Liposomes are small, spherical vesicles which consist of amphiphilic lipids, enclosing an aqueous core. The lipids are predominantly phospholipids which form bilayers similar to those found in biomembranes (see figure 1). In most cases the major component is phosphatidyl choline. Depending on the processing conditions and the chemical composition, liposomes are formed with one or several concentric bilayers.
Figure 1. Chemical structure and schematic representation of a phospholipid (lecithin).
What are the Different Types of Liposomes and How Are They Classified?
Liposomes are often distinguished according to their number of lamellae and size. Small unilamellar vesicles (SUV), large unilamellar vesicles (LUV) and large multilamellar vesicles (MLV) or multivesicular vesicles (MVV) are differentiated (see figure 2). SUVs show a diameter of 20 to approximately 100 nm. LUVs, MLVs, and MVVs range in size from a few hundred nanometers to several microns. The thickness of the membrane (phospholipid bilayer) measures approximately 5 to 6 nm.
Figure 2. Schematic illustration of liposomes of different size and number of lamellae. SUV: Small unilamellar vesicles; LUV: Large unilamellar vesicles; MLV: Multilamellar vesicles; MVV: Multivesicular vesicles.
Processing Methods for Forming Large Liposomes
Large liposomes form spontaneously when phospholipids are dispersed in water above their phase transition temperature. The preparation of SUVs starts usually with MLVs, which then are transformed into small vesicles using an appropriate manufacturing technique, e.g. high-pressure homogenization.
What are Niosomes and Sphingosomes?
Niosomes and sphingosomes are vesicles with a similar structure. In contrast to liposomes, nonionic surfactants, e.g. polyglyceryl alkyl ethers, or sphingolipids make up the bilayer of niosomes and sphingosomes, respectively (see figure 3).
Figure 3. Chemical structure of lipids forming sphingosomes and niosomes.
Liposomes and Hydrophilic, Amphiphilic and Lipophilic Substances
Liposomes which are used as delivery systems, may encapsulate hydrophilic substances in their aqueous core. Amphiphilic and lipophilic substances, e.g. oil soluble UV filters, can be incorporated into the lipid bilayer. Loaded liposomes as well as non-loaded, empty liposomes, are used in cosmetics. The major effect of empty liposomes is an increase in skin humidity.
Benefits of Using Liposomes in Skincare Products
Liposomes frequently favor the disposition of encapsulated active ingredients in the epidermis and dermis, while the permeation rate is decreased. This helps to fix active ingredients to the outermost skin layers as desired for cosmetic products. Simultaneously, the washing out may be delayed so that, for example, aqueous sun care products containing liposome-encapsulated UV filters are water-resistant.
The Size, Composition and the Number of Liposomes Used Determines Their Success in Cosmetic Products
However, these positive effects mentioned above depend on the composition, size, and the amount of liposomes. Therefore, general conclusions are not justified. As far as empty liposomes are concerned, it has recently been discussed that the positive effects are not strongly related to the vesicular nature. The presence of the appropriate lipids (phospholipids, sphingolipids) suffices for cosmetic efficacy.
Risk Assessment and Regulations Concerning the Use of Applied Phospholipids in Food and Cosmetics
The skin compatibility of topically applied phospholipids is very high. There are no restrictions concerning their use in foodstuff and cosmetics, neither in the EU nor for regulations of the US Food and Drug Administration; lecithins are generally accepted as safe (GRAS status - Generally Recognized As Safe). However, it is known that high doses of phospholipids which are applied topically over a longer period may lead to irritations on dry and normal skin. Likewise, it has been mentioned that due to a biochemical feedback mechanism, a long-term application of phospholipids may have an impact on the dermal lipid metabolism.
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