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Abstract
Liposomes were prepared by stepwise extrusion through 5, 1, 0.4, 0.2, 0.1 and 0.05
microm pore sizes using two different filter-extruders, the continuous high pressure
device Dispex Maximator (CE) or alternatively the discontinuous Avestin LiposoFast
(DE). The liposome dispersions obtained were compared in terms of particle size, lamellarity
and encapsulation efficiency of calcein. The liposomes were smaller with CE than DE
at all stages due to higher flow rates and pressure drops, except for final filter
pore size (0.05 microm) where both preparations had similar sizes. The particle size
analysis technique itself had a strong influence on the liposome sizes measured. For
bigger liposomes (extruded through 0.4 microm filters) the Nicomp 370 revealed bigger
volume-based mean particle sizes along with more stringent differences between volume-based
and number-based diameters than the Malvern Zetasizer. In contrast, for small liposomes
extruded through 0.05 microm filters, similar liposome sizes were found no matter
which of the two PCS techniques or cryo-transmission electron microscopy was used.
In congruence to the liposome sizes measured, encapsulation efficiencies were smaller
for CE than DE at all filter stages except the final (0.05 microm). No lipid loss
occurred and lyso-phosphatidylcholine formation was negligible irrespective of which
extrusion technique was used.