Investigating the interfacial properties of electrochemically roughened platinum electrodes for neural stimulation

Graphene-based microelectrodes with bidirectional functionality for next-generation retinal electronic interfaces

Neuroelectronic prostheses are being developed for restoring vision at the retinal level in patients who have lost their sight due to photoreceptor loss. The core component of these devices is the electrode array, which enables interfacing with retinal neurons. Generating the perception of meaningful images requires high-density microelectrode arrays (MEAs) capable of precisely activating targeted retinal neurons. Achieving this precision necessitates the downscaling of electrodes to micrometer dimensions. However, miniaturization increases electrode impedance, which poses challenges by limiting the amount of current that can be delivered, thereby impairing the electrode's capability for effective neural modulation. Additionally, it elevates noise levels, reducing the signal quality of the recorded neural activity. This report focuses on evaluating reduced graphene oxide (rGO) based devices for interfacing with the retina, showcasing their potential in vision restoration. Our findings reveal low impedance and high charge injection limit for microscale rGO electrodes, confirming their suitability for developing next-generation high-density retinal devices. We successfully demonstrated bidirectional interfacing with cell cultures and explanted retinal tissue, enabling the identification and modulation of multiple cells' activity. Additionally, calcium imaging allowed real-time monitoring of retinal cell dynamics, demonstrating a significant reduction in activated areas with small-sized electrodes. Overall, this study lays the groundwork for developing advanced rGO-based MEAs for high-acuity visual prostheses.

Deciphering platinum dissolution in neural stimulation electrodes: Electrochemistry or biology?

Platinum (Pt) is the metal of choice for electrodes in implantable neural prostheses like the cochlear implants, deep brain stimulating devices, and brain-computer interfacing technologies. However, it is well known since the 1970s that Pt dissolution occurs with electrical stimulation. More recent clinical and in vivo studies have shown signs of corrosion in explanted electrode arrays and the presence of Pt-containing particulates in tissue samples. The process of degradation and release of metallic ions and particles can significantly impact on device performance. Moreover, the effects of Pt dissolution products on tissue health and function are still largely unknown. This is due to the highly complex chemistry underlying the dissolution process and the difficulty in decoupling electrical and chemical effects on biological responses. Understanding the mechanisms and effects of Pt dissolution proves challenging as the dissolution process can be influenced by electrical, chemical, physical, and biological factors, all of them highly variable between experimental settings. By evaluating comprehensive findings on Pt dissolution mechanisms reported in the fuel cell field, this review presents a critical analysis of the possible mechanisms that drive Pt dissolution in neural stimulation in vitro and in vivo. Stimulation parameters, such as aggregate charge, charge density, and electrochemical potential can all impact the levels of dissolved Pt. However, chemical factors such as electrolyte types, dissolved gases, and pH can all influence dissolution, confounding the findings of in vitro studies with multiple variables. Biological factors, such as proteins, have been documented to exhibit a mitigating effect on the dissolution process. Other biological factors like cells and fibro-proliferative responses, such as fibrosis and gliosis, impact on electrode properties and are suspected to impact on Pt dissolution. However, the relationship between electrical properties of stimulating electrodes and Pt dissolution remains contentious. Host responses to Pt degradation products are also controversial due to the unknown chemistry of Pt compounds formed and the lack of understanding of Pt distribution in clinical scenarios. The cytotoxicity of Pt produced via electrical stimulation appears similar to Pt-based compounds, including hexachloroplatinates and chemotherapeutic agents like cisplatin. While the levels of Pt produced under clinical and acute stimulation regimes were typically an order of magnitude lower than toxic concentrations observed in vitro, further research is needed to accurately assess the mass balance and type of Pt produced during long-term stimulation and its impact on tissue response. Finally, approaches to mitigating the dissolution process are reviewed. A wide variety of approaches, including stimulation strategies, coating electrode materials, and surface modification techniques to avoid excess charge during stimulation and minimise tissue response, may ultimately support long-term and safe operation of neural stimulating devices.

A Comparative Study on the Effect of Substrate Structure on Electrochemical Performance and Stability of Electrodeposited Platinum and Iridium Oxide Coatings for Neural Electrodes

Implantable electrodes are crucial for stimulation safety and recording quality of neuronal activity. To enhance their electrochemical performance, electrodeposited nanostructured platinum (nanoPt) and iridium oxide (IrOx) have been proposed due to their advantages of in situ deposition and ease of processing. However, their unstable adhesion has been a challenge in practical applications. This study investigated the electrochemical performance and stability of nanoPt and IrOx coatings on hierarchical platinum-iridium (Pt-Ir) substrates prepared by femtosecond laser, compared with the coatings on smooth Pt-Ir substrates. Ultrasonic testing, agarose gel testing, and cyclic voltammetry (CV) testing were used to evaluate the coatings' stability. Results showed that the hierarchical Pt-Ir substrate significantly enhanced the charge-storage capacity of electrodes with both coatings to more than 330 mC/cm2, which was over 75 times that of the smooth Pt-Ir electrode. The hierarchical substrate could also reduce the cracking of nanoPt coatings after ultrasonic, agarose gel and CV testing. Although some shedding was observed in the IrOx coating on the hierarchical substrate after one hour of sonication, it showed good stability in the agarose gel and CV tests. Stable nanoPt and IrOx coatings may not only improve the electrochemical performance but also benefit the function of neurobiochemical detection.

Translational opportunities and challenges of invasive electrodes for neural interfaces

Invasive brain-machine interfaces can restore motor, sensory and cognitive functions. However, their clinical adoption has been hindered by the surgical risk of implantation and by suboptimal long-term reliability. In this Review, we highlight the opportunities and challenges of invasive technology for clinically relevant electrophysiology. Specifically, we discuss the characteristics of neural probes that are most likely to facilitate the clinical translation of invasive neural interfaces, describe the neural signals that can be acquired or produced by intracranial electrodes, the abiotic and biotic factors that contribute to their failure, and emerging neural-interface architectures.

Nanocone-Array-Based Platinum-Iridium Oxide Neural Microelectrodes: Structure, Electrochemistry, Durability and Biocompatibility Study

Neural interfaces provide a window for bio-signal modulation and recording with the assistance of neural microelectrodes. However, shrinking the size of electrodes results in high electrochemical impedance and low capacitance, thus limiting the stimulation/recording efficiency. In order to achieve critical stability and low power consumption, here, nanocone-shaped platinum (Pt) with an extensive surface area is proposed as an adhesive layer on a bare Pt substrate, followed by the deposition of a thin layer of iridium oxide (IrO_x) to fabricate high-performance nanocone-array-based Pt-IrO_x neural microelectrodes (200 μm in diameter). A uniform nanocone-shaped Pt with significant roughness is created via controlling the ratio of NH₄⁺ and Pt⁴⁺ ions in the electrolyte, which can be widely applicable for batch production on multichannel flexible microelectrode arrays (fMEAs) and various substrates with different dimensions. The Pt-IrO_x nanocomposite-coated microelectrode presents a significantly low impedance down to 0.72 ± 0.04 Ω cm² at 1 kHz (reduction of ~92.95%). The cathodic charge storage capacity (CSC_c) and charge injection capacity (CIC) reaches up to 52.44 ± 2.53 mC cm^-2 and 4.39 ± 0.36 mC cm^-2, respectively. Moreover, superior chronic stability and biocompatibility are also observed. The modified microelectrodes significantly enhance the adhesion of microglia, the major immune cells in the central nervous system. Therefore, such a coating strategy presents great potential for biomedical and other practical applications.

Femtosecond laser hierarchical surface restructuring for next generation neural interfacing electrodes and microelectrode arrays

Long-term implantable neural interfacing devices are able to diagnose, monitor, and treat many cardiac, neurological, retinal and hearing disorders through nerve stimulation, as well as sensing and recording electrical signals to and from neural tissue. To improve specificity, functionality, and performance of these devices, the electrodes and microelectrode arrays-that are the basis of most emerging devices-must be further miniaturized and must possess exceptional electrochemical performance and charge exchange characteristics with neural tissue. In this report, we show for the first time that the electrochemical performance of femtosecond-laser hierarchically-restructured electrodes can be tuned to yield unprecedented performance values that significantly exceed those reported in the literature, e.g. charge storage capacity and specific capacitance were shown to have improved by two orders of magnitude and over 700-fold, respectively, compared to un-restructured electrodes. Additionally, correlation amongst laser parameters, electrochemical performance and surface parameters of the electrodes was established, and while performance metrics exhibit a relatively consistent increasing behavior with laser parameters, surface parameters tend to follow a less predictable trend negating a direct relationship between these surface parameters and performance. To answer the question of what drives such performance and tunability, and whether the widely adopted reasoning of increased surface area and roughening of the electrodes are the key contributors to the observed increase in performance, cross-sectional analysis of the electrodes using focused ion beam shows, for the first time, the existence of subsurface features that may have contributed to the observed electrochemical performance enhancements. This report is the first time that such performance enhancement and tunability are reported for femtosecond-laser hierarchically-restructured electrodes for neural interfacing applications.

First Food and Drug Administration Cleared Thin-Film Electrode for Intracranial Stimulation, Recording, and Monitoring of Brain Activity—Part 1: Biocompatibility Testing

Subdural strip and grid invasive electroencephalography electrodes are routinely used for surgical evaluation of patients with drug-resistant epilepsy (DRE). Although these electrodes have been in the United States market for decades (first FDA clearance 1985), their fabrication, materials, and properties have hardly changed. Existing commercially available electrodes are made of silicone, are thick (>0.5 mm), and do not optimally conform to brain convolutions. New thin-film polyimide electrodes (0.08 mm) have been manufactured to address these issues. While different thin-film electrodes are available for research use, to date, only one electrode is cleared by Food and Drug Administration (FDA) for use in clinical practice. This study describes the biocompatibility tests that led to this clearance. Biocompatibility was tested using standard methods according to International Organization for Standardization (ISO) 10993. Electrodes and appropriate control materials were bent, folded, and placed in the appropriate extraction vehicles, or implanted. The extracts were used for in vitro and in vivo tests, to assess the effects of any potential extractable and leachable materials that may be toxic to the body. In vitro studies included cytotoxicity tested in L929 cell line, genotoxicity tested using mouse lymphoma assay (MLA) and Ames assay, and hemolysis tested in rabbit whole blood samples. The results indicated that the electrodes were non-cytotoxic, non-mutagenic, non-clastogenic, and non-hemolytic. In vivo studies included sensitization tested in guinea pigs, irritation tested in rabbits, acute systemic toxicity testing in mice, pyrogenicity tested in rabbits, and a prolonged 28-day subdural implant in sheep. The results indicated that the electrodes induced no sensitization and irritation, no weight loss, and no temperature increase. Histological examination of the sheep brain tissue showed no or minimal immune cell accumulation, necrosis, neovascularization, fibrosis, and astrocyte infiltration, with no differences from the control material. In summary, biocompatibility studies indicated that these new thin-film electrodes are appropriate for human use. As a result, the electrodes were cleared by the FDA for use in clinical practice [510(k) K192764], making it the first thin-film subdural electrode to progress from research to clinic. Its readiness as a commercial product ensures availability to all patients undergoing surgical evaluation for DRE.

Soft Devices for High-Resolution Neuro-Stimulation: The Interplay Between Low-Rigidity and Resolution

The field of neurostimulation has evolved over the last few decades from a crude, low-resolution approach to a highly sophisticated methodology entailing the use of state-of-the-art technologies. Neurostimulation has been tested for a growing number of neurological applications, demonstrating great promise and attracting growing attention in both academia and industry. Despite tremendous progress, long-term stability of the implants, their large dimensions, their rigidity and the methods of their introduction and anchoring to sensitive neural tissue remain challenging. The purpose of this review is to provide a concise introduction to the field of high-resolution neurostimulation from a technological perspective and to focus on opportunities stemming from developments in materials sciences and engineering to reduce device rigidity while optimizing electrode small dimensions. We discuss how these factors may contribute to smaller, lighter, softer and higher electrode density devices.

Neural tissue-microelectrode interaction: Brain micromotion, electrical impedance, and flexible microelectrode insertion

Insertion of a microelectrode into the brain to record/stimulate neurons damages neural tissue and blood vessels and initiates the brain's wound healing response. Due to the large difference between the stiffness of neural tissue and microelectrode, brain micromotion also leads to neural tissue damage and associated local immune response. Over time, following implantation, the brain's response to the tissue damage can result in microelectrode failure. Reducing the microelectrode's cross-sectional dimensions to single-digit microns or using soft materials with elastic modulus close to that of the neural tissue are effective methods to alleviate the neural tissue damage and enhance microelectrode longevity. However, the increase in electrical impedance of the microelectrode caused by reducing the microelectrode contact site's dimensions can decrease the signal-to-noise ratio. Most importantly, the reduced dimensions also lead to a reduction in the critical buckling force, which increases the microelectrode's propensity to buckling during insertion. After discussing brain micromotion, the main source of neural tissue damage, surface modification of the microelectrode contact site is reviewed as a key method for addressing the increase in electrical impedance issue. The review then focuses on recent approaches to aiding insertion of flexible microelectrodes into the brain, including bending stiffness modification, effective length reduction, and application of a magnetic field to pull the electrode. An understanding of the advantages and drawbacks of the developed strategies offers a guide for dealing with the buckling phenomenon during implantation.

Deep brain stimulation: Challenges at the tissue-electrode interface and current solutions

Deep brain stimulation (DBS) is used to treat the motor symptoms of Parkinson's disease patients by stimulating the subthalamic nucleus. However, optimization of DBS is still needed since the performance of the neural electrodes is limited by the body's response to the implant. This review discusses the issues with DBS, such as placement of electrodes, foreign body response, and electrode degradation. The current solutions to these technical issues include modifications to electrode material, coatings, and geometry.

Nanotunnels within Poly(3,4-ethylenedioxythiophene)-Carbon Nanotube Composite for Highly Sensitive Neural Interfacing

Neural electrodes are developed for direct communication with neural tissues for theranostics. Although various strategies have been employed to improve performance, creating an intimate electrode-tissue interface with high electrical fidelity remains a great challenge. Here, we report the rational design of a tunnel-like electrode coating comprising poly(3,4-ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNTs) for highly sensitive neural recording. The coated electrode shows a 50-fold reduction in electrochemical impedance at the biologically relevant frequency of 1 kHz, compared to the bare gold electrode. The incorporation of CNT significantly reinforces the nanotunnel structure and improves coating adhesion by ∼1.5 fold. In vitro primary neuron culture confirms an intimate contact between neurons and the PEDOT-CNT nanotunnel. During acute in vivo nerve recording, the coated electrode enables the capture of high-fidelity neural signals with low susceptibility to electrical noise and reveals the potential for precisely decoding sensory information through mechanical and thermal stimulation. These findings indicate that the PEDOT-CNT nanotunnel composite serves as an active interfacing material for neural electrodes, contributing to neural prosthesis and brain-machine interface.

High-performance Nanocrystal Platinum on Hierarchical Microelectrode for Biochemical Sensing

A high-performance hierarchical microelectrode array was reported, which significantly enlarged the surface area and reduced edge effect of the unevenly distributed electric field, using novel nanocrystal platinum (Pt) surface modification and hierarchical topologies. A hierarchical three-dimensional (3D) nanostructured Pt was introduced on the surface of microelectrodes (diameter Φ=10 pm, and seed pad Φ=200 μm) to reduce the impedance by ~91.2% down to 2.63 kn at 1kHz, and increase the cathodic charge storage capacity (CSCc) by ~60 times up to 217.9 mC.cm^-2 compared with that of bare Pt substrate, demonstrating superior electrochemical performances to the traditional modified electrodes. In addition, the hierarchical microelectrodes exhibited great stability and linearity for glucose sensing with a detection limit of ~1.5 μM, which showed potential applications for neural interface and biochemical sensing.

A review for the peripheral nerve interface designer

Informational density and relative accessibility of the peripheral nervous system make it an attractive site for therapeutic intervention. Electrode-based electrophysiological interfaces with peripheral nerves have been under development since the 1960s and, for several applications, have seen widespread clinical implementation. However, many applications require a combination of neural target resolution and stability which has thus far eluded existing peripheral nerve interfaces (PNIs). With the goal of aiding PNI designers in development of devices that meet the demands of next-generation applications, this review seeks to collect and present practical considerations and best practices which emerge from the literature, including both lessons learned during early PNI development and recent ideas. Fundamental and practical principles guiding PNI design are reviewed, followed by an updated and critical account of existing PNI designs and strategies. Finally, a brief survey of in vitro and in vivo PNI characterization methods is presented.

High-Performance Graphene-Fiber-Based Neural Recording Microelectrodes

Fabrication of flexible and free-standing graphene-fiber- (GF-) based microelectrode arrays with a thin platinum coating, acting as a current collector, results in a structure with low impedance, high surface area, and excellent electrochemical properties. This modification results in a strong synergistic effect between these two constituents leading to a robust and superior hybrid material with better performance than either graphene electrodes or Pt electrodes. The low impedance and porous structure of the GF results in an unrivalled charge injection capacity of 10.34 mC cm^-2 with the ability to record and detect neuronal activity. Furthermore, the thin Pt layer transfers the collected signals along the microelectrode efficiently. In vivo studies show that microelectrodes implanted in the rat cerebral cortex can detect neuronal activity with remarkably high signal-to-noise ratio (SNR) of 9.2 dB in an area as small as an individual neuron.

Climbing-inspired twining electrodes using shape memory for peripheral nerve stimulation and recording

Peripheral neuromodulation has been widely used throughout clinical practices and basic neuroscience research. However, the mechanical and geometrical mismatches at current electrode-nerve interfaces and complicated surgical implantation often induce irreversible neural damage, such as axonal degradation. Here, compatible with traditional 2D planar processing, we propose a 3D twining electrode by integrating stretchable mesh serpentine wires onto a flexible shape memory substrate, which has permanent shape reconfigurability (from 2D to 3D), distinct elastic modulus controllability (from ~100 MPa to ~300 kPa), and shape memory recoverability at body temperature. Similar to the climbing process of twining plants, the temporarily flattened 2D stiff twining electrode can naturally self-climb onto nerves driven by 37°C normal saline and form 3D flexible neural interfaces with minimal constraint on the deforming nerves. In vivo animal experiments, including right vagus nerve stimulation for reducing the heart rate and action potential recording of the sciatic nerve, demonstrate the potential clinical utility.

Corrosion Study of Implanted Tin Electrodes Using Excessive Electrical Stimulation in Minipigs

(1) Background: Titanium nitride (TiN) electrodes have been used for implantable stimulation and sensing electrodes for decades. Nevertheless, there still is a discrepancy between the in vitro and in vivo determined safe charge injection limits. This study investigated the consequences of pulsing implanted electrodes beyond the in vivo safe charge injection limits. (2) Methods: The electrodes were implanted for a month and then pulsed at 20 mA and 50 mA and 200 Hz and 400 Hz. Afterwards, the electrodes were investigated using electrochemical and analytical methods to evaluate whether electrode degradation had occurred. (3) Results: Electrochemical tests showed that electrodes that pulsed at 20 mA and 200 Hz (lowest electrical dose) had a significantly lower charge injection capacity and higher impedance than the other used and unused electrodes. (4) Conclusions: The electrodes pulsed at the lowest electrical dose, for which no tissue damage was found, appeared to have degraded. Electrodes pulsed at higher electrical doses for which tissue damage did occur, on the other hand, show no significant degradation in electrochemical tests compared to unused implanted and not implanted electrodes. It is thus clear that the tissue surrounding the electrode has an influence on the charge injection properties of the electrodes and vice versa.

Progress in the Field of Micro-Electrocorticography

Since the 1940s electrocorticography (ECoG) devices and, more recently, in the last decade, micro-electrocorticography (µECoG) cortical electrode arrays were used for a wide set of experimental and clinical applications, such as epilepsy localization and brain⁻computer interface (BCI) technologies. Miniaturized implantable µECoG devices have the advantage of providing greater-density neural signal acquisition and stimulation capabilities in a minimally invasive fashion. An increased spatial resolution of the µECoG array will be useful for greater specificity diagnosis and treatment of neuronal diseases and the advancement of basic neuroscience and BCI research. In this review, recent achievements of ECoG and µECoG are discussed. The electrode configurations and varying material choices used to design µECoG arrays are discussed, including advantages and disadvantages of µECoG technology compared to electroencephalography (EEG), ECoG, and intracortical electrode arrays. Electrode materials that are the primary focus include platinum, iridium oxide, poly(3,4-ethylenedioxythiophene) (PEDOT), indium tin oxide (ITO), and graphene. We discuss the biological immune response to µECoG devices compared to other electrode array types, the role of µECoG in clinical pathology, and brain⁻computer interface technology. The information presented in this review will be helpful to understand the current status, organize available knowledge, and guide future clinical and research applications of µECoG technologies.

Advances in Penetrating Multichannel Microelectrodes Based on the Utah Array Platform

The Utah electrode array (UEA) and its many derivatives have become a gold standard for high-channel count bi-directional neural interfaces, in particular in human subject applications. The chapter provides a brief overview of leading electrode concepts and the context in which the UEA has to be understood. It goes on to discuss the key advances and developments of the UEA platform in the past 15 years, as well as novel wireless and system integration technologies that will merge into future generations of fully integrated devices. Aspects covered include novel device architectures that allow scaling of channel count and density of electrode contacts, material improvements to substrate, electrode contacts, and encapsulation. Further subjects are adaptations of the UEA platform to support IR and optogenetic simulation as well as an improved understanding of failure modes and methods to test and accelerate degradation in vitro such as to better predict device failure and lifetime in vivo.

Electron transfer processes occurring on platinum neural stimulating electrodes: calculated charge-storage capacities are inaccessible during applied stimulation

OBJECTIVE

Neural prostheses employing platinum electrodes are often constrained by a charge/charge-density parameter known as the Shannon limit. In examining the relationship between charge injection and observed tissue damage, the electrochemistry at the electrode-tissue interface should be considered. The charge-storage capacity (CSC) is often used as a predictor of how much charge an electrode can inject during stimulation, but calculating charge from a steady-state i-E curve (cyclic voltammogram) over the water window misrepresents how electrodes operate during stimulation. We aim to gain insight into why CSC predictions from classic i-E curves overestimate the amount of charge that can be injected during neural stimulation pulsing.

APPROACH

In this study, we use a standard electrochemical technique to investigate how platinum electrochemistry depends on the potentials accessed by the electrode and on the electrolyte composition.

MAIN RESULTS

The experiments indicate: (1) platinum electrodes must be subjected to a 'cleaning' procedure in order to expose the maximum number of surface platinum sites for hydrogen adsorption; (2) the 'cleaned' platinum surface will likely revert to an obstructed condition under typical neural stimulation conditions; (3) irreversible oxygen reduction may occur under neural stimulation conditions, so the consequences of this reaction should be considered; and (4) the presence of the chloride ion (Cl^-) or proteins (bovine serum albumin) inhibits oxide formation and alters H adsorption.

SIGNIFICANCE

These observations help explain why traditional CSC calculations overestimate the charge that can be injected during neural stimulation. The results underscore how careful electrochemical examination of the electrode-electrolyte interface can result in more accurate expectations of electrode performance during applied stimulation.

Flexible 3D carbon nanotubes cuff electrodes as a peripheral nerve interface

The cuff electrode provides a stable interface with peripheral nerves, which has been widely used in basic research and clinical practice. Currently, the cuff electrodes are limited by the planar processing of microfabrication. This paper presents a novel cuff electrode using high-aspect ratio carbon nanotubes (CNTs) integrated on a flexible biocompatible parylene. The microfabrication process unites the high quality vertical CNTs grown at high temperature with the heat sensitive parylene substrate in a highly controllable manner. The fabricated cuff electrodes have been utilized for extracellular nerve stimulation in rats. The experimental results demonstrate the proposed CNT electrode has a better performance than Pt electrode in nerve stimulation. Moreover, the effect of electrode position and stimulation frequency is demonstrated in this paper. This preliminary data indicates that flexible 3D CNTs cuff electrode provides an excellent platform for functional electrical stimulation.

Iridium Oxide-Electrodeposited Nanoporous Gold Multielectrode Array with Enhanced Stimulus Efficacy

A multielectrode array (MEA) was fabricated with electrodes consisting of iridium oxide (IrOx) electrochemically deposited on nanoporous gold (NPG) to improve the moderate charge injection limit (ca. 1 mC cm^-2) of NPG MEA. IrOx was electrodeposited by performing cyclic voltammetry with an IrOx deposition solution. The IrOx was electrodeposited on Au (EIROF/Au) and on NPG (EIROF/NPG) MEA, and the samples were analyzed in terms of the charge injection limit, charge storage capacity (CSC), and electrochemical impedance. The charge injection limit of the EIROF(100-cycled)/NPG MEA was estimated to be 2.3 mC cm^-2 by measuring the voltage transient, and this value is sufficiently greater than the neural damage threshold (ca. 1 mC cm^-2) and is also comparable to that of sputtered IrOx films. Considering the low charge injection limit (<0.1 mC cm^-2) for the EIROF(100-cycled)/Au MEA, the high charge injection limit for the EIROF/NPG MEA was explained to be a result of synergetic combination of the inherently large surface area of the NPG and electrically active EIROF. The EIROF(100-cycled)/NPG exhibited an impedance of 9.7 ± 0.45 kΩ at 1 kHz and a CSC of 8 mC/cm^-2, respectively, obtained via electrochemical impedance spectroscopy and integration of the cathodic current in a cyclic voltammogram. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy are used to conduct an elemental mapping analysis of the cross-sectional structure of the EIROF/NPG and revealed that the EIROF had been uniformly deposited on the surface of the interconnected Au. The efficacy of the improvement in the charge injection limit of the EIROF/NPG MEA was evaluated with rat hippocampal slices. The EIROF/NPG electrodes exhibited a steeper increase in the negative peak amplitude of the field excitatory postsynaptic potentials (fEPSPs), even with an electrical stimulation of a lower amplitude (1-4 V), prolonged negative fEPSPs wave after peak response, and decreased serial reduction of fEPSPs compared to NPG MEA, all of which strongly indicate an improved charge injection for the EIROF/NPG MEA over NPG MEA.

Bioelectric Medicine and Devices for the Treatment of Spinal Cord Injury

Recovery of motor control is paramount for patients living with paralysis following spinal cord injury (SCI). While a cure or regenerative intervention remains on the horizon for the treatment of SCI, a number of neuroprosthetic devices have been employed to treat and mitigate the symptoms of paralysis associated with injuries to the spinal column and associated comorbidities. The recent success of epidural stimulation to restore voluntary motor function in the lower limbs of a small cohort of patients has breathed new life into the promise of electric-based medicine. Recently, a number of new organic and inorganic electronic devices have been developed for brain-computer interfaces to bypass the injury, for neurorehabilitation, bladder and bowel control, and the restoration of motor or sensory control. Herein, we discuss the recent advances in neuroprosthetic devices for treating SCI and highlight future design needs for closed-loop device systems.

Modelling an electrochemically roughened porous platinum electrode for water oxidation

The use of a central composite design to model the roughness of an electrochemically roughened Pt electrode and the surface with a well-defined nanostructure exhibits greatly improved catalytic activity towards oxygen evolution reaction.