Low Contact Impedance Research Articles

Polyvinyl alcohol/polyacrylamide double-network hydrogel-based semi-dry electrodes for robust electroencephalography recording at hairy scalp for noninvasive brain–computer interfaces

Objective. Reliable and user-friendly electrodes can continuously and real-time capture the electroencephalography (EEG) signals, which is essential for real-life brain–computer interfaces (BCIs). This study develops a flexible, durable, and low-contact-impedance polyvinyl alcohol/polyacrylamide double-network hydrogel (PVA/PAM DNH)-based semi-dry electrode for robust EEG recording at hairy scalp. Approach. The PVA/PAM DNHs are developed using a cyclic freeze-thaw strategy and used as a saline reservoir for semi-dry electrodes. The PVA/PAM DNHs steadily deliver trace amounts of saline onto the scalp, enabling low and stable electrode-scalp impedance. The hydrogel also conforms well to the wet scalp, stabilizing the electrode-scalp interface. The feasibility of the real-life BCIs is validated by conducting four classic BCI paradigms on 16 participants. Main results. The results show that the PVA/PAM DNHs with 7.5 wt% PVA achieve a satisfactory trade-off between the saline load-unloading capacity and the compressive strength. The proposed semi-dry electrode exhibits a low contact impedance (18 ± 8.9 kΩ at 10 Hz), a small offset potential (0.46 mV), and negligible potential drift (1.5 ± 0.4 μV min−1). The temporal cross-correlation between the semi-dry and wet electrodes is 0.91, and the spectral coherence is higher than 0.90 at frequencies below 45 Hz. Furthermore, no significant differences are present in BCI classification accuracy between these two typical electrodes. Significance. Based on the durability, rapid setup, wear-comfort, and robust signals of the developed hydrogel, PVA/PAM DNH-based semi-dry electrodes are a promising alternative to wet electrodes in real-life BCIs.

Hydrogel Nanoarchitectonics of a Flexible and Self-Adhesive Electrode for Long-Term Wireless Electroencephalogram Recording and High-Accuracy Sustained Attention Evaluation.

Hydrogels are ideal building blocks to fabricate the next generation of electrodes for acquiring high-quality physiological electrical signals, for example, electroencephalography (EEG). However, collection of EEG signals still suffers from electrode deformation, sweating, extensive body motion and vibration, and environmental interference. Herein, polyvinyl alcohol and polyvinylpyrrolidone are selected to prepare a hydrogel network with tissue-like modulus and excellent flexibility. Additionally, polydopamine nanoparticles, obtained by polydopamine peroxidation, are integrated into the hydrogel to endow them with higher transparency, higher self-adhesion, and lower impedance. Consequently, a multichannel and wirelessly operated hydrogel electrode can establish a conformal and stable interface with tissue and illustrate high channel uniformity, low interfacial contact impedance, low power noise, long-term stability, and a tolerance to sweat and motion. Furthermore, the hydrogel electrode shows the unprecedented ability to classify the recorded high-quality prefrontal EEG signals into seven-category sustained attention with high accuracy (91.5%), having great potential applications in the assessment of human consciousness and in multifunctional diagnoses.

A robust HD-sEMG sensor suitable for convenient acquisition of muscle activity in clinical post-stroke dysphagia

Objective. A flexible high-density surface electromyography (HD-sEMG) sensor combined with an adaptive algorithm was used to collect and analyze the swallowing activities of patients with Post-stroke dysphagia. Approach. The electrode frame, modified electrode, and bonded substrate of the sensor were fabricated using a flexible printed circuit process, controlled drop coating, and molding, respectively. The adaptation algorithm was achieved by using Laplace and Teager-Kaiser energy operators to extract active segments, a cross-correlation coefficient matrix (CCCM) to evaluate synergy, and multi-frame real-time dynamic root mean square (RMS) to visualize spatiotemporal information to screen lesions and level of dysphagia. Finally, support vector machines (SVM) were adopted to explore the classification accuracy of sex, age, and lesion location with small sample sizes. Main results. The sensor not only has a basic low contact impedance (0.262 kΩ) and high signal-to-noise ratio (37.284 ± 1.088 dB) but also achieves other characteristics suitable for clinical applications, such as flexibility (747.67 kPa) and durability (1000 times) balance, simple operation (including initial, repeated, and replacement use), and low cost ($ 15.2). The three conclusions are as follows. CCCM can be used as a criterion for judging the unbalanced muscle region of the patient’s neck and can accurately locate unbalanced muscles. The RMS cloud map provides the time consumption, swallowing times, and unbalanced areas. When the lesion location involves the left and right hemispheres simultaneously, it can be used as an evidence of relatively severely unbalanced areas. The classification accuracy of SVM in terms of sex, age, and lesion location was as high as 100%. Significance. The HD-sEMG sensor in this study and the adaptation algorithm will contribute to the establishment of a larger-scale database in the future to establish more detailed and accurate quantitative standards, which will be the basis for developing more optimized screening mechanisms and rehabilitation assessment methods.

Stretchable, adhesive and low impedance hydrogel prepared by one-pot method used as ECG electrodes

Flexible, self-adhesive and conductive hydrogel has attracted a deal of interest in ECG biosensors. However, the balance between mechanical properties and ionic conductivity of hydrogels is challenging, and there are tremendous difficulties in preparing a multifunctionally integrated hydrogel. Herein, we designed a multi-network conductive hydrogel, the poly(vinylalcohol)/carboxymethylcellulose sodium-poly([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfo-propyl) ammonium hydroxide-co-acrylamide) (PVA/CMC-Na-P(SBMA-co-AM), PVA/CMC-SAM) hydrogel was obtained by thermal initiation and freeze-thaw cycles. Because of the existence of three-dimensional porous structure, its ionic conductivity reaches 0.91 S/m. The elastic modulus is 0.17 MPa, the fracture strain is 105%, and the tensile strength is 0.21 MPa. At the same time, hydrogels can be bound to different surfaces with the adhesion properties, leading to the low contact impedance on human skin surface. Due to the dynamic cross-linking of hydrogen bonds, hydrogels also exhibit obvious self-healing properties. The resistance of hydrogel is sensitive to strain and can accurately monitor subtle physical movements like finger bending, while continuously, steadily and clearly collecting ECG signals. Therefore, the multifunctionally integrated hydrogel can display promising applications in ECG biosensors.

A Flexible Wireless sEMG System for Wearable Muscle Strength and Fatigue Monitoring in Real Time

AbstractThe detection of surface electromyography (sEMG) signals on the skin has attracted increasing attention because of its ability to monitor muscle conditions in a noninvasive manner and thus possesses great application potential to assess athletic status and training efficiency in real time or to evaluate postoperative muscle rehabilitation conveniently. Here, a flexible wireless sEMG monitoring system that consists of a stretchable sEMG epidermal patch and a flexible printed circuit board to provide real‐time evaluation of muscle strength and fatigue is reported. The epidermal patch is designed to have good stretchability and permeability and optimized to ensure a low contact impedance with the skin and minimized background noise for sEMG signal acquisition with high fidelity. Six commonly used time‐domain and two frequency‐domain features extracted from sEMG signals are systematically analyzed, and a strategy for feature selection and pattern identification is proposed that eventually enables the real‐time assessment of muscle strength and fatigue by using an integrated system in a wearable form.

Stretchable and Self-Adhesive PEDOT:PSS/MXene Dry Electrode with Low Contact Impedance for ECG Monitoring

Stretchable and Self-Adhesive PEDOT:PSS/MXene Dry Electrode with Low Contact Impedance for ECG Monitoring

Ultrapermeable and Wet-Adhesive Monolayer Porous Film for Stretchable Epidermal Electrode.

Noninvasive electrophysiological signal monitoring is significant for health care and scientific research. The simultaneous achievement of wet adhesion, stretchability, breathability, and low contact impedance is highly recommended in the epidermal electrode but still challenging. In this work, a monolayer porous film electrode with a pore size and wall thickness of less than ∼10 μm is fabricated via the breath figure method (BFM) and metal sputtering, and it was subsequently applied using epidermal electrophysiological monitoring. The ultrahigh permeability is comparable to the naked skin because the through holes of the monolayer porous film match well with the pores on human skin. The stretchability of 50% is realized with the combination of Au microcrack and the monolayer porous structure. The wet adhesion of 0.17 N/cm is established on the chemical bonding between the electrode and the epidermis. The contact impedance is comparable with the gold standard Ag/AgCl gel electrode, especially after sweating. Stable and precise electrophysiological signals are measured. Especially, the perspiration resistance of the monolayer porous film outperforms that of the gel electrode. The monolayer porous structure provides a new avenue to improve the breathability of the epidermal electronics.

A transient dual-type sensor based on MXene/cellulose nanofibers composite for intelligent sedentary and sitting postures monitoring

Sedentary and poor sitting postures will significantly raise the risk of cardiovascular disease and musculoskeletal disorders in humans. Thus, it is urgent to realize sedentary and sitting postures monitoring through various types of sensors. However, these sensors are commonly rigid, and non-degradable with low sensitivity and sensing range, which greatly restricts their practical applications in seamless, eco-friendly, and accurate monitoring in real-time. Here, we report flexible, degradable, transient, highly sensitive MXene/cellulose nanofibers (CNF) dual-type sensors including pressure sensor and bioelectric sensor. The pressure sensor is fabricated with hierarchical multi-layer MXene/CNF (MMC) as sensing layer, which shows wide detection range (0–950 kPa), high sensitivity (34.6 kPa−1), excellent stability (3000 cycles), transient and degradability. Moreover, the bioelectric sensor is obtained by delaminated MXene/CNF (DMC) sensing film, exhibiting high conductive, low contact impedance in high-quality electrocardiogram signal recording. Furthermore, with the aid of the deep neural network, the dual sensors can be integrated into an intelligent health monitoring system to achieve continous electrocardiogram, heart rate, sedentary, and sitting posture monitoring simultaneously. The system can realize multi-users identification and sitting postures recognition with high accuracy (>97%), showing great potential in health status monitoring with more privacy compared with previous methods.

Investigation on performance of proton exchange membrane electrolyzer with different flow field structures

Proton exchange membrane (PEM) electrolysis provides a sustainable solution for hydrogen generation. As one of the key components, flow field should be properly designed to help distribute reactant evenly across the catalyzed reaction surface area. In this study, three-dimensional models of three flow field structures are built to simulate the internal flow velocity and pressure distribution. The electrolyzer performance of the flow fields at different operating temperature and water pressure is investigated through orthogonal experiments. The output characteristics and electrochemical behavior are investigated by measurement of polarization curves and analysis of electrochemical impedance. It is found that the flow field structure has the most considerable impact on the electrolyzer performance, followed by the operating temperature. The experimental results are consistent with the simulations, indicating that the parallel flow field has lower contact impedance and better mass transfer effect under medium and low current densities due to the large contact area of the channel and the uniform pressure distribution. PEM electrolyzer with the parallel flow field of channel width 1 mm, operating at 60 °C and 0.1 bar anode pressure has the best output performance. Under this condition, the electrolyzer’s potential reaches to 2.1 V at the current density of 1 A/cm2. This work can provide simulation and experimental support for the selection and optimization of PEM electrolyzers’ flow fields and operating conditions.

Vertically aligned conductive metal-organic framework nanowires array composite fiber as efficient solid-contact for wearable potentiometric sweat sensing

Fiber-type solid-contact ion-selective electrodes (SC-ISE) show unique superiorities for flexible and wearable sweat sensors with excellent wearing experiences. However, it is still challenging to construct high-quality fiber-type SC-ISE with robust sensing properties and facile fabrication methods. Here, we propose a new fiber-type flexible potentiometric sensor platform based on the conductive metal-organic framework (MOF) composite carbon nanotubes fiber (CNTF). The vertically aligned Nickel triphenylene-fused metal catecholate (NiCAT) nanowires arrays are in situ grown on CNTF through a bottom-up one-step hydrothermal synthetic approach. Nafion coating is designed to improve the surface hydrophobicity for avoiding the aqueous layer formation at the solid-contact=ion-selective-membrane interface. The well-defined solid-contact transducer of Nafion/NiCAT@CNTF exhibits the greatly enlarged double-layer capacitance (34.39 mF/cm2) attributed to its ordered porous architecture and low contact impedance. Consequently, the as-prepared Na+-ISE displays a sensitive near-Nernstian response of 58.69 ± 0.77 mV/dec, an excellent long-term potential stability with an ultralow drift of 8.1 ± 4.1 μV/h, and a good electrode-to-electrode reproducibility E0 of 308.0 ± 3.1 mV. An integrated wearable band-based sweat sensor device was developed for on-body sweat sodium monitoring during exercise. This platform provides an efficient and facile approach to construct reliable fiber-type potentiometric sensors for wearable applications.

Noninvasive electroencephalogram sensors based on all-solution-processed trapezoidal electrode array

Conventional wet electrodes, such as a silver/silver chloride electrode, are limited for electroencephalogram (EEG) sensors directly attached to the scalp with existing hair due to their incomplete contact and increased impedance. In this study, an all-solution-processed trapezoidal electrode array is demonstrated for highly sensitive and reliable detection of EEG signals even when in direct contact with the scalp. The proposed noninvasive EEG sensors based on nanocomposites consisting of single-wall carbon nanotube random networks incorporated into a gelatin matrix exhibited a relatively low contact impedance of 11.16 × 102 Ω and a high sensitivity of 14.81 dB regardless of existing hair for real-time EEG recording without conductive gels or electrolytes. Furthermore, the origin of such advances induced by the soft and conductive electrode array is investigated by analyzing the effective contact area and signal-to-noise ratio on different scalp positions from 20 different subjects. A trapezoidal EEG electrode penetrates the dense hair and bypasses the hair shaft owing to its deformable shape induced by the soft and flexible nanocomposite film.

C-scheme electron transfer mechanism: An efficient ternary heterojunction photocatalyst carbon quantum dots/Bi/BiOBr with full ohmic contact

With a facile one-pot solvothermal method, an efficient ternary heterojunction photocatalyst carbon quantum dots (CQDs)/Bi/BiOBr is firstly prepared. Ethylene glycol (EG) is used as the solvent and carbon source for the first time. At 190 °C for 3 h, while BiOBr is synthesized, EG is employed to prepare CQDs through bottom-up method. CQDs are grafted by a large number of functional groups with reducibility, which reduce some neighboring BiO+ to metal Bi. By modifying the solvothermal temperature and time, CQDs and Bi are in-situ controllably deposited on the surface of BiOBr microspheres. Due to different Fermi levels and work functions, the interfaces of CQDs, BiOBr and Bi are connected through ohmic junctions with low contact impedance. The hot electrons from Bi with surface plasmon resonance (SPR) properties, and electrons in the CB of BiOBr flow to CQDs, forming a C-scheme electron transfer mechanism. O2− from CQDs and h+ in the VB of BiOBr respectively attack the sites with higher and lower electron density in methyl orange (MO) molecule, resulting in its photodegradation into small molecular products.

A Highly Stretchable and Permeable Liquid Metal Micromesh Conductor by Physical Deposition for Epidermal Electronics.

Stretchable electronics allow functional devices to integrate with human skin seamlessly in an emerging wearable platform termed epidermal electronics. Compliant conductors represent key building components for functional devices. Among the various candidates, gallium-based liquid metals stand out with metallic conductivity and inherent deformability. Currently, the widespread applications of liquid metals in epidermal electronics are hindered by the low steam permeability and hence unpleasant wearing perceptions. In this study, a facile physical deposition approach is established to create a liquid metal micromesh over an elastomer sponge, which exhibits low sheet resistance (∼0.5 Ω sq-1), high stretchability (400% strain), and excellent durability. The porous micromesh shows textile-level permeability to achieve long-term wearing comfort. The conformal interaction of the liquid metal micromesh with the skin gives rise to a low contact impedance. An integrated epidermal sensing sleeve is demonstrated as a human-machine interface to distinguish different hand gestures by recording muscle contractions. The reported stretchable and permeable liquid metal conductor shows promising potentials in next-generation epidermal electronics.

Highly Permeable and Ultrastretchable Liquid Metal Micromesh for Skin-Attachable Electronics

Stretchable electronics represents an emerging technology for next-generation smart wearables toward intimate integration with the human body. In contrast with functional devices constructed over elastomer films with limited moisture permeability, a soft electronic textile may represent the ideal skin-attachable platform to achieve long-term wearing comfort. The advancements in this active area largely hinge on a new generation of permeable conductor. Despite its intrinsic mechanical deformability, gallium-based liquid metal typically represents an impenetrable barrier for gases and liquids. In this study, we introduce a liquid metal micromesh on electrospun microfiber textile as a highly permeable and ultrastretchable conductor. The fabrication process involves dropcasting liquid metal onto an elastomeric microfiber textile followed by high-speed rotation to remove the excessive coating. The liquid metal micromesh exhibits low sheet resistance (0.38 Ω/sq), ultrahigh stretchability (>1000% strain), and mechanical durability. The porous morphology enables a high steam permeability and perception of comfort comparable to those of standard textiles. The conformal interface with the skin gives rise to low contact impedance better than that of state-of-the-art Ag/AgCl gel electrodes. The successful implementation of the liquid micromesh conductor in a multifunctional electronic system demonstrates its practical suitability for a broad range of applications in stretchable and wearable electronics.

Ionic liquid regenerated cellulose membrane electroless plated by silver layer for ECG signal monitoring

Nowadays, more and more attention has been paid to flexible electrodes, which can be used to make wearable devices so as to monitor health signals, such as those in ECG, EMG and EEG. However, the contact impedance of many membrane electrodes is high due to the bad fit between the sensors and the skin, or the poor sensitivity caused by low conductivity. Therefore, we fabricated a skin-like conductive electrode via electroless silver plating on the surface of regenerated cellulose membrane, which was prepared from the cellulose dissolved in ionic liquid [Bmim]Cl. The as-prepared biocompatible electrodes with low skin-electrode contact impedance can be used as a dry electrode for a long time. Its impedance at 700 Hz is only 8 kΩ/cm2 and the conductivity reaches 252 s/cm. After 5 h of wearing, the electrode-skin contact impedance at 700 Hz is only 10 kΩ/cm2 when 0.20 mol/L AgNO3 is used. In short, the prepared electrode can not only ensure stable and clear ECG signals but also can greatly reduce electrode–skin contact impedance when used for long-term health monitoring.

Poly(3,4-ethylenedioxythiophene)-Based Neural Interfaces for Recording and Stimulation: Fundamental Aspects and In Vivo Applications.

Next‐generation neural interfaces for bidirectional communication with the central nervous system aim to achieve the intimate integration with the neural tissue with minimal neuroinflammatory response, high spatio‐temporal resolution, very high sensitivity, and readout stability. The design and manufacturing of devices for low power/low noise neural recording and safe and energy‐efficient stimulation that are, at the same time, conformable to the brain, with matched mechanical properties and biocompatibility, is a convergence area of research where neuroscientists, materials scientists, and nanotechnologists operate synergically. The biotic–abiotic neural interface, however, remains a formidable challenge that prompts for new materials platforms and innovation in device layouts. Conductive polymers (CP) are attractive materials to be interfaced with the neural tissue and to be used as sensing/stimulating electrodes because of their mixed ionic‐electronic conductivity, their low contact impedance, high charge storage capacitance, chemical versatility, and biocompatibility. This manuscript reviews the state‐of‐the‐art of poly(3,4‐ethylenedioxythiophene)‐based neural interfaces for extracellular recording and stimulation, focusing on those technological approaches that are successfully demonstrated in vivo. The aim is to highlight the most reliable and ready‐for‐clinical‐use solutions, in terms of materials technology and recording performance, other than spot major limitations and identify future trends in this field.

Development of a flexible dry electrode based MXene with low contact impedance for biopotential recording

The dry electrode for biopotential recording is urgently need in wearable health monitoring systems, brain-computer interface (BCI), and human–computer interaction (HCI). However, high contact impedances of the electrode and the skin and inadequate shape adaptability of the electrode are challenges for the dry electrode to record signals accurately. Here, we present a flexible dry electrode based on transferring MXene (Ti3C2Tx) ink on a flexible polyimide (PI) substrate for biopotential recording. The electrode exhibits low contact impedances and excellent flexibility to conform the skin well. In vivo biopotential recording demonstrated the flexible dry electrode can record electrocardiograms (ECG), electroencephalograms (EEG), and electrooculogram (EOG) as accurately as the standard electrode (Ag/AgCl electrode). Moreover, our electrode can record ECG accurately, even the subject is in motion. The flexible dry electrode is suitable for wearable biopotential recording and promising to be used in wearable health monitoring systems, BCI, and HCI.

Ten-Hour Stable Noninvasive Brain-Computer Interface Realized by Semidry Hydrogel-Based Electrodes.

Noninvasive brain-computer interface (BCI) has been extensively studied from many aspects in the past decade. In order to broaden the practical applications of BCI technique, it is essential to develop electrodes for electroencephalogram (EEG) collection with advanced characteristics such as high conductivity, long-term effectiveness, and biocompatibility. In this study, we developed a silver-nanowire/PVA hydrogel/melamine sponge (AgPHMS) semidry EEG electrode for long-lasting monitoring of EEG signal. Benefiting from the water storage capacity of PVA hydrogel, the electrolyte solution can be continuously released to the scalp-electrode interface during used. The electrolyte solution can infiltrate the stratum corneum and reduce the scalp-electrode impedance to 10 kΩ-15 kΩ. The flexible structure enables the electrode with mechanical stability, increases the wearing comfort, and reduces the scalp-electrode gap to reduce contact impedance. As a result, a long-term BCI application based on measurements of motion-onset visual evoked potentials (mVEPs) shows that the 3-hour BCI accuracy of the new electrode (77% to 100%) is approximately the same as that of conventional electrodes supported by a conductive gel during the first hour. Furthermore, the BCI system based on the new electrode can retain low contact impedance for 10 hours on scalp, which greatly improved the ability of BCI technique.

Biomimetic integration of tough polymer elastomer with conductive hydrogel for highly stretchable, flexible electronic

Flexible electronics is booming and attracts considerable interest in monitoring physiological activities and health conditions. The development of human-friend flexible electronics with both excellent human movement monitoring and flexible electrode functions at a wide working range remains highly challenging. Herein, we report a biomimetic double-layered multifunctional flexible electronic device composed of a stretchable, tough elastomer covalently coupled with a conductive, double-network hydrogel for monitoring physiological motions. The introduction of optimal physical crosslinking including hydrogen bond and metal coordination in the covalently cross-linked polyurethane elastomer effectively provides favorable flexibility and stretchability. The elastomer-hydrogel integration (EHI) shows ultra-fast responsiveness (10 ms) and resilience (246 ms) as a skin sensor and could effectively detect diverse muscle and joint movements in a highly sensitive and signal waveform-interpreting manner. Further, such a polymer integration with skin shape-adaptive hydrogel and low contact impedance enables real-time high-quality detection of electrocardiogram (ECG) by serving as flexible electrode compared to commercial Ag/AgCl electrodes, and was successfully applied to measure electrooculogram (EOG) and electromyogram (EMG). EHI-based electrodes were feasible and effective for electroencephalogram (EEG) signal detection in brain-computer interfaces (BCI) system. Altogether, this biomimetic EHI electronic device provides an advanced platform for the design of next-generation flexible healthcare diagnostic monitors and soft robots.

Self-adhesive, biodegradable silk-based dry electrodes for epidermal electrophysiological monitoring

Epidermal flexible devices can continuously and real-time record the electrical potential changes from human skin, which is essential for chronic disease diagnosis and smart health monitoring. However, it remains challenging to fabricate high-performance epidermal dry electrodes with excellent self-adhesive while maintaining excellent biodegradability as well as high mechanical performance and low contact impedance. Herein, we fabricate an epidermal dry electrode with high mechanical performance, low contact impedance and excellent self-adhesive by coating polypyrrole (Ppy) conductive layer and calcium (Ca)-modified SF adhesive layer on acid-modified silk (AM-SF)/cellulose nanocrystals (CNC) films. The AM-SF could effectively promote the absorption and intercalation of Ppy into the SF network, endowing the tight integration between the Ppy conductive layer and the AM-SF/CNC films and improving the conductivity of Ppy@AM-SF/CNC films. On the other side, the Ca ions in the Ca-modified SF adhesive layer capture water molecules from the atmosphere and result in a much higher flowability to allow conformal contact between Ca-modified SF and the human skin, leading to significantly low skin-electrode impedance. As a proof-of-concept, the Ppy@AM-SF/CNC electrodes are employed for both electrocardiography (ECG) and electromyography (EMG) recording. Interestingly, this electrode allows detaching from the skin by simple water rinsing to reduce skin-damage for people. Moreover, the Ppy@AM-SF/CNC electrodes possess excellent enzymatic degradability without leaving hazardous substances. This work offers a unique prospective for developing next-generation wearable healthcare devices in the era of personalized healthcare.