Devices in which blood and oxygen are separated by a semipermeable membrane, generally of Teflon or polypropylene, across which gas exchange occurs. The membrane may be arranged as a series of parallel plates or as a number of hollow fibers; in the latter arrangement, the blood may flow inside the fibers, which are surrounded by gas, or the blood may flow outside the fibers and the gas inside the fibers. (Dorland, 28th ed).