Properties of emulsions.

Today's cosmetic emulsions are not simply mixtures of oil, water and emulsifier; they also contain a number of active ingredients functioning to ameliorate the condition of the skin, such as skin hydration or the barrier properties of the stratum corneum (SC). Most of these ingredients will work only if they penetrate the SC or the deeper layers of the epidermis. The extent and rate of penetration depend not only on the diffusion and solubility properties of the ingredient molecule itself, but also on the effect of the cosmetic vehicle on the diffusion process.

Emulsions as Galenic Vehicle

Emulsions are disperse systems in which oil and water are held together in finely distributed form by surface-active substances. Besides oil, water and emulsifiers, emulsions usually contain thickeners for viscosity adjustment. Lipids are frequently used as consistency-imparting agents; they function by building up a lamellar-gel network in the outer phase.[5,13]

In this article I use the term "galenic vehicle" to identify a vehicle that contains pharmaceutical or cosmetic active ingredients. This term, commonly used in Germany and several other European countries, refers to a standard medicinal preparation typically containing one or more active constituents. I use it here to refer to different emulsion formulations.

As intimate mixtures of oil and water, emulsions are eminently suitable for solubilizing pharmaceutical or cosmetic active ingredients, irrespective of whether these active ingredients are lipophilic or hydrophilic. If the desired therapeutic or cosmetic effect is to be achieved, however, it must be ensured that the active ingredient does in fact reach its target. In the case of a topical preparation, this could be, depending on the application, the outermost skin layer (the epidermis), the deeper skin layers or the vascular and lymphatic system.

Penetration Properties of Formulations

The release.of an active ingredient from a topical formulation depends on its passive diffusion into the skin. In principle, this diffusion obeys Fick's law. This takes into account the concentration of the active ingredient in the galenic vehicle, the mobility (diffusion constant) of the active ingredient molecule, and - in the form of the distribution coefficient - the interaction of the active ingredient with the vehicle and the skin.[21,22]

In practice, the application of this simple law is frequently difficult because the composition of a topical formulation only rarely remains constant during the course of the application; volatile components vaporize, water evaporates,[9,10,16] and the mobility and the distribution coefficient of the active ingredient change as a function of its concentration and of the viscosity and phase behavior of the galenic vehicle.

Moreover, the penetration properties of a formulation are also dependent on the interaction of the vehicle with the skin.[1,21] The application of the emulsion can have an occlusive effect under which the hydration of the horny layer increases, resulting in increased penetration. Certain components of the formulation may accumulate in the outermost skin layer, which is the actual diffusion barrier, thus accelerating penetration[4,21] Simple penetration models cannot take these various, mutually interacting effects into account, so an optimal galenical can only be developed on the basis of the results of experimental penetration studies.

On ethical grounds, in vivo studies on humans and animals are carried out only on a limited scale, mainly in connection with dermatics.[18] The search for suitable in vitro models in this field has resulted in a large number of test methods showing a range of success in corresponding to in vivo conditions.[11,20] A significant number of metabolic processes take place in the skin,[21,14] and these can only be taken into

account under in vitro conditions if living and functioning skin (such as perfused skin) is used. The isolated perfused bovine udder skin (B US) model is a good substitute for in vivo tests on humans,[15] and it can be used to study both skin penetration and skin irritation by cosmetic formulations.[19]

Objective

The objective of this work was to study the influence of emulsion type and structure on the penetration of vitamins as cosmetic active ingredients. An emulsion of the w/o type and two different o/w emulsions are compared with an oil solution as a standard. As o/w emulsions, a lamellar cream and a phase inversion temperature (PIT) cream were chosen. The PIT …