Plasma membranes of cells and skin barrier have sub-microscopic lamellar structures that are of particular interest for topical hydro lipid formulations due to their physical, chemical and physiological compatibility. Hence, topical lamellar systems generally are oriented towards their natural models and use phosphatidylcholine, ceramides, sterols and fatty acids as structure forming elements.
Hydrogenated phosphatidylcholine (PC-H) is a relatively inexpensive component in so-called Derma Membrane Structures whose main field of application is the protection of the skin. The fatty acid composition of PC-H, consisting of saturated C18- and C16-acids, determinates the planar structure of the bilayers. The native phosphatidylcholine (PC) fractionated from soybean lecithin via column chromatography with about 80-90% linoleic acid in its fatty acid composition, generates cellular bilayers (liposomes). Liposomes improve the penetration of active agents. Combined with lipids, liposomes can be used to compel nanodispersions with monolayers that can contain lipophilic active agents as well.
The combination with PC and PC-H mixtures allows a smooth adjustment of functions like skin protection and transport of active agents with simultaneously very low wash-out effect. The degree of lamellar structuring is of secondary importance in this context. PC nanodispersions are biodegradable and not subject to the complex safety assessment according to the European Cosmetic Regulation (nanoparticle item).
The stability of lamellar systems is a complex challenge: PC and PC-H inactivate almost all of the preservatives. The polyunsaturated fatty acids of PC in liposomes and nanodispersions need to be protected against the formation of hydroperoxides. PC and PC-H are typical phase transfer catalysts and tend to form lysophosphatidylcholine (hydrolysis of one fatty acid moiety from the glycerin structure), particularly with high temperatures, long storage or high pH-level. Depending on components added to the formulation, transitions to W/O or O/W emulsions may occur which then externally manifest by a different consistency of the formulation. The shearing forces of high speed mixing devices have accelerating effects here. Surface active agents (emulsifiers, tensides) destroy the structures. Not all of additives and cosmetic as well as pharmaceutical agents are compatible. On the other hand, various polysaccharide-based gelling agents as well as carbomers have a stabilizing effect. A significant aspect to be noticed is that these non-compatible substances frequently show intolerances on the skin which means that they are less suitable for the application in lamellar systems for problem skins in the first place. Perfumes as well are inappropriate in formulations with intensified penetration like liposomes or nanodispersions. By contrast, the combination of lamellar systems with natural substances like phytosterols, terpenes, polyphenols, ceramides, amides, vitamins and glycerides of essential fatty acids results in highly effective skin care products.
Non-aqueous formulations with the components of lamellar systems also are appropriate for the application on problem skin.
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Dr. Hans Lautenschläger