ROLES OF VEHICLES FOR SKIN TREATMENT 191 traditional emulsions. In modern times, some emulsifiers (like sucrose esters) or gelling polymers (e.g., taurates) also provide a cooling effect (1). Cosmetic vehicles also are well known to influence the hydration of the stratum cor­ neum, for which at least three different mechanisms have been proposed: First, the cosmetic vehicle can exert a direct hydrating effect by liberating water from the for­ mulation itself (6). In short-term applications, this hydrating effect is more pronounced with formulations containing a high percentage of free water, compared to lipid-rich and low-free-water-containing preparations (7). As expected, the hydrating effect of o/w systems in short-term applications depends primarily on the water activity (unbound water content) of the formulation (5), since only the presence of free (unbound) water insures immediate hydration of the stratum corneum. In contrast, long-term applica­ tions of either w/o or o/w emulsions with different water content revealed hydration of the stratum corneum with the w/o but not with the o/w emulsions (8,9). Thus, although a w/o emulsion may be cosmetically less acceptable (but this is only true with older formulations), such a formulation can be expected to achieve better stratum corneum hydration, especially with prolonged use. It should be mentioned that longlasting hydration can be attributed to unbound water and the organization of lipid bilayers. In general, the structure and composition of modern cosmetic vehicles is so diverse that the attribution of their effect based solely on their o/w or w/o structure is no longer adequate. Second, the occlusive effect of the formulation can influence stratum corneum hydration, especially in long-term applications. A standard model for this occlusive effect is pet­ rolatum (7), for which the highest occlusive effect was detected (5). Water-in-oil emul­ sions with low water content may have occlusive effects similar to those of petrolatum, while w/o emulsions with high water content very rarely have occlusive properties and behave similarly to o/w emulsions (5). Interestingly, even o/w formulations with high water content can exert an occlusion effect after the unbound water has evaporated. The amount and type of emulsifiers have profound effects on water evaporation (from the vehicle), as well as on the incorporation of applied lipid fractions into the stratum corneum lipids and the spatial organization of the formulation residue in the skin. Occlusion is frequently just a temporary effect, since the emulsions may break after topical application on the skin. However, the occlusive effects are not always desirable. For example, in atopic dermatitis, where a formulation with high lipid content is desired, an occlusive effect may enhance discomfort and induce itching. The occlusive effects also may enhance drug penetration, an effect that may or may not be desired (e.g., it is unwanted in sunscreens). A third mechanism by which cosmetic vehicles influence skin hydration is evident when highly hygroscopic compounds like glycerol or hydrotropes like hyaluronic acid or trimethylglycine are applied. By absorbing water either from the cosmetic vehicle itself, from the surface water, or from water evaporation, these agents are then able to increase stratum corneum hydration (10,11). Recent publications have shown that the epidermal water/glycerol transporter aquaporin-3 (AQP3) plays an important role in stratum cor­ neum hydration via glycerol content (12, 13 ). In addition, vehicles can exert an emollient effect (by relipidizing or regreasing). This is of great importance in skin conditions where patients express discomfort due to cracked or rigid skin or rough skin surfaces. It has been suggested that this lipid-supplying effect

192 JOURNAL OF COSMETIC SCIENCE may be of major importance to the plasticity of the skin (14). Moreover, lipid-rich formulations improve skin distensibility, while creams and gels with lower lipid content have a more pronounced effect on (short term) skin hydration, as discussed above (14). The vehicle regreasing effect seems to be limited to the application period, and not a long-term effect, while the related increase in stratum corneum moisture content might be longlasting (10). Finally, it should be noted that compounds with distinct dielectric constants have been shown to influence the electrophysical properties of the stratum corneum, as measured by capacitance- or conductance-based instruments (10). Thus, it is plausible that topi­ cally applied moisturizing creams might be a source of false-positive or false-negative results using these instruments (15). Although a good correlation between capacitance values and water content of the tested creams has been demonstrated (15 ), a sufficient time lag following application of compounds should be allotted before any such mea­ surements are registered. Moreover, the regression curve of vehicle effects should be studied as a function of time. BARRIER PROTECTION AND BARRIER RECOVERY A number of factors are involved in the determination of the effectiveness of dermato­ logical and cosmetic vehicles to protect the skin barrier. Commonly used barrier creams, which are either w/o emulsions or cosmetic vehicles with strongly lipophilic character, are claimed to protect against hydrophilic irritants. On the other hand, barrier creams that are o/w emulsion systems, or act like hydrophilic systems, are mistakenly thought to protect against lipophilic irritants. Dermatological skin protection (especially in work conditions) is based on different product groups in situations where barrier or protective creams are employed. For example, pre-exposure skin care includes the use of o/w and w/o emulsions, tannery substances, zinc oxide, talcum, perfluorpolyethers, chelating agents, and UV protectors (16). However, cleansing products and post-exposure skin care are two other important components of skin protection. The post-exposure skin care is based on dermatological and cosmetic vehicles, long-term moisturizers, fast humec­ tants, and lipid-rich formulations. Although protective effects have been shown using specific test conditions, double-blinded, placebo-controlled, randomized trials are still lacking, especially under conditions that approximate real workplace situations (16). In fact, cumulative stress tests with repetitive application of irritants appear to be the best conditions for approximating work conditions (17-23). The distinction between skin protection and skin care is not always clear. For example, in nurses a barrier cream was compared with its vehicle for effects on clinical improve­ ment. Interestingly, both clinical skin status as well as stratum corneum hydration improved significantly in each treatment group, without evidence of a difference be­ tween the vehicle and the barrier cream groups (24). Thus Berndt and colleagues proposed to abolish the distinctions between skin care and skin protection products (24). Correct instructions for consumer use should be stressed with regard to regular and frequent application of a protection product in order to be effective (25). In addition, a recent study discussed whether claims could be made with respect to protective and preventive properties of topically applied body lotions and barrier creams (26). In this particular study, enhanced stratum corneum hydration, improved barrier function, and a faster barrier recovery were reported after SLS barrier disruption (26).