Working Knowledge: The Hard and the Soft; October 2000; Scientific American Magazine; by Naomi Lubick; 2 Page(s)

In the eyes of millions of Americans float little plastic disks, bending the light that enters their eyes and making their vision clearer. Last year approximately 34 million people in the U.S. wore contact lenses, according to the Contact Lens Council. The majority wear soft (or "hydrogel") lenses, intending to throw them away after one day, two weeks or several months. These lenses need cleaning daily. "Extended wear" soft lenses can be worn continuously-day and night-for up to a week (the Food and Drug Administration is considering 30-day lenses). Fewer than 15 percent of contact lens wearers use what are called rigid gas-permeable lenses, which can last for years if taken care of properly.

These rigid lenses are made of plastic polymers-such as cellulose acetate butyrate, polyacrylate-silicone or silicone elastomers-that do not absorb or attract water. David T. Grubb, a materials scientist at Cornell University, describes their molecular structure as open, with large gaps through which oxygen can pass. Hydrogel polymers, in contrast, are hydrophilic, absorbing or attracting water in amounts that vary from commercial brand to brand. Water moving through the molecular structure of soft lenses carries oxygen with it.