a hollow polysulfonic fiber intended for a selectively transmissive membrane contains a hydrophilic polymer which is cross - linked into a water - insoluble form and is used in the form of a hydrogel containing water . a method for the production of a membrane module consists in spinning a hollow fiber by the use of a spinning solution containing a polysulfonic resin and a hydrophilic polymer , wetting the fiber above the saturated moisture content , and then , with the wet condition maintained , subjecting the fiber to the processes of module molding and radiation induced cross - linking , these processes being performed in either order . the method provides a selectively transmissive hollow polysulfone fiber membrane which displays less elution into water and a sharp molecular weight fractionation , so as to be usable safely in the fields of medicine and foodstuffs .