Large multilamellar liposomes

Figure 14.2 Illustration of a large multilamellar liposome (top) and a small unilamellar liposome (bottom). In each case lipid bilayers are the structural units (inset). From Yang and Alexandridis [890]. Copyright 2000, Elsevier.

EYPC membranes so that the prolonged sonication of large multilamellar liposomes is not able to disperse membranes to form small rmilamellar vesicles (Gabrielska and Gmszecki, 1996). 

Fig. 5. Changes in fluorescence depolarization (p) upon the enzymatic digestion of FITC(0.54)-OPP-50(1.8) as a function of time —/ the free polysaccharide —, the polysaccharide bound to small single-walled liposomes and —0 9 the polysaccharide bound to large multilamellar liposomes, respectively. For detail, see text. Vertical arrows indicate the point when pullulanase was injected after preincubation.

Figure 14. Scanning electron micrograph of large multilamellar polymeric liposomes from (5) (9).

Multilamellar liposomes were prepared by standard methods (6), and prior to incorporation into the delivery systems, were passed through a 0.8 pm filter (Nucleopore, Pleasanton, CA) to remove very large structures and/or aggregates. 

Niven et al. [18] have demonstrated that small liposomes also have a slower release rate than do large multilamellar vesicles following nebulisation. It has also been suggested that liposomes of 50-200mn diameter are optimal for clinical applications, as they tend to avoid phagocytosis by macrophages and still trap useful drug loads [19]. 

Numerous techniques for the preparation of liposomes have been described. Typical procedures involve the hydration of lipid mixtures in buffer, resulting in the formation of large multilamellar vesicles (MLV). These are of limited use in... 

A large body of scientific evidence suggests that carotenoids scavenge and deactivate free radicals both in vitro and in vivo. It has been reported that their antioxidant action is determined by (1) electron transfer reactions and the stability of the antioxidant free radical (2) the interplay with other antioxidants and (3) their structure and the oxygen pressure of the microenvironment. Moreover, the antioxidant activity of carotenoids is characterized by literature data for (1) their relative rate of oxidation by a range of free radicals, or (2) their capacity to inhibit lipid peroxidation in multilamellar liposomes. ... 

It is certain that liposomes had to be present in these experiments (5,8-11]. For instance, Lehmann s book shows an optical micrograph of what is today known as large multilamellar vesicle in 1911 the author called them Kunstliche Zellen , artificial cells [12-14],... 

The electron micrograph in Fig. 11.12 shows the existence of thermodynamically stable vesicles in the L i phase which can be formed as small unilamellar vesicles besides large multilamellar vesicles (liposomes). Depending on the composition of the ternary system the vesicles can also show structural faults like holes (perforated vesicles) [37]. The vesicle phases show a significantly reduced electric conductivity because a part of the water phase together with the ions is included in the interior of the vesicles. With stopped-flow experiments and optical or conductivity readout it is thus possible to determine the permeability of the vesicle membranes for dissolved compoimds [99]. Vesicle phases show often high viscosities and viscoelasticity due to the mutual hindrance of the vesicles in sheared solu-... 

Liposomes are formed due to the amphiphilic character of lipids which assemble into bilayers by the force of hydrophobic interaction. Similar assemblies of lipids form microspheres when neutral lipids, such as triglycerides, are dispersed with phospholipids. Liposomes are conventionally classified into three groups by their morphology, i.e., multilamellar vesicle (MLV), small unilamellar vesicle (SUV), and large unilamellar vesicle (LUV). This classification of liposomes is useful when liposomes are used as models for biomembranes. However, when liposomes are used as capsules for drugs, size and homogeneity of the liposomes are more important than the number of lamellars in a liposome. Therefore, "sized" liposomes are preferred. These are prepared by extrusion through a polycarbonate... 

Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayer membranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome and the lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist of two components ... 

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