Water content (WC) is the percentage of the contact lens mass composed of water when fully hydrated, typically ranging from 30% to over 80% in soft lenses. Many wearers assume that a higher water percentage offers superior comfort and breathability. This perception is only partially correct and largely depends on the specific material used. The relationship between water content, oxygen delivery, and long-term comfort has been fundamentally changed by modern material science.
The Historical Link Between Water and Oxygen Permeability
The earliest soft contact lenses were made from conventional hydrogel polymers, where the lens material itself was not permeable to oxygen. For the cornea to receive the oxygen necessary for its health, it had to be delivered through the water component of the lens. This created a direct, positive correlation: the more water a hydrogel lens contained, the more oxygen could pass through it to the eye’s surface.
In these hydrogel lenses, a water content below 50% could restrict oxygen flow, potentially leading to corneal swelling and other health issues. Manufacturers were driven to create high-water content hydrogels, sometimes exceeding 70% water, to maximize oxygen transmission. This focus on water as the sole oxygen carrier established the popular belief that a high percentage of water was synonymous with a healthier, more breathable lens.
How Silicone Changed the Rulebook for Contact Lenses
The material science of contact lenses advanced significantly with the introduction of silicone hydrogel (SiHy) materials. The silicone component is highly permeable to oxygen, acting as a much more efficient conduit than water. This structural change fundamentally decoupled the lens’s oxygen permeability (Dk/t) from its water content.
A modern SiHy lens with a relatively low water content, sometimes as low as 33%, can deliver significantly more oxygen to the cornea than a traditional hydrogel lens with 70% water content. For example, the maximum oxygen transmissibility of a conventional hydrogel lens is approximately 40 Dk/t, while current SiHy lenses can achieve values well over 100 Dk/t. By relying on silicone for breathability, lens designers are free to optimize the water content for other factors, such as stability and handling.
Dehydration and Surface Issues Associated with Higher Water Content
A trade-off of high water content is the risk of excessive dehydration, which severely compromises comfort. High-water lenses act like a sponge; to maintain their water percentage, they actively draw moisture from the tear film covering the eye’s surface. This water loss accelerates when the wearer is in a dry, air-conditioned, or low-humidity environment.
As the lens loses water, it can tighten on the eye, reducing movement and leading to mid-to-late-day dryness, irritation, and foreign body sensation. Lenses with a high water content and an ionic surface charge are more prone to accumulating protein and lipid deposits from the tear film. These deposits decrease lens clarity and surface wettability, contributing further to discomfort and potentially creating an inflammatory response.
Factors Determining the Right Lens Material for the Wearer
There is no universal “best” water content, as the ideal lens material balances multiple properties tailored to the individual. Suitability depends on factors like the wearer’s tear film quality, occupational environment, and the total hours they wear the lenses each day. For instance, a person with a naturally drier eye or one who works in a climate-controlled office may be more comfortable with a lower water content SiHy lens that resists desiccation.
The final choice balances the lens’s oxygen transmissibility (Dk/t), its water content, and its modulus, which measures the material’s stiffness or flexibility. A lower modulus provides a softer feel, but a higher modulus can make the lens easier to handle during insertion and removal. Finding the right lens requires a professional fitting and a careful assessment of the wearer’s specific ocular needs and daily habits.