Arrane Selvamogan and Rickvir S. Sidhu
Co-First Authors

Introduction

The global burden of blindness affects more than 36 million people worldwide and is projected to rise to 115 million by 2050, largely due to degenerative retinal diseases such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD) (1–3). In response, electrical visual prostheses have emerged as a transformative intervention, offering functional sight restoration through neural stimulation. These devices bypass damaged photoreceptors, activate surviving retinal or cortical neurons, and exploit neuroplasticity within the visual system to restore limited perception. This review compares the leading devices as of 2025, evaluating their efficacy across retinal, cortical, and non-invasive platforms (4).

Device Mechanisms and Implantation Sites

Electrical prostheses are categorised according to their site of action: retinal (epiretinal or subretinal), cortical (direct stimulation of the visual cortex), or non-invasive systems that deliver transorbital current. Patient selection is critical and relies on the structural preservation of visual pathways. Retinal implants require intact inner retinal layers confirmed by OCT imaging to ensure viable bipolar and ganglion cells, while cortical devices are indicated for post-retinal blindness such as advanced glaucoma or optic nerve atrophy (3,4). For retinal prostheses, choroidal perfusion and optic nerve viability significantly predict success, whereas cortical implants depend on preserved retinotopy, often verified using functional MRI to map stimulation zones within the occipital cortex (5–7).

Leading Electrical Visual Prostheses

1. Argus II: The Argus II was the first commercially available epiretinal implant, gaining FDA approval in 2013 for severe RP. It used a 60-electrode array to translate camera-derived images into ganglion cell stimulation, enabling basic light perception and object localisation. Five-year trial data demonstrated functional gains in 60% of patients, particularly in mobility and orientation tasks (8–10). Despite this success, production ceased due to manufacturing costs and limited spatial resolution of approximately 1.8 logMAR, which prevented detailed shape or facial recognition (3). Nevertheless, Argus II remains foundational, having established key surgical protocols and biocompatibility standards that informed subsequent systems such as PRIMA and HARP4k (3).

• Photovoltaic Retinal Implant Microarray (PRIMA): The PRIMA system represents the current frontier of retinal prosthesis research, targeting geographic atrophy in dry AMD. Its 378-pixel photovoltaic chip converts near-infrared light projected onto the retina into electrical signals that stimulate inner retinal neurons (11,12). Clinical studies have demonstrated prosthetic visual acuity of around 0.9 logMAR (20/160), permitting users to identify letters and words using magnification. Four-year follow-up data confirmed stable performance, high implant retention, and minimal adverse effects (11). More recently, a multicentre phase II trial published in the New England Journal of Medicine verified the long-term safety and integration of prosthetic signals with natural fixation patterns, confirming sustained cortical adaptation (12). The ongoing PRIMAvera trial is refining these findings to evaluate combined use of prosthetic and residual natural vision (11,12)

• Orion: This subdural cortical implant bypasses the eye entirely by delivering direct electrical stimulation to the visual cortex through a 60-electrode grid (5–7,13). It is designed for patients with blindness arising from optic nerve or ocular pathology. Phase I feasibility results demonstrated safe chronic implantation, with improvements in localisation and obstacle avoidance. Participants consistently reported stable phosphene perception corresponding to predictable regions of the visual field, confirming preserved cortical retinotopy even after long-term blindness (5–7). One transient seizure was documented, but no vision-threatening events occurred over two years (6,13). The device’s performance underscores cortical prostheses as promising candidates for comprehensive neural visual restoration.

• Repetitive Transorbital Alternating Current Stimulation (rtACS): Among non-invasive options, rtACS applies weak alternating currents across the orbit to promote neuroplastic recovery in residual optic pathways. A randomised, sham-controlled trial confirmed significant improvement in visual field indices, with field expansions of 10–20% and enhanced detection accuracy (14,15). The 2025 VIRON trial further validated these results in glaucoma-related optic atrophy, reporting measurable improvements in perimetric sensitivity and patient-reported quality of life (16). However, a 2025 meta-analysis found substantial interstudy variability, noting non-significant gains in contrast sensitivity and near-vision acuity (17). This variability likely reflects differences in disease chronicity, stimulation intensity, and electrode configuration (14–17).

Future Prospects and Conclusion

Technological innovation continues to push the boundaries of artificial vision. Next-generation systems seek to combine artificial intelligence with high-density electrode arrays exceeding 1,000 pixels to achieve estimated visual acuities near 20/80 (3,17,18). The emerging HARP4k epiretinal implant uses wireless power delivery and ergonomic spherical design to reduce surgical complexity while maintaining stable signal fidelity (18). Concurrently, artificial intelligence-based image preprocessing enhances contrast, motion cues, and edge detection, mirroring natural saliency processing within the visual cortex (18). This evolution toward adaptive, closed-loop prostheses may soon permit real-time tuning of stimulation thresholds based on cortical feedback.

As devices approach clinical translation, ethical and socioeconomic considerations are pivotal. Limited accessibility, high device cost, and uneven global trial distribution remain major barriers (3,17). Proposals for open-source hardware design and international funding collaborations aim to mitigate inequality and expand availability. Ultimately, the convergence of neural engineering, biomaterials, and artificial intelligence is redefining the clinical goal of visual restoration—from symbolic light perception to meaningful functional vision capable of restoring independence and improving quality of life (3,18).

References

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