Analysis of the capacitance of minimally insulated parallel wires implanted in biological tissue.

State of the art bioelectronic implants are using thin cables for therapeutic electrical stimulation. If cable insulation is thin, biological tissue surrounding cables can be unintentionally stimulated. The capacitance of the cable must be much less than the stimulating electrodes to ensure stimulating currents are delivered to the electrode-tissue interface. This work derives and experimentally validates a model to determine the capacitance of parallel cables implanted in biological tissue. Biological tissue has a high relative permittivity, so the capacitance of cabling implanted in the human body depends on cable insulation thickness. Simulations and measurements demonstrate that insulation thickness influences the capacitance of implanted parallel cables across almost two orders of magnitude: from 20 pF/m to 700 pF/m. The results are verified using four different methods: solving the Laplacian numerically from first principles, using a commercially available electrostatic solver, and measuring twelve different parallel pairs of wires using two different potentiostats. Cable capacitance simulations and measurements are performed in air, a porcine blood pool and porcine muscle tissue. The results do not differ by more than 30% for a given cable across simulation and measurement methodologies. The modelling in this work can be used to design cabling for minimally-invasive biomedical implants.

Electrical receptive fields of retinal ganglion cells: Influence of presynaptic neurons.

Implantable retinal stimulators activate surviving neurons to restore a sense of vision in people who have lost their photoreceptors through degenerative diseases. Complex spatial and temporal interactions occur in the retina during multi-electrode stimulation. Due to these complexities, most existing implants activate only a few electrodes at a time, limiting the repertoire of available stimulation patterns. Measuring the spatiotemporal interactions between electrodes and retinal cells, and incorporating them into a model may lead to improved stimulation algorithms that exploit the interactions. Here, we present a computational model that accurately predicts both the spatial and temporal nonlinear interactions of multi-electrode stimulation of rat retinal ganglion cells (RGCs). The model was verified using in vitro recordings of ON, OFF, and ON-OFF RGCs in response to subretinal multi-electrode stimulation with biphasic pulses at three stimulation frequencies (10, 20, 30 Hz). The model gives an estimate of each cell's spatiotemporal electrical receptive fields (ERFs); i.e., the pattern of stimulation leading to excitation or suppression in the neuron. All cells had excitatory ERFs and many also had suppressive sub-regions of their ERFs. We show that the nonlinearities in observed responses arise largely from activation of presynaptic interneurons. When synaptic transmission was blocked, the number of sub-regions of the ERF was reduced, usually to a single excitatory ERF. This suggests that direct cell activation can be modeled accurately by a one-dimensional model with linear interactions between electrodes, whereas indirect stimulation due to summated presynaptic responses is nonlinear.

Wireless induction coils embedded in diamond for power transfer in medical implants.

Wireless power and data transfer to medical implants is a research area where improvements in current state-of-the-art technologies are needed owing to the continuing efforts for miniaturization. At present, lithographical patterning of evaporated metals is widely used for miniature coil fabrication. This method produces coils that are limited to low micron or nanometer thicknesses leading to high impedance values and thus limiting their potential quality. In the present work we describe a novel technique, whereby trenches were milled into a diamond substrate and filled with silver active braze alloy, enabling the manufacture of small, high cross-section, low impedance microcoils capable of transferring up to 10 mW of power up to a distance of 6 mm. As a substitute for a metallic braze line used for hermetic sealing, a continuous metal loop when placed parallel and close to the coil surface reduced power transfer efficiency by 43%, but not significantly, when placed perpendicular to the microcoil surface. Encapsulation of the coil by growth of a further layer of diamond reduced the quality factor by an average of 38%, which can be largely avoided by prior oxygen plasma treatment. Furthermore, an accelerated ageing test after encapsulation showed that these coils are long lasting. Our results thus collectively highlight the feasibility of fabricating a high-cross section, biocompatible and long lasting miniaturized microcoil that could be used in either a neural recording or neuromuscular stimulation device.

A Simple and Accurate Model to Predict Responses to Multi-electrode Stimulation in the Retina.

Implantable electrode arrays are widely used in therapeutic stimulation of the nervous system (e.g. cochlear, retinal, and cortical implants). Currently, most neural prostheses use serial stimulation (i.e. one electrode at a time) despite this severely limiting the repertoire of stimuli that can be applied. Methods to reliably predict the outcome of multi-electrode stimulation have not been available. Here, we demonstrate that a linear-nonlinear model accurately predicts neural responses to arbitrary patterns of stimulation using in vitro recordings from single retinal ganglion cells (RGCs) stimulated with a subretinal multi-electrode array. In the model, the stimulus is projected onto a low-dimensional subspace and then undergoes a nonlinear transformation to produce an estimate of spiking probability. The low-dimensional subspace is estimated using principal components analysis, which gives the neuron's electrical receptive field (ERF), i.e. the electrodes to which the neuron is most sensitive. Our model suggests that stimulation proportional to the ERF yields a higher efficacy given a fixed amount of power when compared to equal amplitude stimulation on up to three electrodes. We find that the model captures the responses of all the cells recorded in the study, suggesting that it will generalize to most cell types in the retina. The model is computationally efficient to evaluate and, therefore, appropriate for future real-time applications including stimulation strategies that make use of recorded neural activity to improve the stimulation strategy.

Development of a Magnetic Attachment Method for Bionic Eye Applications.

Successful visual prostheses require stable, long-term attachment. Epiretinal prostheses, in particular, require attachment methods to fix the prosthesis onto the retina. The most common method is fixation with a retinal tack; however, tacks cause retinal trauma, and surgical proficiency is important to ensure optimal placement of the prosthesis near the macula. Accordingly, alternate attachment methods are required. In this study, we detail a novel method of magnetic attachment for an epiretinal prosthesis using two prostheses components positioned on opposing sides of the retina. The magnetic attachment technique was piloted in a feline animal model (chronic, nonrecovery implantation). We also detail a new method to reliably control the magnet coupling force using heat. It was found that the force exerted upon the tissue that separates the two components could be minimized as the measured force is proportionately smaller at the working distance. We thus detail, for the first time, a surgical method using customized magnets to position and affix an epiretinal prosthesis on the retina. The position of the epiretinal prosthesis is reliable, and its location on the retina is accurately controlled by the placement of a secondary magnet in the suprachoroidal location. The electrode position above the retina is less than 50 microns at the center of the device, although there were pressure points seen at the two edges due to curvature misalignment. The degree of retinal compression found in this study was unacceptably high; nevertheless, the normal structure of the retina remained intact under the electrodes.

Formation of thick aminophenyl films from aminobenzenediazonium ion in the absence of a reduction source.

Aminophenyl films, electrografted to conducting substrates from a solution of the corresponding diazonium ion, are a useful platform for building up functional surfaces. In our hands, reproducible preparation of aminophenyl films via electrografting is difficult, suggesting competing grafting pathways. To investigate the grafting process without the possibility of reduction of the diazonium ion by the substrate, we have used a spin-coated and cured SU-8 substrate that is nonconducting and very smooth (rms surface roughness 0.43 nm). After in situ formation of the aminobenzenediazonium ion (50 mM) in acidic solution, the substrate was added to the solution in the presence and absence of reducing agents (hypophosphorous acid and iron powder). At short reaction times, the films prepared with and without reducing agent have the same thickness and composition (as revealed by X-ray photoelectron spectroscopy). However, in the presence of a reducing agent, films reach a limiting thickness of 7-8 nm after 10 min, whereas, in the absence of a reducing agent, strong film growth continues, giving a film thickness of 14 nm after 120 min. This behavior contrasts with that of other diazonium ions which, in the absence of an applied potential, a reducing agent, or a reducing substrate, give only very thin films after long reaction times.

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.

Patterned forests of vertically-aligned multiwalled carbon nanotubes using metal salt catalyst solutions.

A simple method for producing patterned forests of multiwalled carbon nanotubes (MWCNTs) is described. An aqueous metal salt solution is spin-coated onto a substrate patterned with photoresist by standard methods. The photoresist is removed by acetone washing leaving the acetone-insoluble catalyst pattern on the substrate. Dense forests of vertically aligned (VA) MWCNTs are grown on the patterned catalyst layers by chemical vapour deposition. The procedures have been demonstrated by growing MWCNT forests on two substrates: silicon and conducting graphitic carbon films. The forests adhere strongly to the substrates and when grown directly on carbon film, offer a simple method of preparing MWCNT electrodes.

Spread of activation and interaction between channels with multi-channel optogenetic stimulation in the mouse cochlea.

For individuals with severe to profound hearing loss resulting from irreversibly damaged hair cells, cochlear implants can be used to restore hearing by delivering electrical stimulation directly to the spiral ganglion neurons. However, current spread lowers the spatial resolution of neural activation. Since light can be easily confined, optogenetics is a technique that has the potential to improve the precision of neural activation, whereby visible light is used to stimulate neurons that are modified with light-sensitive opsins. This study compares the spread of neural activity across the inferior colliculus of the auditory midbrain during electrical and optical stimulation in the cochlea of acutely deafened mice with opsin-modified spiral ganglion neurons (H134R variant of the channelrhodopsin-2). Monopolar electrical stimulation was delivered via each of four 0.2 mm wide platinum electrode rings at 0.6 mm centre-to-centre spacing, whereas 453 nm wavelength light was delivered via each of five 0.22 × 0.27 mm micro-light emitting diodes (LEDs) at 0.52 mm centre-to-centre spacing. Channel interactions were also quantified by threshold changes during simultaneous stimulation by pairs of electrodes or micro-LEDs at different distances between the electrodes (0.6, 1.2 and 1.8 mm) or micro-LEDs (0.52, 1.04, 1.56 and 2.08 mm). The spread of activation resulting from single channel optical stimulation was approximately half that of monopolar electrical stimulation as measured at two levels of discrimination above threshold (p<0.001), whereas there was no significant difference between optical stimulation in opsin-modified deafened mice and pure tone acoustic stimulation in normal-hearing mice. During simultaneous micro-LED stimulation, there were minimal channel interactions for all micro-LED spacings tested. For neighbouring micro-LEDs/electrodes, the relative influence on threshold was 13-fold less for optical stimulation compared electrical stimulation (p<0.05). The outcomes of this study show that the higher spatial precision of optogenetic stimulation results in reduced channel interaction compared to electrical stimulation, which could increase the number of independent channels in a cochlear implant. Increased spatial resolution and the ability to activate more than one channel simultaneously could lead to better speech perception in cochlear implant recipients.

Reproducible fabrication of robust, renewable vertically aligned multiwalled carbon nanotube/epoxy composite electrodes.

We describe the reproducible fabrication of robust, vertically aligned multiwalled carbon nanotube (VACNT)/epoxy composite electrodes. The electrodes are characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopies. Low background currents are obtained at the electrodes, and common redox probe molecules and NADH show excellent voltammetric behavior. When electrode performance deteriorates due to fouling, the electrode surfaces can be reproducibly renewed by mechanical polishing followed by O(2) plasma treatment. The electrochemical performance of the electrodes is maintained after more than 100 cycles of use and renewal.

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.

Patterning of metal, carbon, and semiconductor substrates with thin organic films by microcontact printing with aryldiazonium salt inks.

Surface modification through reduction of aryldiazonium salts to give covalently attached layers is a widely investigated procedure. However, realization of potential applications of the layers requires development of patterning methods. Here, we demonstrate that microcontact printing with poly(dimethylsiloxane) stamps inked with aqueous acid solutions of aryldiazonium salts gives stable organic layers on gold, copper, silicon, and graphitic carbon surfaces. Depending on the substrate-diazonium salt combination, the layers range from relatively irregular multilayers to smooth films with close to monolayer thickness. After printing, surface attached aminophenyl and carboxyphenyl groups retain their usual reactivity toward amide bond formation with solution species, and hence, the method is a simple route to patterned, covalently attached, reactive tether layers. Multicomponent patterned films can be prepared by printing a second modifier onto a film-coated surface. Microcontact printing using aryldiazonium salt inks is experimentally very simple and is applicable to the broad range of substrates capable of spontaneously reducing aryldiazonium salts.

Robust forests of vertically aligned carbon nanotubes chemically assembled on carbon substrates.

Forests of vertically aligned carbon nanotubes (VACNTs) have been chemically assembled on carbon surfaces. The structures show excellent stability over a wide potential range and are resistant to degradation from sonication in acid, base, and organic solvent. Acid-treated single-walled carbon nanotubes (SWCNTs) were assembled on amine-terminated tether layers covalently attached to pyrolyzed photoresist films. Tether layers were electrografted to the carbon substrate by reduction of the p-aminobenzenediazonium cation and oxidation of ethylenediamine. The amine-modified surfaces were incubated with cut SWCNTs in the presence of N,N'-dicyclohexylcarbodiimide (DCC), giving forests of vertically aligned carbon nanotubes (VACNTs). The SWCNT assemblies were characterized by scanning electron microscopy, atomic force microscopy, and electrochemistry. Under conditions where the tether layers slow electron transfer between solution-based redox probes and the underlying electrode, the assembly of VACNTs on the tether layer dramatically increases the electron-transfer rate at the surface. The grafting procedure, and hence the preparation of VACNTs, is applicable to a wide range of materials including metals and semiconductors.

Stimulation Strategies for Improving the Resolution of Retinal Prostheses.

Electrical stimulation using implantable devices with arrays of stimulating electrodes is an emerging therapy for neurological diseases. The performance of these devices depends greatly on their ability to activate populations of neurons with high spatiotemporal resolution. To study electrical stimulation of populations of neurons, retina serves as a useful model because the neural network is arranged in a planar array that is easy to access. Moreover, retinal prostheses are under development to restore vision by replacing the function of damaged light sensitive photoreceptors, which makes retinal research directly relevant for curing blindness. Here we provide a progress review on stimulation strategies developed in recent years to improve the resolution of electrical stimulation in retinal prostheses. We focus on studies performed with explanted retinas, in which electrophysiological techniques are the most advanced. We summarize achievements in improving the spatial and temporal resolution of electrical stimulation of the retina and methods to selectively stimulate neurons with different visual functions. Future directions for retinal prostheses development are also discussed, which could provide insights for other types of neuromodulatory devices in which high-resolution electrical stimulation is required.

3D Diamond Electrode Array for High-Acuity Stimulation in Neural Tissue.

Innovations in micro- and nanofabrication technologies enable the manufacture of multielectrode arrays for use in neuromodulation and neural recording. Multielectrode arrays make possible medical implants such as pacemakers, deep-brain stimulators, or visual and hearing aids, to treat numerous neural disorders. An optimal neural interface requires a high density of electrodes to precisely record from and stimulate the nervous system while minimizing the overall size of the array. For example, people with retinal degenerative diseases can benefit from retinal prostheses implanted inside the eye. However, at present the visual acuity provided by such implants is well below the threshold for functional vision, mainly due to the limited spatial resolution. In this work, we present a design of 3D nanostructured conductive diamond electrodes, integrated within a polycrystalline diamond housing, offering a high electrode density and count, which simultaneously satisfies spatial resolution and biocompatibility goals. The array is composed of height adjustable pillar electrodes that are 80 µm in diameter and separated by 150 µm. A holistic characterization of the electrodes was performed and the device tested for stimulation performance in a whole-mounted retina. Electrochemical testing showed impedance of 20 kΩ and a wide water window of 2.47 V. The pillar structure allows the distance between the electrodes and the retinal ganglion cells to be reduced which is key to more confined stimulation at lower current levels, leading to potentially higher-acuity stimulation without damaging retinal tissue.

Minimizing axon bundle activation of retinal ganglion cells with oriented rectangular electrodes.

OBJECTIVE: Retinal prostheses aim to restore vision in patients with retinal degenerative diseases, such as age-related macular degeneration and retinitis pigmentosa. By implanting an array of microelectrodes, such a device creates percepts in patients through electrical stimulation of surviving retinal neurons. A challenge for retinal prostheses when trying to return high quality vision is the unintended activation of retinal ganglion cells through the stimulation of passing axon bundles, which leads to patients reporting large, elongated patches of light instead of focal spots. APPROACH: In this work, we used calcium imaging to record the responses of retinal ganglion cells to electrical stimulation in explanted retina using rectangular electrodes placed with different orientations relative to the axon bundles. MAIN RESULTS: We showed that narrow, rectangular electrodes oriented parallel to the axon bundles can achieve focal stimulation. To further improve the strategy, we studied the impact of different stimulation waveforms and electrode configurations. We found the selectivity for focal stimulation to be higher when using short (33 µs), anodic-first biphasic pulses, with long electrode lengths and at least 50 µm electrode-to-retinal separation. Focal stimulation was, in fact, less selective when the electrodes made direct contact with the retinal surface due to unwanted preferential stimulation of the proximal axon bundles. SIGNIFICANCE: When employed in retinal prostheses, the proposed stimulation strategy is expected to provide improved quality of vision to the blind.

Hybrid diamond/ carbon fiber microelectrodes enable multimodal electrical/chemical neural interfacing.

Implantable medical devices are now in regular use to treat or ameliorate medical conditions, including movement disorders, chronic pain, cardiac arrhythmias, and hearing or vision loss. Aside from offering alternatives to pharmaceuticals, one major advantage of device therapy is the potential to monitor treatment efficacy, disease progression, and perhaps begin to uncover elusive mechanisms of diseases pathology. In an ideal system, neural stimulation, neural recording, and electrochemical sensing would be conducted by the same electrode in the same anatomical region. Carbon fiber (CF) microelectrodes are the appropriate size to achieve this goal and have shown excellent performance, in vivo. Their electrochemical properties, however, are not suitable for neural stimulation and electrochemical sensing. Here, we present a method to deposit high surface area conducting diamond on CF microelectrodes. This unique hybrid microelectrode is capable of recording single-neuron action potentials, delivering effective electrical stimulation pulses, and exhibits excellent electrochemical dopamine detection. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.

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.

Improved visual acuity using a retinal implant and an optimized stimulation strategy.

OBJECTIVE: Retinal prosthetic devices hold great promise for the treatment of retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Through electrical stimulation of the surviving retinal neurons, these devices evoke visual signals that are then relayed to the brain. Currently, the visual prostheses used in clinical trials have few electrodes, thus limiting visual acuity. Electrode arrays with high electrode densities have been developed using novel technologies, including diamond growth and laser machining, and these may provide a more promising route to achieve high visual acuity in blind patients. APPROACH: Here, we studied the potential spatial resolution of electrical stimulation using diamond electrodes. We did this by labeling retinal ganglion cells in whole mount retina with a calcium indicator in wild-type rats and those with retinal degeneration. We imaged the ganglion cell responses to a range of stimulation parameters, including pulse duration and return electrode configuration. MAIN RESULTS: With sub-retinal stimulation, in which electrodes were in contact with the intact or degenerated photoreceptor layer, we found that biphasic pulses of 0.1 ms phase duration and a local return configuration was the most effective in confining the retinal ganglion cell activation patterns, while also remaining within the safety limits of the materials and providing the best power efficiency. SIGNIFICANCE: These results provide an optimized stimulation strategy for retinal implants, which if implemented in a retinal prosthetic is expected to improve the achievable visual acuity.