From Plexiglas to silicone hydrogels—the materials that let you see through to the other side
As a kid who could barely stand eye drops, I had to struggle to muster strength to insert my first contact lens. I was about 13 years old, needed correction only for my left eye, and had to have the lens in place for a couple of hours before an after-school check with my optometrist. It took 30 minutes of staring myself down in the junior high school bathroom mirror to land that first lens.
More than two decades later, inserting my lenses—now needed for both eyes—happens with barely a second thought. I have also hardly given a thought to exactly what it is that I put on my eyes every day. Like most people, I suspect, I tend to think of my lenses as a rather innocuous personal care product. In fact, they’re considered a medical device and are regulated by the U.S. Food & Drug Administration Center for Devices & Radiological Health, along with breast implants, artificial knees, and drug-eluting coronary stents.
The first contact lenses were blown of glass in the 1880s and rested on the white of the eye rather than covering just the cornea; they could be worn for a few hours at most. Plastic lenses were introduced in the 1930s. The first lenses with mass appeal were smaller lenses that covered just the cornea and were made from poly(methyl methacrylate) (PMMA), also known as Plexiglas. Popular through the 1960s, PMMA contacts were notorious for popping out at inconvenient moments. Softer, more comfortable lenses made from poly(hydroxyethyl methacrylate) (poly-HEMA) were introduced in the 1970s.
A key thing to know about eye anatomy is that the cornea does not have blood vessels. Consequently, corneal cells get nutrients from tear fluid and from gelatinous material called the aqueous humor, which is located on the inside of the eye. Corneal cells get their oxygen directly from the air. A big drawback for PMMA and poly-HEMA lenses was that neither is permeable to oxygen. This became a particular problem for the softer poly-HEMA lenses because wearers found them more comfortable than rigid PMMA lenses and people therefore wore them longer. In extreme cases, oxygen deprivation to the cornea can lead to growth of blood vessels into the cornea, threatening eyesight.
From the 1970s to the 1990s, much of contact lens research focused on improving oxygen permeability of lens materials. This effort first led to rigid gas-permeable lenses that incorporated a small amount of silicone for flexibility and oxygen permeability, but many wearers found it hard to adjust to the new lenses. Having started with soft lenses in junior high, I tried rigid gas permeables in high school but went running back to soft lenses within a few weeks.
IN THE 1990s, manufacturers turned to the soft hydrogel materials, silicone hydrogels in particular, that are used in the lenses most common today. Of approximately 30 million contact lens wearers in the U.S., around 98% wear soft lenses, says James Gardner, vice president for marketing at contact lens manufacturer CooperVision.
Hydrogels are networks of water-insoluble polymers that can simultaneously hold large amounts of water. They provide significant advantages over previous lenses in oxygen permeability and comfort—their high water content means that they’ll stay moist and nonirritating even during extended periods of wear.
A major component in some modern lenses is phosphorylcholine, a hydrophilic material that mimics part of the cell membrane. It is commonly used in medical devices to improve biocompatibility, reduce protein adsorption, and reduce inflammatory response. In contact lenses phosphorylcholine also helps to maintain hydration while preventing deposition of lipids and proteins on any dry spots that do develop, Gardner says.
As an alternative to phosphorylcholine, lenses can be engineered to contain longer siloxane chains, which hydrogen bond to water and eliminate the need for added wetting agents. In saline the lenses contain 48% water by weight and are extremely soft, contributing further to comfort.
Common varieties of contact lenses can be replaced daily, weekly, or monthly. The materials used in these lenses are fundamentally the same, Gardner says. The differences lie in the manufacturing. Lenses to be replaced daily are made on very high volume lines set up to maximize production efficiency, and such lenses are typically offered in fewer power increments and perhaps with only one base curve—the radius of the sphere described by the lenses. Lenses replaced less frequently have more options to achieve better sight correction and fit.
At lens manufacturer CIBA Vision, R&D efforts are directed toward incorporating lubricants that elute over time, to maintain that “fresh lens feeling,” says Lynn Winterton, global head of research. For example, a daily disposable lens approved by FDA earlier this year has a lubricant that is forced out with each blink of the eye.
Winterton also says that the company is working on ways to improve the safety of lenses, primarily by looking for materials that don’t create a good breeding ground for bacteria, fungi, or other microorganisms that can cause eye infections. Most of the patent literature involves impregnating lenses with silver, he says, although some people are looking at incorporating active pharmaceutical agents. He notes that companies have to juggle maintaining transparency, avoiding an inflammatory response, and preventing the evolution of drug-resistant microbes. That’s a lot to consider for a product most of us insert in just a blink of an eye.