188 JOURNAL OF COSMETIC SCIENCE limited because kojic acid is labile when exposed to light or heat. Due to decomposition it turns to a yellowish brown. The cyclodextrin complex has an improved stability against coloring in the lapse of time. The cyclodextrin complex of kojic acid also has an enhanced skin-whitening effect (23). Peroxyacetic acid is a liquid that is handled as an aqueous solution. It forms solid complexes with ix- and [3-cyclodextrin. These stable powders are easy to handle and they can be used in cosmetic formulations as skin- lightening substances. These complexes also act as mild oxidants with disinfectant properties (24). Protection against loss by evaporation. Volatile compounds like perfumes (25,26) or com- ponents of them (27) are stabilized against evaporation as cyclodextrin complexes. The complexes may be used in powders or liquid formulations. Even in solid form the perfume complexes are used in perfumes. Applying these suspensions on the skin assures a long-lasting effect by slow release of the perfumes. Long-acting perfumes are marked as Vivace © (Shiseido Co.). SOLUBILIZATION OF THE GUEST MOLECULES A large number of cosmetic components are nearly insoluble in water. Only a few examples, such as vegetables oils, hydrocarbons, higher fatty acids and their esters, vitamins, hormones, antiphlogistics, and preservatives are mentioned. All these chemical substances are able to form inclusion complexes with cyclodextrins. As a result, these complexes are more soluble compared to the pure compounds. This effect can be gen- erally used for the formulation of cosmetics (28). Salicylic acid is used for the cleaning of the skin. Its action is mainly antibacterial and keratolytic. The solubility of the acid or its derivatives in aqueous solution is low. The complex with cyclodextrins is much more soluble. Thus, possibly irritating reactions occurring with the free acid are prevented due to better homogeneity. Complexed with cyclodextrins, its disinfectant, bacteriostatic, and keratolytic properties also increase (29). The cyclodextrin complex of salicylic acid is already commercially available (Lipo Chemicals Inc.). It is used in Bioclin Sebo Care Impure Skin Cream © (Ganassini). In many personal care products triclosan acts as a topical antiseptic and disinfectant. It is nearly insoluble in water, moderately soluble in alkaline solutions, and quite soluble in organic solvents. The cyclodextrin complex (Lipo Chemicals Inc.) is soluble in water, giving a clear solution. Menthol has an effect as a cooling agent in different cosmetic products. Menthol is only slightly soluble in water. To increase the solubility, alcohol is normally added. The cyclodextrin complex is freely soluble in water, giving a clear solution without the presence of alcohols (Lipo Chemicals Inc.). In topical anti-aging formulations, retinol is used. It reduced wrinkles and supports the restoration of UV-damaged tissues. Unfortunately, UV light and oxygen initiate chemi- cal reactions of retinol. Thus, during oxidation some peroxidic toxic intermediates are formed. Also, the solubility of retinol is low in aqueous media. Retinol forms complexes with cyclodextrins that are stable in the presence of light and oxygen. Complexes of retinol acetate and retinol palmitate with [3-cyclodextrin are well known. Also, retinoic acid and some of its derivatives form complexes with cyclodextrins (30). Just recently the formation of complexes with •-cyclodextrin have been reported (31). Retinol-

C¾CLODEXTRINS 189 cyclodextrin complexes are used in the following products: Eucerin Vital Retinol © (Beiersdort), Nutrients & Anti-aging Agents © (Efal), and Dexol A © (Collaborative Labo- ratories). Cyclodextrins are also used to increase the solubility of substances secreted by the skin, and thus they are used in products for skin cleaning. They are able to complex and to dissolve skin fat. The formed fat complex with cyclodextrins can be easily removed from the skin. Therefore cyclodextrins, together with other ingredients, are in a product named Gesichtstonic © (Annemarie B6rlind). ELIMINATION OF UNDESIRED ODORS Dihydroxyacetone is used as tanning agent. It is not stable in aqueous solution. More important is the unpleasant odor of dihydroxyacetone, which is difficult to mask by perfumes. This odor vanishes using the cyclodextrin complex. The slow release of di- hydroxyacetone from the complex results in a more uniform tanning of the skin. This cyclodextrin complex is used in Ultrasun Selftan © (Ultrasun) and in Self-Action Super Tan For Face © (Est6e Lauder). Glutathione shows various physiological activities. For example, it inhibits melanin pigment formation. Thus it may be used for skin whitening and skin-improving effects. Unfortunately, glutathione generates an offensive odor upon use in cosmetic formula- tions. The glutathione complex with cyclodextrin is free of the odor of glutathione and has the same effect on the skin as glutathione (32). Mercapto compounds are often used in waving lotions. Due to the presence of mercapto derivatives, an extremely unpleasant odor is generated during application. This odor is often masked in part by the addition of perfumes to the lotion. A more efficient possibility is the use of cyclodextrins. Due to complex formation with the mercapto compounds, the unpleasant odor is eliminated (33). Chamomile extracts or oil have an antiphlogistic, bacteriostatic, and wound-healing effect. These formulations often have an intense and unpleasant odor. This odor is reduced by complexation with cyclodextrins. The antifiammatory activity of chamomile extracts remains unaffected (34). Deodorants are used to control malodors formed by microbial degradation of sweat. Cyclodextrins and mixtures of cyclodextrins can be used in deodorant sticks. The cy- clodextrins are able to complex perspiration malodors (35). IMPROVEMENT OF HANDLING Liquid or oily substances as powders. The oily o•-tocopherol acts as an antioxidant. It is used in many personal care products to protect the surface of the stratum corneum from damage caused by free radicals. As a cyclodextrin complex it is not soluble in water or oil. It should be used as a powder or suspended in lotions and creams (Lipo Chemicals Inc.). The formation of complexes with cyclodextrins serves to enhance the stability of tocopherol (36,37). Octyl methoxycinnamate is used as an oil-soluble sunscreen. As a cyclodextrin complex it is also not soluble in water or oil. The complex can be used in formulations as a powder