Liposomes

Liposomes are spherical vesicles whose membranes consist of one (unilamellar) or more (oligolamellar, multilamellar) bilayers of phosphatidylcholine. Sometimes, especially in patents, reference is made not about liposomes but about ''vesicles with an internal aqueous phase.'' The vesicles can differ in size (diameter about 15-3500 nm) and shape (single and fused particles). At a given chemical composition, these parameters strongly depend on the process of preparation. Very often the preparations are metastable. That means the state of free enthalpy is not in an equilibrium with the environment. As a result the vesicles change their lamellarity, size, size distribution, and shape with time. For example, small vesicles tend to form larger ones and large vesicles smaller ones. Fortunately this is mostly not critical for quality because the properties of the phosphatidylcholine, which the vesicles are based on, remain unchanged as a rule. Nevertheless the stability seems to be best in a range of about 100 to 300 nm. That is the case of pure aqueous dispersions of highly enriched (80-100%) soy phosphatidylcholine.

In a complete formulation together with further ingredients, other influences like compatibility, concentration of salts, amphiphilics, and lipophilics play an important role. Therefore, it is often very difficult to prove the existence of liposomes, e.g., in a gel phase or a creamy matrix. However, this is more a marketing problem than a problem of effectiveness of the formulation. Today we can assume that the effectiveness of phosphati-dylcholine is based more on the total chemical composition of the cosmetic product and less on the existence or nonexistence of the added liposomes. This may seem curious, but is in fact the reality.

Of course, formulations are very effective in particular when consisting of pure liposomal dispersions bearing lipophilic additives in the membrane spheres and/or hydro-philics in the internal and external aqueous phases within the range of their bearing capacity. In this respect, there has been an intensive search to increase the encapsulation capacity of liposomes for lipids because consumers are used to applying lipid-rich creams. Efforts were made to add emulsifier to the liposomal dispersions to stabilize higher amounts of lipids. Formulators now know that the compatibility of liposomes with regard to emulsifi-ers is generally limited, more or less. On the other hand, additional emulsifiers have a weakening effect on the barrier affinity of phosphatidylcholine. They cause the phosphati-dylcholine and the lipids to be more easily removed from the skin while washing. In this respect there is only one rational consideration: to make use of nanoemulsions consisting of phosphatidylcholine and lipids instead of liposomes. Nanoemulsions are a consequence of the observation that oil droplets can fuse with liposomes when the capacity of bilayers for lipids is exhausted [8]. Further increasing the lipid/phosphatidylcholine ratio and using high-pressure homogenizers lead to nanoemulsions. Nanoemulsions consist of emulsionlike oil droplets surrounded by a monolayer of phosphatidylcholine. The advantage of nanoemulsions is that they allow formulations to tolerate more lipids and remain stable. Also, additional emulsifiers are not needed.

Liposomal dispersions based on unsaturated phosphatidylcholine are lacking in stability against oxidation. Like linoleic esters and linoleic glycerides, these dispersions have to be stabilized by antioxidants. Thinking naturally, a complex of Vitamin C and E (respectively, their derivatives like acetates and palmitates) can be used with success. In some cases, phosphatidylcholine and urea seem to stabilize each other [9,10]. Moreover, agents that are able to mask traces of radical-forming ions of heavy metals, like iron, can be

added. Such additives are chelators like citrates, phosphonates, or EDTA. Alternatively, the unsaturated phosphatidylcholine can be substituted by a saturated one like DPPC or hydrogenated soy phosphatidylcholine, which should be favored with regard to its price. Because of the higher phase-transition temperature, liposomal dispersions based on hydrogenated material are more sophisticated in their preparation and are reserved for pharmacological applications as a rule. An interesting new development in the field of cosmetic compositions with hydrogenated soy phosphatidylcholine is the Derma Membrane Structure (DMS)-technology [11]. DMS stands for cream bases (technically the creams are gels) containing hydrogenated soy phosphatidylcholine, sebum-compatible medium chain triglycerides (MCT), shea butter, and squalane. In addition to liposomal dispersions and nanoemulsions, DMS is a third way to formulate phosphatidylcholine with hydrophilic and lipophilic compounds free of further emulsifiers (Fig. 3). DMS is water- and sweatproof and therefore suitable for skin protection and sun creams without using sili-cones or mineral oil additives. It can easily be transformed into other final products by stirring at room temperature together with liquid lipids and/or aqueous phases.

As previously mentioned, DMS is predestined for skin protection, but by addition of nanoemulsions and/or liposomal dispersions DMS can easily be enriched by unsaturated phosphatidylcholine containing esterified linoleic acid. The resulting products are creamy, stable, and anticomedogenic. The effect of pure DMS basic creams on skin moisturizing, smoothing, and tightening are still significant several days after finishing the application.

Liposomes, nanoemulsions, and DMS have to be preserved. This may be a problem, because phosphatidylcholine (lecithin) inactivates most of the conventional preservatives [12]. On the other hand, preservatives should not be penetrated in the skin to prevent irritation and sensitization. Therefore, glycols like propyleneglycol, glycerol, butylenegly-col, pentyleneglycol, hexyleneglycol, sorbitol, and their mixtures are the compounds of choice. These polyols show a moisturizing effect at the same time.

One of the reasons to substitute phosphatidylcholine by polyglycerols and other synthetic derivatives at the beginning of the liposomal developments was its hydrolytic instability in aqueous preparations for longer periods of time and at higher temperatures. In fact phosphatidylcholine, like other glycerides, is attacked by water to form lysophos-phatidylcholine and free fatty acids. But the cleavage of the glyceride bond occurs mainly at a pH greater than 7, so formulations in the range of pH 5.5 to 7 are sufficiently stable

Liposomes Nanoemulsions

Figure 3 Formulations with phosphatidylcholine free of further emulsifiers.

Liposomes Nanoemulsions

Figure 3 Formulations with phosphatidylcholine free of further emulsifiers.

for most purposes. It is possible that hydrolysis depends on the amount of additional surface active compounds. That is another reason to use liposomal dispersions without additional emulsifiers.

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