Biopotential Electrodes Research Articles

Sensing and Control Strategies Used in FES Systems Aimed at Assistance and Rehabilitation of Foot Drop: A Systematic Literature Review.

Functional electrical stimulation (FES) is a rehabilitation and assistive technique used for stroke survivors. FES systems mainly consist of sensors, a control algorithm, and a stimulation unit. However, there is a critical need to reassess sensing and control techniques in FES systems to enhance their efficiency. This SLR was carried out following the PRISMA 2020 statement. Four databases (PubMed, Scopus, Web of Science, Wiley Online Library) from 2010 to 2024 were searched using terms related to sensing and control strategies in FES systems. A total of 322 articles were chosen in the first stage, while only 60 of them remained after the final filtering stage. This systematic review mainly focused on sensor techniques and control strategies to deliver FES. The most commonly used sensors reported were inertial measurement units (IMUs), 45% (27); biopotential electrodes, 36.7% (22); vision-based systems, 18.3% (11); and switches, 18.3% (11). The control strategy most reported is closed-loop; however, most of the current commercial FES systems employ open-loop strategies due to their simplicity. Three main factors were identified that should be considered when choosing a sensor for gait-oriented FES systems: wearability, accuracy, and affordability. We believe that the combination of computer vision systems with artificial intelligence-based control algorithms can contribute to the development of minimally invasive and personalized FES systems for the gait rehabilitation of patients with FDS.

Multifunctional sensors for surface synchronized monitoring of Pressure/Biopotential signals and motion recognition

Surface biopotential dry electrodes are an important part of long-term healthcare health monitoring systems. During long-term dynamic monitoring, pressure fluctuations between the skin/electrode can affect the acquisition of ECG signals and reduce the stability of the system. In this study, by printing an interactive circuit on the back of the Fructus xanthii-inspired barb structure (FXbs) Ag/AgCl-TPU dry electrodes, the micro-nanocone PANI/PMMA flexible conductive film is encapsulated on the back of the FXbs dry electrode, and a multifunctional sensor and monitoring clothing are constructed, which can synchronously collect pressure signals and ECG signals, and can monitor human movements. In the simulation instrument and human skin surface, the influence of pressure fluctuation on the quality of ECG signal acquisition under different pressure, speed, and motion states is explored. In the simulation instrument, when the change rate of piezoresistive reaches the maximum value of 51.41 %, the fluctuation range of ECG is the smallest, which is 0.99 mV. On the surface of human skin, the change rate of piezoresistive and the fluctuation range of ECG in the turning state reach the maximum, which is 5.02 % and 5.99 mV, respectively. It can provide a reference to study the influence of pressure fluctuation on ECG signals on the surface of human skin, and at the same time, it can provide an experimental method and basis for the study of the dynamic noise mechanism at the electrode/electrolyte interface.

Design, Fabrication, and Evaluation of 3D Biopotential Electrodes and Intelligent Garment System for Sports Monitoring.

This study presents the development and evaluation of an innovative intelligent garment system, incorporating 3D knitted silver biopotential electrodes, designed for long-term sports monitoring. By integrating advanced textile engineering with wearable monitoring technologies, we introduce a novel approach to real-time physiological signal acquisition, focusing on enhancing athletic performance analysis and fatigue detection. Utilizing low-resistance silver fibers, our electrodes demonstrate significantly reduced skin-to-electrode impedance, facilitating improved signal quality and reliability, especially during physical activities. The garment system, embedded with these electrodes, offers a non-invasive, comfortable solution for continuous ECG and EMG monitoring, addressing the limitations of traditional Ag/AgCl electrodes, such as skin irritation and signal degradation over time. Through various experimentation, including impedance measurements and biosignal acquisition during cycling activities, we validate the system's effectiveness in capturing high-quality physiological data. Our findings illustrate the electrodes' superior performance in both dry and wet conditions. This study not only advances the field of intelligent garments and biopotential monitoring, but also provides valuable insights for the application of intelligent sports wearables in the future.

Plant Tattoo Sensor Array for Leaf Relative Water Content, Surface Temperature, and Bioelectric Potential Monitoring

AbstractThis study presents a wearable plant tattoo sensor array designed for continuous monitoring of leaf temperature, relative water content, and biopotential. Current plant wearable sensor technologies often require relatively bulky substrates for sensor support and adhesives for leaf attachment, which potentially can hinder plant growth and affect long‐term measurements. The multifunctional tattoo sensor array overcomes these issues by adhering directly to the leaf surface without the need for additional supporting structures or glues. This array includes a biopotential electrode, a resistive temperature sensor, and an impedimetric water content sensor, all constructed using laminated gold‐on‐polymer thin‐film patterns. Due to their mechanical flexibility, stretchability, and conformability, the sensors can seamlessly attach to leaves via van der Waals force. Performances of these sensors are evaluated to explore plant responses under diverse growth environments. This sensor array is capable of both short‐term and long‐term monitoring, offering continuous data and detailed insights into plant physiological responses to various stress conditions.

Dry-Transferable Photoresist Enabled Reliable Conformal Patterning for Ultrathin Flexible Electronics.

Photolithographic techniques, which are widely used in the silicon-based semiconductor industry, enable the manufacture of high-yield and high-resolution features at the micrometer and nanometer scales. However, conventional photolithographic processes cannot accommodate the micro/nanofabrication of flexible and stretchable electronics. In this study, a microfabrication approach that uses a synthesized, environmentally friendly, and dry-transferable photoresist to enable the reliable conformal manufacturing of thin-film electronics is reported, which is also compatible with the existing cleanroom processes. Photoresists with high-resolution, high-density, and multiscale patterns can be transferred onto various substrates in a defect-free and conformal-contact manner, thus enabling multiple wafer reuses. Theoretical studies are conducted to investigate the damage-free peel-off mechanism of the proposed approach. The in situ fabrication of various electrical components, including ultralight and ultrathin biopotential electrodes, has been demonstrated, which offer lower interfacial impedance, durability, and stability, and the components are applied to collect electromyography signals with superior signal-to-noise ratio (SNR) and quality. Additionally, an exemplary demonstration of a human-machine interface indicates the potential of these electrodes in many emerging applications, including healthcare, sensing, and artificial intelligence.

Characterizing the Impedance Properties of Dry E-Textile Electrodes Based on Contact Force and Perspiration.

Biopotential electrodes play an integral role within smart wearables and clothing in capturing vital signals like electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG). This study focuses on dry e-textile electrodes (E1-E6) and a laser-cut knit electrode (E7), to assess their impedance characteristics under varying contact forces and moisture conditions. Synthetic perspiration was applied using a moisture management tester and impedance was measured before and after exposure, followed by a 24 h controlled drying period. Concurrently, the signal-to-noise ratio (SNR) of the dry electrode was evaluated during ECG data collection on a healthy participant. Our findings revealed that, prior to moisture exposure, the impedance of electrodes E7, E5, and E2 was below 200 ohm, dropping to below 120 ohm post-exposure. Embroidered electrodes E6 and E4 exhibited an over 25% decrease in mean impedance after moisture exposure, indicating the impact of stitch design and moisture on impedance. Following the controlled drying, certain electrodes (E1, E2, E3, and E4) experienced an over 30% increase in mean impedance. Overall, knit electrode E7, and embroidered electrodes E2 and E6, demonstrated superior performance in terms of impedance, moisture retention, and ECG signal quality, revealing promising avenues for future biopotential electrode designs.

Stretchable, self-adhesive and self-healing ionic gels with double-network structure used as biopotential electrodes for ECG monitoring

Stretchable, self-adhesive and self-healing ionic gels with double-network structure used as biopotential electrodes for ECG monitoring

NIR light-induced rapid self-healing hydrogel toward multifunctional applications in sensing

Flexible conductive hydrogels with self-healing ability have attracted great attention in the fields of wearable devices and healthcare monitoring due to their recoverability and sustainability. Herein, the application of inorganic antimony sulfide (Sb2S3) as photothermal agent is extended to the field of self-healing hydrogel for the first time. A near-infrared (NIR) light-induced rapid self-healing double-network conductive hydrogel (SPOH gel) via on-demand irradiation was fabricated by incorporating dopamine (DA)-modified polypyrrole (PPy) coated Sb2S3 nanorod (Sb2S3 @PPy-DA) into the polymer matrix of polyvinyl alcohol (PVA)/poly(N-(2-hydroxyethyl)acrylamide) (pHEAA) in a water-glycerol dispersion medium. The SPOH gel with adhesive and anti-freezing properties exhibited robust tensile strength (1.25 MPa), large elongation (620%), high interfacial toughness (251 J/m2), and outstanding sensitivity (GF = 4.97). The incorporation of Sb2S3 @PPy-DA for SPOH gel endowed high conductivity (2.26 S/m) and promoted rapid self-healing process under NIR light irradiation (within 90 s). Moreover, SPOH gel could be widely applied to various applications, including monitoring various human motions as strain sensor, detecting electrocardiogram signals as biopotential electrodes, and harvesting energy as self-powered device. The multifunctional SPOH gel with the integrated attractive capabilities of rapid self-reparability and real-time physiological signal detection, providing a promising route to develop wearable smart devices in motion monitoring, healthcare management, and energy harvesting applications.

A Nanoporous Carbon‐MXene Heterostructured Nanocomposite‐Based Epidermal Patch for Real‐Time Biopotentials and Sweat Glucose Monitoring

AbstractDespite substantial progress in the development of wearable and flexible monitoring systems that conform to the epidermis, most designs focus on either physiological signs such as the electrocardiogram (ECG) results, respiration rate, or metabolites, and ignore the dynamic fluctuations of pH and temperature in sweat during on‐body tests. An advanced butterfly‐inspired hybrid epidermal biosensing (bi‐HEB) patch is presented here, which is interfaced with a custom‐developed miniaturized monitoring system. The patch incorporates a novel transducing layer of nanoporous carbon and MXene (NPC@MXene) for sensitive and durable detection of biomarkers in sweat. The bi‐HEB patch is composed of a glucose biosensor accompanied by pH and temperature sensors to precisely quantify glucose as well as two biopotential electrodes, allowing real‐time recording of electrophysiological (EP) signals. The NPC@MXene‐based glucose biosensor demonstrates an excellent sensitivity of 100.85 µAmm−1 cm−2 within physiological levels (0.003−1.5 mm), and variations in pH and temperature during on‐body perspiration monitoring are calibrated by employing a correction approach. In parallel, the EP electrodes exhibit skin‐electrode contact impedance and biopotential signals similar to those of conventional Ag/AgCl electrodes. Finally, the bi‐HEB patch integrated wearable system is used to accurately monitor sweat glucose and ECG of human subjects participating in indoor physical activities.

Stretchable and Self-Adhesive PEDOT:PSS Blend with High Sweat Tolerance as Conformal Biopotential Dry Electrodes.

Dry epidermal electrodes that can always form conformal contact with skin can be used for continuous long-term biopotential monitoring, which can provide vital information for disease diagnosis and rehabilitation. But, this application has been limited by the poor contact of dry electrodes on wet skin. Herein, we report a biocompatible fully organic dry electrode that can form conformal contact with both dry and wet skin even during physical movement. The dry electrodes are prepared by drop casting an aqueous solution consisting of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), poly(vinyl alcohol) (PVA), tannic acid (TA), and ethylene glycol (EG). The electrodes can exhibit a conductivity of 122 S cm-1 and a mechanical stretchability of 54%. Moreover, they are self-adhesive to not only dry skin but also wet skin. As a result, they can exhibit a lower contact impedance to skin than commercial Ag/AgCl gel electrodes on both dry and sweat skins. They can be used as dry epidermal electrodes to accurately detect biopotential signals including electrocardiogram (ECG) and electromyogram (EMG) on both dry and wet skins for the users at rest or during physical movement. This is the first time to demonstrate dry epidermal electrodes self-adhesive to wet skin for accurate biopotential detection.

Self-adhesive and anti-fatigue cellulose-polyacrylate ionogels prepared by ultraviolet curing used as biopotential electrodes

Conductive hydrogels have been extensively studied because of flexibility and skin-like capability to be used as biopotential electrodes for wearable health monitoring. However, they may suffer from poor mechanical properties and stability problems when used in practical applications caused by water evaporation. Herein, we prepared self-adhesive, transparent, flexible and robust ionic gels that can conformal contact with the skin used as biopotential electrodes for precise health monitoring. Cellulose based iogels were prepared by dissolving cellulose using [Bmim]Cl at 100 °C followed by in situ Ultraviolet light photopolymerization of acrylic acid by adding a mixture of acrylic acid and 2-hydroxy-2-methylpropiophenone. Cellulose/polyacrylic acid-based ionic gels-2 (BCELIG-2) has a Young's modulus of 0.2 MPa, a strain at break of 226 %, a modulus of elasticity of 0.1 MPa, and a toughness of 22.5 MJ m−3. Fixing the strain at 40 %, the ionic gels can recover to their original length after ten tensile-unloading cycles. They can accurately detect subtle physical motions such as arterial pulsations, which can provide important cardiovascular information.

3d Printed Bio-potential Dry Electrodes.

The most commonly used bio-potential electrodes are silver/silver chlorides gel-electrodes (Ag/AgCl electrodes). However, wet electrodes are not suited for daily usage and long-term monitoring. Dry electrodes are the best choice for long-term measurements. Dry electrodes can be fabricated using various methods and materials; one of these methods is three dimensional (3d) printing. 3d printing is a cost-effective method and can be easily used in mass production. This paper will present 3d printed bio-potential dry electrodes using five different commercially available fused deposition modeling (FDM) electrically conductive filaments. Then, these printed electrodes will be tested and validated using electrocardiogram (ECG) portable acquisition system. Three out of five filaments were printed successfully. Collected ECG data from these electrodes showed acceptable quality with Signal to Noise Ratios (SNRs) ranging around 20 dB when compared with wet electrodes collected raw data as a reference. In conclusion, this study designed and printed a 100% 3d printed electrodes from three different FDM filaments which could be used efficiently for ECG signal monitoring in resting position.

High-Performance Flexible Microneedle Array as a Low-Impedance Surface Biopotential Dry Electrode for Wearable Electrophysiological Recording and Polysomnography

Highlights Polyimide-based flexible microneedle array (PI-MNA) electrodes realize high electrical/mechanical performance and are compatible with wearable wireless recording systems.The normalized electrode–skin interface impedance (EII) of the PI-MNA electrodes reaches 0.98 kΩ cm2 at 1 kHz and 1.50 kΩ cm2 at 10 Hz, approximately 1/250 of clinical standard electrodes.This is the first report on the clinical study of microneedle electrodes. The PI-MNA electrodes are applied to clinical long-term continuous monitoring for polysomnography.Microneedle array (MNA) electrodes are an effective solution to achieve high-quality surface biopotential recording without the coordination of conductive gel and are thus very suitable for long-term wearable applications. Existing schemes are limited by flexibility, biosafety, and manufacturing costs, which create large barriers for wider applications. Here, we present a novel flexible MNA electrode that can simultaneously achieve flexibility of the substrate to fit a curved body surface, robustness of microneedles to penetrate the skin without fracture, and a simplified process to allow mass production. The compatibility with wearable wireless systems and the short preparation time of the electrodes significantly improves the comfort and convenience of electrophysiological recording. The normalized electrode–skin contact impedance reaches 0.98 kΩ cm2 at 1 kHz and 1.50 kΩ cm2 at 10 Hz, a record low value compared to previous reports and approximately 1/250 of the standard electrodes. The morphology, biosafety, and electrical/mechanical properties are fully characterized, and wearable recordings with a high signal-to-noise ratio and low motion artifacts are realized. The first reported clinical study of microneedle electrodes for surface electrophysiological monitoring was conducted in tens of healthy and sleep-disordered subjects with 44 nights of recording (over 8 h per night), providing substantial evidence that the electrodes can be leveraged to substitute for clinical standard electrodes.

Ultrasoft Porous 3D Conductive Dry Electrodes for Electrophysiological Sensing and Myoelectric Control.

Biopotential electrodes have found broad applications in health monitoring, human-machine interactions, and rehabilitation. Here, we report the fabrication and applications of ultrasoft breathable dry electrodes that can address several challenges for their long-term wearable applications - skin compatibility, wearability, and long-term stability. The proposed electrodes rely on porous and conductive silver nanowire based nanocomposites as the robust mechanical and electrical interface. The highly conductive and conformable structure eliminates the necessity of conductive gel while establishing a sufficiently low electrode-skin impedance for high-fidelity electrophysiological sensing. The introduction of gas-permeable structures via a simple and scalable method based on sacrificial templates improves breathability and skin compatibility for applications requiring long-term skin contact. Such conformable and breathable dry electrodes allow for efficient and unobtrusive monitoring of heart, muscle, and brain activities. In addition, based on the muscle activities captured by the electrodes and a musculoskeletal model, electromyogram-based neural-machine interfaces were realized, illustrating the great potential for prosthesis control, neurorehabilitation, and virtual reality.

Recent Progress in Printed Physical Sensing Electronics for Wearable Health-Monitoring Devices: A Review

Printed electronics (PEs), as a fast-growing advanced manufacturing technology in recent years, plays an essential role in development of wearable electronic sensors, flexible displays, human-machine interaction, and thin electronics owing to its low cost, high throughput, and the possibility to be fabricated on diverse thin substrates with good flexibility and stretchability. Various PEs have been developed, such as physical sensing devices, electrochemical sensors, wearable supercapacitors and energy harvesters, stretchable electrodes, thin-film transistors (TFTs), and printed transceiving circuits. In this work, recent progress on physical sensing devices and their interface circuits developed by the printing process are reviewed in terms of their functions, printing methods, materials, and performance. The printed physical sensors used for monitoring the basic biometric parameters through physical sensing mechanisms, such as temperature sensors for skin temperature, pressure sensors for human pulse wave, strain sensors for human motions, biopotential electrodes for electrocardiogram signals, and multiple-function physical sensing platforms, have been studied and highlighted with their good designs and advanced materials. The state-of-the-art printed interface circuits for the wearable sensors such as interconnects, TFTs, digital circuits, amplifiers, oscillators, and antennas have been reviewed in terms of their structures and functions from basic to advanced levels. The challenge and suggestions of the printed wearable devices for future developments are discussed and end with a conclusion.

3D‐Knit Dry Electrodes using Conductive Elastomeric Fibers for Long‐Term Continuous Electrophysiological Monitoring

AbstractRecent advances in telemedicine and personalized healthcare have motivated new developments in wearable technologies targeting continuous monitoring of biosignals. Common limitations of wearables for continuous monitoring include durability and breathability of their biopotential electrodes. This paper tackles this challenge by proposing flexible, breathable, and washable dry textile electrodes made of conductive elastomeric filaments (CEFs). First, candidate CEF fibers are characterized. Using an industrial knitting machine, CEF fibers are then directly knitted into textile electrodes. To assess their performance in more realistic circumstances, smart garments with textile electrodes are knitted. Electrocardiograms (ECGs) are acquired using an underwear garment and electrooculograms (EOGs) are acquired using a headband. ECGs and EOGs with textile electrodes are found to have comparable fidelity to that of the gold standard gel electrodes. CEF electrodes are also resistant to repeated wash and dry cycles (30×) and continue to acquire high‐fidelity biosignals. Smart underwear garments are also used to perform continuous ECG measurements in five participants over 24 h of unrestricted daily activities. Results demonstrate the success of these garments in performing high fidelity continuous ECG monitoring. Collectively, these results present CEF electrodes as a promising scalable solution to the challenges of wearable technologies for long‐term continuous electrophysiological monitoring applications.

Development of fabric electrode for bio-potential signal acquisition in wearable health monitoring and effect of perspiration on signal acquisition

Development of a gel-free bio-potential electrode for the wearable health monitoring applications is a challenging goal. A conductive fabric electrode can replace the traditional conductive gel electrode. This paper describes the development of a conductive fabric electrode with regard to its potential use for electrocardiogram (ECG) acquisition. Since direct contact between the conductive fabric and human skin will be involved, an investigation on the effect of perspiration on the electrical conductivity of fabric is critical. Hence, the developed electrode was treated with alkaline (pH=8.0) and acidic (pH=4.3) perspiration for 3, 8 and 40 h to study the effect of perspiration on the conductivity and surface morphology. The acquired ECG signals were analysed with respect to morphology and frequency distribution. Conductivity tests were carried out on the perspiration-treated test electrodes by two probe method and surface resistivity meter. The ECG signals of volunteers were also recorded. The results showed a slight decrease in conductivity but without affecting the morphology and the quality of ECG signal. Leached silver content in the acid perspiration-treated solution was found to be 0.117 ppm as determined by Atomic absorption spectroscopy. The result shows that soft conducting textile materials can indeed be used as an electrode for ECG acquisition. This is a novel type of gel-free fabric electrode for long term wearable health monitoring applications including space application.

Me-Doped Ti-Me Intermetallic Thin Films Used for Dry Biopotential Electrodes: A Comparative Case Study.

In a new era for digital health, dry electrodes for biopotential measurement enable the monitoring of essential vital functions outside of specialized healthcare centers. In this paper, a new type of nanostructured titanium-based thin film is proposed, revealing improved biopotential sensing performance and overcoming several of the limitations of conventional gel-based electrodes such as reusability, durability, biocompatibility, and comfort. The thin films were deposited on stainless steel (SS) discs and polyurethane (PU) substrates to be used as dry electrodes, for non-invasive monitoring of body surface biopotentials. Four different Ti–Me (Me = Al, Cu, Ag, or Au) metallic binary systems were prepared by magnetron sputtering. The morphology of the resulting Ti–Me systems was found to be dependent on the chemical composition of the films, specifically on the type and amount of Me. The existence of crystalline intermetallic phases or glassy amorphous structures also revealed a strong influence on the morphological features developed by the different systems. The electrodes were tested in an in-vivo study on 20 volunteers during sports activity, allowing study of the application-specific characteristics of the dry electrodes, based on Ti–Me intermetallic thin films, and evaluation of the impact of the electrode–skin impedance on biopotential sensing. The electrode–skin impedance results support the reusability and the high degree of reliability of the Ti–Me dry electrodes. The Ti–Al films revealed the least performance as biopotential electrodes, while the Ti–Au system provided excellent results very close to the Ag/AgCl reference electrodes.

Self-adhesive, stretchable, and dry silver nanorods embedded polydimethylsiloxane biopotential electrodes for electrocardiography

Biopotential signals are extremely useful in assessing organ function and diagnosing diseases. This study proposes a highly flexible, conductive, antibacterial, and self-adhesive silver nanorods (AgNRs) embedded polydimethylsiloxane (PDMS) dry electrode for long-term electrocardiogram (ECG) monitoring with portable instrumentation. We use a unique glancing angle deposition method to fabricate AgNRs and embedded them in a biocompatible PDMS matrix. These electrodes do not cause skin irritation even after several hours of use and work efficiently without skin preparation. These AgNRs-PDMS electrodes have an electrical resistivity of 10−7 Ω m and a skin contact impedance of 93.9 ± 0.7 kΩ to 6.2 ± 3.7 kΩ for frequencies ranging from 40 Hz to 1 kHz, which is around 18% less than most conventional Ag/AgCl wet electrodes. The fabricated electrodes have a signal-to-noise ratio of 12 dB, comparable to that of a conventional Ag/AgCl electrode. A signal acquisition circuit is designed to detect ECG signals combined with proposed dry electrodes and a wireless monitoring device. Finally, real-time ECG signals are displayed on a mobile phone via an Android application. These AgNRs-PDMS dry electrodes, in combination with the portable wireless device, may be used in future clinical studies that require real-time and long-term ECG monitoring.

Evaluating electronic and structural properties of Au and Cu substituted AgCl electrode for application in surface electromyography

The material content of electrodes is very much important for improving the signal quality of surface electromyography (sEMG) in various environments, in particular, long-term monitoring. The electronic and structural properties of silver/silver chloride (Ag/AgCl), a popular biopotential electrode for sEMG recording, were studied under different conditions by X-ray absorption fine structure (XAFS) spectroscopy calculations. The effects of substitution of good-conducting elements (Cu and Au) on Ag coordination were tested at different temperatures as well as possible defect conditions. In the Au-substituted AgCl material, an extension of the Ag K-edge spectra was detected due to the rich quantum symmetry of Au valence level. Hence, it was determined that Au atoms bond with Cl atoms stronger than Ag atoms. Additionally, it was determined that Au atoms also changed the ionic properties of Ag atoms by losing Ag-Cl bonds.