Retinal prosthesis

Whole-eye transplantation: Current challenges and future perspectives

Whole-eye transplantation emerges as a frontier in ophthalmology, promising a transformative approach to irreversible blindness. Despite advancements, formidable challenges persist. Preservation of donor eye viability post-enucleation necessitates meticulous surgical techniques to optimize retinal integrity and ganglion cell survival. Overcoming the inhibitory milieu of the central nervous system for successful optic nerve regeneration remains elusive, prompting the exploration of neurotrophic support and immunomodulatory interventions. Immunological tolerance, paramount for graft acceptance, confronts the distinctive immunogenicity of ocular tissues, driving research into targeted immunosuppression strategies. Ethical and legal considerations underscore the necessity for stringent standards and ethical frameworks. Interdisciplinary collaboration and ongoing research endeavors are imperative to navigate these complexities. Biomaterials, stem cell therapies, and precision immunomodulation represent promising avenues in this pursuit. Ultimately, the aim of this review is to critically assess the current landscape of whole-eye transplantation, elucidating the challenges and advancements while delineating future directions for research and clinical practice. Through concerted efforts, whole-eye transplantation stands to revolutionize ophthalmic care, offering hope for restored vision and enhanced quality of life for those afflicted with blindness.

New frontiers of retinal therapeutic intervention: a critical analysis of novel approaches

A recent wave of pharmacologic and technologic innovations has revolutionized our management of retinal diseases. Many of these advancements have demonstrated efficacy and can increase the quality of life while potentially reducing complications and decreasing the burden of care for patients. Some advances, such as longer-acting anti-vascular endothelial growth factor agents, port delivery systems, gene therapy, and retinal prosthetics have been approved by the US Food and Drug Administration, and are available for clinical use. Countless other therapeutics are in various stages of development, promising a bright future for further improvements in the management of the retinal disease. Herein, we have highlighted several important novel therapies and therapeutic approaches and examine the opportunities and limitations offered by these innovations at the new frontier. KEY MESSAGESNumerous pharmacologic and technologic advancements have been emerging, providing a higher treatment efficacy while decreasing the burden and associated side effects.Anti-vascular endothelial growth factor (anti-VEGF) and its longer-acting agents have dramatically improved visual outcomes and have become a mainstay treatment in various retinal diseases.Gene therapy and retinal prosthesis implantation in the treatment of congenital retinal dystrophy can accomplish the partial restoration of vision and improved daily function in patients with blindness, an unprecedented success in the field of retina.

Retinal ganglion cell desensitization is mitigated by varying parameter constant excitation pulse trains

Retinal prostheses partially restore vision in patients blinded by retinitis pigmentosa (RP) and age-related macular degeneration (AMD). One issue that limits the effectiveness of retinal stimulation is the desensitization of the retina response to repeated pulses. Rapid fading of percepts is reported in clinical studies. We studied the retinal output evoked by fixed pulse trains vs. pulse trains that have variable parameters pulse-to-pulse. We used the current clamp to record RGC spiking in the isolated mouse retina. Trains of biphasic current pulses at different frequencies and amplitudes were applied. The main results we report are: (1) RGC desensitization was induced by increasing stimulus frequency, but was unrelated to stimulus amplitude. Desensitization persisted when the 20 Hz stimulation pulses were applied to the retinal ganglion cells at 65 μA, 85 μA, and 105 μA. Subsequent pulses in the train evoked fewer spikes. There was no obvious desensitization when 2 Hz stimulation pulse trains were applied. (2) Blocking inhibitory GABA_A receptor increased spontaneous activity but did not reduce desensitization. (3) Pulse trains with constant charge or excitation (based on strength-duration curves) but varying pulse width, amplitude, and shape increased the number of evoked spikes/pulse throughout the pulse train. This suggests that retinal desensitization can be partially overcome by introducing variability into each pulse.

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

Selective Activation of Retinal Ganglion Cell Subtypes Through Targeted Electrical Stimulation Parameters

To restore vision to the low vision, epiretinal implants have been developed to electrically stimulate the healthy retinal ganglion cells (RGCs) in the degenerate retina. Given the diversity of retinal ganglion cells as well as the difference in their visual function, selective activation of RGCs subtypes can significantly improve the quality of the restored vision. Our recent results demonstrated that with the proper modulation of the current amplitude, small D1-bistratified cells with the contribution to blue/yellow color opponent pathway can be selectively activated at high frequency (200 Hz). The computational results correlated with the clinical findings revealing the blue sensation of 5/7 subjects with epiretinal implants at high frequency. Here we further explored the impacts of alterations in pulse duration and interphase gap on the response of RGCs at high frequency. We used the developed RGCs, A2-monostratified and D1-bistratified, and examined their response to a range of pulse durations (0.1−1.2 ms) and interphase gaps (0−1 ms). We found that the use of short pulse durations with no interphase gap at high frequency increases the differential response of RGCs, offering better opportunities for selective activation of D1 cells. The presence of the interphase gap has shown to reduce the overall differential response of RGCs. We also explored how the low density of calcium channels enhances the responsiveness of RGCs at high frequency.

Mechanisms underlying activation of retinal bipolar cells through targeted electrical stimulation: a computational study

Objective. Retinal implants have been developed to electrically stimulate healthy retinal neurons in the progressively degenerated retina. Several stimulation approaches have been proposed to improve the visual percept induced in patients with retinal prostheses. We introduce a computational model capable of simulating the effects of electrical stimulation on retinal neurons. Leveraging this computational platform, we delve into the underlying mechanisms influencing the sensitivity of retinal neurons' response to various stimulus waveforms.Approach. We implemented a model of spiking bipolar cells (BCs) in the magnocellular pathway of the primate retina, diffuse BC subtypes (DB4), and utilized our multiscale admittance method (AM)-NEURON computational platform to characterize the response of BCs to epiretinal electrical stimulation with monophasic, symmetric, and asymmetric biphasic pulses.Main results. Our investigations yielded four notable results: (a) the latency of BCs increases as stimulation pulse duration lengthens; conversely, this latency decreases as the current amplitude increases. (b) Stimulation with a long anodic-first symmetric biphasic pulse (duration > 8 ms) results in a significant decrease in spiking threshold compared to stimulation with similar cathodic-first pulses (from 98.2 to 57.5µA). (c) The hyperpolarization-activated cyclic nucleotide-gated channel was a prominent contributor to the reduced threshold of BCs in response to long anodic-first stimulus pulses. (d) Finally, extending the study to asymmetric waveforms, our results predict a lower BCs threshold using asymmetric long anodic-first pulses compared to that of asymmetric short cathodic-first stimulation.Significance. This study predicts the effects of several stimulation parameters on spiking BCs response to electrical stimulation. Of importance, our findings shed light on mechanisms underlying the experimental observations from the literature, thus highlighting the capability of the methodology to predict and guide the development of electrical stimulation protocols to generate a desired biological response, thereby constituting an ideal testbed for the development of electroceutical devices.

Automatic Identification of Axon Bundle Activation for Epiretinal Prosthesis

Objective:

Retinal prostheses must be able to activate cells in a selective way in order to restore high-fidelity vision. However, inadvertent activation of far-away retinal ganglion cells (RGCs) through electrical stimulation of axon bundles can produce irregular and poorly controlled percepts, limiting artificial vision. In this work, we aim to provide an algorithmic solution to the problem of detecting axon bundle activation with a bi-directional epiretinal prostheses.

Methods:

The algorithm utilizes electrical recordings to determine the stimulation current amplitudes above which axon bundle activation occurs. Bundle activation is defined as the axonal stimulation of RGCs with unknown soma and receptive field locations, typically beyond the electrode array. The method exploits spatiotemporal characteristics of electrically-evoked spikes to overcome the challenge of detecting small axonal spikes.

Results:

The algorithm was validated using large-scale, single-electrode and short pulse, ex vivo stimulation and recording experiments in macaque retina, by comparing algorithmically and manually identified bundle activation thresholds. For 88% of the electrodes analyzed, the threshold identified by the algorithm was within ±10% of the manually identified threshold, with a correlation coefficient of 0.95.

Conclusion:

This works presents a simple, accurate and efficient algorithm to detect axon bundle activation in epiretinal prostheses.

Significance:

The algorithm could be used in a closed-loop manner by a future epiretinal prosthesis to reduce poorly controlled visual percepts associated with bundle activation. Activation of distant cells via axonal stimulation will likely occur in other types of retinal implants and cortical implants, and the method may therefore be broadly applicable.

Spatially patterned bi-electrode epiretinal stimulation for axon avoidance at cellular resolution

Objective.Epiretinal prostheses are designed to restore vision to people blinded by photoreceptor degenerative diseases by stimulating surviving retinal ganglion cells (RGCs), which carry visual signals to the brain. However, inadvertent stimulation of RGCs at their axons can result in non-focal visual percepts, limiting the quality of artificial vision. Theoretical work has suggested that axon activation can be avoided with current stimulation designed to minimize the second spatial derivative of the induced extracellular voltage along the axon. However, this approach has not been verified experimentally at the resolution of single cells.Approach.In this work, a custom multi-electrode array (512 electrodes, 10μm diameter, 60μm pitch) was used to stimulate and record RGCs in macaque retinaex vivoat single-cell, single-spike resolution. RGC activation thresholds resulting from bi-electrode stimulation, which consisted of bipolar currents simultaneously delivered through two electrodes straddling an axon, were compared to activation thresholds from traditional single-electrode stimulation.Main results.On average, across three retinal preparations, the bi-electrode stimulation strategy reduced somatic activation thresholds (∼21%) while increasing axonal activation thresholds (∼14%), thus favoring selective somatic activation. Furthermore, individual examples revealed rescued selective activation of somas that was not possible with any individual electrode.Significance.This work suggests that a bi-electrode epiretinal stimulation strategy can reduce inadvertent axonal activation at cellular resolution, for high-fidelity artificial vision.

Improved performance and safety from Argus II retinal prosthesis post-approval study in France

PURPOSE

To evaluate the post-approval long-term outcomes of the Argus II Retinal Prosthesis, with a specific focus on its functional visual benefit in patients' daily activities.

METHODS

Eighteen patients with bare light perception due to end-stage retinitis pigmentosa were included in a French prospective, multicentre, single-arm study and followed for 2 years. Visual benefit in patients' daily activities was monitored through the use of the Functional Low-vision Observer Rated Assessment (FLORA), and the final score at 2 years was the primary effectiveness outcome. Standardized visual assessments were also performed. Device- or procedure-related adverse events were recorded.

RESULTS

Seventeen subjects completed the study. Positive impacts of the Argus II system on functional vision and well-being were demonstrated for over 70% of subjects on the FLORA. Among the daily activities/tasks tested, finding doorways was one of the most statistically significantly improved tasks (p < 0.001), along with estimating the size of an obstacle (p < 0.001), visually locating a place setting on a dining table (p < 0.001) and visually locating people in a non-crowded setting (p < 0.001). Visual function was improved on most standardized tests. Only two device- or procedure-related serious adverse events were observed (one vitreous haemorrhage and one endophthalmitis, both resolved with treatment). No explantation was required.

CONCLUSION

This first report of a completed post-approval study of Argus II with a two-year follow-up demonstrates the safety and effectiveness of the Argus II System in a real-world cohort of patients and further highlights its real functional benefit in implanted patients' daily activities.

Modeling ON Cone Bipolar Cells for Electrical Stimulation

Retinal prosthetic systems have been developed to help blind patients suffering from retinal degenerative diseases gain some useful form of vision. Various experimental and computational studies have been performed to test electrical stimulation strategies that can improve the performance of these devices. Detailed computational models of retinal neurons, such as retinal ganglion cells (RGCs) and bipolar cells (BCs), allow us to explore the mechanisms underlying the response of cells to electrical stimulation. While electrophysiological studies have shown the presence of voltage-gated ionic channels in different regions of BCs, many of the existing cone BCs models are assumed to be passive or only contain calcium channels at the synaptic terminals. We have utilized our Admittance Method (AM)-NEURON computational platform to implement a more realistic model of ON-BCs. Our model closely replicates the recent patch-clamp experiments directly measuring the response of ON-BCs to epiretinal electrical stimulation and thereby predicts the regional distributions of the ionic channels. Our computational results further indicate that outward potassium current strongly contributes to the depolarizing voltage transient of ON-BCs in response to electrical stimulation.

The role of gap junctions in cell death and neuromodulation in the retina

Vision altering diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, myopia, retinal vascular disease, traumatic brain injuries and others cripple many lives and are projected to continue to cause anguish in the foreseeable future. Gap junctions serve as an emerging target for neuromodulation and possible regeneration as they directly connect healthy and/or diseased cells, thereby playing a crucial role in pathophysiology. Since they are permeable for macromolecules, able to cross the cellular barriers, they show duality in illness as a cause and as a therapeutic target. In this review, we take recent advancements in gap junction neuromodulation (pharmacological blockade, gene therapy, electrical and light stimulation) into account, to show the gap junction's role in neuronal cell death and the possible routes of rescuing neuronal and glial cells in the retina succeeding illness or injury.

Effects of Depth-Based Object Isolation in Simulated Retinal Prosthetic Vision

Visual retinal prostheses aim to restore vision for blind individuals who suffer from outer retinal degenerative diseases, such as retinitis pigmentosa and age-related macular degeneration. Perception through retinal prostheses is very limited, but it can be improved by applying object isolation. We used an object isolation algorithm based on integral imaging to isolate objects of interest according to their depth from the camera and applied image processing manipulation to the isolated-object images. Subsequently, we applied a spatial prosthetic vision simulation that converted the isolated-object images to phosphene images. We compared the phosphene images for two types of input images, the original image (before applying object isolation), and the isolated-object image to illustrate the effects of object isolation on simulated prosthetic vision without and with multiple spatial variations of phosphenes, such as size and shape variations, spatial shifts, and dropout rate. The results show an improvement in the perceived shape, contrast, and dynamic range (number of gray levels) of objects in the phosphene image.

Advancement in Nanostructure-Based Tissue-Engineered Biomaterials for Retinal Degenerative Diseases

The review intends to overview a wide range of nanostructured natural, synthetic and biological membrane implants for tissue engineering to help in retinal degenerative diseases. Herein, we discuss the transplantation strategies and the new development of material in combination with cells such as induced pluripotent stem cells (iPSC), mature retinal cells, adult stem cells, retinal progenitors, fetal retinal cells, or retinal pigment epithelial (RPE) sheets, etc. to be delivered into the subretinal space. Retinitis pigmentosa and age-related macular degeneration (AMD) are the most common retinal diseases resulting in vision impairment or blindness by permanent loss in photoreceptor cells. Currently, there are no therapies that can repair permanent vision loss, and the available treatments can only delay the advancement of retinal degeneration. The delivery of cell-based nanostructure scaffolds has been presented to enrich cell survival and direct cell differentiation in a range of retinal degenerative models. In this review, we sum up the research findings on different types of nanostructure scaffolds/substrate or material-based implants, with or without cells, used to deliver into the subretinal space for retinal diseases. Though, clinical and pre-clinical trials are still needed for these transplants to be used as a clinical treatment method for retinal degeneration.

First Human Results With the 256 Channel Intelligent Micro Implant Eye (IMIE 256)

Purpose

To report on the safety and efficacy of the 256-channel Intelligent Micro Implant Eye epiretinal prosthesis system (IMIE 256).

Methods

The IMIE 256 implants were implanted in the right eyes of five subjects with end-stage retinitis pigmentosa. Following implantation, the subjects underwent visual rehabilitation training for 90 days, and their visual performance was evaluated using the grating visual acuity test, Tumbling E visual acuity test, direction of motion, square localization, and orientation and mobility test. To evaluate the safety of the IMIE 256, all adverse events were recorded.

Results

Subjects performed significantly better on all evaluations with the IMIE 256 system on as compared with the performance at baseline or with the system off. There was a steady improvement in performance at each observation interval, indicating that the training and/or practice helped the subjects use the IMIE 256. There were two serious adverse events—electrode array movement and low intraocular pressure in one subject, which resolved with surgery. There were no other adverse events observed except those expected in the course of postoperative healing.

Conclusions

These results show an improved safety and efficacy profile compared with that of the Argus II implant. Further clinical trials are needed to confirm these results in a larger number of subjects and over longer durations.

Translational Relevance

To our knowledge, this study reports the first in-human data from a high-density (256 electrodes) epiretinal implant to restore sight to a subset of blind patients.

Eighty-Five Percent of Improved Optical Power Delivery to Epiretinal Prostheses Using Rigid Body Compensation Algorithm

Vision impairment caused by degenerative retinal pathologies such as age-related macular degeneration can be treated using retinal implants. Such devices receive power and data using cables passing through a permanent surgical incision in the eye wall (sclera), which increases the risk to patients and surgical costs. A recently developed retinal implant design eliminates the necessity of the implant cable using a photonic power converter (PPC), which receives optical power and data through the pupil and is directed by an ellipsoidal reflector and micro-electromechanical mirror. We present a misalignment compensation algorithm model that accounts for rigid-body motions of the reflector relative to the eye and applies the correction to the mirror coordinates in the presence of angular misalignment of the reflector. We demonstrate that up to 85% of the nominal optical power can be delivered to the implant with axial reflector misalignments up to 30 deg using the compensation algorithm.

Model-based comparison of current flow in rod bipolar cells of healthy and early-stage degenerated retina

Retinal degenerative diseases, such as retinitis pigmentosa, are generally thought to initiate with the loss of photoreceptors, though recent work suggests that plasticity and remodeling occurs prior to photoreceptor cell loss. This degeneration subsequently leads to death of other retinal neurons, creating functional alterations and extensive remodeling of retinal networks. Retinal prosthetic devices stimulate the surviving retinal cells by applying external current using implanted electrodes. Although these devices restore partial vision, the quality of restored vision is limited. Further knowledge about the precise changes in degenerated retina as the disease progresses is essential to understand how current flows in retinas undergoing degenerative disease and to improve the performance of retinal prostheses. We developed computational models that describe current flow from rod photoreceptors to rod bipolar cells (RodBCs) in the healthy and early-stage degenerated retina. Morphologically accurate models of retinal cells with their synapses are constructed based on retinal connectome datasets, created using serial section transmission electron microscopy (TEM) images of 70 nm-thick slices of either healthy (RC1) or early-stage degenerated (RPC1) rabbit retina. The passive membrane and active ion currents of each cell are implemented using conductance-based models in the Neuron simulation environment. In response to photocurrent input at rod photoreceptors, the simulated membrane potential at RodBCs in early degenerate tissue is approximately 10-20 mV lower than that of RodBCs of that observed in wild type retina. Results presented here suggest that although RodBCs in RPC1 show early, altered morphology compared to RC1, the lower membrane potential is primarily a consequence of reduced rod photoreceptor input to RodBCs in the degenerated retina. Frequency response and step input analyses suggest that individual cell responses of RodBCs in either healthy or early-degenerated retina, prior to substantial photoreceptor cell loss, do not differ significantly.

Color and cellular selectivity of retinal ganglion cell subtypes through frequency modulation of electrical stimulation

Epiretinal prostheses aim at electrically stimulating the inner most surviving retinal cells-retinal ganglion cells (RGCs)-to restore partial sight to the blind. Recent tests in patients with epiretinal implants have revealed that electrical stimulation of the retina results in the percept of color of the elicited phosphenes, which depends on the frequency of stimulation. This paper presents computational results that are predictive of this finding and further support our understanding of the mechanisms of color encoding in electrical stimulation of retina, which could prove pivotal for the design of advanced retinal prosthetics that elicit both percept and color. This provides, for the first time, a directly applicable "amplitude-frequency" stimulation strategy to "encode color" in future retinal prosthetics through a predictive computational tool to selectively target small bistratified cells, which have been shown to contribute to "blue-yellow" color opponency in the retinal circuitry. The presented results are validated with experimental data reported in the literature and correlated with findings in blind patients with a retinal prosthetic implant collected by our group.

Computational Modeling of Spatially Selective Retinal Stimulation With Temporally Interfering Electric Fields

Retinal electrical stimulation is a widely utilized method to restore visual function for patients with retinal degenerative diseases. Transcorneal electrical stimulation (TES) represents an effective way to improve the visual function due to its potential neuroprotective effect. However, TES with single electrode fails to spatially and selectively stimulate retinal neurons. Herein, a computational modeling method was proposed to explore the feasibility of spatially selective retinal stimulation via temporally interfering electric fields. An eyeball model with multiple electrodes was constructed to simulate the interferential electric fields with various electrode montages and current ratios. The results demonstrated that the temporal interference (TI) stimulation would gradually generate an increasingly localized high-intensity region on retina as the return electrodes moved towards the posterior of the eyeball and got closer. Additionally, the position of the convergent region could be modulated by regulating the current ratio of different electrode channels. The TI strategy with multisite and steerable stimulation can stimulate local retinal region with certain convergence and a relatively large stimulation range, which would be a feasible approach for the spatially selective retinal neuromodulation.

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.

Microtopographical patterns promote different responses in fibroblasts and Schwann cells: A possible feature for neural implants

The chronic reliability of bioelectronic neural interfaces has been challenged by foreign body reactions (FBRs) resulting in fibrotic encapsulation and poor integration with neural tissue. Engineered microtopographies could alleviate these challenges by manipulating cellular responses to the implanted device. Parallel microchannels have been shown to modulate neuronal cell alignment and axonal growth, and Sharklet™ microtopographies of targeted feature sizes can modulate bio-adhesion of an array of bacteria, marine organisms, and epithelial cells due to their unique geometry. We hypothesized that a Sharklet™ micropattern could be identified that inhibited fibroblasts partially responsible for FBR while promoting Schwann cell proliferation and alignment. in vitro cell assays were used to screen the effect of Sharklet™ and channel micropatterns of varying dimensions from 2 to 20 μm on fibroblast and Schwann cell metrics (e.g., morphology/alignment, nuclei count, metabolic activity), and a hierarchical analysis of variance was used to compare treatments. In general, Schwann cells were found to be more metabolically active and aligned than fibroblasts when compared between the same pattern. 20 μm wide channels spaced 2 μm apart were found to promote Schwann cell attachment and alignment while simultaneously inhibiting fibroblasts and warrant further in vivo study on neural interface devices. No statistically significant trends between cellular responses and geometrical parameters were identified because mammalian cells can change their morphology dependent on their environment in a manner dissimilar to bacteria. Our results showed although surface patterning is a strong physical tool for modulating cell behavior, responses to micropatterns are highly dependent on the cell type.

Bio-Compatibility and Bio-Insulation of Implantable Electrode Prosthesis Ameliorated by A-174 Silane Primed Parylene-C Deposited Embedment

Microelectrodes for pain management, neural prosthesis or assistances have a huge medical demand, such as the application of pain management chip or retinal prosthesis addressed on age-related macular degeneration (AMD) and the retinitis pigmentosa (RP). Due to lifelong implanted in human body and direct adhesion of neural tissues, the electrodes and associated insulation materials should possess an ideal bio-compatibility, including non-cytotoxicity and no safety concern elicited by immune responses. Our goal intended to develop retinal prosthesis, an electrical circuit chip used for assisting neural electrons transmission on retina and ameliorating the retinal disability. Therefore, based on the ISO 10993 guidance for implantable medical devices, the electrode prosthesis with insulation material has to conduct bio-compatibility assessment including cytotoxicity, hemolysis, (skin) irritation and pathological implantation examinations. In this study, we manufactured inter-digitated electrode (IDE) chips mimic the electrode prosthesis through photolithography. The titanium and platinum composites were deposited onto a silicon wafer to prepare an electric circuit to mimic the electrode used in retinal prosthesis manufacture, which further be encapsulated to examine the bio-compatibility in compliance with ISO 10993 and ASTM guidance specifically for implantable medical devices. Parylene-C, polyimide and silicon carbide were selected as materials for electrode encapsulation in comparison. Our data revealed parylene-C coating showed a significant excellence on bio-insulation and bio-compatibility specifically addressed on implantable neuron stimulatory devices and provided an economic procedure to package the electrode prosthesis. Therefore, parylene C encapsulation should serve as a consideration for future application on retinal prosthesis manufacture and examination.

Targeted Stimulation of Retinal Ganglion Cells in Epiretinal Prostheses: A Multiscale Computational Study

Retinal prostheses aim at restoring partial sight to patients that are blind due to retinal degenerative diseases by electrically stimulating the surviving healthy retinal neurons. Ideally, the electrical stimulation of the retina is intended to induce localized, focused, percepts only; however, some epiretinal implant subjects have reported seeing elongated phosphenes in a single electrode stimulation due to the axonal activation of retinal ganglion cells (RGCs). This issue can be addressed by properly devising stimulation waveforms so that the possibility of inducing axonal activation of RGCs is minimized. While strategies to devise electrical stimulation waveforms to achieve a focal RGCs response have been reported in literature, the underlying mechanisms are not well understood. This article intends to address this gap; we developed morphologically and biophysically realistic computational models of two classified RGCs: D1-bistratified and A2-monostratified. Computational results suggest that the sodium channel band (SOCB) is less sensitive to modulations in stimulation parameters than the distal axon (DA), and DA stimulus threshold is less sensitive to physiological differences among RGCs. Therefore, over a range of RGCs distal axon diameters, short-pulse symmetric biphasic waveforms can enhance the stimulation threshold difference between the SOCB and the DA. Appropriately designed waveforms can avoid axonal activation of RGCs, implying a consequential reduction of undesired strikes in the visual field.

Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis

We present a retrospective of unique micro-fabrication problems and solutions that were encountered through over 10 years of retinal prosthesis product development, first for the Boston Retinal Implant Project initiated at the Massachusetts Institute of Technology and at Harvard Medical School’s teaching hospital, the Massachusetts Eye and Ear—and later at the startup company Bionic Eye Technologies, by some of the same personnel. These efforts culminated in the fabrication and assembly of 256+ channel visual prosthesis devices having flexible multi-electrode arrays that were successfully implanted sub-retinally in mini-pig animal models as part of our pre-clinical testing program. We report on the processing of the flexible multi-layered, planar and penetrating high-density electrode arrays, surgical tools for sub-retinal implantation, and other parts such as coil supports that facilitated the implantation of the peri-ocular device components. We begin with an overview of the implantable portion of our visual prosthesis system design, and describe in detail the micro-fabrication methods for creating the parts of our system that were assembled outside of our hermetically-sealed electronics package. We also note the unique surgical challenges that sub-retinal implantation of our micro-fabricated components presented, and how some of those issues were addressed through design, materials selection, and fabrication approaches.

Current perspectives on the safe electrical stimulation of peripheral nerves with platinum electrodes

This review details some peripheral nervous system (PNS) targets and electrode designs used for electrical stimulation. It investigates limitations in current knowledge of safe electrical stimulation and possible future electrode developments. Current PNS targets are large, leading to poor resolution and off-target side-effects. Most clinical devices are platinum or platinum/iridium embedded in an insulation material. Their safety is usually guided by the Shannon plot, which is not valid for the PNS. New electrode designs are needed to target smaller nerve fibers, enabling higher resolution electrical therapies with fewer off-target side-effects. Damage can occur through biological and electrochemical mechanisms. Greater mechanistic understanding is required to ensure safe and efficacious, long-term electrical stimulation with new electrode materials, geometries and stimulation waveforms.

Stimulus waveform design for decreasing charge and increasing stimulation selectivity in retinal prostheses

Retinal degenerative diseases, such as retinitis pigmentosa, begin with damage to the photoreceptor layer of the retina. In the absence of presynaptic input from photoreceptors, networks of electrically coupled AII amacrine and cone bipolar cells have been observed to exhibit oscillatory behaviour and result in spontaneous firing of ganglion cells. This ganglion cell activity could interfere with external stimuli provided by retinal prosthetic devices and potentially degrade their performance. In this work, the authors computationally investigate stimulus waveform designs, which can improve the performance of retinal prostheses by suppressing undesired spontaneous firing of ganglion cells and generating precise temporal spiking patterns. They utilise a multi-scale computational model for electrical stimulation of degenerated retina based on the admittance method and NEURON simulation environments. They present a class of asymmetric biphasic pulses that can generate precise ganglion cell firing patterns with up to 55% lower current requirements compared to traditional symmetric biphasic pulses. This lower current results in activation of only proximal ganglion cells, provides more focused stimulation and lowers the risk of tissue damage.

Retinal Drug Delivery: Rethinking Outcomes for the Efficient Replication of Retinal Behavior

The retina is a highly organized structure that is considered to be "an approachable part of the brain." It is attracting the interest of development scientists, as it provides a model neurovascular system. Over the last few years, we have been witnessing significant development in the knowledge of the mechanisms that induce the shape of the retinal vascular system, as well as knowledge of disease processes that lead to retina degeneration. Knowledge and understanding of how our vision works are crucial to creating a hardware-adaptive computational model that can replicate retinal behavior. The neuronal system is nonlinear and very intricate. It is thus instrumental to have a clear view of the neurophysiological and neuroanatomic processes and to take into account the underlying principles that govern the process of hardware transformation to produce an appropriate model that can be mapped to a physical device. The mechanistic and integrated computational models have enormous potential toward helping to understand disease mechanisms and to explain the associations identified in large model-free data sets. The approach used is modulated and based on different models of drug administration, including the geometry of the eye. This work aimed to review the recently used mathematical models to map a directed retinal network.

The combination of brain-computer interfaces and artificial intelligence: applications and challenges

Brain-computer interfaces (BCIs) have shown great prospects as real-time bidirectional links between living brains and actuators. Artificial intelligence (AI), which can advance the analysis and decoding of neural activity, has turbocharged the field of BCIs. Over the past decade, a wide range of BCI applications with AI assistance have emerged. These "smart" BCIs including motor and sensory BCIs have shown notable clinical success, improved the quality of paralyzed patients' lives, expanded the athletic ability of common people and accelerated the evolution of robots and neurophysiological discoveries. However, despite technological improvements, challenges remain with regard to the long training periods, real-time feedback, and monitoring of BCIs. In this article, the authors review the current state of AI as applied to BCIs and describe advances in BCI applications, their challenges and where they could be headed in the future.

Retinal Prosthetic Approaches to Enhance Visual Perception for Blind Patients

Retinal prostheses are implantable devices that aim to restore the vision of blind patients suffering from retinal degeneration, mainly by artificially stimulating the remaining retinal neurons. Some retinal prostheses have successfully reached the stage of clinical trials; however, these devices can only restore vision partially and remain insufficient to enable patients to conduct everyday life independently. The visual acuity of the artificial vision is limited by various factors from both engineering and physiological perspectives. To overcome those issues and further enhance the visual resolution of retinal prostheses, a variety of retinal prosthetic approaches have been proposed, based on optimization of the geometries of electrode arrays and stimulation pulse parameters. Other retinal stimulation modalities such as optics, ultrasound, and magnetics have also been utilized to address the limitations in conventional electrical stimulation. Although none of these approaches have been clinically proven to fully restore the function of a degenerated retina, the extensive efforts made in this field have demonstrated a series of encouraging findings for the next generation of retinal prostheses, and these could potentially enhance the visual acuity of retinal prostheses. In this article, a comprehensive and up-to-date overview of retinal prosthetic strategies is provided, with a specific focus on a quantitative assessment of visual acuity results from various retinal stimulation technologies. The aim is to highlight future directions toward high-resolution retinal prostheses.

A Three-Dimensional Microelectrode Array to Generate Virtual Electrodes for Epiretinal Prosthesis Based on a Modeling Study

Despite many advances in the development of retinal prostheses, clinical reports show that current retinal prosthesis subjects can only perceive prosthetic vision with poor visual acuity. A possible approach for improving visual acuity is to produce virtual electrodes (VEs) through electric field modulation. Generating controllable and localized VEs is a crucial factor in effectively improving the perceptive resolution of the retinal prostheses. In this paper, we aimed to design a microelectrode array (MEA) that can produce converged and controllable VEs by current steering stimulation strategies. Through computational modeling, we designed a three-dimensional concentric ring–disc MEA and evaluated its performance with different stimulation strategies. Our simulation results showed that electrode–retina distance (ERD) and inter-electrode distance (IED) can dramatically affect the distribution of electric field. Also the converged VEs could be produced when the parameters of the three-dimensional MEA were appropriately set. VE sites can be controlled by manipulating the proportion of current on each adjacent electrode in a current steering group (CSG). In addition, spatial localization of electrical stimulation can be greatly improved under quasi-monopolar (QMP) stimulation. This study may provide support for future application of VEs in epiretinal prosthesis for potentially increasing the visual acuity of prosthetic vision.

A Feasibility Study on Optically Transparent Encapsulation for Implantable Neural Prostheses

Optically transparent encapsulation is presented with results from long-term reliability and light transmission tests. This technology is required in certain implantable neural prostheses that demand the transmission of optical signals through an encapsulating material, such as in retinal implants or in optogenetic applications. In this study, biocompatible film-type cyclic olefin polymers (COPs) with low moisture absorption (<; 0.01 %) and high light transmission (92 %) are utilized as encapsulating materials based on thermal lamination. The reliability of COP encapsulation is characterized through accelerated soak tests in a 75 °C saline solution to measure the leakage currents from encapsulated inter-digitated electrodes. These tests had been done for 211 days with the estimated lifetime of 8.05 years at 37 °C. In addition, the optical properties of a thermally laminated COP film sample in relation to its thickness are evaluated by an experimental setup which uses projected line patterns on an image sensor. The light transmittance of COP film samples thinner than 376 μm exceeded 91.69 %, and the minimum distinguishable line pitch was 47.6 μm at a thickness of 26 μm. These results validate the feasibility of optically transparent encapsulation using COPs and may contribute to its use in future implantable neural prostheses.

Vertically integrated photo junction-field-effect transistor pixels for retinal prosthesis

Optoelectronic retinal prostheses transduce light into electrical current for neural stimulation. We introduce a novel optoelectronic pixel architecture consisting of a vertically integrated photo junction-field-effect transistor (Photo-JFET) and neural stimulating electrode. Experimental measurements demonstrate that optically addressed Photo-JFET pixels utilize phototransistive gain to produce a broad range of neural stimulation current and can effectively stimulate retinal neurons in vitro. The compact nature of the Photo-JFET pixel can enable high resolution retinal prostheses with the smallest reported optoelectronic pixel size to help restore high visual acuity in patients with degenerative retinal diseases.

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.

Acute Rabbit Eye Model for Testing Subretinal Prostheses

Purpose

Subretinal prostheses are a novel technology for restoring useful vision in patients with retinitis pigmentosa or age-related macular degeneration. We characterize the surgical implantation technique and functional time window of an acute rabbit eye model for testing of human subretinal prostheses.

Methods

Retinal prostheses were implanted subretinally in 26 rabbits using a two-step technique. Fundus imaging, fluorescein fundus angiography, and optical coherence topography (OCT) were conducted postoperatively from days 1 to 21 to monitor prosthesis positioning and retinal anatomic changes.

Results

Successful implantation and excellent retina apposition were achieved in 84.6% of the rabbits. OCTs showed the overlying retina at full thickness for the first 2 days after implantation. Histology confirmed intact inner layers of the overlying retina until day 3. Progressive atrophy of the overlying retina was revealed by repeated OCTs; approximately 40% of the retina thickness remained on postoperative days 5 and 6.

Conclusions

The two-step implantation technique works well for the rabbit eye model with human prostheses. Rabbit retina may be used for acute electrophysiologic testing of a retinal prosthesis, but is unsuitable for chronic studies due to the merangiotic retina and its limited time window of validity.

Translational Relevance

The improved efficacy in prosthesis surgery using this technique will circumvent the challenges in animal models that provide human-like features critical for the transition into human clinical trials.

Human enhancement through the lens of experimental and speculative neurotechnologies

Human enhancement deals with improving on and overcoming limitations of the human body and mind. Pharmaceutical compounds that alter consciousness and cognitive performance have been used and discussed for a long time. The prospect of neurotechnological applications such as brain-steered devices or using invasive and noninvasive electromagnetic stimulations of the human brain, however, has received less attention-especially outside of therapeutic practices-and remains relatively unexplored. Reflection and debates about neurotechnology for human enhancement are limited and remain predominantly with neurotech engineers, science-fiction enthusiasts and a small circle of academics in the field of neuroethics. It is well known, and described as the Collingridge dilemma, that at an early stage of development, changes can easily be enacted, but the need for changes can hardly be foreseen. Once the technology is entrenched, opportunities and risks start to materialize, and the need to adapt and change is clearly visible. However, carrying out these changes at such a late stage, in turn, becomes very difficult, tremendously expensive, and sometimes practically impossible. In this manuscript, we compile and categorize an overview of existing experimental and speculative applications of neurotechnologies, with the aim to find out, if these real or diegetic prototypes could be used to better understand the paths these applications are forging. In particular, we will investigate what kind of tools, motivations, and normative goals underpin experimental implementations by neurohackers, speculative designers and artists.

Biologically-inspired image processing in computational retina models

Retinal Prosthesis (RP) is an approach to restore vision, using an implanted device to electrically stimulate the retina. A fundamental problem in RP is to translate the visual scene to retina neural spike patterns, mimicking the computations normally done by retina neural circuits. Towards the perspective of improved RP interventions, we propose a Computer Vision (CV) image preprocessing method based on Retinal Ganglion Cells functions and then use the method to reproduce retina output with a standard Generalized Integrate & Fire (GIF) neuron model. "Virtual Retina" simulation software is used to provide the stimulus-retina response data to train and test our model. We use a sequence of natural images as model input and show that models using the proposed CV image preprocessing outperform models using raw image intensity (interspike-interval distance 0.17 vs 0.27). This result is aligned with our hypothesis that raw image intensity is an improper image representation for Retinal Ganglion Cells response prediction.

Molecular Therapies for Inherited Retinal Diseases-Current Standing, Opportunities and Challenges

Inherited retinal diseases (IRDs) are both genetically and clinically highly heterogeneous and have long been considered incurable. Following the successful development of a gene augmentation therapy for biallelic RPE65-associated IRD, this view has changed. As a result, many different therapeutic approaches are currently being developed, in particular a large variety of molecular therapies. These are depending on the severity of the retinal degeneration, knowledge of the pathophysiological mechanism underlying each subtype of IRD, and the therapeutic target molecule. DNA therapies include approaches such as gene augmentation therapy, genome editing and optogenetics. For some genetic subtypes of IRD, RNA therapies and compound therapies have also shown considerable therapeutic potential. In this review, we summarize the current state-of-the-art of various therapeutic approaches, including the pros and cons of each strategy, and outline the future challenges that lie ahead in the combat against IRDs.

Development of an image biosensor based on an optogenetically engineered cell for visual prostheses

Visual prostheses provide blind patients with artificial vision via electrical stimulation of surviving visual cells resulting in partial restoration of vision in many patients. However, high-resolution visual perception, long-term biocompatibility and safety remain the significant challenges of existing visual prostheses. Here, we present a novel method to develop a new visual prosthesis using living cells as integrated electronics and implantable microelectrodes. The living cells modified with channelrhodopsin-2 showed excellent light-sensitive properties and encoded image information with cellular deformations triggered by light stimulation. The photoresponsive properties of the cells were determined using a single pixel imaging system, which indicated that the cells can act as a good light-sensitive biosensor. Additionally, the imaging feasibility of the cells was further validated through successful and clear imaging of several object scenes using the same system. This work represents a step toward the design and use of living cells as an image biosensor for the development of a new generation of high-resolution visual prostheses.

Deep reinforcement learning for task-based feature learning in prosthetic vision

Developing hand-crafted visual features to enhance perception with prosthetic vision devices can often miss important aspects of a given task. Retinal implants suffer from the need to create low-dimensional features for elaborate tasks such as navigation in complex environments. Using Deep Reinforcement Learning (DRL), visual features are learnt through task-based simulations that remove the ambiguity of inferring the visual information most crucial to a specific activity. Learning task-based features ensures that the visual information is salient to the tasks an implant recipient may be undertaking and eliminates potentially redundant features. In this paper, we focus specifically on basic orientation and mobility, and the methods for feature learning and visualisation in structured 3D environments. We propose a new model for learning visual features through task-based simulations and show that learnt features can be transferred directly to real RGB-D images. We demonstrate this new scalable approach for feature learning in simulation and open the possibility for more complex simulations of more complex tasks in the future.

Temporal Neuromodulation of Retinal Ganglion Cells by Low-Frequency Focused Ultrasound Stimulation

Significant progress has been made recently in treating neurological blindness using implantable visual prostheses. However, implantable medical devices are highly invasive and subject to many safety, efficacy, and cost issues. The discovery that ultrasound (US) may be useful as a noninvasive neuromodulation tool has aroused great interest in the field of acoustic retinal prostheses (ARPs). We have investigated the responsiveness of rat retinal ganglion cells (RGCs) to low-frequency focused US stimulation (LFUS) at 2.25 MHz and characterized the neurophysiological properties of US responses by performing in vitro multielectrode array recordings. The results show that LFUS can reliably activate RGCs. The US-induced responses did not correspond to the standard light responses and varied greatly among cell types. Moreover, dual-peak responses to US stimulation were observed that have not been reported previously. The temporal response properties of RGCs, including their latency, firing rate, and response type, were modulated by the acoustic intensity. These findings suggest the presence of a temporal neuromodulation effect of LFUS and potentially open a new avenue in the development of ARP.

In Vivo Photovoltaic Performance of a Silicon Nanowire Photodiode-Based Retinal Prosthesis

Purpose

For more than 20 years, there has been an international, multidisciplinary effort to develop retinal prostheses to restore functional vision to patients blinded by retinal degeneration. We developed a novel subretinal prosthesis with 1512 optically addressed silicon nanowire photodiodes, which transduce incident light into an electrical stimulation of the remaining retinal circuitry. This study was conducted to evaluate the efficacy of optically driving the subretinal prosthesis to produce visual cortex activation via electrical stimulation of the retina.

Methods

We measured electrically evoked potential responses (EEPs) in rabbit visual cortex in response to illumination of the subretinal nanowire prosthesis with pulsed 852-nm infrared (IR) light. We compared the EEP responses to visually evoked potential responses (VEPs) to pulsed 532-nm visible light (positive control) and pulsed 852-nm IR light (negative control).

Results

Activating the devices with IR light produced EEP responses with a significantly higher trough-to-peak amplitude (54.17 ± 33.4 μV) than IR light alone (24.07 ± 22.1 μV) or background cortical activity (23.22 ± 17.2 μV). EEP latencies were significantly faster than focal VEP latencies. Focal VEPs produced significantly higher amplitudes (94.88 ± 43.3 μV) than EEPs. We also demonstrated how an electrode placed on the cornea can be used as a noninvasive method to monitor the function of the implant.

Conclusions

These results show that subretinal electrical stimulation with nanowire electrodes can elicit EEPs in the visual cortex, providing evidence for the viability of a subretinal nanowire prosthetic approach for vision restoration.

Toward a Bidirectional Communication Between Retinal Cells and a Prosthetic Device - A Proof of Concept

Background: Significant progress toward the recovery of useful vision in blind patients with severe degenerative retinal diseases caused by photoreceptor death has been achieved with the development of visual prostheses that stimulate the retina electrically. However, currently used prostheses do not provide feedback about the retinal activity before and upon stimulation and do not adjust to changes during the remodeling processes in the retina. Both features are desirable to improve the efficiency of the electrical stimulation (ES) therapy offered by these devices. Accordingly, devices that not only enable ES but at the same time provide information about the retinal activity are beneficial. Given the above, a bidirectional communication strategy, in which inner retinal cells are stimulated and the output neurons of the retina, the ganglion cells, are recorded using penetrating microelectrode arrays (MEAs) is proposed.

Methods: Custom-made penetrating MEAs with four silicon-based shanks, each one with three or four iridium oxide electrodes specifically designed to match retinal dimensions were used to record the activity of light-adapted wildtype mice retinas and degenerated retinas from rd10 mice in vitro. In addition, responses to high potassium concentration and to light stimulation in wildtype retinas were examined. Furthermore, voltage-controlled ES was performed.

Results: The spiking activity of retinal ganglion cells (RGCs) was recorded at different depths of penetration inside the retina. Physiological responses during an increase of the extracellular potassium concentration and phasic and tonic responses during light stimulation were captured. Moreover, pathologic rhythmic activity was recorded from degenerated retinas. Finally, ES of the inner retina and simultaneous recording of the activity of RGCs was accomplished.

Conclusion: The access to different layers of the retina with penetrating electrodes while recording at the same time the spiking activity of RGCs broadens the use and the field of action of multi-shank and multi-site penetrating MEAs for retinal applications. It enables a bidirectional strategy to stimulate inner retinal cells electrically and to record from the spiking RGCs simultaneously (BiMEA). This opens the possibility of a feedback loop system to acknowledge the success of ES carried out by retinal prostheses.

A Novel Racing Array Transducer for Noninvasive Ultrasonic Retinal Stimulation: A Simulation Study

Neurostimulation has proved to be an effective method for the restoration of visual perception lost due to retinal diseases. However, the clinically available retinal neurostimulation method is based on invasive electrodes, making it a high-cost and high-risk procedure. Recently, ultrasound has been demonstrated to be an effective way to achieve noninvasive neurostimulation. In this work, a novel racing array transducer with a contact lens shape is proposed for ultrasonic retinal stimulation. The transducer is flexible and placed outside the eyeball, similar to the application of a contact lens. Ultrasound emitted from the transducer can reach the retina without passing through the lens, thus greatly minimizing the acoustic absorption in the lens. The discretized Rayleigh–Sommerfeld method was employed for the acoustic field simulation, and patterned stimulation was achieved. A 5 MHz racing array transducer with different element numbers was simulated to optimize the array configuration. The results show that a 512-element racing array is the most appropriate configuration considering the necessary tradeoff between the element number and the stimulation resolution. The stimulation resolution at a focus of 24 mm is about 0.6 mm. The obtained results indicate that the proposed racing array design of the ultrasound transducer can improve the feasibility of an ultrasound retinal prosthesis.

Analysis of upper threshold mechanisms of spherical neurons during extracellular stimulation

Exceeding a certain stimulation strength can prevent the generation of somatic action potentials, as has been demonstrated in vitro with extracellularly stimulated dorsal root ganglion cells as well as retinal ganglion cells. This phenomenon, termed upper threshold, is currently thought to be a consequence of sodium current reversal in strongly depolarized regions. Here we analyze the contribution of membrane kinetics, using spherical model neurons that are stimulated externally with a microelectrode, in more detail. During extracellular pulse application, the electric field depolarizes one part and hyperpolarizes the other part of the cell. Strong transmembrane currents are generated only in the active depolarized region, changing the overall polarization level. The asymmetric membrane voltage distribution caused by the stimulus strongly influences the cell's behavior during and even after the stimulus. Effects on membrane voltage and transmembrane currents during and after the stimulus are shown and discussed in detail. Aside from the sodium current reversal, two more key mechanisms were identified in causing the upper threshold: strong potassium currents and inactivation of sodium channels. The contributions of the mechanisms involved strongly depend on cell properties, stimulus parameters, and other factors such as temperature. The conclusions presented here are based on several retinal ganglion cell models of the Fohlmeister group, a model with original Hodgkin-Huxley membrane, and a pyramidal cell model. NEW & NOTEWORTHY The upper threshold phenomenon in extracellular stimulation is analyzed in detail for spherical cells. Three main mechanisms were identified that prevent the generation of action potentials at high stimulation strengths: 1) strong potassium currents, 2) inactivating sodium ion channels, and 3) sodium current reversal. Ion channel kinetics in retinal ganglion cells, pyramidal cells, and the original Hodgkin-Huxley model were investigated under the influence of an extracellular stimulus.

A Fully Integrated RF-Powered Energy-Replenishing Current-Controlled Stimulator

This paper presents a fully-integrated current-controlled stimulator that is powered directly from on-chip coil antenna and achieves adiabatic energy-replenishing operation without any bulky external components. Adiabatic supply voltages, which can reach a differential range of up to 7.2 V, are directly generated from an on-chip 190-MHz resonant LC tank via a self-cascading/folding rectifier network, bypassing the losses that would otherwise be introduced by the 0.8 V system supply-generating rectifier and regulator. The stimulator occupies 0.22 mm ² in a 180 nm silicon-on-insulator process and produces differential currents up to 145  μA. Using a charge replenishing scheme, the stimulator redirects the charges accumulated across the electrodes to the system power supplies for 63.1% of stimulation energy recycling. To benchmark the efficiency of stimulation, a figure of merit termed the stimulator efficiency factor (SEF) is introduced. The adiabatic power rails and energy replenishment scheme enabled our stimulator to achieve an SEF of 6.0.

Disintegrin-Integrin Binding for Attachment of Polymer Substrate to the Retina

Objective:

We propose a novel attachment method for retinal tissue that utilizes silicone modified with bioactive molecules.

Design:

This is an experimental study divided into an in vitro section performed in cadaveric pig eyes and an in vivo section performed in rabbits.

Subjects:

During in vitro experiments 36 cadaveric pig eyes were used. During in vivo experiments 4 rabbits were used.

Methods:

Different types of silicone went through a laser irradiation process to determine if binding sites for disintegrins could be created. Laser treated silicones that showed disintegrin binding were evaluated with in vitro testing for retina-silicone attachment. The best silicone binding in vitro was implanted into a rabbit’s eye after a full vitrectomy was performed. Post-operative exams were done every two weeks to evaluate placement, attachment and sterilization method. After three months animals were euthanized and eye was enucleated for histology analysis.

Main Outcome Measures:

Attachment strength between silicone-disintegrin-retina, and signs of endophthalmitis during in vivo studies for biocompatibility purposes.

Results:

A technique to successfully lase and produce an active area on the silicone surface was described. Scanning electron microscope (SEM) images were evaluated to assess physical ablation/debris field area on the surface, definition of edges, evenness, and symmetry of the lased area allowing us to select MED 4800 silicone family for further testing. Cell culture experiments showed disintegrin binding to the silicone active area. In vitro experiments with cadaveric eyes were performed to test retina-silicone attachment. MED 4860 showed strongest attachment to the retina and it was used during in vivo experiments. A sterilization protocol was tested and proved to be reliable for bioactive materials. Disintegrin coated silicone showed attachment in 2 of 4 rabbits during the 3-month implant period. The adhesion was persistent until reversed with plasmin. All rabbits were implanted for 3 months regardless of attachment, to test the feasibility of the sterilization method. None of the rabbits developed any type of eye infection during the implant period.

Conclusion:

We successfully lased and produced an active area on the silicone surface to allow disintegrin-silicone binding. Different silicones interact differently with the laser energy, and this is reflected in the strength of the silicone-disintegrin-retina attachment.

Retinitis pigmentosa: recent advances and future directions in diagnosis and management

PURPOSE OF REVIEW

Retinitis pigmentosa is a group of genetically diverse inherited blinding disorders for which there are no treatments. Owing to recent advances in imaging technology, DNA sequencing, gene therapy, and stem cell biology, clinical trials have multiplied and the landscape is rapidly changing. This review provides a relevant and timely update of current trends and future directions for the diagnosis and management of this disease.

RECENT FINDINGS

This review will highlight the use of retinal imaging to measure progression of disease, next-generation sequencing for genetic diagnosis, the use of electronic retinal implants as well as noninvasive digital low-vision aids, and the current state of preclinical and clinical research with gene therapy and cell-based therapies.

SUMMARY

Retinitis pigmentosa has historically been an untreatable condition. Recent advances have allowed for limited improvement in visual outcomes for select patients. Retinal degenerative disease is on the cutting edge of regenerative medicine. Gene therapy and stem cell therapeutic strategies are currently under investigation and are expected to radically impact management of inherited retinal disease in the coming years. VIDEO ABSTRACT: http://links.lww.com/MOP/A33.

Augmented reality powers a cognitive assistant for the blind

To restore vision for the blind, several prosthetic approaches have been explored that convey raw images to the brain. So far, these schemes all suffer from a lack of bandwidth. An alternate approach would restore vision at the cognitive level, bypassing the need to convey sensory data. A wearable computer captures video and other data, extracts important scene knowledge, and conveys that to the user in compact form. Here, we implement an intuitive user interface for such a device using augmented reality: each object in the environment has a voice and communicates with the user on command. With minimal training, this system supports many aspects of visual cognition: obstacle avoidance, scene understanding, formation and recall of spatial memories, navigation. Blind subjects can traverse an unfamiliar multi-story building on their first attempt. To spur further development in this domain, we developed an open-source environment for standardized benchmarking of visual assistive devices.

Epiretinal stimulation with local returns enhances selectivity at cellular resolution

OBJECTIVE

Epiretinal prostheses are designed to restore vision in people blinded by photoreceptor degenerative diseases, by directly activating retinal ganglion cells (RGCs) using an electrode array implanted on the retina. In present-day clinical devices, current spread from the stimulating electrode to a distant return electrode often results in the activation of many cells, potentially limiting the quality of artificial vision. In the laboratory, epiretinal activation of RGCs with cellular resolution has been demonstrated with small electrodes, but distant returns may still cause undesirable current spread. Here, the ability of local return stimulation to improve the selective activation of RGCs at cellular resolution was evaluated.

APPROACH

A custom multi-electrode array (512 electrodes, 10 μm diameter, 60 μm pitch) was used to simultaneously stimulate and record from RGCs in isolated primate retina. Stimulation near the RGC soma with a single electrode and a distant return was compared to stimulation in which the return was provided by six neighboring electrodes.

MAIN RESULTS

Local return stimulation enhanced the capability to activate cells near the central electrode (<30 μm) while avoiding cells farther away (>30 μm). This resulted in an improved ability to selectively activate ON and OFF cells, including cells encoding immediately adjacent regions in the visual field.

SIGNIFICANCE

These results suggest that a device that restricts the electric field through local returns could optimize activation of neurons at cellular resolution, improving the quality of artificial vision.