Why lenses get coated
Scleral lenses are made of rigid gas permeable plastics. Tears don’t always spread evenly over those surfaces, especially in people with dry eye or oily-gland problems along the lid margins. When the tear film over the lens breaks up, vision can turn hazy and the lens can feel dry or gritty. Deposits from the tears can also build up on the surface. Surface treatments try to make the lens more “wettable,” so tears spread and stay put.
Two terms come up most:
- Plasma treatment. A surface process applied to gas permeable lenses by the lab. In a lab study using a powerful microscope, plasma-treated lenses had smoother surfaces than untreated lenses, both new and after three months of wear. But smoother surfaces weren’t linked to how comfortable people found the lenses.[3]
- Hydra-PEG. A coating based on polyethylene glycol (PEG), a water-attracting polymer, applied to the lens surface. A 2023 review describes it as improving wettability and lubrication and reducing protein and lipid deposits.[6]
What the studies show
The best evidence for scleral lenses is one small, well-designed study:
- Dry eye, scleral lens wearers. Twenty-one people with dry eye who were uncomfortable in their scleral lenses wore an untreated lens and a Hydra-PEG-treated lens of the same design for 30 days each, in random order, with neither they nor the examiners knowing which was which. With the treated lens, comfort, dry eye symptoms, staining of the eye’s surface, the frequency of foggy vision, and comfortable wearing time all improved significantly.[1]
Other evidence is weaker or less directly relevant:
- New rigid lens wearers. In 119 people trying small corneal gas permeable lenses for the first time, a Hydra-PEG plasma coating didn’t improve comfort or redness over the first 20 minutes.[2] That’s a different lens type and a very short test, so it says little about long-term scleral lens wear.
- PROSE case reports. In one of two case reports, a PEG-based coating on a PROSE device more than doubled how long the tear film over the lens stayed stable, from about 4.6 to 11.5 seconds. The authors suggest this kind of measurement could help decide when a device needs recoating or replacing.[4]
Where coatings fall short
Coatings address the front surface of the lens. They don’t fix debris building up in the fluid behind the lens, a poor fit, or untreated dry eye. In a study of 48 habitual scleral lens wearers, lens material, coatings, diameter, and solutions together explained only part of who had midday fogging, and no single factor made a significant difference by itself. People with fogging had dry eye symptom scores similar to severe dry eye.[5]
Coatings can also wear off over time, which is why recoating and replacement come up.[4] If you notice your lens beading water, looking hazy, or feeling dry sooner in the day than it used to, tell your fitter. For more on fogging and haze, see vision still not perfect with sclerals?, and for your routine, cleaning and disinfecting.
Common questions
How do I know if my lens is coated?
Ask your fitter or check your lens order. Coatings aren't visible to the eye. If your lens is coated, ask which cleaners and solutions are compatible with it, and whether anything in your routine could wear it off sooner.
Can a coating be added to the lenses I already have?
Sometimes lenses are sent back to be recoated or reconditioned, but whether that's possible depends on the lens and the lab. Ask your fitter.
My lens fogs up by midday. Will a coating fix it?
It might help if the problem is film on the front of the lens, especially with dry eye. If the fog is debris building up in the fluid behind the lens, a coating won't address that. Your fitter can help work out which it is.