The Use of Cross-Flow Microfiltration in Purification of Liposomes

Abstract

ABSTRACTThe use of cross-flow microfiltration for purification of liposome vesicles is studied. The liposomes are concentrated while the phospholipid monomers are rejected or permeated through the filter membrane during filtration. Effects of operating conditions, such as cross-flow velocity and filtration pressure, on the filtration rate, the cake properties, the rejection coefficient of phospholipid, and the separation efficiency are discussed. The experimental results show that the filter cake formed by liposome vesicles exhibits a very high compressibility and specific filtration resistance. Cake resistance plays a major role on the overall filtration resistance. The filtration rate attenuates quickly at the beginning of filtration, due to the deposition and the compression of liposome vesicles. After a period of time, the filtration rate gradually approaches a pseudo-steady value. An increase in applied filtration pressure causes an increase in the filtration rate at the pseudo-steady state, the membrane and the cake resistances, and the separation efficiency. On the other hand, the filtration rate at a pseudo-steady state increases, whereas the cake resistance decreases with the increase in cross-flow velocity. However, the effects of operating conditions on internal membrane fouling are negligible. A theory based on the basic filtration equation and the force balance model of particle deposition is proposed for estimating the filtration rate at a pseudo-steady state. The experimental data agree very well with he model calculations. A lower cross-flow velocity or filtration pressure gives the phospholipid monomers more opportunities to penetrate the cake and the membrane into filtrate. Therefore, the rejection coefficient increases with the increase in filtration pressure or crossflow velocity. Compared with the conventional “dead-end” cake filtration, the use of cross-flow microfiltration in the purification of liposomes is an efficient method.