Conducting polymer coated neural recording electrodes

Bioelectronic Direct Current Stimulation at the Transition Between Reversible and Irreversible Charge Transfer

Many biological processes rely on endogenous electric fields (EFs), including tissue regeneration, cell development, wound healing, and cancer metastasis. Mimicking these biological EFs by applying external direct current stimulation (DCS) is therefore the key to many new therapeutic strategies. During DCS, the charge transfer from electrode to tissue relies on a combination of reversible and irreversible electrochemical processes, which may generate toxic or bio-altering substances, including metal ions and reactive oxygen species (ROS). Poly(3,4-ethylenedioxythiophene) (PEDOT) based electrodes are emerging as suitable candidates for DCS to improve biocompatibility compared to metals. This work addresses whether PEDOT electrodes can be tailored to favor reversible biocompatible charge transfer. To this end, different PEDOT formulations and their respective back electrodes are studied using cyclic voltammetry, chronopotentiometry, and direct measurements of H2 O2 and O2 . This combination of electrochemical methods sheds light on the time dynamics of reversible and irreversible charge transfer and the relationship between capacitance and ROS generation. The results presented here show that although all electrode materials investigated generate ROS, the onset of ROS can be delayed by increasing the electrode's capacitance via PEDOT coating, which has implications for future bioelectronic devices that allow longer reversibly driven pulse durations during DCS.

Highly Sensitive and Omnidirectionally Stretchable Bioelectrode Arrays for In Vivo Neural Interfacing

Flexible electrode array, a new-generation neural microelectrode, is a crucial tool for information exchange between living tissues and external electronics. Till date, advances in flexible neural microelectrodes are limited because of their high impedance and poor mechanical consistency at tissue interfaces. Herein, a highly sensitive and omnidirectionally stretchable polymeric electrode array (PEA) is introduced. Micropyramid-nanowire composite structures are constructed to increase the effective surface area of PEA, achieving an exponential reduction in impedance compared with gold (Au) and flat polypyrrole electrodes. Moreover, for the first time, a suspended umbrella structure to enable PEA with omnidirectional stretchability of up to ≈20% is designed. The PEA can withstand 1000 cycles of mechanical loads without decrease in performance. As a proof of concept, PEA is conformally attached to a rat heart and tibialis anterior muscle, and electrophysiological signals (electrocardiogram and electromyogram) of the rat are successfully recorded. This strategy provides a new perspective toward highly sensitive and omnidirectionally stretchable PEA that can facilitate the practical application of neural electrodes.

Electrically responsive release of proteins from conducting polymer hydrogels

Electrically modulated delivery of proteins provides an avenue to target local tissues specifically and tune the dose to the application. This approach prolongs and enhances activity at the target site whilst reducing off-target effects associated with systemic drug delivery. The work presented here explores an electrically active composite material comprising of a biocompatible hydrogel, gelatin methacryloyl (GelMA) and a conducting polymer, poly(3,4-ethylenedioxythiophene), generating a conducting polymer hydrogel. In this paper, the key characteristics of electroactivity, mechanical properties, and morphology are characterized using electrochemistry techniques, atomic force, and scanning electron microscopy. Cytocompatibility is established through exposure of human cells to the materials. By applying different electrical-stimuli, the short-term release profiles of a model protein can be controlled over 4 h, demonstrating tunable delivery patterns. This is followed by extended-release studies over 21 days which reveal a bimodal delivery mechanism influenced by both GelMA degradation and electrical stimulation events. This data demonstrates an electroactive and cytocompatible material suitable for the delivery of protein payloads over 3 weeks. This material is well suited for use as a treatment delivery platform in tissue engineering applications where targeted and spatio-temporal controlled delivery of therapeutic proteins is required. STATEMENT OF SIGNIFICANCE: Growth factor use in tissue engineering typically requires sustained and tunable delivery to generate optimal outcomes. While conducting polymer hydrogels (CPH) have been explored for the electrically responsive release of small bioactives, we report on a CPH capable of releasing a protein payload in response to electrical stimulus. The composite material combines the benefits of soft hydrogels acting as a drug reservoir and redox-active properties from the conducting polymer enabling electrical responsiveness. The CPH is able to sustain protein delivery over 3 weeks, with electrical stimulus used to modulate release. The described material is well suited as a treatment delivery platform to deliver large quantities of proteins in applications where spatio-temporal delivery patterns are paramount.

Semiconducting electrodes for neural interfacing: a review

In the past 50 years, the advent of electronic technology to directly interface with neural tissue has transformed the fields of medicine and biology. Devices that restore or even replace impaired bodily functions, such as deep brain stimulators and cochlear implants, have ushered in a new treatment era for previously intractable conditions. Meanwhile, electrodes for recording and stimulating neural activity have allowed researchers to unravel the vast complexities of the human nervous system. Recent advances in semiconducting materials have allowed effective interfaces between electrodes and neuronal tissue through novel devices and structures. Often these are unattainable using conventional metallic electrodes. These have translated into advances in research and treatment. The development of semiconducting materials opens new avenues in neural interfacing. This review considers this emerging class of electrodes and how it can facilitate electrical, optical, and chemical sensing and modulation with high spatial and temporal precision. Semiconducting electrodes have advanced electrically based neural interfacing technologies owing to their unique electrochemical and photo-electrochemical attributes. Key operation modalities, namely sensing and stimulation in electrical, biochemical, and optical domains, are discussed, highlighting their contrast to metallic electrodes from the application and characterization perspective.

Methods of poly(3,4)-ethylenedioxithiophene (PEDOT) electrodeposition on metal electrodes for neural stimulation and recording

Conductive polymers are of great interest in the field of neural electrodes because of their potential to improve the interfacial properties of electrodes. In particular, the conductive polymer poly (3,4)-ethylenedioxithiophene (PEDOT) has been widely studied for neural applications.Objective:This review compares methods for electrodeposition of PEDOT on metal neural electrodes, and analyses the effects of deposition methods on morphology and electrochemical performance.Approach:Electrochemical performances were analysed against several deposition method choices, including deposition charge density and co-ion, and correlations were explained to morphological and structural arguments as well as characterisation methods choices.Main results:Coating thickness and charge storage capacity are positively correlated with PEDOT electrodeposition charge density. We also show that PEDOT coated electrode impedance at 1 kHz, the only consistently reported impedance quantity, is strongly dependent upon electrode radius across a wide range of studies, because PEDOT coatings reduces the reactance of the complex impedance, conferring a more resistive behaviour to electrodes (at 1 kHz) dominated by the solution resistance and electrode geometry. This review also summarises how PEDOT co-ion choice affects coating structure and morphology and shows that co-ions notably influence the charge injection limit but have a limited influence on charge storage capacity and impedance. Finally we discuss the possible influence of characterisation methods to assess the robustness of comparisons between published results using different methods of characterisation.Significance:This review aims to serve as a common basis for researchers working with PEDOT by showing the effects of deposition methods on electrochemical performance, and aims to set a standard for accurate and uniform reporting of methods.

Active and passive drug release by self-assembled lubricin (PRG4) anti-fouling coatings

Electroactive polymers (EAPs) have been investigated as materials for use in a range of biomedical applications, ranging from cell culture, electrical stimulation of cultured cells as well as controlled delivery of growth factors and drugs. Despite their excellent drug delivery ability, EAPs are susceptible to biofouling thus they often require surface functionalisation with antifouling coatings to limit unwanted non-specific protein adsorption. Here we demonstrate the surface modification of para toluene sulfonate (pTS) doped polypyrrole with the glycoprotein lubricin (LUB) to produce a self-assembled coating that both prevents surface biofouling while also serving as a high-capacity reservoir for cationic drugs which can then be released passively via diffusion or actively via an applied electrical potential. We carried out our investigation in two parts where we initially assessed the antifouling and cationic drug delivery ability of LUB tethered on a gold surface using quartz crystal microbalance with dissipation monitoring (QCM) to monitor molecular interactions occurring on a gold sensor surface. After confirming the ability of tethered LUB nano brush layers on a gold surface, we introduced an electrochemically grown EAP layer to act as the immobilisation surface for LUB before subsequently introducing the cationic drug doxorubicin hydrochloride (DOX). The release of cationic drug was then investigated under passive and electrochemically stimulated conditions. High-performance liquid chromatography (HPLC) was then carried out to quantify the amount of DOX released. It was shown that the amount of DOX released from nano brush layers of LUB tethered on gold and EAP surfaces could be increased by up to 30% per minute by applying a positive electrochemically stimulating pulse at 0.8 V for one minute. Using bovine serum albumin (BSA), we show that DOX loaded LUB tethered on para toluene sulfonic acid (pTS) doped polypyrrole retained antifouling ability of up to 75% when compared to unloaded tethered LUB. This work demonstrates the unique, novel ability of tethered LUB to actively participate in the delivery of cationic therapeutics on different substrate surfaces. This study could lead to the development of versatile multifunctional biomaterials for use in wide range of biomedical applications, such as dual drug delivery and lubricating coatings, dual drug delivery and antifouling coatings, cellular recording and stimulation.

Vitamin C-reduced graphene oxide improves the performance and stability of multimodal neural microelectrodes

Nanocarbons are often employed as coatings for neural electrodes to enhance surface area. However, processing and integrating them into microfabrication flows requires complex and harmful chemical and heating conditions. This article presents a safe, scalable, cost-effective method to produce reduced graphene oxide (rGO) coatings using vitamin C (VC) as the reducing agent. We spray coat GO + VC mixtures onto target substrates, and then heat samples for 15 min at 150°C. The resulting rGO films have conductivities of ∼44 S cm⁻¹, and are easily integrated into an ad hoc microfabrication flow. The rGO/Au microelectrodes show ∼8x lower impedance and ∼400x higher capacitance than bare Au, resulting in significantly enhanced charge storage and injection capacity. We subsequently use rGO/Au arrays to detect dopamine in vitro, and to map cortical activity intraoperatively over rat whisker barrel cortex, demonstrating that conductive VC-rGO coatings improve the performance and stability of multimodal microelectrodes for different applications.

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Easy, scalable, and safe reduction method to create rGO films with vitamin C

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VC-rGO coatings improve the performance of bare gold microelectrodes in vitro

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VC-rGO coatings enable the voltammetric detection of dopamine on the microscale

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rGO/Au electrode arrays enable high-resolution microscale recording in vivo

Strategies for interface issues and challenges of neural electrodes

Neural electrodes, as a bridge for bidirectional communication between the body and external devices, are crucial means for detecting and controlling nerve activity. The electrodes play a vital role in monitoring the state of neural systems or influencing it to treat disease or restore functions. To achieve high-resolution, safe and long-term stable nerve recording and stimulation, a neural electrode with excellent electrochemical performance (e.g., impedance, charge storage capacity, charge injection limit), and good biocompatibility and stability is required. Here, the charge transfer process in the tissues, the electrode-tissue interfaces and the electrode materials are discussed respectively. Subsequently, the latest research methods and strategies for improving the electrochemical performance and biocompatibility of neural electrodes are reviewed. Finally, the challenges in the development of neural electrodes are proposed. It is expected that the development of neural electrodes will offer new opportunities for the evolution of neural prosthesis, bioelectronic medicine, brain science, and so on.

Electrostatic Interaction-Based High Tissue Adhesive, Stretchable Microelectrode Arrays for the Electrophysiological Interface

The drift or fall of stretchable neural microelectrodes from the surface of wet and dynamic tissues severely hampers the adoption of microelectrodes for electrophysiological signal monitoring. Endowing the stretchable electrodes with adhesive ability is an effective way to overcome these problems. Current adhesives form tough adhesion to tissues by covalent interaction, which decreases the biocompatibility of the adhesives. Here, we fabricate a strong electrostatic adhesive (noncovalent interaction), highly conformal, stretchable microelectrode arrays (MEAs) for the electrophysiological interface. This MEA was composed of polypyrrole (PPy) as the electrode material and hydrogel as the stretchable substrate [the cross-linked and copolymerized hydrogel of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), gelatin, chitosan, 2-methoxyethyl acrylate, and acrylic acid is named PAGMA]. Strong and stable electrostatic adhesion (85 kPa) and high stretchability (100%) allow for the integration of PPy MEAs based on the PAGMA hydrogel substrate (PPy-PAGMA MEAs) on diverse wet dynamic tissues. Additionally, by adjusting the concentration of AMPS in PAGMA, the hydrogel (PAGMA-1) can produce tough adhesion to many inorganic and elastomer materials. Finally, the PPy-PAGMA MEAs were toughly and conformally adhered on the rat's subcutaneous muscle and beating heart, and the rat's electrophysiological signals were successfully recorded. The development of these adhesive MEAs offers a promising strategy to establish stable and compliant electrode-tissue interfaces.

Comparison of the In Vitro and In Vivo Electrochemical Performance of Bionic Electrodes

The electrochemical performance of platinum electrodes was assessed in vitro and in vivo to determine the impact of electrode implantation and the relevance of in vitro testing in predicting in vivo behaviour. A significant change in electrochemical response was seen after electrode polarisation. As a result, initial in vitro measurements were poor predictors of subsequent measurements performed in vitro or in vivo. Charge storage capacity and charge density measurements from initial voltammetric measurements were not correlated with subsequent measurements. Electrode implantation also affected the electrochemical impedance. The typically reported impedance at 1 kHz was a very poor predictor of electrode performance. Lower frequencies were significantly more dependent on electrode properties, while higher frequencies were dependent on solution properties. Stronger correlations in impedance at low frequencies were seen between in vitro and in vivo measurements after electrode activation had occurred. Implanting the electrode increased the resistance of the electrochemical circuit, with bone having a higher resistivity than soft tissue. In contrast, protein fouling and fibrous tissue formation had a minimal impact on electrochemical response. In vivo electrochemical measurements also typically use a quasi-reference electrode, may operate in a 2-electrode system, and suffer from uncompensated resistance. The impact of these experimental conditions on electrochemical performance and the relevance of in vitro electrode assessment is discussed. Recommended in vitro testing protocols for assessing bionic electrodes are presented.

Soft Bioelectronics Based on Nanomaterials

Recent advances in nanostructured materials and unconventional device designs have transformed the bioelectronics from a rigid and bulky form into a soft and ultrathin form and brought enormous advantages to the bioelectronics. For example, mechanical deformability of the soft bioelectronics and thus its conformal contact onto soft curved organs such as brain, heart, and skin have allowed researchers to measure high-quality biosignals, deliver real-time feedback treatments, and lower long-term side-effects in vivo. Here, we review various materials, fabrication methods, and device strategies for flexible and stretchable electronics, especially focusing on soft biointegrated electronics using nanomaterials and their composites. First, we summarize top-down material processing and bottom-up synthesis methods of various nanomaterials. Next, we discuss state-of-the-art technologies for intrinsically stretchable nanocomposites composed of nanostructured materials incorporated in elastomers or hydrogels. We also briefly discuss unconventional device design strategies for soft bioelectronics. Then individual device components for soft bioelectronics, such as biosensing, data storage, display, therapeutic stimulation, and power supply devices, are introduced. Afterward, representative application examples of the soft bioelectronics are described. A brief summary with a discussion on remaining challenges concludes the review.

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.

Improving Deep Brain Stimulation Electrode Performance in vivo Through Use of Conductive Hydrogel Coatings

Active implantable neurological devices like deep brain stimulators have been used over the past few decades to treat movement disorders such as those in people with Parkinson’s disease and more recently, in psychiatric conditions like obsessive compulsive disorder. Electrode-tissue interfaces that support safe and effective targeting of specific brain regions are critical to success of these devices. Development of directional electrodes that activate smaller volumes of brain tissue requires electrodes to operate safely with higher charge densities. Coatings such as conductive hydrogels (CHs) provide lower impedances and higher charge injection limits (CILs) than standard platinum electrodes and support safer application of smaller electrode sizes. The aim of this study was to examine the chronic in vivo performance of a new low swelling CH coating that supports higher safe charge densities than traditional platinum electrodes. A range of hydrogel blends were engineered and their swelling and electrical performance compared. Electrochemical performance and stability of high and low swelling formulations were compared during insertion into a model brain in vitro and the formulation with lower swelling characteristics was chosen for the in vivo study. CH-coated or uncoated Pt electrode arrays were implanted into the brains of 14 rats, and their electrochemical performance was tested weekly for 8 weeks. Tissue response and neural survival was assessed histologically following electrode array removal. CH coating resulted in significantly lower voltage transient impedance, higher CIL, lower electrochemical impedance spectroscopy, and higher charge storage capacity compared to uncoated Pt electrodes in vivo, and this advantage was maintained over the 8-week implantation. There was no significant difference in evoked potential thresholds, signal-to-noise ratio, tissue response or neural survival between CH-coated and uncoated Pt groups. The significant electrochemical advantage and stability of CH coating in the brain supports the suitability of this coating technology for future development of smaller, higher fidelity electrode arrays with higher charge density requirement.

3D Electrodes for Bioelectronics

In recent studies related to bioelectronics, significant efforts have been made to form 3D electrodes to increase the effective surface area or to optimize the transfer of signals at tissue-electrode interfaces. Although bioelectronic devices with 2D and flat electrode structures have been used extensively for monitoring biological signals, these 2D planar electrodes have made it difficult to form biocompatible and uniform interfaces with nonplanar and soft biological systems (at the cellular or tissue levels). Especially, recent biomedical applications have been expanding rapidly toward 3D organoids and the deep tissues of living animals, and 3D bioelectrodes are getting significant attention because they can reach the deep regions of various 3D tissues. An overview of recent studies on 3D bioelectronic devices, such as the use of electrical stimulations and the recording of neural signals from biological subjects, is presented. Subsequently, the recent developments in materials and fabrication processing to 3D micro- and nanostructures are introduced, followed by broad applications of these 3D bioelectronic devices at various in vitro and in vivo conditions.

Redox Polymers for Tissue Engineering

This review will focus on the targeted design, synthesis and application of redox polymers for use in regenerative medicine and tissue engineering. We define redox polymers to encompass a variety of polymeric materials, from the multifunctional conjugated conducting polymers to graphene and its derivatives, and have been adopted for use in the engineering of several types of stimulus responsive tissues. We will review the fundamental properties of organic conducting polymers (OCPs) and graphene, and how their properties are being tailored to enhance material - biological interfacing. We will highlight the recent development of high-resolution 3D fabrication processes suitable for biomaterials, and how the fabrication of intricate scaffolds at biologically relevant scales is providing exciting opportunities for the application of redox polymers for both in-vitro and in-vivo tissue engineering. We will discuss the application of OCPs in the controlled delivery of bioactive compounds, and the electrical and mechanical stimulation of cells to drive behaviour and processes towards the generation of specific functional tissue. We will highlight the relatively recent advances in the use of graphene and the exploitation of its physicochemical and electrical properties in tissue engineering. Finally, we will look forward at the future of organic conductors in tissue engineering applications, and where the combination of materials development and fabrication processes will next unite to provide future breakthroughs.

Understanding Charge Transfer on the Clinically Used Conical Utah Electrode Array: Charge Storage Capacity, Electrochemical Impedance Spectroscopy and Effective Electrode Area

OBJECTIVE

The Utah electrode is used for pre/clinical studies on neural recording and stimulation. Anecdotal and empirical reports on their performance have been made, resulting in variable testing methods. An in depth investigation was performed to understand the electrochemical behaviour and charge transfer mechanisms occurring on these clinically important electrodes.

APPROACH

Platinum and iridium electrodes were assessed by cyclic voltammetry and electrochemical impedance spectroscopy. The effective electrode area was measured by reduction of Ru(NH3)63+.

MAIN RESULTS

Pristine Utah electrodes have little to no oxide present and the surface roughness is very low. Pristine iridium electrodes pass charge through capacitance and oxide formation. Hydride and anion adsorption occurs on the platinum electrode. Anodic current oxidises both metal surfaces, altering the charge transfer mechanisms at the electrode-solution interface. The charge storage capacity depends on measurement technique and electrode structure, providing no information on charge transfer mechanisms. Electrode oxidation increases pseudocapacitance, reducing impedance. Charge transfer was non-homogeneous, most likely due to the electrode geometry enhancing charge density at the electrode tip and base. Oxidation of the electrode surface enhanced charge transfer inhomogeneity. The effective electrode area could be measured by reduction of Ru(NH3)63+ and calculated with a finite cone geometry.

SIGNIFICANCE

Increasing electrode pseudocapacitance, demonstrated by metal oxidation, reduces impedance. Increasing electrode capacitance offers a potential route to reducing thermal noise and increasing signal-to-noise ratio of neural recording. The effective electrode area of conical electrodes can be measured. The charge density of the conical electrode was greater than expected on a planar disc electrode, indicating modification of electrode geometry can increase an electrodes safe charge injection capacity. In vivo electrochemical measurements often don't include sufficient details to understand the electrode behaviour. Electrode oxidation most likely accounts for a significant amount of variation in previously published Utah electrode impedance data.

Technological Challenges in the Development of Optogenetic Closed-Loop Therapy Approaches in Epilepsy and Related Network Disorders of the Brain

Epilepsy is a chronic, neurological disorder affecting millions of people every year. The current available pharmacological and surgical treatments are lacking in overall efficacy and cause side-effects like cognitive impairment, depression, tremor, abnormal liver and kidney function. In recent years, the application of optogenetic implants have shown promise to target aberrant neuronal circuits in epilepsy with the advantage of both high spatial and temporal resolution and high cell-specificity, a feature that could tackle both the efficacy and side-effect problems in epilepsy treatment. Optrodes consist of electrodes to record local field potentials and an optical component to modulate neurons via activation of opsin expressed by these neurons. The goal of optogenetics in epilepsy is to interrupt seizure activity in its earliest state, providing a so-called closed-loop therapeutic intervention. The chronic implantation in vivo poses specific demands for the engineering of therapeutic optrodes. Enzymatic degradation and glial encapsulation of implants may compromise long-term recording and sufficient illumination of the opsin-expressing neural tissue. Engineering efforts for optimal optrode design have to be directed towards limitation of the foreign body reaction by reducing the implant's elastic modulus and overall size, while still providing stable long-term recording and large-area illumination, and guaranteeing successful intracerebral implantation. This paper presents an overview of the challenges and recent advances in the field of electrode design, neural-tissue illumination, and neural-probe implantation, with the goal of identifying a suitable candidate to be incorporated in a therapeutic approach for long-term treatment of epilepsy patients.

Benchmarking the Performance of Electropolymerized Poly(3,4-ethylenedioxythiophene) Electrodes for Neural Interfacing

The development of electronics adept at interfacing with the nervous system is an ever-growing effort, leading to discoveries in fundamental neuroscience applied in clinical setting. Highly capacitive and electrochemically stable electronic materials are paramount for these advances. A systematic study is presented where copolymers based on 3,4-ethylenedioxythiophene (EDOT) and its hydroxyl-terminated counterpart (EDOTOH) are electropolymerized in an aqueous solution in the presence of various counter anions and additives. Amongst the conducting materials developed, the copolymer p(EDOT-ran-EDOTOH) doped with perchlorate in the presence of ethylene glycol shows high specific capacitance (105 F g^-1 ), and capacitance retention (85%) over 1000 galvanostatic charge-discharge cycles. A microelectrode array-based on this material is fabricated and primary cortical neurons are cultured therein for several days. The microelectrodes electrically stimulate targeted neuronal networks and record their activity with high signal-to-noise ratio. The stability of charge injection capacity of the material is validated via long-term pulsing experiments. While providing insights on the effect of additives and dopants on the electrochemical performance and operational stability of electropolymerized conducting polymers, this study highlights the importance of high capacitance accompanied with stability to achieve high performance electrodes for biological interfacing.

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.

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.

Miniaturized Paper-Supported 3D Cell-Based Electrochemical Sensor for Bacterial Lipopolysaccharide Detection

Sensitive detection of lipopolysaccharides (LPSs), which are present on the outer wall of Gram-negative bacteria, is important to reflect the degree of bacterial contamination in food. For indirect assessment of the LPS content, a miniaturized electrochemical cell sensor consisting of a screen-printed paper electrode, a three-dimensional cells-in-gels-in-paper culture system, and a conductive jacket device was developed for in situ detection of nitric oxide released from LPS-treated mouse macrophage cells (Raw264.7). Nafion/polypyrrole/graphene oxide with excellent selectivity, high conductivity, and good biocompatibility functionalized on the working electrode via electrochemical polymerization could enhance sensing. Raw264.7 cells encapsulated in the alginate hydrogel were immobilized on a Nafion/polypyrrole/graphene oxide/screen-printed carbon electrode in paper fibers as a biorecognition element. Differential impulse voltammetry was employed to record the current signal as-influenced by LPS. Results indicated that LPS from Salmonella enterica serotype Enteritidis caused a significant increase in peak current, varying from 1 × 10^-2 to 1 × 10⁴ ng/mL, dose-dependently. This assay had a detection limit of 3.5 × 10^-3 ng/mL with a linear detection range of 1 × 10^-2 to 3 ng/mL. These results were confirmed by analysis of nitric oxide released from Raw264.7 via the Griess method. The miniaturized sensor was ultimately applied to detect LPSs in fruit juice samples. The results indicated that the method exhibited high recovery and relative standard deviation lower than 2.65% and LPSs in samples contaminated with 10²-10⁵ CFU/mL bacteria could be detected, which proved the practical value of the sensor. Thus, a novel, low-cost, and highly sensitive approach for LPS detection was developed, providing a method to assess Gram-negative bacteria contamination in food.

NanoPt-A Nanostructured Electrode Coating for Neural Recording and Microstimulation

Bioelectronic devices, interfacing neural tissue for therapeutic, diagnostic, or rehabilitation purposes, rely on small electrode contacts in order to achieve highly sophisticated communication at the neural interface. Reliable recording and safe stimulation with small electrodes, however, are limited when conventional electrode metallizations are used, demanding the development of new materials to enable future progress within bioelectronics. In this study, we present a versatile process for the realization of nanostructured platinum (nanoPt) coatings with a high electrochemically active surface area, showing promising biocompatibility and providing low impedance, high charge injection capacity, and outstanding long-term stability both for recording and stimulation. The proposed electrochemical fabrication process offers exceptional control over the nanoPt deposition, allowing the realization of specific coating morphologies such as small grains, pyramids, or nanoflakes, and can moreover be scaled up to wafer level or batch fabrication under economic process conditions. The suitability of nanoPt as a coating for neural interfaces is here demonstrated, in vitro and in vivo, revealing superior stimulation performance under chronic conditions. Thus, nanoPt offers promising qualities as an advanced neural interface coating which moreover extends to the numerous application fields where a large (electro)chemically active surface area contributes to increased efficiency.

Magnesium-based biodegradable microelectrodes for neural recording

This article reports fabrication, characterization, degradation and electrical properties of biodegradable magnesium (Mg) microwires coated with two functional polymers, and the first in vivo evidence on the feasibility of Mg-based biodegradable microelectrodes for neural recording. Conductive poly(3,4‑ethylenedioxythiophene) (PEDOT) coating was first electrochemically deposited onto Mg microwire surface, and insulating biodegradable poly(glycerol sebacate) (PGS) was then spray-coated onto PEDOT surface to improve the overall properties of microelectrode. The assembled PGS/PEDOT-coated Mg microelectrodes showed high homogeneity in coating thickness, surface morphology and composition before and after in vivo recording. The charge storage capacity (CSC) of PGS/PEDOT-coated Mg microwire (1.72 mC/cm²) was nearly 5 times higher than the standard platinum (Pt) microwire widely used in implantable electrodes. The Mg-based microelectrode demonstrated excellent neural-recording capability and stability during in vivo multi-unit neural recordings in the auditory cortex of a mouse. Specifically, the Mg-based electrode showed clear and stable onset response, and excellent signal-to-noise ratio during spontaneous-activity recordings and three repeats of stimulus-evoked recordings at two different anatomical locations in the auditory cortex. During 10 days of immersion in artificial cerebrospinal fluid (aCSF) in vitro, PGS/PEDOT-coated Mg microelectrodes showed slower degradation and less change in impedance than PEDOT-coated Mg electrodes. The biodegradable PGS coating protected the PEDOT coating from delamination, and prolonged the mechanical integrity and electrical properties of Mg-based microelectrode. Mg-based novel microelectrodes should be further studied toward clinical translation because they can potentially eliminate the risks and costs associated with secondary surgeries for removal of failed or no longer needed electrodes.

Conducting Polymer-Based Composite Materials for Therapeutic Implantations: From Advanced Drug Delivery System to Minimally Invasive Electronics

Conducting polymer-based composites have recently becoming popular in both academic research and industrial practices due to their high conductivity, ease of process, and tunable electrical properties. The multifunctional conducting polymer-based composites demonstrated great application potential for in vivo therapeutics and implantable electronics, including drug delivery, neural interfacing, and minimally invasive electronics. In this review article, the state-of-the-art conducting polymer-based composites in the mentioned biological fields are discussed and summarized. The recent progress on the synthesis, structure, properties, and application of the conducting polymer-based composites is presented, aimed at revealing the structure-property relationship and the corresponding functional applications of the conducting polymer-based composites. Furthermore, key issues and challenges regarding the implantation performance of these composites are highlighted in this paper.

Implantable Neural Interfaces and Wearable Tactile Systems for Bidirectional Neuroprosthetics Systems

Neuroprosthetics and neuromodulation represent a promising field for several related applications in the central and peripheral nervous system, such as the treatment of neurological disorders, the control of external robotic devices, and the restoration of lost tactile functions. These actions are allowed by the neural interface, a miniaturized implantable device that most commonly exploits electrical energy to fulfill these operations. A neural interface must be biocompatible, stable over time, low invasive, and highly selective; the challenge is to develop a safe, compact, and reliable tool for clinical applications. In case of anatomical impairments, neuroprosthetics is bound to the need of exploring the surrounding environment by fast-responsive and highly sensitive artificial tactile sensors that mimic the natural sense of touch. Tactile sensors and neural interfaces are closely interconnected since the readouts from the first are required to convey information to the neural implantable apparatus. The role of these devices is pivotal hence technical improvements are essential to ensure a secure system to be eventually adopted in daily life. This review highlights the fundamental criteria for the design and microfabrication of neural interfaces and artificial tactile sensors, their use in clinical applications, and future enhancements for the release of a second generation of devices.

Inkjet-printed PEDOT:PSS multi-electrode arrays for low-cost in vitro electrophysiology

Multi-electrode arrays (MEAs) have become a key element in the study of cellular phenomena in vitro. Common modern MEAs are still based on costly microfabrication techniques, making them expensive tools that researchers are pushed to reuse, compromising the reproducibility and the quality of the acquired data. There is a need to develop novel fabrication strategies, able to produce disposable devices that incorporate advanced technologies beyond the standard metal electrodes on rigid substrates. Here we present an innovative fabrication process for the production of polymer-based flexible MEAs. The device fabrication exploited inkjet printing, as this low-cost manufacturing method allows for an easy and reliable patterning of conducting polymers. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was used as the sole conductive element of the MEAs. The physical structure and the electrical properties of the plastic/printed MEAs (pMEAs) were characterised, showing a low impedance that is maintained also in the long term. The biocompatibility of the devices was demonstrated, and their capability to successfully establish a tight coupling with cells was proved. Furthermore, the pMEAs were used to monitor the extracellular potentials from cardiac cell cultures and to record high quality electrophysiological signals from them. Our results validate the use of pMEAs as in vitro electrophysiology platforms, pushing for the adoption of innovative fabrication techniques and the use of new materials for the production of MEAs.

Electrodeposited PEDOT:Nafion Composite for Neural Recording and Stimulation

Microelectrode arrays are used for recording and stimulation in neurosciences both in vitro and in vivo. The electrodeposition of conductive polymers, such as poly(3,4-ethylene dioxythiophene) (PEDOT), is widely adopted to improve both the in vivo recording and the charge injection limit of metallic microelectrodes. The workhorse of conductive polymers in the neurosciences is PEDOT:PSS, where PSS represents polystyrene-sulfonate. In this paper, the counterion is the fluorinated polymer Nafion, so the composite PEDOT:Nafion is deposited onto a flexible neural microelectrode array. PEDOT:Nafion coated electrodes exhibit comparable in vivo recording capability to the reference PEDOT:PSS, providing a large signal-to-noise ratio in a murine animal model. Importantly, PEDOT:Nafion exhibits a minimized polarization during electrical stimulation, thereby resulting in an improved charge injection limit equal to 4.4 mC cm^-2 , almost 80% larger than the 2.5 mC cm^-2 that is observed for PEDOT:PSS.

Predicting neural recording performance of implantable electrodes

Recordings of neural activity can be used to aid communication, control prosthetic devices or alleviate disease symptoms. Chronic recordings require a high signal-to-noise ratio that is stable for years. Current cortical devices generally fail within months to years after implantation. Development of novel devices to increase lifetime requires valid testing protocols and a knowledge of the critical parameters controlling electrophysiological performance. Here we present electrochemical and electrophysiological protocols for assessing implantable electrodes. Biological noise from neural recording has significant impact on signal-to-noise ratio. A recently developed surgical approach was utilised to reduce biological noise. This allowed correlation of electrochemical and electrophysiological behaviour. The impedance versus frequency of modified electrodes was non-linear. It was found that impedance at low frequencies was a stronger predictor of electrophysiological performance than the typically reported impedance at 1 kHz. Low frequency impedance is a function of electrode area, and a strong correlation of electrode area with electrophysiological response was also seen. Use of these standardised testing protocols will allow future devices to be compared before transfer to preclinical and clinical trials.

Applications of PEDOT in bioelectronic medicine

The widespread use of conducting polymers, especially poly(3,4-ethylene dioxythiophene) (PEDOT), within the space of bioelectronics has enabled improvements, both in terms of electrochemistry and functional versatility, of conventional metallic electrodes. This short review aims to provide an overview of how PEDOT coatings have contributed to functionalizing existing bioelectronics, the challenges which meet conducting polymer coatings from a regulatory and stability point of view and the possibilities to bring PEDOT-based coatings into large-scale clinical applications. Finally, their potential use for enabling new technologies for the field of bioelectronics as biodegradable, stretchable and slow-stimulation materials will be discussed.

Facile Synthesis of a 3,4-Ethylene-Dioxythiophene (EDOT) Derivative for Ease of Bio-Functionalization of the Conducting Polymer PEDOT

In the pursuit of conducting polymer based bio-functional devices, a cost-effective and high yield synthesis method for a versatile monomer is desired. We report here a new synthesis strategy for a versatile monomer 2-methylene-2,3-dihydrothieno (3,4-b) (1,4) dioxine, or 3,4-ethylenedioxythiophene with a exomethylene side group (EDOT-EM). Compared to the previously reported synthesis route, the new strategy uses less steps, with faster reaction rate, and higher yield. The presence of EM group opens up endless possibility for derivatization via either hydro-alkoxy addition or thiol-ene click chemistry. EDOT-EM could be polymerized into stable and low impedance PEDOT-EM polymer using electro-polymerization method on different conducting substrates at both macro and micro scales. Facile post-functionalization of PEDOT-EM with molecules of varying size and functionality (from small molecules to DNAs and proteins) was achieved. The new synthetic route of EDOT-EM and the ease of post-functionalization of PEDOT-EM will greatly accelerate the use of conducting polymer in a broad range of organic electronics and bioelectronics applications.

Development and Characterization of PEDOT:PSS/Alginate Soft Microelectrodes for Application in Neuroprosthetics

Reducing the mechanical mismatch between the stiffness of a neural implant and the softness of the neural tissue is still an open challenge in neuroprosthetics. The emergence of conductive hydrogels in the last few years has considerably widened the spectrum of possibilities to tackle this issue. Nevertheless, despite the advancements in this field, further improvements in the fabrication of conductive hydrogel-based electrodes are still required. In this work, we report the fabrication of a conductive hydrogel-based microelectrode array for neural recording using a hybrid material composed of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), and alginate. The mechanical properties of the conductive hydrogel have been investigated using imaging techniques, while the electrode arrays have been electrochemically characterized at each fabrication step, and successfully validated both in vitro and in vivo. The presence of the conductive hydrogel, selectively electrodeposited onto the platinum microelectrodes, allowed achieving superior electrochemical characteristics, leading to a lower electrical noise during recordings. These findings represent an advancement in the design of soft conductive electrodes for neuroprosthetic applications.

A Micropatterned Multielectrode Shell for 3D Spatiotemporal Recording from Live Cells

Microelectrode arrays (MEAs) have proved to be useful tools for characterizing electrically active cells such as cardiomyocytes and neurons. While there exist a number of integrated electronic chips for recording from small populations or even single cells, they rely primarily on the interface between the cells and 2D flat electrodes. Here, an approach that utilizes residual stress-based self-folding to create individually addressable multielectrode interfaces that wrap around the cell in 3D and function as an electrical shell-like recording device is described. These devices are optically transparent, allowing for simultaneous fluorescence imaging. Cell viability is maintained during and after electrode wrapping around the cel and chemicals can diffuse into and out of the self-folding devices. It is further shown that 3D spatiotemporal recordings are possible and that the action potentials recorded from cultured neonatal rat ventricular cardiomyocytes display significantly higher signal-to-noise ratios in comparison with signals recorded with planar extracellular electrodes. It is anticipated that this device can provide the foundation for the development of new-generation MEAs where dynamic electrode-cell interfacing and recording substitutes the traditional method using static electrodes.

Evaluating the in vivo glial response to miniaturized parylene cortical probes coated with an ultra-fast degrading polymer to aid insertion

OBJECTIVE

Despite the feasibility of short-term neural recordings using implantable microelectrodes, attaining reliable, chronic recordings remains a challenge. Most neural recording devices suffer from a long-term tissue response, including gliosis, at the device-tissue interface. It was hypothesized that smaller, more flexible intracortical probes would limit gliosis by providing a better mechanical match with surrounding tissue.

APPROACH

This paper describes the in vivo evaluation of flexible parylene microprobes designed to improve the interface with the adjacent neural tissue to limit gliosis and thereby allow for improved recording longevity. The probes were coated with an ultrafast degrading tyrosine-derived polycarbonate (E5005(2K)) polymer that provides temporary mechanical support for device implantation, yet degrades within 2 h post-implantation. A parametric study of probes of varying dimensions and polymer coating thicknesses were implanted in rat brains. The glial tissue response and neuronal loss were assessed from 72 h to 24 weeks post-implantation via immunohistochemistry.

MAIN RESULTS

Experimental results suggest that both probe and polymer coating sizes affect the extent of gliosis. When an appropriate sized coating dimension (100 µm  ×  100 µm) and small probe (30 µm  ×  5 µm) was implanted, a minimal post-implantation glial response was observed. No discernible gliosis was detected when compared to tissue where a sham control consisting of a solid degradable polymer shuttle of the same dimensions was inserted. A larger polymer coating (200 µm  ×  200 µm) device induced a more severe glial response at later time points, suggesting that the initial insertion trauma can affect gliosis even when the polymer shuttle degrades rapidly. A larger degree of gliosis was also observed when comparing a larger sized probe (80 µm  ×  5 µm) to a smaller probe (30 µm  ×  5 µm) using the same polymer coating size (100 µm  ×  100 µm). There was no significant neuronal loss around the implantation sites for most device candidates except the group with largest polymer coating and probe sizes.

SIGNIFICANCE

These results suggest that: (1) the degree of mechanical trauma at device implantation and mechanical mismatches at the probe-tissue interface affect long term gliosis; (2) smaller, more flexible probes may minimize the glial response to provide improved tissue biocompatibility when used for chronic neural signal recording; and (3) some degree of glial scarring did not significantly affect neuronal distribution around the probe.

PEDOT doped with algal, mammalian and synthetic dopants: polymer properties, protein and cell interactions, and influence of electrical stimulation on neuronal cell differentiation

PEDOT films were electrochemically polymerised with synthetic and biological dopants, characterised, and their interactions with proteins and neuronal cells investigated.

Chemical Vapor Deposition of Thin, Conductive, and Fouling-Resistant Polymeric Films

Fouling has been a persistent issue within applications ranging from membrane separation to biomedical implantation. Research to date focuses on fouling-resistant coatings, where electrical conductivity is unnecessary. In this study, we report the synthesis of multifunctional thin films with both fouling resistance and electrical conductivity for their potential applications in the electrolysis-based self-cleaning of separation membranes and in the field of bioelectronics. This unique combination of properties results in multifunctional coatings that are a zwitterionic derivative of intrinsically conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) synthesized via oxidative chemical vapor deposition (oCVD). Their fouling resistance is shown to be comparable to that of known dielectric fouling-resistant surfaces, such as a poly(4-vinylpyridine)-co-divinylbenzene (p4VP-DVB)-derived zwitterionic coating, an amphiphilic poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-2-hydroxyethyl methacrylate) (pPFDA-HEMA) coating, and a glass surface, and are far superior to the fouling resistance of gold or polydimethylsiloxane (PDMS) surfaces. The fouling resistances of seven surfaces are quantitatively characterized by molecular force probe (MFP) analysis. In addition, four-point probe electrical measurements, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), variable-angle spectroscopic ellipsometry (VASE), profilometry, water contact angle (WCA) measurements, surface ζ-potential measurements, and atomic force microscopy (AFM) were employed to characterize the physiochemical properties and morphology of the different surfaces.

Taking Electrons out of Bioelectronics: From Bioprotonic Transistors to Ion Channels

From cell-to-cell communication to metabolic reactions, ions and protons (H+) play a central role in many biological processes. Examples of H+ in action include oxidative phosphorylation, acid sensitive ion channels, and pH dependent enzymatic reactions. To monitor and control biological reactions in biology and medicine, it is desirable to have electronic devices with ionic and protonic currents. Here, we summarize our latest efforts on bioprotonic devices that monitor and control a current of H+ in physiological conditions, and discuss future potential applications. Specifically, we describe the integration of these devices with enzymatic logic gates, bioluminescent reactions, and ion channels.

Biocompatibility study of three distinct carbon pastes for application as electrode material in neural stimulations and recordings

Neural interfaces hold great promise for research and treatment of a wide variety of neurological diseases. Medical electrodes are designed to interface with the nervous system and provide control signals for neural prostheses. We fabricated previously a hook-up neural electrode. Here we investigate the in vitro cytotoxicity of three commercial carbon pastes used for printing the conductor tracks of this electrode. At first, the carbon pastes were characterized with respect to their microstructure and chemical composition. SEM images showed a grainy texture that is associated to the carbon/graphite microparticles dispersed by the polymeric binder. All the three pastes contained in major proportions carbon and in different proportions other elements. The surface roughness analysis evidenced differences in the smoothness of the carbon paste surfaces. Sterilization procedures did not alter the microstructure or surface morphology of the pastes. Finally, cell viability based on -(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test and fluorescence staining experiments proved non-cytotoxicity and suitability of the studied carbon pastes as electrode material for measuring neural activity during surgeries (up to a certain time period).

Correlation of Impedance and Effective Electrode Area of Iridium Oxide Neural Electrodes

Iridium oxide is routinely used for bionic applications owing to its high charge injection capacity. The electrode impedance at 1 kHz is typically reported to predict neural recording performance. In this article, the impedance of activated iridium oxide films (AIROFs) has been examined. The impedance of unactivated iridium electrodes was half that of platinum electrodes of similar geometry, indicating some iridium oxide was present on the electrode surface. A two time constant equivalent circuit was used to model the impedance of activated iridium. The impedance at low and intermediate frequencies decreased with increasing number of activation pulses and total activation charge. The impedance at 12 Hz correlated with the steady-state diffusion electroactive area. The impedance at 12 Hz also correlated with the charge density of the electrode. The high charge density and low impedance of AIROFs may provide improved neural stimulation and recording properties compared with typically used platinum electrodes.

Chronic in vivo stability assessment of carbon fiber microelectrode arrays

OBJECTIVE

Individual carbon fiber microelectrodes can record unit activity in both acute and semi-chronic (∼1 month) implants. Additionally, new methods have been developed to insert a 16 channel array of carbon fiber microelectrodes. Before assessing the in vivo long-term viability of these arrays, accelerated soak tests were carried out to determine the most stable site coating material. Next, a multi-animal, multi-month, chronic implantation study was carried out with carbon fiber microelectrode arrays and silicon electrodes.

APPROACH

Carbon fibers were first functionalized with one of two different formulations of PEDOT and subjected to accelerated aging in a heated water bath. After determining the best PEDOT formula to use, carbon fiber arrays were chronically implanted in rat motor cortex. Some rodents were also implanted with a single silicon electrode, while others received both. At the end of the study a subset of animals were perfused and the brain tissue sliced. Tissue sections were stained for astrocytes, microglia, and neurons. The local reactive responses were assessed using qualitative and quantitative methods.

MAIN RESULTS

Electrophysiology recordings showed the carbon fibers detecting unit activity for at least 3 months with average amplitudes of ∼200 μV. Histology analysis showed the carbon fiber arrays with a minimal to non-existent glial scarring response with no adverse effects on neuronal density. Silicon electrodes showed large glial scarring that impacted neuronal counts.

SIGNIFICANCE

This study has validated the use of carbon fiber microelectrode arrays as a chronic neural recording technology. These electrodes have demonstrated the ability to detect single units with high amplitude over 3 months, and show the potential to record for even longer periods. In addition, the minimal reactive response should hold stable indefinitely, as any response by the immune system may reach a steady state after 12 weeks.

In Vivo Neural Recording and Electrochemical Performance of Microelectrode Arrays Modified by Rough-Surfaced AuPt Alloy Nanoparticles with Nanoporosity

In order to reduce the impedance and improve in vivo neural recording performance of our developed Michigan type silicon electrodes, rough-surfaced AuPt alloy nanoparticles with nanoporosity were deposited on gold microelectrode sites through electro-co-deposition of Au-Pt-Cu alloy nanoparticles, followed by chemical dealloying Cu. The AuPt alloy nanoparticles modified gold microelectrode sites were characterized by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and in vivo neural recording experiment. The SEM images showed that the prepared AuPt alloy nanoparticles exhibited cauliflower-like shapes and possessed very rough surfaces with many different sizes of pores. Average impedance of rough-surfaced AuPt alloy nanoparticles modified sites was 0.23 MΩ at 1 kHz, which was only 4.7% of that of bare gold microelectrode sites (4.9 MΩ), and corresponding in vitro background noise in the range of 1 Hz to 7500 Hz decreased to 7.5 μ V rms from 34.1 μ V rms at bare gold microelectrode sites. Spontaneous spike signal recording was used to evaluate in vivo neural recording performance of modified microelectrode sites, and results showed that rough-surfaced AuPt alloy nanoparticles modified microelectrode sites exhibited higher average spike signal-to-noise ratio (SNR) of 4.8 in lateral globus pallidus (GPe) due to lower background noise compared to control microelectrodes. Electro-co-deposition of Au-Pt-Cu alloy nanoparticles combined with chemical dealloying Cu was a convenient way for increasing the effective surface area of microelectrode sites, which could reduce electrode impedance and improve the quality of in vivo spike signal recording.

The Evolution of Neuroprosthetic Interfaces

The ideal neuroprosthetic interface permits high-quality neural recording and stimulation of the nervous system while reliably providing clinical benefits over chronic periods. Although current technologies have made notable strides in this direction, significant improvements must be made to better achieve these design goals and satisfy clinical needs. This article provides an overview of the state of neuroprosthetic interfaces, starting with the design and placement of these interfaces before exploring the stimulation and recording platforms yielded from contemporary research. Finally, we outline emerging research trends in an effort to explore the potential next generation of neuroprosthetic interfaces.

Biofunctionalized Conducting Polymer/Carbon Microfiber Electrodes for Ultrasensitive Neural Recordings

Carbon microfibers (MFs) coated with conducting polymers may provide a solution for long-term recording of activity from individual or small groups of neurons. Attaching cell adhesion molecules to the electro-sensitive surface might further improve electrode-neuron contact, thus enhancing signal stability and fidelity. We fabricated biofunctionalized microelectrodes consisting of 7-μm diameter carbon MFs coated with poly(3,4-ethylenedioxythiophene) doped with poly[(4-styrenesulfonic acid)-co-(maleic acid)] (

PEDOT

PSS-co-MA), and linked N-Cadherin to the polymer surface. These electrodes were tested for recording artificially generated electric potentials, as well as multiunit activity (MUA), sharp wave-ripple complexes (SWRs), and field excitatory postsynaptic potentials (fEPSPs) in rat hippocampal slices. The effects of electrode length and functionalization were compared.

PEDOT

PSS-co-MA coating improved electric current detection and reduced the electrical noise but had no significant effect on the amplitude of recorded biopotentials. Surface biofunctionalization lowered the electric current flow, and further reduced the electrical noise. Additionally, it increased the amplitude of the recorded MUA, finally doubling the signal-to-noise ratio achieved with bare carbon MFs. Biofunctionalization benefits were apparent only for potentials from cells putatively adjacent to the microelectrode. Analysis of fEPSPs excluded adverse effects of functionalized electrodes in basal synaptic transmission. These results demonstrate the possibility of enhancing the amplitude and signal-to-noise ratio of neural recordings by coating the microelectrodes with conducting polymers modified with neural cell adhesion molecules, and support the use of biofunctionalized MFs in advanced neuroprosthetic devices.

Conducting Polymers for Neural Prosthetic and Neural Interface Applications

Neural-interfacing devices are an artificial mechanism for restoring or supplementing the function of the nervous system, lost as a result of injury or disease. Conducting polymers (CPs) are gaining significant attention due to their capacity to meet the performance criteria of a number of neuronal therapies including recording and stimulating neural activity, the regeneration of neural tissue and the delivery of bioactive molecules for mediating device-tissue interactions. CPs form a flexible platform technology that enables the development of tailored materials for a range of neuronal diagnostic and treatment therapies. In this review, the application of CPs for neural prostheses and other neural interfacing devices is discussed, with a specific focus on neural recording, neural stimulation, neural regeneration, and therapeutic drug delivery.