challenging environments where chemical inertia is important compared to soft lenses. Soft lenses Soft lenses are more flexible than rigid lenses, and can be gently rolled or folded without damaging the lens. While rigid lenses require a period of adaptation before comfort is achieved, new soft lens wearers typically report lens awareness rather than pain or discomfort. Hydrogel lenses rely on their water content to transmit oxygen through the lens to the cornea. As a result, higher water content lenses allowed more oxygen to the cornea. In 1998, silicone hydrogel, or Si-hy lenses became available. These materials 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, oxygen permeability of silicone hydrogels is not tied to the lenses' water content. 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, thus less "wettable" – factors that can influence comfort of lens use. New manufacturing techniques and changes to multipurpose solutions have minimized these effects. Those new techniques are often broken down into 3 generations: 1st generation (plasma coating): A surface modification process called plasma coating alters the lens surface's hydrophobic nature; 2nd generation (wetting agents): Another technique incorporates internal rewetting agents to make the lens surface hydrophilic; 3rd generation (inherently wettable): A third process uses longer backbone polymer chains that results in less cross linking and increased wetting without surface alterations or additive agents. Current brands of soft lenses are either traditional hydrogel or silicone hydrogel. Because of drastic differences in oxygen permeability, replacement schedule, and other design characteristics, it is very important to follow the instructions of the eye care professional prescribing the lenses. Hybrid A small number of hybrid lenses exist. Typically these contact lenses consist of a rigid center and a soft "skirt". A similar technique is the "piggybacking" of a smaller, rigid lens on the surface of a larger, soft lens. These techniques are often chosen to give the vision correction benefits of a rigid lens and the comfort of a soft lens. Wear schedule A "daily wear" (DW) contact lens is designed to be worn for one day and removed