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.
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.
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.
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.”