Neural electrodes enable the recording and stimulation of bioelectrical activity in the nervous system. This technology provides neuroscientists with the means to probe the functionality of neural ...circuitry in both health and disease. In addition, neural electrodes can deliver therapeutic stimulation for the relief of debilitating symptoms associated with neurological disorders such as Parkinson’s disease and may serve as the basis for the restoration of sensory perception through peripheral nerve and brain regions after disease or injury. Lastly, microscale neural electrodes recording signals associated with volitional movement in paralyzed individuals can be decoded for controlling external devices and prosthetic limbs or driving the stimulation of paralyzed muscles for functional movements. In spite of the promise of neural electrodes for a range of applications, chronic performance remains a goal for long-term basic science studies, as well as clinical applications. New perspectives and opportunities from fields including tissue biomechanics, materials science, and biological mechanisms of inflammation and neurodegeneration are critical to advances in neural electrode technology. This Special Issue will address the state-of-the-art knowledge and emerging opportunities for the development and demonstration of advanced neural electrodes.
Bioelectrical or electrophysiological signals generated by living cells or tissues during daily physiological activities are closely related to the state of the body and organ functions, and ...therefore are widely used in clinical diagnosis, health monitoring, intelligent control and human-computer interaction. Ag/AgCl electrodes with wet conductive gels are widely used to pick up these bioelectrical signals using electrodes and record them in the form of electroencephalograms, electrocardiograms, electromyography, electrooculograms, etc. However, the inconvenience, instability and infection problems resulting from the use of gel with Ag/AgCl wet electrodes can't meet the needs of long-term signal acquisition, especially in wearable applications. Hence, focus has shifted toward the study of dry electrodes that can work without gels or adhesives. In this paper, a retrospective overview of the development of dry electrodes used for monitoring bioelectrical signals is provided, including the sensing principles, material selection, device preparation, and measurement performance. In addition, the challenges regarding the limitations of materials, fabrication technologies and wearable performance of dry electrodes are discussed. Finally, the development obstacles and application advantages of different dry electrodes are analyzed to make a comparison and reveal research directions for future studies.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Arrays of electrodes for recording and stimulating the brain are used throughout clinical medicine and basic neuroscience research, yet are unable to sample large areas of the brain while maintaining ...high spatial resolution because of the need to individually wire each passive sensor at the electrode-tissue interface. To overcome this constraint, we developed new devices that integrate ultrathin and flexible silicon nanomembrane transistors into the electrode array, enabling new dense arrays of thousands of amplified and multiplexed sensors that are connected using fewer wires. We used this system to record spatial properties of cat brain activity in vivo, including sleep spindles, single-trial visual evoked responses and electrographic seizures. We found that seizures may manifest as recurrent spiral waves that propagate in the neocortex. The developments reported here herald a new generation of diagnostic and therapeutic brain-machine interface devices.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Emerging real-world EEG applications require gel-free electrodes, which have to break through technical bottlenecks and achieve satisfactory electrode-skin impedance. It is crucial to understand the ...electrical properties of the electrode-skin interface. In this work, the electrode-skin impedance of bioelectrodes (wet, semi-dry, and dry) has been studied systemically, concerning not only magnitude but stability. Various factors have been investigated including types of electrodes, skin locations, pressure, skin abrasion, and electrode contact area. The electrode-skin impedance always decreases in the following order: forearm, scalp and forehead for all electrodes. Compared with the impedances of wet electrodes and semi-dry electrodes, the dry electrode impedances are significantly higher (58.50±64.16kΩcm2) and unstable (impedance variation 31.2±31.3kΩ/10min). Even worse, the dry electrode impedance variation between six subjects is considerably large (57.5–540.0kΩ). As a result, no satisfactory EEG signals could be obtained. Moreover, the dry electrode impedances are lowered significantly under pressure or after skin abrasion. Accordingly, alpha rhythms from the dry electrodes appeared with the assistance of pressure or skin abrasion. These findings provide insights for the development of new gel-free electrodes to complement the emerging new EEG applications, such as brain-computer interfaces and wearable EEGs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
It follows from critical evaluation of possibilities and limitations of modern voltammetric/amperometric methods that one of the biggest obstacles in their practical applications in real sample ...analysis is connected with electrode passivation/fouling by electrode reaction products and/or matrix components. This review summarizes possibilities how to minimise these problems in the field of detection of small organic molecules and critically compares their potential and acceptability in practical laboratories. Attention is focused on simple and fast electrode surface renewal, the use of disposable electrodes just for one and/or few measurements, surface modification minimising electrode fouling, measuring in flowing systems, application of rotating disc electrode, the use of novel separation methods preventing access of passivating particles to electrode surface and the novel electrode materials more resistant toward passivation. An attempt is made to predict further development in this field and to stress the need for more systematic and less random research resulting in new measuring protocols less amenable to complications connected with electrode passivation.
The oxygen reduction reaction (ORR) is important due to high electrochemical potential. Multicopper oxidases (e.g., bilirubin oxidase (BOx)) belong to a class of enzymes that catalyze ORR with low ...overpotential, and have attracted significant attention in designing bioelectronic devices, viz., biosensors and enzymatic cathodes of biofuel cells. In this work, we present a new concept – an enzyme driven pH electrode system composed of two electrodes: a pH-sensitive BOx-based working electrode and pH-insensitive, a BOx-based reference electrode. ORR-driven pH monitoring was conducted by measuring the potential difference between the two electrodes. Moreover, the electrode system was self-supported and self-calibrating, and no additional electrodes (Pt counter or Ag/AgCl reference) were needed for measurements. The BOx-based pH sensor was tested in continuous and real-time pH monitoring with high accuracy. Also, we present an in-depth mechanistic study explaining the electrode potential-pH dependence, which brings new insights into the operational principle of bilirubin oxidase at different pH.
•An enzymatic pH biosensor based on bilirubin oxidase was developed.•Electrode system was self-supported and self-calibrating.•Electrode system was tested in continuous and real-time pH monitoring.•Mechanistic study, explaining the electrode potential-pH dependence was conducted.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Equivalent circuit models for three typical EEG electrodes were proposed.•Area-normalized impedance was used throughout the whole analysis.•Impedance fitting with mild pressure and skin abrasion ...conditions were investigated.•Inspiration for development of novel conductive-gel-free electrodes was discussed.
The electrode-scalp impedance of six subjects is studied by electrochemical impedance spectroscopy using three representative electrodes, namely wet electrodes, semi-dry electrodes, and dry electrodes. With the analysis of proposed equivalent models and area-normalized impedance (kΩ cm2), the specific electrode double layer resistances are 530 Ω cm2 for all electrodes, their specific contact resistances Res are 80 Ω cm2, 114 Ω cm2 and 20,077 Ω cm2, and their specific skin resistances Rs are 867 Ω cm2, 1730 Ω cm2 and 46,145 Ω cm2 for these three representative electrodes respectively. When pressure was applied on dry electrodes, Res reduced by 47% and Rs reduced by 81%. When pressure was applied on semi-dry electrodes, Rs reduced by 50% (Res is not applicable for semi-dry electrodes). After application of scalp abrasion for dry electrodes, Res reduced by 36% and Rs reduced by 82%. After application of scalp abrasion for semi-dry electrodes, Rs reduced by 64%. Seeking novel interface materials may break through the bottleneck for dry electrodes development in terms of integrated materials, mechanical support and electronic circuitry, while semi-dry electrodes have much development space to meet a balance of quick setup, comfortable wear, and satisfactory signal quality in many medical and out-clinic applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•Fast, simple, reproducible method to fabricate Ag/Ag2S micro-electrode by reacting Ag strip/wire with sulfur vapor at 105 °C.•No encapsulation needed: compact, small ...footprint.•Compatible with polar solvents and ionic liquids.•Repeatable and stable performance over years.•Optimal morphology corresponds to slightly porous Ag2S film of intermediate thickness.
Reliable electrochemical measurements depend on the availability of robust reference electrodes (RE) with well-defined potentials. While many reliable REs are known, they are not applicable in certain demanding media such as ionic liquids, nor in small confined spaces. Here, we describe the fabrication of a simple yet robust Ag/Ag2S micro-reference electrode (μ-RE) where a micron-thick Ag2S layer is formed by isothermal reaction with sulfur vapor. Scanning electron microscopy, X-ray photoemission spectroscopy, and spectroscopic ellipsometry characterization reveals that the optimal morphology corresponds to a slightly porous Ag2S film. We demonstrate that the Ag/Ag2S μ-RE can be operated in and cycled through a wide variety of polar organic solvents, including common protic solvents (EtOH), aprotic solvents (ACN, DMSO, NMP, DMF) and ionic liquids (EMIM-TFSI, BMP-TFSI), with short equilibration time (tens of seconds) and little drift (<20 mV), without requiring encapsulation, protective liquid junctions, nor special conditioning. A redox potential of 0.54 ± 0.02 V was obtained for ferrocene in acetonitrile, which places this RE at 0.08 V vs Ag/AgCl. We have also successfully embedded the electrode inside the CR2032 coin cell to perform cyclic voltammetry of battery materials. These results underpin the suitability of this simple micro-reference electrode for a wide variety of electrochemical measurements in demanding and/or miniaturized environments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Electrical stimulation of nervous structures is a widely used experimental and clinical method to probe neural circuits, perform diagnostics, or treat neurological disorders. The recent introduction ...of soft materials to design electrodes that conform to and mimic neural tissue led to neural interfaces with improved functionality and biointegration. The shift from stiff to soft electrode materials requires adaptation of the models and characterization methods to understand and predict electrode performance. This guideline aims at providing (1) an overview of the most common techniques to test soft electrodes in vitro and in vivo; (2) a step-by-step design of a complete study protocol, from the lab bench to in vivo experiments; (3) a case study illustrating the characterization of soft spinal electrodes in rodents; and (4) examples of how interpreting characterization data can inform experimental decisions. Comprehensive characterization is paramount to advancing soft neurotechnology that meets the requisites for long-term functionality in vivo.
Schiavone et al. describe the use of current characterization techniques to study electrode systems for neural stimulation, with focus on the specificities of soft technology. The authors provide guidelines to design complete in vivo characterization studies and interpret characterization data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The performance of rechargeable lithium (Li) batteries is highly correlated with the structure of solid electrolyte interphase (SEI). The properties of a working anode are vital factors in ...determining the structure of SEI; however, the correspondingly poor understanding hinders the rational regulation of SEI. Herein, the electrode potential and anode material, two critical properties of an anode, in dictating the structural evolution of SEI were investigated theoretically and experimentally. The anode potential is identified as a crucial role in dictating the SEI structure. The anode potential determines the reduction products in the electrolyte, ultimately giving rise to the mosaic and bilayer SEI structure at high and low potential, respectively. In contrast, the anode material does not cause a significant change in the SEI structure. This work discloses the crucial role of electrode potential in dictating SEI structure and provides rational guidance to regulate SEI structure.
The role of electrode potential and anode material, two critical properties of a working anode, in dictating the structural evolution of solid electrolyte interphase (SEI) was investigated theoretically and experimentally, which provides rational guidance to regulate SEI structure.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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