An in-vivo plasmapheresis and/or in-vivo ultrafiltration membrane comprises
a plurality of 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.
