Interim results from the international trial of Second Sight's visual prosthesis

Potential of ultrasound stimulation and sonogenetics in vision restoration: a narrative review

Vision restoration presents a considerable challenge in the realm of regenerative medicine, while recent progress in ultrasound stimulation has displayed potential as a non-invasive therapeutic approach. This narrative review offers a comprehensive overview of current research on ultrasound-stimulated neuromodulation, emphasizing its potential as a treatment modality for various nerve injuries. By examining of the efficacy of different types of ultrasound stimulation in modulating peripheral and optic nerves, we can delve into their underlying molecular mechanisms. Furthermore, the review underscores the potential of sonogenetics in vision restoration, which involves leveraging pharmacological and genetic manipulations to inhibit or enhance the expression of related mechanosensitive channels, thereby modulating the strength of the ultrasound response. We also address how methods such as viral transcription can be utilized to render specific neurons or organs highly responsive to ultrasound, leading to significantly improved therapeutic outcomes.

Understanding responses to multi-electrode epiretinal stimulation using a biophysical model

Objective.Neural interfaces are designed to evoke specific patterns of electrical activity in populations of neurons by stimulating with many electrodes. However, currents passed simultaneously through multiple electrodes often combine nonlinearly to drive neural responses, making evoked responses difficult to predict and control. This response nonlinearity could arise from the interaction of many excitable sites in each cell, any of which can produce a spike. However, this multi-site activation hypothesis is difficult to verify experimentally.Approach.We developed a biophysical model to study retinal ganglion cell responses to multi-electrode stimulation and validated it using data collected fromex vivopreparations of the macaque retina using a microelectrode array (512 electrodes; 30µm pitch; 10µm diameter).Results.First, the model was validated by using it to reproduce essential empirical findings from single-electrode recording and stimulation, including recorded spike voltage waveforms at multiple locations and sigmoidal responses to injected current. Then, stimulation with two electrodes was modeled to test how the positioning of the electrodes relative to the cell affected the degree of response nonlinearity. Currents passed through pairs of electrodes positioned near the cell body or far from the axon (>40µm) exhibited approximately linear summation in evoking spikes. Currents passed through pairs of electrodes close to the axon summed linearly when their locations along the axon were similar, and nonlinearly otherwise. Over a range of electrode placements, several distinct, localized spike initiation sites were observed, and the number of these sites covaried with the degree of response nonlinearity. Similar trends were observed for three-electrode stimuli. All of these trends in the simulation were consistent with experimental observations.Significance.These findings support the multi-site activation hypothesis for nonlinear activation of neurons, providing a biophysical interpretation of previous experimental results and potentially enabling more efficient use of multi-electrode stimuli in future neural implants.

Modulating synaptic plasticity with metal-organic framework for information-filterable artificial retina

Neuroprosthetics equipped with artificial synapses hold promise to address some most intricate medical problems, such as human sensory disorders. Yet, it is necessitated and of paramount importance for neuroprosthetics to be able to differentiate significant and insignificant signals. Here, we present an information-filterable artificial retina system that integrates artificial synapses with a signal-integration device for signal perception and processing with attention. The synaptic weight modulation is rendered through metal-organic framework (MOF) layers, where distinct short-term and long-term properties are predominantly determined by MOF's pore diameter and functionality. Specifically, four types of isoreticular Zr-based MOFs that share Zr6O4(OH)4 secondary building units have been systematically examined. It is demonstrated that small pore diameters enhance short-term properties, while large pores, which are characterized by increased ion affinity, sustain long-term properties. Moreover, we demonstrated a 6 × 6 pixel artificial retina by incorporating both short-term and long-term artificial synapses with a signal-integration device. Signal summation by the signal-integration device enables attention-based information processing. The information-filterable artificial retina system developed here emulates human perception processes and holds promise in the fields of neuroprosthetics and advanced artificial intelligence.

Retinitis Pigmentosa and Therapeutic Candidates

Retinitis pigmentosa (RP) is a class of inherited retinal dystrophies (IRDs) that involves the degeneration of retinal photoreceptor cells and results in progressive vision loss. It was identified and named in 1857. For over 100 years, treatment of RP was generally limited to modifications in diet, management of cystoid macular edema, and supportive care for low vision. Over the last several decades, advances in technology and our understanding of the human genome have led to a host of new therapeutic candidates for the treatment of RP. This includes gene and cell therapy, optogenetics, neuroprotective agents, and electronic retinal implants. In this article, we summarize both the traditional and novel therapeutic modalities for the treatment of retinitis pigmentosa.

Feasibility Assessment of an Optically Powered Digital Retinal Prosthesis Architecture for Retinal Ganglion Cell Stimulation

Clinical trials previously demonstrated the notable capacity to elicit visual percepts in blind patients affected with retinal diseases by electrically stimulating the remaining neurons on the retina. However, these implants restored very limited visual acuity and required transcutaneous cables traversing the eyeball, leading to reduced reliability and complex surgery with high postoperative infection risks. To overcome the limitations imposed by cables, a retinal implant architecture in which near-infrared illumination carries both power and data through the pupil to a digital stimulation controller is presented. A high efficiency multi-junction photovoltaic cell transduces the optical power to a CMOS stimulator capable of delivering flexible interleaved sequential stimulation through a diamond microelectrode array. To demonstrate the capacity to elicit a neural response with this approach while complying with the optical irradiance limit at the pupil, fluorescence imaging with a calcium indicator is used on a degenerate rat retina. The power delivered by the laser at the permissible irradiance of 4 mW/mm2 at 850 nm is shown to be sufficient to both power the stimulator ASIC and elicit a response in retinal ganglion cells (RGCs), with the ability to generate of up to 35 000 pulses per second at the average stimulation threshold. This confirms the feasibility of generating a response in RGCs with an infrared-powered digital architecture capable of delivering complex sequential stimulation patterns at high repetition rates, albeit with some limitations.

Transcorneal electrical stimulation restores DNA methylation changes in retinal degeneration

Background

Retinal degeneration is a major cause of irreversible blindness. Stimulation with controlled low-level electrical fields, such as transcorneal electrical stimulation (TES), has recently been postulated as a therapeutic strategy. With promising results, there is a need for detailed molecular characterization of the therapeutic effects of TES.

Methods

Controlled, non-invasive TES was delivered using a custom contact lens electrode to the retinas of Royal College of Surgeons (RCS) rats, a model of retinal degeneration. DNA methylation in the retina, brain and cell-free DNA in plasma was assessed by reduced representation bisulfite sequencing (RRBS) and gene expression by RNA sequencing.

Results

TES induced DNA methylation and gene expression changes implicated in neuroprotection in the retina of RCS rats. We devised an epigenomic-based retinal health score, derived from DNA methylation changes observed with disease progression in RCS rats, and showed that TES improved the epigenomic health of the retina. TES also induced DNA methylation changes in the superior colliculus: the brain which is involved in integrating visual signaling. Lastly, we demonstrated that TES-induced retinal DNA methylation changes were detectable in cell-free DNA derived from plasma.

Conclusion

TES induced DNA methylation changes with therapeutic effects, which can be measured in circulation. Based on these changes, we were able to devise a liquid biopsy biomarker for retinal health. These findings shed light on the therapeutic potential and molecular underpinnings of TES, and provide a foundation for the further development of TES to improve the retinal health of patients with degenerative eye diseases.

Harms associated with retinal implantation of a stimulating electrode array to treat outer-retinal degeneration: a systematic review and meta-analysis of safety

Retinal implantation of an electrode array is an emerging treatment for vision loss caused by outer-retinal degeneration. This article collects and analyses harms associated with the treatment reported in the peer-reviewed literature, thus enabling informed decision-making by patients, clinicians, researchers, engineers, and policymakers. We searched MEDLINE, Embase, and clinical trials registries for peer-reviewed journal articles reporting harms outcomes. We extracted data from articles including study design, definitions of 'serious adverse event', and timing of adverse events. We applied the McMaster tool to these articles to assess the risk of bias in harms assessment and reporting. Our searches returned 585 abstracts. We reviewed the full text of 59 articles describing 11 different devices. McMaster scores ranged from 3 to 12 (maximum 15; higher scores indicate less risk). We compiled a comprehensive list of all serious and non-serious adverse events associated with retinal implantation. Several harms were common across devices. Our meta-analysis showed that serious adverse events are log-uniformly distributed throughout follow-up. Improved reporting and further clinical studies are needed to develop a reliable safety profile of retinal implantation. Our findings will help guide the design, conduct, and reporting of future clinical trials of retinal implantation and other emerging treatments for vision loss. (PROSPERO registration: CRD42022308123.).

Understanding responses to multi-electrode epiretinal stimulation using a biophysical model

Objective

Neural interfaces are designed to evoke specific patterns of electrical activity in populations of neurons by stimulating with many electrodes. However, currents passed simultaneously through multiple electrodes often combine nonlinearly to drive neural responses, making evoked responses difficult to predict and control. This response nonlinearity could arise from the interaction of many excitable sites in each cell, any of which can produce a spike. However, this multi-site activation hypothesis is difficult to verify experimentally.

Approach

We developed a biophysical model to study retinal ganglion cell (RGC) responses to multi-electrode stimulation and validated it using data collected from ex vivo preparations of the macaque retina using a microelectrode array (512 electrodes; 30µm pitch; 10µm diameter).

Results

First, the model was validated by using it to reproduce essential empirical findings from single-electrode recording and stimulation, including recorded spike voltage waveforms at multiple locations and sigmoidal responses to injected current. Then, stimulation with two electrodes was modeled to test how the positioning of the electrodes relative to the cell affected the degree of response nonlinearity. Currents passed through pairs of electrodes positioned near the cell body or far from the axon (>40 µm) exhibited approximately linear summation in evoking spikes. Currents passed through pairs of electrodes close to the axon summed linearly when their locations along the axon were similar, and nonlinearly otherwise. Over a range of electrode placements, several distinct, localized spike initiation sites were observed, and the number of these sites covaried with the degree of response nonlinearity. Similar trends were observed for three-electrode stimuli. All of these trends in the simulation were consistent with experimental observations. Significance . These findings support the multi-site activation hypothesis for nonlinear activation of neurons, providing a biophysical interpretation of previous experimental results and potentially enabling more efficient use of multi-electrode stimuli in future neural implants.

Precise control of neural activity using dynamically optimized electrical stimulation

Neural implants have the potential to restore lost sensory function by electrically evoking the complex naturalistic activity patterns of neural populations. However, it can be difficult to predict and control evoked neural responses to simultaneous multi-electrode stimulation due to nonlinearity of the responses. We present a solution to this problem and demonstrate its utility in the context of a bidirectional retinal implant for restoring vision. A dynamically optimized stimulation approach encodes incoming visual stimuli into a rapid, greedily chosen, temporally dithered and spatially multiplexed sequence of simple stimulation patterns. Stimuli are selected to optimize the reconstruction of the visual stimulus from the evoked responses. Temporal dithering exploits the slow time scales of downstream neural processing, and spatial multiplexing exploits the independence of responses generated by distant electrodes. The approach was evaluated using an experimental laboratory prototype of a retinal implant: large-scale, high-resolution multi-electrode stimulation and recording of macaque and rat retinal ganglion cells ex vivo. The dynamically optimized stimulation approach substantially enhanced performance compared to existing approaches based on static mapping between visual stimulus intensity and current amplitude. The modular framework enabled parallel extensions to naturalistic viewing conditions, incorporation of perceptual similarity measures, and efficient implementation for an implantable device. A direct closed-loop test of the approach supported its potential use in vision restoration.

Wide-angle simulated artificial vision enhances spatial navigation and object interaction in a naturalistic environment

Objective. Vision restoration approaches, such as prosthetics and optogenetics, provide visual perception to blind individuals in clinical settings. Yet their effectiveness in daily life remains a challenge. Stereotyped quantitative tests used in clinical trials often fail to translate into practical, everyday applications. On the one hand, assessing real-life benefits during clinical trials is complicated by environmental complexity, reproducibility issues, and safety concerns. On the other hand, predicting behavioral benefits of restorative therapies in naturalistic environments may be a crucial step before starting clinical trials to minimize patient discomfort and unmet expectations.Approach. To address this, we leverage advancements in virtual reality technology to conduct a fully immersive and ecologically valid task within a physical artificial street environment. As a case study, we assess the impact of the visual field size in simulated artificial vision for common outdoor tasks.Main results. We show that a wide visual angle (45°) enhances participants' ability to navigate and solve tasks more effectively, safely, and efficiently. Moreover, it promotes their learning and generalization capability. Concurrently, it changes the visual exploration behavior and facilitates a more accurate mental representation of the environment. Further increasing the visual angle beyond this value does not yield significant additional improvements in most metrics.Significance. We present a methodology combining augmented reality with a naturalistic environment, enabling participants to perceive the world as patients with retinal implants would and to interact physically with it. Combining augmented reality in naturalistic environments is a valuable framework for low vision and vision restoration research.

Graphene-based microelectrodes with bidirectional functionality for next-generation retinal electronic interfaces

Neuroelectronic prostheses are being developed for restoring vision at the retinal level in patients who have lost their sight due to photoreceptor loss. The core component of these devices is the electrode array, which enables interfacing with retinal neurons. Generating the perception of meaningful images requires high-density microelectrode arrays (MEAs) capable of precisely activating targeted retinal neurons. Achieving this precision necessitates the downscaling of electrodes to micrometer dimensions. However, miniaturization increases electrode impedance, which poses challenges by limiting the amount of current that can be delivered, thereby impairing the electrode's capability for effective neural modulation. Additionally, it elevates noise levels, reducing the signal quality of the recorded neural activity. This report focuses on evaluating reduced graphene oxide (rGO) based devices for interfacing with the retina, showcasing their potential in vision restoration. Our findings reveal low impedance and high charge injection limit for microscale rGO electrodes, confirming their suitability for developing next-generation high-density retinal devices. We successfully demonstrated bidirectional interfacing with cell cultures and explanted retinal tissue, enabling the identification and modulation of multiple cells' activity. Additionally, calcium imaging allowed real-time monitoring of retinal cell dynamics, demonstrating a significant reduction in activated areas with small-sized electrodes. Overall, this study lays the groundwork for developing advanced rGO-based MEAs for high-acuity visual prostheses.

Wireless and Opto-Stimulated Flexible Implants: Artificial Retina Constructed by Ferroelectric BiFeO3-BaTiO3/P(VDF-TrFE) Composites

The degeneration of retinal photoreceptors is one of the primary causes of blindness, and the implantation of retinal prostheses offers hope for vision restoration in individuals who are completely blind. Flexible bioelectronic devices present a promising avenue for the next generation of retinal prostheses owing to their soft mechanical properties and tissue friendliness. In this study, we developed flexible composite films of ferroelectric BiFeO3-BaTiO3 (BFO-BTO) particles synthesized by the hydrothermal method and ferroelectric poly(vinyldene difluoride-trifluoroethylene) (P(VDF-TrFE)) polymer and investigated their applications in artificial retinas. Owing to the coupling of the photothermal effect of BFO-BTO particles and the pyroelectric effect of the P(VDF-TrFE) polymer, the composite films demonstrate a strong photoelectric response (a maximum peak-to-peak photovoltage > 80 V under blue light of 100 mW/cm2) in a wide wavelength range of light (from visible to infrared) with the inherent flexibility and ease of preparation, making it an attractive candidate for artificial retinal applications. Experimental results showed that blind rats implanted with artificial retinas of the composites display light-responsive behavior, showcasing the effectiveness of vision restoration. This study demonstrates a novel approach for employing ferroelectric materials in vision restoration and offers insights into future artificial retina design.

Stochastic resonance in the sensory systems and its applications in neural prosthetics

Noise is generally considered to be detrimental. In the right conditions, however, noise can improve signal detection or information transmission. This counterintuitive phenomenon is called stochastic resonance (SR). SR has generated significant interdisciplinary interest, particularly in physics, engineering, and medical and environmental sciences. In this review, we discuss a growing empirical literature that suggests that noise at the right intensity may improve the detection and processing of auditory, sensorimotor, and visual stimuli. We focus particularly on applications of SR in sensory biology and investigate whether SR-based technologies present a pathway to improve outcomes for individuals living with sensory impairments. We conclude that there is considerable evidence supporting the application of SR in developing sensory prosthetics. However, the progression of SR-based technologies is variable across the sensory modalities. We suggest opportunities for further advancements in each modality, considering the best approaches to maximise benefits and capitalise on progress already made. Overall, SR can offer opportunities to improve existing technologies or to motivate innovations.

Prosthetic Visual Acuity with the PRIMA Subretinal Microchip in Patients with Atrophic Age-Related Macular Degeneration at 4 Years Follow-up

Objective

To assess the efficacy and safety of the PRIMA neurostimulation system with a subretinal microchip for improving visual acuity (VA) in patients with geographic atrophy (GA) due to age-related macular degeneration (AMD) at 48-months postimplantation.

Design

Feasibility clinical trial of the PRIMA subretinal prosthesis in patients with atrophic AMD, measuring best-corrected ETDRS VA (Clinicaltrials.govNCT03333954).

Subjects

Five patients with GA, no foveal light perception, and VA of logarithm of the minimum angle of resolution (logMAR) 1.3 to 1.7 (20/400-20/1000) in their worse-seeing "study" eye.

Methods

In patients subretinally implanted with a photovoltaic neurostimulation array containing 378 pixels of 100 μm in size, the VA was measured with and without the PRIMA system using ETDRS charts at 1 m. The system's external components, augmented reality glasses, and pocket computer provide image processing capabilities, including zoom.

Main Outcome Measures

Visual acuity using ETDRS charts with and without the system, as well as light sensitivity in the central visual field, measured by Octopus perimetry. Anatomical outcomes demonstrated by fundus photography and OCT up to 48 months postimplantation.

Results

All 5 subjects met the primary end point of light perception elicited by the implant in the scotoma area. In 1 patient, the implant was incorrectly inserted into the choroid. One subject died 18 months postimplantation due to study-unrelated reasons. ETDRS VA results for the remaining 3 subjects are reported here. Without zoom, VA closely matched the pixel size of the implant: 1.17 ± 0.13 pixels, corresponding to a mean logMAR of 1.39, or Snellen of 20/500, ranging from 20/438 to 20/565. Using zoom at 48 months, subjects improved their VA by 32 ETDRS letters versus baseline (standard error 5.1) 95% confidence intervals (13.4, 49.9; P < 0.0001). Natural peripheral visual function in the treated eye did not decline after surgery or during the 48-month follow-up period (P = 0.08).

Conclusions

Subretinal implantation of PRIMA in subjects with GA experiencing profound vision loss due to AMD is feasible and well tolerated, with no reduction of natural peripheral vision up to 48 months. Prosthetic central vision provided by photovoltaic neurostimulation enabled patients to reliably recognize letters and sequences of letters, and with zoom, it improved VA of up to 8 ETDRS lines.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Retinoic Acid-Dependent Loss of Synaptic Output from Bipolar Cells Impairs Visual Information Processing in Inherited Retinal Degeneration

In retinitis pigmentosa (RP), rod and cone photoreceptors degenerate, depriving downstream neurons of light-sensitive input, leading to vision impairment or blindness. Although downstream neurons survive, some undergo morphological and physiological remodeling. Bipolar cells (BCs) link photoreceptors, which sense light, to retinal ganglion cells (RGCs), which send information to the brain. While photoreceptor loss disrupts input synapses to BCs, whether BC output synapses remodel has remained unknown. Here we report that synaptic output from BCs plummets in RP mouse models of both sexes owing to loss of voltage-gated Ca2+ channels. Remodeling reduces the reliability of synaptic output to repeated optogenetic stimuli, causing RGC firing to fail at high-stimulus frequencies. Fortunately, functional remodeling of BCs can be reversed by inhibiting the retinoic acid receptor (RAR). RAR inhibitors targeted to BCs present a new therapeutic opportunity for mitigating detrimental effects of remodeling on signals initiated either by surviving photoreceptors or by vision-restoring tools.

Multimodal Imaging of Optic Nerve Head in Retinitis Pigmentosa

BACKGROUND

A pallor optic nerve head (ONH) is one of the three features of retinitis pigmentosa (RP). This study aimed to assess the ONH prospectively by color tone, presence of hyper-reflective tissue, blood flow, retinal nerve fiber layer (RNFL) thickness, ganglion cell complex (GCC) and investigate the change in these parameters with and without ONH pallor.

METHODS

The presence of ONH pallor was assessed by three independent examiners through careful examination using fundus photographs. The presence of a hyper-reflective structure on the ONH was carefully evaluated using a volume scan optical coherence tomography (OCT). RNFL thickness and ellipsoid zone (EZ) width around the macula were also evaluated by OCT. Laser speckle flowgraphy was used to measure the mean blur rate of the entire ONH area, which was subsequently divided into the vessel area (MV) and tissue area (MT).

RESULTS

Twenty-eight eyes of 28 patients with RP (55.4 ± 16.23 years of age) were included. The pale ONH was observed in 10 (35%) eyes. Hyper-reflective structures were observed in seven (25%) eyes. No significant correlation was found between the pale ONH and the presence of a hyper-reflective structure (Pearson's chi-squared test, p = .364). The average of the ONH area, MV, and MT was 8.65 ± 3.08 AU, 17.81 ± 7.54 AU, and 6.4 ± 2.66 AU, respectively, which significantly decreased in patients with pallor ONH (all p < .05). The global RNFL thickness was 73.54 ± 18.82 μm. The nasal and superior quadrants and global RNFL thickness in patients with a pale ONH were significantly thinner than in patients without a pale ONH (all p < .05). The global and superior and inferior GCC thickness in patients with a pale ONH were significantly thinner than in patients without a pale ONH(all p < .05).There was no difference in the EZ width between patients with and without a pale ONH (p = .107).

CONCLUSION

We conducted multiple assessments of the ONH in RP patients and investigated its clinical significance. Our findings suggest that ONH pallor may indicate a comprehensive change that emerges alongside the progression of retinal degeneration in RP.

TRIAL REGISTRATION

This trial was retrospectively registered in the UMIN Clinical Trial Registry (UMIN ID: 000048168).

Comparison of modulation efficiency between normal and degenerated primate retina

With electrical stimulation, retinal prostheses bypass dysfunctional photoreceptors and activate the surviving bipolar or retinal ganglion cells (RGCs). Therefore, the effective modulation of RGCs is crucial for developing retinal prostheses. Substantial research has been performed on the ability of an electrical stimulus to generate a reliable RGC response. However, different experimental conditions show varying levels of how well the electrical stimulation evokes RGC spikes. Therefore, in this study, we attempted to extract an indicator to understand how the electrical stimulation effectively evokes RGC spikes. Six cynomolgus monkeys were used: three as controls and three as an N-methyl-N-nitrosourea (MNU)-induced retinal degeneration model. The retinal recordings were performed using 8 × 8 multi-electrode arrays (MEAs). Electrical stimulation consisted of symmetrical biphasic pulses of varying amplitudes and durations. The number of stimulation conditions that resulted in significantly higher post-stimulation firing rates than pre-stimulus firing rates was defined as the modulation efficiency ratio (MER). The MER was significantly lower in degenerated retinas than in normal retinas. We investigated the relationship between the variables and the MER in normal and degenerated primate RGCs. External variables, such as duration and inter-electrode distance, and internal variables, such as average firing rates and statistics (mean, standard deviation, and coefficient of variation [CV]) of inter-spike intervals (ISIs) of spontaneous spikes, were used. External variables had similar effects on MER in normal and degenerated RGCs. In contrast, internal variables affected MER differently in normal and degenerated RGCs. While in normal RGCs, they were not related to MER, in degenerated RGCs, the mean ISIs were positively correlated with MER, and the CV of ISIs was negatively correlated with MER. The most important variable affecting MER was the mean ISI. A shorter ISI indicates hyperactive firing in the degenerated retina, which prevents electrical stimulation from evoking more RGCs. We believe that this hyperactivity in degenerated retinas results in a lower MER than that in the normal retina. Our findings can be used to optimize the selection of stimulation channels for in vitro MEA experiments and practical calibration methods to achieve higher efficiency when testing retinal prostheses.

Design and simulation of artificial retinal stimulation IC with switched capacitor using Si nanowire optical properties

This study introduces an approach for converting the current from a sensor into controllable voltage. To this end, a switched-capacitor structure was integrated to provide efficient current-to-voltage conversion. The generated voltage was further regulated by an operational amplifier current source, enhancing stability and precision. An n-type metal oxide semiconductor field-effect transistor structure under an H-bridge was integrated into the system to achieve fine-tuned control over current stimulation. This component contributed to voltage regulation and enabled bi-directional control of current flow, offering versatility in adjusting current amplitudes using working and counter electrodes. This dynamic control mechanism was pivotal for effectively controlling the intensity of current stimulation. We applied Verilog-A modeling to simulate the optical characteristics of Si nanowires. The proposed system efficiently converted sensor-derived current into voltage using a switched-capacitor structure. Simultaneously, the precision was enhanced via operational amplifier regulation and n-type metal-oxide-semiconductor field-effect transistor-based H-bridge control. The simulation showed a current stimulus amplitude ranging from 2 to 13 μA for a variable photocurrent of Si nanowires (Rex: 10 kΩ, pulse: 100 Hz, 1 ms). The ability to finely control current stimulation intensity holds promise for diverse applications requiring accurate and adjustable current manipulation. This study contributes to the growing field of sensor technology by offering a unique perspective on the integration of nanostructures and electronic components for an enhanced control and functionality.

Visual Cortical Thickness Increases with Prolonged Artificial Vision Restoration

The Argus II retinal prosthesis restores visual perception to late blind patients. It has been shown that structural changes occur in the brain due to late-onset blindness, including cortical thinning in visual regions of the brain. Following vision restoration, it is not yet known whether these visual regions are reinvigorated and regain a normal cortical thickness or retain the diminished thickness from blindness. We evaluated the cortical thicknesses of ten Argus II Retinal Prostheses patients, ten blind patients, and thirteen sighted participants. The Argus II patients on average had a thicker left Cuneus Cortex and Lateral Occipital Cortex relative to the blind patients. The duration of the Argus II use (time since implant in active users) significantly partially correlated with thicker visual cortical regions in the left hemisphere. Furthermore, in the two case studies (scanned before and after implantation), the patient with longer device use (44.5 months) had an increase in the cortical thickness of visual regions, whereas the shorter-using patient did not (6.5 months). Finally, a third case, scanned at three time points post-implantation, showed an increase in cortical thickness in the Lateral Occipital Cortex between 43.5 and 57 months, which was maintained even after 3 years of disuse (106 months). Overall, the Argus II patients' cortical thickness was on average significantly rejuvenated in two higher visual regions and, patients using the implant for a longer duration had thicker visual regions. This research raises the possibility of structural plasticity reversing visual cortical atrophy in late-blind patients with prolonged vision restoration.

Photovoltaic, wireless wide-field epiretinal prosthesis to treat retinitis pigmentosa

PURPOSE

To develop and evaluate a photovoltaic, wireless wide-field epiretinal prosthesis for the treatment of retinitis pigmentosa.

METHODS

A mosaic array of thinned silicon-based photodiodes with integrated thin-film stimulation electrodes was fabricated with a flexible polyimide substrate film to form a film-based miniaturized electronic system with wireless optical power and signal transmission and integrated electrostimulation. Manufactured implants were characterized with respect to their optoelectronic performance and biocompatibility following DIN EN ISO 10993.

RESULTS

A 14 mm diameter prosthesis containing 1276 pixels with a maximum sensitivity at a near infrared wavelength of 905 nm and maximized stimulation current density 30-50 μm below the electrodes was developed for direct activation of retinal ganglion cells during epiretinal stimulation. Fabricated prostheses demonstrated mucosal tolerance and the preservation of both metabolic activity, proliferation and membrane integrity of human fibroblasts as well as the retinal functions of bovine retinas. Illumination of the prosthesis, which was placed epiretinally on an isolated perfused bovine retina, with infrared light resulted in electrophysiological recordings reminiscent of an a-wave (hyperpolarization) and b-wave (depolarization).

CONCLUSIONS

A photovoltaic, wireless wide-field epiretinal prosthesis for the treatment of retinitis pigmentosa using near infrared light for signal transmission was designed, manufactured and its biocompatibility and functionality demonstrated in vitro and ex vivo.

The mechanism of human color vision and potential implanted devices for artificial color vision

Vision plays a major role in perceiving external stimuli and information in our daily lives. The neural mechanism of color vision is complicated, involving the co-ordinated functions of a variety of cells, such as retinal cells and lateral geniculate nucleus cells, as well as multiple levels of the visual cortex. In this work, we reviewed the history of experimental and theoretical studies on this issue, from the fundamental functions of the individual cells of the visual system to the coding in the transmission of neural signals and sophisticated brain processes at different levels. We discuss various hypotheses, models, and theories related to the color vision mechanism and present some suggestions for developing novel implanted devices that may help restore color vision in visually impaired people or introduce artificial color vision to those who need it.

Retinal prosthesis edge detection (RPED) algorithm: Low-power and improved visual acuity strategy for artificial retinal implants

This paper proposes a retinal prosthesis edge detection (RPED) algorithm that can achieve high visual acuity and low power. Retinal prostheses have been used to stimulate retinal tissue by injecting charge via an electrode array, thereby artificially restoring the vision of visually impaired patients. The retinal prosthetic chip, which generates biphasic current pulses, should be located in the foveal area measuring 5 mm × 5 mm. When a high-density stimulation pixel array is realized in a limited area, the distance between the stimulation pixels narrows, resulting in current dispersion and high-power dissipation related to heat generation. Various edge detection methods have been proposed over the past decade to reduce these deleterious effects and achieve high-resolution pixels. However, conventional methods have the disadvantages of high-power consumption and long data processing times because many pixels are activated to detect edges. In this study, we propose a novel RPED algorithm that has a higher visual acuity and less power consumption despite using fewer active pixels than existing techniques. To verify the performance of the devised RPED algorithm, the peak signal-to-noise ratio and structural similarity index map, which evaluates the quantitative numerical value of the image are employed and compared with the Sobel, Canny, and past edge detection algorithms in MATLAB. Finally, we demonstrate the effectiveness of the proposed RPED algorithm using a 1600-pixel retinal stimulation chip fabricated using a 0.35 μm complementary metal-oxide-semiconductor process.

Information Transmission through Biotic-Abiotic Interfaces to Restore or Enhance Human Function

Advancements in reliable information transfer across biotic-abiotic interfaces have enabled the restoration of lost human function. For example, communication between neuronal cells and electrical devices restores the ability to walk to a tetraplegic patient and vision to patients blinded by retinal disease. These impactful medical achievements are aided by tailored biotic-abiotic interfaces that maximize information transfer fidelity by considering the physical properties of the underlying biological and synthetic components. This Review develops a modular framework to define and describe the engineering of biotic and abiotic components as well as the design of interfaces to facilitate biotic-abiotic information transfer using light or electricity. Delineating the properties of the biotic, interface, and abiotic components that enable communication can serve as a guide for future research in this highly interdisciplinary field. Application of synthetic biology to engineer light-sensitive proteins has facilitated the control of neural signaling and the restoration of rudimentary vision after retinal blindness. Electrophysiological methodologies that use brain-computer interfaces and stimulating implants to bypass spinal column injuries have led to the rehabilitation of limb movement and walking ability. Cellular interfacing methodologies and on-chip learning capability have been made possible by organic transistors that mimic the information processing capacity of neurons. The collaboration of molecular biologists, material scientists, and electrical engineers in the emerging field of biotic-abiotic interfacing will lead to the development of prosthetics capable of responding to thought and experiencing touch sensation via direct integration into the human nervous system. Further interdisciplinary research will improve electrical and optical interfacing technologies for the restoration of vision, offering greater visual acuity and potentially color vision in the near future.

Enhanced organic photovoltaic-based retinal prosthesis using a cathode-modified structure with plasmonic silver nanoparticles: a computational study

Introduction

Organic interfaces have recently emerged as a breakthrough trend in biomedical applications, demonstrating exceptional performance in stimulating retinal neuronal cells owing to their high flexibility and compatibility with tissues. However, the primary challenge associated with organic photovoltaics is their low efficiency compared to that of their inorganic counterparts. Among different approaches, embedding plasmonic metal nanoparticles (NPs) in active or buffer layers can efficiently improve photovoltaic cell performance.

Methods

A cathode decorated with silver nanoparticles is introduced to increase the absorption Phenomenon and improve the interface performance as a computational study. In addition to embedding spherical silver nanoparticles in the active layer (A-AgNPs), a monolayer array of spherical AgNPs in the cathode electrode (K-AgNPs) is incorporated. In this configuration, the large K-AgNPs play dual roles: acting as cathode electrode and serving as plasmonic centers to increase light trapping and absorption. The bulk heterojunction PCPDTBT:PCBM is chosen as the active layer due to its favorable electronic properties.

Results

Our computational analysis demonstrates a notable 10% enhancement in the photovoltaic cell current density for the developed structure with K-AgNPs in contrast to without them. Additionally, the simulation results reveal that the modeled device achieves a two-fold efficiency of the bare photovoltaic cell (without A-AgNPs and K-AgNPs), which is particularly evident at a low intensity of 0.26 mW/mm2.

Discussion

This study aims to propose an efficient epiretinal prosthesis structure using a different strategy for plasmonic effects rather than conventional methods, such as incorporating NPs into the active or buffer layer. This structure can prevent the harmful side effects of using large metal NPs (r > 10 nm) in the active layer during exciton quenching, charge trapping, and recombination, which deteriorate the power conversion efficiency (PCE).

Measuring spatial visual loss in rats by retinotopic mapping of the superior colliculus using a novel multi-electrode array technique

BACKGROUND

The retinotopic map property of the superior colliculus (SC) is a reliable indicator of visual functional changes in rodents. Electrophysiological mapping of the SC using a single electrode has been employed for measuring visual function in rat and mouse disease models. Single electrode mapping is highly laborious requiring long-term exposure to the SC surface and prolonged anesthetic conditions that can adversely affect the mapping data.

NEW METHOD

To avoid the above-mentioned issues, we fabricated a fifty-six (56) electrode multi-electrode array (MEA) for rapid and reliable visual functional mapping of the SC. Since SC is a dome-shaped structure, the array was made of electrodes with dissimilar tip lengths to enable simultaneous and uniform penetration of the SC.

RESULTS

SC mapping using the new MEA was conducted in retinal degenerate (RD) Royal College of Surgeons (RCS) rats and rats with focal retinal damage induced by green diode laser. For SC mapping, the MEA was advanced into the SC surface and the visual activities were recorded during full-filed light stimulation of the eye. Based on the morphological examination, the MEA electrodes covered most of the exposed SC area and penetrated the SC surface at a relatively uniform depth. MEA mapping in RCS rats (n=9) demonstrated progressive development of a scotoma in the SC that corresponded to the degree of photoreceptor loss. MEA mapping in the laser damaged rats demonstrated the presence of a scotoma in the SC area that corresponded to the location of retinal laser injury.

COMPARISON WITH EXISTING METHODS AND CONCLUSIONS

The use of MEA for SC mapping is advantageous over single electrode recording by enabling faster recordings and reducing anesthesia time. This study establishes the feasibility of the MEA technique for rapid and efficient SC mapping, particularly advantageous for evaluating therapeutic effects in retinal degenerate rat disease models.

Asymmetric Activation of ON and OFF Pathways in the Degenerated Retina

Retinal prosthetics are one of the leading therapeutic strategies to restore lost vision in patients with retinitis pigmentosa and age-related macular degeneration. Much work has described patterns of spiking in retinal ganglion cells (RGCs) in response to electrical stimulation, but less work has examined the underlying retinal circuitry that is activated by electrical stimulation to drive these responses. Surprisingly, little is known about the role of inhibition in generating electrical responses or how inhibition might be altered during degeneration. Using whole-cell voltage-clamp recordings during subretinal electrical stimulation in the rd10 and wild-type (wt) retina, we found electrically evoked synaptic inputs differed between ON and OFF RGC populations, with ON cells receiving mostly excitation and OFF cells receiving mostly inhibition and very little excitation. We found that the inhibition of OFF bipolar cells limits excitation in OFF RGCs, and a majority of both pre- and postsynaptic inhibition in the OFF pathway arises from glycinergic amacrine cells, and the stimulation of the ON pathway contributes to inhibitory inputs to the RGC. We also show that this presynaptic inhibition in the OFF pathway is greater in the rd10 retina, compared with that in the wt retina.

Avoidance of axonal stimulation with sinusoidal epiretinal stimulation

Objective.Neuromodulation, particularly electrical stimulation, necessitates high spatial resolution to achieve artificial vision with high acuity. In epiretinal implants, this is hindered by the undesired activation of distal axons. Here, we investigate focal and axonal activation of retinal ganglion cells (RGCs) in epiretinal configuration for different sinusoidal stimulation frequencies.Approach.RGC responses to epiretinal sinusoidal stimulation at frequencies between 40 and 100 Hz were tested inex-vivophotoreceptor degenerated (rd10) isolated retinae. Experiments were conducted using a high-density CMOS-based microelectrode array, which allows to localize RGC cell bodies and axons at high spatial resolution.Main results.We report current and charge density thresholds for focal and distal axon activation at stimulation frequencies of 40, 60, 80, and 100 Hz for an electrode size with an effective area of 0.01 mm2. Activation of distal axons is avoided up to a stimulation amplitude of 0.23µA (corresponding to 17.3µC cm-2) at 40 Hz and up to a stimulation amplitude of 0.28µA (14.8µC cm-2) at 60 Hz. The threshold ratio between focal and axonal activation increases from 1.1 for 100 Hz up to 1.6 for 60 Hz, while at 40 Hz stimulation frequency, almost no axonal responses were detected in the tested intensity range. With the use of synaptic blockers, we demonstrate the underlying direct activation mechanism of the ganglion cells. Finally, using high-resolution electrical imaging and label-free electrophysiological axon tracking, we demonstrate the extent of activation in axon bundles.Significance.Our results can be exploited to define a spatially selective stimulation strategy avoiding axonal activation in future retinal implants, thereby solving one of the major limitations of artificial vision. The results may be extended to other fields of neuroprosthetics to achieve selective focal electrical stimulation.

Honeycomb-Patterned Graphene Microelectrodes: A Promising Approach for Safe and Effective Retinal Stimulation Based on Electro-Thermo-Mechanical Modeling and Simulation

The main objective of the present study is to use graphene as electrode neural interface material to design novel microelectrodes topology for retinal prosthesis and investigate device operation safety based on the computational framework. The study's first part establishes the electrode material selection based on electrochemical impedance and the equivalent circuit model. The second part of the study is modeling at the microelectrode-tissue level to investigate the potential distribution, generated resistive heat dissipation, and thermally induced stress in the tissue due to electrical stimulation. The formulation of Joule heating and thermal expansion between microelectrode-tissue-interface employing finite element method modeling is based on the three coupled equations, specifically Ohm's law, Navier's equation, and Fourier equation. Electrochemical simulation results of electrode material reveal that single-layer and few-layer graphene-based microelectrode has a specific impedance in the range of 0.02- [Formula: see text], comparable to platinum counterparts. The microelectrode of [Formula: see text] size can stimulate retinal tissue with a threshold current in the range of 8.7- [Formula: see text]. Such stimulation with the observed microelectrode size indicates that both microelectrodes and retinal tissue stay structurally intact, and the device is thermally and mechanically stable, functioning within the safety limit. The results reveal the viability of high-density graphene-based microelectrodes for improved interface as stimulating electrodes to acquire higher visual acuity. Furthermore, the novel microelectrodes design configuration in the honeycomb pattern gives the retinal tissue non-invasive heating and minimal stress upon electrical stimulation. Thus, it paves the path to designing a graphene-based microelectrode array for retinal prosthesis for further in vitro or in vivo studies.

Wireless Battery-free and Fully Implantable Organ Interfaces

Advances in soft materials, miniaturized electronics, sensors, stimulators, radios, and battery-free power supplies are resulting in a new generation of fully implantable organ interfaces that leverage volumetric reduction and soft mechanics by eliminating electrochemical power storage. This device class offers the ability to provide high-fidelity readouts of physiological processes, enables stimulation, and allows control over organs to realize new therapeutic and diagnostic paradigms. Driven by seamless integration with connected infrastructure, these devices enable personalized digital medicine. Key to advances are carefully designed material, electrophysical, electrochemical, and electromagnetic systems that form implantables with mechanical properties closely matched to the target organ to deliver functionality that supports high-fidelity sensors and stimulators. The elimination of electrochemical power supplies enables control over device operation, anywhere from acute, to lifetimes matching the target subject with physical dimensions that supports imperceptible operation. This review provides a comprehensive overview of the basic building blocks of battery-free organ interfaces and related topics such as implantation, delivery, sterilization, and user acceptance. State of the art examples categorized by organ system and an outlook of interconnection and advanced strategies for computation leveraging the consistent power influx to elevate functionality of this device class over current battery-powered strategies is highlighted.

Electrical Input Filters of Ganglion Cells in Wild Type and Degenerating rd10 Mouse Retina as a Template for Selective Electrical Stimulation

Bionic vision systems are currently limited by indiscriminate activation of all retinal ganglion cells (RGCs)- despite the dozens of known RGC types which each encode a different visual message. Here, we use spike-triggered averaging to explore how electrical responsiveness varies across RGC types toward the goal of using this variation to create type-selective electrical stimuli. A battery of visual stimuli and a randomly distributed sequence of electrical pulses were delivered to healthy and degenerating (4-week-old rd10) mouse retinas. Ganglion cell spike trains were recorded during stimulation using a 60-channel microelectrode array. Hierarchical clustering divided the recorded RGC populations according to their visual and electrical response patterns. Novel electrical stimuli were presented to assess type-specific selectivity. In healthy retinas, responses fell into 35 visual patterns and 14 electrical patterns. In degenerating retinas, responses fell into 12 visual and 23 electrical patterns. Few correspondences between electrical and visual response patterns were found except for the known correspondence of ON visual type with upward deflecting electrical type and OFF cells with downward electrical profiles. Further refinement of the approach presented here may yet yield the elusive nuances necessary for type-selective stimulation. This study greatly deepens our understanding of electrical input filters in the context of detailed visual response characterization and includes the most complete examination yet of degenerating electrical input filters.

ISCEV and IPS guideline for the full-field stimulus test (FST)

The full-field stimulus test (FST) is a psychophysical technique designed for the measurement of visual function in low vision. The method involves the use of a ganzfeld stimulator, as used in routine full-field electroretinography, to deliver full-field flashes of light. This guideline was developed jointly by the International Society for Clinical Electrophysiology of Vision (ISCEV) and Imaging and Perimetry Society (IPS) in order to provide technical information, promote consistency of testing and reporting, and encourage convergence of methods for FST. It is intended to aid practitioners and guide the formulation of FST protocols, with a view to future standardisation.

Full-field stimulus threshold testing: a scoping review of current practice

The full-field stimulus threshold (FST) is a psychophysical measure of whole-field retinal light sensitivity. It can assess residual visual function in patients with severe retinal disease and is increasingly being adopted as an endpoint in clinical trials. FST applications in routine ophthalmology clinics are also growing, but as yet there is no formalised standard guidance for measuring FST. This scoping review explored current variability in FST conduct and reporting, with an aim to inform further evidence synthesis and consensus guidance. A comprehensive electronic search and review of the literature was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-analysis Extension for Scoping Reviews (PRISMA-ScR) checklist. Key source, participant, methodology and outcomes data from 85 included sources were qualitatively and quantitatively compared and summarised. Data from 85 sources highlight how the variability and insufficient reporting of FST methodology, including parameters such as units of flash luminance, colour, duration, test strategy and dark adaptation, can hinder comparison and interpretation of clinical significance across centres. The review also highlights an unmet need for paediatric-specific considerations for test optimisation. Further evidence synthesis, empirical research or structured panel consultation may be required to establish coherent standardised guidance on FST methodology and context or condition dependent modifications. Consistent reporting of core elements, most crucially the flash luminance equivalence to 0 dB reference level is a first step. The development of criteria for quality assurance, calibration and age-appropriate reference data generation may further strengthen rigour of measurement.

Artificial Vision: The High-Frequency Electrical Stimulation of the Blind Mouse Retina Decay Spike Generation and Electrogenically Clamped Intracellular Ca2+ at Elevated Levels

BACKGROUND

The electrical stimulation (stim) of retinal neurons enables blind patients to experience limited artificial vision. A rapid response outage of the stimulated ganglion cells (GCs) allows for a low visual sensation rate. Hence, to elucidate the underlying mechanism, we investigated different stim parameters and the role of the neuromodulator calcium (Ca2+).

METHODS

Subretinal stim was applied on retinal explants (blind rd1 mouse) using multielectrode arrays (MEAs) or single metal electrodes, and the GC activity was recorded using Ca2+ imaging or MEA, respectively. Stim parameters, including voltage, phase polarity, and frequency, were investigated using specific blockers.

RESULTS

At lower stim frequencies (<5 Hz), GCs responded synaptically according to the stim pulses (stim: biphasic, cathodic-first, -1.6/+1.5 V). In contrast, higher stim frequencies (≥5 Hz) also activated GCs directly and induced a rapid GC spike response outage (<500 ms, MEA recordings), while in Ca2+ imaging at the same frequencies, increased intracellular Ca2+ levels were observed.

CONCLUSIONS

Our study elucidated the mechanisms involved in stim-dependent GC spike response outage: sustained high-frequency stim-induced spike outage, accompanied by electrogenically clamped intracellular Ca2+ levels at elevated levels. These findings will guide future studies optimizing stim paradigms for electrical implant applications for interfacing neurons.

A Novel Surgical Approach for Big Sheet Allogenic Retinal Pigment Epithelium-Bruch Membrane Complex Transplantation Into the Subretinal Space

PURPOSE

Allogenic transplantation of retinal pigmented epithelium monolayer sheet has experienced bottlenecks due to imperfect surgical techniques. In this study, we developed a novel approach for allogenic transplantation of big sheets of retinal pigment epithelium (RPE)-Bruch membrane complex.

METHODS

RPE-Bruch membrane complex sheets of 5 × 6 mm2 to 10 × 10 mm2 were taken from donated eyes. Through a novel approach, the sheets of RPE-Bruch membrane complex were transplanted into the subretinal space of eight eyes (8 patients) with late-stage retinitis pigmentosa. The patients were followed up for 5 ± 2 months.

RESULTS

All RPE-Bruch membrane complexes were successfully inserted into the subretinal space during the surgery. Follow-up examinations also showed that the grafts attached well to the transplantation site. No rejection or retinal detachment was found.

CONCLUSION

Through our technique, big sheets of allogenic RPE-Bruch membrane complexes could be implanted into the subretinal space smoothly. This novel approach may be useful for big sheet of allogenic RPE-derived or stem cells-derived RPE transplantation in the treatment of RP and other retinal dystrophic diseases.

Effects on Retinal Stimulation of the Geometry and the Insertion Location of Penetrating Electrodes

Retinal implants have been developed and implanted to restore vision from outer retinal degeneration, but their performance is still limited due to the poor spatial resolution. To improve the localization of stimulation, microelectrodes in various three-dimensional (3D) shapes have been investigated. In particular, computational simulation is crucial for optimizing the performance of a novel microelectrode design before actual fabrication. However, most previous studies have assumed a uniform conductivity for the entire retina without testing the effect of electrodes placement in different layers. In this study, we used the finite element method to simulate electric fields created by 3D microelectrodes of three different designs in a retina model with a stratified conductivity profile. The three electrode designs included two conventional shapes - a conical electrode (CE) and a pillar electrode (PE); we also proposed a novel structure of pillar electrode with an insulating wall (PEIW). A quantitative comparison of these designs shows the PEIW generates a stronger and more confined electric field with the same current injection, which is preferred for high-resolution retinal prostheses. Moreover, our results demonstrate both the magnitude and the shape of potential distribution generated by a penetrating electrode depend not only on the geometry, but also substantially on the insertion depth of the electrode. Although epiretinal insertions are mainly discussed, we also compared results for subretinal insertions. The results provide valuable insights for improving the spatial resolution of retinal implants using 3D penetrating microelectrodes and highlight the importance of considering the heterogeneity of conductivities in the retina.

Electrophysiological properties of layer 2/3 pyramidal neurons in the primary visual cortex of a retinitis pigmentosa mouse model (rd10)

Retinal degeneration is one of the main causes of visual impairment and blindness. One group of retinal degenerative diseases, leading to the loss of photoreceptors, is collectively termed retinitis pigmentosa. In this group of diseases, the remaining retina is largely spared from initial cell death making retinal ganglion cells an interesting target for vision restoration methods. However, it is unknown how downstream brain areas, in particular the visual cortex, are affected by the progression of blindness. Visual deprivation studies have shown dramatic changes in the electrophysiological properties of visual cortex neurons, but changes on a cellular level in retinitis pigmentosa have not been investigated yet. Therefore, we used the rd10 mouse model to perform patch-clamp recordings of pyramidal neurons in layer 2/3 of the primary visual cortex to screen for potential changes in electrophysiological properties resulting from retinal degeneration. Compared to wild-type C57BL/6 mice, we only found an increase in intrinsic excitability around the time point of maximal retinal degeneration. In addition, we saw an increase in the current amplitude of spontaneous putative inhibitory events after a longer progression of retinal degeneration. However, we did not observe a long-lasting shift in excitability after prolonged retinal degeneration. Together, our results provide evidence of an intact visual cortex with promising potential for future therapeutic strategies to restore vision.

Suppressing Retinal Remodeling to Mitigate Vision Loss in Photoreceptor Degenerative Disorders

Rod and cone photoreceptors degenerate in retinitis pigmentosa and age-related macular degeneration, robbing the visual system of light-triggered signals necessary for sight. However, changes in the retina do not stop with the photoreceptors. A stereotypical set of morphological and physiological changes, known as remodeling, occur in downstream retinal neurons. Some aspects of remodeling are homeostatic, with structural or functional changes compensating for partial loss of visual inputs. However, other aspects are nonhomeostatic, corrupting retinal information processing to obscure vision mediated naturally by surviving photoreceptors or artificially by vision-restoration technologies. In this review, I consider the mechanism of remodeling and its consequences for residual and restored visual function; discuss the role of retinoic acid, a critical molecular trigger of detrimental remodeling; and discuss strategies for suppressing retinoic acid biosynthesis or signaling as therapeutic possibilities for mitigating vision loss.

Electronic Retinal Prostheses

Retinal prostheses are a promising means for restoring sight to patients blinded by photoreceptor atrophy. They introduce visual information by electrical stimulation of the surviving inner retinal neurons. Subretinal implants target the graded-response secondary neurons, primarily the bipolar cells, which then transfer the information to the ganglion cells via the retinal neural network. Therefore, many features of natural retinal signal processing can be preserved in this approach if the inner retinal network is retained. Epiretinal implants stimulate primarily the ganglion cells, and hence should encode the visual information in spiking patterns, which, ideally, should match the target cell types. Currently, subretinal arrays are being developed primarily for restoration of central vision in patients impaired by age-related macular degeneration (AMD), while epiretinal implants-for patients blinded by retinitis pigmentosa, where the inner retina is less preserved. This review describes the concepts and technologies, preclinical characterization of prosthetic vision and clinical outcomes, and provides a glimpse into future developments.

A systematic review of extended reality (XR) for understanding and augmenting vision loss

Over the past decade, extended reality (XR) has emerged as an assistive technology not only to augment residual vision of people losing their sight but also to study the rudimentary vision restored to blind people by a visual neuroprosthesis. A defining quality of these XR technologies is their ability to update the stimulus based on the user's eye, head, or body movements. To make the best use of these emerging technologies, it is valuable and timely to understand the state of this research and identify any shortcomings that are present. Here we present a systematic literature review of 227 publications from 106 different venues assessing the potential of XR technology to further visual accessibility. In contrast to other reviews, we sample studies from multiple scientific disciplines, focus on technology that augments a person's residual vision, and require studies to feature a quantitative evaluation with appropriate end users. We summarize prominent findings from different XR research areas, show how the landscape has changed over the past decade, and identify scientific gaps in the literature. Specifically, we highlight the need for real-world validation, the broadening of end-user participation, and a more nuanced understanding of the usability of different XR-based accessibility aids.

Bioengineering strategies for restoring vision

Late-stage retinal degenerative disease involving photoreceptor loss can be treated by optogenetic therapy, cell transplantation and retinal prostheses. These approaches aim to restore light sensitivity to the retina as well as visual perception by integrating neuronal responses for transmission to the cortex. In age-related macular degeneration, some cell-based therapies also aim to restore photoreceptor-supporting tissue to prevent complete photoreceptor loss. In the earlier stages of degeneration, gene-replacement therapy could attenuate retinal-disease progression and reverse loss of function. And gene-editing strategies aim to correct the underlying genetic defects. In this Review, we highlight the most promising gene therapies, cell therapies and retinal prostheses for the treatment of retinal disease, discuss the benefits and drawbacks of each treatment strategy and the factors influencing whether functional tissue is reconstructed and repaired or replaced with an electronic device, and summarize upcoming technologies for enhancing the restoration of vision.

Modeling of dielectric resonator antenna array for retina photoreceptors

The retina encompasses several cone and rod photoreceptors at fovea region i.e., 90 million cells of rod photoreceptors and 4.5million cells of cone photoreceptors. The overall photoreceptors determine the vision of every human. An electromagnetic dielectric resonator antenna has been presented for retina photoreceptors in order to model them at fovea and its peripheral retina with the respected angular spectrum. Three coloring primary system of human eye (R, G, B) can be realized based on the model. Three miscellaneous models i.e., simple, graphene coated, and interdigital models have been presented in this paper. The nonlinear property of interdigital structures is one of the best advantages to use for creating the capacitor. The capacitance property helps improving the upper band of visible spectrum. The absorption of light for graphene as an energy harvesting material and its conversion into electrochemical signals is making it one of the best models. The mentioned three electromagnetic models of human photoreceptors have been expressed as a receiver antenna. The proposed electromagnetic models based on dielectric resonator antenna (DRA) are being analyzed for cones and rods photoreceptors of retina in the human eye by Finite Integral Method (FIM) utilized by CST MWS. The results show that the models are so fine for vision spectrum due to its localized near field enhancement property. The results indicate fine parameters of S 11 (return loss below -10 dB) with invaluable resonants in a wide range of frequencies from 405 THz to 790 THz (vision spectrum), appropriate S 21 (insertion loss 3-dB bandwidth), very good field distribution of electric and magnetic fields for flowing the power and electrochemical signals. Finally, mfERG clinical and experimental results validate the numeric results by the normalized output to input ratio of these models and it points out that these models can stimulate the electrochemical signals in photoreceptor cells for the best suiting of realizing the new retinal implants.

Evaluation of the Long-Term Clinical Results of 3 Patients Implanted with the Argus II Retinal Prosthesis

This study presents the long-term clinical results of Argus II retinal prosthesis implantation in eyes with light perception and projection in 3 patients with end-stage retinitis pigmentosa. No conjunctival erosion, hypotony, or implant displacement was observed during postoperative follow-up. The electrical threshold values were lower in the macular region and higher close to the tack fixation region and peripherally. Optical coherence tomography scans showed fibrosis and retinoschisis formation at the retina-implant interface in two cases. This was attributed to mechanical and electrical effects on the tissue due to the active daily use of the system and the electrodes' proximity to the retina. The patients were able to integrate the system into their daily lives and perform activities that they could not do before. Studies on retinal prostheses for the rehabilitation of hereditary retinal diseases are ongoing, so both social and clinical observations and experiences related to the implant are valuable.

Retinitis Pigmentosa: Novel Therapeutic Targets and Drug Development

Retinitis pigmentosa (RP) is a heterogeneous group of hereditary diseases characterized by progressive degeneration of retinal photoreceptors leading to progressive visual decline. It is the most common type of inherited retinal dystrophy and has a high burden on both patients and society. This condition causes gradual loss of vision, with its typical manifestations including nyctalopia, concentric visual field loss, and ultimately bilateral central vision loss. It is one of the leading causes of visual disability and blindness in people under 60 years old and affects over 1.5 million people worldwide. There is currently no curative treatment for people with RP, and only a small group of patients with confirmed RPE65 mutations are eligible to receive the only gene therapy on the market: voretigene neparvovec. The current therapeutic armamentarium is limited to retinoids, vitamin A supplements, protection from sunlight, visual aids, and medical and surgical interventions to treat ophthalmic comorbidities, which only aim to slow down the progression of the disease. Considering such a limited therapeutic landscape, there is an urgent need for developing new and individualized therapeutic modalities targeting retinal degeneration. Although the heterogeneity of gene mutations involved in RP makes its target treatment development difficult, recent fundamental studies showed promising progress in elucidation of the photoreceptor degeneration mechanism. The discovery of novel molecule therapeutics that can selectively target specific receptors or specific pathways will serve as a solid foundation for advanced drug development. This article is a review of recent progress in novel treatment of RP focusing on preclinical stage fundamental research on molecular targets, which will serve as a starting point for advanced drug development. We will review the alterations in the molecular pathways involved in the development of RP, mainly those regarding endoplasmic reticulum (ER) stress and apoptotic pathways, maintenance of the redox balance, and genomic stability. We will then discuss the therapeutic approaches under development, such as gene and cell therapy, as well as the recent literature identifying novel potential drug targets for RP.

Therapeutic landscape for inherited ocular diseases: Current and emerging therapies

Inherited ocular diseases comprise a heterogeneous group of rare and complex diseases, including inherited retinal diseases (IRDs) and inherited optic neuropathies. Recent success in adeno-associated virus-based gene therapy, voretigene neparvovec (Luxturna^®) for RPE65-related IRDs, has heralded rapid evolution in gene therapy platform technologies and strategies, from gene augmentation to RNA editing, as well as gene agnostic approaches such as optogenetics. This review discusses the fundamentals underlying the mode of inheritance, natural history studies and clinical trial outcomes, as well as current and emerging therapies covering gene therapy strategies, cell-based therapies and bionic vision.

2D Semiconductor Based Flexible Photoresponsive Ring Oscillators for Artificial Vision Pixels

Artificial retina implantation provides an effective and feasible attempt for vision recovery in addition to retinal transplantation. The most advanced artificial retinas ever developed based on silicon technology are rigid and thus less compatible with the biosystem. Here we demonstrate flexible photoresponsive ring oscillators (PROs) based on the 2D semiconductor MoS2 for artificial retinas. Under natural light illuminations, arrayed PROs on flexible substrates serving as vision pixels can efficiently output light-intensity-dependent electrical pulses that are processable and transmittable in the human visual nerve system. Such PROs can work under low supply voltages below 1 V with a record-low power consumption, e.g. only 12.4 nW at a light intensity of 10 mW/cm2, decreased by ∼500 times compared with that of the state-of-the-art silicon devices. Such flexible artificial retinas with a simple device structure, high light-to-signal conversion efficiency, ultralow power consumption, and high tunability provide an alternative prosthesis for further clinical trials.

Focal electrical stimulation of human retinal ganglion cells for vision restoration

Objective. Vision restoration with retinal implants is limited by indiscriminate simultaneous activation of many cells and cell types, which is incompatible with reproducing the neural code of the retina. Recent work has shown that primate retinal ganglion cells (RGCs), which transmit visual information to the brain, can be directly electrically activated with single-cell, single-spike, cell-type precision - however, this possibility has never been tested in the human retina. In this study we aim to characterize, for the first time, direct in situ extracellular electrical stimulation of individual human RGCs.Approach. Extracellular electrical stimulation of individual human RGCs was conducted in three human retinas ex vivo using a custom large-scale, multi-electrode array capable of simultaneous recording and stimulation. Measured activation properties were compared directly to extensive results from macaque.Main results. Precise activation was in many cases possible without activating overlying axon bundles, at low stimulation current levels similar to those used in macaque. The major RGC types could be identified and targeted based on their distinctive electrical signatures. The measured electrical activation properties of RGCs, combined with a dynamic stimulation algorithm, was sufficient to produce an evoked visual signal that was nearly optimal given the constraints of the interface.Significance. These results suggest the possibility of high-fidelity vision restoration in humans using bi-directional epiretinal implants.

Challenges of Treatment Methodologies and the Future of Gene Therapy and Stem Cell Therapy to Treat Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a heterogeneous group of hereditary retinal degenerations for which there is currently no cure. Studies investigating the use of gene therapy, gene editing, and stem cells as potential treatment strategies have shown promising results in animal models and some early clinical trials. Even still, major barriers still exist, including the ability to develop therapies that can target the wide range of mutational etiologies and phenotypic presentations that encompass RP. Additionally, effective screening and early diagnosis are crucial for maximum therapeutic potential, especially because many therapeutic agents require a baseline level photoreceptor function.

High spatial resolution artificial vision inferred from the spiking output of retinal ganglion cells stimulated by optogenetic and electrical means

With vision impairment affecting millions of people world-wide, various strategies aiming at vision restoration are being undertaken. Thanks to decades of extensive research, electrical stimulation approaches to vision restoration began to undergo clinical trials. Quite recently, another technique employing optogenetic therapy emerged as a possible alternative. Both artificial vision restoration strategies reported poor spatial resolution so far. In this article, we compared the spatial resolution inferred ex vivo under ideal conditions using a computational model analysis of the retinal ganglion cell (RGC) spiking activity. The RGC spiking was stimulated in epiretinal configuration by either optogenetic or electrical means. RGCs activity was recorded from the ex vivo retina of transgenic late-stage photoreceptor-degenerated mice (rd10) using a high-density Complementary Metal Oxide Semiconductor (CMOS) based microelectrode array. The majority of retinal samples were stimulated by both, optogenetic and electrical stimuli using a spatial grating stimulus. A population-level analysis of the spiking activity of identified RGCs was performed and the spatial resolution achieved through electrical and optogenetic photo-stimulation was inferred using a support vector machine classifier. The best f1 score of the classifier for the electrical stimulation in epiretinal configuration was 86% for 32 micron wide gratings and increased to 100% for 128 microns. For optogenetically activated cells, we obtained high f1 scores of 82% for 10 microns grid width for a photo-stimulation frequency of 2.5 Hz and 73% for a photo-stimulation frequency of 10 Hz. A subsequent analysis, considering only the RGCs modulated in both electrical and optogenetic stimulation protocols revealed no significant difference in the prediction accuracy between the two stimulation modalities. The results presented here indicate that a high spatial resolution can be achieved for electrical or optogenetic artificial stimulation using the activated retinal ganglion cell output.

Ultrasound Retinal Stimulation: A Mini-Review of Recent Developments

Ultrasound neuromodulation is an emerging technology. A significant amount of effort has been devoted to investigating the feasibility of noninvasive ultrasound retinal stimulation. Recent studies have shown that ultrasound can activate neurons in healthy and degenerated retinas. Specifically, high-frequency ultrasound can evoke localized neuron responses and generate patterns in visual circuits. In this review, we recapitulate pilot studies on ultrasound retinal stimulation, compare it with other neuromodulation technologies, and discuss its advantages and limitations. An overview of the opportunities and challenges to develop a noninvasive retinal prosthesis using high-frequency ultrasound is also provided.