Low Electrode Impedance Research Articles

A profile on the WISE cortical strip for intraoperative neurophysiological monitoring

ABSTRACT Introduction During intraoperative neurophysiological monitoring in neurosurgery, brain electrodes are placed to record electrocorticography or to inject current for direct cortical stimulation. A low impedance electrode may improve signal quality. Areas covered We review here a brain electrode (WISE Cortical Strip, WCS®), where a thin polymer strip embeds platinum nanoparticles to create conductive electrode contacts. The low impedance contacts enable a high signal-to-noise ratio, allowing for better detection of small signals such as high-frequency oscillations (HFO). The softness of the WCS may hinder sliding the electrode under the dura or advancing it to deeper structures as the hippocampus but assures conformability with the cortex even in the resection cavity. We provide an extensive review on WCS including a market overview, an introduction to the device (mechanistics, cost aspects, performance standards, safety and contraindications) and an overview of the available pre- and post-approval data. Expert opinion The WCS improves signal detection by lower impedance and better conformability to the cortex. The higher signal-to-noise ratio improves the detection of challenging signals. The softness of the electrode may be a disadvantage in some applications and an advantage in others.

Low impedance electrodes improve detection of high frequency oscillations in the intracranial EEG

ObjectiveEpileptic fast ripple oscillations (FR, 250–500 Hz) indicate epileptogenic tissue with high specificity. However, their low amplitude makes detection demanding against noise. Since thermal noise is reduced by low impedance electrodes (LoZ), we investigate here whether this noise reduction is relevant in the FR frequency range. MethodsWe analyzed intracranial electrocorticography during neurosurgery of 10 patients where a low impedance electrode was compared to a standard electrode (HiZ) with equal surface area during stimulation of the somatosensory evoked potential, which evokes a robust response in the FR frequency range. To estimate the noise level, we computed the difference between sweep 2n and sweep 2n + 1 for all sweeps. ResultsThe power spectral density of the noise spectrum improved for the LoZ over all frequencies. In the FR range, the median noise level improved from HiZ (0.153 µV) to LoZ (0.089 µV). For evoked FR, the detection rate improved (91% for HiZ vs. 100% for LoZ). ConclusionsLow impedance electrodes for intracranial EEG reduce noise in the FR frequency range and may thereby improve FR detection. SignificanceImproving the measurement chain may enhance the diagnostic value of FR as biomarkers for epileptogenic tissue.

Neural probe integrated with low-impedance electrodes implemented using vertically aligned carbon nanotubes for three-dimensional mapping of neural signals

High-quality recordings of neural interfaces are crucial in advancing neuroscience research, but conventional electrodes with highly conductive materials have limitations due to their limited effective surface area. Here, we develop a neural probe integrated with an electrode implemented using vertically aligned carbon nanotubes (VACNTs) that remarkably improves recording capability. By directly synthesizing VACNT bundles on the electrode array of the neural probe, we successfully fabricate an electrode with the lowest area-specific impedance. Owing to the dramatically lowered impedance of the VACNT electrode, its signal-to-noise ratio increases by more than 3 times compared with that of a black Pt electrode with low impedance. Moreover, the number of spikes observed in the measured signals increases by more than 4 times. Notably, our VACNT electrode enables accurate mapping of the spatial distribution of neurons, demonstrating its potential to significantly advance neuroscience research through massive neural recordings with improved mapping capability. This powerful technology that maximizes electrode performance could pave the way for new discoveries and insights in the field of neuroscience.

Inkjet-Printed Graphene-Modified Aluminum Current Collector for High-Voltage Lithium-Ion Battery

One of the problems plaguing the development of Li-ion batteries that can operate at high cell voltages (i.e., beyond 4.2 V; vs Li/Li+) is the corrosion of the cathode current collector, which causes electrochemical instability and impedance build-up. In this regard, here, we demonstrate the benefits of using inkjet printing technology to uniformly coat the Al current collector (AlCC) with a graphene layer to suppress the corrosion degradation of the current collector at the cathode side. The graphene ink is prepared via solvent exfoliation, and then a thin layer of graphene is coated (∼268 nm in thickness) via inkjet printing on AlCC, which is further annealed in Ar at 350 °C to enhance the electrical conductivity. The thickness and mass loading of the graphene film on Al foil is controlled by the number of printing cycles. The corrosion properties of the bare and printed graphene-coated AlCCs (Gr-AlCC) were evaluated by cyclic voltammetry within 1–5 V (vs Li/Li+) using a Li-ion battery electrolyte. This clearly showed the extensive corrosion degradation of the bare AlCC, which was, however, nearly completely suppressed in the presence of the printed graphene coating. Upon usage of these AlCCs for the galvanostatic cycling of homemade Li-NMC-based cathodes in Li “half” cells, the Gr-AlCC resulted in excellent cyclic stability pertaining to ∼90% capacity retention after 100 cycles (@C/5) despite the usage of 4.5 V as the upper cut-off. By contrast, the bare AlCC resulted in only ∼68% capacity retention under the same conditions. Furthermore, the usage of Gr-AlCC facilitated considerably superior rate capability and lower electrode impedance as compared to the bare AlCC counterpart. This highlights the importance of such passivation/protection of current collectors with conducting coatings and the utility of the scalable-cum-versatile inkjet printing technology for the same.

P-128 Low impedance electrodes improve detection of high frequency oscillations in intracranial EEG

P-128 Low impedance electrodes improve detection of high frequency oscillations in intracranial EEG

Oscillatory discharges in the auditory midbrain of the big brown bat contribute to coding of echo delay.

Subsequent to his breakthrough discovery of delay-tuned neurons in the bat's auditory midbrain and cortex, Albert Feng proposed that neural computations for echo delay involve intrinsic oscillatory discharges generated in the inferior colliculus (IC). To explore further the presence of these neural oscillations, we recorded multiple unit activity with a novel annular low impedance electrode from the IC of anesthetized big brown bats and Seba's short-tailed fruit bats. In both species, responses to tones, noise bursts, and FM sweeps contain long latency components, extending up to 60ms post-stimulus onset, organized in periodic, oscillatory-like patterns at frequencies of 360-740Hz. Latencies of this oscillatory activity resemble the wide distributions of single neuron response latencies in the IC. In big brown bats, oscillations lasting up to 30ms after pulse onset emerge in response to single FM pulse-echo pairs, at particular pulse-echo delays. Oscillatory responses to pulses and evoked responses to echoes overlap extensively at short echo delays (5-7ms), creating interference-like patterns. At longer echo delays, responses are separately evident to both pulses and echoes, with less overlap. These results extend Feng's reports of IC oscillations, and point to different processing mechanisms underlying perception of short vs long echo delays.

In situ diagnosis and simultaneous treatment of cardiac diseases using a single-device platform

The in situ diagnosis of cardiac activities with simultaneous therapeutic electrical stimulation of the heart is key to preventing cardiac arrhythmia. Here, we present an unconventional single-device platform that enables in situ monitoring even in a wet condition and control of beating heart motions without interferences to the recording signal. This platform consists of the active-matrix array of pressure-sensitive transistors for detecting cardiac beatings, biocompatible, low-impedance electrodes for cardiac stimulations, and an alginate-based hydrogel adhesive for attaching this platform conformally to the epicardium. In contrast to conventional electrophysiological sensing using electrodes, the pressure-sensitive transistors measured mechanophysiological characteristics by monitoring the spatiotemporal distributions of cardiac pressures during heart beating motions. In vivo tests show mechanophysiological readings having good correlation with electrocardiography and negligible interference with the electrical artifacts caused during cardiac stimulations. This platform can therapeutically synchronize the rhythm of abnormal heartbeats through efficient pacing of cardiac arrhythmia.

New Microfluidic System for Electrochemical Impedance Spectroscopy Assessment of Cell Culture Performance: Design and Development of New Electrode Material.

Electrochemical impedance spectroscopy (EIS) is widely accepted as an effective and non-destructive method to assess cell health during cell-culture. However, there is a lack of compact devices compatible with microfluidic integration and microscopy that could provide the real-time and non-invasive monitoring of cell-cultures using EIS. In this paper, we reported the design and characterization of a modular EIS testing system based on a patented technology. This device was fabricated using easily processable methodologies including screen-printing of the impedance electrodes and molding or micromachining of the cell culture chamber with an easy assembly procedure. Accordingly, to obtain processable, biocompatible and sterilizable electrode materials that lower the impact of interfacial impedance on TEER (Transepithelial electrical resistance) measurements, and to enable concomitant microscopy observations, we optimized the formulation of the electrode inks and the design of the EIS electrodes, respectively. First, electrode materials were based on carbon biocompatible inks enriched with IrOx particles to obtain low interfacial impedance electrodes approaching the performances of classical non-biocompatible Ag/AgCl second-species electrodes. Secondly, we proposed three original electrode designs, which were compared to classical disk electrodes that were optically compatible with microscopy. We assessed the impact of the electrode design on the response of the impedance sensor using COMSOL Multiphysics. Finally, the performance of the impedance spectroscopy devices was assessed in vitro using human airway epithelial cell cultures.

Characterization of electrically conductive, printable ink based on alginate hydrogel and graphene nanoplatelets

In recent years, there has been an increasing interest in electrically conductive hydrogels for a wide range of biomedical applications, like tissue engineering or biosensors. In this study, we present a cost-effective conductive hydrogel based on alginate and graphene nanoplatelets for extrusion-based bioprinters. The hydrogel is prepared under ambient conditions avoiding high temperatures detrimental for cell culture environments. Investigation of the hydrogel revealed a conductivity of up to 7.5 S/cm, depending on the ratio of platelets. Furthermore, in vitro tests with human embyronic kidney cells - as an example cell type - showed good adhesion of the cells to the surface of the conductive hydrogel. Electrochemical measurements revealed a low electrode impedance which is desirable for the extracellular recording, but also low electrode capacitance, which is unfavorable for electrical stimulation purposes. Therefore, future experiments with the graphene nanoplatelets-based hydrogels will focus on electrodes for biosensors and extracellular recordings of neurons or cardiac myocytes.

A Carbon-Based Biosensing Platform for Simultaneously Measuring the Contraction and Electrophysiology of iPSC-Cardiomyocyte Monolayers.

Heart beating is triggered by the generation and propagation of action potentials through the myocardium, resulting in the synchronous contraction of cardiomyocytes. This process highlights the importance of electrical and mechanical coordination in organ function. Investigating the pathogenesis of heart diseases and potential therapeutic actions in vitro requires biosensing technologies which allow for long-term and simultaneous measurement of the contractility and electrophysiology of cardiomyocytes. However, the adoption of current biosensing approaches for functional measurement of in vitro cardiac models is hampered by low sensitivity, difficulties in achieving multifunctional detection, and costly manufacturing processes. Leveraging carbon-based nanomaterials, we developed a biosensing platform that is capable of performing on-chip and simultaneous measurement of contractility and electrophysiology of human induced pluripotent stem-cell-derived cardiomyocyte (iPSC-CM) monolayers. This platform integrates with a flexible thin-film cantilever embedded with a carbon black (CB)-PDMS strain sensor for high-sensitivity contraction measurement and four pure carbon nanotube (CNT) electrodes for the detection of extracellular field potentials with low electrode impedance. Cardiac functional properties including contractile stress, beating rate, beating rhythm, and extracellular field potential were evaluated to quantify iPSC-CM responses to common cardiotropic agents. In addition, an in vitro model of drug-induced cardiac arrhythmia was established to further validate the platform for disease modeling and drug testing.

Rapid and Scalable Fabrication of Low Impedance, 3D Dry Electrodes for Physiological Sensing

AbstractMedical electrophysiological sensors that can study the body and diagnose diseases depend on consistently low impedance electrode–skin interfaces. Clinical‐standard wet electrodes use hydrogels and skin abrasion to improve the interface and thus the recorded signal quality. These electrodes are challenging to self‐administer and impede in‐home care. Wearable dry electrodes are more practical; however, they show higher impedances than wet electrodes and are costly to customize. This work presents a fabrication method for rapidly producing low impedance, anatomically fit dry electrodes that do not require hydrogels. By using electroless copper and gold plating with 3D printing, biocompatible electrodes can be optimized for individuals at a fraction of the cost of existing vacuum deposition‐based techniques. Example 3D dry electrodes made with this process are evaluated alongside clinical‐standard devices in typical scenarios to compare electrical performance and comfort. The resulting dry electrodes exhibit an average electrode‐skin impedance of 66.7 kΩ at 50 Hz and DC offset of −20 mV without any hydrogel, which, when area normalized, is within the range achieved by wet electrodes without skin abrasion.

Compositional and morphological properties of platinum-iridium electrodeposited on carbon fiber microelectrodes

Objective. Neural interfaces based on carbon fiber (CF) electrodes have demonstrated key positive attributes such as minimal foreign body response and mechanical strength to self-insert in brain tissue. However, carbon does not form a low impedance electrode interface with neural tissue. Electrodeposited platinum iridium (PtIr) has been used to improve electrode interface properties for metallic bioelectrodes. Approach. In this study, a PtIr electrodeposition process has been performed on CF microelectrode arrays to improve the interfacial properties of these arrays. We study the film morphology and composition as well as electrode durability and impedance. Results. A PtIr coating with a composition of 70% Pt, 30% Ir and a thickness of ∼400 nm was observed. Pt and Ir were evenly distributed within the film. Impedance was decreased by 89% @ 1 kHz. Accelerated soak testing in a heated (T = 50 ∘C) saline solution showed impedance increase (@ 1 kHz) of ∼12% after 36 days (89 equivalent) of soaking.

Stretchable gold nanowire-based cuff electrodes for low-voltage peripheral nerve stimulation

Objective. Electrical stimulation of the peripheral nervous system (PNS) can treat various diseases and disorders, including the healing process after nerve injury. A major challenge when designing electrodes for PNS stimulation is the mechanical mismatch between the nerve and the device, which can lead to non-conformal contact, tissue damage and inefficient stimulation due to current leakage. Soft and stretchable cuff electrodes promise to tackle these challenges but often have limited performance and rely on unconventional materials. The aim of this study is to develop a high performance soft and stretchable cuff electrode based on inert materials for low-voltage nerve stimulation. Approach. We developed 50 µm thick stretchable cuff electrodes based on silicone rubber, gold nanowire conductors and platinum coated nanowire electrodes. The electrode performance was characterized under strain cycling to assess the durability of the electrodes. The stimulation capability of the cuff electrodes was evaluated in an in vivo sciatic nerve rat model by measuring the electromyography response to various stimulation pulses. Main results. The stretchable cuff electrodes showed excellent stability for 50% strain cycling and one million stimulation pulses. Saturated homogeneous stimulation of the sciatic nerve was achieved at only 200 mV due to the excellent conformability of the electrodes, the low conductor resistance (0.3 Ohm sq−1), and the low electrode impedance. Significance. The developed stretchable cuff electrode combines favourable mechanical properties and good electrode performance with inert and stable materials, making it ideal for low power supply applications within bioelectronic medicine.

Low-Impedance Low-Artifact PEDOT: PSS-Coated Graphene Electrodes Towards High Density Optogenetic Electrophysiology

Graphene electrode arrays hold promise for in vivo optogenetic electrophysiology. In such studies, it would be ideal to enable spatial oversampling with a high-density array, which is challenging to maintain b oth low electrode impedance a nd low light-induced artifact. To this end, we present a 28- $\mu \text{m}$ pitched, poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-coated graphene electrode array, which was found to feature both low electrode impedance and low artifact in optogenetic electrophysiology. The resulting array was able to record the optogenetically stimulated signals, whose amplitudes increased with the stimulus intensity and maximized next to the responsive cell. Such PEDOT:PSS-coated graphene electrodes are suitable to ultimately form a flexible array for high density optogenetic electrophysiology in vivo .

Sputtered porous Pt for wafer-scale manufacture of low-impedance flexible microelectrodes

Objective. Recording electrical activity from individual cells in vivo is a key technology for basic neuroscience and has growing clinical applications. To maximize the number of independent recording channels as well as the longevity, and quality of these recordings, researchers often turn to small and flexible electrodes that minimize tissue damage and can isolate signals from individual neurons. One challenge when creating these small electrodes, however, is to maintain a low interfacial impedance by applying a surface coating that is stable in tissue and does not significantly complicate the fabrication process. Approach. Here we use a high-pressure Pt sputtering process to create low-impedance electrodes at the wafer scale using standard microfabrication equipment. Main results. We find that direct-sputtered Pt provides a reliable and well-controlled porous coating that reduces the electrode impedance by 5–9 fold compared to flat Pt and is compatible with the microfabrication technologies used to create flexible electrodes. These porous Pt electrodes show reduced thermal noise that matches theoretical predictions. In addition, we show that these electrodes can be implanted into rat cortex, record single unit activity, and be removed all without disrupting the integrity of the coating. We also demonstrate that the shape of the electrode (in addition to the surface area) has a significant effect on the electrode impedance when the feature sizes are on the order of tens of microns. Significance. Overall, porous Pt represents a promising method for manufacturing low-impedance electrodes that can be seamlessly integrated into existing processes for producing flexible neural probes.

Stretchable Low Impedance Electrodes for Bioelectronic Recording from Small Peripheral Nerves

Monitoring of bioelectric signals in peripheral sympathetic nerves of small animal models is crucial to gain understanding of how the autonomic nervous system controls specific body functions related to disease states. Advances in minimally-invasive electrodes for such recordings in chronic conditions rely on electrode materials that show low-impedance ionic/electronic interfaces and elastic mechanical properties compliant with the soft and fragile nerve strands. Here we report a highly stretchable low-impedance electrode realized by microcracked gold films as metallic conductors covered with stretchable conducting polymer composite to facilitate ion-to-electron exchange. The conducting polymer composite based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) obtains its adhesive, low-impedance properties by controlling thickness, plasticizer content and deposition conditions. Atomic Force Microscopy measurements under strain show that the optimized conducting polymer coating is compliant with the micro-crack mechanics of the underlying Au-layer, necessary to absorb the tensile deformation when the electrodes are stretched. We demonstrate functionality of the stretchable electrodes by performing high quality recordings of renal sympathetic nerve activity under chronic conditions in rats.

Lower initial electrode impedances in minimally invasive cochlear implantation

Background: The round window approach and the cochleostomy approach are two widely practiced methods to insert cochlear implant electrode arrays. Yet, there is no consensus on which is more minimally invasive. Objective: To compare the initial electrode impedance (EI) values and the incidence of abnormal electrodes of the round window approach and the cochleostomy approach, and to evaluate the effects of surgical techniques on the intracochlear microenvironment. Material and methods: One hundred and seventy-one patients received a unilateral Nucleus cochlear implant. Eighty-two patients were implanted using the round window approach, and 89 patients were implanted using the cochleostomy approach. EI was measured immediately after closure of the incision. Results: The round window group had lower average initial EI values than the cochleostomy group. For the EI values at each position, statistically significant differences were found in the basal-middle region (Electrode 1–14), but not in the middle-apical region (Electrode 15–22) of the electrode arrays. A lower incidence of high-impedance electrodes was found in the round window group. Conclusions: The round window approach leads to lower initial EI and less disturbance to the intracochlear microenvironment.

Polypyrrole-Carbon Nanotube-FeOOH Composites for Negative Electrodes of Asymmetric Supercapacitors

For the first time, polypyrrole (PPy) coated carbon nanotubes (CNT) were combined with FeOOH to fabricate negative supercapacitor (SCP) electrodes. The synergistic effects of PPy-CNT and FeOOH resulted in enhanced electrochemical performance at high active mass loading of 37 mg cm−2 in a voltage window of −0.8–+0.1 V versus a saturated calomel electrode. Particle extraction through liquid-liquid interface (PELLI) was utilized for the agglomerate-free processing of FeOOH, which improved FeOOH mixing with PPy-CNT and contributed to the enhanced capacitive behavior of the composite. Tetradecylamine (TA) was found to be an efficient extractor for FeOOH. Cyclic voltammetry and impedance spectroscopy data at different electrode potentials provided an insight into the synergistic effects of PPy-CNT and FeOOH, and influence of PELLI on electrode performance. An areal capacitance of 4.5 F cm−2 and nearly ideal capacitive behavior were achieved at a low electrode impedance. The important finding was that capacitances of the negative PPy-CNT-FeOOH and positive MnO2-CNT electrodes can be matched in different voltage windows to fabricate advanced asymmetric devices, which exhibited promising electrochemical performance in a voltage window of 1.6 V at high active mass.

Multimodal Characterization of Neural Networks Using Highly Transparent Electrode Arrays.

Transparent and flexible materials are attractive for a wide range of emerging bioelectronic applications. These include neural interfacing devices for both recording and stimulation, where low electrochemical electrode impedance is valuable. Here the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used to fabricate electrodes that are small enough to allow unencumbered optical access for imaging a large cell population with two-photon (2P) microscopy, yet provide low impedance for simultaneous high quality recordings of neural activity in vivo. To demonstrate this, pathophysiological activity was induced in the mouse cortex using 4-aminopyridine (4AP), and the resulting electrical activity was detected with the PEDOT:PSS-based probe while imaging calcium activity directly below the probe area. The induced calcium activity of the neuronal network as measured by the fluorescence change in the cells correlated well with the electrophysiological recordings from the cortical grid of PEDOT:PSS microelectrodes. Our approach provides a valuable vehicle for complementing classical high temporal resolution electrophysiological analysis with optical imaging.

Functionalized Thick Film Impedance Sensors for Use in In Vitro Cell Culture.

Multi-electrode arrays find application in electrophysiological recordings. The quality of the captured signals depends on the interfacial contact between electrogenic cells and the electronic system. Therefore, it requires reliable low-impedance electrodes. Low-temperature cofired ceramic technology offers a suitable platform for rapid prototyping of biological reactors and can provide both stable fluid supply and integrated bio-hardware interfaces for recordings in electrogenic cell cultures. The 3D assembly of thick film gold electrodes in in vitro bio-reactors has been demonstrated for neuronal recordings. However, especially when dimensions become small, their performance varies strongly. This work investigates the influence of different coatings on thick film gold electrodes with regard to their influence on impedance behavior. PEDOT:PSS layer, titanium oxynitride and laminin coatings are deposited on LTCC gold electrodes using different 2D and 3D MEA chip designs. Their impedance characteristics are compared and discussed. Titanium oxynitride layers emerged as suitable functionalization. Small 86-µm-electrodes have a serial resistance Rs of 32 kOhm and serial capacitance Cs of 4.1 pF at 1 kHz. Thick film gold electrodes with such coatings are thus qualified for signal recording in 3-dimensional in vitro cell cultures.