Optimizing growth and post treatment of diamond for high capacitance neural interfaces

Control of Neuronal Survival and Development Using Conductive Diamond

This study demonstrates the control of neuronal survival and development using nitrogen-doped ultrananocrystalline diamond (N-UNCD). We highlight the role of N-UNCD in regulating neuronal activity via near-infrared illumination, demonstrating the generation of stable photocurrents that enhance neuronal survival and neurite outgrowth and foster a more active, synchronized neuronal network. Whole transcriptome RNA sequencing reveals that diamond substrates improve cellular-substrate interaction by upregulating extracellular matrix and gap junction-related genes. Our findings underscore the potential of conductive diamond as a robust and biocompatible platform for noninvasive and effective neural tissue engineering.

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.

Diamond Supercapacitors: Towards Durable, Safe, and Biocompatible Aqueous-Based Energy Storage

Durable and safe energy storage is required for the next generation of miniature bioelectronic devices, in which aqueous electrolytes are preferred due to the advantages in safety, low cost, and high conductivity. While rechargeable aqueous batteries are among the primary choices with relatively low power requirements, their lifetime is generally limited to a few thousand charging/discharging cycles as the electrode material can degrade due to electrochemical reactions. Electrical double layer capacitors (EDLCs) possess increased cycling stability and power density, although with as-yet lower energy density, due to quick electrical adsorption and desorption of ions without involving chemical reactions. However, in aqueous solution, chemical reactions which cause electrode degradation and produce hazardous species can occur when the voltage is increased beyond its operation window to improve the energy density. Diamond is a durable and biocompatible electrode material for supercapacitors, while at the same time provides a larger voltage window in biological environments. For applications requiring higher energy density, diamond-based pseudocapacitors (PCs) have also been developed, which combine EDLCs with fast electrochemical reactions. Here we inspect the properties of diamond-related materials and discuss their advantages and disadvantages when used as EDLC and PC materials. We argue that further optimization of the diamond surface chemistry and morphology, guided by computational modelling of the interface, can lead to supercapacitors with enhanced performance. We envisage that such diamond-based supercapacitors could be used in a wide range of applications and in particular those requiring high performance in biomedical applications.

Single-Step Fabrication Method toward 3D Printing Composite Diamond-Titanium Interfaces for Neural Applications

Owing to several key attributes, diamond is an attractive candidate material for neural interfacing electrodes. The emergence of additive-manufacturing (AM) of diamond-based materials has addressed multiple challenges associated with the fabrication of diamond electrodes using the conventional chemical vapor deposition (CVD) approach. Unlike the CVD approach, AM methods have enabled the deposition of three-dimensional diamond-based material at room temperature. This work demonstrates the feasibility of using laser metal deposition to fabricate diamond-titanium hybrid electrodes for neuronal interfacing. In addition to exhibiting a high electrochemical capacitance of 1.1 mF cm^-2 and a low electrochemical impedance of 1 kΩ cm² at 1 kHz in physiological saline, these electrodes exhibit a high degree of biocompatibility assessed in vitro using cortical neurons. Furthermore, surface characterization methods show the presence of an oxygen-rich mixed-phase diamond-titanium surface along the grain boundaries. Overall, we demonstrated that our unique approach facilitates printing biocompatible titanium-diamond site-specific coating-free conductive hybrid surfaces using AM, which paves the way to printing customized electrodes and interfacing implantable medical devices.

Advances in Carbon-Based Microfiber Electrodes for Neural Interfacing

Neural interfacing devices using penetrating microelectrode arrays have emerged as an important tool in both neuroscience research and medical applications. These implantable microelectrode arrays enable communication between man-made devices and the nervous system by detecting and/or evoking neuronal activities. Recent years have seen rapid development of electrodes fabricated using flexible, ultrathin carbon-based microfibers. Compared to electrodes fabricated using rigid materials and larger cross-sections, these microfiber electrodes have been shown to reduce foreign body responses after implantation, with improved signal-to-noise ratio for neural recording and enhanced resolution for neural stimulation. Here, we review recent progress of carbon-based microfiber electrodes in terms of material composition and fabrication technology. The remaining challenges and future directions for development of these arrays will also be discussed. Overall, these microfiber electrodes are expected to improve the longevity and reliability of neural interfacing devices.

Laminin coated diamond electrodes for neural stimulation

The performance of many implantable neural stimulation devices is degraded due to the loss of neurons around the electrodes by the body's natural biological responses to a foreign material. Coating of electrodes with biomolecules such as extracellular matrix proteins is one potential route to suppress the adverse responses that lead to loss of implant functionality. Concurrently, however, the electrochemical performance of the stimulating electrode must remain optimal to continue to safely provide sufficient charge for neural stimulation. We have previously found that oxygen plasma treated nitrogen included ultrananocrystalline diamond coated platinum electrodes exhibit superior charge injection capacity and electrochemical stability for neural stimulation (Sikder et al., 2019). To fabricate bioactive diamond electrodes, in this work, laminin, an extracellular matrix protein known to be involved in inter-neuron adhesion and recognition, was used as an example biomolecule. Here, laminin was covalently coupled to diamond electrodes. Electrochemical analysis found that the covalently coupled films were robust and resulted in minimal change to the charge injection capacity of diamond electrodes. The successful binding of laminin and its biological activity was further confirmed using primary rat cortical neuron cultures, and the coated electrodes showed enhanced cell attachment densities and neurite outgrowth. The method proposed in this work is versatile and adaptable to many other biomolecules for producing bioactive diamond electrodes, which are expected to show reduced the inflammatory responses in vivo.

In vivo feasibility of epiretinal stimulation using ultrananocrystalline diamond electrodes

OBJECTIVE

Due to their increased proximity to retinal ganglion cells (RGCs), epiretinal visual prostheses present the opportunity for eliciting phosphenes with low thresholds through direct RGC activation. This study characterised the in vivo performance of a novel prototype monolithic epiretinal prosthesis, containing Nitrogen incorporated ultrananocrystalline (N-UNCD) diamond electrodes.

APPROACH

A prototype implant containing up to twenty-five 120 × 120 µm N-UNCD electrodes was implanted into 16 anaesthetised cats and attached to the retina either using a single tack or via magnetic coupling with a suprachoroidally placed magnet. Multiunit responses to retinal stimulation using charge-balanced biphasic current pulses were recorded acutely in the visual cortex using a multichannel planar array. Several stimulus parameters were varied including; the stimulating electrode, stimulus polarity, phase duration, return configuration and the number of electrodes stimulated simultaneously.

MAIN RESULTS

The rigid nature of the device and its form factor necessitated complex surgical procedures. Surgeries were considered successful in 10/16 animals and cortical responses to single electrode stimulation obtained in eight animals. Clinical imaging and histological outcomes showed severe retinal trauma caused by the device in situ in many instances. Cortical measures were found to significantly depend on the surgical outcomes of individual experiments, phase duration, return configuration and the number of electrodes stimulated simultaneously, but not stimulus polarity. Cortical thresholds were also found to increase over time within an experiment.

SIGNIFICANCE

The study successfully demonstrated that an epiretinal prosthesis containing diamond electrodes could produce cortical activity with high precision, albeit only in a small number of cases. Both surgical approaches were highly challenging in terms of reliable and consistent attachment to and stabilisation against the retina, and often resulted in severe retinal trauma. There are key challenges (device form factor and attachment technique) to be resolved for such a device to progress towards clinical application, as current surgical techniques are unable to address these issues.

Single-step grown boron doped nanocrystalline diamond-carbon nanograss hybrid as an efficient supercapacitor electrode

Direct synthesis of a nano-structured carbon hybrid consisting of vertically aligned carbon nanograsses on top of boron-doped nanocrystalline diamond is demonstrated and the carbon hybrid is further applied as an electrode material for the fabrication of supercapacitors. The hybrid film combines the dual advantages of sp2 (carbon nanograss) and sp3 (nanocrystalline diamond) bonded carbon, possessing not only the excellent electrical characteristics of sp2 carbon but also the exceptional electrochemical stability of sp3 carbon. As a result, the specific capacitance of the as-prepared hybrid material reaches up to 0.4 F cm-2, one of the highest reported in diamond-based supercapacitors. The entire electrochemical results exhibit enhanced electron transfer efficiency with remarkable stability of 95% of capacitance retention even after 10 000 cycles.

In vitro and in vivo biocompatibility and bio-tribological properties of the calcium/amorphous-C composite films for bone tissue engineering application

Carbon-and diamond-like-carbon coated Ti alloys hold great promise for tissue engineering applications. Unfortunately, their strong intrinsic stress leads to the adhesion failure of the films. Herein, a series of a-C films with different Ca content were prepared on Ti₆Al₄V via co-sputtering deposition technology. Homogeneous spherical Ca nanoclusters, with an inner diameter of 2-6 nm, were formed in an amorphous carbon matrix. The addition of Ca induced indistinctive variation in either phase composition or topography. However, the introduction of Ca not only improved the mechanical properties of a-C film but also significantly strengthened its adhesion to osteoblasts. The bio-tribological properties of Ca/a-C films were also assessed using a tribometer in FBS solution. The Ca/a-C films exhibited a low friction coefficient of 0.083 and a low wear rate of 1.02-1.24×10^-6 mm³/Nm. The low coefficient of friction (COF) of the Ca/a-C films indicates their superior mechanical properties, making them the promising target of nanocomposite films used in bio-tribological applications. Well-stretched cells and the developed actin filaments were distinctly observed on the Ca/a-C films in the osteoblast cell adhesion experiments. In addition, the Ca/a-C films promoted cell proliferation and showed high cell viability. After being implanted for 4 weeks, the Ca/a-C implant material still adhered well to the muscle tissue, without inducing hyperergic or inflammatory reactions. Collectively, our results suggest that the Ca/a-C film is an ideal mounting material for bone tissue engineering.

High-performance supercabatteries using graphite@diamond nano-needle capacitor electrodes and redox electrolytes

Supercabatteries have the characteristics of supercapacitors and batteries, namely high power and energy densities as well as long cycle life. To construct them, capacitor electrodes with wide potential windows and/or redox electrolytes are required. Herein, graphite@diamond nano-needles and an aqueous solution of Fe(CN)63-/4- are utilized as the capacitor electrode and the electrolyte, respectively. This diamond capacitor electrode has a nitrogen-doped diamond core and a nano-graphitic shell. In 0.05 M Fe(CN)63-/4- + 1.0 M Na2SO4 aqueous solution, the fabricated supercabattery has a capacitance of 66.65 mF cm-2 at a scan rate of 10 mV s-1. It is stable over 10 000 charge/discharge cycles. The symmetric supercabattery device assembled using a two-electrode system possesses energy and power densities of 10.40 W h kg-1 and 6.96 kW kg-1, respectively. These values are comparable to those of other energy storage devices. Therefore, diamond supercabatteries are promising for many industrial applications.

Research on in vitro and in vivo biocompatibility of the low-friction Ti+C/amorphous carbon gradient multilayer films for hard tissue engineering

Ti+C/amorphous carbon (a-C) gradient multilayer (GM) films are prepared on the Ti-alloy substrates via physical vapor deposition. Transmission electron microscopy revealed that the Ti atoms combine with the a-C film to form a TiC phase in the inner layer and the sputtering current significantly influences the amount of the TiC phase. Further, the mechanical properties of the Ti+C/a-C GM films were obtained using nanoindentation, and the results denoted the significant improvement in the mechanical properties of the a-C film after adding the Ti+C transition layers. The hardness and elastic modulus of the a-C GM films became approximately 31 and 265 GPa, respectively, which were obviously greater than those of the a-C films. The biotribological properties of the a-C GM films in fetal bovine serum (FBS) were verified. The coefficient of friction (COF) and wear rate of the obtained Ti+C/a-C GM film were 0.057 and (1.06-1.24) × 10^-6 mm³/(N m), respectively, which were lower than those of pure a-C and the bare Ti alloy. The excellent mechanical properties of the Ti+C gradient transition layer and the lubricating effect of the FBS medium caused the low COF of the a-C GM films, indicating the potential biotribology applications of the a-C films. The cell apoptosis tests suggested that the a-C GM films promoted cell proliferation and viability. Meanwhile, the a-C-GM-coated implants and muscle tissue combined, and hyperergic and inflammatory reactions were not observed six weeks after implantation. These data indicate that the Ti+C/a-C GM film exhibits good biocompatibility and is an ideal mounting material for bone tissue engineering.

High-Performance Graphene-Fiber-Based Neural Recording Microelectrodes

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

Feasibility of Nitrogen Doped Ultrananocrystalline Diamond Microelectrodes for Electrophysiological Recording From Neural Tissue

Neural prostheses that can monitor the physiological state of a subject are becoming clinically viable through improvements in the capacity to record from neural tissue. However, a significant limitation of current devices is that it is difficult to fabricate electrode arrays that have both high channel counts and the appropriate electrical properties required for neural recordings. In earlier work, we demonstrated nitrogen doped ultrananocrystalline diamond (N-UNCD) can provide efficacious electrical stimulation of neural tissue, with high charge injection capacity, surface stability and biocompatibility. In this work, we expand on this functionality to show that N-UNCD electrodes can also record from neural tissue owing to its low electrochemical impedance. We show that N-UNCD electrodes are highly flexible in their application, with successful recordings of action potentials from single neurons in an in vitro retina preparation, as well as local field potential responses from in vivo visual cortex tissue. Key properties of N-UNCD films, combined with scalability of electrode array fabrication with custom sizes for recording or stimulation along with integration through vertical interconnects to silicon based integrated circuits, may in future form the basis for the fabrication of versatile closed-loop neural prostheses that can both record and stimulate.

Ultrananocrystalline diamond-coated nanoporous membranes support SK-N-SH neuroblastoma epithelial [corrected] cell attachment

Ultrananocrystalline diamond (UNCD) has been demonstrated to have attractive features for biomedical applications and can be combined with nanoporous membranes for applications in drug delivery systems, biosensing, immunoisolation and single molecule analysis. In this study, free-standing nanoporous UNCD membranes with pore sizes of 100 or 400 nm were fabricated by directly depositing ultrathin UNCD films on nanoporous silicon nitride membranes and then etching away silicon nitride using reactive ion etching. Successful deposition of UNCD on the substrate with a novel process was confirmed with Raman spectroscopy, X-ray photoelectron spectroscopy, cross-section scanning electron microscopy (SEM) and transmission electron microscopy. Both sample types exhibited uniform geometry and maintained a clear hexagonal pore arrangement. Cellular attachment of SK-N-SH neuroblastoma endothelial cells was examined using confocal microscopy and SEM. Attachment of SK-N-SH cells onto UNCD membranes on both porous regions and solid surfaces was shown, indicating the potential use of UNCD membranes in biomedical applications such as biosensors and tissue engineering scaffolds.

Polycrystalline Diamond Coating of Additively Manufactured Titanium for Biomedical Applications

Additive manufacturing using selective laser melted titanium (SLM-Ti) is used to create bespoke items across many diverse fields such as medicine, defense, and aerospace. Despite great progress in orthopedic implant applications, such as for "just in time" implants, significant challenges remain with regards to material osseointegration and the susceptibility to bacterial colonization on the implant. Here, we show that polycrystalline diamond coatings on these titanium samples can enhance biological scaffold interaction improving medical implant applicability. The highly conformable coating exhibited excellent bonding to the substrate. Relative to uncoated SLM-Ti, the diamond coated samples showed enhanced mammalian cell growth, enriched apatite deposition, and reduced microbial S. aureus activity. These results open new opportunities for novel coatings on SLM-Ti devices in general and especially show promise for improved biomedical implants.

Charge injection based electrical stimulation on polypyrrole planar electrodes to regulate cellular osteogenic differentiation

This study reveals that the Q_inj on electrodes is a more significant factor than applied voltage for electrical stimulation to regulate cellular osteogenic differentiation, and the charge injection capacity can be tuned by thickness of Ppy.

Diamond thin films: giving biomedical applications a new shine

Progress made in the last two decades in chemical vapour deposition technology has enabled the production of inexpensive, high-quality coatings made from diamond to become a scientific and commercial reality. Two properties of diamond make it a highly desirable candidate material for biomedical applications: first, it is bioinert, meaning that there is minimal immune response when diamond is implanted into the body, and second, its electrical conductivity can be altered in a controlled manner, from insulating to near-metallic. In vitro, diamond can be used as a substrate upon which a range of biological cells can be cultured. In vivo, diamond thin films have been proposed as coatings for implants and prostheses. Here, we review a large body of data regarding the use of diamond substrates for in vitro cell culture. We also detail more recent work exploring diamond-coated implants with the main targets being bone and neural tissue. We conclude that diamond emerges as one of the major new biomaterials of the twenty-first century that could shape the way medical treatment will be performed, especially when invasive procedures are required.

Planar Diamond-Based Multiarrays to Monitor Neurotransmitter Release and Action Potential Firing: New Perspectives in Cellular Neuroscience

High biocompatibility, outstanding electrochemical responsiveness, inertness, and transparency make diamond-based multiarrays (DBMs) first-rate biosensors for in vitro detection of electrochemical and electrical signals from excitable cells together, with potential for in vivo applications as neural interfaces and prostheses. Here, we will review the electrochemical and physical properties of various DBMs and how these devices have been employed for recording released neurotransmitter molecules and all-or-none action potentials from living cells. Specifically, we will overview how DBMs can resolve localized exocytotic events from subcellular compartments using high-density microelectrode arrays (MEAs), or monitoring oxidizable neurotransmitter release from populations of cells in culture and tissue slices using low-density MEAs. Interfacing DBMs with excitable cells is currently leading to the promising opportunity of recording electrical signals as well as creating neuronal interfaces through the same device. Given the recent increasingly growing development of newly available DBMs of various geometries to monitor electrical activity and neurotransmitter release in a variety of excitable and neuronal tissues, the discussion will be limited to planar DBMs.