No, modern contact lenses are not made of glass. The contact lens is a medical device designed to be placed directly on the cornea, the clear, dome-shaped front surface of the eye. Today’s lenses are made from advanced, oxygen-permeable plastic polymers that are engineered to be soft and flexible. This design ensures maximum comfort and promotes eye health by allowing the cornea to breathe.
The Historical Basis for the Glass Question
The concept of altering the eye’s refractive power with a surface placed directly on it dates back to Leonardo da Vinci in 1508. In the late 1800s, the first actual lenses were created from glass. These early glass lenses were large, covering the entire eye and resting on the less sensitive sclera. However, they were extremely uncomfortable and could only be worn for short periods due to a lack of oxygen transmission. German glassblower F.A. Muller and ophthalmologist Adolf Fick developed these first glass-based designs.
A major shift occurred in the late 1930s with the introduction of polymethyl methacrylate (PMMA), a rigid, non-glass plastic. This material replaced glass, being lighter and more durable. PMMA lenses, which dominated the market through the mid-20th century, were hard, inflexible, and completely impermeable to oxygen. Because of their rigid nature, users often perceived them as “hard” or “glass-like,” which limited wearing time to only a few hours a day. The lack of oxygen permeability in both glass and PMMA materials led to complications like corneal edema, or swelling.
The Chemistry of Modern Soft Lenses
The revolution in contact lens materials began in the 1960s with the development of soft lenses made from hydrogels. These polymers, such as hydroxyethyl methacrylate (HEMA), absorb water, and their oxygen permeability is directly tied to this water content. Oxygen is transported through the lens dissolved in the water component of the material. A higher water content, ranging from about 38% to 75%, generally increases oxygen transmission in these traditional soft lenses.
The next significant advancement was the introduction of silicone hydrogels in the late 1990s, combining the comfort of hydrogels with the high oxygen permeability of silicone. In these advanced materials, silicone forms a network that allows oxygen to pass directly through the material itself, independent of the water content. Pure silicone is highly permeable to oxygen, giving silicone hydrogel lenses the ability to transmit up to five times more oxygen than older hydrogel lenses. This innovation solved the problem of balancing comfort (high water) with breathability (high oxygen), which had previously limited soft lens performance.
Material Differences and Eye Health
The choice of contact lens material directly impacts eye health. The cornea is avascular and receives most of its oxygen directly from the atmosphere; thus, a contact lens acts as a barrier. Oxygen transmissibility, known by the value Dk/t, measures how much oxygen can pass through a specific lens, considering both the material’s permeability (Dk) and the lens thickness (t).
For safe daily wear, a lens needs a minimum Dk/t value of around 24 to prevent corneal swelling. Traditional hydrogel lenses, which have Dk/t values up to about 40, meet this requirement. However, the oxygen requirement is much higher for extended or overnight wear because the eyelids are closed.
Silicone hydrogel lenses, with Dk/t values often exceeding 100, were developed to safely meet the increased oxygen demands of closed-eye conditions. Insufficient oxygen can lead to corneal hypoxia, causing complications like corneal edema or the abnormal growth of blood vessels into the clear cornea, known as neovascularization. The water content of the lens also affects comfort; high-water lenses feel comfortable initially but can draw moisture from the eye in low-humidity environments, leading to dryness.