The retina is the light-sensitive part of the eye, which helps us to see. Damage to the light-sensitive cells in the retina is the reason for a great many cases of vision loss.
A common disease of the retina is macular degeneration, where the central part of the retina loses its function. Another is retinitis pigmentosa, where vision loss starts at the periphery and progresses inwards.
In both cases, the remaining structure of the eye, which collects lights and forms images, and then the nerves that carry electrical signals to the brain, are intact. It is the mechanism to convert light into electrical impulses that has failed.
Jing Tang, Nan Qin, Yan Chong, Yupu Diao, Yiliguma, Zhexuan Wang, Min Jiang, Jiayi Zhang and Gengfeng Zheng from Fudan University, Shanghai and a Tian Xue from the Heifi National Laboratory, China, report in the journal, Nature Communications, an advance in the drive to find man-made devices to take the place of the failed light-sensitive cells.
The team of researchers has created a mat made of tiny bits of titanium dioxide wire, dotted with nano-particles of gold, which is able to resolve light falling on the retina with fineness not achieved so far. The device has been tested on laboratory mice and there is promise of helping human patients as well.
When light strikes the photoreceptor cells in the retina of the natural eye, there is a structural change in molecules in a pigment within the cell, and this affects the movement of charged sodium ions, creating electrical tension.
This is the signal that gets transmitted down, to other nerve cells within the retina and then to the brain, as brightness in the part of the visual field that the photoreceptor cell represents.
There are about 120 million photoreceptor cells in the average human eye, and these are connected to some two million nerve terminals leading to the brain.
When the photoreceptor cells are damaged, what we need is a method to generate electrical signals and send them to the nerve cells. One method that has been developed has head-mounted cameras with photocells to generate signals.
Users learn to associate the neural stimuli received to approximate shapes. The implanted electronics, however, is bulky and the surgery to pass cables through the lining of the eye, to transfer signals, is complex. Another system places the camera and related electronics in a chip that is implanted behind the retina.
This device contains tiny photocells to capture light, amplifiers to boost their signal and electrodes to stimulate retinal nerve cells. The surgery is even more complex, the cost in both cases is astronomical and the resolution of images is limited.
An improvement that was reported in 2015 consists of photocells, with basic electronics, mounted on a chip that is implanted with less invasive surgery and communicating directly with the mat of nerve cells.
A chip that was one mm square and implanted behind the retina of experimental rats had 250 photocells, which permitted reasonable resolution of detail.
The current development by the group writing in Nature Communications goes one further and replaces the very photocell with photosensitive material, at the dimensions of nano-metres, which allows far greater resolution of images.
Apart from not needing diodes and other electronics to transfer charge, nano-materials have a different mechanism of sensitivity to light.
The principle of the photocell is that atoms of semiconductor materials, like silicon, have loosely bound electrons in their outer orbits, which can get knocked free when struck by a photon or particle of light.
These electrons represent charge, which can be tapped through one-way gates, called diodes, and initiate electrical effects.
In contrast, light, which is an electromagnetic wave, set up mechanical and electrical oscillations in the material. The dimensions of nano-particles are suitable for resonance, or frequency matching of the light wave and the oscillations in the material, and there can be transfer of energy from the light wave.
An array or matrix of such nano-wires of titanium dioxide was created by depositing the titanium material on a substrate from a solution, followed by heat treatment.
Nano-particles of gold were then deposited on the nano-wires by soaking the array in a solution of a gold compound after washing and annealing. Spikes of nano-wires and a nano-wire spotted with gold can be seen in the picture.
The interesting thing about this array of nano-wires is that while it will give off electric charge when exposed to light, the array can be directly in contact with nerve tissue, to transfer charge in the same way that the natural rods and cones of the eye excite the nerve cells.
While this is similar to the arrangement in the 2015 development, the big difference is that the light sensitive elements are now nano-wires, not full-fledged photocells.
The diameter of the nano-wires was 100 nm (a nanometre is a millionth of a millimetre) and their length was two microns (a micron is a thousandth of a millimetre).
There were about 10 nano-wires to every square micron. This works out to be 10 million nano-wires in every square millimetre. We can see that this is a huge improvement over the 250 photocells we could have in a square millimetre in the work reported in 2015.
The arrays of nano-wires were then tried out, first in experimental mouse retinas in the lab and then after implantation in the retinas of living mice.
The nano-wire implanted retinas showed robust sensitivity to light of all relevant frequencies (colours) both in the retina test as well as in living mice.
“…the pupillary light reflex experiment showed that the sensitivity of blind mice implanted with NW arrays was similar to that in wild-type mice”, the paper says.
What has been designed and manufactured hence mimics the way real rod and cone cells of the eye function, and has several good features flowing from the nano-metre scale dimensions, the paper says.
Although the design of nano-wires still does not make colour vision possible, that is also within reach. The paper says the capability that has been demonstrated point towards adaptation for use as prosthetic devices in human patients.
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