A filter device for being implanted in a blood vessel for carrying out in-vivo
plasma separation comprises one or more elongated hollow tubes and a plurality
of elongated hollow microporous fibers, each fiber having a first and second end
secured to one or more of the elongated hollow tubes with the interior lumen of
each of the fibers communicating with the interior of the one or more of the hollow
tubes, and wherein the fiber wall has a higher mass density zone adjacent to the
outer wall surface and a lower mass density zone adjacent to the inner wall surface.
