Dieter Bachmann

Liposome Preparation by a New High Pressure Homogenizer Gaulin Micron Lab 40

AbstractHigh pressure expansion of lipid dispersions with a lab-scale homogenizer (Gaulin Micron LAB 4 0) was tested to produce forced lecithin hydration and subsequent liposome formation. A single-step liposome preparation method was developed. Lipid composition, pressure and the number of passages as the main process parameters were varied. Homogenizing with the new apparatus leads to small unilamellar vesicles (SUV), when 10% (w/w) lipid dispersions are used. Depending on the lipid composition the resulting vesicles ly from 25 up to 50 nm and show quite narrow particle size distribution. The cumulation of SUVs at their theoretical low size limits was detected after negative staining by transmission electron microscopy. Cryo-electron microscopy (CEM) of very thin vitrified layers of unfixed and unstained dispersions of vesicles confirmed in principle the negative staining data. Erosions of the especially stressed parts of the homogenizer, the valve, were monitored by scanning electron microscopy (SEM). ...

Preparation of liposomes using a Mini-Lab 8.30 H high-pressure homogenizer

Abstract The range of liposome preparation methods has recently been extended by a number of techniques which are based on the use of homogenizers. However, only a few suitable types of apparatus have been proposed so far. This report describes work with a continously working Mini-Lab 8.30 H high-pressure homogenizer of APV Rannie. Homogenization of dispersions of preformed vesicles which were multilamellar and heterogeneous in size resulted in uniform, small unilamellar vesicles (SUVs). The previously described one-step method (Brandl et al., Drug Dev. Ind. Pharm., 16 (1990) 2167–2191) for liposome preparation could also be carried out using the Mini-Lab homogenizer; the resulting vesicles were again very small and homogeneous after one cycle only. Higher homogenization pressures and repeated recirculation led to further reduction in vesicle diameter and heterogeneity. Finally, after about 15 cycles size distributions with peaks of about 30 nm and upper limits below 100 nm could be achieved. With higher lipid concentrations (maximum of 100 mg/ml) size reduction by homogenization was less effective. In the case of cholesterol-containing vesicles, maximum size reduction during repeated homogenization was observed after five to ten cycles. Further recirculation caused vesicle re-growth. The encapsulation efficiency of one-step vesicles for carboxyfluorescein at lipid concentrations (100 mg/ml) was nearly 12%, the entrapped volume being about 0.90 l/mol lipid. Repeated homogenization (up to 30 cycles) reduced these values by about 50%. The erosion of the most stressed part of the apparatus (i.e., the valve) was low, as monitored microscopically. During homogenization a slight leakage from the product compartment into the cooling water compartment was observed. Leakage of cooling water into the product compartment could be seen only during standby periods.

Three-dimensional liposome networks : freeze fracture electron microscopical evaluation of their structure and in vitro analysis of release of hydrophilic markers

Abstract This paper summarises first experiences with highly concentrated, semisolid phospholipid dispersions. Their preparation is based on ‘forced hydration’ of (phospho-)lipid(s) by high-pressure homogenisation in the presence of relatively low amounts of water. The inner structure of the obtained semisolid pastes, as revealed by freeze-fracture electron microscopy, is best described as a matrix of densely packed vesicles. Depending on the lipid content the characteristics of these vesicles range from very homogeneous, small and unilamellar to more heterogeneous in size as well as lamellarity. Although not comparable to ‘classical’ liposome dispersions these multivesicular pastes may be useful as drug carriers. Results from in-vitro release tests demonstrate that they may serve as local depots for controlled release of active compounds. Two release mechanisms are observed occurring at the same time: (1) release of free active molecules via diffusion out of the matrix and (2) budding off of active compound-carrying liposomes from the matrix. Release type and rate are determined among other factors by the phospholipid content of the matrices and thus by their inner structure.