Preparation and characterization of liposomes for encapsulation and targeted release of substances
Liposomes are small hollow particles whose wall is formed by a phospholipid bilayer. Their size ranges from tens of nanometers to hundreds of micrometers. They can encapsulate a wide spectrum of different substances because of their core-shell structure. The substances are slowly released out of the vesicles. The aim of this diploma thesis is to prepare liposomes with an ability to hold chosen substance inside for a sufficient time and to release it by an external stimulus. In the future this ability could be used in medicine for targeted drug delivery.
Experiments in this thesis were done with liposomes of diameter in range of tens to hundreds of nanometers. The liposomes were formed by the hydration of a dried phospholipid layer. They were subsequently processed by two different methods, sonication or extrusion. Sonication is based on homogenization and size decrease of the liposomes under the influence of ultrasound. Particles prepared by this method had usually a diameter of tens of nanometers. The other method involves extrusion of liposomes through a porous membrane. This leads to liposome sizes corresponding to the size of the pores. The particles prepared in this thesis had a diameter of approximately 200 nm.
Release kinetics from the liposomes and ways of acceleration of the leakage rate were studied. A fluorescent dye (carboxyfluorescein) was encapsulated into the liposomes and its leakage rate was measured by florescence spectroscopy. Differences between azolectin (phosholipids from soybeans) and dipalmitoylphosphatidycholine (DPPC) liposomes were shown. The azolectin vesicles were able to hold the dye no longer than 12 hours, contrary to the DPPC vesicles which released only 6% of the dye in 3 weeks. The liposomes were also prepared with an addition of cholesterol, however, an effect of cholesterol on stability was not observed.
An acceleration of the dye release was done by heating and by osmotic shocks. The effect of an increased temperature was studied in more detail. It was found that heating of the DPPC liposomes to 60°C for 10 minutes caused a release more than 20% of the encapsulated dye. Further leakage was slower, a release of another 20% took more than one hour. This acceleration is primarily due to a phase transition of the bilayer. Exceeding the phase change temperature (41°C for DPPC) leads to a transition from solid-like to liquid-like bilayer causing an increased permeability of the membrane.