Since Swedish engineer Arne Larsson received the first fully implanted cardiac pacemaker more than 40 years ago, researchers around the world have looked at ways of improving people’s lives with artificial, bionic devices. One of the most dramatic applications of bionics is the creation of artificial eyes. Early efforts used silicon-based photodetectors, but silicon is toxic to the human body and reacts unfavourably with fluids in the eye. Now, scientists at the Space Vacuum Epitaxy Centre (SVEC) based at the University of Houston, Texas, are using a new material they have developed, tiny ceramic photocells that could detect incoming light and so ‘repair’ malfunctioning human eyes.
Success of Silicon Microdetectors to Date
‘Silicon has not been successful to date, and its use continues to suffer from problems of deterioration of the chip, contamination of the eye and atrophy of the retina,’ says Alex Ignatiev, a Professor at the University of Houston and Director of SVEC. ‘Our ceramic microdetectors should overcome all of these problems.’ In fact, SVEC’s thin, photosensitive ceramic films respond to light as much as rods and cones do.
How the Eye Detects Light
At the back of every healthy human eye are millions of rods and cones - 120 million rods and around six million cones. They act as biological ‘solar cells’ in the retina that convert light into electrical impulses, which then travel along the optic nerve to the brain where images are formed. If these cells degenerate or malfunction, the result is a loss of eyesight.
Diseases of the Eye That Can Potentially Be Cured
‘There are some diseases in which the sensors in the eye, the rods and cones, have deteriorated but all the wiring is still in place,’ says Ignatiev ‘if we could replace those damaged rods and cones with artificial ones, then a person who is retinally blind might be able to regain some of their sight.’ The artificial implants being developed at SVEC are intended to help people with retinal diseases such as macular degeneration and retinitis pigmentosa. Macular degeneration is an age-related disease and usually affects people over 50 years of age. According to Moorfields Eye Hospital in London, UK, it accounts for almost 50% of all visual impairment in the developed world. Retinitis pigmentosa causes the rods and cones in the eyes to malfunction, and tends to be hereditary. In the UK more than 25,000 families have RP, and globally this figure runs into millions, according to the British Retinitis Pigmentosa Society.
Origins of the Technology
The new thin photosensitive ceramic films could offer hope to sufferers, but manufacturing an artificial replacement for millions of rods and cones is no easy task. Crafting the films is a skill SVEC scientists learned from experiments conducted using the Wake Shield Facility (WSF) - a 3.5m diameter disk-shaped platform launched from the space shuttle. The WSF was designed by SVEC engineers to study epitaxial film growth in the ultra-vacuum of space. ‘We grew thin oxide films using atomic oxygen in low-Earth orbit as a natural oxidising agent,’ explains Ignatiev. ‘Those experiments helped us develop the oxide (ceramic) detectors we are now using for the ‘bionic eye’ project.’
Structure of the Microdetectors
The ceramic microdetectors resemble the ultra-thin films found in modern computer chips. The arrays are stacked in a hexagonal structure, which mimics the arrangement of the rods and cones it has been designed to replace. ‘Our work differs from existing work in that we are using a newly developed oxide thin-film ceramic microdetector that does not require encapsulation or wire connections for integration into the human retina.’
How Ceramic Microdetectors Overcome Problems with Silicon Implants
One problem that has plagued silicon implants is solved by the ceramic detectors. The naturally porous structure allows nutrients to flow from the back to the front of the eye, preventing atrophication of the retina. Also, whereas silicon needs to be encapsulated because of its toxicity in the human body, ceramic sensors don’t require any such treatment. ‘There is no need to encapsulate the ceramic microdetectors or have wire connections to the bipolar layer of cells that neighbour the human cones,’ explains Ignatiev. ‘The microdetectors are simply placed into the subretinal space as an array of individual microdetectors, which allows the passage of nutrients from the back of the eye to the interior, thus preventing atrophy of neighbouring cells, which can occur when large encapsulated detectors are used.’
First Generation Artificial Retinas
The first generation of artificial retinas constructed at SVEC consist of 100,000 tiny ceramic microdetectors, each one twentieth the size of a human hair, figure 1. The assemblage is so small that surgeons can’t safely handle it. So instead, the arrays are attached to a polymer film one millimetre by one millimetre in size. The film is designed to dissolve after a couple weeks, leaving the array in position on the retina.
Figure 1. First generation ceramic thin film microdetectors (~30µm in size) attached in a square array onto a polymer carrier for surgical implantation. Human cones (~5-10µm) arranged in a hexagonal array.
Ignatiev hopes that the first human trials will begin later this year. Until this happens biocompatibility tests will continue in rabbits with identification of both ‘rabbit-eye’ and microdetector stability. Dr Charles Garcia is leading the tests from the University of Texas Medical School and St Joesph’s Hospital. ‘We still have to prove that the microdetectors work in vivo,’ says Ignatiev. ‘They output the appropriate voltage amount in the laboratory experimental system, but we don’t know how their voltage output will respond when implanted in the eye.’ Ignatiev and his team of scientists are also unsure how the brain will interpret these ‘foreign’ signals generated by the artificial light sensors, compared with the signals generated naturally by human rods and cones.
There is still an enormous amount of work to be done in developing artificial retinas, and the response so far to the work Ignatiev is involved with has been extremely positive. He believes the immediate goal is to develop a functioning artificial retina with resolution that mimics human sensors. ‘Once this step has been achieved,’ he says, ‘then attention can be brought to bear on colour vision, followed by the replacement of some of the interconnecting neural cells that lead to the optic nerve.’
Around the world, other ‘bionic eyes’ are still being developed. However, all these developments are designed to help repair degenerative diseases of the retina, and not damage to the optic nerve or brain. Therefore, current devices won’t help sufferers of glaucoma, in which there is damage to the optic nerve. ‘Optic nerve replacement is a huge challenge,’ says Ignatiev. ‘There are more than 1.2 million connections, and there will be a long learning curve for microsurgeons to be able to make such connections to artificial parts of the eye.’