Contact lenses are medical devices whose materials determine their comfort, visual performance, and safety. This is especially true for rigid lenses, which are manufactured from specific polymers to maintain their precise shape and provide clear vision. Understanding their composition reveals a history of chemical innovation focused on improving eye health and optical quality.
Clarifying the Term Hard Contacts
The term “hard contacts” generally refers to rigid lenses, encompassing both older and newer types. Historically, this described the first contact lenses made from non-porous plastic. The modern equivalent is the rigid gas permeable (RGP) lens, which is chemically distinct and significantly healthier for the eye. Although the outdated name is often used interchangeably, the materials are fundamentally different.
The Original Material: PMMA
The first successful rigid contact lens material was Poly(methyl methacrylate), commonly known as PMMA or acrylic glass. This polymer was first used in the 1940s due to its excellent optical clarity, light weight, and durability. PMMA is a transparent thermoplastic that provided a strong, stable material for vision correction.
The main limitation of PMMA is that it is virtually impermeable to oxygen. The cornea needs a constant supply of oxygen to remain healthy. Because the PMMA lens was a barrier, the lens had to be made small enough to float on the tear layer, allowing oxygenated tears to exchange underneath with every blink. The lack of oxygen transmission meant PMMA lenses carried a risk of corneal swelling and other health issues, prompting the search for alternative materials.
Composition of Modern Rigid Gas Permeable Lenses
Modern rigid gas permeable (RGP) lenses are complex polymers engineered to allow oxygen to pass directly through the lens material. This gas-permeability is achieved by incorporating specific chemical elements into the polymer backbone. The two primary classes of materials used today are Silicone Acrylates and Fluoro-silicone Acrylates.
First-generation RGP materials were Silicone Acrylates, which introduced siloxane monomers to the acrylic base. The silicone component significantly increases oxygen permeability because oxygen molecules diffuse easily through the silicon-oxygen bonds. This addition was a substantial improvement over PMMA, but higher silicone content could negatively affect the lens’s surface properties, leading to poor wetting.
To further enhance performance, manufacturers developed Fluoro-silicone Acrylates, which incorporate fluorine atoms into the material. The addition of fluorine increases oxygen permeability and improves the surface wetting characteristics of the lens. The ability of a lens to transmit oxygen is quantified by its Dk value; the combination of silicone and fluorine allows modern RGP lenses to achieve very high Dk values, ensuring the cornea receives sufficient oxygen.
Why RGP Materials are Selected
The specific composition of RGP lenses is chosen to maximize patient health and visual clarity. The rigidity ensures the lens maintains its precise curvature, creating a smooth optical surface. This stable shape is particularly effective for correcting astigmatism and other corneal irregularities, often providing sharper vision than possible with softer lens types.
The incorporation of silicone and fluorine provides superior oxygen flow to the cornea, lowering the risk of oxygen deprivation complications. Because RGP materials do not contain water, unlike soft lenses, they are naturally resistant to the buildup of protein and lipid deposits. This deposit resistance makes RGP lenses easier to keep clean and contributes to their exceptional durability, allowing a single pair to last a year or more with proper care.