Millions of people around the world are blind due to corneal
disease or damage. In hopes of making corneal transplants more widely available,
researchers have designed an artificial cornea made from a water-filled polymer
that closely resembles the eye’s natural cornea. Compared with existing
commercially available artificial corneas, the new implant could reduce the
likelihood of infection and other complications that arise from surgery.
Seeing clearly: This hydrogel-based artificial cornea developed by researchers at Stanford University contains microscopic pores that were patterned using photolithography. Once implanted in a patient, cells migrate through the pores and help integrate the artificial cornea with the surrounding tissue.
Approximately 40,000 patients undergo corneal transplant
surgery in the United States every year. The vast majority of these people receive
a replacement cornea from a human donor. Although the surgery has a high
success rate, the supply of donor tissue is limited, and wait lists can be
long. In the developing world, access to donor tissue is even more difficult.
And yet “most cases of corneal blindness are in developing countries,” says Tueng Shen, an expert
in cornea and refractive surgery at the University of Washington Medical Center,
in Seattle.
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To overcome this problem, researchers have been developing
artificial corneas using synthetic materials. The most successful of these to
date is the Dolhman-Doane keratoprosthesis, which received approval from the
U.S. Food and Drug Administration in 1992 and has been used in hundreds of
patients. It consists of a hard, clear plastic core surrounded by human donor
tissue to help attach the cornea to the eye.
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However, because the implant is prone to infection and other
complications, patients must take a lifelong course of antibiotics. As a
result, the artificial cornea is used only as a last resort in patients who
have repeatedly rejected natural donor tissue or who are otherwise not eligible
for such transplant surgery.
Instead of using hard plastic, Stanford University chemical
engineer Curtis Frank and former graduate student David Myung have created
an artificial cornea based on a soft hydrogel. The water-swollen gel is made of
a mesh of two polymer networks. The first network is made of polyethylene
glycol, the second of polyacrylic acid. “It’s like
filling up the holes in the sponge with a second material,” says Frank. “You
can’t separate one from the other. They become inextricably intertwined.”
The resulting clear material is
mechanically robust, despite being 80 percent water. The high water content, explains
Stanford ophthalmologist Christopher Ta, is critical
for allowing glucose and other nutrients to diffuse through the cornea and
encourage the growth of epithelial cells on the implant’s surface. “We
think this is important for minimizing risk of infection,” says Ta. “In the natural cornea, the epithelial layer is
very important for protection.”
For instance, one type of artificial cornea currently
marketed under the name AlphaCor is also based on a hydrogel. Yet the material
contains only half the amount of water as the Stanford implant. As a result, it
can’t support the growth of epithelial cells, which many researchers say could
explain AlphaCor’s high failure rate.
Because the Stanford hydrogel is
inert, cells don’t normally stick to it. So, with the help of Stanford
bioengineer Jennifer
Cochran, the researchers devised a way of tethering collagen to the
artificial cornea’s surface. The collagen, in turn, binds to the epithelial
cells. Cochran is working on incorporating growth factors and other components
of the cell’s natural environment into the material.
Using photolithography, Frank’s
team can also create patterns of microscopic pores around the edges of the
implant. That way, he says, when the cornea is implanted in the patient’s eye,
cells will migrate through the pores, anchor the cornea, and help integrate the
material with the native tissue. This will also reduce the number of sutures
required to keep the artificial cornea in place, says Frank.
Shen, who was not involved in the
Stanford effort, says that the development of new artificial corneas will be
important for solving a critical health problem. However, she wonders whether
the design of these new implants is well suited for use in the developing
world. For instance, hydrogel-based implants might require relatively
complicated surgery. “That could be difficult in terms of training
surgeons abroad,” Shen says. She is also
concerned about the potentially high cost of the materials, whether they can be
applied to large populations, and whether they will require a lot of follow-up
care.
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So far, the Stanford group has
shown that the diffusion of glucose across the material is equal to that of the
human cornea, and preliminary studies in rabbits show that implants can support
the growth of epithelial cells. The researchers say that studies in human patients
are still several years away.
