5.3 Lens Materials, Hyper-Dk Polymers & Surface Coatings
Key Takeaways
- Oxygen permeability (Dk) is a material property, while transmissibility (Dk/t) also depends on lens thickness; a scleral fluid reservoir adds another diffusion resistance.
- Open-eye daily-wear and closed-eye extended-wear oxygen criteria are different, so the Harvitt-Bonanno 125 value must not be applied as a universal scleral-lens target.
- For scleral lenses, use a high-Dk material and minimize lens and reservoir thickness consistent with optics, stability, and corneal clearance.
- Fluorosilicone acrylate chemistry balances oxygen, deposits, wettability, rigidity, and durability; high Dk alone does not determine clinical success.
- Hydra-PEG can improve wettability and deposit resistance, but it can wear or be damaged and does not guarantee elimination of front-surface or reservoir fogging.
5.3 Lens Materials, Oxygen Transmission & Surface Coatings
A specialty lens succeeds only when its material, thickness, geometry, surface, and care system work together. Memorizing the highest Dk brand is not enough: a thick high-Dk lens over a deep fluid reservoir may deliver less oxygen than expected, and a highly permeable surface that does not wet can still produce poor vision and comfort.
Dk and Dk/t
Oxygen permeability, Dk, is an intrinsic material property. D represents diffusion through the polymer and k represents oxygen solubility. Oxygen transmissibility, Dk/t, divides permeability by the local lens thickness t. The same material therefore has lower transmissibility in a thicker high-plus, prism-ballasted, or flexure-resistant zone.
Historical Holden-Mertz and Harvitt-Bonanno criteria addressed different physiological endpoints and wearing conditions. Values near 24 or 35 Fatt units are commonly discussed for open-eye daily wear, while values near 87 or 125 relate to closed-eye extended wear. Do not use the 125 closed-eye value as a universal required combined Dk/t for a daily-wear scleral system.
Add the Fluid Reservoir as a Second Resistance
A scleral system includes the lens and the post-lens tear reservoir. A simplified resistance model is:
1 divided by total Dk/t = lens thickness divided by lens Dk + reservoir thickness divided by tear-fluid Dk
The units must be handled consistently. The model teaches three practical levers:
- select a high-Dk material;
- avoid unnecessary lens thickness while retaining adequate rigidity and optics;
- avoid unnecessary post-lens reservoir depth while maintaining complete corneal and limbal clearance.
Published modeling has recommended high-Dk material, central lens thickness near or below about 250 microns when feasible, and a reservoir near or below about 200 microns to reduce hypoxic swelling. These are modeling-based design goals, not universal pass/fail cutoffs. Actual corneal response, diagnosis, endothelial reserve, wear time, lens design, and measurement method matter.
For example, a 300-micron lens with Dk 30 over a 250-micron reservoir has much greater total oxygen resistance than the same geometry in a Dk 150-plus material. The high-Dk design improves oxygen delivery, but its calculated combined transmissibility does not magically reach the closed-eye 125 criterion. The correct conclusion is comparative: higher Dk and thinner barriers reduce risk.
Material Families
PMMA has essentially no oxygen permeability and is historical. Silicone acrylates introduced silicone for oxygen transport but often had wetting and deposit problems. Fluorosilicone acrylates combine silicone-containing and fluorinated monomers to improve the overall balance of permeability, deposit behavior, wettability, and dimensional stability. Fluorination does not simply “lower surface energy and therefore improve wetting”; surface wetting depends on the complete polymer chemistry and its surface treatment.
When comparing materials, record the measurement method because ISO and Fatt Dk values are not always interchangeable. Also consider modulus, specific gravity, refractive index, ultraviolet absorber, machining characteristics, flexure, and approved wearing indication.
Surface Wetting
Wettability is commonly described by a contact angle: a lower angle generally indicates that liquid spreads more readily. Measurements depend on technique, conditioning, and hydration, so brand values should not be compared without knowing the method.
Oxygen-plasma treatment cleans organic residue and introduces polar surface groups. It can improve initial wetting, but handling, deposits, cleaners, and time can reduce the effect.
Tangible Hydra-PEG is a hydrophilic polymer coating applied to compatible rigid and hybrid lens materials. It can reduce contact angle, improve lubricity, and resist deposits for many wearers. It is not indestructible or permanent. Manufacturer guidance warns that incompatible abrasive, alcohol-based, enzymatic, or extra-strength cleaners can damage the coating. Compatible products currently include Tangible Clean, Unique pH, Boston Simplus, and specified Clear Care systems; current manufacturer instructions control because compatibility lists can change.
A coating may help front-surface wetting but does not by itself eliminate midday reservoir fogging, which can involve inflammation, tear exchange, landing-zone alignment, filling solution, or cellular debris. Distinguish a dry front surface that changes with a blink from particulate behind the lens that remains after blinking.
Care and Modification
Follow the material and coating instructions. A compatible solution may be rubbed when its labeling permits; the problem is incompatible chemistry or abrasive action, not all digital rubbing. Do not polish or modify a coated lens unless the laboratory confirms that the coating will be removed and reapplied. Never expose lenses to tap water.
If a coated lens begins to wet poorly:
- review every product and handling step;
- inspect for scratches, deposits, chips, and coating damage;
- distinguish front-surface dewetting from post-lens fogging;
- clean only with a compatible labeled system;
- consult the laboratory about recoating or replacement rather than promising that a conditioner can rebuild a destroyed coating.
Clinical Selection Checklist
| Question | Why it matters |
|---|---|
| What is the material Dk and measurement convention? | Supports valid comparison |
| What is the lens thickness at the relevant zone? | Determines Dk/t and flexure resistance |
| How deep is the settled reservoir? | Adds oxygen resistance |
| Is the cornea or graft physiologically vulnerable? | Changes monitoring and wearing plan |
| Does the surface wet and resist deposits? | Affects optics, comfort, and adherence |
| Is there a coating? | Controls compatible care and modification |
Exam Traps
- Dk is not Dk/t.
- A high-Dk label does not cancel excessive thickness or clearance.
- Closed-eye extended-wear criteria are not universal daily scleral targets.
- Hydra-PEG may improve a surface but cannot guarantee zero fogging or deposits.
- Manufacturer care instructions outrank a memorized generic cleaner list.
Why is a high-Dk material generally preferred for a scleral lens with a substantial center thickness and fluid reservoir?
Which care action is incompatible with a Hydra-PEG-coated lens?
Which statement about fluorosilicone acrylate materials is most accurate?