The contact lens world is changing fast! According to the experts, in the next few years silicone-hydrogel lenses will dominate the industry. The differences between rigid and soft lenses is blurring as advances in polymer chemistry bring these materials closer together.

Why silicone-hydrogels?

Silicone-hydrogels were developed to solve the basic problems with both soft and rigid contact lenses. Standard hydrogel soft lenses cannot be made with sufficient oxygen permeability for long-time extended wear and standard first generation fluoro-silicone acrylate rigid lenses cannot be made as comfortable as hydrogel soft lenses. Silicone-hydrogel polymers provide the answers to both problems.

Oxygen transport through standard hydrogel lenses depends upon the water content and thickness of a lens. Even 70% water lenses that are 0.04mm thick will only transmit a fraction of the oxygen required by the cornea. Silicone-hydrogel polymers provide an additional mechanism for oxygen transport through the polymer matrix itself.

Combining the best features of soft and rigid lenses

Standard rigid oxygen permeable lenses depend upon surface wettability for comfort. Methacrylic acid (MAA) added to first generation fluoro-silicone acrylate formulations as a wetting agent can produce low wetting angles, however comfort does not approach that of a hydrogel lens. This is because the surface remains rigid while ionized acid groups attract water to the surface. In a hydrogel lens, water is actually absorbed into the polymer creating a comfortable soft and supple surface. This surface is created in rigid silicone-hydrogel lenses - lenses that approach the comfort of hydrogel lenses because the surface is the same.

Silicone-hydrogel polymers combine the best properties of hydrogel and rigid oxygen permeable materials. The idea of polymerizing mixtures of hydrophilic monomers with silicone monomers is difficult to accomplish in the chemical laboratory. It is like mixing oil and water. Mixtures of the most common monomers, HEMA and TRIS, almost always produce opaque polymers. By using other monomers however, regions of compatibility can be found. Researchers have been investigating these regions of compatibility for over twenty years. Recently, these advances have finally reached the contact lens marketplace.

Soft silicone-hydrogels

In soft silicone-hydrogels the monomer ratios are adjusted to produce water contents in the 20% to 50% range. These lenses are very similar to standard hydrogel lenses, with the added advantage of increased oxygen permeability. The oxygen permeability is controlled principally by the amount of silicone monomer in the formulation. In efforts to achieve the highest oxygen permeability possible, the silicone content may be so high that the lens surface may not wet well and may attract deposits. Such lenses may be surface treated to restore optimum surface properties. Currently all silicone-hydrogel lenses are molded, however, custom lathe cut soft and rigid silicone-hydrogel lenses will soon become available.

Rigid silicone-hydrogels

By varying monomer ratios, silicone-hydrogel polymers can be made with a range of water contents from 0 to 60%. At low water contents, the polymers are rigid. There are two types of rigid silicone-hydrogel materials. In one type, the water content and expansion is constrained by decreasing the ratio of hydrophilic monomers and increasing crosslink density. Thus the polymer cannot absorb water into the interior of the matrix, but the surface can hydrate like a hydrogel lens. In the second type, a reactive hydrolyzable monomer is included in the formulation. When the lens is placed in water, the reactive monomer hydrolyzes producing a hydrophilic surface like a hydrogel lens. The reaction cannot proceed into the interior since expansion is constrained by crosslinking. This type has the novel property of regenerating the hydrophilic surface if the lens is damaged or the lens is repolished. There are also hybrid materials that contain both hydrophilic and reactive monomers.