What limits the Dk/t of previous hydrogel lenses prior to silicone hydrogel lenses?

The limitation of Dk/t in previous hydrogel lenses primarily relates to water content and center thickness. Dk, which represents the oxygen permeability of the lens material, and t, the thickness of the lens, together influence how much oxygen can reach the cornea. In hydrogel lenses, the oxygen permeability is largely affected by the water content. Higher water content typically increases oxygen transmission because water acts as a medium for oxygen diffusion. However, hydrogel materials also have physical characteristics that inherently limit the extent to which oxygen can pass through the lens, especially when considering lens thickness.

Thicker lenses will inherently reduce the Dk/t ratio because the pathway for oxygen to reach the cornea becomes longer, thus decreasing oxygen transmission, even if the water content is high. Therefore, both water content and center thickness work together to restrict the oxygen availability to the eye, impacting the overall performance of previous hydrogel lens designs compared to newer silicone hydrogel lenses, which have higher Dk values and allow better oxygen permeability at thinner profiles.