Nearly every soft lens sold today is silicone hydrogel. The material is not a specification-sheet detail — it determines how much oxygen reaches your cornea, how much water the lens loses during the day, and how many hours you can comfortably wear it.
The three families
1. Hydrogel
HEMA-based polymers saturated with water, between 38% and 70% water content. Oxygen reaches the cornea dissolved in that water.
Comfortable on insertion and usually the cheapest. The limitation is structural: water is the only oxygen pathway, so more oxygen requires more water, and more water means a lens that dehydrates faster against dry air. See Dailies AquaComfort Plus for a good example of the category.
2. Silicone hydrogel
Adds silicone to the matrix. Silicone carries oxygen directly through the material, independent of water.
This inverts the old logic: these lenses hold less water (24–48%) and deliver far more oxygen. A conventional hydrogel runs a Dk/t around 20–30; a silicone hydrogel reaches 100–175.
The difference shows up in the last hours of the day, not the first. See the full comparison.
3. Rigid gas permeable materials
Fluorosilicone acrylates, containing no water at all. Oxygen passes through the material, and each blink pumps fresh tears beneath the lens.
They give the sharpest optics available and last years rather than weeks, in exchange for one to two weeks of adaptation. See gas permeable lenses and, for irregular corneas, sclerals.
Dk and Dk/t are not the same thing
The most common confusion when comparing specifications.
- Dk measures the permeability of the raw material
- Dk/t measures how much oxygen crosses that lens, at that thickness
The second is the one that reaches your cornea. An excellent material in a thick lens can transmit less than a mediocre one in a thin lens. And since thickness varies with power, a −6.00 lens transmits less than the same model at −1.00.
A manufacturer publishing only Dk is publishing the flattering number. See the brand comparison.
What else is in the lens
The surface chemistry is half the product:
- Wetting agents — PVP, PVA or similar, either released through the day or built into the matrix
- Surface treatments — pure silicone is hydrophobic, so without treatment the lens would repel your own tears
- UV filters in many designs — though these cover the cornea only, not the lids or the white of the eye, so they do not replace sunglasses
- Handling tint — the faint blue that exists solely so you can find the lens in its solution
What to do with this
Honestly, not much directly: you do not choose the material, the fitting does. It follows from your corneal shape and tear film.
Where it helps is in two conversations:
- If your eyes burn by late afternoon, mention material. Moving from hydrogel to silicone hydrogel resolves a large share of what gets attributed to dry eye.
- If you are offered a cheaper equivalent, ask about material and Dk/t. Equivalents usually match on power, not on oxygen transmission — and base curve geometry differs between brands too, which is why substitution is not permitted under the FTC rule.
Sources
- FDA classification of contact lens materials (groups I–V)
- Literature on oxygen transmissibility (Dk/t) and corneal health
General information. Reviewed by Dr. Teresita Bollar, OD.
