For a silicone hydrogel soft contact lens, Dk does what as water content increases?

• A. Increases
• B. Decreases
• C. Remains constant
• D. Increases then decreases

Answer: (B)

Dk, the oxygen permeability of a contact lens, in silicone hydrogel materials is dominated by the silicone component that forms the continuous network through which oxygen travels. When water content increases, the fraction of water-filled regions within the lens rises while the relative fraction of silicone decreases. Since water provides much less efficient pathways for oxygen diffusion than silicone, the overall permeability drops. So, as water content goes up, Dk decreases. This is the trade-off in silicone hydrogels: higher water content can improve comfort and wettability, but it reduces oxygen permeability because the silicone proportion is reduced.