Retinal prosthesis: an encouraging first decade with major challenges ahead

Structural changes in the retina after implantation of subretinal three-dimensional implants in mini pigs

The retinal structural changes after subretinal implantation of three-dimensional (3D) microelectrodes were investigated in a mini pig. Three types of electrode were implanted into the subretinal spaces of nine mini pigs: 75-μm-high 3D electrodes on a 200-μm-thick right-angled polydimethylsiloxane (PDMS) substrate (group 1); a 140-μm-thick sloped PDMS substrate without electrodes (group 2); and a 140-μm-thick sloped PDMS substrate with 20-μm-high 3D electrodes (group 3). One mini pig was used as a control. Spectral domain–optical coherence tomography (SD–OCT) images were obtained at baseline and 2, 6, and 12 weeks post-surgery. Retinal specimens were immunostained using a tissue-clearing method 3 months post-implantation. The 75-μm-high 3D electrodes progressively penetrated the inner nuclear layer (INL) and touched the inner plexiform layer (IPL) 2 weeks post-surgery. At 6 weeks post-operatively, the electrodes were in contact with the nerve-fiber layer, accompanied by a severe fibrous reaction. In the other groups, the implants remained in place without subretinal migration. Immunostaining showed that retinal ganglion and bipolar cells were preserved without fibrosis over the retinal implants in groups 2 and 3 during the 12-week implantation period. In summary, SD–OCT and immunohistology results showed differences in the extent of reactions, such as fibrosis over the implants and penetration of the electrodes into the inner retinal layer depending on different types of electrodes. A sloped substrate performed better than a right-angled substrate in terms of retinal preservation over the implanted electrodes. The 20-μm-high electrodes showed better structural compatibility than the 75-μm-high 3D electrodes. There was no significant difference between the results of sloped implants without electrodes and 20-μm-high 3D electrodes, indicating that the latter had no adverse effects on retinal tissue.

Retinal prosthetic vision simulation: temporal aspects

Objective. The perception of individuals fitted with retinal prostheses is not fully understood, although several retinal implants have been tested and commercialized. Realistic simulations of perception with retinal implants would be useful for future development and evaluation of such systems.Approach.We implemented a retinal prosthetic vision simulation, including temporal features, which have not been previously simulated. In particular, the simulation included temporal aspects such as persistence and perceptual fading of phosphenes and the electrode activation rate.Main results.The simulated phosphene persistence showed an effective reduction in flickering at low electrode activation rates. Although persistence has a positive effect on static scenes, it smears dynamic scenes. Perceptual fading following continuous stimulation affects prosthetic vision of both static and dynamic scenes by making them disappear completely or partially. However, we showed that perceptual fading of a static stimulus might be countered by head-scanning motions, which together with the persistence revealed the contours of the faded object. We also showed that changing the image polarity may improve simulated prosthetic vision in the presence of persistence and perceptual fading.Significance.Temporal aspects have important roles in prosthetic vision, as illustrated by the simulations. Considering these aspects may improve the future design, the training with, and evaluation of retinal prostheses.

Advances in Neuroscience, Not Devices, Will Determine the Effectiveness of Visual Prostheses

Background: Innovations in engineering and neuroscience have enabled the development of sophisticated visual prosthetic devices. In clinical trials, these devices have provided visual acuities as high as 20/460, enabled coarse navigation, and even allowed for reading of short words. However, long-term commercial viability arguably rests on attaining even better vision and more definitive improvements in tasks of daily living and quality of life. Purpose: Here we review technological and biological obstacles in the implementation of visual prosthetics. Conclusions: Research in the visual prosthetic field has tackled significant technical challenges, including biocompatibility, signal spread through neural tissue, and inadvertent activation of passing axons; however, significant gaps in knowledge remain in the realm of neuroscience, including the neural code of vision and visual plasticity. We assert that further optimization of prosthetic devices alone will not provide markedly improved visual outcomes without significant advances in our understanding of neuroscience.

Implantation, removal and replacement of subretinal electronic implants for restoration of vision in patients with retinitis pigmentosa

PURPOSE OF REVIEW

The purpose of this review is to provide an update on the efforts to restore vision through subretinal implants in patients with degenerative retinal diseases. In addition to the current technique and its latest improvements, it will focus on the surgical technique of implantation as well as explantation and reimplantation.

RECENT FINDINGS

The durability of the current subretinal implant RETINA IMPLANT Alpha AMS has increased substantially compared with the predecessor model RETINA IMPLANT Alpha IMS. According to validated examinations in the laboratory, a median lifetime of 4.7 years will be reached in clinical use; in similar examinations, the previous model has reached only 8 months. Visual function has slightly increased. The surgical technique for subretinal implants is complex and demanding for ophthalmic surgeons, as it is multifaceted and combines novel surgical steps in areas, which are not commonly entered such as the suprachoroidal and the subretinal space. The surgical approach for implantation has matured considerably and has led to successful implantation in 64 patient cases. Surgical challenges are now mainly encountered with the exact subfoveal positioning of the device. The explantation procedure is relatively straight-forward because the implant can be withdrawn in a reverse direction along the already existent subretinal path. Reimplantations, however, are more challenging because some degree of scar tissue may exist along the path of the chip and around the scleral trapdoor. Nevertheless, reimplantations have now been carried out successfully in four patients.

SUMMARY

The new RETINA IMPLANT Alpha AMS shows significantly improved durability compared with the predecessor model RETINA IMPLANT Alpha IMS. The subretinal implant offers excellent visual results but requires experienced surgeons. Explantation of devices is straight-forward, and reimplantations are challenging but have been successful in four patients.

High-amplitude electrical stimulation can reduce elicited neuronal activity in visual prosthesis

Retinal electrostimulation is promising a successful therapy to restore functional vision. However, a narrow stimulating current range exists between retinal neuron excitation and inhibition which may lead to misperformance of visual prostheses. As the conveyance of representation of complex visual scenes may require neighbouring electrodes to be activated simultaneously, electric field summation may contribute to reach this inhibitory threshold. This study used three approaches to assess the implications of relatively high stimulating conditions in visual prostheses: (1) in vivo, using a suprachoroidal prosthesis implanted in a feline model, (2) in vitro through electrostimulation of murine retinal preparations, and (3) in silico by computing the response of a population of retinal ganglion cells. Inhibitory stimulating conditions led to diminished cortical activity in the cat. Stimulus-response relationships showed non-monotonic profiles to increasing stimulating current. This was observed in vitro and in silico as the combined response of groups of neurons (close to the stimulating electrode) being inhibited at certain stimulating amplitudes, whilst other groups (far from the stimulating electrode) being recruited. These findings may explain the halo-like phosphene shapes reported in clinical trials and suggest that simultaneous stimulation in retinal prostheses is limited by the inhibitory threshold of the retinal ganglion cells.

[Visual prostheses]

Sensory neuroprostheses for restoration of vision are a technical approach for treatment of previously untreatable blindness. These systems consist of a technical sensor such as a camera and an implanted multi-electrode array within the visual system. The image information from the sensor is processed with specially designed integrated circuits in such a way that the stimulation pulses can be determined and presented to the implanted multi-electrode matrix. Energy supply and the transfer of the stimulus pulse information is realized either via direct cable connections within the site of the implant or by telemetric inductive links. Currently, two retinal implant systems are approved in the European Union (EU) to be used in blind patients with retinitis pigmentosa. With both systems basic visual functions can be restored. The complication rate is relatively low given the complexity of the surgical procedure. Other systems are still under development but approval studies by several manufacturers and consortia are already in preparation.

Perspectives on: Materials Aspects for Retinal Prostheses

Retinitis pigmentosa and age-related macular degeneration are both incurable eye diseases that lead to blindness due to photoreceptor degeneration. Electrically stimulating the remaining intact nerve cells may generate some useful vision for patients afflicted with these diseases. Various types of retinal prostheses, sub- and epi-retinal electrode arrays, as well as subretinal microphotodiode arrays are considered from a materials and biocompatibility point of view. Other, more innovative approaches to restoring vision, such as microfluidic pumps and activated nanosystems that deliver neurotransmitters in a controlled way and photodynamic therapy are being developed. This article discusses materials aspects of retinal prostheses that are currently in use or under development.

Selectivity of direct and network-mediated stimulation of the retinal ganglion cells with epi-, sub- and intraretinal electrodes

OBJECTIVE

Intra-retinal placement of stimulating electrodes can provide close and stable proximity to target neurons. We assessed improvement in stimulation thresholds and selectivity of the direct and network-mediated retinal stimulation with intraretinal electrodes, compared to epiretinal and subretinal placements.

APPROACH

Stimulation thresholds of the retinal ganglion cells (RGCs) in wild-type rat retina were measured using the patch-clamp technique. Direct and network-mediated responses were discriminated using various synaptic blockers.

MAIN RESULTS

Three types of RGC responses were identified: short latency (SL, τ < 5 ms) originating in RGCs, medium latency (ML, 3 < τ < 70 ms) originating in the inner nuclear layer and long latency (LL, τ > 40 ms) originating in photoreceptors. Cathodic epiretinal stimulation exhibited the lowest threshold for direct RGC response and the highest direct selectivity (network/direct thresholds ratio), exceeding a factor of 3 with pulse durations below 0.5 ms. For network-mediated stimulation, the lowest threshold was obtained with anodic pulses in OPL position, and its network selectivity (direct/network thresholds ratio) increased with pulse duration, exceeding a factor of 4 at 10 ms. Latency of all three types of responses decreased with increasing strength of the stimulus.

SIGNIFICANCE

These results define the optimal range of pulse durations, pulse polarities and electrode placement for the retinal prostheses aiming at direct or network-mediated stimulation of RGCs.

Artificial human vision

Can vision be restored to the blind? As early as 1929 it was discovered that stimulating the visual cortex of an individual led to the perception of spots of light, known as phosphenes [1] . The aim of artificial human vision systems is to attempt to utilize the perception of phosphenes to provide a useful substitute for normal vision. Currently, four locations for electrical stimulation are being investigated; behind the retina (subretinal), in front of the retina (epiretinal), the optic nerve and the visual cortex (using intra- and surface electrodes). This review discusses artificial human vision technology and requirements, and reviews the current development projects.

A fully intraocular high-density self-calibrating epiretinal prosthesis

This paper presents a fully intraocular self-calibrating epiretinal prosthesis with 512 independent channels in 65 nm CMOS. A novel digital calibration technique matches the biphasic currents of each channel independently while the calibration circuitry is shared among every 4 channels. Dual-band telemetry for power and data with on-chip rectifier and clock recovery reduces the number of off-chip components. The rectifier utilizes unidirectional switches to prevent reverse conduction loss in the power transistors and achieves an efficiency > 80%. The data telemetry implements a phase-shift keying (PSK) modulation scheme and supports data rates up to 20 Mb/s. The system occupies an area of 4.5 ×3.1 mm². It features a pixel size of 0.0169 mm² and arbitrary waveform generation per channel. In vitro measurements performed on a Pt/Ir concentric bipolar electrode in phosphate buffered saline (PBS) are presented. A statistical measurement over 40 channels from 5 different chips shows a current mismatch with μ = 1.12 μA and σ = 0.53 μA. The chip is integrated with flexible MEMS origami coils and parylene substrate to provide a fully intraocular implant.

Extraction of retinal tacks from subjects implanted with an epiretinal visual prosthesis

BACKGROUND

Retinal tacks, first developed for the treatment of complex retinal detachments, have more recently been used for the fixation of epiretinal electrode arrays as part of implanted visual prostheses. Here, we report on the clinical experience of extracting four such tacks after chronic implantation. The ability to safely extract retinal tacks ensures that epiretinal devices can be repositioned or removed if necessary.

METHODS

Custom-built, titanium alloy retinal tacks were mechanically removed from the posterior coats after prolonged implantation (up to 19 months). The resulting wound was characterized by clinical evaluation, fundus photography, and fluorescein angiography while being monitored for stability over time. The wounds were also compared to earlier published reports of the healing response around retinal tacks in human subjects.

RESULTS

Tack extraction was accomplished successfully, without complication, in all four subjects. The wound site was readily identified by pale scar tissue. No change in the wound size or appearance was noted over many months of post-operative observation (up to 22 months after explant). No adverse effects on overall ocular health were detected.

CONCLUSION

Extraction of retinal tacks from subjects implanted with epiretinal prostheses can be performed without significant complication. The long-term healing response appears to be stable and localized in eyes afflicted with retinitis pigmentosa or choroideremia. There was also minimal, if any, impact on the local circulatory system. These cases suggest that the use of retinal tacks for anchoring epiretinal visual prostheses does not preclude safe repositioning or removal of the device more than a year after implant.

ASIC design and data communications for the Boston retinal prosthesis

We report on the design and testing of a custom application-specific integrated circuit (ASIC) that has been developed as a key component of the Boston retinal prosthesis. This device has been designed for patients who are blind due to age-related macular degeneration or retinitis pigmentosa. Key safety and communication features of the low-power ASIC are described, as are the highly configurable neural stimulation current waveforms that are delivered to its greater than 256 output electrodes. The ASIC was created using an 0.18 micron Si fabrication process utilizing standard 1.8 volt CMOS transistors as well as 20 volt lightly doped drain FETs. The communication system receives frequency-shift keyed inputs at 6.78 MHz from an implanted secondary coil, and transmits data back to the control unit through a lower-bandwidth channel that employs load-shift keying. The design's safety is ensured by on-board electrode voltage monitoring, stimulus charge limits, error checking of data transmitted to the implant, and comprehensive self-test and performance monitoring features. Each stimulus cycle is initiated by a transmitted word with a full 32-bit error check code. Taken together, these features allow researchers to safely and wirelessly tailor retinal stimulation and vision recovery for each patient.

Corrosion of silicon integrated circuits and lifetime predictions in implantable electronic devices

Corrosion is a prime concern for active implantable devices. In this paper we review the principles underlying the concepts of hermetic packages and encapsulation, used to protect implanted electronics, some of which remain widely overlooked. We discuss how technological advances have created a need to update the way we evaluate the suitability of both protection methods. We demonstrate how lifetime predictability is lost for very small hermetic packages and introduce a single parameter to compare different packages, with an equation to calculate the minimum sensitivity required from a test method to guarantee a given lifetime. In the second part of this paper, we review the literature on the corrosion of encapsulated integrated circuits (ICs) and, following a new analysis of published data, we propose an equation for the pre-corrosion lifetime of implanted ICs, and discuss the influence of the temperature, relative humidity, encapsulation and field-strength. As any new protection will be tested under accelerated conditions, we demonstrate the sensitivity of acceleration factors to some inaccurately known parameters. These results are relevant for any application of electronics working in a moist environment. Our comparison of encapsulation and hermetic packages suggests that both concepts may be suitable for future implants.

The Detection of Motion by Blind Subjects With the Epiretinal 60-Electrode (Argus II) Retinal Prosthesis

OBJECTIVE

To investigate the ability of 28 blind subjects implanted with a 60-electrode Argus II (Second Sight Medical Products Inc) retinal prosthesis system to detect the direction of a moving object.

METHODS

Blind subjects (bare light perception or worse in both eyes) with retinitis pigmentosa were implanted with the Argus II prosthesis as part of a phase 1/2 feasibility study at multiple clinical sites worldwide. The experiment measured their ability to detect the direction of motion of a high-contrast moving bar on a flatscreen monitor in 3 conditions: with the prosthesis system on and a 1-to-1 mapping of spatial information, with the system off, and with the system on but with randomly scrambled spatial information.

RESULTS

Fifteen subjects (54%) were able to perform the task significantly better with their prosthesis system than they were with their residual vision, 2 subjects had significantly better performance with their residual vision, and no difference was found for 11 subjects. Of the 15 better-performing subjects, 11 were available for follow-up testing, and 10 of them had significantly better performance with normal rather than with scrambled spatial information.

CONCLUSIONS

This work demonstrates that blind subjects implanted with the Argus II retinal prosthesis were able to perform a motion detection task they could not do with their native vision, confirming that electrical stimulation of the retina provides spatial information from synchronized activation of multiple electrodes.

TRIAL REGISTRATION

clinicaltrials.gov Identifier:NCT00407602

Artificial vision with wirelessly powered subretinal electronic implant alpha-IMS

This study aims at substituting the essential functions of photoreceptors in patients who are blind owing to untreatable forms of hereditary retinal degenerations. A microelectronic neuroprosthetic device, powered via transdermal inductive transmission, carrying 1500 independent microphotodiode-amplifier-electrode elements on a 9 mm² chip, was subretinally implanted in nine blind patients. Light perception (8/9), light localization (7/9), motion detection (5/9, angular speed up to 35 deg s⁻¹), grating acuity measurement (6/9, up to 3.3 cycles per degree) and visual acuity measurement with Landolt C-rings (2/9) up to Snellen visual acuity of 20/546 (corresponding to decimal 0.037 or corresponding to 1.43 logMAR (minimum angle of resolution)) were restored via the subretinal implant. Additionally, the identification, localization and discrimination of objects improved significantly (n = 8; p < 0.05 for each subtest) in repeated tests over a nine-month period. Three subjects were able to read letters spontaneously and one subject was able to read letters after training in an alternative-force choice test. Five subjects reported implant-mediated visual perceptions in daily life within a field of 15° of visual angle. Control tests were performed each time with the implant's power source switched off. These data show that subretinal implants can restore visual functions that are useful for daily life.

Photovoltaic retinal prosthesis: implant fabrication and performance

The objective of this work is to develop and test a photovoltaic retinal prosthesis for restoring sight to patients blinded by degenerative retinal diseases. A silicon photodiode array for subretinal stimulation has been fabricated by a silicon-integrated-circuit/MEMS process. Each pixel in the two-dimensional array contains three series-connected photodiodes, which photovoltaically convert pulsed near-infrared light into bi-phasic current to stimulate nearby retinal neurons without wired power connections. The device thickness is chosen to be 30 µm to absorb a significant portion of light while still being thin enough for subretinal implantation. Active and return electrodes confine current near each pixel and are sputter coated with iridium oxide to enhance charge injection levels and provide a stable neural interface. Pixels are separated by 5 µm wide trenches to electrically isolate them and to allow nutrient diffusion through the device. Three sizes of pixels (280, 140 and 70 µm) with active electrodes of 80, 40 and 20 µm diameter were fabricated. The turn-on voltages of the one-diode, two-series-connected diode and three-series-connected diode structures are approximately 0.6, 1.2 and 1.8 V, respectively. The measured photo-responsivity per diode at 880 nm wavelength is ∼0.36 A W(-1), at zero voltage bias and scales with the exposed silicon area. For all three pixel sizes, the reverse-bias dark current is sufficiently low (<100 pA) for our application. Pixels of all three sizes reliably elicit retinal responses at safe near-infrared light irradiances, with good acceptance of the photodiode array in the subretinal space. The fabricated device delivers efficient retinal stimulation at safe near-infrared light irradiances without any wired power connections, which greatly simplifies the implantation procedure. Presence of the return electrodes in each pixel helps to localize the current, and thereby improves resolution.

Effects of subretinal electrical stimulation in mer-KO mice

PURPOSE

Subretinal electrical stimulation (SES) from microphotodiode arrays protects photoreceptors in the RCS rat model of retinitis pigmentosa. The authors examined whether mer(kd) mice, which share a Mertk mutation with RCS rats, showed similar neuroprotective effects from SES.

METHODS

Mer(kd) mice were implanted with a microphotodiode array at postnatal day (P) 14. Weekly electroretinograms (ERGs) followed by retinal histology at week 4 were compared with those of age-matched controls. RT-PCR for fibroblast growth factor beta (Fgf2), ciliary nerve trophic factor (Cntf), glial-derived neurotrophic factor (Gdnf), insulin growth factor 1 (Igf1), and glial fibrillary acidic protein (Gfap) was performed on retinas at 1 week after surgery. Rates of degeneration using ERG parameters were compared between mer(kd) mice and RCS rats from P28 to P42.

RESULTS

SES-treated mer(kd) mice showed no differences in ERG a- and b-wave amplitudes or photoreceptor numbers compared with controls. However, the expression of Fgf2 and Cntf was greater (6.5 ± 1.9- and 2.5 ± 0.5-fold, respectively; P < 0.02) in SES-treated mer(kd) retinas. Rates of degeneration were faster for dark-adapted maximal b-wave, log σ, and oscillatory potentials in mer(kd) mice than in RCS rats.

CONCLUSIONS

Although SES upregulated Fgf2 in mer(kd) retinas, as reported previously for RCS retinas, this was not accompanied by neuroprotection of photoreceptors. Comparisons of ERG responses from mer(kd) mice and RCS rats across different ages showed inner retinal dysfunction in mer(kd) mice but not in RCS rats. This inner retinal dysfunction and the faster rate of degeneration in mer(kd) mice may produce a retinal environment that is not responsive to neuroprotection from SES.

Encoding visual information in retinal ganglion cells with prosthetic stimulation

Retinal prostheses aim to restore functional vision to those blinded by outer retinal diseases using electric stimulation of surviving retinal neurons. The ability to replicate the spatiotemporal pattern of ganglion cell spike trains present under normal viewing conditions is presumably an important factor for restoring high-quality vision. In order to replicate such activity with a retinal prosthesis, it is important to consider both how visual information is encoded in ganglion cell spike trains, and how retinal neurons respond to electric stimulation. The goal of the current review is to bring together these two concepts in order to guide the development of more effective stimulation strategies. We review the experiments to date that have studied how retinal neurons respond to electric stimulation and discuss these findings in the context of known retinal signaling strategies. The results from such in vitro studies reveal the advantages and disadvantages of activating the ganglion cell directly with the electric stimulus (direct activation) as compared to activation of neurons that are presynaptic to the ganglion cell (indirect activation). While direct activation allows high temporal but low spatial resolution, indirect activation yields improved spatial resolution but poor temporal resolution. Finally, we use knowledge gained from in vitro experiments to infer the patterns of elicited activity in ongoing human trials, providing insights into some of the factors limiting the quality of prosthetic vision.

Novel Interface to Biological Systems for Retinal Prosthetics

The development of retinal prostheses requires a method for interconnecting an imaging system to the retina. Such a system must be able to individually address and stimulate retinal neurons, a significant advance from current technology. As a step toward this goal, we present a novel electronic-to-biologic interface using microfabricated apertures in a silicon substrate. Apertures are created in a thin silicon nitride membrane, after which the surface is appropriately modified to support cell growth. Excitable cells are seeded on the device and imaged using Ca2+-sensitive fluorescent dyes in either an inverted or confocal microscope. Using rat pheochromocytoma (PC12) cells, we show the ability to stimulate locally through the apertures. The device allows for the stimulation of cells at precise locations, a necessary requirement for future high-resolution retinal prostheses.

Estimation of phosphene spatial variability for visual prosthesis applications

Visual prostheses are the focus of intensive research efforts to restore some measure of useful vision to blind or near-blind patients. The development of such technology is being guided to an extent by tools that simulate prosthesis behavior for healthy sighted subjects in order to assess system requirements and configurations. These simulators, however, typically assume purely deterministic phosphene properties and thus do not apply any variability to phosphene size, intensity, or location. We address this issue by presenting data on phosphene variability measured in a blind human subject fitted with an optic nerve prosthesis. In order to correct for normal limitations in human-pointing accuracy, the experimental conditions were repeated with sighted subjects. We conclude that identical optic nerve stimulations can result in phosphenes whose perceived locations vary by up to 5 degrees of deviation angle and 10 degrees of position angle. The consistency of phosphenes presented in the peripheral field of view can vary by an additional 3 degrees.

Glutamate stimulation of retinal ganglion cells in normal and s334ter-4 rat retinas: a candidate for a neurotransmitter-based retinal prosthesis

PURPOSE. To investigate the suitability of glutamate as a potential agent for a neurotransmitter-based retinal prosthesis. METHODS. Retinal ganglion cells (RGCs) from P35-70 albino Sprague-Dawley (normal) and P60-254 S334ter-4 (photoreceptor degeneration) rats were recorded extracellularly in flattened eye cup preparations, to assess their responses to glutamate, applied locally via micropipettes. RESULTS. Brief local application of glutamate effectively excited RGCs in both normal and degenerated retinas. Epiretinal surface application of glutamate was less likely to excite RGCs than was subsurface application (20 microm below the epiretinal surface). Glutamate evoked RGC firing rates, and the response patterns were similar for epiretinal surface and subsurface applications. Subsurface application of 2 mM glutamate effectively excited cells within 130 microm of the ejection sites. Response latencies averaged 281 ms and were significantly longer for OFF RGCs than for ON RGCs in normal retinas (P = 0.025). Suppression of activity was observed at shorter latencies ( approximately 100 ms) after glutamate application in most of the spontaneously active RGCs. Responses to each glutamate application were similar, and the duration of activity was directly dependent on the duration of application. RGC responses varied from recurrent high-frequency bursts to sustained firing at rates above 40 spikes/s, in normal and degenerated retinas. Paired, sequential applications of glutamate evoked two distinguishable responses, with interstimulus intervals as low as 200 ms. Overall, RGC response sensitivity to glutamate was similar in normal and degenerated retinas. CONCLUSIONS. Glutamate is an excellent candidate for a neurotransmitter-based retinal prosthesis, as its local application effectively stimulates RGCs with high spatial and temporal resolution.

Preliminary 6 month results from the Argus II epiretinal prosthesis feasibility study

The Argus II 60 channel epiretinal prosthesis has been developed in order to provide partial restoration of vision to subjects blinded from outer retinal degenerative disease. To date the device has been implanted in 21 subjects as part of a feasibility study. In 6 month post-implantation door finding and line tracking orientation and mobility testing, subjects have shown improvements of 86% and 73%, respectively, for system on vs. system off. In high-contrast Square Localization tests using a touch screen monitor 87% of tested subjects performed significantly better with the system on compared with off. These preliminary results show that the Argus II system provides some functional vision to blind subjects.

Magnetic resonance imaging of the retina

This paper reviews recent developments in high-resolution magnetic resonance imaging (MRI) and its application to image anatomy, physiology, and function in the retina of animals. It describes technical issues and solutions in performing retinal MRI, anatomical MRI, blood oxygenation level-dependent functional MRI (fMRI), and blood-flow MRI both of normal retinas and of retinal degeneration. MRI offers unique advantages over existing retinal imaging techniques, including the ability to image multiple layers without depth limitation and to provide multiple clinically relevant data in a single setting. Retinal MRI has the potential to complement existing retinal imaging techniques.

Structure and Function of Embryonic Rat Retinal Sheet Transplants

PURPOSE

To evaluate retinal sheet transplants in S334ter-line-3 retinal degenerate rats by comparing visual responses recorded electrophysiologically with morphology based on light and electron microscopy.

METHODS

S334ter-line-3 retinal degenerate rats (n = 7) received retinal sheet transplants between postnatal days 28 and 31. The donor tissue was derived from transgenic embryonic day 19 (E19) rat retinae expressing human placental alkaline phosphatase (hPAP). Fresh retinal sheets were gently transplanted into the subretinal space of the left eye with the help of a custom-made implantation tool. Selected rats (n = 5) were subjected to electrophysiologic evaluation of visual responses from the superior colliculus about 84-121 days after surgery. Transplanted eyes were processed for light microscopy (LM) and electron microscopy (EM) evaluations.

RESULTS

All the transplanted rats that were evaluated for visual responses in the brain showed responses to very low light stimulation (-3.42 to -2.8 log cd/m(2)) of the eye in a small area of the superior colliculus corresponding with the placement of the transplant in the host retina. Histologic evaluation showed that most of the transplants contained well-laminated areas with correct polarity in the subretinal space. Inside the transplant areas, rosettes of photoreceptors with inner and outer segments were found. In the laminated areas, the outer segments of photoreceptors were facing the host retinal pigment epithelium (RPE). Immunohistochemical evaluation of hPAP donor cells revealed areas with specific staining of the transplants in the subretinal space. Electron microscopic evaluation showed a glial demarcation membrane between the host and the transplant, however, processes originating from the transplant were observed inside the host retina.

CONCLUSIONS

Sheets of E19 rat retina transplanted into the subretinal space of S334ter-line-3 rats survived without immune rejection and continued to show visual function when tested after 3 months. Well-developed photoreceptors and many synapse types were seen within the transplants. hPAP staining showed a certain degree of integration between the host retina and the transplant suggesting that transplanted photoreceptors contributed to the restored light sensitivity.

Development and implantation of a minimally invasive wireless subretinal neurostimulator

A wirelessly operated, minimally invasive retinal prosthesis was developed for preclinical chronic implantation studies in Yucatan minipig models. The implant conforms to the outer wall of the eye and drives a microfabricated polyimide stimulating electrode array with sputtered iridium oxide electrodes. This array is implanted in the subretinal space using a specially designed ab externo surgical technique that fixes the bulk of the prosthesis to the outer surface of the sclera. The implanted device is fabricated on a host polyimide flexible circuit. It consists of a 15-channel stimulator chip, secondary power and data receiving coils, and discrete power supply components. The completed device is encapsulated in poly(dimethylsiloxane) except for the reference/counter electrode and the thin electrode array. In vitro testing was performed to verify the performance of the system in biological saline using a custom RF transmitter circuit and primary coils. Stimulation patterns as well as pulse strength, duration, and frequency were programmed wirelessly using custom software and a graphical user interface. Wireless operation of the retinal implant has been verified both in vitro and in vivo in three pigs for more than seven months, the latter by measuring stimulus artifacts on the eye surface using contact lens electrodes.

Layer-specific anatomical, physiological and functional MRI of the retina

Most retinal imaging has been performed using optical techniques. This paper reviews alternative retinal imaging methods based on MRI performed with spatial resolution sufficient to resolve multiple well-defined retinal layers. The development of these MRI technologies to study retinal anatomy, physiology (blood flow, blood volume, and oxygenation) and function, and their applications to the study of normal retinas, retinal degeneration and diabetic retinopathy in animal models are discussed. Although the spatiotemporal resolution of MRI is poorer than that of optical imaging techniques, it is unhampered by media opacity and can thus image all retinal and pararetinal structures, and has the potential to provide multiple unique clinically relevant data in a single setting and could thus complement existing retinal imaging techniques. In turn, the highly structured retina with well-defined layers is an excellent model for advancing emerging high-resolution anatomical, physiological and functional MRI technologies.

Blood flow magnetic resonance imaging of retinal degeneration

PURPOSE

This study aims to investigate quantitative basal blood flow as well as hypercapnia- and hyperoxia-induced blood flow changes in the retinas of the Royal College of Surgeons (RCS) rats with spontaneous retinal degeneration, and to compare with those of normal rat retinas.

METHODS

Experiments were performed on male RCS rats at post-natal days P90 (n=4) and P220 (n=5), and on age-matched controls at P90 (n=7) and P220 (n=6). Hyperoxic (100% O(2)) and hypercapnic (5% CO(2), 21% O(2), balance N(2)) challenges were used to modulate blood flow. Quantitative baseline blood flow, and hypercapnia- and hyperoxia-induced blood flow changes in the retinas were imaged using continuous arterial spin labeling MRI at 90 x 90 x 1500 microm.

RESULTS

In the normal rat retinas, basal blood flow of the whole-retina was 5.5 mL/gram per min, significantly higher than those reported in the brain (approximately 1 mL/gram per min). Hyperoxia decreased blood flow due to vasoconstriction and hypercapnia increased blood flow due to vasodilation in the normal retinas. In the RCS rat retinas, basal blood flow was diminished significantly (P<0.05). Interestingly, absolute hyperoxia- and hypercapnia-induced blood flow changes in the RCS retinas were not statistically different from those in the normal retinas (P>0.05). However, blood flow percent changes in RCS retinas were significantly larger than in normal retinas due to lower basal blood flow in the RCS retinas.

CONCLUSIONS

Retinal degeneration markedly reduces basal blood flow but does not appear to impair vascular reactivity. These data also suggest caution when interpreting relative stimulus-evoked functional MRI changes in diseased states where basal parameters are significantly perturbed. Quantitative blood flow MRI may serve as a valuable tool to study the retina without depth limitation.

Vision improvement in retinal degeneration patients by implantation of retina together with retinal pigment epithelium

PURPOSE

To demonstrate efficacy and safety of the implantation of neural retinal progenitor cell layers (sheets) with its retinal pigment epithelium (RPE) in retinitis pigmentosa (RP) and dry age-related macular degeneration (AMD) patients with 20/200 or worse vision in the surgery eye.

DESIGN

Interventional nonrandomized clinical trial.

METHODS

Ten patients (six RP, four AMD) received retinal implants in one eye and were followed in a phase II trial conducted in a clinical practice setting. Early Treatment Diabetic Retinopathy Study (EDTRS) was the primary outcome measure. All implant recipients and nine of 10 tissue donors were deoxyribonucleic acids typed.

RESULTS

Seven patients (three RP, four AMD) showed improved EDTRS visual acuity (VA) scores. Three of these patients (one RP, two AMD) showed improvement in both eyes to the same extent. Vision in one RP patient remained the same, while vision in two RP patients decreased. One RP patient has maintained an improvement in vision from 20/800 to 20/200 ETDRS for more than five years; at the six-year examination, it was still maintained at 20/320 while the nonsurgery eye had deteriorated to hand motion vision. This patient also showed a 22.72% increase in light sensitivity at five years compared to microperimetry results at two years; the other patients showed no improved sensitivity. Although no match was found between donors and recipients, no rejection of the implanted tissue was observed clinically.

CONCLUSIONS

Seven (70%) of 10 patients showed improved VA. This outcome provides clinical evidence of the safety and beneficial effect of retinal implants and corroborates results in animal models of retinal degeneration.

Therapeutic potential of computer to cerebral cortex implantable devices

In this article, an overview of some of the latest developments in the field of cerebral cortex to computer interfacing (CCCI) is given. This is posed in the more general context of Brain-Computer Interfaces in order to assess advantages and disadvantages. The emphasis is clearly placed on practical studies that have been undertaken and reported on, as opposed to those speculated, simulated or proposed as future projects. Related areas are discussed briefly only in the context of their contribution to the studies being undertaken. The area of focus is notably the use of invasive implant technology, where a connection is made directly with the cerebral cortex and/or nervous system. Tests and experimentation which do not involve human subjects are invariably carried out a priori to indicate the eventual possibilities before human subjects are themselves involved. Some of the more pertinent animal studies from this area are discussed. The paper goes on to describe human experimentation, in which neural implants have linked the human nervous system bidirectionally with technology and the internet. A view is taken as to the prospects for the future for CCCI, in terms of its broad therapeutic role.

Impedance-based retinal contact imaging as an aid for the placement of high resolution epiretinal prostheses

An important factor in effective stimulation of the retina is close contact with the retina. The design of the electrode surface and the placement of the electrode against the retina both affect the degree of contact with the retina. We have addressed the design factor by creating a curved surface 3200-electrode array. The placement factor we have addressed by use of an impedance sensitive feedback from the array. The feedback is in the form of an image showing contact with the retina, where greater pixel intensity indicates greater impedance and thus closer contact with the retina. In this paper, we present qualitative and quantitative assessments of the relationship between impedance and the device output as well as an in vivo demonstration of contact imaging. In addition, we evaluated the three-dimensional profile of the stimulation voltage distribution to assess the importance of close retinal contact for high resolution stimulation.

Towards the bionic eye--the retina implant: surgical, opthalmological and histopathological perspectives

Degenerations of the outer retina such as retinitis pigmentosa (RP) lead to blindness due to photoreceptor loss. There is a secondary loss of inner retinal cells but significant numbers of bipolar and ganglion cells remain intact for many years. Currently, no therapeutic option to restore vision in these blind subjects is available. Short-term pattern electrical stimulation of the retina using implanted electrode arrays in subjects blind from RP showed that ambulatory vision and limited character recognition are possible. To produce artificial vision by electrical retinal stimulation, a wireless intraocular visual prosthesis was developed. Images of the environment, taken by a camera are pre-processed by an external visual encoder. The stimulus patterns are transmitted to the implanted device wirelessly and electrical impulses are released by microcontact electrodes onto the retinal surface. Towards a human application, the biocompatibility of the utilised materials and the feasibility of the surgical implantation procedure were stated. In acute stimulation tests, thresholds were determined and proved to be within a safe range. The local and retinotopic activation of the visual cortex measured by optical imaging of intrinsic signals was demonstrated upon electrical retinal stimulation with a completely wireless and remotely controlled retinal implant. Potential obstacles are reviewed and further steps towards a successful prosthesis development are discussed.