A NOVEL METHOD FOR MEASURING HOLLOW-FIBER MEMBRANE-PERMEABILITY IN A GAS-LIQUID SYSTEM

Designing an effective intravenous membrane oxygenator requires select ing hollow fiber membranes (HFMs) that present minimal resistance to g as exchange over extended periods of time. Microporous fiber membranes , as used in extracorporeal oxygenators, offer a minimal exchange resi stance, but one that diminishes with time because of fiber wetting and subsequent serum leakage. Potentially attractive alternatives are com posite HFMs, which inhibit fiber wetting and serum leakage by incorpor ating a true membrane layer within their porous walls. To evaluate com posite and other HFMs, the authors developed a simple apparatus and me thod for measuring HFM permeability in a gas-liquid system under condi tions relevant to intravenous oxygenation. The system requires only a small volume of liquid that is mixed with a pitched blade impeller dri ven by a direct current motor at controlled rates. Mass flux is measur ed from the gas flow exiting the fibers, eliminating the necessity of measuring any liquid side conditions. The authors measured the CO2 exc hange permeabilities of Mitsubishi MHF 200L composite HFMs, KPF 280E m icroporous HFMs, and KPF 190 microporous HFMs. The membrane permeabili ties to CO2 were 9.3 x 10(-5) ml/cm(2)/sec/cmHg for the MHF 200L fiber , 4.7 x 10(-4) ml/cm(2)/sec/cmHg for the KPF 280E fiber, and 2.8 x 10( -4) ml/cm(2)/sec/cmHg for the KPF 190 fiber. From these results it is concluded that 1) because of liquid-fiber surface interactions, the pe rmeabilities of the microporous fibers are several orders of magnitude less than would be measured for completely gas filled pores, emphasiz ing the importance of measuring microporous fiber permeability in a ga s-liquid system; and 2) the liquid diffusional boundary layer adjacent to the fibers generated by the pitched blade impeller is unique to ea ch fiber, resulting in different boundary layer characterizations.