What is the difference between hydrogel and silicone hydrogel lenses?

Both are soft lenses, but the material chemistry differs.

• Hydrogel: water-based plastic. Soft and comfortable, but oxygen has to dissolve through the water to reach the cornea. Dk/t values typically 20–30.
• Silicone hydrogel: combines silicone (very high oxygen permeability) with water content. Dk/t values 80–160. The modern default since ~2005.

Dk/t under 24 leads to chronic low-grade hypoxia (oxygen starvation) of the cornea, which over years can promote neovascularisation. That's why silicone hydrogel has displaced traditional hydrogel for most healthy wearers.

Hydrogel still has a place:

• Very dry eyes — some hydrogels feel "wetter" because they don't carry the slight silicone surface that some people sense.
• Very brief, occasional wear — under 8 hours, oxygen isn't the limiting factor.

If your fitter offers both, prefer silicone hydrogel for full-day daily use.