Chemical stimulation of rat retinal neurons: feasibility of an epiretinal neurotransmitter-based prosthesis

A hybrid bioelectronic retina-probe interface for object recognition

Retina converts light stimuli into spike firings, encoding abundant visual information critical for both fundamental studies of the visual system and therapies for visual diseases. However, probing these spikes directly from the retina is hindered by limited recording channels, insufficient contact between the retina and electrodes, and short operational lifetimes. In this study, we developed a perforated and flexible microelectrode array to achieve a robust retina-probe interface, ensuring high-quality detection of spike firings from hundreds of neurons. Leveraging the retina's natural light-sensing ability, we created a hybrid bioelectronic system that enables image recognition through machine learning integration. We systematically explored the system's spatial resolution, and demonstrated its capability to recognize different colors and light intensities. Importantly, due to the perforated structure, the hybrid system maintained over 94 % accuracy in distinguishing light on/off conditions for 9 h ex vivo. Finally, inspired by the eye's configuration, we developed a bioelectronic mimic eye capable of recognizing objects in real environments. This work demonstrated that the hybrid bioelectronic retina-probe interface is effective not only for light sensing but also for efficient image and object recognition.

Gold Nanorod-Coated Hydrogel Brush Valves in Macroporous Silicon Membranes for NIR-Driven Localized Chemical Modulation

A two-dimensional array of microfluidic ports with remote-controlled valve actuation is of great interest for applications involving localized chemical stimulation. Herein, a macroporous silicon-based platform where each pore contains an independently controllable valve made from poly(N-isopropylacrylamide) (PNIPAM) brushes is proposed. These valves are coated with silica-encapsulated gold nanorods (GNRs) for NIR-actuated switching capability. The layer-by-layer (LBL) electrostatic deposition technique was used to attach the GNRs to the PNIPAM brushes. The deposition of GNRs was confirmed by dark-field optical microscopy, and the localized surface plasmon resonance (LSPR) of the deposited GNRs was analyzed using UV-Vis spectra. To evaluate the chemical release behaviors, fluorescein dye was employed as a model substance. The chemical release properties, like OFF-state diffusion through the valve, the ratio between ON-state and OFF-state chemical release, and the rapidness of chemical modulation of the valve, were investigated, varying the PNIPAM brush thickness. The results indicate that enhancing the thickness of the PNIPAM brush in our platform improves control over the chemical modulation properties. However, excessive increases in brush length may lead to entanglement, which negatively impacts the chemical modulation efficiency.

A tissue-engineered neural interface with photothermal functionality

Neural interfaces are well-established as a tool to understand the behaviour of the nervous system via recording and stimulation of living neurons, as well as serving as neural prostheses. Conventional neural interfaces based on metals and carbon-based materials are generally optimised for high conductivity; however, a mechanical mismatch between the interface and the neural environment can significantly reduce long-term neuromodulation efficacy by causing an inflammatory response. This paper presents a soft composite material made of gelatin methacryloyl (GelMA) containing graphene oxide (GO) conjugated with gold nanorods (AuNRs). The soft hydrogel presents stiffness within the neural environment range of modulus below 5 kPa, while the AuNRs, when exposed to light in the near infrared range, provide a photothermal response that can be used to improve the spatial and temporal precision of neuromodulation. These favourable properties can be maintained at safer optical power levels when combined with electrical stimulation. In this paper we provide mechanical and biological characterization of the optical activity of the GO-AuNR composite hydrogel. The optical functionality of the material has been evaluated via photothermal stimulation of explanted rat retinal tissue. The outcomes achieved with this study encourage further investigation into optical and electrical costimulation parameters for a range of biomedical applications.

Graphene-based microfluidic perforated microelectrode arrays for retinal electrophysiological studies

Perforated microelectrode arrays (pMEAs) have become essential tools for ex vivo retinal electrophysiological studies. pMEAs increase the nutrient supply to the explant and alleviate the accentuated curvature of the retina, allowing for long-term culture and intimate contacts between the retina and electrodes for electrophysiological measurements. However, commercial pMEAs are not compatible with in situ high-resolution optical imaging and lack the capability of controlling the local microenvironment, which are highly desirable features for relating function to anatomy and probing physiological and pathological mechanisms in retina. Here we report on microfluidic pMEAs (μpMEAs) that combine transparent graphene electrodes and the capability of locally delivering chemical stimulation. We demonstrate the potential of μpMEAs by measuring the electrical response of ganglion cells to locally delivered high K+ stimulation under controlled microenvironments. Importantly, the capability for high-resolution confocal imaging of the retina tissue on top of the graphene electrodes allows for further analyses of the electrical signal source. The new capabilities provided by μpMEAs could allow for retinal electrophysiology assays to address key questions in retinal circuitry studies.

Progress on Designing a Chemical Retinal Prosthesis

The last major review of progress toward a chemical retinal prosthesis was a decade ago. Many important advancements have been made since then with the aim of producing an implantable device for animal testing. We review that work here discussing the potential advantages a chemical retinal prosthesis may possess, the spatial and temporal resolutions it might provide, the materials from which an implant might be constructed and its likely effectiveness in stimulating the retina in a natural fashion. Consideration is also given to implant biocompatibility, excitotoxicity of dispensed glutamate and known changes to photoreceptor degenerate retinas.

Neuromodulation using electroosmosis

Objective.Our laboratory has proposed chemical stimulation of retinal neurons using exogenous glutamate as a biomimetic strategy for treating vision loss caused by photoreceptor (PR) degenerative diseases. Although our previousin-vitrostudies using pneumatic actuation indicate that chemical retinal stimulation is achievable, an actuation technology that is amenable to microfabrication, as needed for anin-vivoimplantable device, has yet to be realized. In this study, we sought to evaluate electroosmotic flow (EOF) as a mechanism for delivering small quantities of glutamate to the retina. EOF has great potential for miniaturization.Approach.An EOF device to dispense small quantities of glutamate was constructed and its ability to drive retinal output tested in anin-vitropreparation of PR degenerate rat retina.Main results.We built and tested an EOF microfluidic system, with 3D printed and off-the-shelf components, capable of injecting small volumes of glutamate in a pulsatile fashion when a low voltage control signal was applied. With this device, we produced excitatory and inhibitory spike rate responses in PR degenerate rat retinae. Glutamate evoked spike rate responses were also observed to be voltage-dependent and localized to the site of injection.Significance.The EOF device performed similarly to a previously tested conventional pneumatic microinjector as a means of chemically stimulating the retina while eliminating the moving plunger of the pneumatic microinjector that would be difficult to miniaturize and parallelize. Although not implantable, the prototype device presented here as a proof of concept indicates that a retinal prosthetic based on EOF-driven chemical stimulation is a viable and worthwhile goal. EOF should have similar advantages for controlled dispensing of charged neurochemicals at any neural interface.

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.

Mechanical Stimulation of the Retina: Therapeutic Feasibility and Cellular Mechanism

Retinal prostheses that seek to restore vision by artificially stimulating retinal neurons with electrical current are an emerging treatment for photoreceptor degenerative diseases but face difficulties achieving naturalistic vision with high spatial resolution. Here, we report the unexpected discovery of a technique for mechanically stimulating retinal neurons with the potential to bypass the limitations of electrical stimulation. We found that pulsatile injections of standard Ames medium solution into explanted retinas of wild type rats under certain injection conditions (pulse-width > 50ms at 0.69 kPa pressure) elicit spatially localized retinal responses similar to light-evoked responses. The same injections made into photoreceptor degenerated retinas of transgenic S334ter-3 rats also elicit robust neural responses. We investigated the cellular mechanism causing these responses, by repeating the injections after treating the retinas with a pharmacological blocker of the transient receptor potential vanilloid (TRPV) channel group, a common mechanoreceptor found on retinal neurons, and observed a significant reduction in retinal ganglion cell spike rate response amplitudes. Together, these data reveal that therapeutic mechanical stimulation of the retina, occurring in part through TRPV channel activation, is feasible and this little explored neurostimulation paradigm could be useful in stimulating photoreceptor degenerated retinas for vision restoration.

Investigation of Injection Depth for Subretinal Delivery of Exogenous Glutamate to Restore Vision via Biomimetic Chemical Neuromodulation

Chemical neuromodulation of the retina using native neurotransmitters to biomimetically activate target retinal neurons through chemical synapses is a promising biomimetic alternative to electrical stimulation for restoring vision in blindness caused by photoreceptor degenerative diseases. Recent research has shown that subretinal chemical stimulation could be advantageous for treating photoreceptor degenerative diseases but many of the parameters for achieving efficacious chemical neuromodulation are yet to be explored. In this paper, we investigated how the depth at which neurotransmitter is injected subretinally affects the success rate, spike rate characteristics (i.e., amplitude, response latency, and time width), and spatial resolution of chemical stimulation in wild-type Long Evans and photoreceptor degenerated S334ter-3 transgenic rat retinas in vitro. We compared the responses to injections of glutamate at the subretinal surface and two subsurface depths near the outer and inner plexiform layers and found that while injections at all depths elicited robust retinal ganglion cell responses, they differed significantly in terms of the spike rate characteristics and spatial resolutions across injection depths. Shallow subsurface injections near the outer plexiform layer evoked the highest spike rate amplitudes and had the highest spatial resolution and success rates, while deep subsurface injections near the inner plexiform layer elicited the shortest latencies and narrowest time widths. Our results suggest that surface injections are suboptimal for subretinal chemical neuromodulation, while shallow subsurface and deep subsurface injections may optimize high spatial and high temporal resolution, respectively. These findings have great significance for the design and development of a potential neurotransmitter-based subretinal prosthesis.

Electrical activation of degenerated photoreceptors in blind mouse retina elicited network-mediated responses in different types of ganglion cells

Electrical (e-) stimulation is explored in schemes to rescue the vision of blind people, e.g. those affected by Retinitis Pigmentosa (RP). We e-activated subretinally the surviving degenerated photoreceptors (d-Phrs) of the rd1 mouse (RP model) and evoked visual responses in the blind retina. The e-stimulation was applied with a single platinum/iridium electrode. The d-Phrs (calcium-imaging) and ganglion cells (GC) activity (MEA-recording) were recorded in simultaneous multilayer recordings. The findings of this study confirm that the d-Phrs responded to e-stimulation and modulated the retinal network-activity. The application of blockers revealed that the synaptic interactions were dependent on voltage-gated calcium channels and mediated by the transmitters glutamate and GABA. Moreover, the gap junctions coupled networks promoted the lateral-spread of the e-evoked activity in the outer (~60 µm) and inner (~120 µm) retina. The activated GCs were identified as subtypes of the ON, OFF and ON-OFF classes. In conclusion, d-Phrs are the ideal interface partners for implants to elicit enhanced visual responses at higher temporal and spatial resolution. Furthermore, the retina's intact circuity at the onset of complete blindness makes it a tempting target when considering the implantation of implants into young patients to provide a seamless transition from blinding to chip-aided vision.

Feasibility study for a glutamate driven subretinal prosthesis: local subretinal application of glutamate on blind retina evoke network-mediated responses in different types of ganglion cells

OBJECTIVE

A feasibility study for a transmitter based subretinal prosthesis, generating visual responses in blind mouse retina is presented.

APPROACH

Degenerated rd1 mouse retina were stimulated in subretinal configuration by local glutamate (Glu) or NMDA application via micropipettes (~1.5 μm) and thereby the outer retinal activity was recorded by calcium-imaging or the ganglion cell (GC) activity was recorded by the multi-electrode array system. The network mediated activation of GC via bipolar cells was approved by the administration of Glu receptor blockers.

MAIN RESULTS

Data of the degenerated and blind rd1 mouse retina reveals that the outer retina is Glu sensitive and that the subretinal Glu stimulation promotes network mediated GC responses. Analysis of the spatial activity-spread indicates that the Glu induced cell activation radius in the outer retina (~12.5 μm) and postsynaptically activated GC (~40 μm) is focal to the stimulation pipette tip. Moreover, the application of NMDA in subretinal space also evoked network mediated GC responses. The Glu-activated GC were identified as ON-OFF, OFF and two ON cells types.

SIGNIFICANCE

This study evaluates the prerequisite for the function of a transmitter based implant, that after the loss of the photoreceptors, the remnant blind retinal network is Glu sensitive and functional, positively. The differential activation of ON (hyperpolarisation) and OFF (depolarisation) bipolar cells by transmitter Glu is a unique feature and of high interest for retinal implants. Therefore, the respective bipolar cell types could only be driven by glutamatergic stimulation accurately and not by electrical stimulation. The preserved functionality of the blind retina at the onset of complete blindness is motivating to continue research on a transmitter-based prosthesis. Since the artificial Glu stimulation mimics the natural retinal input, early implantation of a Glu-prosthesis might delay the devastating retinal remodelling positively, due to the neuronal-plasticity.

Methodology for Biomimetic Chemical Neuromodulation of Rat Retinas with the Neurotransmitter Glutamate In Vitro

Photoreceptor degenerative diseases cause irreparable blindness through the progressive loss of photoreceptor cells in the retina. Retinal prostheses are an emerging treatment for photoreceptor degenerative diseases that seek to restore vision by artificially stimulating the surviving retinal neurons in the hope of eliciting comprehensible visual perception in patients. Current retinal prostheses have demonstrated success in restoring limited vision to patients using an array of electrodes to electrically stimulate the retina but face substantial physical barriers in restoring high acuity, natural vision to patients. Chemical neurostimulation using native neurotransmitters is a biomimetic alternative to electrical stimulation and could bypass the fundamental limitations associated with retinal prostheses using electrical neurostimulation. Specifically, chemical neurostimulation has the potential to restore more natural vision with comparable or better visual acuities to patients by injecting very small quantities of neurotransmitters, the same natural agents of communication used by retinal chemical synapses, at much finer resolution than current electrical prostheses. However, as a relatively unexplored stimulation paradigm, there is no established protocol for achieving chemical stimulation of the retina in vitro. The purpose of this work is to provide a detailed framework for accomplishing chemical stimulation of the retina for investigators who wish to study the potential of chemical neuromodulation of the retina or similar neural tissues in vitro. In this work, we describe the experimental setup and methodology for eliciting retinal ganglion cell (RGC) spike responses similar to visual light responses in wild-type and photoreceptor-degenerated wholemount rat retinas by injecting controlled volumes of the neurotransmitter glutamate into the subretinal space using glass micropipettes and a custom multiport microfluidic device. This methodology and protocol are general enough to be adapted for neuromodulation using other neurotransmitters or even other neural tissues.

Advances in Retinal Prosthetic Research: A Systematic Review of Engineering and Clinical Characteristics of Current Prosthetic Initiatives

PURPOSE

To date, reviews of retinal prostheses have focused primarily on devices undergoing human trials in the Western Hemisphere and fail to capture significant advances in materials and engineering research in countries such as Japan and Korea, as well as projects in early stages of development. To address these gaps, this systematic review examines worldwide advances in retinal prosthetic research, evaluates engineering characteristics and clinical progress of contemporary device initiatives, and identifies potential directions for future research in the field of retinal prosthetics.

METHODS

A literature search using PubMed, Google Scholar, and IEEExplore was conducted following the PRISMA Guidelines for Systematic Review. Inclusion criteria were peer-reviewed papers demonstrating progress in human or animal trials and papers discussing the prosthetic engineering design. For each initiative, a description of the device, its engineering considerations, and recent clinical results were provided.

RESULTS

Ten prosthetic initiatives met our inclusion criteria and were organized by stimulation location. Of these initiatives, four have recently completed human trials, three are undergoing multi- or single-center human trials, and three are undergoing preclinical animal testing. Only the Argus II (FDA 2013, CE 2011) has obtained FDA approval for use in the United States; the Alpha-IMS (CE 2013) has achieved the highest visual acuity using a Landolt-C test to date and is the only device presently undergoing a multicenter clinical trial.

CONCLUSION

Several distinct approaches to retinal stimulation have been successful in eliciting visual precepts in animals and/or humans. However, many clinical needs are still not met and engineering challenges must be addressed before a retinal prosthesis with the capability to fully and safely restore functional vision can be realized.

Prototype chemical synapse chip for spatially patterned neurotransmitter stimulation of the retina ex vivo

Biomimetic stimulation of the retina with neurotransmitters, the natural agents of communication at chemical synapses, could be more effective than electrical stimulation for treating blindness from photoreceptor degenerative diseases. Recent studies have demonstrated the feasibility of neurotransmitter stimulation by injecting glutamate, a primary retinal neurotransmitter, into the retina at isolated single sites. Here, we demonstrate spatially patterned multisite stimulation of the retina with glutamate, offering the first experimental evidence for applicability of this strategy for translating visual patterns into afferent neural signals. To accomplish pattern stimulation, we fabricated a special microfluidic device comprising an array of independently addressable microports connected to tiny on-chip glutamate reservoirs via microchannels. The device prefilled with glutamate was interfaced with explanted rat retinas placed over a multielectrode array (MEA) with the retinal ganglion cells (RGC) contacting the electrodes and photoreceptor surface contacting the microports. By independently and simultaneously activating a subset of the microports with modulated pressure pulses, small boluses of glutamate were convectively injected at multiple sites in alphabet patterns over the photoreceptor surface. We found that the glutamate-driven RGC responses recorded through the MEA system were robust and spatially laid out in patterns strongly resembling the injection patterns. The stimulations were also highly localized with spatial resolutions comparable to or better than electrical retinal prostheses. Our findings suggest that surface stimulation of the retina with neurotransmitters in pixelated patterns of visual images is feasible and an artificial chemical synapse chip based on this approach could potentially circumvent the limitations of electrical retinal prostheses.

Differential stimulation of the retina with subretinally injected exogenous neurotransmitter: A biomimetic alternative to electrical stimulation

Subretinal stimulation of the retina with neurotransmitters, the normal means of conveying visual information, is a potentially better alternative to electrical stimulation widely used in current retinal prostheses for treating blindness from photoreceptor degenerative diseases. Yet, no subretinal electrical or chemical stimulation study has stimulated the OFF and ON pathways differentially through inner retinal activation. Here, we demonstrate the feasibility of differentially stimulating retinal ganglion cells (RGCs) through the inner nuclear layer of the retina with glutamate, a primary neurotransmitter chemical, in a biomimetic way. We show that controlled pulsatile delivery of glutamate into the subsurface of explanted wild-type rat retinas elicits highly localized simultaneous inhibitory and excitatory spike rate responses in OFF and ON RGCs. We also present the spatiotemporal characteristics of RGC responses to subretinally injected glutamate and the therapeutic stimulation parameters. Our findings could pave the way for future development of a neurotransmitter-based subretinal prosthesis offering more naturalistic vision and better visual acuity than electrical prostheses.

Can Nanofluidic Chemical Release Enable Fast, High Resolution Neurotransmitter-Based Neurostimulation?

Artificial chemical stimulation could provide improvements over electrical neurostimulation. Physiological neurotransmission between neurons relies on the nanoscale release and propagation of specific chemical signals to spatially-localized receptors. Current knowledge of nanoscale fluid dynamics and nanofluidic technology allows us to envision artificial mechanisms to achieve fast, high resolution neurotransmitter release. Substantial technological development is required to reach this goal. Nanofluidic technology—rather than microfluidic—will be necessary; this should come as no surprise given the nanofluidic nature of neurotransmission. This perspective reviews the state of the art of high resolution electrical neuroprostheses and their anticipated limitations. Chemical release rates from nanopores are compared to rates achieved at synapses and with iontophoresis. A review of microfluidic technology justifies the analysis that microfluidic control of chemical release would be insufficient. Novel nanofluidic mechanisms are discussed, and we propose that hydrophobic gating may allow control of chemical release suitable for mimicking neurotransmission. The limited understanding of hydrophobic gating in artificial nanopores and the challenges of fabrication and large-scale integration of nanofluidic components are emphasized. Development of suitable nanofluidic technology will require dedicated, long-term efforts over many years.