Creating a Silicone Hydrogel Contact
BY LYNN C. WINTERTON, PHD
Since the earliest days of soft contact lenses,
materials scientists have sought a lens that would resemble wearing no vision correction at all. I believe an ideal lens could be worn safely and comfortably round-the-clock for at least a month at a time, with none of the daily hassles of handling, application, removal, cleaning and storage.
Designing a continuous wear lens presented daunting challenges, and it has taken the industry nearly 30 years to achieve this goal this time.
First and foremost, a continuous wear lens must transmit oxygen to the cornea in quantity and effect so that any overnight corneal swelling is similar to that during no lens wear in a closed-eye condition. This assures that hypoxia cannot occur. Hypoxia can cause epithelial changes, which include reduced resistance to bacterial infection. Harvitt and Bonanno have postulated that the no hypoxia threshold requires a contact lens oxygen transmission of 125
Dk/t or greater. Additional research by La Hood et al (1989) and Papas (1998) supports this value, with further studies by Mueller et al concluding that "the minimum
Dk/t to prevent corneal swelling beyond that which occurs with no lens on the eye appears to be greater than 110, but less than 175." This is four to six times greater than the most widely used extended wear hydrogels available today.
Polymer chemists have long understood that introducing silicone into a lens matrix greatly increases the material's oxygen transmissibility. But silicone adds undesirable material properties that must be overcome. This is the prevailing reason why the first generation of overnight lenses that incorporated silicone failed in the early 1970s. Overcoming the downsides of silicone incorporation was a huge obstacle.
Lens Design Issues
Other lens design challenges are to make a lens that provides:
- Mechanical properties that make it center on the cornea
- Easy handling
- Resistance to lens adherance
- Sufficient movement to promote flushing of debris that might accumulate under a lens
Every lens must remain stable for its intended life expectancy, especially while in contact with ocular tissue. It must maintain its refractive properties, structural balance and surface character as well as its desired mechanical properties. Finally the lens must be biocompatible,
wettable, deposit resistant and permeable to oxygen and metabolic gases.
Designing Night & Day
CIBA Vision's continuous wear development strategy focused on blending silicone, fluorine and water to form the body of the lens, then creating a surface layer to mask the undesirable traits of silicone polymers.
We determined that a low water content in the lens matrix helped maximize oxygen flow and comfort while minimizing lens dehydration during use. Higher water contents actually diminish oxygen transmission because silicone transmits six times more oxygen than water. Also, as water content increases, lens dehydration also increases. Our goal was to keep the water content as low as possible while assuring flexibility and biocompatibility.
Our resultant material, lotrafilcon A, is a two-phase material that combines a flourosiloxane phase for high oxygen transport and a hydrogel phase for movement and comfort. Its
Dk/t value is 175, which exceeds the 125 value required to eliminate hypoxia during overnight wear even in thicker, higher-power lens parameters. The high oxygen transmission eliminates metabolic changes caused by oxygen starvation that may produce a myopic shift over time. The lens material exhibits little dehydration in use, which helps promote all-day comfort and a stable refractive power. While the lower water content makes the lens somewhat stiffer, any initial lens awareness usually dissipates as the lens shapes itself to the cornea.
The fluorine in Night & Day creates an oxygen reservoir in the lens because it absorbs and binds oxygen gas. Materials containing fluorine also tend to resist deposits.
Silicone Hydrogel Lens Surface
After CIBA Vision researchers found the right blend of silicone, fluorine and water, the next challenge was to invent a surface treatment to mask the undesirable properties of silicone and assure the lens' ability to withstand 30 days of continuous wear in the ocular environment. We knew the surface needed to be highly oxygen permeable, smooth and flexible for refractive correction and comfort. Further, it must resist protein and lipid build-up and be wettable and compatible with ocular tissue. Finally, the surface must remain permanently bonded to the body of the lens and remain intact throughout its intended life of 30 days and nights.
CIBA Vision created an ultra-thin surface layer rather than modifying the surface molecules of the lens body. We believe this maximizes surface performance and that this surface layer approach is a key factor in the successful performance of the lens. The surface layer satisfies all of the requirements of continuous wear it is uniform, flexible, permeable, wettable and resists deposits. The surface layer is ten thousand times thinner than the tear film. It is permanently bound to the lens body and cannot degrade during use.
Focus Night & Day represents six years of materials research by an international team of
researchers. The Australian Contact Lens Research Unit, which has been a pioneer in increasing the safety of overnight wear, contributed important input.
Dr. Winterton is a Research Fellow at CIBA Vision. He has published and lectured extensively on the material properties of contact lenses.
Contact Lens Spectrum, Issue: April 2003