In vivo neuronal action potential recordings via three-dimensional microscale needle-electrode arrays

Interfacing with the Brain: How Nanotechnology Can Contribute

Interfacing artificial devices with the human brain is the central goal of neurotechnology. Yet, our imaginations are often limited by currently available paradigms and technologies. Suggestions for brain-machine interfaces have changed over time, along with the available technology. Mechanical levers and cable winches were used to move parts of the brain during the mechanical age. Sophisticated electronic wiring and remote control have arisen during the electronic age, ultimately leading to plug-and-play computer interfaces. Nonetheless, our brains are so complex that these visions, until recently, largely remained unreachable dreams. The general problem, thus far, is that most of our technology is mechanically and/or electrically engineered, whereas the brain is a living, dynamic entity. As a result, these worlds are difficult to interface with one another. Nanotechnology, which encompasses engineered solid-state objects and integrated circuits, excels at small length scales of single to a few hundred nanometers and, thus, matches the sizes of biomolecules, biomolecular assemblies, and parts of cells. Consequently, we envision nanomaterials and nanotools as opportunities to interface with the brain in alternative ways. Here, we review the existing literature on the use of nanotechnology in brain-machine interfaces and look forward in discussing perspectives and limitations based on the authors' expertise across a range of complementary disciplines─from neuroscience, engineering, physics, and chemistry to biology and medicine, computer science and mathematics, and social science and jurisprudence. We focus on nanotechnology but also include information from related fields when useful and complementary.

Endovascular neural stimulation with platinum and platinum black modified electrodes

Recent work has shown the ability to record neural behaviour in pre-clinical studies from an endovascular location for over a year. Previous work on stimulating neural tissue from an endovascular location has also shown motor-evoked responses in sheep. However, endovascular stimulation requires high currents and can result in electrode degradation. This study aimed to modify an endovascular electrode to increase its charge injection capacity for efficacious neural stimulation. The platinum endovascular electrode was modified with platinum black and characterised by electrochemical and microscopic techniques. The stability of the electrode coating was assessed after a 7-day continuous stimulation paradigm. Modelling of the neural activating function was performed for central and peripheral neural anatomy with both electrode materials. Platinum black coatings had a substantially larger electroactive area than uncoated platinum. This resulted in increased electrode admittance, charge storage capacity and charge injection capacity while reducing the total impedance at 10 Hz and polarisation voltage. The coated electrode was comparatively more electrochemically stable than uncoated platinum following the 7-day continuous stimulation protocol. Modelling of the neural activating function indicated a substantial increase in the electrode-neuron distance which could be safely stimulated using platinum black coated electrodes. By comparison of electrochemical response with neural modelling, we have demonstrated the feasibility of safe stimulation of neural tissue using an endovascular neural interface, opening the possibility of a new, minimally invasive neural stimulation paradigm.

Magnetic assembly of microwires on a flexible substrate for minimally invasive electrophysiological recording

Understanding the neural system in the brain requires the detection of signals from the tissue. Microscale electrodes enable high spatiotemporal neural recording, whereas traditional microelectrodes cause material and geometry mismatches between the electrode and the tissue, leading to injury and signal loss during recording. In this study, we propose a fabrication technique that uses magnetic force to facilitate assembly of vertical microscale wire-electrodes on a flexible substrate. Two-channel 15-μm-diameter and 400-μm-length nickel-microwire electrodes on a 5-μm-thick flexible parylene film are designed and fabricated. Impedance characteristics of these electrodes are <500 kΩ at 1 kHz, with output/input signal amplitude ratios of over 90%. In vivo neural recording in mice demonstrates that both local field potentials and action potentials are detected through each wire electrode, confirming the minimal invasiveness during the electrode penetration and through immunohistochemical tissue analysis.

Highly stretchable and customizable microneedle electrode arrays for intramuscular electromyography

Stretchable three-dimensional (3D) penetrating microelectrode arrays have potential utility in various fields, including neuroscience, tissue engineering, and wearable bioelectronics. These 3D microelectrode arrays can penetrate and conform to dynamically deforming tissues, thereby facilitating targeted sensing and stimulation of interior regions in a minimally invasive manner. However, fabricating custom stretchable 3D microelectrode arrays presents material integration and patterning challenges. In this study, we present the design, fabrication, and applications of stretchable microneedle electrode arrays (SMNEAs) for sensing local intramuscular electromyography signals ex vivo. We use a unique hybrid fabrication scheme based on laser micromachining, microfabrication, and transfer printing to enable scalable fabrication of individually addressable SMNEA with high device stretchability (60 to 90%). The electrode geometries and recording regions, impedance, array layout, and length distribution are highly customizable. We demonstrate the use of SMNEAs as bioelectronic interfaces in recording intramuscular electromyography from various muscle groups in the buccal mass of Aplysia.

Low-invasive neural recording in mouse models with diabetes via an ultrasmall needle-electrode

Diabetes is known to cause a variety of complications, having a high correlation with Alzheimer's disease. Electrophysiological recording using a microscale needle electrode is a promising technology for the study, however, diabetic brain tissue is more difficult to record neuronal activities than normal tissue due to these complications including the development of cerebrovascular disease. Here we show an electrophysiological methodology for diabetic db/db mice (+Leprdb/+Leprdb) using a 4-μm-tip diameter needle-electrode device. The needle electrode minimized the tissue injury when compared to a typical larger metal electrode, as confirmed by bleeding during penetration. The proposed electrode device showed both acute and chronic in vivo recording capabilities for diabetic mice while reducing the glial cells' responses. Because of these device characteristics, the 4-μm-tip diameter needle-electrode will allow electrophysiological studies on diabetes models of not only mice, as proven in this study, but also other animals.

NeuroActivityToolkit-Toolbox for Quantitative Analysis of Miniature Fluorescent Microscopy Data

The visualization of neuronal activity in vivo is an urgent task in modern neuroscience. It allows neurobiologists to obtain a large amount of information about neuronal network architecture and connections between neurons. The miniscope technique might help to determine changes that occurred in the network due to external stimuli and various conditions: processes of learning, stress, epileptic seizures and neurodegenerative diseases. Furthermore, using the miniscope method, functional changes in the early stages of such disorders could be detected. The miniscope has become a modern approach for recording hundreds to thousands of neurons simultaneously in a certain brain area of a freely behaving animal. Nevertheless, the analysis and interpretation of the large recorded data is still a nontrivial task. There are a few well-working algorithms for miniscope data preprocessing and calcium trace extraction. However, software for further high-level quantitative analysis of neuronal calcium signals is not publicly available. NeuroActivityToolkit is a toolbox that provides diverse statistical metrics calculation, reflecting the neuronal network properties such as the number of neuronal activations per minute, amount of simultaneously co-active neurons, etc. In addition, the module for analyzing neuronal pairwise correlations is implemented. Moreover, one can visualize and characterize neuronal network states and detect changes in 2D coordinates using PCA analysis. This toolbox, which is deposited in a public software repository, is accompanied by a detailed tutorial and is highly valuable for the statistical interpretation of miniscope data in a wide range of experimental tasks.

Combining PEDOT:PSS Polymer Coating with Metallic 3D Nanowires Electrodes to Achieve High Electrochemical Performances for Neuronal Interfacing Applications

This study presents a novel approach to improve the performance of microelectrode arrays (MEAs) used for electrophysiological studies of neuronal networks. The integration of 3D nanowires (NWs) with MEAs increases the surface-to-volume ratio, which enables subcellular interactions and high-resolution neuronal signal recording. However, these devices suffer from high initial interface impedance and limited charge transfer capacity due to their small effective area. To overcome these limitations, the integration of conductive polymer coatings, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is investigated as a mean of improving the charge transfer capacity and biocompatibility of MEAs. The study combines platinum silicide-based metallic 3D nanowires electrodes with electrodeposited PEDOT:PSS coatings to deposit ultra-thin (<50 nm) layers of conductive polymer onto metallic electrodes with very high selectivity. The polymer-coated electrodes were fully characterized electrochemically and morphologically to establish a direct relationship between synthesis conditions, morphology, and conductive features. Results show that PEDOT-coated electrodes exhibit thickness-dependent improved stimulation and recording performances, offering new perspectives for neuronal interfacing with optimal cell engulfment to enable the study of neuronal activity with acute spatial and signal resolution at the sub-cellular level.

Design and fabrication of a microelectrode array for studying epileptiform discharges from rodents

Local field potentials, the extracellular electrical activities from brain regions, provide clinically relevant information about the status of neurophysiological conditions, including epilepsy. In this study, a 13-channel silicon-based single-shank microelectrode array (MEA) was designed and fabricated to record local field potentials (LFPs) from the different depths of a rat's brain. A titanium/gold layer was patterned as electrodes on an oxidized silicon substrate, and silicon dioxide was deposited as a passivation layer. The fabricated array was implanted in the somatosensory cortex of the right hemisphere of an anesthetized rat. The developed MEA was interfaced with an OpenBCI Cyton Daisy Biosensing Board to acquire the local field potentials. The LFPs were acquired at three different neurophysiological conditions, including baseline signals, chemically-induced epileptiform discharges, and recovered baseline signals after anti-epileptic drug (AED) administration. Further, time-frequency analyses were performed on the acquired biopotentials to study the difference in spatiotemporal features. The processed signals and time-frequency analyses clearly distinguish between pre-convulsant and post-AED baselines and evoked epileptiform discharges.

In Vivo Penetrating Microelectrodes for Brain Electrophysiology

In recent decades, microelectrodes have been widely used in neuroscience to understand the mechanisms behind brain functions, as well as the relationship between neural activity and behavior, perception and cognition. However, the recording of neuronal activity over a long period of time is limited for various reasons. In this review, we briefly consider the types of penetrating chronic microelectrodes, as well as the conductive and insulating materials for microelectrode manufacturing. Additionally, we consider the effects of penetrating microelectrode implantation on brain tissue. In conclusion, we review recent advances in the field of in vivo microelectrodes.

Nanoneedle-Electrode Devices for In Vivo Recording of Extracellular Action Potentials

Microscale needle-like electrode technologies offer in vivo extracellular recording with a high spatiotemporal resolution. Further miniaturization of needles to nanoscale minimizes tissue injuries; however, a reduced electrode area increases electrical impedance that degrades the quality of neuronal signal recording. We overcome this limitation by fabricating a 300 nm tip diameter and 200 μm long needle electrode where the amplitude gain with a high-impedance electrode (>15 MΩ, 1 kHz) was improved from 0.54 (-5.4 dB) to 0.89 (-1.0 dB) by stacking it on an amplifier module of source follower. The nanoelectrode provided the recording of both local field potential (<300 Hz) and action potential (>500 Hz) in the mouse cortex, in contrast to the electrode without the amplifier. These results suggest that microelectrodes can be further minimized by the proposed amplifier configuration for low-invasive recording and electrophysiological studies in submicron areas in tissues, such as dendrites and axons.

In-Plane Si Microneedles: Fabrication, Characterization, Modeling and Applications

Microneedles are getting more and more attention in research and commercialization since their advancement in the 1990s due to the advantages over traditional hypodermic needles such as minimum invasiveness, low material and fabrication cost, and precise needle geometry control, etc. The design and fabrication of microneedles depend on various factors such as the type of materials used, fabrication planes and techniques, needle structures, etc. In the past years, in-plane and out-of-plane microneedle technologies made by silicon (Si), polymer, metal, and other materials have been developed for numerous biomedical applications including drug delivery, sample collections, medical diagnostics, and bio-sensing. Among these microneedle technologies, in-plane Si microneedles excel by the inherent properties of Si such as mechanical strength, wear resistance, biocompatibility, and structural advantages of in-plane configuration such as a wide range of length, readiness of integration with other supporting components, and complementary metal-oxide-semiconductor (CMOS) compatible fabrication. This article aims to provide a review of in-plane Si microneedles with a focus on fabrication techniques, theoretical and numerical analysis, experimental characterization of structural and fluidic behaviors, major applications, potential challenges, and future prospects.

Nanoscale-tipped wire array injections transfer DNA directly into brain cells ex vivo and in vivo

Genetic modification to restore cell functions in the brain can be performed through the delivery of biomolecules in a minimally invasive manner into live neuronal cells within brain tissues. However, conventional nanoscale needles are too short (lengths of ~10 µm) to target neuronal cells in ~1‐mm‐thick brain tissues because the neuronal cells are located deep within the tissue. Here, we report the use of nanoscale‐tipped wire (NTW) arrays with diameters < 100 nm and wire lengths of ~200 µm to address biomolecule delivery issues. The NTW arrays were manufactured by growth of silicon microwire arrays and nanotip formation. This technique uses pinpoint, multiple‐cell DNA injections in deep areas of brain tissues, enabling target cells to be marked by fluorescent protein (FP) expression vectors. This technique has potential for use for electrophysiological recordings and biological transfection into neuronal cells. Herein, simply pressing an NTW array delivers and expresses plasmid DNA in multiple‐cultured cells and multiple‐neuronal cells within a brain slice with reduced cell damage. Additionally, DNA transfection is demonstrated using brain cells ex vivo and in vivo. Moreover, knockdown of a critical clock gene after injecting a short hairpin RNA (shRNA) and a genome‐editing vector demonstrates the potential to genetically alter the function of living brain cells, for example, pacemaker cells of the mammalian circadian rhythms. Overall, our NTW array injection technique enables genetic and functional modification of living cells in deep brain tissue areas, both ex vivo and in vivo.

The Future of Neuroscience: Flexible and Wireless Implantable Neural Electronics

Neurological diseases are a prevalent cause of global mortality and are of growing concern when considering an ageing global population. Traditional treatments are accompanied by serious side effects including repeated treatment sessions, invasive surgeries, or infections. For example, in the case of deep brain stimulation, large, stiff, and battery powered neural probes recruit thousands of neurons with each pulse, and can invoke a vigorous immune response. This paper presents challenges in engineering and neuroscience in developing miniaturized and biointegrated alternatives, in the form of microelectrode probes. Progress in design and topology of neural implants has shifted the goal post toward highly specific recording and stimulation, targeting small groups of neurons and reducing the foreign body response with biomimetic design principles. Implantable device design recommendations, fabrication techniques, and clinical evaluation of the impact flexible, integrated probes will have on the treatment of neurological disorders are provided in this report. The choice of biocompatible material dictates fabrication techniques as novel methods reduce the complexity of manufacture. Wireless power, the final hurdle to truly implantable neural interfaces, is discussed. These aspects are the driving force behind continued research: significant breakthroughs in any one of these areas will revolutionize the treatment of neurological disorders.

Three-micrometer-diameter needle electrode with an amplifier for extracellular in vivo recordings

Microscale needle-electrode devices offer neuronal signal recording capability in brain tissue; however, using needles of smaller geometry to minimize tissue damage causes degradation of electrical properties, including high electrical impedance and low signal-to-noise ratio (SNR) recording. We overcome these limitations using a device assembly technique that uses a single needle-topped amplifier package, called STACK, within a device of ∼1 × 1 mm2 Based on silicon (Si) growth technology, a <3-µm-tip-diameter, 400-µm-length needle electrode was fabricated on a Si block as the module. The high electrical impedance characteristics of the needle electrode were improved by stacking it on the other module of the amplifier. The STACK device exhibited a voltage gain of >0.98 (-0.175 dB), enabling recording of the local field potential and action potentials from the mouse brain in vivo with an improved SNR of 6.2. Additionally, the device allowed us to use a Bluetooth module to demonstrate wireless recording of these neuronal signals; the chronic experiment was also conducted using STACK-implanted mice.

Recent Advances in Materials, Devices, and Systems for Neural Interfaces

Technologies capable of establishing intimate, long-lived optical/electrical interfaces to neural systems will play critical roles in neuroscience research and in the development of nonpharmacological treatments for neurological disorders. The development of high-density interfaces to 3D populations of neurons across entire tissue systems in living animals, including human subjects, represents a grand challenge for the field, where advanced biocompatible materials and engineered structures for electrodes and light emitters will be essential. This review summarizes recent progress in these directions, with an emphasis on the most promising demonstrated concepts, materials, devices, and systems. The article begins with an overview of electrode materials with enhanced electrical and/or mechanical performance, in forms ranging from planar films, to micro/nanostructured surfaces, to 3D porous frameworks and soft composites. Subsequent sections highlight integration with active materials and components for multiplexed addressing, local amplification, wireless data transmission, and power harvesting, with multimodal operation in soft, shape-conformal systems. These advances establish the foundations for scalable architectures in optical/electrical neural interfaces of the future, where a blurring of the lines between biotic and abiotic systems will catalyze profound progress in neuroscience research and in human health/well-being.

Design and Fabrication of a Three-Dimensional Multi-Electrode Array for Neuron Electrophysiology

Neural recording and stimulation with high spatial and temporal resolution are highly desirable in the study of neurocommunication and diseases. Planar multiple microelectrode arrays (MEA) or quasi-three-dimensional (3D) MEA with fixed height have been proposed by many researchers and become commercially available. In this paper, we present the design, fabrication, and test of a novel true 3D multiple electrode array for brain slice stimulation and recording. This MEA is composed of 105 microelectrodes with 50 μm diameter and 125 μm center-to-center spacing integrated in a 1.2 × 1.2 mm2 area. This "true" 3D MEA allows us to precisely position the individual electrodes by piezoelectric-based actuators to penetrate the inactive tissue layer and to approach the active neurons so as to optimize the recording and stimulation of electrical field potential. The capability to stimulate nerve fibers and record postsynaptic field potentials was demonstrated in an experiment using mouse brain hippocampus slice.

A Materials Roadmap to Functional Neural Interface Design

Advancement in neurotechnologies for electrophysiology, neurochemical sensing, neuromodulation, and optogenetics are revolutionizing scientific understanding of the brain while enabling treatments, cures, and preventative measures for a variety of neurological disorders. The grand challenge in neural interface engineering is to seamlessly integrate the interface between neurobiology and engineered technology, to record from and modulate neurons over chronic timescales. However, the biological inflammatory response to implants, neural degeneration, and long-term material stability diminish the quality of interface overtime. Recent advances in functional materials have been aimed at engineering solutions for chronic neural interfaces. Yet, the development and deployment of neural interfaces designed from novel materials have introduced new challenges that have largely avoided being addressed. Many engineering efforts that solely focus on optimizing individual probe design parameters, such as softness or flexibility, downplay critical multi-dimensional interactions between different physical properties of the device that contribute to overall performance and biocompatibility. Moreover, the use of these new materials present substantial new difficulties that must be addressed before regulatory approval for use in human patients will be achievable. In this review, the interdependence of different electrode components are highlighted to demonstrate the current materials-based challenges facing the field of neural interface engineering.

Connectivity inference from neural recording data: Challenges, mathematical bases and research directions

This article presents a review of computational methods for connectivity inference from neural activity data derived from multi-electrode recordings or fluorescence imaging. We first identify biophysical and technical challenges in connectivity inference along the data processing pipeline. We then review connectivity inference methods based on two major mathematical foundations, namely, descriptive model-free approaches and generative model-based approaches. We investigate representative studies in both categories and clarify which challenges have been addressed by which method. We further identify critical open issues and possible research directions.

Single 5 μm diameter needle electrode block modules for unit recordings in vivo

Investigations into mechanisms in various cortical areas can be greatly improved and supported by stable recording of single neuronal activity. In this study, fine silicon wire electrodes (diameter 3 μm, length 160 μm) are fabricated by vapor–liquid–solid (VLS) growth with the aim of stabilizing recording and reducing the invasiveness on the measurement procedure. The electrode is fabricated on a modular 1 ×  1 mm² conductive silicon block that can be assembled into a number of different device packages, for example on rigid or flexible printed circuit boards (PCB). After plating with a 5 μm diameter platinum black, the needle exhibits an electrical impedance of ~100 kΩ at 1 kHz in saline. The in vivo recording capability of the device is demonstrated using mice, and spike signals with peak-to-peak amplitudes of 200−300 μV in the range 0.5−3 kHz are stably detected, including single-unit activities in cortical layer 2/3. In addition, the device packaged with a flexible PCB shows stable unit recordings for 98.5 min (n = 4). Consequently, our modular, low-invasive needle electrode block devices present an effective route for single-unit recordings in vivo, as well as demonstrating adaptability in device design for a diverse range of experiments.

Portable closed-loop optogenetic stimulation device

This paper presents a closed-loop optogenetic stimulation device to achieve online modulation of neurons. The device is designed to be mountable on small rodents in pre-clinical settings. Considering the size of rodents and the need for portability, a single-piece self-contained device is developed which allows real-time photostimulation based on detected neuronal states. It consists of three components: a neural recorder, a control algorithm, and an optogenetic stimulator. The neural recorder which is realized by analogue circuitry measures the neural signal. The on-off control algorithm analyses the neural signal and controls the stimulation of the target neurons. The optogenetic stimulator performs sampling and digitization of the detected neural signal, runs the control algorithm, and manages the operation of the light source. The configurable neural recorder is capable of 64 dB amplification in the frequency range of 300 Hz to 6 KHz. The outcome of bench testing of the device is reported. The device is portable and headmountable which makes it suitable for use with small rodents in pre-clinical trials.

Nanoscale-Tipped High-Aspect-Ratio Vertical Microneedle Electrodes for Intracellular Recordings

Intracellular recording nanoscale electrode devices provide the advantages of a high spatial resolution and high sensitivity. However, the length of nanowire/nanotube-based nanoelectrodes is currently limited to <10 μm long due to fabrication issues for high-aspect-ratio nanoelectrodes. The concept reported here can address the technological limitations by fabricating >100 μm long nanoscale-tipped electrodes, which show intracellular recording capability.

In vivo optical modulation of neural signals using monolithically integrated two-dimensional neural probe arrays

Integration of stimulation modalities (e.g. electrical, optical, and chemical) on a large array of neural probes can enable an investigation of important underlying mechanisms of brain disorders that is not possible through neural recordings alone. Furthermore, it is important to achieve this integration of multiple functionalities in a compact structure to utilize a large number of the mouse models. Here we present a successful optical modulation of in vivo neural signals of a transgenic mouse through our compact 2D MEMS neural array (optrodes). Using a novel fabrication method that embeds a lower cladding layer in a silicon substrate, we achieved a thin silicon 2D optrode array that is capable of delivering light to multiple sites using SU-8 as a waveguide core. Without additional modification to the microelectrodes, the measured impedance of the multiple microelectrodes was below 1 MΩ at 1 kHz. In addition, with a low background noise level (± 25 μV), neural spikes from different individual neurons were recorded on each microelectrode. Lastly, we successfully used our optrodes to modulate the neural activity of a transgenic mouse through optical stimulation. These results demonstrate the functionality of the 2D optrode array and its potential as a next-generation tool for optogenetic applications.

Dissolvable base scaffolds allow tissue penetration of high-aspect-ratio flexible microneedles

Microscale needle technology is important in electrophysiological studies, drug/chemical delivery systems, optogenetic applications, and so on. In this study, dissolvable needle-base scaffold realizes penetration of high-aspect-ratio flexible microneedles (e.g., <5 μm diameter and >500 μm length) into biological tissues. This methodology, which is applicable to numerous high-aspect-ratio flexible microneedles, should reduce the invasiveness and provide safer tissue penetrations than conventional approaches.

One-touch-activated blood multidiagnostic system using a minimally invasive hollow microneedle integrated with a paper-based sensor

The development of real-time innocuous blood diagnosis has been a long-standing goal in healthcare; an improved, miniature, all-in-one point-of-care testing (POCT) system with low cost and simplified operation is highly desired. Here, we present a one-touch-activated blood multidiagnostic system (OBMS) involving the synergistic integration of a hollow microneedle and paper-based sensor, providing a number of unique characteristics for simplifying the design of microsystems and enhancing user performance. In this OBMS, all functions of blood collection, serum separation, and detection were sequentially automated in one single device that only required one-touch activation by finger-power without additional operations. For the first time, we successfully demonstrated the operation of this system in vivo in glucose and cholesterol diagnosis, showing a great possibility for human clinical application and commercialization. Additionally, this novel system offers a new approach for the use of microneedles and paper sensors as promising intelligent elements in future real-time healthcare monitoring devices.

Microfabrication, characterization and in vivo MRI compatibility of diamond microelectrodes array for neural interfacing

Neural interfacing still requires highly stable and biocompatible materials, in particular for in vivo applications. Indeed, most of the currently used materials are degraded and/or encapsulated by the proximal tissue leading to a loss of efficiency. Here, we considered boron doped diamond microelectrodes to address this issue and we evaluated the performances of a diamond microelectrode array. We described the microfabrication process of the device and discuss its functionalities. We characterized its electrochemical performances by cyclic voltammetry and impedance spectroscopy in saline buffer and observed the typical diamond electrode electrochemical properties, wide potential window and low background current, allowing efficient electrochemical detection. The charge storage capacitance and the modulus of the electrochemical impedance were found to remain in the same range as platinum electrodes used for standard commercial devices. Finally we observed a reduced Magnetic Resonance Imaging artifact when the device was implanted on a rat cortex, suggesting that boron doped-diamond is a very promising electrode material allowing functional imaging.