Abstract
The development of an electrocardiogram (ECG) monitoring garment that ensures monitoring accuracy, convenience, and wearing comfort is crucial for enabling timely diagnosis and prevention of heart disease. The article provides a comprehensive overview of the monitoring principles and historical evolution of ECG monitoring garment, delineates the preparation methods for textile electrodes, summarizes the evaluation techniques for ECG monitoring garment, and examines various factors that influence monitoring performance and garment durability, including the electrode material, size, shape, position, fixation method, pressurization mode, and contact pressure between the electrode and the skin. The results showed that the hybrid electrode made from multiple materials demonstrated better overall performance. Additionally, most existing studies focus on single-lead ECG monitoring, with the study subjects primarily consisting of healthy men. Therefore, future research should focus on developing textile electrodes that are flexible, skin-friendly, highly elastic, and cost-effective; examining the combined effects of textile electrodes and garment structure on ECG monitoring performance; and quantifying the relationship between body characteristics and electrode positions across varying body shapes. These conclusions provide critical insights for the enhancement of electrode design, structural optimization, and size grading in ECG monitoring garment.
Introduction
Heart disease remains a significant threat to human life, and it is an important factor in the deaths of the elderly. 1 Therefore, the implementation of dynamic monitoring and rapid diagnosis is of great significance for the prevention of various heart diseases. Electrocardiography is essential for assessing cardiac health, and acquiring clear electrocardiogram (ECG) signals is vital for the accurate analysis and diagnosis of heart disease.2,3 Currently, ECG signals are primarily acquired using medical Ag/AgCl electrodes coated with conductive gels. 4 This type of electrode is referred to as wet electrodes in this paper due to the conductive gels present between the electrodes and the skin. The conductive gels in wet electrodes ensure good contact between the skin and the electrode, facilitating a smooth flow of electricity from the skin to the electrode. 5 Additionally, the conductive gels help secure the electrode to the skin, reducing sliding and separation, which results in stable impedance between the skin and the electrode. Therefore, wet electrodes help produce high-quality ECG signals and are relatively resistant to motion artifacts.
However, prolonged monitoring can lead to the desiccation of the conductive gel, which in turn attenuates signal quality. 6 Additionally, the prolonged adhesion of gel electrodes to the skin may lead to dermatological issues, including redness, swelling, and allergic reactions.7,8 And, traditional ECG monitoring equipment is also characterized by significant bulk and weight. In light of these limitations, there is an urgent need to develop flexible, skin-friendly dry electrodes and to integrate these electrodes with small communication elements within garments to create ECG monitoring apparel. The lightweight ECG monitoring garment provides long-term heart health monitoring, thereby improving quality of life for elderly individuals and patients in non-hospital environments.
In recent years, the wearable device market has demonstrated a strong growth trend. According to a scientific research report, global shipments of smart wearable devices market reached 520 million units in 2023.9,10 At the same time, the application of wearable devices in healthcare is rapidly increasing, particularly in the areas of health management and disease prevention. The rapid growth of this market has driven swift innovation in wearable technology. In terms of personal health management, the functions of wearable devices are continually expanding, providing not only basic features such as heart rate and blood oxygen monitoring but also advanced functions like ECG and EEG monitoring. 11 Additionally, wearable devices enable early disease prediction and personalized health guidance through real-time data analysis and visual presentations of reports. 12
Currently, many research institutions have developed garments for single-lead and multi-lead ECG monitoring. Dry electrodes in ECG garments are made from various flexible, skin-friendly conductive materials, such as woven or knitted fabric electrodes, embroidered electrodes, printed electrodes, and non-fabric electrodes. The conductivity of dry electrodes is closely related to their material composition and manufacturing process. Notably, while dry electrodes can provide stable conductivity, human skin is a viscoelastic material with a curved surface, which can lead to insufficient contact between the skin and the electrode and result in high or unstable impedance at the skin–electrode interface. Simultaneously, due to the influence of human movement, there may be a small area of separation and lateral slip between the skin and the electrode, which also affects the interface impedance at the skin-electrode interface, resulting in motion artifacts in ECG signals. 13 In addition, human sweating is one of the significant causes of motion artifacts in ECG signals. To address these issues, numerous studies have investigated the factors that influence the monitoring performance of the ECG garment, such as the material, area, and fixation method of electrode, and the contact pressure between the electrodes and the skin. However, these studies primarily involve a multi-factorial analysis using a self-developed ECG garment, which may lead to a biased perspective on the findings and a lack of comprehensive systematic evaluation. Therefore, these conclusions drawn are still insufficient to directly guide the development and production of ECG garments.
To gain a more comprehensive understanding of the development of ECG monitoring garment and the current research deficiencies, we conducted a review study. A total of 572 articles on ECG monitoring garments, published between January 2008 and June 2024, were retrieved from Web of Science, Google Scholar, and PubMed. Key words include ECG monitoring garment (or clothing, suit) and textile electrode. After eliminating duplicate and invalid literature (such as reports and comments), a total of 51 articles in English, as well as 7 articles in Chinese, were included in this review. Based on a review of the existing literature, the article elucidated the monitoring principles and developmental trajectory of ECG monitoring devices, provided an overview of the fabrication techniques for textile electrodes, outlined the evaluation methodologies for ECG monitoring garments, and comprehensively investigated the factors influencing the monitoring performance and durability of the garments. The primary objective of this systematic literature review is to develop comprehensive and practical guidelines to advance the design, development, and commercialization of ECG monitoring garments. These guidelines focus on addressing essential factors, including monitoring performance, durability, user comfort, and seamless integration with existing healthcare systems. This review synthesizes current knowledge, identifies emerging technological trends, and analyzes challenges within the field to provide actionable insights for future innovation.
Mult-Leads ECG Monitoring Equipment
ECG monitoring devices are instruments capable of monitoring variations in surface potential within the cardiac electric field in real time. The devices comprise wet electrodes, wires, signal amplifiers, filters, transmission equipment, and an electrocardiograph. 14 Wet electrodes are attached to the surface of the human body to measure ECG signals. The signals are subsequently amplified, filtered, and transmitted to the electrocardiograph to record the electrocardiogram. 15
The electrodes and their connecting wires constitute an electrocardiogram circuit that is an electrocardiogram lead. 16 Different electrocardiogram leads enable the observation of the heart’s electrical activity from various perspectives. 14 The number of leads available is directly correlated with the variety of perspectives from which the heart can be observed. For instance, a single-lead ECG is limited to diagnosing arrhythmias and heart rate variability, while a 12-lead ECG can also identify conditions such as myocardial ischemia, myocardial infarction, and coronary artery disease.17–19 In clinical practice, a standard 12-lead ECG actually comprises three standard limb leads (I, II, and III), six precordial (chest) leads (V1–V6), and three augmented unipolar limb leads (aVR, aVL, and aVF). 15 The specific electrode attachment sites for various ECG leads, along with their corresponding cardiac observation angles, are illustrated in Figure 1.
The electrode position and the viewing angle of various ECG leads.
The ECG equipment can be classified into two categories based on the state of human activity during the monitoring process: static and dynamic. A static ECG device is used to monitor an individual’s ECG signals while they are in a resting state over a short duration. However, detecting certain cardiac diseases within such a limited timeframe can be quite challenging. To address this limitation, Holter et al. introduced a new type of electrocardiogram, known as the dynamic ECG, which continuously recorded the heart’s electrical activity over an extended period, typically 24 h. 20 Subsequently, this innovation led to the development of dynamic ECG devices. Static ECG devices typically monitor limb leads by attaching electrodes to the limbs of the body, which allows for the assessment of the heart’s electrical activity. However, in dynamic monitoring, limb movement may introduce motion artifacts into the ECG signal. To address this issue, the study by Mason and Likar proposed a new configuration for the limb leads, known as the Mason–Likar limb system, which involved relocating the electrodes from the limbs to the trunk. 21 Currently, most dynamic ECG equipment utilizes this electrode placement strategy.
Traditional ECG equipment faces several challenges, including its large size, significant weight, rigid electrodes, limited flexibility, poor ventilation, and suboptimal tactile properties. As a result, there has been a progressive development of lightweight, flexible, and skin-friendly multi-lead ECG monitoring garments. Specifically, the textile electrode replaces the metal wet electrode, while the flexible wire replaces the rigid wire, significantly reducing skin irritation and enhancing wearer comfort. Furthermore, to enhance the diagnostic capabilities for heart disease, the number of leads in commercially available ECG garments has increased from single-lead to multi-lead configurations.14,22 This advancement has progressively elevated the diagnostic functionality to a level comparable to that of medical-grade equipment. However, to the best of our knowledge, current dry electrodes still cannot collect ECG signals with a quality comparable to that of wet electrodes, especially when individuals are monitored in their daily lives while in motion. A number of recent research findings support this fact. For example, a study by An et al. compared textile silver electrodes, made of 99% silver-plated knitted fabric, with wet electrodes and found that the wet electrodes produced clearer ECG signals than textile silver electrodes when the subject was in a standing position. 23 Meanwhile, they found that increasing the size of the textile silver electrode can reduce the quality gap of the ECG signal compared to wet electrodes. Additionally, the study by Qin et al. compared silver nanowire electrodes with wet electrodes and found that the ECG signals monitored by the silver nanowire electrodes exhibited more significant baseline drift and waveform distortion during walking. 24 Similarly, the study by Le et al. compared six types of silver-plated, nylon-based textile electrodes with varying structures to wet electrodes and found that the wet electrodes exhibited the lowest coefficient of variation across R-R intervals under movement conditions. 25 These studies indicate that a gap still exists in ECG signal quality between dry and wet electrodes. Therefore, improving the quality of ECG signals monitored by dry electrodes under dynamic and static conditions presents a significant challenge in the research and development of ECG garments.
Preparation Methods of Textile Electrodes
The textile electrode is a crucial component of ECG garments, which play a pivotal role in monitoring cardiac performance. Based on processing technology, the preparation methods for textile electrodes can be primarily categorized into two types. One method is conductive yarn weaving, which involves integrating conductive yarns into textile electrodes through various processes. The other method is conductive ink coating, which involves applying conductive ink to the fabric surface using a variety of techniques.
Preparation Methods of Conductive Yarn Weaving
Conductive yarns encompass materials such as copper, nickel, and silver,4,5 with silver exhibiting superior electrical conductivity, antibacterial properties, and deodorization capabilities,26,27 making it an ideal choice for use in electrodes. Consequently, electrodes fabricated from silver yarn are among the most prevalent in the market. Furthermore, based on their fabric structure, textile electrodes made of silver yarn can be categorized into two types: woven electrodes and knitted electrodes.
Woven Fabric Electrodes
Woven fabrics typically possess the following characteristics: a stable structure, tight interweaving of warp and weft yarns, and resistance to deformation. Many studies have found that textile electrodes made from woven fabrics perform well in monitoring ECG signals from the human body under static conditions, such as while sitting and standing. Song et al. employed polyester filament as the warp yarn and silver yarn as the weft yarn to fabricate woven fabric electrodes using a double-sided Jacquard process. 28 They found that the signal-to-noise ratio (SNR) of ECG signals monitored from these electrodes were relatively high when individuals were in static states. However, the SNR were not satisfactory when individuals were in dynamic states. Dong et al. conducted a comparative analysis of both woven fabric electrodes featuring plain and satin weave structures, utilizing identical silver yarn and maintaining consistent warp and weft densities. 29 The results revealed that the electrode with a satin weave exhibited superior conductivity and produced clearer ECG signals compared to the plain weave. Dong et al. suggested that the observed phenomenon can be attributed to two main factors: the lower interweaving density of warp and weft yarns in satin weave, and the long conductive float lines on the fabric surface. These factors increase the contact area between the skin and conductive yarns, thereby producing lower impedance at the fabric–skin interface. 29 Furthermore, Xiao et al. conducted a comparative study of woven fabric electrodes made from plain and honeycomb structures, discovering that the fabric electrodes with honeycomb structures showed higher impedances at the fabric-skin interface and produced distinct ECG signals. 30 This finding was attributed to the presence of numerous micro-grooves on the surface of the honeycomb fabric, which reduce the contact area between the fabric and the skin, thereby increasing the impedance at the fabric-skin interface.
Knitted Fabric Electrodes
Knitted fabrics typically possess the following characteristics: a loose structure, good tensile elasticity, and high ductility. Several studies have found that electrodes made from knitted fabrics perform more effectively than those made from woven fabrics in monitoring ECG signals from the human body under dynamic conditions, such as while walking and running. Rajanna et al. conducted an investigation comparing the two structures (woven and knitted) of textile electrodes and discovered that the quality of the ECG signals obtained from knitted fabric electrodes was higher than that obtained from woven fabric electrodes under dynamic conditions. 31 Fink et al. examined two electrodes produced through distinct weaving processes. 32 One electrode involved a knitted fabric made from silver-plated nylon yarn, while the other entailed an embroidered fabric in which silver yarn was stitched onto the knitted fabric. The results showed that the embroidered electrode had better ECG signal quality under static and dynamic conditions.
Comparison between Woven and Knitted Fabric Electrodes
In summary, the fabric structure affects the texture, surface characteristics, and tensile properties of the textile electrode. For the woven fabric electrode, the compact structure, combined with the closely arranged conductive yarns on the surface, facilitates full contact between the skin and the conductive yarns, thereby reducing the impedance at the fabric-skin interface. Under static conditions, there is no relative slip between the skin and the electrode. The current flowing through the skin from the woven fabric electrode is relatively large and stable, resulting in high-quality ECG signals. On the contrary, under dynamic conditions, the limited elasticity of the woven fabric electrode can easily cause it to shift on the skin surface, resulting in fluctuations in interface resistance and the production of motion artifacts. Therefore, the quality of the ECG signals is compromised under dynamic conditions. For the knitted fabric electrode, the loose structure and the loose arrangement of conductive yarns decrease the contact area between the skin and the yarns, which increases the impedance at the fabric-skin interface. Therefore, under static conditions, the impedance of the knitted fabric electrode at the skin–fabric interface is greater than that of the woven fabric electrode, resulting in a lower quality of the ECG signals. However, the stretchability and ductility of the knitted fabric electrode are higher than those of the woven fabric electrode. There may be intermittent slipping between the skin and the electrode when the human body is in motion. The knitted fabric electrode, which is a highly elastic, can effectively alleviate relative slip between the skin and the electrode, reduce fluctuations in impedance at the skin–fabric interface, and minimize motion artifacts in the ECG signals. Therefore, the quality of the ECG signal monitored from the knitted fabric electrode is superior to that of the woven fabric electrode under dynamic conditions. In addition, the embroidered electrode mentioned in the study of Fink et al. consists of a layer of conductive yarn tightly sewn on the surface of the knitted fabric. This electrode not only retains the high elasticity of the knitted fabric but also improves the density of conductive yarn on the fabric’s surface, thereby reducing the impedance at the skin–fabric interface. Therefore, it demonstrates strong performance in monitoring ECG signals under static and dynamic conditions.
Preparation Methods of Conductive Ink Coating
Based on the composition of the conductive ink, it can be categorized into silver ink, carbon ink, poly(3,4-ethylenedioxthiophene) (PEDOT:PSS) ink, and mixed ink. The coating methods vary depending on the specific types of electrodes used.
Silver Ink Electrodes
The components of silver inks mainly include silver, Ag/AgCl, silver nanoparticles (AgNW). Typically, silver inks are combined with a polyurethane (TPU) solution, which is then applied to the surface of fabrics to create electrodes. Tada et al. mixed silver ink with a TPU solution and coated it onto the surface of elastic fabric to fabricate a silver electrode that possesses a degree of elasticity. 33 Similarly, Yokus et al. and Bu et al. utilized screen printing to apply Ag/AgCl ink and silver ink to the surfaces of stretchable nonwoven fabric and TPU film, respectively, resulting in the formation of Ag/AgCl-nonwoven electrodes and Ag-TPU electrodes, both exhibiting certain elastic properties.34,35 The findings indicate that this type of elastic electrode exhibits superior stability in dynamic monitoring. Qin et al. applied a coating of AgNW onto the surface of polydimethylsiloxane (PDMS) to fabricate the AgNW electrode. 24 PDMS is characterized by its excellent flexibility, ductility, and biocompatibility, which it makes an ideal polymer for preparing composite materials. The results demonstrate that the AgNW electrode performs effectively under static monitoring conditions. However, under dynamic conditions, the electrocardiogram (ECG) signal was prone to baseline drift and waveform distortion.
Carbon Ink Electrodes
In general, carbon conductive materials can be mixed with PDMS to create carbon electrodes using thermocoagulation technology. The components of carbon inks typically include carbon nanotube (CNT), graphite powder (GP), and graphene (GN), each contributing unique properties to the ink’s performance.36–38
Using CNT as a substrate, many scholars have developed carbon electrodes.39–41 For instance, Jung et al. employed a cylinder agitator to disperse a high-concentration CNT solution into a PDMS matrix, subsequently fabricating CNT-PDMS electrodes using thermocoagulation technology. 39 Their study demonstrated that, after 7 days of continuous wear, the ECG signal remained stable, and no significant skin irritation or swelling was observed. Similarly, Li et al. integrated a composite film of CNT and TPU onto the surface of knitted fabrics to create textile dry electrodes with excellent tensile properties. 42 Furthermore, Tas et al. produced MWCNT-PDMS electrodes by coating a composite material made of multi-walled carbon nanotubes (MWCNT) and PDMS onto the surface of a polyimide film using thermocoagulation technology. 40 To achieve a uniform dispersion of MWCNT on the surface of viscous PDMS, Chi et al. utilized n-hexane as a dispersant and employed ultrasonic dispersion techniques to fabricate MWCNT-PDMS electrodes. 41 Their findings indicated that during prolonged continuous monitoring, the ECG signal from these electrodes remained stable without significant attenuation, and no adverse skin reactions, such as redness or swelling, were observed.
Using GP as a substrate, Lee et al. mixed it with a polyvinyl alcohol (PVA) film-forming agent and applied the mixture to the skin surface where the electrode was attached. After drying, the GP-PVA electrode conformed to the curved surface of the skin, thereby enhancing the actual contact area between the electrode and the skin, which in turn reduced the contact impedance. 43 Furthermore, Cheng et al. prepared a GP-PDMS electrode by attaching graphite powder to the surface of PDMS using thermocoagulation technology. The GP-PDMS electrode has characteristics of low modulus, high adhesion, and thin thickness. It was found that, under static and dynamic conditions, the ECG waveforms monitored by the GP-PDMS electrode were similar to those of wet electrodes. 44
Using GN as a substrate, Sun et al. attached GN to the surface of a porous material to create a dry electrode with high permeability. They found that, under static conditions, the quality of ECG recordings from the chest lead using the porous GN electrode was comparable to that of the wet electrode. 45 Furthermore, Celik et al. deposited it onto the surface of medical Ag/AgCl electrodes via chemical vapor deposition to create GN-Ag/AgCl electrodes. 46 Although the GN-Ag/AgCl electrode was not flexible, it was capable of detecting a clear ECG signal, achieving an 8% increase in the signal-to-noise ratio without the need for conductive gel. Therefore, this method could serve as a reference for the future development of textile electrodes.
Among the three kinds of carbon materials examined, GN exhibits superior electrical conductivity, has a well-established preparation process, and is a moderate priced (higher than GP but lower than CNT). Consequently, graphene holds significant potential for future advancements in electrode technology.
PEDOT: PSS Ink Electrodes
PEDOT, a polymer material developed by Bayer company in Germany, exhibits high physical stability and electrical conductivity. Its water dispersion (PEDOT:PSS) demonstrates excellent processability, making it widely applicable in the field of smart textiles.47,48
There are two primary methods for preparing PEDOT:PSS in situ polymerization and gas phase polymerization. In situ polymerization involved impregnating the fabric with a solution containing 3,4-ethylenedioxythiophene (EDOT) and an oxidant, followed by polymerization at a low temperature (below 10°C). Subsequently, the fabric was removed from the solution and dried to produce a PEDOT:PSS electrode. 49 Notably, this low-temperature environment can effectively prolong the polymerization time, which is conducive to the uniform adhesion of PEDOT:PSS on the fabric surface.
Gas phase polymerization involves impregnating the fabric with an oxidant solution, followed by exposure to the gas phase EDOT monomer and the subsequent polymerization of the electrode within a vacuum reactor. In comparison to in situ polymerization, the gas phase polymerization does not require a low-temperature environment and thereby offers a more straightforward processing technology. Additionally, this method eliminates the need for organic solvents and thus conserves materials more effectively.
Utilizing gas phase polymerization, Trindade et al. prepared a PEDOT:PSS electrode on polyester fabric, which was subsequently sewn onto an elastic chest strap for ECG signal measurement. 50 However, this method may require sewing the electrodes twice, which complicates the sewing process and reduces electrical conductivity. To address this issue, Takamatsu et al. proposed a method to directly print PEDOT:PSS electrodes onto the fabric. 51 The detailed procedure was as follows: (1) create a hollow electrode template and coat it with PDMS; (2) position the template over the polyester fabric to facilitate the transfer of PDMS onto it, enhancing the hydrophobic performance in the coated area; and (3) coat PEDOT:PSS onto the surface of the polyester fabric and allow it to dry. Finally, the PEDOT:PSS coating is applied to the non-PMDS region, while the PDMS coating is washed away to create the PEDOT:PSS electrode. According to the study by Takamatsu et al., ECG signals obtained using this electrode with conductive gel exhibited superior performance compared to those acquired with the wet electrode. Furthermore, Ankhili et al. enhanced the viscosity of the PEDOT:PSS solution applied to the fabric through a chemical modification technique, thereby improving the wash resistance of the electrode. 52 Agua et al. developed a novel PEDOT:PSS-DVS electrode by introducing divinyl sulfone (DVS) as a crosslinker, which significantly improved the electrode’s stability in water without compromising its conductivity. 53 Lo et al. incorporated polyethylene oxide into the PEDOT:PSS solution to enhance the tensile properties of the ink, thereby improving the elasticity of the electrodes. 54
Hybrid Ink Electrodes
The hybrid ink electrode is fabricated by sequentially coating multiple layers of different types of inks onto the surface of a fabric. Shathi et al. integrated the material properties of PEDOT:PSS (including physical stability, biocompatibility, and high conductivity) with reduced graphene oxide (rGO) (notable for its high electron mobility and surface area ratio). Specifically, rGO was applied to the fabric surface using jet dyeing technology, followed by the coating of PEDOT:PSS onto the rGO surface, resulting in the formation of a PEDOT: PSS-rGO electrode.55,56 A comparative analysis between the PEDOT:PSS-rGO electrode and the PEDOT:PSS electrode revealed that the former exhibited superior conductivity after undergoing repeated cycles of washing, stretching, and bending. Furthermore, Huang et al. created a carbon ink electrode synthesized from carbon black (CB), rGO, and polyurethane. They then coated the surface of the carbon ink electrode with a PEDOT:PSS film, creating a hybrid ink electrode. 57 The hybrid electrode has high electrical conductivity, mechanical durability, and hydrophobicity. The quality of the ECG signal monitored by the hybrid ink electrode is comparable to that of the wet electrode. Zahed et al. combined PEDOT:PSS with GN to create a hybrid ink electrode with high flexibility and conductivity. 58 When compared to the wet electrode, it was found that the SNR of the ECG signals was essentially similar to that of the wet electrode.
Comparison Among the Different Ink Electrodes
Currently, there are few comparative studies on the monitoring performance of electrodes made from these four inks, with most studies focusing on comparing newly developed ink electrodes to traditional wet electrodes. Because the monitoring performance of an electrode is difficult to determine based solely on the ink material, it depends more on factors such as the ink composition ratio, the production process, and the size, thickness, and shape of the electrode. From a review of the literature, we found that the hybrid ink electrode made of PEPOT:PSS ink and carbon ink exhibits better electrical conductivity and improved quality in ECG monitoring. In contrast, the conductivity of the silver ink electrode is poor, resulting in subpar ECG monitoring quality. Among the electrodes studied, the carbon ink electrode demonstrates good conductivity due to its high specific surface area and electron mobility, while the PEPOT:PSS ink electrode offers superior biocompatibility, flexibility, and ductility. 59 Furthermore, Meziance et al. and Joutsen et al. compared the ECG signal quality between carbon-based and silver-based electrodes, respectively, and found that the signal quality recorded by the silver-based electrodes was poor under both static and dynamic conditions.60,61 The conclusions drawn about different ink electrodes exhibit a certain degree of directivity and are not universal. Further extensive experimental studies are needed to obtain more reliable research findings.
Measurement Methods for Monitoring Performance
Currently, there is no standardized methodology for assessing the monitoring performance of ECG clothing. Based on different evaluation objectives, the evaluation methods can be categorized into two primary types: direct evaluation and indirect evaluation.
Direct Evaluation
The direct evaluation involves either observing the clarity of the ECG signals or comparing them with the measurement results obtained from medical Ag/AgCl electrodes under the conditions of various body postures, actions, and movements. From 2010 to 2024, the experimental parameters and evaluation indicators used in the direct evaluation method for ECG monitoring equipment are presented in Table 1.
Experimental parameters and evaluation indexes of evaluation research of ECG equipment.
SNR = signal-to-noise ratio; QRS-complex = the complex of Q-wave, R-wave, and S-wave.
The subjects recruited in these experiments were in good health. The sample size for this type of assessment is relatively small, usually consist of one to three subjects, predominantly male participants. The positions include standing, sitting, and supine. The actions are primarily designed based on the activities of daily living (ADL) and the instrumental activities of daily living (IADL) and encompass actions such as horizontal forearm rotation, lateral arm swing, forward arm swing, horizontal torso rotation, lateral torso bend, and anterior–posterior torso bend. The movements include walking, jogging, running, stepping forward and backward, lateral stepping, squatting, and ladder climbing.
The evaluation indexes can be categorized into two categories: qualitative and quantitative. Qualitative indexes primarily assess the observability of distinct ECG waveforms, such as the P-wave, R-wave, S-wave, T-wave, and QRS-complex. In contrast, quantitative indexes provide specific measurements of these qualitative aspects, encompassing parameters such as SNR, R-wave detection rate, baseline deviation, kurtosis and skewness of QRS-complex, and correlation coefficients between textile electrodes and medical electrodes across both time and frequency domain indexes.
The SNR is defined as the ratio of the intensity of the received effective signal to the intensity of the noise signal. It is noteworthy that researchers have varying definitions of what constitutes effective signals and noise signals in ECG readings. For instance, Trindade et al. used the most significant QRS-complex as the setting standard, defined the peak-to-peak value of the QRS-complex as effective signal, and defined the average amplitude on the baseline over 100 as noise signal. 50 Conversely, Lee et al. used the minimum P-wave as the setting standard, indicating that analyzing ECG signals becomes challenging if the noise exceeds the amplitude of the P-wave. 43 Consequently, the average amplitude of the P-wave is defined as the effective signal, and the peak standard deviation of the fluctuating signal between the T-wave and the P-wave is defined as noise signal. The R-wave detection rate is defined as the ratio of the number of correctly identified R-waves in an ECG to the total number of R-waves present.35,64 The amplitude of the R-wave, which reflects ventricular depolarization activity on an ECG, plays a crucial role in identifying other characteristic waves and in classifying arrhythmias. Therefore, it serves as a significant indicator in the evaluation of ECG results. Baseline deviation refers to changes in the baseline position of the ECG signal. 31 Typically, the ECG baseline is a smooth horizontal line, symbolizing the heart’s electrical activity at rest. A larger offset in the ECG signal baseline can lead to decreased accuracy in the monitoring signal. 69 Le et al. discovered that the kurtosis (K = 27.5) and skewness (S = 4.1) of a clear PQRST-complex were significantly larger, while the kurtosis (K < 5) and skewness (S < 1) of a noisy PQRST-complex were smaller. 25 Consequently, these two statistical measures were used as indicators for the evaluation of ECG signals. Specifically, an increase in the values of kurtosis and skewness was associated with a clearer signal. Furthermore, the researchers conducted a comparative analysis of the measurement results obtained from ECG clothing and traditional ECG equipment. This was done to determine the correlation between the two sets of results under the same index. A higher correlation coefficient implies a better quality of monitoring for the ECG clothing.
In conclusion, the focus of ECG evaluation research has predominantly been on a number of male subjects, which may limit the generalizability of the findings. Furthermore, numerous studies have evaluated the performance of ECG clothing in various postures, actions, and movements; however, there is a noticeable lack of research investigating the effects of perspiration from the human body on ECG clothing. Currently, the evaluation metrics for ECG clothing are comprehensive, allowing for the assessment of its monitoring performance through quantitative indices. Future research should strive to include a more diverse range of participants, encompassing various genders, ages, and body types, to enhance the universality of the research results for ECG clothing.
Indirect Evaluation
The indirect evaluation involves assessing the performance of ECG monitoring by quantifying non-ECG indicators, including electrode conductivity, contact impedance between electrode and skin, and electrode displacement. The conductivity of the electrode can reflect the clarity of the ECG signal. An increase in electrode conductivity results in a decrease in ECG lead resistance, which enhances the detectability of ECG signal fluctuations. The conductivity of the electrode is characterized by sheet resistance, which is defined as the ratio of electrode’s resistance to its geometric factors, 70 and is often measured using the two-probe method or four-probe method.71,72 A smaller contact impedance between the electrode and the skin can lead to reduced ECG lead resistance and a clearer ECG signal. The contact impedance is usually measured by an impedance analyzer.63,73 The electrode displacement refers to the relative displacement of the electrode on the skin as the human body moves. An increase in electrode displacement is directly proportional to the prominence of the motion artifact in the ECG signal. The electrode displacement is primarily measured using either a digital camera or a displacement sensor, depending on the required precision and application.62,74
Influencing Factors in ECG Monitoring Performance
The Electrode Material
The material of the electrode significantly influences its conductivity; thus, it plays a crucial role in determining the quality of ECG signal. Electrodes are primarily classified into three types based on their material composition: silver, carbon, and PEDOT:PSS. Current research suggests that these three types of electrodes have not yet achieved the same monitoring performances as wet electrodes used in conjunction with conductive gels. 75 Among them, the silver electrode closely resembles the wet electrode in terms of material composition and exhibits superior conductivity; carbon exhibits commendable electrical conductivity, and its proficiency in electrode monitoring performance is equally noteworthy; conversely, PEDOT:PSS exhibits relatively poor conductivity and weak monitoring performance.
Furthermore, it is important to note that the conductivity of the electrode depends on the composition of the conductive material. Tasneem et al. conducted a study on three types of CNT electrodes with varying mass fractions (4.0–12.0 wt%) and discovered a correlation between the mass fraction of CNT and electrode resistance, with the latter decreasing as the former increased. 40 Similarly, Shathi et al. explored the impact of rGO coating on electrode resistance and found that an increase in rGO coating led to a decrease in resistance. 55 Specifically, upon the application of ten coatings, the electrode resistance experiences a significant reduction from 180 kΩ to 120 Ω, indicating a decrease in resistance of approximately 1500-fold. Ankhili et al. conducted a comprehensive study on four distinct types of PEDOT:PSS electrodes with varying mass fractions (6.3–12.8 wt%). 76 Their findings revealed that the electrode conductivity increased in correspondence with an increase in mass fraction. Notably, when the mass fraction reached 12.8%, the conductivity of the electrode was essentially equivalent to that of the wet electrode. Consequently, it is evident that the composition of the conductive material serves as a critical parameter affecting the electrode’s conductivity.
The Electrode Size
The electrode size significantly influences the contact impedance between the electrode and the skin, which in turn impacts the ECG monitoring performance. A study conducted by Cai et al. examined five types of round carbon electrodes with varying diameters (2.0–6.0 cm), revealing that contact impedance decreased as the electrode size increased. 67 Concurrently, a comparison was made with the wet electrode (diameter = 2.0 cm), demonstrating that the interface resistance of the electrode was lower than that of the wet electrode when the electrode size exceeded 2 cm. Similarly, Yokus et al., Tasneem et al., and Lee et al. corroborated the findings of Cai et al. through their research on round electrodes of varying diameters (0.5–3.0 cm) and composed of different materials (silver and carbon).34,40,43
The studies mentioned above revealed that the electrode size positively influences the ECG monitoring performance. However, it is important to acknowledge that the electrodes currently being developed often face issues such as rigidity, lack of breathability, and limited stretchability. Additionally, an excessively large electrode size can cause discomfort upon contact. Therefore, considerations of comfort should be incorporated into the design of the electrode size.
The Electrode Shape
The influence of electrode shape on ECG monitoring performance was further examined, while maintaining a consistent electrode size. Tasneem et al. conducted a study involving two carbon electrodes of varying rectangular shapes (5.0* 0.4 cm and 4.0* 0.5 cm), and found no significant variation in the ECG signals detected by either electrode. 40 Conversely, Zhao et al. conducted research on three silver electrodes of differing shapes (square, round and oval), and discovered that, given the same area, the ECG signals monitored by oval electrodes were more distinct. 77 It is evident that there is no definitive consensus on the impact of electrode shape, likely due to the interplay among the shape, material, and size of the electrode.
The Electrode Position
Generally, the configuration of electrode positioning in an ECG garment adheres to the standard 12-lead system used in ECG. However, several studies have investigated the quality of ECG signals obtained from various positions on the human body. Cho et al. designed an ECG system based on a tightly fitting waistcoat, involving 56 electrodes (diameter = 1.2 cm) distributed across the front and back of the body. 65 The electrodes were arranged in a grid with a distance of 6 cm both horizontally and vertically between adjacent electrodes in the ECG system. As illustrated in Figure 2, the blue area represents the electrode position with high ECG signal quality; the larger the blue area, the clearer the ECG signal. Their findings indicated that the electrodes positioned below the anterior and posterior aspects of the pectoralis major muscle on both sides were the least influenced by movement, thereby producing the clearest ECG signals. Yokus et al. conducted an investigation into the ECG signals from two electrodes positioned at varying distances on the chest circumference line. 34 Specifically, the right electrode was centrally positioned on the chest, while the left electrode was placed on the second rib, near the left arm. The right electrode was sequentially moved to the left, creating distances of 2.0, 4.0, 6.0, 8.0, 10.0, and 12.0 cm between the two electrodes, respectively. Their findings indicated that as the spacing between the electrodes increased, the SNR of the ECG signals initially rose before subsequently declining, peaking when the electrode spacing was 8 cm. 25
Distribution of electrode position of the ECG monitoring garment.
Contact Pressure Between the Electrode and the Skin
The contact pressure between electrode and skin affects the actual contact area between the two, which in turn influences the contact impedance and the quality of the electrocardiogram signal. The regulation of contact pressure primarily occurs through two modes: gasket pressurization and strap pressurization, as shown in Figure 3.
The two modes of regulating pressure: (a) Gasket pressurization and (b) strap pressurization.
Gasket Pressurization
Gasket pressurization refers to the insertion of a gasket between the garment and the electrode, which enhances the stretching deformation of the garment by adjusting the thickness of the gasket, thus increasing the contact pressure between the garment and the electrode. Tada et al. manipulated the contact pressure by inserting gaskets made from different materials with the same thickness of 1.5 cm, and discovered that the contact impedance between the fabric and the skin decreased as the contact pressure (0–7 kPa) increased. 33 They also found that in the absence of contact pressure, the ECG signals were challenging to discern during standing, walking, and jogging; and in the presence of contact pressure, the noise signals in the ECG from V1, V2, V4, and V6 leads decreased during walking, while the noise signals in the ECG from V1 and V2 leads were relatively large and that from V4 and V6 leads were relatively small during jogging. Moreover, Cai et al. further examined the impact of contact pressure on ECG signals by inserting sponge gaskets of varying thicknesses (0–1.5 cm). 67 Their study found that an increase in contact pressure initially enhanced the clarity of ECG signals; however, beyond a certain point, the clarity began to decrease. The optimal ECG signal was achieved with a gasket thickness of 0.5 cm. The study by Cai et al. indicated that a sponge that is too thick could cause an arc contact between the electrode and the skin, which would increase contact impedance and reduce signal quality. Based on the findings of Rajanna et al., 31 as the pressure exceeded 5 kPa, the quality of the ECG signal was less affected by further increases in pressure. In addition to the thickness of the gasket, Cömert et al. also investigated the influence of gasket material and found that, under the same pressure conditions, five different gasket materials had no significant impact on ECG signals. 64
Strap Pressurization
Strap pressurization refers to the adjustment of strap tightness in the garment, enhancing the stretching deformation of the garment, thus increasing the contact pressure between the garment and the electrode. Cömert et al. conducted a study on the performance of silver electrodes in ECG monitoring under varying contact pressures ranging from 655 to 3325 Pa. 64 Their findings indicated that as contact pressure increased, the contact impedance between the skin and the electrode decreased, leading to a reduction in motion artifacts in ECG signals. Notably, within the pressure range from 1995 to 2660 Pa, the ECG signals were the clearest. However, beyond this range, there was a slight increase in contact impedance and a corresponding decrease in ECG quality. Similarly, Dong et al. conducted research on the ECG signals produced by four types of silver electrodes at varying contact pressures (2, 5, and 10 kPa) and suggested that increased pressure correspondingly improved the quality of the ECG signals, with optimal signal quality observed at a pressure of 10 kPa. 29
In conclusion, regardless of the mode of pressurization employed, increasing the contact pressure can significantly enhance the quality of the ECG signal. However, it is important to note that the optimal pressure threshold varies depending on the pressurization mode and the electrode material. Furthermore, excessive pressure can potentially lead to a sense of constraint in specific areas of the human body. Consequently, investigating the correlation between pressurization mode, electrode material, and contact pressure, as well as establishing the optimal pressure threshold under varying conditions, holds substantial significance for the advancement of ECG monitoring garment.
The Fixation Method of the Electrode
Under identical contact pressure, the fixation method of the electrode within the garment was also examined. Cho et al. developed four methods for fixing electrodes via the strip, including “chest-belt type,”“cross type,”“X type,” and “curved X type,” as depicted in Figure 4(a). 62 Their research determined that across six different body actions (horizontal forearm rotation, lateral arm swing, forward arm swing, horizontal torso rotation, lateral torso bend, and anterior-posterior torso bend), the electrode offset of the “cross type” was minimal, exerting a negligible impact on the ECG signal. Conversely, the electrode offset of the “X type” was significantly larger, resulting in a more pronounced motion artifact of the ECG signal. Moreover, Yan et al. created three types of electrode fixation based on sports underwear (“fold line type,”“curve line type,” and “oblique line type,” as shown in Figure 4(b)), 74 discovering that, under the same conditions as those in Cho et al.’s study, the electrode offset of the “fold line type” was smaller (<2.7 cm). It is evident that, under the same pressure, different methods of electrode fixation can also affect the quality of the ECG signal.
Fixation methods for the electrodes: (a) Four methods of fixing electrodes as studied by Cho et al. and (b) three methods of fixing electrodes as studied by Yan et al.
Influencing Factors in the Durability of ECG Monitoring Garment
Factors in Washing Aspect
In the laundering process, the electrodes within the ECG garment are likely to experience some degree of degradation due to mechanical and chemical actions, leading to a decline in conductivity, which in turn impacts the precision of monitoring in wearable technology. Currently, the primary factors being investigated in relation to washing include washing times, washing temperature, and detergent type.
Washing Times
Trindade et al. conducted a study on the machine washing of three different electrodes, one PEDOT:PSS electrode and two medical electrodes, and discovered that the sheet resistance of each electrode increased to varying extents post-washing. 50 Likewise, Bu et al. subjected a silver electrode to 40 washing cycles and observed an increase in electrode resistance with each subsequent wash. Shathi et al. washed three PEODT: PSS-rGO electrodes (P-rGO-1, P-rGO-2, P-rGO-3) multiple times and found that the resistance of these electrodes increased with the number of washing cycles. 35 For example, after 25 washes, the resistance of the P-rGO-1 electrode increased from 85 to 190 kΩ. The resistance of the P-rGO-2 electrode (P-rGO-1 treated with 5.0 wt% dimethyl sulfone) and the P-rGO-3 electrode (P-rGO-1 treated with 5.0 wt% ethylene glycol) increased from 30 to 70 Ω and from 20 to 90 sΩ, respectively.
In summary, as the number of washing cycles increases, the electrode resistances of different materials also rise, and with the degree of increase primarily depending on the conductivity of the electrode material. Certain research suggests that applying a hydrophobic coating to the electrode surface can significantly mitigate the deterioration of the electrode after washing.75,78 However, it is crucial to consider that if the hydrophobic film lacks electrical conductivity, it could potentially compromise the overall conductivity of the electrode.
Washing Temperature
Trindade et al. conducted an experiment involving the machine washing of three types of electrodes (one PEDOT:PSS electrode and two medical electrodes) at three different temperatures (23°C, 40°C and 60°C), and found that the electrode resistance decreased with increasing washing temperature. 50 Furthermore, Trindade et al. discovered that the impact of washing temperature on electrode resistance was contingent upon the type of electrode material. For instance, the resistance of the PEDOT:PSS electrode was more susceptible to changes in varying washing temperatures compared to the other two types of medical electrodes. Consequently, based on this result from the study by Trindade et al., it is advisable to clean electrodes at elevated temperatures to improve the durability of the ECG monitoring garment. 50
Detergent Type
The effects of various detergents, both acidic and alkaline, on electrode conductivity were also examined. Some researchers found that under alkaline detergent conditions, the resistances of the PEDOT:PSS electrode and the medical electrode increased considerably after machine washing, while under acidic detergent conditions, the conductivity of each electrode was less affected. 50 Therefore, we recommend using acidic detergent when cleaning the ECG monitoring garment.
Factors From Mechanical Action
Since human body movement can cause mechanical losses in the electrodes and wires within clothing, such as stretching, bending, and friction, this can affect the longevity of ECG monitoring garments. Shathi et al. conducted an experiment involving the repeated bending of a self-developed PEDOT:PSS-rGO electrode for a total of 130 times. 55 Their findings indicated a marginal decrease in electrode conductivity post-bending; however, the overall stability remained relatively unaltered. In a similar study, Bu et al. conducted an experiment involving the repeated stretching of a silver-based electrode and found a slight increase in electrode resistance that correlated with the frequency of stretching. 35 Concurrently, the research by Bu et al. indicated that when the electrode was in the initial stretch, there was no substantial disparity between the electrode resistance in the stretched state (i.e. elongated to twice the electrode’s original size) and in the non-stretched state; when the stretching times are below 250, the electrode resistance in the non-stretched state marginally surpasses that in the stretched state; and when the stretching times exceed 250, the electrode resistance in the non-stretched state is marginally inferior to that in the stretched state. In summary, under the action of mechanical force on the ECG monitoring garment, the electrode resistance and ECG monitoring performance change; however, the influence is minimal.
Conclusions and Prospectives
This review elucidated the principles of monitoring and the developmental process associated with ECG monitoring equipment, provided a comprehensive summary of the methodology involved in the preparation of textile electrodes and the evaluation techniques for ECG monitoring garments, and examined the factors influencing the monitoring performance and durability of ECG clothing. Based on the analysis above, future research can be pursued from three distinct perspectives.
The first prospect is to engineer flexible, skin-friendly electrodes that exhibit superior conductivity, high tensile elasticity, and economical manufacturing costs. In our study, we elaborated on the advantages and disadvantages of various textile electrodes and their preparation methods. We found that textile electrodes developed from a single material exhibit various defects, such as the lack of elasticity in silver fabric electrodes, the high cost of carbon nanotube ink electrodes, and the poor conductivity of PEDOT:PSS electrodes. In contrast, textile electrodes made from composite materials, such as PEDOT:PSS and CNT mixed ink electrodes, demonstrate superior overall performance. We found that the textile electrode developed by a single material will have a variety of defects, such as the lack of elasticity of the silver fabric electrode, the high cost of the carbon nanotube ink electrode, and the poor conductivity of the PEDOT:PSS electrode, etc., while the textile electrode developed by the composite material can show a more powerful comprehensive performance, such as PEDOT:PSS and GN mixed ink electrodes. However, most of these electrodes are made from multi-layer coating materials, such as a PEDOT:PSS film at the bottom and a CNT coating on top. Although this type of electrode can improve biocompatibility and conductivity to some extent, its air permeability is insufficient. Therefore, in the future, we can start from a microscopic point of view by first spinning conductive material into yarn, and then weave it with spandex yarn into conductive fabric using three-dimensional weaving technology, and finally coat the surface of the conductive fabric with graphene using the microporous film process to create electrocardiogram electrodes with excellent overall properties, such as conductivity, permeability, and tensile strength.
The second prospect is to explore the interaction effects of material characteristic, fixation method, pressurization mode, and contact pressure on ECG monitoring performance at different body parts. A comprehensive analysis of the factors influencing the monitoring performance has been conducted in the study, and revealed that the electrode’s material characteristics, fixation methods, pressurization modes, and contact pressure had an interactive impact on ECG clothing functionality. However, existing studies have only conducted a comprehensive analysis of the aforementioned multifactorial aspects at a few points on the chest (V1 and V2), while the configuration of clothing parameters on other parts of the human body has not been discussed in detail. Therefore, in the future, it is necessary to analyze the 10 points (RA, LA, RL, LL, and V1 to V6) in the standard 12-lead ECG one by one and summarize the internal influences of various factors at each electrode point. Under the premise of clearly defining the position and performance of each electrode, the fixation method, pressurization mode, and optimal threshold of contact pressure need to be further quantified, thereby optimizing the configuration of the ECG monitoring garment.
The third prospect involves quantifying the relationship between body characteristics and electrode position across varying body shapes, and explicitly elucidating the rules for size grading of ECG monitoring garments. Upon reviewing the evaluation methodologies for ECG monitoring clothing, it was observed that the demographic in these studies predominantly consisted of young males, with a notable underrepresentation of women and the elderly. However, it is important to highlight that the elderly population is at an increased risk of cardiac disease. Consequently, future research should aim to conduct a comprehensive quantitative analysis investigating the correlation between body characteristics and electrode position across diverse population groups, particularly focusing on the elderly demographic. This would facilitate the establishment of standardized guidelines for ECG monitoring clothing tailored to different body types, thereby broadening the scope of its applicability.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Xi’an Polytechnic University Doctoral Scientific Research Initial Fund Project (BS202310).
