Mini-Review Article
Zhila Vazifehasl 1, Sara Salatin 2,*
1 University of Mohaghegh Ardabili, Ardabil, Iran.
2 Research Center for Pharmaceutical Nanotechnology and Student Research Committee, Tabriz University of Medical Sciences, Tabriz, Iran.
Abstract
Liposomes recently gained more attention in scientist’s research programs. Their biocompatibility together the fine size and hydrophobic and hydrophilic appeal make them as talented drug delivery systems. There are a variety of methods to prepare these valuable particles. In this paper, we had a brief view of the preparation techniques for liposomes. The data summarized herein tried to show some of the current problems in liposomes preparations to exposed innovative prospects for future studies.
Keywords: Liposomes, Vesicle techniques, Solvent dispersion, the Detergent removal method
Liposomes are synthetic sphere-shaped vesicles formed of different natural and safe phospholipids. Thanks to their fine dimensions, hydrophobicity/hydrophilicity and biocompatibility, liposomes are deliberated as effective systems for drug delivery purposes [1]. Characteristics of liposomes are related to the lipid structure, size, surface charge as well as the technique of preparation. In addition, the bilayer constituents control the degree of rigidity as well as the charge of the bilayer. In this regard, studies suggest that unstable and permeable bilayers formed from unsaturated phosphatidylcholine classes from natural bases, while the saturated phospholipids contains long acyl chains (dipalmitoylphosphatidyl choline) procedure a rigid and slightly impermeable bilayer construction [2, 3].
The liposomal vesicles originated from hydrated phospholipids contain a hydrophilic head group and two hydrophobic tail chains. Phospholipid molecules form bilayers in aquatic media spontaneously. Within the bilayer, the hydrophilic heads (polar groups) form a hydrophilic surface whereas the hydrophobic tails form a water-free zone [4]. Phospholipid bilayers surround the dispersing aqueous medium and form vesicles while heating or shaking mechanically. Polar groups of phospholipids faced the inner and outer aqueous phase, however, their hydrophobic ends are placed within the bilayer. The structure of the liposomes highlights their capability of encapsulating hydrophilic and hydrophobic substances inside the inner hydrophilic core and the hydrophobic lipid bilayers, respectively [4, 5].
2.1 Sonication
This method is perhaps the most broadly applied technique today for the formation of small unilamellar liposome vesicle (SUV). For this, multilamellar liposome vesicle (MLV) are sonicated using either a probe sonicator or a bath type sonicator under an inert atmosphere. The chief drawbacks of this technique are probability of pollution from probe, probable degradation of phospholipids and encapsulated compounds, very small volume/encapsulation efficacy, and occurrence of MLVs together with SUVs [6].
Two sonication procedures have been reported so far:
2.2. French pressure cell: based on extrusion
This process includes the extrusion of MLV over a fine hole [6]. The main property of the this system is that the proteins do not look to be meaningfully pretentious throughout the sonication system [7]. Notably, French press vesicle seems to recall captured solutes fundamentally longer than SUVs do, generated by detergent removal or sonication [8]. The technique is based on moderate handling of unstable materials. This approach provides several interesting compensations over sonication technique [9]. The obtained liposomes are slightly bigger than sonicated SUVs. However, the working volumes are moderately low and it is difficult to achieve high temperature [6].
2.3. Freeze-thawed liposomes
Reports have shown that the SUVs are quickly frozen and melted slowly. The brief sonication is able to disperse aggregates to LUV. The unilamellar vesicles is formed by fusion of SUV during the freeze-thawing process [10]. This mechanism is significantly limited by enhancing the phospholipid concentration and the ionic strength of media. The encapsulation efficacies were found between 20% and 30% [11].
3.1. Solvent vaporization
A lipid solution with mixture of diethyl ether or ether-methanol is slowly inserted into an aqueous solution of a substance to be entrapped at 55°C to 65°C or under decreased pressure. The solvent is removed by evaporation under vacuum to form liposomes. However, in this technique the population is polydisperse (70 to 200 nm) and compounds to be encapsulated to organic solvents must be exposed to high temperature [12].
3.2. Ethanol injection
Lipids dissolved in ethanol are quickly inserted into a buffer solution where MLVs are formed at once. The main drawbacks of this technique are that liposomes are very thinned, the population is polydisperse (30 to 110 nm), the elimination of all ethanol is problematic because ethanol and water form an azeotropic mixture, and inactivation of different active macromolecules may occur in the existence of even low quantities of ethanol [13].
3.3. Reverse phase evaporation technique
This technique has shown a good growth in the area of liposomes, allowing the production of liposomes with a high aqueous space-to-lipid ratio and an ability to capture a large amount of the aqueous material for the first time. It is founded on the development of inverted micelles that are occurred from a buffered aqueous phase including the water-soluble molecules and an organic phase after sonication. The gentle elimination of the organic solvent results in the alteration of inverted micelles into a viscid gel. In this process, at a critical point the gel form collapses, and some of the inverted micelles disintegrate. The extra remained phospholipids in the medium uses to the creation of an organized bilayer around the remaining micelles, leading to the formation of liposomes. The prepared liposomes by reverse phase evaporation technique can be formed from various lipid preparations and its volume-to-lipid ratios are 4-times higher than multilamellar liposomes or hand-shaken method [14].
Using a two-phase system sonication method the w/o emulsion is made that is comprising phospholipids in an organic solvent. Then, the organic solvents are separated during continued rotary under decreased pressure, causing in the formation of liposomes. Here, a high percentage of encapsulation efficiency can be gained in a situation of low ionic strength. This system has widely been applied to entrap large, small and macromolecules. The chief problem of the process is the exposure of the compounds to be entrapped to organic solvents and to short-lived phases of sonication, resulting in the denaturation of proteins or the breakage of DNA strands [15]. Handa et al. presented a modified reverse phase evaporation method in which the liposomes showed high entrapment efficiency of 80 percent [15].
4.1. Dialysis
Indeed, critical micelle concentrations (cmc) of detergents have great role for different applications. Detergents are widely applied as solubilizing agents to dissolve lipids. When the detergent is removed by dialysis, the micelles finally combine to form LUVs. A commercial version of dialysis system, named LipoPrep (Diachema AG, Switzerland), is obtainable for the removal of detergents [16].
4.2. Detergent absorption
Detergent absorption is obtained by shuddering a mixed solution of micelles with beaded organic polystyrene absorbers such as and Bio-beads. These absorbers can eradicate detergents with a very low cmc, which are not wholly exhausted [17].
4.3. Gel-permeation chromatography
The detergent is eliminated by size-exclusion chromatography. The liposomes do not pass through the holes of the beads and percolate over the inter-bead spaces the interbead spaces of the column. The separation of detergent monomers from liposomes remains very good at slow flow rates. A substantial amount of amphiphilic lipids is adsorbed by swollen polysaccharide beads, thereby, pre-treatment is necessary that it can be performed with pre-saturation of the column using lipids by blank liposome suspensions [18].
4.4. Dilution
The fine size together the polydispersity property increase significantly after dilution of aqueous mixed micellar solution of phospholipids and detergent with buffer, and a impulsive change from polydisperse micelles to vesicles happens when the system is diluted [19, 20].
Liposomes seem to be acceptable transporters for various agents in a variety of fields of medicine. We reviewed the preparation techniques for liposomes in this study. The information gathered herein tried to display some of the current problems in liposomes preparations to show novel prospects for future studies.
Acknowledgments
The author state that there is no financial support for this study.
Conflict of interest
The author has no conflict of interest for this study.
References
HOW TO CITE
Vazifehasl, Z., & Salatin, S. (2019). Liposomes; an update on preparation techniques. Journal of Advanced Chemical and Pharmaceutical Materials (JACPM), 2(2), 133-137. Retrieved from http://advchempharm.ir/journal/index.php/JACPM/article/view/113
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