339 Application of Reverse Micelles in Cosmetic Formulations the cosmetic products. Nowadays, cosmetic products often incorporate natural bioactive ingredients such as proteins and plant extracts (59, 60). Reverse micelle extraction can be used to extract and purify these bioactive compounds. Reverse micelle extraction has many advantages compared to conventional liquid-liquid extraction methods. It allows the use of safer organic solvents, avoids formation of stable emulsion during extraction, and protects bioactive compounds from degradation due to direct contact with solvents. It is highly selective, easy to operate, conducted under mild conditions, and is a fast process. It also has the potential for large scale and continuous operation. Several important factors that must be considered during reverse micelle extraction are surfactant concentration, salt concentration, solution pH, and phase volume ratio. In general, these parameters should be Figure 3. Penetration routes: across SC (upper image), through transcellular route (solid-line arrow), and intercellular route (dotted-line arrow) into epidermis and dermis (lower image) through (1) sweat pores, (2) SC layer and (3) hair follicles (30).

340 JOURNAL OF COSMETIC SCIENCE optimized to maximize the interactions between reverse micelles and bioactive compounds during forward extraction. On the other hand, the interactions should be minimized during backward extraction. Reverse micelle had been used for extraction of jacalin from jackfruit (61), bromelain from pineapple peel (62), miraculin from miracle fruit (28), soybean protein (63), tea polysaccharides (64), lignin peroxidase and manganese peroxidase (65), as well as many other biomolecules. Reverse micelles have been used to extract rhodamine B in lipsticks for analytical purposes (66). It is an efficient extraction method that can be used in quality control of cosmetic products. Besides using for extraction, reverse micelles can be used to form nanoparticles for cosmetic formulations. Gold and silver nanoparticles are attractive active ingredients in cosmetic formulations. They have good antimicrobial activity and wound-healing properties. Reverse micelles can be used to prepare gold nanoparticles (67, 68) and silver nanoparticles (69, 70). Zinc oxide nanoparticles, which are commonly used as UV filters in cosmetic products, can be produced using reverse micelles (71). Other non-metal nanoparticles that are included in cosmetic formulations, such as chitosan nanoparticles, can also be prepared using reverse micelles (72). Therefore, reverse micelles not only have the potential to be included directly in cosmetic formulations, but they are also useful for preparation of various active ingredients used in cosmetic products. CONCLUSION Reverse micelle systems are widely used in downstream processing for extraction and the purification of bioactive compounds. Reverse micelle systems have many advantages such as ease of production, high encapsulation efficiency, and the protection of active ingredients. It can be prepared using safe, biocompatible, nontoxic, and environmentally friendly materials. Reverse micelle-based formulations can be designed to achieve specific rheological properties, desired skin permeation properties, targeted delivery of active ingredients, and sustained release of active ingredients. Therefore, reverse micelles have great potential as carriers of active ingredients in cosmetic formulations. To date, few studies have reported direct implementation of reverse micelles in cosmetic formulations. Reverse micelles are also used for preparation of active ingredients and the analysis of cosmetic products. Reverse micelle technology is promising in the cosmetic sector. More research needs to be conducted to study the interactions between reverse micelles and the active ingredients in cosmetic. This knowledge is important to identify their potential applications and help cosmetic formulators design innovative products based on reverse micelle formulations. Studies on preparation of reverse micelle formulations using natural materials such as essential oils and biosurfactants are needed as the current trend is toward more sustainable and environmentally friendly products. The skin penetration properties and delivery of active ingredients by reverse micelle formulations also need more investigation. This is important to optimize the functions of cosmetic products and to ensure that no negative side effects arise after application of the products. ACKNOWLEDGMENTS The authors appreciate the support by the Malaysia-Japan International Institute of Technology (MJIIT), University Teknologi Malaysia (UTM), CRG grants (08G08, 08G09)