Rigid Electrode Research Articles

Co-doped 1T‐MoS2 nanosheets anchored on carbon cloth as self-supporting anode for high-performance lithium storage

The widespread use of wearable devices has triggered a huge demand for flexible power source. Flexible lithium-ion batteries (LIBs) are widely regarded as an ideal power source for wearable devices because of their high energy density and long cycle life, but their application is limited by traditional rigid electrodes. A promising strategy is to prepare flexible electrodes by synthesising active materials with high capacity on flexible carbon substrates. In this study, Co-doped 1T‐MoS2 nanosheets were successfully anchored on a pretreatment carbon cloth (CC) by using a facile one-step solvothermal method, which is denoted as Co-MoS2/CC. Benefiting from the high conductivity and large layer spacing of the metallic 1T‐MoS2, self-supporting CC framework, doping of cobalt and their synergistic effect, when used as a self-supporting anode for LIBs, Co-MoS2/CC delivers a high reversible capacity of 1392 mAh g−1 at 0.1 A g−1 and retains 611 mAh g−1 at 2 A g−1 over 300 cycles. Furthermore, it delivers a remarkable rate performance of 230 mAh g−1 at 10 A g−1. The facile and scalable synthesis method can also be applied to other similar flexible free-standing electrodes for the application of electrochemical energy storage.

Stable and low-resistance polydopamine methacrylamide-polyacrylamide hydrogel for brain-computer interface

Signal drift and performance instability of brain-computer interface devices induced by the interface failure between rigid metal electrodes and soft human skin hinder the precise data acquisition of electroencephalogram (EEG). Thus, it is desirable to achieve a robust interface for brain-computer interface devices. Here, a kind of polydopamine methacrylamide-polyacrylamide (PDMA-PAAM) hydrogel is developed. To improve the adhesion, dopamine is introduced into the polyacrylamide hydrogel, through the amino and catechol groups of dopamine in an organic-inorganic interface to build a covalent and non-covalent interaction. A strong attachment and an effective modulus transition system can be formed between the metal electrodes and human skin, so that the peeling force between the PDMA-PAAM hydrogel and the porcine skin can reach 22 N m−1. In addition, the stable conductivity and long-term operating life of the PDMA-PAAM hydrogel for more than 60 days at room temperature are achieved by adding sodium chloride (NaCl) and glycerol, respectively. The PDMA-PAAM hydrogel membrane fabricated in this work is integrated onto a flexible Au electrode applied in a brain-computer interface. In comparison, the collected EEG signal intensity and waveform are consistent with that of the commercial counterparts. And obviously, the flexible electrode with PDMA-PAAM hydrogel membrane is demonstrated to enable a more stable and user-friendly interface.

Localizing epileptic focus and assessing electrical stimulus effects on epilepsy in rats using stretchable micro electrocorticogram electrodes

Epilepsy is a chronic nervous disease with increasing incidence worldwide, while the accurate localization of epileptic focus and the corresponding treatment are still challenging due to the lack of effective tools to monitor and modulate the related brain neurological activities. In this work, stretchable micro electrocorticogram (mECoG) electrodes are developed and used to investigate penicillin-induced epilepsy in rats. The electrodes possess excellent stretchability, conformality, anti-interference ability and sufficient resolution to successfully monitor electroencephalogram (EEG) signals, which is superior to traditional rigid polyimide-based electrodes. Characteristic epileptic spike waves are detected and analyzed to study the epileptic focus and electrical stimulus effects during epileptic seizures. It is found that the spike waves occur first in the visual cortex which is likely to be the epileptic focus. Epileptic spike wave frequency quickly increases to 1.07 Hz where it reaches a plateau and remains stable. There is no dominant brain hemisphere that would show early warning of epileptic seizures. Electrical stimuli for various times are applied after administering penicillin. It is found that 15 min of electrical stimulus has the best restraining effect on epileptic seizures. The mECoG electrodes developed in this study show potentials for applications in stretchable biomedical devices.

Reverse Electrowetting-on-Dielectric Energy Harvesting Using 3-D Printed Flexible Electrodes for Self-Powered Wearable Sensors

Reverse electrowetting-on-dielectric (REWOD) presents a unique low-frequency motion energy harvesting method to self-power wearable sensors for human health monitoring in real-time. However, previous studies in REWOD energy harvesting have mainly focused on enhancing power density using rigid electrodes, which due to their rigidity, are not ideal to be implemented as wearable energy harvesting or motion sensing applications from human motion activities. To be considered as the next generation self-powered wearable sensors, self-powered wearable sensors should operate continuously and sustainably in any kind of electrode bending motion and without an external power source. Herein, we demonstrate ac current generation by repeatedly modulating a liquid droplet of microliter volume on an elastomer-based three-dimensional printed flexible electrode without any external bias source. Flexible electrodes were fabricated by deposition of ∼200 nm of titanium (Ti) and an additional ∼200 nm of aluminum oxide (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) along with ∼50 nm of chromium (Cr) as an interlayer between the metal and the dielectric layers, and all of these on a flexible polydimethylsiloxane substrate. Using a 50 μL electrolyte droplet and 2.5 mm of electrode displacement at a low-frequency motion of 1–2 Hz, a maximum peak-to-peak ac current of ∼340 nA was measured. Although significant progress has yet to be made toward implementing REWOD energy harvesting as a reliable source of power for wearable self-powered systems, this letter signifies the proof-of-concept in implementing flexible electrodes in REWOD energy harvesting.

Adhesive and Hydrophobic Bilayer Hydrogel Enabled On‐Skin Biosensors for High‐Fidelity Classification of Human Emotion

AbstractTraditional human emotion recognition is based on electroencephalogram (EEG) data collection technologies which rely on plenty of rigid electrodes and lack anti‐interference, wearing comfort, and portability. Moreover, a significant distribution difference in EEG data also results in low classification accuracy. Here, on‐skin biosensors with adhesive and hydrophobic bilayer hydrogel (AHBH) as interfaces for high accuracy emotion classification are proposed. The AHBH achieves remarkable adhesion (59.7 N m−1) by combining the adhesion mechanism of catechol groups and electrostatic attraction. Meanwhile, based on the synergistic effects of hydrophobic group rearrangements and surface energy reduction, the AHB‐hydrophobic layer exhibits 133.87° water contact angles through hydrophobic treatment of only 0.5 h. Hydrogen and electrostatic bonds are also introduced to form a seamless adhesive‐hydrophobic hydrogel interface and inhibit adhesion attenuation, respectively. With the AHBH as an ideal device/skin interface, the biosensor can reliably collect high‐quality electrophysiological signals even under vibration, sweating, and long‐lasting monitoring condition. Furthermore, the on‐skin electrodes, data processing, and wireless modules are integrated into a portable headband for EEG‐based emotion classification. A domain adaptive neural network based on the transfer learning technique is introduced to alleviate the effect of domain shift and achieve high classification accuracy.

The electrical response and self-sensing of the fully flexible PVC gel actuator based on flexible electrodes

There has recently been a substantial increase in interest in research on PVC gel actuators because of its numerous advantages over traditional actuators, such as low mass density, large actuation strain, great compliance, and simple manufacturing. However, the current PVC gel actuator is mainly based on the research of rigid electrodes, and there are few researches based on flexible electrodes. In this paper, a fully flexible actuator based on PVC gel film and carbon grease electrode has been prepared. The mechanical properties of the prepared PVC gels with different plasticizer ratios were tested. Three commonly used hyperelastic models (Neo-Hookean, Mooney-Rivlin and Yeoh models) were used to characterize the mechanical properties of PVC gels, and only the Yeoh model was found to be better for the characterization of the mechanical properties of PVC gels. In addition, we used the Prony series to successfully characterize the viscoelasticity of PVC gels. Then, the fully flexible PVC gel actuator (FFPGA) with long-time static and dynamic response characteristics was studied. It was found that the FFPGA has strong viscoelasticity when working. Finally, we proposed to simplify the FFPGA into a capacitor and designed a high-pass filter in the driving circuit. The self-sensing function of the FFPGA was achieved by superimposing a high-frequency low-voltage sensing signal on the low-frequency high-voltage driving signal as the input signal.

All-in-one asymmetric micro-supercapacitor with Negative Poisson's ratio structure based on versatile electrospun nanofibers

It is very necessary to design electrodes with satisfactory electrochemical performance and robust mechanical properties for flexible energy storage devices. Herein, a self-supporting electrode with Negative Poisson's Ratio (NPR) structure was fabricated by combining electrospinning and laser direct writing technology, which overcame the limitation that the traditional carbon-based materials cannot be stretched and avoided the integration problem of rigid electrodes and soft substrates in the conventional stretchable devices. Furthermore, a quasi-solid-state asymmetric micro-supercapacitor with NPR structure was assembled by an as-spun separator coated with gel electrolyte and two composite electrodes composed of carbon nanofibers and nanoarrays ([email protected]2O3 nanorods and [email protected]2 nanosheets). The apparatus not only reached an extended voltage window of 2 V, but also achieved an area energy density of 26 μWh·cm−2 at a power density of 0.55 mW·cm−2, which was superior to other wearable micro-supercapacitors.

High Interfacial Thermal Stability of Flexible Flake-Structured Aluminum Thin-Film Electrodes for Bi2Te3-Based Thermoelectric Devices.

Environmental thermal energy harvesting based on thermoelectric devices is greatly significant to the advancement of next-generation self-powered wearable electronic devices. However, the rigid electrodes and interface diffusion of electrodes/thermoelectric materials would lead to the wearable discomfort and performance degradation of the thermoelectric device. Here, a flake-structured Al thin-film electrode with high conductivity and excellent reliability is prepared by regulating the microstructure and crystallinity of the films. The as-prepared Al thin film not only maintains its robustness after 1000 bending cycles but also does not delaminate from the substrate when subjected to the 3M tape test, exhibiting excellent flexibility and adhesion to substrate. By comparing with the annealed interface of the double-layer Cu/Bi2Te3 film, the interface of the heat-treated Al/Bi2Te3 film has almost no element diffusion, demonstrating high interfacial thermal stability. Moreover, a thermoelectric temperature sensor based on the Al thin-film electrode is prepared. The sensitivity of the annealed sensor is still linear, and it can stably monitor the temperature variation, showing high reliability. This discovery could provide a facile and effective strategy to achieving highly reliable thermoelectric devices and flexible electronic devices without any additional diffusion barriers.

On an electrostatic micropump with a rigorous mathematical model

The novel electrostatic micropump model for applications such as in biomedical drug delivery is presented. The geometrical arrangement of the coupling rigid electrodes lets us exploit our mathematically rigorous boundary integral equation formulation and its solution. Thus, the charge densities involving the fringe effects on the plates are obtained by means of analytical regularization method (ARM) constructed for annular strips earlier. The efficiency of the constructed method is demonstrated with respect to the direct integral equation solvers implemented via the entire domain Galerkin method and point matching. The main physical characteristics of the suggested system and their deviation from that of infinitely large plane approximation are discussed.

Research Progress on the Flexibility of an Implantable Neural Microelectrode.

Neural microelectrode is the important bridge of information exchange between the human body and machines. By recording and transmitting nerve signals with electrodes, people can control the external machines. At the same time, using electrodes to electrically stimulate nerve tissue, people with long-term brain diseases will be safely and reliably treated. Young’s modulus of the traditional rigid electrode probe is not matched well with that of biological tissue, and tissue immune rejection is easy to generate, resulting in the electrode not being able to achieve long-term safety and reliable working. In recent years, the choice of flexible materials and design of electrode structures can achieve modulus matching between electrode and biological tissue, and tissue damage is decreased. This review discusses nerve microelectrodes based on flexible electrode materials and substrate materials. Simultaneously, different structural designs of neural microelectrodes are reviewed. However, flexible electrode probes are difficult to implant into the brain. Only with the aid of certain auxiliary devices, can the implant be safe and reliable. The implantation method of the nerve microelectrode is also reviewed.

Cold plasma from flexible and conformable paper-based electrodes for fresh produce sanitation: Evaluation of microbial inactivation and quality changes

Two types of cold plasma generating paper-based electrodes (CPPE) made with flexible metallized paper were developed to decontaminate fresh produce. As opposed to conventional rigid volume dielectric barrier discharge electrodes currently used for in-package plasma generation for which voltage increases as a function of thickness, two flexible CPPE configurations based on surface dielectric barrier discharge provided high local concentrations of plasma with less voltage which was independent of the thickness of the package. For decontaminating E. coli and Listeria innocua, a circular design for spinach and a kirigami and origami based two-cone design for tomato were used. Generation of ozone and other reactive oxygen and nitrogen species by both CPPE were identified. Activation of the circular CPPE for 10 min reduced E. coli by 4.6 ± 0.6 log CFU per spinach leaf and 4.6 ± 0.5 log CFU per tomato, whereas for Listeria innocua the reductions were 4.8 ± 1.7 log CFU per spinach leaf and 2.0 ± 0.4 log CFU per tomato, respectively. No changes in quality attributes were observed in the post-treated tomatoes, while surface browning was observed on spinach leaves. The results demonstrated the effectiveness of the innovative CPPE for in-package sanitation of fresh produce and its impact on quality. Package conformability, surface properties, and bacterial characteristics played important role in overall inactivation efficacy.

Fully printed and multifunctional graphene-based wearable e-textiles for personalized healthcare applications

SummaryWearable e-textiles have gained huge tractions due to their potential for non-invasive health monitoring. However, manufacturing of multifunctional wearable e-textiles remains challenging, due to poor performance, comfortability, scalability, and cost. Here, we report a fully printed, highly conductive, flexible, and machine-washable e-textiles platform that stores energy and monitor physiological conditions including bio-signals. The approach includes highly scalable printing of graphene-based inks on a rough and flexible textile substrate, followed by a fine encapsulation to produce highly conductive machine-washable e-textiles platform. The produced e-textiles are extremely flexible, conformal, and can detect activities of various body parts. The printed in-plane supercapacitor provides an aerial capacitance of ∼3.2 mFcm−2 (stability ∼10,000 cycles). We demonstrate such e-textiles to record brain activity (an electroencephalogram, EEG) and find comparable to conventional rigid electrodes. This could potentially lead to a multifunctional garment of graphene-based e-textiles that can act as flexible and wearable sensors powered by the energy stored in graphene-based textile supercapacitors.

Contact resistance based tactile sensor using covalently cross-linked graphene aerogels.

A movable electrical contact between two materials is one of the most fundamental, simple, and common components in electronics that is used for binary control of a conducting path in an electrical circuit. Here, variable contact resistance between a highly elastic graphene aerogel and a rigid metal electrode is used for the analysis of non-binary pushing and pulling mechanical forces acting on the contact, enabling superior strain and pressure measurements. The variable contact resistance based electromechanical sensors demonstrate superfast, ultrasensitive and quantitative measurements of compressive and tensile stress from -1.18 MPa to 0.55 MPa. The sensors can operate over the temperature range of -60 to 100 °C, cover the whole skin and human motion range, and determine the weight of a grasped object. The measurement of such high forces has only been possible due to the high-temperature induced covalent cross-linking of graphene in the aerogel that provides high strength, durability, and fast response (<0.5 ms) to the sensing element. The study demonstrates the great potential of the contact resistance-controlled sensing, which enables high-precision and reliable measurement of strain and pressure over a remarkable large sensing range, providing new opportunities for applications in human-machine interfaces, robotics, flexible electronics, and haptic technology.

Boosting uniform charge distribution using 3D rigid electrodes with interconnected gyroid channels to achieve stable and reliable zinc-ion batteries

3D gyroid electrodes with excellent connectivity and rigidity boost uniform charge distribution within the 3D structure, achieving stable and reliable zinc ion batteries.

Patterned Sandwich-Type Silver Nanowire-Based Flexible Electrode by Photolithography.

Silver nanowires (AgNWs) are one of the important flexible electrode material candidates that can replace brittle indium tin oxide (ITO). In this work, we demonstrated novel patterned sandwich-type AgNW-based transparent electrodes easily prepared using the photolithography method for application in flexible devices. A cross-linked underlayer was introduced to increase the adhesion properties between a poly(ethylene terephthalate) substrate and AgNWs, and as a result, a uniform AgNW layer was easily deposited. Finally, the AgNW layer could be easily patterned by introducing a photocross-linkable upper layer without lift-off, dry transfer, and removal methods. A mixture of poly(sodium-4-styrene sulfonate) (PSS-Na+) and 2,4-hexadiyne-1,6-diol (HDD), which is a component of the upper layer, exhibited good cross-linking properties as well as excellent adhesion to the AgNW layer. Through the above method, it was possible to easily fabricate a patterned electrode with smooth surface morphology. Moreover, AgNW-based patterned electrodes exhibit good optical and electrical properties (Rs = 29.8 Ω/□, T550 nm = 94.6%), making them suitable for optoelectronic devices. Flexible polymer solar cells (PSCs) using patterned AgNW electrodes showed a high power conversion efficiency of over 10%, which is comparable to that of PSCs using rigid ITO electrodes. In addition, the high mechanical stability of AgNW-based PSCs was confirmed by bending experiments.

Organic Electrochemical Transistors for In Vivo Bioelectronics

Organic electrochemical transistors (OECTs) are presently a focus of intense research and hold great potential in expanding the horizons of the bioelectronics industry. The notable characteristics of OECTs, including their electrolyte-gating, which offers intimate interfacing with biological environments, and aqueous stability, make them particularly suitable to be operated within a living organism (in vivo). Unlike the existing in vivo bioelectronic devices, mostly based on rigid metal electrodes, OECTs form a soft mechanical contact with the biological milieu and ensure a high signal-to-noise ratio because of their powerful amplification capability. Such features make OECTs particularly desirable for a wide range of in vivo applications, including electrophysiological recordings, neuron stimulation, and neurotransmitter detection, and regulation of plant processes in vivo. In this review, a systematic compilation of the in vivo applications is presented that are addressed by the OECT technology. First, the operating mechanisms, and the device design and materials design principles of OECTs are examined, and then multiple examples are provided from the literature while identifying the unique device properties that enable the application progress. Finally, one critically looks at the future of the OECT technology for in vivo bioelectronic applications.

Wearable eutectic gallium-indium liquid fuel cells

With the rapid growth of wearable electronics, the development of wearable fuel cells as a smart power source is receiving ever more attention due to their high energy conversion efficiency, modest operating temperature, and ease of handling. To address the most notable limiting factor, the rigid electrodes, of fuel cells, herein, a eutectic gallium-indium liquid metal with excellent deformability and redox ability has been employed to wearable and rechargeable fuel cells with high performance. Thanks to the optimized Ga/In ratio, which is achieved by balancing the anticorrosion and electrochemical activity of the liquid metal anode, the power density of the fuel cell is as high as 72.8 mW cm−2; to our knowledge, this is the highest power density among existing wearable liquid fuel cells at room temperature. Due to the stable redox properties of liquid metal, the fuel cell was stably cycled for 96 h at 2 mA cm−2 as a rechargeable metal-air battery. Meanwhile, running in feed mode to maintain the proportion of Ga in the anode, the fuel cell and the rechargeable liquid metal fuel cell exhibited stable discharging and cycling performances, respectively, and delivered exemplary performances under various flexibility and stretchability measurements. Based on the fluent and renewable liquid metal anode, this novel high-performance wearable liquid fuel cell shows great promise as a shape-variable energy supply for bionic soft robots and wearable devices.

Textile-Based Sensors for Biosignal Detection and Monitoring.

Biosignals often have to be detected in sports or for medical reasons. Typical biosignals are pulse and ECG (electrocardiogram), breathing, blood pressure, skin temperature, oxygen saturation, bioimpedance, etc. Typically, scientists attempt to measure these biosignals noninvasively, i.e., with electrodes or other sensors, detecting electric signals, measuring optical or chemical information. While short-time measurements or monitoring of patients in a hospital can be performed by systems based on common rigid electrodes, usually containing a large amount of wiring, long-term measurements on mobile patients or athletes necessitate other equipment. Here, textile-based sensors and textile-integrated data connections are preferred to avoid skin irritations and other unnecessary limitations of the monitored person. In this review, we give an overview of recent progress in textile-based electrodes for electrical measurements and new developments in textile-based chemical and other sensors for detection and monitoring of biosignals.

Flexible iron oxide supercapacitor electrodes by photonic processing

We report the preparation of flexible and nano-porous iron oxide-reduced graphitic oxide (Fe2O3–rGO) electrodes using a novel photonic processing method. Due to this unique technique, high-temperature thermal processing could be accomplished on inexpensive and low-temperature substrates instantaneously as opposed to longer processing times of conventional thermal processing. The nano-porous morphology of the electrode not only accommodates the volume changes of the electrode but also facilitates the transport of the electrolyte ions into the electrodes. The as-prepared electrode showed excellent electrochemical performance with an initial specific capacitance of 179 F/g at 2 A/g. Moreover, it exhibited excellent specific capacitance retention after 5000 cycles (70%), revealing its superior cyclic stability. Along with having specific capacitance comparable to that of rigid electrodes, the as-prepared electrode is bendable and lightweight, signifying its potential application in foldable and wearable consumer electronic devices which require continuous energy supply while going through physical deformation.Graphic abstract

Conformal electrodes for on‐skin digitalization

AbstractOn‐skin digitalization, streamlining the concept of the “human to device to cyberspace” platform, has attracted great attention due to its vital function in remote medicine and human–cyber interfaces. Beyond traditional rigid electrodes, soft electrodes with conformal and comfortable interfaces are essential for long‐term and high‐fidelity signal acquisition. In addition, the on‐skin data processing systems will get rid of complex cables toward a vision of fascinating form, being lightweight, skin‐friendly and even imperceptible. Although numerous soft materials and devices with mechanical tolerance have been developed, the study of conformal electrodes and on‐skin digital integrated systems are still in infancy. Here, the requirements and designs of conformal electrodes, the emerging opportunities and challenges from multichannel/multifunctional sensors to a whole new on‐skin sensing platform are highlighted.