Freeze-fracture Transmission Electron Microscopy (TEM) was used to show that sonication does not homogeneously disrupt liposome dispersions to form vesicles. Many large multilamellar particles remain intact after sonication and small, unilamellar vesicles are present after just 10 s of exposure. Small vesicles appear to coexist with large liposomes even before sonication. The mechanical and thermal stresses induced by sonication nucleate liquid crystalline defects in the liposomes, including edge and screw dislocations and +1 disclinations, but the Dupin cyclide structure of unsonicated liposomes is still recognizable in the larger particles after sonication. Defects in the bilayer organization may provide pathways for enhanced transport within the liposome, as well as from the liposome interior to exterior. A screw dislocation-catalyzed mechanism of liposome-to-vesicle conversion is proposed that accounts for the TEM observations.
Bibliographical noteFunding Information:
I would also like to recognize the financial support of a National Science Foundation Fellowship and a University of Minnesota Doctoral Disserta- tion Fellowship. Received for publication 14 June 1985 and in final form 17 December 1985.