A plasmapheresis and/or ultrafiltration membrane comprises elongated
hollow fibers each fiber having an interior lumen extending along the
fiber length, the fiber wall having a plurality of zones between the
inner and outer wall surfaces, each of the zones having a mass density
different than the mass density of an adjacent zone. The fiber wall is
characterized by having a lower mass density zone at the inner wall
surface and a higher mass density zone at the outer wall surface. The
fiber is further characterized by having an average elongation breaking
force strength of at least about 0.2 lbs. and an average elongation of at
least about 45%.