Soft contact lenses are flexible, water-containing discs worn directly on the cornea to correct vision. Unlike hard lenses, their soft and pliable nature allows them to conform to the eye’s surface, providing a high level of comfort. The development of these lenses has involved a continuous search for materials that balance oxygen flow, moisture retention, and durability. Understanding their composition reveals how manufacturers have worked to improve both the health and comfort of contact lens wear.
The Original Soft Lens Material: Hydrogels
The first widely adopted soft contact lenses were made from hydrogels, introduced in the early 1970s. The primary component was a polymer called poly(hydroxyethyl methacrylate), or pHEMA, a soft plastic that becomes flexible when hydrated. Hydrogel materials are defined by their ability to absorb and hold water within their polymer structure.
Since the pHEMA polymer has very low oxygen permeability, water content became the sole mechanism for transporting oxygen to the cornea. Oxygen dissolves into the water held by the lens and then diffuses through this water-rich phase to reach the eye. Consequently, manufacturers designed hydrogels with higher water content to increase oxygen passage.
While higher water content improved oxygen delivery, it introduced a trade-off. Lenses with more water were generally more fragile and dehydrated rapidly during long periods of wear or in dry environments. As water evaporated, the lens material would tighten, potentially reducing comfort and oxygen flow. This limitation meant hydrogel lenses were less suitable for extended or overnight wear, restricting oxygen levels below what the cornea requires for health.
The Modern Standard: Silicone Hydrogels
The limitations of hydrogels led to a major material advancement: the development of silicone hydrogels (SiHy), which are now considered the modern standard. These lenses incorporate silicone, a highly oxygen-permeable material, directly into the polymer structure. The introduction of silicone fundamentally changed the way oxygen moves through the lens, making it largely independent of the water content.
Silicone polymers create microscopic channels that allow oxygen to pass through the lens material itself, rather than relying solely on the water component. This allows up to five times more oxygen to reach the eye compared to conventional hydrogels. The high oxygen permeability of SiHy lenses significantly reduces the risk of hypoxia-related issues, such as corneal swelling and redness.
Modern silicone hydrogels are a composite material, designed to balance the benefits of both components. The silicone ensures high oxygen flow for corneal health, especially during extended wear, while the hydrogel component, often supplemented with wetting agents, maintains comfort and surface wettability. While early SiHy lenses were sometimes stiffer due to the silicone, newer generations have successfully lowered the stiffness while retaining high oxygen permeability, providing a material that is both healthier for the eye and comfortable for the wearer.
Critical Performance Factors: Water Content and Oxygen Flow
Lens performance is determined by two measurable properties: water content and oxygen flow. Water content is expressed as the percentage of water by mass in the fully hydrated lens. This factor primarily influences the lens’s initial softness, flexibility, comfort upon insertion, and its tendency to dry out.
The flow of oxygen is measured by oxygen transmissibility, or Dk/t, where ‘Dk’ is the oxygen permeability of the material and ‘t’ is the lens thickness. Since the cornea is avascular (lacking blood vessels), it relies on oxygen diffusing directly from the air through the tear film and the contact lens. A higher Dk/t value indicates the lens allows more oxygen to reach the eye, which is required for maintaining corneal health.
To prevent corneal swelling and hypoxic changes during open-eye wear, a lens needs a Dk/t value of at least 24. Lenses approved for extended or overnight wear require a much higher transmissibility, often cited around 87 or more. For traditional hydrogels, Dk/t is directly tied to water percentage. However, for silicone hydrogels, the Dk value comes primarily from the silicone, allowing for high oxygen flow even with moderate water content. Manufacturers must precisely engineer the material composition to achieve the specific balance of moisture for comfort and high Dk/t for eye health.