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Visual prosthesis

A visual prosthesis, also known as a bionic eye, is an implantable microelectronic medical device that restores rudimentary vision to blind individuals by electrically stimulating surviving neurons in the visual system—such as the retina, optic nerve, lateral geniculate nucleus (LGN), or visual cortex—to elicit perceptions of light called phosphenes.^[1]^[2] These devices bypass damaged photoreceptors in conditions like retinitis pigmentosa or age-related macular degeneration, where inner retinal layers or higher visual pathways remain viable, enabling users to perceive patterns for basic tasks such as navigation and object recognition.^[3]^[4] Visual prostheses operate through a combination of external and internal components: an outward-facing camera captures visual input, which is processed by an external unit into electrical signals, then transmitted wirelessly to an implanted electrode array that delivers targeted pulses to neural tissue.^[2]^[1] This bio-inspired encoding leverages neural plasticity, allowing the brain to adapt and interpret the artificial signals over time, though current systems provide low-resolution vision limited by electrode count and stimulation precision.^[1]^[4] Prostheses are categorized by stimulation site, with retinal types being the most developed: epiretinal implants like the Argus II (60 electrodes) attach to the inner retinal surface, while subretinal ones like the PRIMA (378 electrodes) are placed beneath the retina to interface with bipolar cells.^[2]^[4] Non-retinal options include optic nerve cuffs, LGN arrays, and cortical prostheses like the Orion (60 electrodes) or ICVP (400 electrodes), which target the visual cortex for broader applicability in cases of complete optic nerve damage.^[1]^[3] The foundational principles trace to 18th-century experiments with electrical nerve stimulation, evolving through mid-20th-century cortical trials by researchers like Brindley and Dobelle into today's multidisciplinary efforts involving engineering, neuroscience, and ophthalmology.^[2]^[1] As of 2025, the Argus II received FDA approval in 2013 but production and support ceased in 2020 due to commercial challenges; its assets were acquired by Cortigent in 2023 for potential further development, while small-scale trials of advanced systems like PRIMA (improvements of ≥0.2 LogMAR) and ICVP (e.g., LogMAR 2.33) demonstrate progress amid global blindness affecting approximately 43 million people as of 2020, projected to nearly triple to 115 million by 2050.^[4] In 2025, PRIMA's pivotal trial demonstrated meaningful vision restoration in patients with geographic atrophy secondary to AMD, with 80% showing ≥0.2 LogMAR improvement; Science Corporation applied for CE marking. The Orion study reached completion milestones, and ICVP confirmed stable two-year performance. Over 20 research programs worldwide address limitations like biocompatibility, power delivery, and surgical risks through innovations such as AI-enhanced image processing for saliency detection and closed-loop stimulation.^[4]^[3]^[5]^[6]^[7]^[8]

Fundamentals

Definition and purpose

A visual prosthesis, also known as a bionic eye, is an implantable medical device designed to restore partial vision in individuals who are blind by electrically stimulating surviving neurons in the visual pathway, thereby generating artificial visual perceptions called phosphenes—spots of light perceived in the absence of actual visual input.^[1]^[3]^[9] These devices bypass damaged components of the visual system, such as the retina or optic nerve, to directly interface with intact neural structures downstream, eliciting focal patterns of light that mimic basic visual signals.^[1] Unlike natural vision, which relies on photoreceptors to convert light into neural impulses, visual prostheses convert external images captured by a camera into electrical pulses delivered to the target neurons.^[10] The primary purpose of visual prostheses is to enable functional vision that supports essential daily activities for patients with untreatable forms of blindness, including object recognition, obstacle avoidance during navigation, and reading of large-print text.^[10]^[11] By providing this rudimentary form of sight, the devices aim to improve independence and quality of life, though the resulting vision remains far from the high-acuity perception of sighted individuals.^[9] Current systems focus on delivering safe, reliable stimulation to achieve these goals without restoring full anatomical or physiological normality.^[1] In distinction from traditional visual aids like glasses or magnifiers, which enhance or correct remaining natural vision, visual prostheses directly interface with neural tissue to compensate for profound loss, functioning as a neurotechnological bridge rather than an optical correction.^[3] They do not cure the underlying pathology but offer low-resolution vision, typically composed of 60 to 1,000 discrete phosphenes that form coarse patterns, limited by the number of stimulating electrodes.^[12] These prostheses primarily target populations with outer retinal degenerations or optic nerve damage, such as those affected by retinitis pigmentosa (RP), age-related macular degeneration (AMD), or optic neuropathies, where inner retinal layers or post-retinal pathways remain viable; they are not yet optimized for cortical blindness involving higher visual centers.^[13]^[14]^[15]

Historical development

The concept of visual prostheses originated in the early 20th century with experiments on direct electrical stimulation of the visual cortex. In 1929, German neurosurgeon Otfrid Foerster demonstrated that applying electrical currents to the exposed occipital cortex under local anesthesia elicited perceptions of light spots, known as phosphenes, in patients, laying the groundwork for artificial vision restoration.^[1] Advancements accelerated in the 1960s with the first attempts at intracortical electrode arrays. In 1968, Giles Brindley and William Lewin implanted a wireless 80-electrode array into the medial occipital cortex of a blind volunteer, successfully inducing patterned phosphenes that allowed rudimentary shape recognition, though the device was removed after three months due to infection risks.^[16] The 1970s and 1980s saw the development of more refined cortical implants, notably the Dobelle Eye system. Pioneered by William Dobelle at the University of Utah, the first human implantation occurred in 1974, using 37 platinum electrodes on the visual cortex connected to an external camera; by 1978, upgrades enabled a patient to read block letters at a distance of several feet through phosphene patterns.^[17] The 1990s marked a pivotal shift toward retinal-based approaches, driven by recognition of the retina's role in diseases like retinitis pigmentosa. Ophthalmologist Alan Chow developed the Artificial Silicon Retina (ASR), a subretinal photovoltaic microchip; animal tests in rabbits during the mid-1990s confirmed its ability to stimulate surviving retinal cells without external power, leading to the first human implants in 2000.^[18] Concurrently, the Argus project emerged from Second Sight Medical Products, founded in 1996, with initial DARPA funding in the late 1990s supporting epiretinal electrode array prototypes tested in animals.^[19] Clinical progress intensified in the 2000s and 2010s, culminating in regulatory approvals. The Argus II epiretinal prosthesis received FDA humanitarian device exemption in February 2013 for adults with severe retinitis pigmentosa, enabling perception of light, motion, and large objects in over 100 implanted patients worldwide.^[20] In Europe, the subretinal Alpha IMS implant by Retina Implant AG gained CE mark approval in July 2013, restoring pattern recognition and letter identification in clinical trials for retinitis pigmentosa patients; however, Retina Implant AG ceased operations in 2019.^[21]^[22] The 2020s have expanded applications to age-related macular degeneration (AMD), with ongoing trials of subretinal photovoltaic implants. The PRIMA system, developed by Stanford's Daniel Palanker and licensed to Science Corp., underwent pivotal trials from 2021 to 2025 in patients with geographic atrophy secondary to AMD; 12-month results showed approximately 80% of the 32 assessed participants (out of 38 implanted) achieving meaningful central vision improvement, including reading letters, numbers, and words.^[5]^[23] In parallel, Stanford researchers demonstrated the feasibility of upgrading existing subretinal photovoltaic implants in situ to smaller 22 µm pixels with enhanced hexagonal photodiodes in 2025, potentially improving prosthetic visual acuity from ~20/438 (original PRIMA in humans) to up to 20/80 based on animal studies and models.^[24]

Biological Foundations

Visual system anatomy

The human visual system begins with the entry of light through the cornea and lens, which focuses it onto the retina at the back of the eye. The retina, a thin layer of neural tissue, contains photoreceptor cells—rods and cones—that convert light into electrical signals. Rods, numbering about 120 million per eye, are highly sensitive to low light levels and enable vision in dim conditions, while cones, around 6 million per eye, provide color vision and high acuity in bright light, with three types sensitive to short (blue), medium (green), and long (red) wavelengths. These photoreceptors synapse with bipolar cells, which in turn connect to retinal ganglion cells (RGCs), the output neurons of the retina whose axons form the optic nerve. This initial processing in the retina involves lateral inhibition via horizontal and amacrine cells to enhance contrast and edge detection. The retina's layered structure is crucial for understanding prosthesis targeting strategies. It consists of several distinct layers: the outer nuclear layer housing photoreceptor nuclei, the inner nuclear layer containing bipolar, horizontal, and amacrine cell bodies, and the ganglion cell layer with RGC somata. Photoreceptors extend outer segments into the photoreceptor layer for light capture, while their inner segments connect to bipolar cells in the outer plexiform layer. Epiretinal implants, placed on the inner retinal surface, stimulate RGCs directly to bypass damaged photoreceptors, whereas subretinal implants position electrodes closer to the outer nuclear layer to activate remaining bipolar cells or residual photoreceptors. The inner plexiform layer facilitates synaptic interactions between bipolar cells and RGCs, supporting parallel processing pathways such as the magnocellular (motion-sensitive) and parvocellular (color/detail-sensitive) streams. Beyond the retina, the optic nerve comprises approximately 1.2 million RGC axons per eye, which converge at the optic disc and exit the eye to form the optic chiasm, where nasal fibers cross to the contralateral side. These axons then travel through the optic tract to the lateral geniculate nucleus (LGN) in the thalamus, a relay station organized into six layers that segregate inputs by eye and cell type (parvo- and magnocellular). The LGN refines signals through excitatory and inhibitory inputs before projecting via the optic radiations to the primary visual cortex (V1) in the occipital lobe. V1, also known as the striate cortex, features a retinotopic organization where adjacent retinal points map to nearby cortical areas, enabling the generation of phosphenes—perceived spots of light—from direct stimulation. Visual information in this pathway is encoded both spatially and temporally to represent the external world efficiently. Spatially, RGCs exhibit receptive fields with center-surround antagonism, allowing detection of local contrasts; this organization persists in LGN and V1, where orientation-selective neurons in V1 respond to edges at specific angles via simple and complex cell tuning. Temporally, signals propagate with varying latencies—photoreceptor responses occur in milliseconds, while RGC firing rates modulate at up to 100 Hz for dynamic scenes—facilitating motion perception. In prosthesis design, this encoding informs phosphene mapping, where electrode arrays mimic retinotopic layouts to evoke patterned perceptions approximating visual scenes, though with limited resolution due to electrode counts typically under 1000.

Targeted visual impairments

Visual prostheses are designed to address specific forms of blindness where damage occurs at discrete points along the visual pathway, enabling targeted stimulation of surviving neural elements to restore rudimentary vision. The primary indications include retinal degenerations, optic nerve pathologies, and post-retinal cortical damage, each corresponding to distinct segments of the pathway: the outer retina for degenerations, the optic nerve for axonal losses, and the visual cortex for higher-order impairments. Suitability depends on the preservation of downstream neural structures, such as inner retinal layers for retinal-based approaches or cortical tissue for all prosthesis types.^[25]^[26] Retinal degenerations, particularly retinitis pigmentosa (RP) and age-related macular degeneration (AMD), represent the most common targets due to their impact on photoreceptors while often sparing inner retinal ganglion cells and the optic nerve, allowing retinal prostheses to electrically stimulate these viable elements. In RP, progressive loss of rod and cone photoreceptors leads to peripheral and eventual central vision decline, with the disease affecting approximately 1 in 4000 individuals worldwide. AMD primarily damages the macula, causing central vision loss and legal blindness in advanced stages, with a global prevalence of around 200 million people in 2025.^[27] These conditions account for a significant portion of prosthesis-eligible patients, as the intact inner retina supports effective phosphene generation via epiretinal or subretinal stimulation.^[26] Optic nerve damage, arising from glaucoma, ischemic events, or trauma, destroys retinal ganglion cell axons, blocking signals from the retina to the brain and precluding retinal prostheses; optic nerve prostheses offer a potential bypass by directly stimulating preserved axonal bundles, though this remains challenging due to the nerve's compact, non-laminated structure that complicates selective activation. Glaucoma, the predominant cause of such damage, affects an estimated 80 million people globally, disproportionately in older populations and regions like Asia and Africa.^[28]^[15] Post-retinal blindness, typically from cortical lesions due to stroke, tumor, or injury, leaves the retina and optic nerve intact but disrupts visual processing in the occipital cortex, necessitating cortical prostheses that implant arrays directly onto the visual cortex surface or depth to elicit perceptions independent of earlier pathway integrity. Such cases are rarer than retinal or optic neuropathies, and early visual prosthesis devices excluded total cortical blindness owing to uncertainties in mapping complex cortical representations.^[25] Overall suitability criteria emphasize preserved inner retina and optic nerve for retinal prostheses, viable optic nerve segments for optic nerve approaches, and functional visual cortex across all types to ensure signal propagation and perceptual interpretation.^[29]

Technological Principles

Electrical stimulation techniques

Electrical stimulation techniques in visual prostheses involve delivering controlled electrical pulses to neural tissues within the visual pathway to elicit artificial visual perceptions, bypassing damaged photoreceptors by directly activating surviving neurons such as retinal ganglion cells, optic nerve fibers, or cortical neurons.^[30] These methods rely on precise pulse parameters to mimic natural neural firing while minimizing tissue damage and ensuring charge balance.^[31] The standard approach uses biphasic pulse stimulation, consisting of cathodic (negative) and anodic (positive) phases to deliver equal and opposite charges, thereby preventing net charge accumulation that could lead to electrochemical reactions and tissue damage.^[30] Typical parameters include current amplitudes of 0.1-10 mA, pulse durations of 100-500 μs per phase, and frequencies of 20-60 Hz, adjusted based on the target neural structure (lower for retinal, higher for cortical) to achieve suprathreshold activation without exceeding safety limits.^[31]^[32] Direct electrical stimulation via these biphasic pulses elicits phosphenes, which are perceptual spots of light corresponding to the activated neural populations.^[33] Spatial patterns of phosphenes are generated using multi-electrode arrays, with configurations ranging from small grids like 5x5 electrodes for early prototypes to larger arrays up to 60x60 for higher-resolution systems, allowing rudimentary form and object recognition through patterned activation.^[34] Encoding strategies translate captured visual information into stimulation patterns, with analog methods modulating pulse amplitude or duration proportionally to light intensity for grayscale representation, while digital approaches use patterned spike trains to convey binary or discrete information.^[3] Temporal coding enhances these strategies by varying pulse timing or frequency to encode motion and dynamic features, improving the prosthesis's ability to convey moving stimuli.^[35] An alternative approach, particularly in subretinal prostheses like PRIMA, uses photovoltaic arrays powered by projected infrared light from external glasses, converting light directly into electrical stimulation without traditional pulses or inductive powering, enabling fully wireless operation as of 2025.^[23] Power delivery for these implants employs wireless inductive coupling through radio-frequency (RF) telemetry, enabling battery-free operation by transferring energy across the skin via external and internal coils, thus avoiding the risks of implanted batteries.^[36] Coil design optimizes efficiency using basic principles such as power transfer approximated by P = \frac{V^2}{R}, where V is the induced voltage and R the load resistance, to ensure sufficient power for sustained stimulation without excessive heating.^[37]

Implant components and materials

Visual prostheses rely on electrode arrays designed to interface directly with neural tissue, typically featuring 60 to 1000 electrodes with diameters ranging from 100 to 500 μm to enable targeted stimulation while minimizing tissue damage.^[38]^[39] These electrodes often employ platinum-iridium tips for their conductivity and stability, coated with iridium oxide to enhance charge injection capacity exceeding 2 mC/cm², which supports safe and effective neural activation.^[40]^[41] The arrays are mounted on flexible polyimide substrates, which conform to the curved geometry of the retina or cortex, reducing mechanical stress and improving long-term integration.^[40]^[42] Hermetic packaging is essential to shield internal electronics from biofluids, utilizing titanium or ceramic enclosures that maintain integrity over years of implantation.^[40]^[43] These enclosures, often sealed via laser welding, incorporate feedthroughs—such as platinum-iridium wires embedded in ceramics—to enable electrical connections while preventing moisture ingress and corrosion.^[40]^[44] External components facilitate the capture and processing of visual information, including camera-equipped glasses that record scenes in real time.^[38] A video processing unit (VPU) then converts these images into electrical signals, with wireless transmission via radiofrequency (RF) telemetry or optical projection to the implant for control and power management.^[38]^[15] Biocompatibility is ensured through rigorous testing per ISO 10993 standards, evaluating cytotoxicity, sensitization, and material interactions with tissue.^[45]^[46] Materials like iridium oxide exhibit low degradation rates, maintaining charge injection performance without significant loss over extended stimulation periods.^[41]^[47] Miniaturization has progressed dramatically since the bulky, wired systems of the 1990s, evolving toward fully wireless implants by 2025 with volumes under 1 cm³, such as titanium-cased neurostimulators measuring approximately 11 mm × 11 mm × 2 mm.^[40]^[15] This trend enhances patient comfort and enables intraocular or intracortical placement without percutaneous connections.^[48]^[4]

Types of Prostheses

Retinal-based prostheses

Retinal-based prostheses target the retina to restore partial vision in patients with outer retinal degeneration, such as retinitis pigmentosa (RP) or age-related macular degeneration (AMD), where inner retinal layers remain viable. These devices stimulate surviving retinal neurons electrically or optically, bypassing damaged photoreceptors to elicit phosphenes—perceived points of light that form crude visual percepts. Approaches differ by implantation site: epiretinal on the inner retinal surface, subretinal beneath the retina, and suprachoroidal between the sclera and choroid.^[26] Epiretinal prostheses are positioned on the inner surface of the retina, directly stimulating retinal ganglion cells. The electrode array is typically affixed using a retinal tack to ensure stable contact during eye movements.^[26] A representative example is the Argus II system, which features a 60-electrode array and relies on an external camera mounted on glasses to capture images, processed into stimulation patterns transmitted wirelessly to the implant.^[49] This configuration suits RP, where inner retinal preservation allows ganglion cell activation.^[26] Subretinal prostheses are implanted under the retina, aiming to replace photoreceptor function by stimulating bipolar and horizontal cells. These often use photovoltaic arrays that convert projected infrared light into electrical signals without requiring percutaneous wiring or external power for stimulation.^[24] The PRIMA system exemplifies this approach, with an array of approximately 378 pixels (100 μm each) designed for central vision restoration in AMD patients with geographic atrophy; the PRIMAvera pivotal trial (NCT04676854), completed in 2025, demonstrated safety and meaningful vision restoration, including reading short words and letter sequences, with 81% of participants achieving ≥0.2 logMAR improvement (mean 0.49 logMAR) at 12 months, enabling reading up to 5 lines on an eye chart and acuities improving to around 20/100 with processing (pixel-limited ~20/500 without zoom).^[50]^[51]^[52] Suprachoroidal prostheses are placed in the space between the sclera and choroid, enabling transchoroidal stimulation of the retina with reduced risk of direct retinal trauma. This location facilitates safer implantation via an ab externo approach.^[53] The Australian-developed second-generation 44-channel suprachoroidal retinal prosthesis by Bionic Vision Technologies completed its clinical trial in 2025, demonstrating safety, surgical stability over 2 years, and improvements in functional vision such as face and chair detection in RP patients, representing advancements in the 2020s through broad retinal activation.^[54]^[53] Comparatively, epiretinal systems like Argus II have achieved visual acuities up to 20/1260 in RP patients, enabling basic object detection via 60 phosphenes.^[49] Subretinal devices, such as PRIMA, offer higher potential resolution (up to ~378 phosphenes) and better suitability for AMD, with 2025 trial results showing improved reading capabilities and acuities around 20/100 with image processing.^[50]^[52] Suprachoroidal approaches provide 44 phosphenes with safer profiles but lower resolution currently, though ongoing developments aim to expand electrode counts toward 2000 phosphenes across retinal prostheses for enhanced pattern recognition.^[53] Overall, phosphene counts limit resolution to low-acuity vision, with epiretinal favoring RP and subretinal AMD due to targeted cell stimulation.^[26] Surgical implantation for epiretinal and subretinal prostheses typically involves pars plana vitrectomy to access the vitreous cavity, followed by array placement and retinal reattachment.^[26] Epiretinal arrays are tacked onto the macula post-vitrectomy, while subretinal devices require a small retinotomy for insertion under the retina.^[55] In contrast, suprachoroidal surgery is less invasive, avoiding vitrectomy through scleral incision and choroidal exposure for array positioning.^[53]

Optic nerve prostheses

Optic nerve prostheses stimulate the optic nerve directly to elicit visual perceptions in individuals with preserved retinal ganglion cells but damage further along the visual pathway. These devices typically employ either penetrating electrodes inserted into the nerve or epineural cuff electrodes wrapped around its surface to target bundles of axons. The design aims for selective activation of axon groups to approximate the topographic organization of the visual field, with electrode arrays often featuring 4 to 24 channels to enable multi-site stimulation.^[56]^[57] Pioneering experiments in the early 1990s, led by Claude Veraart and colleagues in Belgium, demonstrated the feasibility of this approach. In 1998, a self-sizing spiral cuff electrode with four contacts was chronically implanted around the intraorbital portion of the optic nerve in a volunteer blind due to retinitis pigmentosa, successfully eliciting phosphenes—small, localized spots of light—distributed across the visual field. Subsequent work under the OPTIVIP project in the 2000s expanded on this, incorporating telemetry-controlled neurostimulators to allow pattern recognition tasks, such as identifying simple shapes, though the system remained experimental and was not commercialized by 2025.^[58]^[59] A key advantage of optic nerve prostheses is their ability to bypass retinal degeneration, making them suitable for conditions like glaucoma or optic nerve trauma where the retina remains functional. The nerve's bundled, organized fibers also offer potential for higher resolution mapping compared to more diffuse structures, with evoked potentials closely resembling normal visual responses.^[57]^[56] However, these devices face significant limitations, including the risk of nerve damage from penetrating electrodes or chronic compression from cuffs, which can lead to fiber degeneration. Clinical trials have been limited, with fewer participants than retinal or cortical approaches, and challenges in achieving fine selectivity among the optic nerve's approximately 1.2 million fibers have hindered progress toward practical vision restoration.^[60]^[57] Stimulation parameters are tailored to the bundled axon structure, typically requiring lower currents of 100–1000 μA per pulse due to the proximity and density of fibers, with durations around 200–400 μs and frequencies up to 40 Hz to generate stable percepts. Resulting phosphene patterns form clusters rather than isolated points, offering less precise localization than cortical stimulation but enabling basic tasks like object localization.^[61]^[58]

Cortical prostheses

Cortical visual prostheses represent a class of intracortical implants designed to restore rudimentary vision by directly stimulating neurons in the brain's visual cortex, circumventing damage to the anterior visual pathway such as the retina or optic nerve. These devices are particularly suited for individuals with profound blindness resulting from conditions affecting the eye or optic nerve, as they target the primary visual cortex (V1) where visual processing remains intact. By delivering targeted electrical pulses, they elicit perceptions of light known as phosphenes, forming basic patterns that users can learn to interpret over time.^[62]^[34] Implantation typically occurs in the primary visual cortex (V1), located along the calcarine sulcus in the occipital lobe, via a neurosurgical procedure involving craniotomy to access the brain surface. Common electrode arrays include the Utah array, a penetrating microelectrode system with 96 to 1,000 electrodes that extend 1-1.5 mm into the cortical tissue to reach neurons in layers II and III, or surface-based microelectrocorticography (microECoG) arrays for less invasive epicortical stimulation. These arrays, often iridium oxide-coated for biocompatibility, are positioned to align with the retinotopic organization of V1, where spatial mapping preserves the eye's topographic representation of the visual field.^[4]^[63]^[64] The core mechanism involves direct electrical activation of cortical neurons, bypassing the eye and optic nerve to generate artificial visual signals that propagate through higher visual pathways. Retinotopic mapping guides electrode placement and stimulation patterns, allowing the creation of phosphene grids that correspond to visual field locations; for instance, configurations ranging from 10x10 to 100x100 phosphenes can simulate low-resolution images by modulating pulse amplitude, duration, and frequency to control phosphene size, brightness, and position. This approach leverages the brain's plasticity, enabling blind users to adapt to these elicited percepts through training, though the resulting vision remains far below normal acuity.^[34]^[65] Pioneering work includes Giles Brindley's 1968 implantation of a wireless array of 80 radio receivers connected to surface electrodes on the occipital pole of a blind volunteer, marking the first demonstration of phosphene induction via cortical stimulation and an early milestone in the historical development of visual prostheses. In the 1970s through the 2000s, William Dobelle advanced the field with systems featuring 68 platinum disc electrodes on the visual cortex surface, linked to a camera for real-time input, enabling subjects to recognize simple shapes like letters and objects through patterned phosphene arrays. The Orion Visual Cortical Prosthesis System, developed by Second Sight Medical Products (now part of Vivani Medical and Cortigent), completed its early feasibility trial in 2025 with a 400-electrode intracortical array, demonstrating dynamic phosphene patterns in profoundly blind participants. A pivotal trial is planned to commence in 2027. Another example is the Intracortical Visual Prosthesis (ICVP), featuring 400 wireless microelectrodes, which completed initial feasibility testing in 2025, eliciting stable phosphenes in a blind participant.^[66]^[67]^[68]^[6]^[4] A primary advantage of cortical prostheses is their broad applicability to blindness from diverse causes, including retinal degeneration or optic nerve damage, as long as the visual cortex remains viable, unlike retinal implants limited to specific pathologies. Additionally, these devices demonstrate stable long-term performance, with implants functioning effectively for decades without significant degradation, supported by the cortex's relative protection from ocular comorbidities.^[69]^[70] Despite these benefits, challenges persist, including the need for invasive craniotomy, which carries risks of infection, hemorrhage, and neurological deficits. Current systems are also constrained to low-resolution output, equivalent to approximately 20/2000 visual acuity, producing coarse phosphene grids insufficient for fine details like reading or facial recognition, necessitating further advances in electrode density and stimulation precision.^[25]^[70]

Notable Devices and Projects

Epiretinal systems

Epiretinal prostheses stimulate the ganglion cells on the inner surface of the retina, bypassing damaged photoreceptors to restore basic visual function in patients with retinitis pigmentosa (RP), where inner retinal layers remain viable.^[71] The most prominent example is the Argus II system, developed by Second Sight Medical Products, which consists of a 60-electrode array fabricated from platinum-iridium with 200 μm electrode diameters and 575 μm center-to-center spacing.^[72] The device received FDA approval in 2013 as a humanitarian device exemption for severe RP patients with bare or no light perception. As of 2019, more than 350 patients worldwide had received the Argus II implant, demonstrating its clinical adoption.^[73] Clinical outcomes with the Argus II range from perception of light and motion to recognition of large letters and short words, with the best visual acuity measured at 20/1260 (equivalent to 1.8 logMAR).^[74] In trials involving 30 participants, approximately 50% could identify letters on high-contrast charts, and top performers read paragraphs of text at reduced speeds.^[75] Functional benefits included significant improvements in orientation and mobility tasks; for instance, patients achieved up to 89% success in localizing doors and lines with the system activated compared to residual vision alone.^[76] Patients require extensive training to interpret the resulting phosphenes—spots of light elicited by electrical stimulation—which typically spans 3-6 months post-implantation, involving rehabilitation programs to optimize device use for daily activities like navigation.^[73] The system has shown long-term stability, with no device failures reported over 88 subject-years in early trials and sustained benefits in visual tasks up to five years.^[77] Following Second Sight's financial challenges and bankruptcy proceedings in 2020, commercial production of the Argus II ceased, but the technology was licensed to Cortigent in 2023, enabling continued academic and research applications into 2025.^[78] Another notable effort is the Boston Retinal Implant Project, which is developing an epiretinal/subretinal hybrid prosthesis aimed at higher-resolution stimulation, remaining in preclinical testing as of 2025 with plans for FDA investigational device exemption submission.^[79]

Subretinal systems

Subretinal visual prostheses are implanted beneath the retina to directly stimulate the surviving inner retinal layers, bypassing damaged photoreceptors in conditions such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). These devices typically employ two main designs: active pixel arrays, which use integrated electronics like photodiodes and amplifiers to convert light into electrical pulses, and photovoltaic arrays, which generate current wirelessly from projected infrared light without onboard power sources. This positioning allows for closer proximity to bipolar and ganglion cells, potentially leveraging natural eye movements for improved phosphene alignment and visual perception.^[80] The Alpha IMS, approved with CE marking in 2013, and its successor, the Alpha AMS, approved with CE marking in 2016, both developed by Retina Implant AG, represent early active pixel subretinal systems targeted at end-stage RP. The implant features a 3 mm × 3 mm microchip with 1,500 independent pixels, each measuring 70 μm × 70 μm, containing a photodiode, amplifier, and stimulating electrode to deliver biphasic pulses based on local light intensity. The device restored basic visual functions such as light localization and object recognition in clinical trials involving blind patients, with five of six participants showing improved performance. Long-term data indicate device stability, with predicted clinical lifetime of approximately five years and ongoing function observed up to 24 months post-implantation in multicenter studies.^[81]^[82]^[83]^[84]^[85] The PRIMA system, originally developed by Pixium Vision and advanced by Science Corporation in collaboration with Stanford Medicine, exemplifies a wireless photovoltaic approach for dry AMD with geographic atrophy. The 2 mm × 2 mm implant consists of 378 pixels, each 100 μm in size, arranged in a hexagonal pattern to convert near-infrared light—projected via specialized glasses with an integrated camera—into localized electrical stimulation of the inner retina. In a 2025 phase 1/2 clinical trial published in the New England Journal of Medicine, 38 patients with baseline visual acuity of at least 1.2 logMAR (approximately 20/320) received the implant; at 12 months, 81% (26 of 32 assessed) achieved a clinically meaningful improvement of ≥0.2 logMAR, enabling tasks such as reading words and recognizing faces.^[51]^[23]^[86] Subretinal positioning in these systems enhances the integration of prosthetic phosphenes with residual peripheral vision and natural saccades, contributing to more intuitive navigation and object detection compared to surface-mounted alternatives. A 2025 Stanford-led variant of the PRIMA implant incorporates enhanced pixel designs with amorphous silicon resistors to enable smaller 20 μm pixels and up to 10,000 elements, tested in preclinical models to boost resolution for macular degeneration patients.^[24]^[87] Surgical implantation of subretinal prostheses involves pars plana vitrectomy, creation of a localized retinal flap or bleb via subretinal fluid injection to accommodate the device, and precise positioning under the macula using intraoperative imaging. Complications are generally low, with retinal detachment occurring in fewer than 10% of cases across reported series, often managed successfully with reattachment procedures; other events include transient inflammation and vitreous hemorrhage, resolving within months in most patients.^[84]^[86]

Other innovative approaches

The Implantable Miniature Telescope (IMT), developed by VisionCare Ophthalmic Technologies, is an intraocular lens system designed for patients with end-stage age-related macular degeneration (AMD). It functions as a fixed-focus Galilean telescope implanted in one eye, providing magnification of 2.2x to 2.7x to project images onto healthier peripheral retina surrounding the central scotoma, thereby improving central vision for tasks like reading and face recognition.^[88] The U.S. Food and Drug Administration (FDA) approved the IMT in 2010 for patients aged 75 and older with bilateral severe to profound vision loss (20/160 to 20/800), expanding the indication in 2014 to include those aged 65 and older.^[89] Clinical trials demonstrated sustained improvements in distance and near visual acuity, with over 60% of patients gaining at least three lines on the eye chart at five years post-implantation.^[90] By 2025, the original IMT and its smaller-incision successor (SING IMT) had been implanted in more than 750 patients across the U.S. and Europe, primarily enhancing distance vision and quality of life in suitable AMD cases.^[91] The Artificial Silicon Retina (ASR), developed by Optobionics Corporation, represented an early subretinal photovoltaic implant aimed at restoring light perception in patients with retinitis pigmentosa (RP). This passive microchip, approximately 2 mm in diameter with 5,000 independent microphotodiodes and electrodes, converted incident light directly into electrical stimulation without external power or cameras, targeting the outer retina to elicit phosphenes.^[26] FDA-approved for a Phase I/IIa trial in 2000, the device was implanted in 16 patients during the early 2000s, with reports of modest visual gains such as improved light detection and basic motion perception in some participants. However, the project faced challenges including limited resolution and biocompatibility issues, leading to discontinuation in the mid-2010s after Optobionics ceased operations. The Dobelle Eye, pioneered by biomedical engineer William H. Dobelle, was an experimental cortical visual prosthesis that interfaced directly with the visual cortex to provide patterned phosphenes for navigation. Implanted in a blind volunteer in 1978 and activated in 2000, the system used a head-mounted video camera processed by external electronics to deliver electrical pulses via 68 platinum electrodes on the occipital cortex surface, enabling the patient to recognize letters, words, and simple shapes for independent mobility over short distances. The prosthesis allowed functional vision equivalent to recognizing large objects at close range, but recurrent infections necessitated removal of the external connectors post-2004 following Dobelle's death, halting further development.^[92] The Orion Visual Cortical Prosthesis, developed by Cortigent (formerly Second Sight), is an intracortical implant targeting the visual cortex with 60 penetrating electrodes. As of 2025, early feasibility trials have shown six implanted patients achieving phosphene perception for basic navigation and object detection, with no serious adverse events reported over one year.^[93] Emerging hybrid approaches have explored suprachoroidal placement to combine retinal stimulation with broader field coverage. Bionic Vision Technologies (BVT) in Australia developed a second-generation suprachoroidal-retinal prosthesis, implanted between the sclera and choroid to stimulate surviving bipolar cells in RP patients, producing wide-view phosphenes spanning up to 55 degrees.^[53] Initial Phase I trials in the 2010s involved three participants with a 24-electrode array, demonstrating safe implantation and basic perceptual abilities like light localization; subsequent 44-channel trials through 2024 reported substantial functional improvements in navigation and object detection for four additional subjects.^[94]^[95]

Challenges and Future Directions

Technical and surgical challenges

One major technical challenge in visual prostheses is achieving long-term biocompatibility, as implanted electrodes often provoke chronic inflammation and glial encapsulation, leading to increased impedance that can degrade stimulation efficacy. For instance, electrode encapsulation can cause a twofold rise in impedance in vivo due to biological responses, limiting charge delivery and neural interface stability. Mitigation strategies include conductive polymer coatings like poly(3,4-ethylenedioxythiophene) (PEDOT), which significantly reduce impedance (by up to two orders of magnitude) in vitro and enhance charge injection limits by 15- to 35-fold, promoting better tissue integration and reducing inflammatory responses in retinal models.^[30]^[96] Power delivery and thermal management pose significant hurdles, particularly for high-channel devices relying on inductive coupling, where efficiencies often fall below 12% in retinal systems due to coil misalignment from eye movements. Tissue heating must remain below 1°C to comply with FDA safety guidelines for implantable devices, as excessive dissipation can cause damage; current inductive links achieve this but limit power to avoid exceeding ocular thermal thresholds. By 2025, wireless photovoltaic implants like the PRIMA system have advanced power transfer without percutaneous wires, enabling subretinal placement and reducing infection risks associated with external connections. As of November 2025, the PRIMA system is advancing toward regulatory approval following positive pivotal trial results.^[36]^[97]^[23]^[86] Surgical implantation carries notable risks, including retinal detachment in epiretinal approaches, occurring in approximately 3% of cases as seen in Argus II trials, often due to traction from array tacking. Cortical prostheses involve craniotomy, with infection rates ranging from 1% to 10%, exacerbated by foreign body implantation and potential cerebrospinal fluid leakage. Recent 2025 wireless innovations, such as fully internalized subretinal chips, minimize wired penetrations, thereby lowering overall surgical complications compared to earlier tethered designs.^[98]^[99] Resolution remains constrained by phosphene blending, where overlapping or indistinct light spots from adjacent electrodes hinder clear pattern recognition, limiting useful vision to basic object detection rather than fine acuity. Current devices support up to around 400 electrodes, far short of the 1 million needed for natural-like resolution, with 2025 prototypes aiming for 1000 channels but still facing scalability issues from tissue response and power constraints.^[96]^[100] Device longevity is challenged by failures such as component degradation or recurrent erosions, with the Argus II showing a ~20% explant rate (6 of 30 implants) within 5 years, primarily due to conjunctival issues or patient-elected removal despite overall stability in 80% of cases.^[76]

Clinical outcomes and patient experiences

Clinical trials of retinal prostheses have demonstrated modest gains in visual acuity, particularly in light perception and basic localization tasks. In the Argus II system trials from 2013 to 2020, approximately 81% of participants performed better on square localization tests with the device activated compared to off, enabling improved detection of high-contrast objects. Grating visual acuity assessments showed that 27-48% of patients achieved 2.9 logMAR or better, equivalent to roughly 20/1260 Snellen acuity, though no full restoration of normal vision was observed. More recent advancements, such as the PRIMA subretinal implant trial reported in 2025, yielded stronger outcomes for patients with geographic atrophy due to age-related macular degeneration (AMD); 81% of the 32 participants completing 12-month follow-up exhibited at least 0.2 logMAR improvement in visual acuity, with 84% regaining the ability to read sentences or paragraphs.^[101]^[76]^[86]^[102] Functional benefits extend to enhanced mobility and performance of daily tasks, though outcomes vary by device and patient. Recipients of the Argus II reported significant improvements in navigating obstacle courses and avoiding low-lying hazards, with post-implantation mobility scores outperforming preoperative baselines in structured assessments. In simulated and real-world studies, visual prostheses facilitated better object recognition and counting, aiding activities like self-grooming and food preparation, with up to 80% of subjects deriving benefit across functional vision tasks. The PRIMA system similarly supported practical gains, including shape discrimination and basic environmental orientation, contributing to sustained central vision improvements in AMD patients over one year.^[103]^[70]^[104] Patient adaptation to visual prostheses typically involves intensive rehabilitation programs lasting 3 to 12 months, focusing on perceptual learning and device optimization to maximize utility. Surveys of Argus II implantees indicate mixed psychological impacts, with average satisfaction ratings around 6 out of 10 and about 50% expressing they would opt for reimplantation despite challenges like cognitive strain from interpreting phosphene patterns. Many report gains in perceived independence, with qualitative feedback highlighting enhanced quality of life through better navigation in familiar settings, though unmet expectations regarding vision clarity can lead to frustration. In broader patient reports, up to 70% note improvements in daily autonomy, underscoring the role of ongoing training in fostering adaptation.^[105]^[73]^[15] Adverse events associated with implantation are generally manageable, with most resolving post-surgery. In Argus II trials, serious events occurred in about 31% of patients, including vitreous hemorrhage and conjunctival erosion, but temporary visual field perturbations were noted in some cases without permanent loss. The 2025 PRIMA study for AMD patients reported 26 serious adverse events across 19 participants, predominantly within the first two months and resolving in 95% of early cases, with no long-term compromise to the restored central vision. Overall, these implants maintain functional stability, though monitoring for transient field loss remains essential.^[73]^[86] Key metrics for evaluating outcomes include Snellen visual acuity scales for resolution and contrast sensitivity for perceptual clarity, though prostheses do not achieve normal-range performance. By 2025, the best reported prosthetic acuity hovers around 20/500 equivalent, as seen in PRIMA's logMAR gains translating to functional reading without full restoration. These measures emphasize partial, task-specific enhancements rather than comprehensive sight recovery.^[106]^[86]

Ethical considerations and emerging trends

The development and deployment of visual prostheses raise significant ethical concerns regarding equity and access. The Argus II retinal prosthesis, for instance, costs approximately $150,000 in the United States, excluding surgical and rehabilitation expenses, which poses a substantial barrier for many patients.^[107] High development and implementation costs further exacerbate global disparities, with over one billion people worldwide lacking access to essential assistive technologies, particularly in low- and middle-income countries where resources for advanced medical devices are limited.^[96]^[108] Informed consent processes for visual prosthesis implantation present unique challenges due to the partial and unpredictable nature of restored vision, as well as potential psychological impacts. Patients must weigh risks such as surgical complications and device failure against benefits like basic light perception or object recognition, which may not fully restore functional sight.^[109] Additionally, the integration of bionic devices can impose a psychological burden, including altered self-perception and the stigma of a "cyborg" identity, which may affect social interactions and personal autonomy.^[110]^[111] Beyond restoration, visual prostheses hold potential for human enhancement, such as enabling perception of infrared light, which exceeds natural human visual capabilities. For example, tellurium nanowire-based retinal implants have demonstrated infrared vision in animal models, raising questions about equitable access to such augmentations and their societal implications.^[112] Regulatory debates continue, with the U.S. Food and Drug Administration emphasizing safety and efficacy in regenerative therapies, though specific guidelines for enhancement features remain evolving as of 2025.^[113] Emerging trends in visual prosthesis technology increasingly incorporate artificial intelligence (AI) for image processing to enhance user outcomes. AI-driven saliency extraction algorithms prioritize key visual elements, optimizing stimulation patterns to improve perception in low-resolution prosthetic vision.^[4] Recent studies indicate these approaches can significantly boost task performance, such as object detection, by focusing on salient features.^[114] Nanotechnology-based electrodes, including carbon nanotubes, offer improved biocompatibility and finer resolution for retinal stimulation.^[115] Hybrid bioelectronic systems, combining synthetic electrodes with biological components like neuronal cells, are also advancing, enabling more natural signal integration.^[116] Looking ahead, combinations of gene therapy and visual prostheses show promise for synergistic vision restoration. Optogenetic gene therapies, which sensitize remaining retinal cells to light, can complement electronic implants to enhance overall efficacy in degenerative diseases.^[117] In cortical prostheses, AI decoding of neural signals is poised to enable higher-resolution vision, with projections for systems eliciting thousands of phosphenes to support complex scene perception by 2030.^[4]^[118]

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We observed gradual remodelling of the inner retina to incorporate CNT assemblies. Electrophysiological recordings demonstrate a progressive increase in ...Carbon Nanotube Electrodes... · 2. Materials And Methods · 3. Results
[110]
https://www.frontiersin.org/journals/psychology/articles/10.3389/fpsyg.2018.02251/full
[111]
Visual prostheses, optogenetics, stem cell and gene therapies - NIH
In this manuscript, the author described four potential approaches currently to treat vision impairments, diseases or blindness, and carefully compared their ...
[112]
Selective activation of mesoscale functional circuits via multichannel ...
Jan 27, 2025 · To restore vision in the blind, advances in visual cortical prosthetics (VCPs) have offered high-channel-count electrical interfaces. Here, we ...