Editing Contact lens (section)

====Soft lenses====
{{see also|List of soft contact lens materials}}
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.<ref>FDA Premarket Notification for [http://acuvue-now.com/latest-news/news-flash/121-green-light-for-1-day-acuvue-trueye.html "new silicone hydrogel lens for daily wear"] {{webarchive|url=https://web.archive.org/web/20081003075751/http://acuvue-now.com/latest-news/news-flash/121-green-light-for-1-day-acuvue-trueye.html |date=3 October 2008 }} 'July 2008.</ref>

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. When comparing traditional hydrogel soft lens contacts with silicone hydrogel versions, there is no clear evidence to recommend a superior lens.<ref>{{Cite journal |last=Haworth |first=Kristina |last2=Travis |first2=Darian |last3=Leslie |first3=Louis |last4=Fuller |first4=Daniel |last5=Pucker |first5=Andrew D |date=2023-09-19 |editor-last=Cochrane Eyes and Vision Group |title=Silicone hydrogel versus hydrogel soft contact lenses for differences in patient-reported eye comfort and safety |journal=Cochrane Database of Systematic Reviews |language=en |volume=2023 |issue=9 |pages=CD014791 |doi=10.1002/14651858.CD014791.pub2 |pmc=10507745 |pmid=37724689}}</ref>

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:<ref name="szcz">{{Cite web |last=Szczotka-Flynn |first=Loretta |date=1 May 2008 |title=Looking at Silicone Hydrogels Across Generations |url=http://www.optometricmanagement.com/articleviewer.aspx?articleid=101727 |url-status=live |archive-url=https://web.archive.org/web/20110719173206/http://www.optometric.com/article.aspx?article=101727 |archive-date=19 July 2011 |access-date=5 October 2021 |website=www.optometricmanagement.com}}</ref><ref name="chou">{{Cite web |last=Chou |first=Brian |date=1 June 2008 |title=The Evolution of Silicone Hydrogel Lenses |url=https://www.clspectrum.com/issues/2008/june-2008/the-evolution-of-silicone-hydrogel-lenses |url-status=live |archive-url=https://web.archive.org/web/20161230160857/https://www.clspectrum.com/issues/2008/june-2008/the-evolution-of-silicone-hydrogel-lenses |archive-date=30 December 2016 |access-date=5 October 2021 |website=www.clspectrum.com}}</ref>

* 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.