Currently in a Phase I Trial, the bioartificial liver support system employs a hollow-fiber bioreactor cartridge that is loaded with hepatocytes. In one version of the reactor the hollow fibers act as an immunoisolation barrier, serving to prevent direct contact of patient blood flowing on the interior of the fibers with the hepatocytes on the exterior of the fibers. In an even more exciting development, the hollow fibers are being used as an artificial capillary network, passing plasma from the interior of one fiber, through the cellular layer and Into another fiber after metabolism.