Soft Contact Lenses
While rigid lenses have been around for about 120 years, soft lenses are a much more recent development. The principal breakthrough in soft lenses made by Otto Wichterle led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the approval of the “Soflens” daily material (polymacon) by the United States FDA in 1971. Soft lenses are immediately comfortable, while rigid lenses require a period of adaptation before full comfort is achieved. The biggest improvements to soft lens polymers have been increasing oxygen permeability, lens wetability, and overall comfort.
In 1998, silicone hydrogels became available. Silicone hydrogels have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. Because silicone allows more oxygen permeability than water, the oxygen permeability of silicone hydrogels is not tied to the water content of the lens. Lenses have now been developed with so much oxygen permeability that they are approved for overnight wear (extended wear). Lenses approved for daily wear are also available in silicone hydrogel materials.
Disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic and less “wettable.” These factors can influence the comfort of the lens. New manufacturing techniques and changes to multipurpose solutions have minimized these effects. A surface modification processes called plasma coating alters the hydrophobic nature of the lens surface. Another technique incorporates internal rewetting agents to make the lens surface hydrophilic. A third process uses longer backbone polymer chains that results in less cross linking and increased wetting without surface alterations or additive agents.