Stanford News: Multidisciplinary Team Develops Devices to Restore Vision
Wednesday November 19, 1:00 pm ET
STANFORD, Calif.--(BUSINESS WIRE)--Nov. 19, 2003--An interdisciplinary team
of experts is working to reverse the effects of age-related macular
degeneration -- the leading cause of blindness among Americans over age 65.
The researchers plan to use eye tissue transplants for patients who still
have some vision and prosthetic chips for those who have lost all vision.
"This is a very optimistic and ambitious project," said chemical engineer
Stacey Bent, who with ophthalmologist Harvey Fishman jointly leads the
research efforts.
On Saturday, Fishman presented the first complete design for a chip that
functions like the natural retina of the eye. It uses chemicals to transmit
nerve impulses to the brain. He spoke at the annual meeting of the American
Academy of Ophthalmology in Anaheim. Also this week, two of Bent's graduate
students, Christina Lee and Neville Mehenti, are presenting the group's work
at the annual meeting of the American Institute of Chemical Engineers in San
Francisco.
"Optimistically, human trials of the tissue transplant could begin within
the next six months," Fishman said. The retinal prosthesis is a longer-term
project -- trials could begin in two to three years. The team already has
successfully implanted prototype devices into animals and is refining the
surgical techniques to prevent complications such as bleeding or retinal
detachments.
Tissue transplant
In a healthy eye, vision occurs when light-sensitive cells in the retina
convert light into electrical signals that the optic nerve then transmits to
the brain. These cells receive nutrients and excrete waste through a thin
layer of cells that covers them. In age-related macular degeneration, this
life-giving layer degrades over time, leading to the eventual death of the
cells beneath.
Patients with the disease typically lose central vision. In about 80 percent
of those patients, some underlying cells remain alive although the cover
layer has degraded. The team is recreating the protective cell layer using
cells and tissues from other parts of the eye. This involves removing the
tissue that normally covers the eye lens and using it as a support membrane
on which to grow healthy cells taken from the iris. The iris cells are
capable of growing into different types of cells that perform different
functions. The lens tissue can be replaced with an artificial lens, as is
routinely done during cataract surgery. The newly created layer would then
be transplanted into the retina. Since only the patient's own tissues and
cells are used, this type of transplantation reduces the possibility that
the immune system will reject the implant.
The major challenge to this approach is getting the transplanted layer of
cells to look and act like the naturally occurring layer. The cells need to
be densely packed onto the membrane and perform the necessary feeding and
waste-removal functions. Bent and her team of engineers are devising ways to
modify how the iris cells cluster on the surface of the lens capsule tissue
using some of the same techniques used to make patterns on a computer chip.
They also are monitoring the biological function of the cells. At the same
time, the surgeons are developing and testing microsurgical techniques for
transplanting the newly developed materials into the eye.
"That's actually why it's such a fun project, because it's not just
academic," said Bent. "These problems have to be approached in both
directions -- the engineering and the medical side of it." Fishman says that
without the contributions of experts such as ophthalmology Professor Mark
Blumenkranz in retinal surgery, and others in fields such as physics,
chemistry and engineering, the work would not have been possible. "This is
the new generation of super highly collaborative scientists," he said.
'The Holy Grail of Prostheses'
For the remaining 20 percent of patients with age-related macular
degeneration, all the light-sensitive cells have died. In those cases, a
pinpoint-sized electronic device capable of receiving light and translating
it into nerve-stimulating signals would be implanted into the eye. Bent
called it "the holy grail of prostheses - it's coming up with something
electronic that could take the place of something that's naturally there but
is having problems because of disease."
Bent said the investigators are working toward the most "physiologically
correct" kind of prosthesis. They want to stimulate the nerve cells with
chemicals, in the same way that neurons work naturally. When hit with light,
the prosthesis would release a burst of neurotransmitter chemicals through a
system of tiny valves. Those chemicals would stimulate the neurons.
Retinal prosthetic devices developed by others over the last five years
stimulate the nerve cells with metal electrodes. But Fishman is concerned
about the long-term effects of constantly hitting cells with electrical
currents. The early retinal prostheses also have been relatively large and
positioned far away from the neurons, so the electricity affects everything
in the vicinity rather than focusing on the nerve cells. Fishman compares
the process to hitting the nerve cell over the head with a large electrode
hammer. "Maybe we can tickle the retina instead," he said.
To do this, the researchers will develop a chip from soft polymer material
that can conform to the curvature of the back of the eye. This material
would be better suited to the task than a traditional silicon chip.
Researchers are developing techniques for extending the nerve cell branches
so they can be close enough to the chip to be stimulated individually.
The initial work on the project was made possible by a grant from Bio-X and
continued through industry support from VISX Inc., a California-based
company that specializes in the design, manufacture and marketing of
proprietary laser vision correction technologies. The investigators have
applied for additional funding from the National Institutes of Health to
continue their work.
The new technologies being developed to solve vision problems may find
applications in other areas of medical research for conditions that affect
many more people. "We are developing tissue engineering ways to regenerate
nerve cells and to release drugs in very selective ways," Fishman said.
"This has tremendous implications for the field of drug delivery in the eye
and other parts of the body including the brain." He believes that
neurodegenerative diseases such as Parkinson's and Alzheimer's may benefit
from the technologies being developed.
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