Function Of Vehicles Direct Intrinsic Effect

The term vehicle is used in pharmaceutics as well as in cosmetics in the area of formulation. In general, this term implies differentiation between active and inactive principles. The active principle is embedded into a matrix, the vehicle. With the aid of the vehicle the active principle is delivered to the application site or to the target organ, respectively, where the desired effect is achieved. As a matter of fact, however, when dermatological and cosmetical preparations are applied, sharp differentiation between active and inactive principle is generally not possible because of the so-called vehicle effect.

The aim of application of both a pharmaceutical preparation as well as a cosmetic topical care product is to achieve a desired effect. Pharmaceutical preparations are effective because of a pharmacologically active compound delivered with the aid of a vehicle, whereas cosmetic formulations are not allowed to contain such compounds. Nevertheless, an effect is also achieved by a cosmetic preparation—not any systemic or central or curative effect—but a caring or preventing effect mainly on skin, hair, or nails. This effect may be achieved either by cosmetically active ingredients or by the vehicle itself on the site of application, i.e., on the skin in most cases. In contrast to pharmaceutics, in cosmetics the vehicle is of greater importance.

Depending on the composition, a vehicle is used to exert mainly five types of effects on the skin, briefly described in following sections.

Cleansing

The most common and probably oldest use of cosmetic preparations is to clean the human body. In our modern time and society, not just soap but a variety of sophisticated cosmetic cleansing products are available.

Decoration

Decoration serves to produce a pleasing appearance by minimizing facial defects of color or shape and unobtrusively enhancing and directing attention toward better points [1]. Decorative cosmetic preparations are not the main object of this chapter on vehicles, although similar principles have to be considered for decorative cosmetic preparations.

Care

Probably more cosmetic preparations are applied to care for the outermost organs of the body, i.e., skin, hair, and nails, than to decorate these organs. Care of skin, hair, and nails and improvement of their state is an important function of an applied cosmetic product. Application of an appropriate vehicle may be fully sufficient for care of the body.

Hydration

The state of dry skin may be treated by applying a cosmetic product. In this case the skin is hydrated by application of an appropriate vehicle containing specific components that are able to reduce the transepidermal water loss. This results in an increase in water in the stratum corneum and a smoother surface of the skin.

Protection

A further important function of cosmetic vehicles is to build up a protective layer against external potentially damaging factors that could come into contact with the body. Especially in recent years the protective and preventive function of vehicles has become increasingly important, because of an increase of various external harmful factors or at least higher awareness about them (e.g., air pollution, UV radiation).

Delivery of Actives

From a stringent medicinal and legal point of view, a cosmetic preparation must not contain any (pharmacologically) active substance or ingredient that treats or prevents disease or alters the structure or function of the human body [2]. That means just the vehicle is effective directly at the site of application. This is in contrast to pharmaceutical vehicles, which in principle should serve as pure vehicles delivering active substances to the target organ and showing no effect on the body. However, in reality there are no such distinct |

but floating boundaries. Therefore, cosmetic vehicles can also be considered as means containing cosmetic actives that are applied to the outermost layer of the body. Furthermore, many cosmetically used substances are bifunctional: first they constitute the vehicle structure and second they show a positive effect on the skin status when applied.

Carrying Actives to Target (Targeting) 2

Going even one step further, cosmetic vehicles can also be considered and used as carriers s for cosmetic actives which, after application, are carried and delivered to the specified target sites, i.e., to legally allowed targets in deeper regions of skin. However, this is only allowed if no systemic, physiological, or pharmacological effect is achieved and the product has shown to be safe.

Delivering active substances to these targets requires the right concentration of actives in the formulation to achieve the optimal release rate and desired distribution of active substances between the vehicle and the target site. That means the vehicle should penetrate (superficially) into the stratum corneum and release the active substance at the optimal rate (immediate or sustained for depot effect) at the target site where the desired effect is achieved.

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