Characterization of supported lipid membranes towards the development of nano-sized drug carriers for hyperthermia applications
Sammanfattning: So called thermosensitive liposomal drug carriers for temperature-controlled drug release under hyperthermal conditions have recently attracted great attention and advanced lipid membrane - nanoparticle assemblies are emerging at an increasing rate to meet the demands for multifunctional drug carriers in medicine. The rational design of these drug delivery systems will require understanding of lipid membrane physicochemical properties under conditions relevant to the medical treatment. Towards this aim, the present thesis is devoted to studies of the formation and the function of lipid membrane coatings on planar solid supports, focusing on lipid compositions yielding a gel to liquid phase transition temperature in the range 40 - 45 °C. In the first study, DPPC (glass transition temperature, Tg = 41 oC) liposomes ranging from 90 nm to 160 nm in diameter were prepared and used for studies of the formation of supported lipid membrane on silica using the quartz crystal microbalance with dissipation (QCM-D) technique. It was found that, at a temperature (50 oC) well above the glass transition temperature, DPPC liposomes smaller than 100 nm spontaneously rupture on the surface at a critical surface coverage, following a well-established pathway. DPPC liposomes larger than 160 nm do not rupture on a silica surface at 50 oC, but can overcome the threshold to form membranes when first adsorbed to the surface at 22 oC, after which the temperature is increased to 50 oC. This study contributes to the understanding of the liposome-to-membrane formation process, where the critical coverage of adsorbed liposomes and the liposome shape play important roles. In the second study, a new method of preparing asymmetric lipid membranes on solid surfaces by combining two leaflets in different phase states is demonstrated. It includes a proof-of-concept, where phase transition induced flip-flop between the lipid leaflets is employed to control what lipid head groups are presented at the membrane surface. The process was monitored by QCM-D and dual polarization interferometry (DPI). The asymmetric structure was stable at a temperature below the effective Tg of the lower leaflet, while lipid flip-flop was induced upon increasing of the temperature above the effective Tg. Transmembrane lipid exchange was demonstrated by detecting, through streptavidin binding, biotinylated lipids appearing at the surface of the top leaflet after ‘temperature-activation’ of an asymmetric structure where these lipids were first located in the lower leaflet. The understanding of the fundamental physicochemical properties of lipid membranes, in particular the distinct transition between the gel phase and the fluid phase in response to temperature, is an important key to successful designs of hyperthermia-responsive nanodrugs for medical applications. The described strategy for constructing asymmetric membranes on flat titania surfaces provides a guide to ‘smart’ drug carriers upon transfer to spherical titania nanoparticle templates. The presented results also suggest that QCM-D can be a useful method to measure liposome content release, which is important in drug carrier development.
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