Abstract: A hollow fiber formed of hydrophilic material and having permeable walls is treated to enhance the permeability. Reactions used to treat the fiber have a gas inside and gas outside, a gas inside and liquid outside, a liquid inside and gas outside, and a liquid inside and a liquid outside. The treatments generally involve the plasticizing of the fiber material while having the inside of the fiber at a higher pressure than the outside. These treatments produce a structural change in the fiber from a flexible tube having strong walls to expanded, rigid and somewhat fragile walls. Where more than a short length of the treated fiber is to be used, an open mesh cage is used to confine the fiber to a desired volume during treatment and to protect it subsequently. A technique for joining flexible tubing to the treated fiber is presented. The treated fiber may be used in apparatus and methods involving osmosis and reverse osmosis.