Photo courtesy Optobionics
Only 2 mm in diameter and thinner than a
human hair, this silicon chip may restore
sight. |
Even if you wear eyeglasses, your eyes are functioning at a
level good enough to recognize the small letters on this page.
Text on most computer screens is about 3 millimeters tall and
2 mm wide ( .12 X .08 inches). As you read this one sentence,
you are probably oblivious to the thousands of pieces of
visual information that your eyes are gathering each second.
Just in the retina alone, there are millions of cells at work
right now acting as photoreceptors reacting to light, similar
to how a camera works to capture images on film.
The retina is a thin layer of neural tissue that lines the
back wall inside the eye. Some of these cells act to receive
light, while others interpret the information and send
messages to the brain through the optic nerve. This is part of
the process that enables you to see. In damaged or
dysfunctional retina, the photoreceptors stop working, causing
blindness. By some estimates, there are more than 10 million
people worldwide affected by retinal diseases that lead to
loss of vision.
Until now, those who lost their vision to retinal disease
would have had little hope of regaining their sight, but
technological breakthroughs could soon give back the gift of
sight. Several groups of scientists have already developed
silicon microchips that can create artificial vision!
In this issue of How Stuff Will Work, we will examine
how your retinas work and how blindness caused by retinal
disease no longer means the loss of vision.
How Your Retina Works
The eye is one of the
most amazing organs in the body. To understand how artificial
vision is created, it's important to know about the important
role that the retina plays in how you see (see How Corrective
Lenses Work to learn more about how your vision works).
Here is a simple explanation of what happens when you look at
an object:
- Scattered light from the object enters through the
cornea.
- The light is projected onto the retina.
- The retina sends messages to the brain through the optic
nerve.
- The brain interprets what the object is.
The anatomy of the eye
|
The retina is complex in itself. This thin membrane at the
back of the eye is a vital part of your ability to see. Its
main function is to receive and transmit images to the brain.
These are the three main types of cells in the eye that help
perform this function:
- Rods
- Cones
- Ganglion Cells
There are about 125 million rods
and cones within the retina that act as the eye's
photoreceptors. Rods are the most numerous of the two
photoreceptors, outnumbering cones 18 to 1. Rods are able to
function in low light (they can detect a single photon) and
can create black and white images without much light. Once
enough light is available (for example, daylight or artificial
light in a room), cones give us the ability to see color and
detail of objects. Cones are responsible for allowing you to
read this article, because they allow us to see at a high
resolution.
The information received by the rods and cones are then
transmitted to the nearly 1 million ganglion cells in the
retina. These ganglion cells interpret the messages from the
rods and cones and send the information on to the brain by way
of the optic nerve.
There are a number of retinal diseases that attack these
cells, which can lead to blindness. The most notable of these
diseases are retinitis pigmentosa and age-related
macular degeneration. Both of these diseases attack the
retina, rendering the rods and cones inoperative, causing
either loss of peripheral vision or total blindness. However,
it's been found that neither of these retinal diseases affect
the ganglion cells or the optic nerve. This means that if
scientists can develop artificial cones and rods, information
could still be sent to the brain for interpretation.
Creating Artificial Sight
Photo courtesy Optobionics.
The dot above the date on this penny is the
full size of the Artificial Silicon Retina.
|
The current path that scientists are taking to create
artificial vision received a jolt in 1988, when Dr. Mark
Humayun demonstrated that a blind person could be made to see
light by stimulating the nerve ganglia behind the retina with
an electrical current. This test proved that the nerves behind
the retina still functioned even when the retina had
degenerated. Based on this information, scientists set out to
create a device that could translate images and electrical
pulses that could restore vision.
Today, such a device is very close to becoming available to
the millions of people who have lost their vision to retinal
disease. In fact, there are at least two silicon microchip
devices that are being developed, and one has already been
implanted in the eyes of three blind patients. The concept for
both devices is similar, with each being:
- Small enough to be implanted in the eye
- Supplied with a continuous source of power
- Biocompatible with the surrounding eye tissue
Perhaps the most promising of these two silicon
devices is the artificial silicon retina (ASR)
developed by Optobionics.
As you can see in the picture at the top of this page the ASR
is an extremely tiny device, smaller than the surface of a
pencil eraser. It has a diameter of just 2 mm (.078 inch) and
is thinner than a human hair. There is good reason for its
microscopic size. In order for an artificial retina to work it
has to be small enough so that doctors can transplant it in
the eye without damaging the other structures within the eye.
On June 28 and 29, doctors at the University of Illinois at
Chicago Medical Center and the Central DuPage Hospital,
Winfield, Ill., implanted the first artificial retinas in the
eyes of blind patients who had lost nearly all of their vision
from retinitis pigmentosa. Preliminary tests from these
FDA-approved surgeries have determined that the device has
been biocompatible with each patient's eyes so far. It could
be months before doctors know the results of these surgeries.
They expect that the patients will be able to regain some
vision that would allow them to see rough black and white
images, but not with any detail or color
The ASR contains about 3,500 microscopic solar
cells that are able to convert light into electrical
pulses, mimicking the function of cones and rods. To implant
this device into the eye, surgeons make three tiny incisions
no larger than the diameter of a needle in the white part of
the eye. Through these incisions, the surgeons introduce a
miniature cutting and vacuuming device that removes the gel in
the middle of the eye and replaces it with saline. Next, a
pinpoint opening is made in the retina through which they
inject fluid to lift up a portion of the retina from the back
of the eye, which creates a small pocket in the subretinal
space for the device to fit in. The retina is then resealed
over the ASR.
Photo courtesy Optobionics.
Here you can see where the ASR is placed
between the outer and inner retinal layers.
|
For any microchip
to work it needs power, and the amazing thing about the ASR is
that it receives all of its needed power from the light
entering the eye. As you learned before, light that enters the
eye is directed at the retina. This means that with the ASR
implant in place behind the retina, it receives all of the
light entering the eye. This solar energy eliminates the need
for any wires, batteries or other secondary devices to supply
power.
Another microchip device that would restore partial vision
is currently in development by a team of researchers from Johns
Hopkins University, North
Carolina State University and the University
of North Carolina-Chapel Hill. Called the artificial
retina component chip (ARCC), this device is quite similar
to the ASR. Both are made of silicon and both are powered by
solar energy. The ARCC is also a very small device measuring 2
mm square and a thickness of .02 millimeters (.00078 inch).
There are significant differences between the devices,
however.
Unlike the ASR which is placed between layers of retinal
tissue, the ARCC is placed on top of the retina. Because it is
so thin, light entering the eye is allowed to pass through the
device to strike the photosensors on the back of the chip.
However, this light is not the power source for the ARCC.
Instead, a secondary device attached to a pair of common
eyeglasses directs a laser at the chip's solar cells to
provide power. The laser would have to be powered by a small
battery pack.
According to researchers, the ARCC will give blind patients
the ability to see 10 by 10 pixel images, which is about the
size of a single letter on this page. However, researchers
have said that they could eventually develop a version of the
chip that would allow 250 by 250 pixel array, which would
allow those who were once blind to read a newspaper.
Lots More Information!
