temperatures (between 200°C and 400°C). The laser ablation process of graphene synthesis is usually conducted at high temperatures (around 3000°C to 5000°C). Temperature, precursor, gas environment, pressure—these synthesis conditions determine the properties and quality of graphene. Various techniques, such as Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM), are used to understand and characterise graphene and its derived materials. TEM is used to visualise the layers of graphene and the number of layers in few-layer graphene and graphene nanosheets. TEM is also employed to detect defects, such as graphene edges, grain boundaries and vacancies. After TEM characterisation, the stacking order and rotational stacking faults can be investigated in bilayers or multilayers of graphene. After chemical modifications or functionalisation, the structural variation of graphene-derived materials can be observed using TEM. SEM is employed to analyse the surface morphology of graphene films and nanosheets and to visualise graphene-based composites and their dispersion within matrices. Also, Density Functional Theory (DFT) and ab initio molecular dynamics play crucial roles in comprehending and fabricating nanomaterial structures with complexities akin to those of nano-graphene and graphene-derived materials (Kakanakova-Georgieva et al., 2021; Sangiovanni et al., 2023).

2.1.2. Applications of graphene

Due to its unique properties, graphene can be used in various fields of application such as solar cells (Patchkovskii et al., 2005), hydrogen storage (Brownson & Banks, 2012), supercapacitors (Eda & Chhowalla, 2010; Yoo et al., 2011), electronics (Raju et al., 2014), strain sensors (Bunch et al., 2007), electromechanical systems (Schwierz, 2010), field effect transistor (Papageorgiou et al., 2015) and high end-composite materials (Potts et al., 2011; Shen et al., 2012). Graphene is a zero-gap semimetal with a two-dimensional planar structure, making its use in many applications difficult. Hence, graphene is being processed and made into different forms, for example, quantum dots (Jia et al., 2011) and nano-ribbons (Xue et al., 2012) and are used in foams (Wu et al., 2012; Xu et al., 2010) and in hydrogels (Song et al., 2020) for energy and biological applications. These are also used in semiconductor devices and for designing supercapacitor electrodes. With high power densities and quick charge/discharge cycles, supercapacitors, also known as ultra-capacitors or electrochemical capacitors, store energy through quick and reversible charge accumulation at the electrode-electrolyte interface. Graphene-based supercapacitor electrodes take advantage of the unique characteristics of graphene to achieve excellent performance and efficiency.

Graphene has recently been getting much attention in wearable biomedical electronics due to its unique physical, chemical, mechanical and electrical properties (Kabiri Ameri et al., 2017). Since graphene demonstrates high conformability (Lou et al., 2016; Qiao et al., 2018), when it comes in contact with human skin, it is mostly employed as electrode material in flexible electrodes. The highly conformal nature of graphene-based flexible electrodes enhances the SNR (Celik et al., 2016), reduces skin-electrode contact impedance (Qiao et al., 2020) and improves the quality of the signal (Xu et al., 2019). Hence, graphene is considered ideal for flexible ECG electrodes for high-quality, long-term ECG signal monitoring.

2.1.3. Thermal stability of graphene-based electrodes

In high-temperature environments, graphene-based electrodes need to be thermally stable. Understanding how graphene behaves when exposed to elevated temperatures is essential to ensure that graphene electrodes perform reliably and last for a long time. Graphene exhibits exceptional thermal conductivity as a single layer of carbon atoms arranged in a two-dimensional lattice. Graphene is promising for various thermal management applications due to its high thermal conductivity. The thermal stability of graphene is dependent on several factors. Thermal stability is strongly influenced by the quality of graphene used in electrodes. Thermal stability tends to be higher for graphene with fewer defects and impurities (Goyenola et al., 2011). For graphene to be thermally stable, it must be attached to a substrate. Graphene’s stability can be affected by faults or stress induced by some substrates during temperature changes. Graphene is often thermally annealed to enhance its thermal stability and quality. Graphene structure becomes robust at higher temperatures by annealing to repair defects and remove impurities. A high temperature and oxygen-containing environment can cause graphene to oxidise. The electrical and thermal characteristics of electrodes made of graphene can be deteriorated by oxidation. Graphene’s structural integrity and characteristics may be impacted by the intercalation of foreign atoms or molecules at high temperatures. The graphene structure—single-layer, bilayer, or few-layer- can also affect the material’s thermal stability. In conclusion, although graphene has excellent thermal conductivity and stability at low temperatures, its performance at high temperatures (beyond a few hundred degrees Celsius) depends on the aforementioned parameters. Increasing the thermal stability of graphene-based electrodes and extending their use in high-temperature environments, such as in electronics, aerospace, and energy storage devices, can be accomplished by optimising graphene quality, substrate selection, and thermal treatment.

3. Methodologies for fabricating graphene-based ECG electrodes

Screen printing is one of the methods for fabricating graphene-based ECG electrodes. Screen-printed electrodes have uniform graphene coating on their surface, reducing noise in the ECG signal. Dip coating and drop casting are cost-effective and fast methods for fabricating dry ECG electrodes. Mostly, cotton fabrics are used in these two processes. A very high-quality coating of graphene layer can be transferred on the surface of the desired substrate following the wet transfer method. Large-area graphene is grown on the copper substrate via the CVD technique, even though CVD is a physical method to obtain thin films. After being transferred on the substrate, the CVD-grown large-area graphene is cut into desired shapes using a mechanical cutter plotter in the wet transfer and dry patterning process. In the spray coating technique, the printing material passes through a nozzle, forming a fine aerosol. In the spin coating method, the fabricated electrodes have a uniform thickness. Other fabrication techniques are discussed in detail in this paper.

3.1. Screen printing

In the screen-printing method, ink, composite, or dye is directly placed onto a substrate. In this process, at first, the impression which needs to be printed is transferred onto the screen (which is nothing but a thin fabric) with the help of photography. The ink or composite is smeared all over the screen. It flows through the unblocked parts and gets deposited on to the substrate. The ink is impermeable to those nonprinting parts where the fabric is used as a stencil. The method is simple and cost-effective. Mass fabrication of electrodes can be done by this process. A study on the works of the researchers who have used screen printing to fabricate graphene-based electrodes is discussed below and is shown in Figure 1.

Figure 1. Fabrication of electrodes using screen printing technique.

In 2019, Xu et al. (Goyenola et al., 2011) fabricated graphene-based screen-printed dry ECG electrode. In the fabrication process, at first, an aqueous solution of Carboxymethyl Cellulose (CMC) was synthesised and mixed with graphene nanoplatelets to prepare fine graphene ink. Commercial cotton fabric was selected as the substrate. An aluminum alloy screen printer is used for screen printing. After screen printing the cotton textile with graphene ink, the fabrics are annealed for 30 min inside an oven at 80°C. This cycle was repeated to obtain a better quality of the electrodes. The fabricated electrode shows excellent performance. The Pearson correlation co-efficient between this graphene-based screen-printed electrode and Ag/AgCl electrode is found to be as high as 99.47%.

In the same year, Xu et al., (2020) fabricated another type of graphene ink-based screen-printed dry ECG electrode. A coating of graphene conductive ink was deposited over the textile substrate using thermal transfer technology before screen printing the graphene ink over the textile. The screen-printed electrodes were annealed for 30 mins at 80°C. After one printing process, the sheet resistance of the resultant electrode was observed to be 418 ± 21 Ω/sq. There was a sharp decrease in sheet resistance after four printing processes which was reported to be 96 ± 8 Ω/sq

A transparent, wearable and non-invasive electrode for ECG signal monitoring composed of graphene oxide (GO) and silver nano-wires (AgNWs) was described by Xu et al., (2019). These types of hybrid electrodes were fabricated layer-by-layer by screen printing method onto polyethylene terephthalate (PET) substrate. The fabricated electrodes were of good chemical stability, mechanical flexibility and electrical conductivity. At first, the solution of AgNWs dispersed in ethanol was screen printed on the PET substrate, followed by annealing at 130°C for 20 mins. In the next step, a layer of GO was deposited on the electrode by screen printing and further annealed for 30 mins at 130°C. After two and three printing processes, the sheet resistance of the electrode after deposition of AgNWs drastically decreases from 2.4 × 10⁷ Ω/sq to 350 Ω/sq and 35 Ω/sq. After depositing the GO layer on the top of AgNWs film, the sheet resistance of the screen-printed electrode changes from 35 Ω/sq to 28 Ω/sq and 11.9 Ω/sq after one and two printing processes, respectively.

Sinha et al., (2019) have demonstrated the fabrication of a novel, dry, highly conducting ECG electrode composed of poly(ethylene terephthalate) (PET) which is a nonwoven fabric. It was first coated with graphene/graphite and then with poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT: PSS) which was doped with dimethyl sulfoxide (DMSO). A layer of graphite/graphene was deposited on the top of PET nonwoven fabric by interfacial trapping technique. The sheet resistance of the electrode was reported to be 5.5 Ω/sq. The SNR value of the graphene and PEDOT: PSS hybrid screen printed electrode was found to be 23.71 dB when used for the ECG signal. The electrical properties of the electrodes fabricated by screen printing method are summarized in Table 1.

Table 1. Electrical properties of the electrodes fabricated by screen printing method

Sl. No.	Material used for electrode fabrication	Ref.	Sheet resistance	Quality of ECG signal
1.	Fine graphene ink prepared from graphene nanoplatelets	(Goyenola et al., 2011)	–	The Pearson correlation co-efficient between this electrode and Ag/AgCl electrode is found to be 99.47%.
2.	Fine graphene ink screen printed onto textile fabric precoated with a polymer layer	(Xu et al., 2020)	418 ± 21 Ω/sq (after one printing process) 96 ± 8 Ω/sq (after four printing process)	–
3.	Graphene oxide (GO) and silver nanowires (AgNWs)	(Xu et al., 2019)	28Ω/sq (after one printing process) and 11.9 Ω/sq (after two printing process)	–
4.	Graphene/graphite and PEDOT:PSS	(Sinha et al., 2019)	5.5 Ω/sq	SNR = 23.71 dB

3.2. Dip coating

Dip coating is one of the earliest wet chemical thin film deposition processes (Brinker et al., 2013). In this process, at a uniform speed, the substrate is immersed into the precursor solution and kept until the substrate becomes completely wet. Next, the substrate is taken out of the precursor. A thin layer of the precursor solution deposits on the top of the substrate. In the next step, the substrate undergoes heat treatment for drying. The fabrication of electrodes with the dip coating method is illustrated in Figure 2.

Figure 2. Fabrication of electrodes following the dip coating method.

(Yapici et al., 2015) developed novel graphene-clad textile-based electrodes in 2015. These types of electrodes are dry, reusable, comfortable, wearable and can be easily integrated into any personal clothing. Textiles or fiber mats such as nylon which are hydrophobic were chosen as substrate, and by dipping them into dilute GO suspension, the fabrics are cladded with graphene oxide. The textile was kept inside an oven at 80°C to dry the electrode for a few hours. An average value of the cross-correlation of 97% was found when the signals extracted from traditional wet electrodes and fabricated graphene-clad textile electrodes were compared. In the frequency range of 1 Hz to 1 kHz, the impedance of the GO clad textile was measured to be in the range of 87.5 kΩ to 11.6 kΩ. In the same frequency range, the impedance of the traditional wet electrode was reported to be in the range of 50.9 kΩ to 2.2 kΩ. Before the coating of the graphene layer, the conductivity of the nylon was reported to be 6 × 10⁻¹² S/cm which reduces to 4.5 S/cm after the graphene coating.

In 2017, Lam et al., (2017) fabricated flexible ECG electrodes with high electrical conductivity by dipping pre-treated cotton fabrics into a graphene solution. After seven layers of coating of the conductive material, the sheet resistance of the flexible electrodes was reported to be 11.51Ω/sq.

To fabricate high-performance and multifunctional graphene-based epidermal strain sensors and bioelectrodes, Yun et al., (2017) have reported a cost-effective fabrication method by placing electroconductive sheets of reduced graphene oxide(RGO) on the top of porous and elastic poly(dimethylsiloxane)(PDMS). Several pores were created by steam etching on the PDMS thin film. A uniform layer of GO was coated on the top of this film by dipping it in an aqueous GO solution. The sheet resistance of the GOpPDMS film was observed to be approximately 1.5 kΩ/sq.

Yapici & Alkhidir, (2017) have reported developing textile-based, wearable smart medical garments to monitor ECG signals. The prototype comprises flexible graphene clad textile-based bands as ECG sensors that can be worn around the neck or wrists, lithium-ion batteries to supply power, and ECG signal acquisition and transmission system. Pieces of nylon textile were dipped into the GO suspension and were thermally treated. The graphene oxide of the GO-clad textile was reduced, resulting in a highly conducting graphene-clad textile. When compared between the wearable electrodes and conventional wet ECG electrodes, the average value of cross-correlation of 88% was achieved for the whole waveform and the maximum value of 97% was measured between two P-QRS-T segments. The electrical properties of electrodes fabricated by the dip coating method are summarized in Table 2.

Table 2. Electrical properties of electrodes fabricated by the dip coating method

Sl. No.	Material used for electrode fabrication	Quality of ECG signal	Sheet resistance/Conductivity	Ref.	Impedance
1.	Reduced graphene oxide clad textile	The Pearson correlation co-efficient between this electrode and Ag/AgCl electrode is found out to be 97%.	Conductivity 4.5 S/cm	(Yapici et al., 2015)	87.5 kΩ to 11.6 kΩ in the frequency range of 1Hz to 1kHz. For Ag/AgCl electrode 5.9 kΩ to 2.2 kΩ in the same frequency range
2.	Graphene and PEDOT:PDMS coated textile electrode	-	Sheet resistance 11.51 Ω/sq	(Lam et al., 2017)	-
3.	Graphene oxide- porous PDMS thin film	-	Sheet resistance 1.5 kΩ/sq	(Yun et al., 2017)	-
4.	Reduced graphene oxide clad textile as wearable medical garment	Cross-correlation of 88% for the whole waveform and the maximum value of 97% was measured between two P-QRS-T segments when compared between wearable electrodes and conventional wet ECG electrodes	-	(Yapici & Alkhidir, 2017)	87.5 kΩ to 55 kΩ in the frequency range of 10–50 Hz. For Ag/AgCl electrode.9 kΩ to 20 kΩ in the same frequency range 50.9 kΩ to 20 kΩ in the same frequency range

3.3. Wet transfer method

The most cost-efficient and successful bottom-up method for synthesising large-area graphene is chemical vapor deposition (CVD). In this process, large-area graphene crystals are grown on top of transition metal catalyst substrates like nickel (Ni) or copper (Cu). After this, the grown layer of graphene on the metal substrate is transferred to the desired substrate (without compromising the quality of as-grown graphene) for various photonics and electronics applications. While transferring the graphene layer from one substrate to another, the etching solution or water causes cracks, wrinkles or ripples on the graphene layer due to its surface tension. To protect the graphene layer, a polymer layer is deposited on top of it for protection which is known as the sacrificial layer or supporting layer. The most used supporting layer is Polymethyl methacrylate (PMMA). It is nothing but a flexible thin film. The fabrication of electrode using the wet transfer method is shown in Figure 3.

Figure 3. Electrode fabrication using the wet transfer method.

Celik et al., (2016) have demonstrated the fabrication of dry electrodes following the CVD technique where Ag/AgCl electrode was coated with graphene. Following the CVD growth technique, monolayer graphene was synthesised on top of a copper substrate using methane (CH4) and hydrogen (H2) gases (Celik et al., 2016; Kidambi et al., 2012; Li et al., 2009). To transfer the monolayer graphene from Cu substrate to Ag layer, Cu substrate was etched away, and Polymethyl methacrylate (PMMA) was spin-coated onto the graphene layer, which acts as a supporting layer via a wet chemical approach. The supporting layer was removed after transferring the graphene layer onto the Ag layer (Meng et al., 2016; Strudwick et al., 2015). After coating graphene, the electrical conductivity of the dry electrode increased from 2.48 × 10² S/m to 6.94 × 10⁴ S/m.

In 2016, Lou et al., (2016) described developing a flexible, dry graphene-based ECG electrode and a wireless portable ECG monitoring system. The dry flexible electrodes were fabricated by growing graphene films on copper foils via CVD. Next, the graphene films were transferred onto the top of the flexible PET substrate by the wet transfer method. The electrodes were connected with the ECG signal acquisition system by attaching silver wires to the graphene layer of the electrodes with conductive silver pulp. The fabricated graphene electrode could acquire almost all typical features and characteristics of ECG signals. The SNR values were higher than commercial Ag/AgCl electrodes when measured in different motion states. Even after wearing the electrodes for one weeklong, over time, degradation in signal quality was not observed.

In 2021, Zhao et al., (2021) reported epidermal dry electrodes of 100 nm thickness which could monitor electrophysiological signals. It exhibits conformable adhesion to the skin. CVD-grown graphene electronic tattoo having a thickness of 463 ± 30 nm (when poly (methyl methacrylate) (PMMA) was used as a supporting layer) exhibited optical transparency of approximately 85%, stretchability of ~ 40% and sheet resistance of 1994.33 ± 264 Ω/sq. But using PMMA polymer as a supporting layer could lead to defects, cracks or contamination in the graphene layer during wet transfer, which degrades the quality of CVD grown graphene layer and reduces its electrical conductivity (Yu et al., 2011). Hence, Zhao and his co-workers have fabricated CVD-grown graphene-based highly conductive, ultrathin and transparent dry electrodes where PEDOT: PSS has been used as a supporting polymer layer for sensing electrophysiological signal. These dry epidermal electrodes fabricated by PEDOT: PSS transferred CVD-grown graphene film (PTG) having a thickness of ~100 nm exhibited optical transparency of ~ 80%, high electro-mechanical stability and sheet resistance of ~ 45Ω/sq. Compared with PMMA/graphene electronic tattoo, electrochemical impedance was reported to be reduced by 1.3 times in the case of PTG electrodes when interfaced with skin. The electrical properties of electrodes fabricated by wet transfer method are summarized in Table 3.

Table 3. Electrical properties of electrodes fabricated by wet transfer method

Sl. No.	Material used for electrode fabrication	SNR Value of ECG signal	Sheet resistance or conductivity	Ref.	Skin Contact Impedance
1.	CVD grown graphene layer transferred on Ag layer	Graphene coated Ambu type electrodes- 27.03 dB Wet Ag/AgCl Ambu type electrodes- 25.21 dB Graphene coated Covidien type electrode- 25.32 dB Wet Ag/AgCl Covidien type electrodes- 21.23 dB	Conductivity 6.94×10^4S/m	(Celik et al., 2016)	In the frequency range of 20 Hz to 1 kHz -Commercial Ag/AgCl electrodes - from 445.05 kΩ to 13.82 kΩGraphene coated electrodes - from 65.82 kΩ to 5.10 kΩ Commercial Ag/AgCl electrodes - from 445.05 kΩ to 13.82 kΩ Graphene coated electrodes - from 65.82 kΩ to 5.10 kΩ
2.	CVD grown graphene layer transferred on PET substrate	Commercial Ag/AgCl electrodes- 31dB Graphene coated PET electrode- 31.6 dB	-	(Lou et al., 2016)	In the frequency range of 10 Hz to 1kHz –Commercial Ag/AgCl electrodes - from 55.1 kΩ to 18.2 kΩGraphene-based electrode - from 281.8 kΩ to 24.8 kΩ Commercial Ag/AgCl electrodes - from 55.1 kΩ to 18.2 kΩ Graphene-based electrode - from 281.8 kΩ to 24.8 kΩ
3.	PEDOT:PSS transferred CVD grown graphene film(PTG)	-	Sheet resistance ~45 Ω/sq	(Zhao et al., 2021)	Conventional Ag/AgCl electrodes- 45 kΩ at 100HzPTG electrodes- 32 kΩ at 100 Hz PTG electrodes- 32 kΩ at 100 Hz

3.4. Wet transfer and dry patterning

In the wet transfer method, copper is etched, the grown layer of graphene on the copper substrate is transferred to the desired substrate without any degradation in the quality of grown graphene and the continuous structure of large area CVD grown graphene can be maintained. In the dry patterning process, a programmable mechanical cutter plotter is used for carving out specially designed filamentary serpentine shaped CVD grown graphene films. Chemical contamination can be avoided in the case of a dry patterning process in comparison with photolithography. Not only that, but dry patterning is also more cost- and time-effective. The electrode fabrication following wet transfer and dry patterning technique is illustrated in Figure 4.

Figure 4. Electrode fabrication following wet transfer and dry patterning technique (Qiao et al., 2018).

In 2017, Lu et al., (2017) invented a fully dry, “cut-and-paste”, time- and cost-effective technique for manufacturing epidermal electronics within a short period of time. They have reported about transparent, thin epidermal graphene electrodes, which can be attached directly to the epidermal layer of human skin temporarily like a tattoo and detect biopotential; it can generate ECG signal. The ECG signals detected by these electrodes can be comparable with the ECG signal detected by conventional wet electrodes. Researchers have used commercially available thin metalised polymer sheets in this method rather than following high vacuum metal deposition process. It shows 85% optical transparency, more than 40% stretchability and thickness of 463 ± 30 nm.

In the same year, Ameri et al. (Qiao et al., 2018) evaluated the performance of the above-mentioned graphene electronic tattoo (GET) or graphene epidermal electronic system as dry flexible ECG electrodes. They have reported that the skin-electrode impedance of the GET and wet commercial Ag/AgCl electrodes was comparable even though the surface area of graphene-based skin hydration sensor (GSHS) was ten times smaller than the conventional Ag/AgCl electrodes.

In 2019, Sel et al., (2019) proposed the fabrication of skin-conformable, wearable, transparent, soft and ultrathin graphene-based electronic tattoos (GETs), which can be applied to monitor ECG signals in the long term. GETs offer better breathability, additional comfort, sensing robustness, improved skin adherence, and good contact impedance compared to commercial wet electrodes. The electrical properties of electrodes fabricated by wet transfer and dry patterning method are summarized in Table 4.

Table 4. Electrical properties of electrodes fabricated by wet transfer and dry patterning method

Sl. No	ECG electrodes	SNR of the ECG signal	Bio-Z measurements	Ref.	Skin Contact Impedance
1.	Graphene electronic tattoo (GET)	Graphene-based electrophysiological sensors (GEPS)- 15.22 dB Conventional Ag/AgCl electrodes- 11dB	-	(Lu et al., 2017; Qiao et al., 2018)	The skin contact impedance of graphene epidermal electronic system was comparable with conventional wet Ag/AgCl electrodes
2.	Bilayer graphene electronic tattoo(BiGET)	-	Average standard deviation – BiGET- 3.6 mΩ Ag/AgCl − 4.1mΩ	(Sel et al., 2019)	The fabricated bilayer GETs offered skin contact impedance of 10kΩ at 10kHz which is comparatively lower than the commercial Ag/AgCl electrodes.

3.5. Spray coating

The printing material passes through the nozzle forming a fine aerosol. This is the working principle of spray coating (Steirer et al., 2009) and is illustared in Figure 5. Spray coating can be affected by a few parameters such as flow rate of printing ink through the nozzle, distance between airbrush and sample, substrate temperature, pressure, spray duration, blend solution concentration, number of times the substrate is sprayed and cosolvent mixture (Park et al., 2011)– (Zahed et al., 2020).

Figure 5. Fabrication of electrodes using spray coating technique.

Stephens-Fripp et al. described the fabrication of a flexible, reusable, 3D-printed concentric ECG electrode spray-coated with graphene ink (Stephens-Fripp et al., 2018) in 2018. Using Ninjaflex material, three basic structures of the electrode; outer electrode, separator and inner electrode were 3D printed. Using a spray gun, ECG electrodes were manufactured by spraying graphene solution onto the surface of 3D-printed circular-shaped structures. The sheet resistance of the graphene spray-coated electrode was measured to be 903.5 ± 262.15 Ω/sq.

In 2020, Zahed et al., (2020) developed conductive dry and highly flexible ECG electrodes. In this case, laser-induced graphene (LIG) mechanically loaded with PEDOT:PSS, a biocompatible polymer, was used as electrode material. PEDOT: PSS was spray-coated to increase the electrical conductivity and robustness of the electrode on the surface of LIG. The electrical conductivity of the PEDOT:PSS spray-coated LIG electrodes was measured to be 164.2 Scm⁻¹which is considerably high. The skin contact impedance of the electrode at 10 Hz was reported to be 385 KΩ, and the surface resistivity was measured as 17.4 Ω/sq.

3.6. Spin coating

In the spin coating method, thin coatings are rapidly deposited on top of comparatively flat substrates as shown in Figure 6. The flat substrate, which needs spin-coated, is placed onto a rotatable fixture. Then, the coating material coated onto the slat substrate is distributed on its surface. The constant rotation of the substrate forms a uniform layer of coating (Birnie et al., 2004).

Figure 6. Spin coating technique for fabricating electrodes.

In 2020, Du et al., (2020) designed a novel stretchable and transparent graphene ECG electrode where molybdenum chloride (MoCl₅) is intercalated in between assembled few-layer graphene (FLG) in a confined or semi-sealing atmosphere. The PMMA/graphene film was transferred on single-layer graphene, and the PMMA layer was later separated. Thus bi-layer graphene (BLG) was prepared. In the next step, a layer of MoCl₅solution was deposited on the top of BLG by spin coating for 30 s at 2000 rpm to make the BLG surface doped. At 80% transmittance, the fabricated electrode has exhibited sheet resistance of 40 Ω/sq, which is considerably low. When this electrode was used to measure the ECG signal, all significant characteristic peaks of the ECG signal were observed very well, and the signal was comparable with the ECG signal obtained from the wet get electrodes.

3.7. Drop casting

In the drop-casting method, drops of desired particles dispersed in liquid are deposited on the top of the electrode, which needs to be modified. This process in shown in Figure 7.

Figure 7. Electrode fabrication using drop casting technique.

(Liu et al., 2018) have described a cost-effective and simple technique for fabricating graphene-based polymer film for monitoring ECG signals. In this method, the graphene suspension was deposited on the surface of a filter membrane by drop casting method. The graphene-coated filter paper was attached to a glass slide or plate using double-sided tape. A mixture of curing agents and liquid PDMS in the 1:10 ratio was deposited on the graphene-coated membrane. After curing was done, the filter membrane was removed from the graphene and PDMS layer, creating an interface of the graphene-PDMS layer.

In 2021, to analyse the cardiac function and real-time ECG signal acquisition, Prasad et al., (2021) demonstrated the fabrication process of graphene nano-ribbons (GNR) using polyvinyl alcohol (PVA). Different PVA/GNR suspension compositions were prepared and drop cast on the top of precleaned conventional Ag/AgCl electrodes. After drying, they obtained a 3 mm thick, adherent PVA/GNR dry ECG electrode. In the case of PVA/GNR electrodes, it was observed that the SNR value changed according to the different content of GNR.

Murastov et al., (2020) have described the fabrication of laser-reduced graphene oxide (rGO) and PET-based bioelectrodes, which can overcome issues such as low SNR value of dry electrodes, omitting the use of hydrogel on the interface of skin and Ag/AgCl bioelectrodes. In this process, GO dispersion was drop cast on the top and bottom surface of the PET substrate. This bioelectrode’s average skin contact impedance was reported to be 4 kΩ.

In 2021, Wei et al., (2021) presented the fabrication of wearable graphene-based textiles (GT) following laser scribing and thermal transfer technology. For fabricating GT, the techniques the researchers follow are—drop-casting of graphene oxide (GO), laser-scribing, removal of GO layer and thermal transfer process. The ECG signals extracted from the GT and Ag/AgCl electrodes were comparable. The signal obtained from the GT electrode was accurate and highly stable. All characteristic peaks of the ECG signal are observed clearly in the ECG signal extracted from the GT electrodes.

In 2022, Maithani et al., (2022) proposed a wearable dry ECG electrode based on antibacterial, highly flexible, conductive silver nanorods (AgNRs) and a composite matrix comprising RGO-PDMS. On the top of the AgNRs-coated Si substrate, RGO-PDMS mixture was drop cast and cured. The ECG signal obtained from AgNRs/RGO-PDMS electrode was of high quality, distinguishable and stable.

Following cost-effective, facile laser writing and drop casting methods, Zahed et al., (2021) have fabricated a novel flexible 3D porous graphene (3DPG) electrode incorporating polyaziridine-encapsulated phosphorene (PEP). To increase the electrical conductivity and mechanical robustness of the 3DPG electrodes, the prepared PEP solution was drop casted on to the active regions of the 3DPG electrode.

Ali et al., (2022) demonstrated the fabrication of graphene nanoplatelets-based wearable and flexible textile electrodes for ECG signal monitoring. These highly conductive electrodes having low skin contact impedance have better conformability with the skin. The fabricated electrodes are superhydrophobic and can be used even after several times of washing without any degradation in the signal quality. In this paper, researchers have blended graphene nanoplatelets (GNPs) nanofillers with polyvinylidene fluoride (PVDF), and clean fabric textile was coated with this mixture. Adding PVDF to the GNPs solution makes the conductive electrodes superhydrophobic (Bidsorkhi et al., 2020). The electrical properties of electrodes fabricated by drop casting method are summarized in Table 5.

Table 5. Electrical properties of electrodes fabricated by drop casting method

Sl. No.	ECG electrode	Quality of the ECG signal	Ref.	Skin contact Impedance
1.	Graphene based PDMS electrode	The average value of the R-wave Ag/AgCl electrode- 0.704 mV Graphene polymer film electrode- 0.662 mV	(Liu et al., 2018)	In the frequency range of .1 Hz to 1kHz, Commercial wet gel electrodes- from 1kΩ to 200 Ω and Graphene-based polymer electrode- from 20k Ω to 30k Ω
2.	Graphene nanoribbon and PVA based electrode (PVA/GNR)	SNR value Ag/AgCl electrodes: 62.22dB PVA/GNR (1%) electrodes: 59.99 dB	(Prasad et al., 2021)	In the frequency range of 10Hz to 1kHzWet Ag/AgCl electrodes from 400 Ω to 250 Ω PVA/GNR electrode- constant skin contact impedance value Wet Ag/AgCl electrodes from 400 Ω to 250 Ω PVA/GNR electrode- constant skin contact impedance value
3.	Laser-reduced graphene oxide(rGO) and PET based electrode	SNR value for rGO bioelectrode is above 70 dB	(Murastov et al., 2020)	4k Ω
4.	Graphene based textile electrode	The ECG signals extracted from GT electrode and Ag/AgCl electrode were comparable.	(Wei et al., 2021)	-
5.	AgNRs/RGO-PDMS flexible dry electrodes	High-quality, distinguishable and stable ECG signal	(Maithani et al., 2022)	In the frequency range of 40Hz to 1 kHzwet Ag/AgCl electrodes- from 28.4 ± 2.7 k Ω to 7.6 ± 0.9 kΩAgNRs/RGO-PDMS flexible dry electrodes - from 7.1 ± .7 k Ω to 5.6 ± 1.7 k Ω wet Ag/AgCl electrodes- from 28.4 ± 2.7 k Ω to 7.6 ± 0 0.9 k Ω AgNRs/RGO-PDMS flexible dry electrodes - from 70.1 ± 0.7 k Ω to 5.6 ± 1.7 k Ω
6.	Flexible 3D porous graphene (3DPG) electrode incorporating PEP	SNR value - 3DPG electrode- 8 dB PEP/3DPG electrode- 13.5 dB Wet Ag/AgCl electrodes- 13.9dB.	(Zahed et al., 2021)	In the frequency range of 4 Hz to 1 kHzPEP/3DPG electrodes- from 198.2k Ω to 90.35k Ω Wet Ag/AgCl electrodes- from 1.68 M Ω to 189.6k Ω PEP/3DPG electrodes- from 198.2k Ω to 90.35k Ω Wet Ag/AgCl electrodes- from 1.68 M Ω to 189.6k Ω
7.	Graphene nanoplatelets (GNPs) nanofillers with polyvinylidene fluoride (PVDF) coated clean fabric textile	SNR value - GNP/PVDF (10%) electrode − 36.5 dB GNP/PVDF (13%) electrode − 38.3 dB Wet Ag/AgCl electrode- 34.6dB.	(Ali et al., 2022)	At 20HzGNP/PVDF (10%) − 98.9k Ω GNP/PVDF (13%)- 98.32k Ω GNP/PVDF (10%) − 98.9k Ω GNP/PVDF (13%)- 98.32k Ω

3.8. Inkjet printing

For liquid phase materials, inkjet printing is one of the best techniques for material conserving deposition (Singh et al., 2010). The process is illustrated in Figure 8. The inks or materials deposited following this technique comprise a solute dispersed or dissolved in a solvent. In this process, a predetermined quantity of ink present in a chamber, is ejected due to a sudden change in the chamber volume from nozzle via piezoelectric action. When external voltage is applied, liquid filled chamber is contracted. The sudden, quasi-adiabatic reduction in chamber volume causes shock wave throughout the liquid, which causes the ejection of liquid drop out of the nozzle. The drop ejected from the nozzle then falls and spreads on the substrate under air resistance and gravity. The substrate is then dried following solvent evaporation (Lim et al., 2008). Viscosity, a function of the polymer’s molar mass, controls the spreading of the drop and the final printed shape on the substrate.

Figure 8. Fabrication of electrodes using ink-jet printing technique.

In 2017, Karim et al., (2017) have reported fabrication of inkjet-printed environment-friendly, comfortable and breathable graphene e-textiles which are surface pre-treated with organic nanoparticles. In case of inkjet printing of the surface pre-treat, ink or coating material can be deposited on a specific targeted position which is advantageous compared to other techniques such as curtain coating and screen printing. The surface pre-treatment decreases the chance of heat-sensitive fabrics getting damaged since the pre-treatment acts as receptor of water-contained rGO inks, reducing the drying temperature of the fabric as low as 100°C. An organic nanoparticle which is hydroxyl functionalised cross-linked styrene/divinylbenzene was inkjet printed on the surface of the textile fabric. Then this fabric was coated with rGO water-based ink. In the presence of polyvinyl alcohol (PVA), the rGO water-based ink was synthesised using L-ascorbic acid which is a green and nontoxic reducing agent. In comparison with untreated textiles, the sheet resistance of the graphene-based inkjet-printed e-textiles decreased from 1.09 × 10⁶ Ω/sq to 2.14 × 10³ Ω/sq due to inkjet printing onto the pre-treated coating. The SNR value of the extracted ECG signal when this fabricated electrode was used was measured to be 21 dB.

3.9. Electrospinning

Continuous nanofibers of various materials can be fabricated by electrospinning method, which is illustrated in Figure 9. Nanofibers are generated when a viscoelastic solution is uniaxially stretched in electrospinning (Teo & Ramakrishna, 2006). For uniaxial stretching of the solution, electrostatic forces are employed in the case of electrospinning, unlike melt spinning and dry spinning, which are conventional fibre spinning techniques. Unless the electrospinning jet filled with the viscoelastic solution gets emptied, the formation of nanofibers is continued. During the electrospinning process, as soon as the applied voltage is greater than the critical voltage, which is 5kV, the repulsive force within the viscoelastic charged solution overcomes the surface tension of the solution, and the solution erupts from the tip of the spinneret in the form of a jet. The nanofibers generated during this process form a nonwoven mesh at the targeted and grounded collector.

Figure 9. Electrospinning technique for electrode fabrication.

(Huang & Chiu, 2021) have fabricated a flexible and stretchable nanofiber carbon film for sensing ECG signal via electrospinning method in 2021. The carbon film collector for electrospinning was synthesised using reduced graphene oxide (rGO). Nano-dispersed carbon black and nanofibers were coated on its surface through electrospinning. Mixing and electrospinning, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polyvinylidene difluoride (PVDF), the nanofibers were synthesised. The nanofiber deposited on the surface of the carbon film increases surface hydrophobicity and skin contact area. The carbon plate collector coated with nanofiber acts as flexible and stretchable nanofiber carbon electrode. The electrode exhibits good flexibility, excellent electrical conductivity, high mechanical durability and high stability. The prepared electrode’s surface resistance was 2.5 × 101 Ω/sq, which is very low. The surface of the electrode was reported to be hydrophobic, having a water contact angle of 146°. When this electrode was used for ECG signal monitoring, all characteristic peaks such as P, QRS and T can be clearly distinguished. This electrode can be integrated into wearable smart clothing and can be used to measure ECG signals for long term and under various exercising conditions.

3.10. Patterning

In this process, using polyimide, various graphene-derived structures are obtained. CO2 laser machining system is used to synthesise graphene from polyimide. As soon as laser rays incise on Kapton HN polyimide surface, it causes temperature elevation. The rise in temperature leads to local ablation of the PI. The conductivity of the graphene prepared from this process can be compared with the CVD grown graphene.

Using laser writing technology, Yang and Zhang et al. fabricated reusable, patterned and robust graphene-based dry wearable electrodes where PDMS is used as substrate (Yang et al., 2021). The fabricated electrodes were proven to show reusability and excellent stability for long-term ECG signal monitoring even after 10,000 bending test cycles. The prepared LIG/PDMS electrode’s skin contact impedance was measured to be 112kΩ at 20 Hz. The value of skin contact impedance of commercial Ag/AgCl electrodes was found to be 78kΩ at 20 Hz. The SNR value of the ECG signal extracted from the prepared LIG/PDMS electrode and conventional Ag/AgCl electrode was measured to be 32 dB and 33 dB, respectively.

In 2020 Toral et al., (2020) presented a research paper based on flexible paper electrodes applied in the wearable electronics field. In this paper, they have fabricated three types of electrodes based on silver chloride, carbon inks and laser-induced graphene (LIG). Their performance was recorded during acquisition of various electrophysiological signals. They have fabricated one screen printed electrode and one LIG electrode and have compared these two electrodes with conventional Ag/AgCl electrodes during EMG, ECG and EOG signal acquisition. The sheet resistance of the LIG electrode was reported to be 360 Ω/cm². In comparison with the ECG signal extracted from the conventional wet electrodes, the ECG signal extracted from the fabricated LIG electrodes was 96.78% accurate.

In higher plant cells, cytoplasm and plasmodesmata are together called symplasm. Cytoplasm is surrounded by a cell wall, and plasmodesmata form an intercellular communication among adjacent cytoplasm. Qiao et al., (2020) have demonstrated the fabrication of electronic skin based on silver nanowires (AgNWs) bridged laser scribed graphene oxide. Silver nanowires (AgNWs) form a connection among laser scribed graphene oxide (LSGO), like how plasmodesmata connect one cytoplasm to another by forming nanochannels. AgNWs bridged graphene electronic skin (GES) exhibited better measuring range and mechanical sensitivity than pure GES. The GES performs excellently as a wearable, real-time and physiological monitoring system. Hence, it is used for real-time ECG and EEG signal monitoring. Biomimicking symplasm, the researchers have fabricated the AgNWs bridging LSGO-based multifunctional programmable patterned electronic skin (El-Kady & Kaner, 2013; Qiao et al., 2018; Tian et al., 2014), which are cost-effective and have high biocompatibility and flexibility. Besides monitoring ECG and EEG signals, it can also detect physiological signals such as joint movement, blinking, respiration and pulse due to its superior electrophysiological and mechanical characteristics. The network of AgNWs in LSGO enhances the range and sensitivity of the GES when it acts as a mechanical sensor. The performance of GES is also enhanced due to the bridging of AgNWs.

3.11. Exhaust dyeing

Exhaust dyeing is considered one of the most common and popular dyeing techniques. In this method, the dye molecules are transferred from a larger volume dye bath to the targeted material needing to be dyed. Conductive e-textiles can be fabricated and applied in biomedical health monitoring following this process.

Using the exhaust dyeing method, a stretchable, comfortable and highly flexible conductive e-textile have been developed by Akter Shathi et al., (2020) in 2020. The sheet resistance of the rGO coated fabric was measured to be 2.5 MΩ, and the value was reduced to 180kΩ after a few dyeing cycles. After the deposition of PEDOT:PSS, the sheet resistance value further decreased from 180kΩ to 120 Ω. The SNR value of the obtained ECG signal when the fabricated rGO electrodes coated with PEDOT: PSS were used, was reported to be 21.76 dB.

3.12. Vacuum filtration and drying

In the vacuum filtration method, desired graphene oxide solution mixed with other materials is filtered with the help of a vacuum pump using a nylon membrane filter paper placed on top of a filtration beaker. After filtration, the nylon membrane paper is dried properly and used as dry wearable electrodes. Materials other than Nylon membrane can also be used for filtration. Electrode fabrication using vacuum filtration and drying technique is depicted in Figure 10.

Figure 10. Electrode fabrication using vacuum filtration and drying technique.

Using carboxylic-functionalised multiwalled carbon nanotube composites (f@MWCNTs) and chemically modified graphene (CG), Hossain and Heo et al. have developed flexible paper-based ECG electrodes (Hossain et al., 2019). Carboxylic functionalised MWCNT and chemically modified graphene composites were synthesised via solvothermal technique. The synthesised composites were deposited on the surface of nylon filter papers via drop casting and at a certain pressure, the CG-f@MWCNT coated filter papers were subjected to vacuum filtration and drying. The skin contact impedance of the conventional wet electrodes was reported to decrease from 45 kΩ to 15.05 kΩ when the frequency increased from 10 Hz to 1 kHz. In the case of 25% CG-f@MWCNT electrodes, the average value of sheet resistance was measured to be 75 Ω/sq.

Using polyhydroxyalkanoate (PHA) as flexible substrate and chemically derived graphene as sensing material, Suvarnaphaet et al., (2019) developed electrode patch of graphene/PHA which can be used for ECG signal monitoring. Both layers of the graphene/PHA electrode patch i.e., conductive Graphene layer and flexible PHA bioplastic substrate layer have excellent biodegradability and biocompatibility (Liao et al., 2011; Priyadarsini, et al., 2018; Suvarnaphaet & Pechprasarn, 2017). The flexible PHA substrate was fabricated by electrospinning method. The white PHA nanofiber which was obtained via electrospinning was shaped as scaffold membrane. Using fritted vacuum filter, the prepared conductive graphene dispersion was poured on the top of the PHA for filtering. The resulting black thin film was changed into nanofibers by self-assembling. The ECG signal extracted from the graphene/PHA thin film was similar to the ECG signal obtained when wet Ag/AgCl electrodes were used.

In 2017, Das et al., (2017) fabricated chemically reduced graphene oxide (CRGO) based dry, conductive, flexible electrode to monitor ECG signal long term using a touch sensing mechanism. The CRGO paper exhibited surface resistivity of 28 Ω/sq, which is considerably low. While measuring ECG signals, the fabricated dry wearable CRGO electrodes performed excellently.

Das and Park et al. have proposed fabricating a dry, flexible, reusable, low-cost and biocompatible epidermal sensor using thermally reduced graphene oxide and nylon membrane (TRGO/NM) (Das et al., 2020). The prepared thermally reduced graphene oxide and nylon membrane-based epidermal sensor has demonstrated sheet resistance of 40 Ω/sq; at low frequency, skin contact impedance of the fabricated electrode was observed to be 20k Ω.

3.13. Other methods

A non-disposable, durable, robust and transparent graphene skin electrode was developed by (Qiu et al., 2020). The developed graphene-based skin electrode is reported to detect various electrophysiological signals of the body. The solidification process of filiform saliva inspired the fabrication process of this electrode. In the fabricated conductive graphene skin electrodes, annealed phenolic resin (PR) was deposited onto monolayer CVD graphene by electrospinning which was later semi-embedded into styrene-ethylene-butylene-styrene (SEBS) elastomer. Electro-spun polymer fibers strongly interact with CVD-grown monolayer graphene due to annealing at high temperatures. The annealing and semi-embedding process help the conductive film mimic the avian nest structure. The as-prepared conductive film exhibited 83% transmittance and sheet resistance of 150 Ω/sq.

Using silver nanocomposite and patterned 3D porous laser-induced graphene, Xuan et al., (2018). have developed conductive and highly stretchable electrodes following a simple, novel and inexpensive fabrication process. Even under mechanical vibrations, the fabricated electrode demonstrated stable and high electrical conductivity due to the presence of well-patterned AgNW film in between the LIG and PDMS layer. The stretchable electrode was fabricated by placing 3D network of the silver nanowire (AgNW)/LIG multi-layered film on a flexible PDMS substrate. The stretchable electrode can function as an ECG sensor, non-invasive glucose sensor, and pH sensor.

For neonatal monitoring, Chen et al., (2020) have proposed a wearable, flexible and non-invasive sensor system to provide comfortable neonatal care along with long-term vital signs monitoring. The sensor system comprises two parts, i.e., smart vest and cloud platform. The smart vest is composed of novel stretchable polydimethylsiloxane-Graphene (PDMS-Graphene) based sensor which can detect the respiration signal of the neonatal, dry textile-based electrodes for ECG signal monitoring and inertial measurement units (IMUs) for obtaining movement information of the new-born. The textile electrodes for ECG signal monitoring comprise regular cotton, e-textile and metal lead button. The regular cotton layer is sandwiched between two e-textile layers, and the metal lead button is connected to the regular cotton layer.

Xiao and Wu et al. have fabricated a novel hydrogel-based ECG electrode which exhibits excellent conductive and mechanical properties (Xiao et al., 2017). The conductive hydrogel was synthesised using polyethylene glycol (PEG), graphene oxide (GO) nanoparticles and polyvinyl alcohol (PVA) following cyclic freezing-thawing method. The solution was then poured on the top of nonwoven fabric kept inside an organic glass mold and subjected to freezing. The researchers have made a cylindrical shaped PEG/PVA/GO hydrogel and measured its resistance. Integrating four hydrogel electrodes, Bluetooth, a chip and electronic circuit, a wearable belt have been fabricated by the researchers for ECG signal acquisition system.

Creating porous nanographene structures on the top of flexible PI substrate via photothermal production, Romero et al., (2019). reported an inexpensive, one-step fabrication method of flexible electrodes that can be utilised for long term, ubiquitous and continuous ECG signal monitoring. Here, researchers have used low power laser diode to form foam of porous graphene (PG) on the PI substrate. Without any pre-treatment (which is required before laser treatment in case of fabrication of laser reduced graphene oxide (Romero et al., 2018) or lithographic masks, laser-induced nanographene aggregates (LINA), having excellent conductive property and high precision, were patterned on the flexible PI substrates. Since this is a one-step fabrication process, mass fabrication of electrodes can be done at low cost. The sheet resistance of the electrodes was reported to be 250 Ω/sq. The crest factor (peak-to-RMS ratio) of the ECG signals obtained from the LINA and commercial wet Ag/AgCl electrodes were reported to be 11.47 dB and 11.40 dB, respectively.

Wearable and compact graphene elastomer electrodes were developed by Asadi et al., (2021). in 2020. They have used porous and highly compliant graphene sponge (GS) as flexible electrodes which can act as ECG sensor. GS has a unique combination of excellent mechanical and electrical properties. GS is a soft, ultralight, elastomeric material (Wang et al., 2017). The elastic modulus of GS is 1,000,000 times lower compared to PDMS (Silverman et al., 2006). Hence, GS provides excellent contact with the skin, leading to noise reduction in the ECG signal. GS is stable at high temperatures, chemically inert (Suzuki et al., 1999), and highly conductive (Doytchinova et al., 2017). Via the freeze-casting method, GS electrodes were fabricated. The graphene synthesised in this process is inferior compared to the graphene produced via CVD. But this fabrication process is scalable, fast and cheap, producing graphene with high mechanical flexibility and excellent electrical conductivity. The SNR value of the obtained ECG signal when GS electrodes were used was measured to be 20.3 dB.

4. Conclusion

The stretchable and flexible electronics field has been getting huge attention from researchers recently. The continuous progress in this area has brought a revolution in medicine. The synthesis and processing of reproducible and consistent graphene is a huge problem for graphene-based ECG sensors and electrodes. Several advancements are required to control the graphene layer numbers, overcome mechanical restrictions, and construct devices with unique structures. In general, the wearable sensor consists of two major components- a) conductive mesh, which captures the signal, and b) stretchable/flexible substrate, which protects the conductive mesh and offers stretchability/flexibility to the sensor. Hence, the sensor materials are selected wisely, and the structures are designed carefully not to compromise the sensor’s performance. Although the graphene-based electrodes are flexible, stress corrosion cracking (SCC) might still appear on the graphene layer due to several environmental conditions during ECG monitoring. This may cause an increase in electrode impedance, causing degradation of the quality of the acquired signal. This review paper discusses a comparative study of several techniques and procedures for fabricating graphene-based ECG electrodes. The methods are screen printing, dip coating, wet transfer, wet transfer and dry patterning, spray coating, spin coating, drop casting, inkjet printing, electrospinning etc. Mechanisms of all procedures are described along with illustrations, merits- demerits and feasibility in manufacturing. Screen printing is simple and cost-effective, but has limited resolution. Dip coating is easy to control thickness and uniformity but is not very scalable. Drop casting is simple and low-cost, but it also has limited scalability. Wet transfer offers high-quality graphene and large-area deposition but requires expensive equipment and a complex process. Electrospinning is versatile and scalable, but has limited resolution. Dry patterning offers high resolution and precise control of electrode shape and size, but is complex and not very scalable. Spin coating is simple and low cost but has limited scalability and is difficult to control thickness and uniformity. Spray coating offers large-area deposition and high-quality graphene, but requires expensive equipment and a complex process. Inkjet printing is material-conserving and has a high resolution, but it is limited in scalability and requires specialised ink. Many researchers have already paid attention to this, but further research is required.

The choice of fabrication method depends on the specific requirements of the ECG sensor, such as the desired electrode shape, size, and thickness, as well as the substrate material and compatibility with the target application. Researchers must carefully consider these factors and optimise the fabrication process to achieve reproducible and consistent graphene-based electrodes for ECG signal monitoring. With continued advancements in fabrication techniques and materials, graphene-based electrodes have the potential to revolutionise the field of healthcare by enabling flexible, wearable, and non-invasive ECG monitoring for improved patient outcomes.

Abbreviations

BLG	=	Bi-Layer Graphene
CG	=	Chemically modified Graphene
CRGO	=	Chemically Reduced Graphene Oxide
CVD	=	Chemical Vapor Deposition
ECG	=	Electrocardiogram
EEG	=	Electroencephalogram
EMG	=	Electromyography
EOG	=	Electrooculogram
GES	=	Graphene Electronic
GET	=	Graphene Electronic Tattoo
GNR	=	Graphene Nano-Ribbons
GO	=	Graphene Oxide
GS	=	Graphene Sponge
GT	=	Graphene-Based Textiles
LIG	=	Laser Induced Graphene
LINA	=	Laser Induced Nanographene Aggregates
PDMS	=	Polydimethylsiloxane
PEDOT	=	Poly(3,4-Ethylenedio-xythiophene)
PEG	=	Polyethylene Glycol
PEP	=	Polyaziridine-Encapsulated Phosphorene
PET	=	Polyethylene Terephthalate
PHA	=	Polyhydroxyalkanoate
PMMA	=	Polymethyl Methacrylate
PSS	=	Poly(Styrenesulfonate)
PVA	=	Polyvinyl Alcohol
PVDF	=	Polyvinylidene Fluoride
rGO	=	Reduced Graphene Oxide
RMS	=	Root Mean Square
SEM	=	Scanning Electron Microscopy
SNR	=	Signal To Noise Ratio
TEM	=	Transmission Electron Microscopy

Author contributions

Conceptualisation, RD and RPC; methodology, PS; software, BA and BPD; validation, PS.; formal analysis, PS, and BA; investigation, AGS, MO and NB; resources, BA, BPD, and RPC; data curation, RD and RPC; writing—original draft preparation, RD, RPC, and PS; supervision, AS, and PN.; project administration, BA; funding acquisition, AS, and PN writing—review and editing: PS, BA, RD and BPD; coordination, PN; All authors have read and agreed to the published version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).