Design and validation of a foldable and photovoltaic wide-field epiretinal prosthesis

Biomechanics characterization of an implantable ultrathin intracortical electrode through finite element method

Neural electrodes are widely used in brain-computer interfaces and neuroprosthesis for the treatment of various neurological disorders. However, as components that come into direct contact with neural tissue, implanted neural electrodes could cause mechanical damage during surgical insertions or while inside the brain. Thus, accurately and timely assessing this damage was vital for chronic implantation, which posed a significant challenge. This study aimed to evaluate the biomechanical effects and clinical application risks of a polyimide-based ultrathin flexible intracortical microelectrode through the finite element method (FEM). It analyzed the electrode-brain biomechanical effects during the electrode's insertion process and under steady-state acceleration with the electrode inside the brain. Furthermore, the study examined the impact of factors including implantation depth (ranging from 5 to 5000 μm), cortical thickness (0.5 mm, 2.5 mm, and 4.5 mm), step displacement (from 1 to 5 μm) during insertion, and acceleration direction (vertical and horizontal) on the electrode's biomechanical effects. The primary findings showed that the 98th percentile Von Mises Strain (ε98) and Von Mises Stress (σ98) in the region of interest (ROI) decreased dual-exponentially with increasing implantation depth and increased linearly with larger step displacements. Compared to the Von Mises strain threshold of 14.65%, as proposed by Sahoo et al., indicating a 50% risk of diffuse axonal injury (DAI), it was recommended to limit the initial step displacement during insertion to 1 μm, increasing to 5 μm at deeper locations (over 500 μm) to balance safety and efficiency. Additionally, it was found that cortical thickness had a negligible impact during insertion and while experiencing steady-state acceleration in vivo, with the three fitted curves almost coinciding when cortical thicknesses were 0.5 mm, 2.5 mm, and 4.5 mm. The flexible electrode exhibited excellent mechanical performance under steady-state acceleration in vivo, with ε98 being less than 0.3% and σ98 being less than 50 Pa, although it was more sensitive to horizontal acceleration. Thus, it could be concluded that during long-duration accelerations from transportation modes such as elevators and high-speed trains, the electrode's mechanical effects on brain tissue could be neglected, demonstrating long-term mechanical stability. This research was significant for guiding surgical insertion and clinical applications of flexible electrodes.

Poly(3-hexylthiophene) as a versatile semiconducting polymer for cutting-edge bioelectronics

Semiconducting polymers (SPs), widely used in organic optoelectronics, are gaining interest in bioelectronics owing to their intrinsic optical properties, conductivity, biocompatibility, flexibility, and chemical tunability. Among them, poly(3-hexylthiophene) (P3HT) has attracted great attention as a versatile SP, being both optically active and conductive, for the fabrication of smart materials (e.g., films and nanoparticles), allowing the modulation of their performance and final biomedical applications. This review article provides an overview of the design of different kinds of P3HT-based materials, from chemical properties to structural engineering, to be used as key opto-electronic components in the development of opto-transducers for the modulation of cell fate, as well as biosensors such as organic electrochemical transistors (OECTs) and organic field effect transistors (OEFTs). Finally, their foremost applications in the biomedical field ranging from tissue engineering to biosensing will be discussed, including the future perspectives of P3HT derivatives towards cutting-edge applications for bioelectronics, in which optoceutics plays a key role.

Metal-Organic Frameworks Modified Organic Bulk Heterojunction Interfaces for Effective Nongenetic Neuromodulation

Photoactive organic semiconductors, such as bulk heterojunctions (BHJs) of donor-acceptor pairs, are promising for building flexible devices for nongenetic and precise optical neuromodulation. However, the full potential of the diverse compositions and functionalities of BHJs has yet to be explored for neuromodulation due to their unsatisfactory interfaces with soft biotissues, which hinder signal transduction, tissue adhesion, and biocompatibility. Here, we address these challenges by introducing an interfacial layer composed of conductive and porous metal-organic frameworks (MOFs). The MOFs layer enhances charge injection capacity at the interface by >400 times and ensures tight and biocompatible junction between BHJs and biological materials. These improvements enable efficient electrical-to-ionic signal transduction for various BHJs, supporting reliable nongenetic modulation of cultured mouse hippocampal neurons under deep-red and near-infrared light. Moreover, flexible devices made from MOFs-modified BHJs allow for the in vivo stimulation of rat sciatic nerves at an ultralow light intensity threshold (0.01 mW mm-2), 700 times lower than that required for unmodified devices. This interfacial engineering with porous MOFs can expand the material toolbox of BHJs-based photocapacitors and unlock more functionalities for neuromodulation and prosthetic biointerfaces.

Perovskite Quantum Dot-Based Photovoltaic Biointerface for Photostimulation of Neurons

OBJECTIVE

A promising avenue for vision restoration against retinal degeneration is the use of semiconductor-based photovoltaic biointerfaces to substitute natural photoreceptors. Instead of silicon, perovskite has emerged as an exciting material for solar energy harvesting, and its nanocrystalline forms generally offer better stability than their bulk counterparts in addition to the distinct synthesis and fabrication steps.

METHODS

Herein, we synthesize tetramethylammonium lead iodide (TMAPbI3) perovskite quantum dots (QDs) as a novel photoactive material for photovoltaic biointerfaces. While the TMAPbI3 quantum dots and electrolyte interface induces Faradaic photocurrent under light illumination, the heterojunction with P3HT converts the charge-transfer process to a safe capacitive photocurrent with an improved ionic responsivity of 17.4 mA/W.

SIGNIFICANCE

The integration of the 18-nm quantum dot thickness shows good biocompatibility with primary cultures of hippocampal neurons and the photoresponse of the biointerface triggered photostimulation of the neurons. The rise of perovskite materials can stimulate novel forms of photovoltaic retina implants.

Nitrogen-Doped Ultrananocrystalline Diamond - Optoelectronic Biointerface for Wireless Neuronal Stimulation

This study presents a semiconducting optoelectronic system for light-controlled non-genetic neuronal stimulation using visible light. The system architecture is entirely wireless, comprising a thin film of nitrogen-doped ultrananocrystalline diamond directly grown on a semiconducting silicon substrate. When immersed in a physiological medium and subjected to pulsed illumination in the visible (595 nm) or near-infrared wavelength (808 nm) range, charge accumulation at the device-medium interface induces a transient ionic displacement current capable of electrically stimulating neurons with high temporal resolution. With a measured photoresponsivity of 7.5 mA W-1, the efficacy of this biointerface is demonstrated through optoelectronic stimulation of degenerate rat retinas using 595 nm irradiation, pulse durations of 50-500 ms, and irradiance levels of 1.1-4.3 mW mm-2, all below the safe ocular threshold. This work presents the pioneering utilization of a diamond-based optoelectronic platform, capable of generating sufficiently large photocurrents for neuronal stimulation in the retina.

Machine Learning Techniques for Simulating Human Psychophysical Testing of Low-Resolution Phosphene Face Images in Artificial Vision

To evaluate the quality of artificial visual percepts generated by emerging methodologies, researchers often rely on labor-intensive and tedious human psychophysical experiments. These experiments necessitate repeated iterations upon any major/minor modifications in the hardware/software configurations. Here, the capacity of standard machine learning (ML) models is investigated to accurately replicate quaternary match-to-sample tasks using low-resolution facial images represented by arrays of phosphenes as input stimuli. Initially, the performance of the ML models trained to approximate innate human facial recognition abilities across a dataset comprising 3600 phosphene images of human faces is analyzed. Subsequently, due to the time constraints and the potential for subject fatigue, the psychophysical test is limited to presenting only 720 low-resolution phosphene images to 36 human subjects. Notably, the superior model adeptly mirrors the behavioral trend of human subjects, offering precise predictions for 8 out of 9 phosphene quality levels on the overlapping test queries. Subsequently, human recognition performances for untested phosphene images are predicted, streamlining the process and minimizing the need for additional psychophysical tests. The findings underscore the transformative potential of ML in reshaping the research paradigm of visual prosthetics, facilitating the expedited advancement of prostheses.

Design, Fabrication, and Surgical Testing of the 3D-Printed Large-Array Port-System for the Implantation of Large Epiretinal Stimulators

Purpose

In the treatment of blindness causing retinal dystrophies, that is, retinitis pigmentosa (RP), retinal implants showed promising results. Recently, larger devices restoring a greater visual field were introduced. With larger size, implantation surgery became more difficult. In this study, a novel implantation device was developed, fabricated, and tested in implantation surgeries. The goal was to demonstrate a reproducible, safe, and, in comparison, superior implantation method.

Methods

The novel implantation device 3D-Printed Large-Array Port-System (3D-PLAPS) was designed using computer-aided design software. Anatomic dimensions of rabbit, pig, and human eyes were collected from anatomic and histological data sources. The 3D-PLAPS were 3D-printed. In cadaveric porcine and rabbit eyes, 3D-PLAPS was used to implant large epiretinal stimulators developed by this group. A standardized surgical procedure was established. Intraocular pressure (IOP) was measured.

Results

The 3D-PLAPS implantation device was designed with a length of 8.4 mm and adapted to the curvature of normal sighted human eyes with a diameter of 24.0 mm. The elliptical aperture is 7.0 mm in length and 1.0 mm in width at its widest points. Marginal apertures for scleral fixation were added. A closing plug was introduced. Design and dimensions were adapted for rabbit eyes. During surgery, the 3D-PLAPS improved ocular stability, sealed the incision, and withstood an elevated IOP. It was suitable for foldable stimulators with a diameter of up to 14.0 mm.

Conclusions

The 3D-PLAPS implantation device showed feasibility in implantation of large epiretinal stimulators and possibly also facilitates repositioning of stimulating arrays in acute experiments without the necessity for additional surgical steps.

Translational Relevance

The 3D printing and CAD software are used to applied surgery for large epiretinal stimulators.

Ultrathin rubbery bio-optoelectronic stimulators for untethered cardiac stimulation

Untethered electrical stimulation or pacing of the heart is of critical importance in addressing the pressing needs of cardiovascular diseases in both clinical therapies and fundamental studies. Among various stimulation methods, light illumination-induced electrical stimulation via photoelectric effect without any genetic modifications to beating cells/tissues or whole heart has profound benefits. However, a critical bottleneck lies in the lack of a suitable material with tissue-like mechanical softness and deformability and sufficient optoelectronic performances toward effective stimulation. Here, we introduce an ultrathin (<500 nm), stretchy, and self-adhesive rubbery bio-optoelectronic stimulator (RBOES) in a bilayer construct of a rubbery semiconducting nanofilm and a transparent, stretchable gold nanomesh conductor. The RBOES could maintain its optoelectronic performance when it was stretched by 20%. The RBOES was validated to effectively accelerate the beating of the human induced pluripotent stem cell-derived cardiomyocytes. Furthermore, acceleration of ex vivo perfused rat hearts by optoelectronic stimulation with the self-adhered RBOES was achieved with repetitive pulsed light illumination.

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.

Highly Branched Au Superparticles as Efficient Photothermal Transducers for Optical Neuromodulation

Precise neuromodulation is critical for interrogating cellular communication and treating neurological diseases. Nanoscale transducers have emerged as effective interfaces to exert photothermal effects and modulate neural activities with a high spatiotemporal resolution. Ideal materials for this application should possess strong light absorption, high photothermal conversion efficiency, and great biocompatibility for clinical translation. Here, we show that the structurally designed 3D Au superparticles with a highly branched morphology can be promising candidates for nongenetic and remote neuromodulation. The structure-induced blackbody-like absorption endows Au superparticles with photothermal conversion efficiency over 90%, much higher than that of conventional Au nanorods. With the biocompatible polydopamine ligands, Au superparticles can be readily interfaced with primary mouse hippocampal neurons and other cells and can photostimulate or inhibit their activities in both cell networks or with a single-cell resolution. These findings highlight the importance of structural designs as powerful tools to promote the performance of plasmonic materials in neuromodulation and related research of neuroscience and neuroengineering.

A Current Development of Energy Harvesting Systems for Energy-Independent Bioimplantable Biosensors

Biosensors have emerged as vital tools for the detection and monitoring of essential biological information. However, their efficiency is often constrained by limitations in the power supply. To address this challenge, energy harvesting systems have gained prominence. These off-grid, independent systems harness energy from the surrounding environment, providing a sustainable solution for powering biosensors autonomously. This continuous power source overcomes critical constraints, ensuring uninterrupted operation and seamless data collection. In this article, a comprehensive review of recent literature on energy harvesting-based biosensors is presented. Various techniques and technologies are critically examined, including optical, mechanical, thermal, and wireless power transfer, focusing on their applications and optimization. Furthermore, the immense potential of these energy harvesting-driven biosensors is highlighted across diverse fields, such as medicine, environmental surveillance, and biosignal analysis. By exploring the integration of energy harvesting systems, this review underscores their pivotal role in advancing biosensor technology. These innovations promise improved efficiency, reduced environmental impact, and broader applicability, marking significant progress in the field of biosensors.

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.

Recent Advances in Implantable Neural Interfaces for Multimodal Electrical Neuromodulation

Electrical neuromodulation plays a pivotal role in enhancing patient outcomes among individuals suffering from neurological disorders. Implantable neural interfaces are vital components of the electrical neuromodulation system to ensure desirable performance; However, conventional devices are limited to a single function and are constructed with bulky and rigid materials, which often leads to mechanical incompatibility with soft tissue and an inability to adapt to the dynamic and complex 3D structures of biological systems. In addition, current implantable neural interfaces utilized in clinical settings primarily rely on wire-based techniques, which are associated with complications such as increased risk of infection, limited positioning options, and movement restrictions. Here, the state-of-art applications of electrical neuromodulation are presented. Material schemes and device structures that can be employed to develop robust and multifunctional neural interfaces, including flexibility, stretchability, biodegradability, self-healing, self-rolling, or morphing are discussed. Furthermore, multimodal wireless neuromodulation techniques, including optoelectronics, mechano-electrics, magnetoelectrics, inductive coupling, and electrochemically based self-powered devices are reviewed. In the end, future perspectives are given.

Coupling of photovoltaics with neurostimulation electrodes-optical to electrolytic transduction

Objective.The wireless transfer of power for driving implantable neural stimulation devices has garnered significant attention in the bioelectronics field. This study explores the potential of photovoltaic (PV) power transfer, utilizing tissue-penetrating deep-red light-a novel and promising approach that has received less attention compared to traditional induction or ultrasound techniques. Our objective is to critically assess key parameters for directly powering neurostimulation electrodes with PVs, converting light impulses into neurostimulation currents.Approach.We systematically investigate varying PV cell size, optional series configurations, and coupling with microelectrodes fabricated from a range of materials such as Pt, TiN, IrOx, Ti, W, PtOx, Au, or poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate). Additionally, two types of PVs, ultrathin organic PVs and monocrystalline silicon PVs, are compared. These combinations are employed to drive pairs of electrodes with different sizes and impedances. The readout method involves measuring electrolytic current using a straightforward amplifier circuit.Main results.Optimal PV selection is crucial, necessitating sufficiently large PV cells to generate the desired photocurrent. Arranging PVs in series is essential to produce the appropriate voltage for driving current across electrode/electrolyte impedances. By carefully choosing the PV arrangement and electrode type, it becomes possible to emulate electrical stimulation protocols in terms of charge and frequency. An important consideration is whether the circuit is photovoltage-limited or photocurrent-limited. High charge-injection capacity electrodes made from pseudo-faradaic materials impose a photocurrent limit, while more capacitive materials like Pt are photovoltage-limited. Although organic PVs exhibit lower efficiency than silicon PVs, in many practical scenarios, stimulation current is primarily limited by the electrodes rather than the PV driver, leading to potential parity between the two types.Significance.This study provides a foundational guide for designing a PV-powered neurostimulation circuit. The insights gained are applicable to bothin vitroandin vivoapplications, offering a resource to the neural engineering community.

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.

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).

Advances in conductive hydrogels for neural recording and stimulation

The brain-computer interface (BCI) allows the human or animal brain to directly interact with the external environment through the neural interfaces, thus playing the role of monitoring, protecting, improving/restoring, enhancing, and replacing. Recording electrophysiological information such as brain neural signals is of great importance in health monitoring and disease diagnosis. According to the electrode position, it can be divided into non-implantable, semi-implantable, and implantable. Among them, implantable neural electrodes can obtain the highest-quality electrophysiological information, so they have the most promising application. However, due to the chemo-mechanical mismatch between devices and tissues, the adverse foreign body response and performance loss over time seriously restrict the development and application of implantable neural electrodes. Given the challenges, conductive hydrogel-based neural electrodes have recently attracted much attention, owing to many advantages such as good mechanical match with the native tissues, negligible foreign body response, and minimal signal attenuation. This review mainly focuses on the current development of conductive hydrogels as a biocompatible framework for neural tissue and conductivity-supporting substrates for the transmission of electrical signals of neural tissue to speed up electrical regeneration and their applications in neural sensing and recording as well as stimulation.

Noninvasive imaging-guided ultrasonic neurostimulation with arbitrary 2D patterns and its application for high-quality vision restoration

Retinal degeneration, a leading cause of irreversible low vision and blindness globally, can be partially addressed by retina prostheses which stimulate remaining neurons in the retina. However, existing electrode-based treatments are invasive, posing substantial risks to patients and healthcare providers. Here, we introduce a completely noninvasive ultrasonic retina prosthesis, featuring a customized ultrasound two-dimensional array which allows for simultaneous imaging and stimulation. With synchronous three-dimensional imaging guidance and auto-alignment technology, ultrasonic retina prosthesis can generate programmed ultrasound waves to dynamically and precisely form arbitrary wave patterns on the retina. Neuron responses in the brain's visual center mirrored these patterns, evidencing successful artificial vision creation, which was further corroborated in behavior experiments. Quantitative analysis of the spatial-temporal resolution and field of view demonstrated advanced performance of ultrasonic retina prosthesis and elucidated the biophysical mechanism of retinal stimulation. As a noninvasive blindness prosthesis, ultrasonic retina prosthesis could lead to a more effective, widely acceptable treatment for blind patients. Its real-time imaging-guided stimulation strategy with a single ultrasound array, could also benefit ultrasound neurostimulation in other diseases.

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.

Monolithic silicon for high spatiotemporal translational photostimulation

Electrode-based electrical stimulation underpins several clinical bioelectronic devices, including deep-brain stimulators1,2 and cardiac pacemakers3. However, leadless multisite stimulation is constrained by the technical difficulties and spatial-access limitations of electrode arrays. Optogenetics offers optically controlled random access with high spatiotemporal capabilities, but clinical translation poses challenges4-6. Here we show tunable spatiotemporal photostimulation of cardiac systems using a non-genetic platform based on semiconductor-enabled biomodulation interfaces. Through spatiotemporal profiling of photoelectrochemical currents, we assess the magnitude, precision, accuracy and resolution of photostimulation in four leadless silicon-based monolithic photoelectrochemical devices. We demonstrate the optoelectronic capabilities of the devices through optical overdrive pacing of cultured cardiomyocytes (CMs) targeting several regions and spatial extents, isolated rat hearts in a Langendorff apparatus, in vivo rat hearts in an ischaemia model and an in vivo mouse heart model with transthoracic optical pacing. We also perform the first, to our knowledge, optical override pacing and multisite pacing of a pig heart in vivo. Our systems are readily adaptable for minimally invasive clinical procedures using our custom endoscopic delivery device, with which we demonstrate closed-thoracic operations and endoscopic optical stimulation. Our results indicate the clinical potential of the leadless, lightweight and multisite photostimulation platform as a pacemaker in cardiac resynchronization therapy (CRT), in which lead-placement complications are common.

An actor-model framework for visual sensory encoding

A fundamental challenge in neuroengineering is determining a proper artificial input to a sensory system that yields the desired perception. In neuroprosthetics, this process is known as artificial sensory encoding, and it holds a crucial role in prosthetic devices restoring sensory perception in individuals with disabilities. For example, in visual prostheses, one key aspect of artificial image encoding is to downsample images captured by a camera to a size matching the number of inputs and resolution of the prosthesis. Here, we show that downsampling an image using the inherent computation of the retinal network yields better performance compared to learning-free downsampling methods. We have validated a learning-based approach (actor-model framework) that exploits the signal transformation from photoreceptors to retinal ganglion cells measured in explanted mouse retinas. The actor-model framework generates downsampled images eliciting a neuronal response in-silico and ex-vivo with higher neuronal reliability than the one produced by a learning-free approach. During the learning process, the actor network learns to optimize contrast and the kernel's weights. This methodological approach might guide future artificial image encoding strategies for visual prostheses. Ultimately, this framework could be applicable for encoding strategies in other sensory prostheses such as cochlear or limb.

An update on visual prosthesis

PURPOSE

To review the available evidence on the different retinal and visual prostheses for patients with retinitis pigmentosa and new implants for other indications including dry age-related macular degeneration.

METHODS

The PubMed, GoogleScholar, ScienceDirect, and ClinicalTrials databases were the main resources used to conduct the medical literature search. An extensive search was performed to identify relevant articles concerning the worldwide advances in retinal prosthesis, clinical trials, status of devices and potential future directions up to December 2022.

RESULTS

Thirteen devices were found to be current and were ordered by stimulation location. Six have active clinical trials. Four have been discontinued, including the Alpha IMS, Alpha AMS, IRIS II, and ARGUS II which had FDA and CE mark approval. Future directions will be presented in the review.

CONCLUSION

This review provides an update of retinal prosthetic devices, both current and discontinued. While some devices have achieved visual perception in animals and/or humans, the main issues impeding the commercialization of these devices include: increased length of time to observe outcomes, difficulties in finding validated meaures for use in studies, unknown long-term effects, lack of funding, and a low amount of patients simultaneously diagnosed with RP lacking other comorbid conditions. The ARGUS II did get FDA and CE mark approval so it was deemed safe and also effective. However, the company became more focused on a visual cortical implant. Future efforts are headed towards more biocompatible, safe, and efficacious devices.

MnO2 Nanoflower Integrated Optoelectronic Biointerfaces for Photostimulation of Neurons

Optoelectronic biointerfaces have gained significant interest for wireless and electrical control of neurons. Three-dimentional (3D) pseudocapacitive nanomaterials with large surface areas and interconnected porous structures have great potential for optoelectronic biointerfaces that can fulfill the requirement of high electrode-electrolyte capacitance to effectively transduce light into stimulating ionic currents. In this study, the integration of 3D manganese dioxide (MnO2 ) nanoflowers into flexible optoelectronic biointerfaces for safe and efficient photostimulation of neurons is demonstrated. MnO2 nanoflowers are grown via chemical bath deposition on the return electrode, which has a MnO2 seed layer deposited via cyclic voltammetry. They facilitate a high interfacial capacitance (larger than 10 mF cm-2 ) and photogenerated charge density (over 20 µC cm-2 ) under low light intensity (1 mW mm-2 ). MnO2 nanoflowers induce safe capacitive currents with reversible Faradaic reactions and do not cause any toxicity on hippocampal neurons in vitro, making them a promising material for biointerfacing with electrogenic cells. Patch-clamp electrophysiology is recorded in the whole-cell configuration of hippocampal neurons, and the optoelectronic biointerfaces trigger repetitive and rapid firing of action potentials in response to light pulse trains. This study points out the potential of electrochemically-deposited 3D pseudocapacitive nanomaterials as a robust building block for optoelectronic control of neurons.

Emerging trends in the development of flexible optrode arrays for electrophysiology

Optical-electrode (optrode) arrays use light to modulate excitable biological tissues and/or transduce bioelectrical signals into the optical domain. Light offers several advantages over electrical wiring, including the ability to encode multiple data channels within a single beam. This approach is at the forefront of innovation aimed at increasing spatial resolution and channel count in multichannel electrophysiology systems. This review presents an overview of devices and material systems that utilize light for electrophysiology recording and stimulation. The work focuses on the current and emerging methods and their applications, and provides a detailed discussion of the design and fabrication of flexible arrayed devices. Optrode arrays feature components non-existent in conventional multi-electrode arrays, such as waveguides, optical circuitry, light-emitting diodes, and optoelectronic and light-sensitive functional materials, packaged in planar, penetrating, or endoscopic forms. Often these are combined with dielectric and conductive structures and, less frequently, with multi-functional sensors. While creating flexible optrode arrays is feasible and necessary to minimize tissue-device mechanical mismatch, key factors must be considered for regulatory approval and clinical use. These include the biocompatibility of optical and photonic components. Additionally, material selection should match the operating wavelength of the specific electrophysiology application, minimizing light scattering and optical losses under physiologically induced stresses and strains. Flexible and soft variants of traditionally rigid photonic circuitry for passive optical multiplexing should be developed to advance the field. We evaluate fabrication techniques against these requirements. We foresee a future whereby established telecommunications techniques are engineered into flexible optrode arrays to enable unprecedented large-scale high-resolution electrophysiology systems.

Eye-Mimicked Neural Network Composed of Photosensitive Neural Spheroids with Human Opsin Proteins

An in vitro model, composed of the short-wavelength human opsins and rhodopsins, is created. Two types of photosensitive neural spheroids are transfected for selective reaction under bluish-purple and green lights. These are employed to two devices with intact neuron and neural-spheroid to study the interaction. By photostimulation, the photosensitive spheroid initiated photoactivation, and the signal generated from its body is transmitted to adjacent neural networks. Specifically, the signal traveled through the axon bundle in narrow gap from photosensitive spheroid to intact spheroid as an eye-to-brain model including optic nerve. The whole process with photosensitive spheroid is monitored by calcium ion detecting fluorescence images. The results of this study can be applied to examine vision restoration and novel photosensitive biological systems with spectral sensitivity.

Computational analysis of efficient organic solar cell-based retinal prosthesis using plasmonic gold nanoparticles

Introduction

Photovoltaic restoration of vision, especially in conjunction with the use of silicon photodiodes, has gained attention for use in patients affected by blindness due to retinal layer disease. Although the use of silicon photodiodes offers miniaturization of the implant unit and increase in the stimulation channel, the implant unit may suffer from the fracture of these brittle photodiodes when mechanical pressure exerted.

Methods

We present an organic solar cell (OSC)-based retinal prosthesis in which spherical gold nanoparticles (AuNPs) are embedded into the active layer to increase the efficiency of the bioelectric interface.

Results

We demonstrate computationally that a modeled OSC incorporating spherical AuNPs has three times higher efficiency than that of a bare OSC presented before for retinal prostheses. Our AuNP based OSC was able to activate the neuron at the minimum light intensity of 0.26 mW/mm2, which is lower than that of the bare OSC.

Discussion

The use of AuNPs in OSC allows device miniaturization or lowering of the light exposure required for neural activation using a photovoltaic retinal prosthesis, which can generally be applied in a broad range of neural prostheses.

Integrated High-Temporal-Resolution and High-Density Subretinal Prosthesis Using a Correlated Double-Sampling Technique

This paper presents a 1600-pixel integrated neural stimulator with a correlated double-sampling readout (DSR) circuit for a subretinal prosthesis. The retinal stimulation chip inserted beneath the photoreceptor layer comprises an array of an active pixel sensor (APS) and biphasic pulse shaper. The DSR circuit achieves a high signal-to-noise ratio (SNR) of the APS with a short integration time to simultaneously improve the temporal and spatial resolutions of restored vision. This DSR circuit is adopted along with a 5 × 5-pixel tile, which reduces pixel size and improves the SNR by increasing the area occupied by storage capacitors. Moreover, a low-mismatch reference generator enables a low standard deviation between individual pulse shapers. The 1600-pixel retinal chip, fabricated using the 0.18 μm 1P6M CMOS process, occupies a total area of 4.3 mm × 3.3 mm and dissipates an average power of 3.4 mW; this was demonstrated by determining the stimulus current patterns corresponding to the illuminations of an LCD projector. Experimental results show that the proposed high-density stimulation array chip can achieve a high temporal resolution owing to its short integration time.

Colloidal self-assembly of soft neural interfaces from injectable photovoltaic microdevices

Biomimetic retinas with a wide field of view and high resolution are in demand for neuroprosthetics and robot vision. Conventional neural prostheses are manufactured outside the application area and implanted as a complete device using invasive surgery. Here, a minimally invasive strategy based on in situ self-assembly of photovoltaic microdevices (PVMs) is presented. The photoelectricity transduced by PVMs upon visible light illumination reaches the intensity levels that could effectively activate the retinal ganglion cell layers. The geometry and multilayered architecture of the PVMs along with the tunability of their physical properties such as size and stiffness allow several routes for initiating a self-assembly process. The spatial distribution and packing density of the PVMs within the assembled device are modulated through concentration, liquid discharge speed, and coordinated self-assembly steps. Subsequent injection of a photocurable and transparent polymer facilitates tissue integration and reinforces the cohesion of the device. Taken together, the presented methodology introduces three unique features: minimally invasive implantation, personalized visual field and acuity, and a device geometry adaptable to retina topography.

Retinal Prostheses: Engineering and Clinical Perspectives for Vision Restoration

A retinal prosthesis, also known as a bionic eye, is a device that can be implanted to partially restore vision in patients with retinal diseases that have resulted in the loss of photoreceptors (e.g., age-related macular degeneration and retinitis pigmentosa). Recently, there have been major breakthroughs in retinal prosthesis technology, with the creation of numerous types of implants, including epiretinal, subretinal, and suprachoroidal sensors. These devices can stimulate the remaining cells in the retina with electric signals to create a visual sensation. A literature review of the pre-clinical and clinical studies published between 2017 and 2023 is conducted. This narrative review delves into the retinal anatomy, physiology, pathology, and principles underlying electronic retinal prostheses. Engineering aspects are explored, including electrode-retina alignment, electrode size and material, charge density, resolution limits, spatial selectivity, and bidirectional closed-loop systems. This article also discusses clinical aspects, focusing on safety, adverse events, visual function, outcomes, and the importance of rehabilitation programs. Moreover, there is ongoing debate over whether implantable retinal devices still offer a promising approach for the treatment of retinal diseases, considering the recent emergence of cell-based and gene-based therapies as well as optogenetics. This review compares retinal prostheses with these alternative therapies, providing a balanced perspective on their advantages and limitations. The recent advancements in retinal prosthesis technology are also outlined, emphasizing progress in engineering and the outlook of retinal prostheses. While acknowledging the challenges and complexities of the technology, this article highlights the significant potential of retinal prostheses for vision restoration in individuals with retinal diseases and calls for continued research and development to refine and enhance their performance, ultimately improving patient outcomes and quality of life.

Physical mechanisms of emerging neuromodulation modalities

One of the ultimate goals of neurostimulation field is to design materials, devices and systems that can simultaneously achieve safe, effective and tether-free operation. For that, understanding the working mechanisms and potential applicability of neurostimulation techniques is important to develop noninvasive, enhanced, and multi-modal control of neural activity. Here, we review direct and transduction-based neurostimulation techniques by discussing their interaction mechanisms with neurons via electrical, mechanical, and thermal means. We show how each technique targets modulation of specific ion channels (e.g. voltage-gated, mechanosensitive, heat-sensitive) by exploiting fundamental wave properties (e.g. interference) or engineering nanomaterial-based systems for efficient energy transduction. Overall, our review provides a detailed mechanistic understanding of neurostimulation techniques together with their applications toin vitro, in vivo, and translational studies to guide the researchers toward developing more advanced systems in terms of noninvasiveness, spatiotemporal resolution, and clinical applicability.

SpikeSEE: An energy-efficient dynamic scenes processing framework for retinal prostheses

Intelligent and low-power retinal prostheses are highly demanded in this era, where wearable and implantable devices are used for numerous healthcare applications. In this paper, we propose an energy-efficient dynamic scenes processing framework (SpikeSEE) that combines a spike representation encoding technique and a bio-inspired spiking recurrent neural network (SRNN) model to achieve intelligent processing and extreme low-power computation for retinal prostheses. The spike representation encoding technique could interpret dynamic scenes with sparse spike trains, decreasing the data volume. The SRNN model, inspired by the human retina's special structure and spike processing method, is adopted to predict the response of ganglion cells to dynamic scenes. Experimental results show that the Pearson correlation coefficient of the proposed SRNN model achieves 0.93, which outperforms the state-of-the-art processing framework for retinal prostheses. Thanks to the spike representation and SRNN processing, the model can extract visual features in a multiplication-free fashion. The framework achieves 8 times power reduction compared with the convolutional recurrent neural network (CRNN) processing-based framework. Our proposed SpikeSEE predicts the response of ganglion cells more accurately with lower energy consumption, which alleviates the precision and power issues of retinal prostheses and provides a potential solution for wearable or implantable prostheses.

Full-field, conformal epiretinal electrode array using hydrogel and polymer hybrid technology

Shape-morphable electrode arrays can form 3D surfaces to conform to complex neural anatomy and provide consistent positioning needed for next-generation neural interfaces. Retinal prostheses need a curved interface to match the spherical eye and a coverage of several cm to restore peripheral vision. We fabricated a full-field array that can (1) cover a visual field of 57° based on electrode position and of 113° based on the substrate size; (2) fold to form a compact shape for implantation; (3) self-deploy into a curvature fitting the eye after implantation. The full-field array consists of multiple polymer layers, specifically, a sandwich structure of elastomer/polyimide-based-electrode/elastomer, coated on one side with hydrogel. Electrodeposition of high-surface-area platinum/iridium alloy significantly improved the electrical properties of the electrodes. Hydrogel over-coating reduced electrode performance, but the electrodes retained better properties than those without platinum/iridium. The full-field array was rolled into a compact shape and, once implanted into ex vivo pig eyes, restored to a 3D curved surface. The full-field retinal array provides significant coverage of the retina while allowing surgical implantation through an incision 33% of the final device diameter. The shape-changing material platform can be used with other neural interfaces that require conformability to complex neuroanatomy.

Semiconducting electrodes for neural interfacing: a review

In the past 50 years, the advent of electronic technology to directly interface with neural tissue has transformed the fields of medicine and biology. Devices that restore or even replace impaired bodily functions, such as deep brain stimulators and cochlear implants, have ushered in a new treatment era for previously intractable conditions. Meanwhile, electrodes for recording and stimulating neural activity have allowed researchers to unravel the vast complexities of the human nervous system. Recent advances in semiconducting materials have allowed effective interfaces between electrodes and neuronal tissue through novel devices and structures. Often these are unattainable using conventional metallic electrodes. These have translated into advances in research and treatment. The development of semiconducting materials opens new avenues in neural interfacing. This review considers this emerging class of electrodes and how it can facilitate electrical, optical, and chemical sensing and modulation with high spatial and temporal precision. Semiconducting electrodes have advanced electrically based neural interfacing technologies owing to their unique electrochemical and photo-electrochemical attributes. Key operation modalities, namely sensing and stimulation in electrical, biochemical, and optical domains, are discussed, highlighting their contrast to metallic electrodes from the application and characterization perspective.

Artificial intelligence techniques for retinal prostheses: a comprehensive review and future direction

Objective. Retinal prostheses are promising devices to restore vision for patients with severe age-related macular degeneration or retinitis pigmentosa disease. The visual processing mechanism embodied in retinal prostheses play an important role in the restoration effect. Its performance depends on our understanding of the retina's working mechanism and the evolvement of computer vision models. Recently, remarkable progress has been made in the field of processing algorithm for retinal prostheses where the new discovery of the retina's working principle and state-of-the-arts computer vision models are combined together.Approach. We investigated the related research on artificial intelligence techniques for retinal prostheses. The processing algorithm in these studies could be attributed to three types: computer vision-related methods, biophysical models, and deep learning models.Main results. In this review, we first illustrate the structure and function of the normal and degenerated retina, then demonstrate the vision rehabilitation mechanism of three representative retinal prostheses. It is necessary to summarize the computational frameworks abstracted from the normal retina. In addition, the development and feature of three types of different processing algorithms are summarized. Finally, we analyze the bottleneck in existing algorithms and propose our prospect about the future directions to improve the restoration effect.Significance. This review systematically summarizes existing processing models for predicting the response of the retina to external stimuli. What's more, the suggestions for future direction may inspire researchers in this field to design better algorithms for retinal prostheses.

Conformable neural interface based on off-stoichiometry thiol-ene-epoxy thermosets

Off-stoichiometry thiol-ene-epoxy (OSTE+) thermosets show low permeability to gases and little absorption of dissolved molecules, allow direct low-temperature dry bonding without surface treatments, have a low Young's modulus, and can be manufactured via UV polymerisation. For these reasons, OSTE+ thermosets have recently gained attention for the rapid prototyping of microfluidic chips. Moreover, their compatibility with standard clean-room processes and outstanding mechanical properties make OSTE+ an excellent candidate as a novel material for neural implants. Here we exploit OSTE+ to manufacture a conformable multilayer micro-electrocorticography array with 16 platinum electrodes coated with platinum black. The mechanical properties allow conformability to curved surfaces such as the brain. The low permeability and strong adhesion between layers improve the stability of the device. Acute experiments in mice show the multimodal capacity of the array to record and stimulate the neural tissue by smoothly conforming to the mouse cortex. Devices are not cytotoxic, and immunohistochemistry stainings reveal only modest foreign body reaction after two and six weeks of chronic implantation. This work introduces OSTE+ as a promising material for implantable neural interfaces.

Object recognition and localization enhancement in visual prostheses: a real-time mixed reality simulation

Blindness is a main threat that affects the daily life activities of any human. Visual prostheses have been introduced to provide artificial vision to the blind with the aim of allowing them to restore confidence and independence. In this article, we propose an approach that involves four image enhancement techniques to facilitate object recognition and localization for visual prostheses users. These techniques are clip art representation of the objects, edge sharpening, corner enhancement and electrode dropout handling. The proposed techniques are tested in a real-time mixed reality simulation environment that mimics vision perceived by visual prostheses users. Twelve experiments were conducted to measure the performance of the participants in object recognition and localization. The experiments involved single objects, multiple objects and navigation. To evaluate the performance of the participants in objects recognition, we measure their recognition time, recognition accuracy and confidence level. For object localization, two metrics were used to measure the performance of the participants which are the grasping attempt time and the grasping accuracy. The results demonstrate that using all enhancement techniques simultaneously gives higher accuracy, higher confidence level and less time for recognizing and grasping objects in comparison to not applying the enhancement techniques or applying pair-wise combinations of them. Visual prostheses could benefit from the proposed approach to provide users with an enhanced perception.

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.

Towards aSmart Bionic Eye: AI-powered artificial vision for the treatment of incurable blindness

Objective.How can we return a functional form of sight to people who are living with incurable blindness? Despite recent advances in the development of visual neuroprostheses, the quality of current prosthetic vision is still rudimentary and does not differ much across different device technologies.Approach.Rather than aiming to represent the visual scene as naturally as possible, aSmart Bionic Eyecould provide visual augmentations through the means of artificial intelligence-based scene understanding, tailored to specific real-world tasks that are known to affect the quality of life of people who are blind, such as face recognition, outdoor navigation, and self-care.Main results.Complementary to existing research aiming to restore natural vision, we propose a patient-centered approach to incorporate deep learning-based visual augmentations into the next generation of devices.Significance.The ability of a visual prosthesis to support everyday tasks might make the difference between abandoned technology and a widely adopted next-generation neuroprosthetic device.

The Relative Importance of Depth Cues and Semantic Edges for Indoor Mobility Using Simulated Prosthetic Vision in Immersive Virtual Reality

Visual neuroprostheses (bionic eyes) have the potential to treat degenerative eye diseases that often result in low vision or complete blindness. These devices rely on an external camera to capture the visual scene, which is then translated frame-by-frame into an electrical stimulation pattern that is sent to the implant in the eye. To highlight more meaningful information in the scene, recent studies have tested the effectiveness of deep-learning based computer vision techniques, such as depth estimation to highlight nearby obstacles (DepthOnly mode) and semantic edge detection to outline important objects in the scene (EdgesOnly mode). However, nobody has yet attempted to combine the two, either by presenting them together (EdgesAndDepth) or by giving the user the ability to flexibly switch between them (EdgesOrDepth). Here, we used a neurobiologically inspired model of simulated prosthetic vision (SPV) in an immersive virtual reality (VR) environment to test the relative importance of semantic edges and relative depth cues to support the ability to avoid obstacles and identify objects. We found that participants were significantly better at avoiding obstacles using depth-based cues as opposed to relying on edge information alone, and that roughly half the participants preferred the flexibility to switch between modes (EdgesOrDepth). This study highlights the relative importance of depth cues for SPV mobility and is an important first step towards a visual neuroprosthesis that uses computer vision to improve a user's scene understanding.

Noninvasive Stimulation of Peripheral Nerves using Temporally-Interfering Electrical Fields

Electrical stimulation of peripheral nerves is a cornerstone of bioelectronic medicine. Effective ways to accomplish peripheral nerve stimulation (PNS) noninvasively without surgically implanted devices are enabling for fundamental research and clinical translation. Here, it is demonstrated how relatively high-frequency sine-wave carriers (3 kHz) emitted by two pairs of cutaneous electrodes can temporally interfere at deep peripheral nerve targets. The effective stimulation frequency is equal to the offset frequency (0.5 - 4 Hz) between the two carriers. This principle of temporal interference nerve stimulation (TINS) in vivo using the murine sciatic nerve model is validated. Effective actuation is delivered at significantly lower current amplitudes than standard transcutaneous electrical stimulation. Further, how flexible and conformable on-skin multielectrode arrays can facilitate precise alignment of TINS onto a nerve is demonstrated. This method is simple, relying on the repurposing of existing clinically-approved hardware. TINS opens the possibility of precise noninvasive stimulation with depth and efficiency previously impossible with transcutaneous techniques.

Factors affecting two-point discrimination in Argus II patients

Two of the main obstacles to the development of epiretinal prosthesis technology are electrodes that require current amplitudes above safety limits to reliably elicit percepts, and a failure to consistently elicit pattern vision. Here, we explored the causes of high current amplitude thresholds and poor spatial resolution within the Argus II epiretinal implant. We measured current amplitude thresholds and two-point discrimination (the ability to determine whether one or two electrodes had been stimulated) in 3 blind participants implanted with Argus II devices. Our data and simulations show that axonal stimulation, lift and retinal damage all play a role in reducing performance in the Argus 2, by either limiting sensitivity and/or reducing spatial resolution. Understanding the relative role of these various factors will be critical for developing and surgically implanting devices that can successfully subserve pattern vision.

PVGAN: A generative adversarial network for object simplification in prosthetic vision

OBJECTIVE

By means of electrical stimulation of the visual system, visual prostheses provide promising solution for blind patients through partial restoration of their vision. Despite the great success achieved so far in this field, the limited resolution of the perceived vision using these devices hinders the ability of visual prostheses users to correctly recognize viewed objects. Accordingly, we propose a deep learning approach based on Generative Adversarial Networks (GANs), termed PVGAN, to enhance object recognition for the implanted patients by representing objects in the field of view based on a corresponding simplified clip art version.

APPROACH

To assess the performance, an axon map model was used to simulate prosthetic vision in experiments involving normally-sighted participants. In these experiments, four types of image representation were examined. The first and second types comprised presenting phosphene simulation of real images containing the actual high-resolution object, and presenting phosphene simulation of the real image followed by the clip art image, respectively. The other two types were utilized to evaluate the performance in the case of electrode dropout, where the third type comprised presenting phosphene simulation of only clip art images without electrode dropout, while the fourth type involved clip art images with electrode dropout.

MAIN RESULTS

The performance was measured through three evaluation metrics which are the accuracy of the participants in recognizing the objects, the time taken by the participants to correctly recognize the object, and the confidence level of the participants in the recognition process. Results demonstrate that representing the objects using clip art images generated by the PVGAN model results in a significant enhancement in the speed and confidence of the subjects in recognizing the objects.

SIGNIFICANCE

These results demonstrate the utility of using GANs in enhancing the quality of images perceived using prosthetic vision.

In Vitro Cytotoxicity of D18 and Y6 as Potential Organic Photovoltaic Materials for Retinal Prostheses

Millions of people worldwide are diagnosed with retinal dystrophies such as retinitis pigmentosa and age-related macular degeneration. A retinal prosthesis using organic photovoltaic (OPV) semiconductors is a promising therapeutic device to restore vision to patients at the late onset of the disease. However, an appropriate cytotoxicity approach has to be employed on the OPV materials before using them as retinal implants. In this study, we followed ISO standards to assess the cytotoxicity of D18, Y6, PFN-Br and PDIN individually, and as mixtures of D18/Y6, D18/Y6/PFN-Br and D18/Y6/PDIN. These materials were proven for their high performance as organic solar cells. Human RPE cells were put in direct and indirect contact with these materials to analyze their cytotoxicity by the MTT assay, apoptosis by flow cytometry, and measurements of cell morphology and proliferation by immunofluorescence. We also assessed electrophysiological recordings on mouse retinal explants via microelectrode arrays (MEAs) coated with D18/Y6. In contrast to PFN-Br and PDIN, all in vitro experiments show no cytotoxicity of D18 and Y6 alone or as a D18/Y6 mixture. We conclude that D18/Y6 is safe to be subsequently investigated as a retinal prosthesis.

POLYRETINA restores light responses in vivo in blind Göttingen minipigs

Retinal prostheses hold the potential for artificial vision in blind people affected by incurable diseases of the outer retinal layer. Available technologies provide only a small field of view: a significant limitation for totally blind people. To overcome this problem, we recently proposed a large and high-density photovoltaic epiretinal device, known as POLYRETINA. Here, we report the in vivo assessment of POLYRETINA. First, we characterise a model of chemically-induced blindness in Göttingen minipigs. Then, we develop and test a minimally invasive injection procedure to insert the large epiretinal implant into the eye. Last, we show that POLYRETINA restores light-evoked cortical responses in blind animals at safe irradiance levels. These results indicate that POLYRETINA holds the potential for artificial vision in totally blind patients affected by retinitis pigmentosa.

Artificial Visual Information Produced by Retinal Prostheses

Numerous retinal prosthetic systems have demonstrated somewhat useful vision can be restored to individuals who had lost their sight due to outer retinal degenerative diseases. Earlier prosthetic studies have mostly focused on the confinement of electrical stimulation for improved spatial resolution and/or the biased stimulation of specific retinal ganglion cell (RGC) types for selective activation of retinal ON/OFF pathway for enhanced visual percepts. To better replicate normal vision, it would be also crucial to consider information transmission by spiking activities arising in the RGC population since an incredible amount of visual information is transferred from the eye to the brain. In previous studies, however, it has not been well explored how much artificial visual information is created in response to electrical stimuli delivered by microelectrodes. In the present work, we discuss the importance of the neural information for high-quality artificial vision. First, we summarize the previous literatures which have computed information transmission rates from spiking activities of RGCs in response to visual stimuli. Second, we exemplify a couple of studies which computed the neural information from electrically evoked responses. Third, we briefly introduce how information rates can be computed in the representative two ways - direct method and reconstruction method. Fourth, we introduce in silico approaches modeling artificial retinal neural networks to explore the relationship between amount of information and the spiking patterns. Lastly, we conclude our review with clinical implications to emphasize the necessity of considering visual information transmission for further improvement of retinal prosthetics.

Chronic electrical stimulation of peripheral nerves via deep-red light transduced by an implanted organic photocapacitor

Implantable devices for the wireless modulation of neural tissue need to be designed for reliability, safety and reduced invasiveness. Here we report chronic electrical stimulation of the sciatic nerve in rats by an implanted organic electrolytic photocapacitor that transduces deep-red light into electrical signals. The photocapacitor relies on commercially available semiconducting non-toxic pigments and is integrated in a conformable 0.1-mm³ thin-film cuff. In freely moving rats, fixation of the cuff around the sciatic nerve, 10 mm below the surface of the skin, allowed stimulation (via 50-1,000-μs pulses of deep-red light at wavelengths of 638 nm or 660 nm) of the nerve for over 100 days. The robustness, biocompatibility, low volume and high-performance characteristics of organic electrolytic photocapacitors may facilitate the wireless chronic stimulation of peripheral nerves.

Electrical Stimulation of Neurons with Quantum Dots via Near-Infrared Light

Photovoltaic biointerfaces offer wireless and battery-free bioelectronic medicine via photomodulation of neurons. Near-infrared (NIR) light enables communication with neurons inside the deep tissue and application of high photon flux within the ocular safety limit of light exposure. For that, nonsilicon biointerfaces are highly demanded for thin and flexible operation. Here, we devised a flexible quantum dot (QD)-based photovoltaic biointerface that stimulates cells within the spectral tissue transparency window by using NIR light (λ = 780 nm). Integration of an ultrathin QD layer of 25 nm into a multilayered photovoltaic architecture enables transduction of NIR light to safe capacitive ionic currents that leads to reproducible action potentials on primary hippocampal neurons with high success rates. The biointerfaces exhibit low in vitro toxicity and robust photoelectrical performance under different stability tests. Our findings show that colloidal quantum dots can be used in wireless bioelectronic medicine for brain, heart, and retina.

Light Controlled Signaling Initiated by Subretinal Semiconducting-Polymer Layer in Developing-Blind-Retina Mimics the Response of the Neonatal Retina

Optoelectronic semiconducting polymer material interfaced with a blind-developing chick-retina (E13-E18) in subretinal configuration reveals a response to full-field flash stimulus that resembles an elicited response from natural photoreceptors in a mature chick retina. The response manifests as evoked-firing of action potentials was recorded using a multi-electrode array in contact with the retinal ganglion layer. Characteristics of increasing features in the signal unfold during different retina-development stages and highlight the emerging network mediated pathways typically present in the vision process of the artificial photoreceptor interfaced retina.