Abstract: Selectively permeable asymmetric membranes are disclosed which are suitable for use in hemodialysis processes and the like. The membranes are produced from hydrophobic and hydrophilic polymers, and include a three-layered structure including a first dense thin skin layer which includes uniform pores of a size whereby proteins having a molecular weight of at least that of albumin are substantially rejected therefrom, an intermediate second layer in the form of a sponge and having a higher diffusive permeability than the first layer, and a third layer with a finger-like structure which provides mechanical stability. These membranes are preferably formed in the shape of hollow fibers, with the first layer as the inner layer thereof.

Abstract: Plasmapheresis membranes of hydrophilic polycarbonates are disclosed, comprising polycondensates of bisphenol A and a polyalkylene oxide. These membranes which have a regular pore structure with smoothly rounded pores with openings without sharp edges, have an average pore diameter of about 0.01 to 0.8.mu., a porosity preferably above 65%, and a thickness of between about 50 and 300.mu.. Processes for preparing these membranes are also disclosed, including casting a 3 to 20 weight percent polycarbonate polymer solution onto a smooth surface, contacting the layer with a gelling medium prior to precipitation of the membrane, and subsequent precipitation of the gelled layer to form the membrane.