My eye doctor told me that traditional soft contact lenses are made from Hydrogel Polymers which is soft-water containing plastic. I was switched to the 30 day contact lenses which contains Silicone Hydrogel. Silicone Hydrogel contact lenses contain less water so they won't get as dehydrated while in your eyes. With Hydrogel Polymers the plastic itself is not oxygen premeable, so water performs the job of carrying water to the lens through the eye. Water can carry so much oxygen and the more water a lens contain the greater the risk it will dehydrate after long periods of time.