Soft contact lenses are medical devices designed for vision correction, manufactured from highly flexible, water-retaining plastics known as polymers. These materials are engineered to be soft and conform precisely to the eye’s surface, providing comfort and maintaining the necessary optical shape for clear sight. The core function of these polymer formulations is to balance flexibility, comfort, and the movement of oxygen to the eye. The resulting lens structure is a water-loving network that allows gases and fluids to pass through, keeping the corneal tissue healthy during wear.
Hydrogels The Foundational Material
The first generation of soft contact lenses utilized a polymer based on hydroxyethyl methacrylate (HEMA), which forms a flexible, water-absorbing material called a hydrogel. These materials operate on the principle that water molecules within the lens matrix are the sole carriers for oxygen diffusion from the air to the cornea. Early hydrogel compositions had a water content ranging from approximately 38% up to 80%. This dependency on water introduced a significant limitation, particularly for extended or overnight wear, as lenses with high water content tended to dehydrate rapidly in dry environments. This dehydration caused discomfort and reduced oxygen transmission, meaning the oxygen levels provided were generally sufficient only for daily wear.
The Shift to Silicone Hydrogels
To overcome the oxygen limitations of traditional hydrogels, scientists introduced silicone into the polymer composition, leading to the development of silicone hydrogels. This new material blends the original water-loving hydrogel components with highly gas-permeable silicone compounds. The benefit of incorporating silicone is that it allows oxygen to pass directly through the solid material itself, rather than relying solely on water content. This shift in oxygen transport dramatically increased the amount of oxygen that could reach the cornea, measured as oxygen transmissibility (Dk/t). Modern silicone hydrogel lenses achieve significantly higher Dk/t values than their predecessors, making them the dominant material in the market. The high oxygen flow enabled by the silicone component allows for safer extended and continuous wear schedules.
How Material Properties Influence Comfort and Wear
The user experience of a soft contact lens is determined by the interplay of physical and chemical properties engineered into the material.
Oxygen Transmissibility (Dk/t)
Oxygen transmissibility (Dk/t) quantifies the amount of oxygen that can pass through a specific thickness of the lens. Since the cornea lacks blood vessels and must draw oxygen from the air, a Dk/t value of at least 25 Fatt units is required for daily wear to prevent corneal swelling.
Water Content
Water content, expressed as a percentage, influences comfort, though its role differs between material types. In silicone hydrogels, water content primarily affects material flexibility and fluid transport across the lens surface, rather than oxygen delivery. Lenses with very high water content can sometimes cause discomfort by drawing moisture from the eye in low-humidity environments.
Mechanical Modulus
The mechanical stiffness of the lens is measured by its modulus, which is the material’s resistance to deformation. A higher modulus makes the lens easier to handle, insert, and remove because it holds its shape better. However, a material that is too stiff can lead to irritation or poor fit by resisting conformity to the cornea and eyelid. Conversely, a lower modulus lens is softer and more comfortable but can be challenging for new wearers to manipulate.