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Bionic eyes — realities and aspirations

$currentPage/@nodeName Schematic of a retinal prosthesis Image: Bionic Vision Australia

Dr Lauren N Ayton
BOptom PhD GCOT FACO

Dr Chi D Luu  
BOrth  PhD  GradDipEpidem

Dr Penelope J Allen
MBBS  FRANZCO

Professor
Robyn H Guymer 
MBBS  PhD  FRANZCO

Centre for Eye Research Australia,
The University of Melbourne,
Royal Victorian Eye and Ear Hospital*

Traditionally, most ophthalmic interventions have been targeted at delaying the progression of vision loss, such as ocular hypotensives for glaucoma and laser photocoagulation for proliferative diabetic retinopathy, with no ability to restore lost vision. Anti-VEGF treatments for neovascular AMD have changed this, with the ability to regain some vision if the intervention is applied urgently.1 Until recently, it has not been possible to intervene in cases of long-standing vision loss, such as in diseases like retinitis pigmentosa and choroideremia.

Due to recent technological advances, this is about to change. The advent of techniques such as gene therapy,2 stem cell implantation3 and visual prostheses (`bionic eyes') has the potential to restore vision in people who have had profound vision loss for many years. Of these techniques, visual prostheses have had the greatest commercial advances, with the Second Sight Argus II bionic eye device now available for sale in the USA and Europe.

Visual prostheses convert visual information into electrical impulses, in a similar way that the cochlear implants have worked to restore hearing to the deaf. There are three main categories of visual prosthesis, classified by the location of the electrode array: cortical, optic nerve or retinal.

The first visual prostheses were cortical implants, developed in the 1930s after the German ophthalmologist Carl Förster discovered that direct electrical stimulation of the visual cortex caused blind patients to detect a spot of light, known as a phosphene.4 There was little advancement in this area until the 1960s, when Australian inventor Graham Tassicker patented a photo-sensitive selenium cell that could be placed subretinally to evoke visual phosphenes.5 This discovery reinvigorated the visual prosthetic research field and rekindled the idea of using a visual prosthesis to restore vision to the blind.

Three approaches

Cortical implants work by placing penetrating or surface electrodes directly in the primary visual cortex which, until recently, led to inherent limitations in resolution and long-term instability of cortical electrodes. Recent improvements in electrode configuration and material biocompatibility have improved the feasibility of these devices. One such cortical device is being developed in Melbourne by the Monash Vision Group.

Another option for a visual prosthesis is direct electrical stimulation of the optic nerve. Two techniques have been developed, either using a cuff electrode activating numerous optic nerve fibres at once, giving the perception of large indistinct phosphenes,6 or a more targeted penetrating micro-electrode array.7

The retinal approach, where the electrode array is placed in direct proximity to the retina, has proved most popular among research groups and has arguably seen the most advances. Until the 1970s, placement on the retina was not a viable option due to the complexities of retinal surgery but as surgical methods improved, the possibility of placing electrodes within the eye became a feasible option.8 Retinal implants can be placed in different locations near the retina, variously epiretinally, subretinally or supra-choroidally (Figure 1).

The causes of blindness that may be able to benefit from bionic eye technology depend on the location of the implanted electrodes. Cortical prostheses stimulate the brain directly and so they do not need an intact globe or optic nerve.

As such, it is predicted that cortical implants will be able to work in most causes of blindness, including glaucoma and traumatic vision loss. The disadvantage with this method is that the device does not use the image processing capabilities of the retina. Also, due to the cortical topography and electrode limitations, it is theoretically more difficult to generate localised high resolution images.

On the other hand, retinal implants will use the remaining visual pathway (residual inner retina to cortex) to help in the processing of the electrically stimulated phosphene image. Retinal implants theoretically can be used only in patients who have some remaining inner retinal cells and optic nerve function, such as retinitis pigmentosa and choroideremia. Both of these retinal diseases specifically affect the outer retinal cells (photoreceptor layer), leaving the inner retina and optic nerve relatively intact.

Retinal prosthesis

The main components of the retinal prosthesis system consist of a camera mounted on a pair of glasses, an external computer microprocessor, a battery and a silicon chip with an array of electrodes for retinal stimulation (Feature Image). The video camera captures an image which is then transformed by the microprocessor into electrical signals. The signal will be coded so that information like edge detection and brightness can be relayed by adjusting variables such as the level of current, rate or duration of electrical stimulation.

The external processor then sends the signals, either by a cable or wirelessly, to the implanted electronic microchip where it stimulates an electrode array. This electrode array is placed near the surviving inner retinal ganglion cells, which are responsible for taking the signal from the electrode to the brain via the optic nerve.

Due to the form of electrical stimulation, the vision from a bionic eye implant will be perceived as spots of light, or `phosphenes'. While the phosphenes will be able to be manipulated to improve the visual potential, it will not be the same as normal human vision. At this stage of research, the expected visual outcomes are still moderate.

It is unlikely that patients will be able to read again with the first generation of bionic eye implants; however, it is believed that there will be significant improvements in the level of functional vision. This would mean that patients would be able to identify objects, have improved orientation and mobility performance and enjoy safer independent travel.

Current clinical trials

There is a number of groups that are currently completing clinical trials into the safety and efficacy of visual prostheses. All trials currently involve patients with profound vision loss (bare light perception or worse in both eyes) from retinitis pigmentosa or choroideremia. The most advanced research comes from the Second Sight group in the USA, which now has commercial approval to sell its epiretinal 60 electrode device (Argus II) in both Europe and the USA. Second Sight has shown that this device can allow some patients to perform simple daily living tasks and read large letters.9,10 Similar outcomes have been shown in subretinal implants by the Retina Implant AG group in Germany.11,12

Supra-choroidal implant

Both epiretinal and subretinal visual prostheses require a high level of ophthalmic surgical skill to implant and have had problems with long-term safety stability. Bionic Vision Australia has developed a supra-choroidal implant. The advantage of this positioning is that the surgery is less complicated and the device remains stable over time13 as it is `sandwiched' between the choroid and sclera.16,17

The supra-choroidal location also means that the device wires need to pass only from the intraorbital space into the supra-choroidal space, and not into the interior of the globe, which reduces the chances of endophthalmitis. We are currently undertaking the first human clinical trials of this supra-choroidal implant, which has shown great success to date and clinical findings will be published in the coming months.**

Bionic Vision Australia is developing two future devices, one with 96 electrodes and one with 256, which will provide increased resolution and improved patient outcomes. Novel stimulation strategy techniques such as current steering will further optimise the efficacy of these devices. While the Bionic Vision Australia device will not be the first retinal prosthesis to enter the market, it is hoped that when it is released, it will be a highly functional device that will deliver superior visual return to the patients.

910-Figure-1

Figure 1. Potential locations for retinal visual prostheses  
Image: Bionic Vision Australia

Future work and challenges

The development of a bionic eye is one of the most difficult technological challenges that biomedical engineering has faced to date, requiring multidisciplinary input from engineers, surgeons, clinicians, material scientists and basic scientists. Devices must be hermetically sealed to keep biological fluids away from the electronics, electrically safe, and of appropriate size and shape to be implanted into the eye.

At the same time, they must be able to conduct electricity in a reliable and repeatable manner, requiring advanced power and wiring solutions. While the technological challenges are significant, the available resources and knowledge in the current scientific community mean that they are not insurmountable.

Another challenge facing visual prostheses researchers is the question of how best to assess functional outcomes following bionic eye implantation. At this stage of the technological development, it is unlikely that patients will be able to read a standard visual acuity chart. It is believed that the three most important aspects of improvement that will require quantitative measurement are independent mobility, functional vision in activities of daily living and reported improvements in quality of life.

For example, there have been reports of subjects with retinal prosthetic implants being able to correctly describe and name objects like a fork or knife on a table, geometric patterns and different fruits.12 These measurable tasks relate directly to improvements in everyday visual functioning and will be vital to assess in future clinical trials. At present, there are no standardised assessment tests for recipients of bionic eye implants but it is hoped that an international collaboration in the future will determine the most effective and sensitive tests.14

The aims for future work in this area are to improve the visual prosthesis devices, by a combination of increasing the number of electrodes on the arrays and more advanced stimulation strategy and image processing of the video input. It is hoped that bionic eyes may become a widely clinically available option for people with profound vision loss.

* All authors are researchers for the Bionic Vision Australia program. Dr Ayton is the clinical research co-ordinator, Dr Luu is the surgical and clinical program manager, Dr Allen is the surgical program leader and Professor Guymer is the clinical program leader.

** Full details of the Australian trial can be accessed at www.clinicaltrials.gov/ct2/show/NCT01603576.

References are available from j.megahan@optometrists.asn.au, subject: Bionic eyes, Low Vision Primer.



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