## 4 Formulations of Green Tea Extracts and EGCG in Pharmaceutical and Cosmetic Applications  
Due to antioxidant and antiaging activities, green tea extracts, including catechins such as EGCG, have been used for the preparation of many advanced drug and cosmetic formulations for topical and other applications (Figure 1). Results of many studies on the topical application of catechins revealed significant advantages of this method of administration, which was found superior to oral or intravenous ones [28]. Model creams (oil-in-water emulsions), ointments, hydrogels, and some microsphere suspensions, e.g.,niosomes and ethosomes containing EGCG, were assessed for their photostability, skin penetration and retention (bioavailability), stratum corneum, and tranepidermal water content and loss, skin elasticity. However, hydrogel formulations of EGCG were studied not only for topical but also for oral delivery systems. For example, [PERSON] et al. prepared ionic hydrogel consisting of gelatin-\(\gamma\)-polyglutamic acid [29]. The obtained biopolymer showed significant protection of EGCG at different pH values present in the gastrointestinal tract environment. Several studies reported on the increase of the transdermal and intradermal concentration of EGCG. For example, the permeation abilities study performed by [PERSON] et al. using various cosmetic formulations revealed that the highest amounts of delivered catechin were found in the stratum corneum [30]. In contrast, significant loss of EGCG concentration was noted in deeper tissues. Following that observation, [PERSON] et al. examined 10% of EGCG in hydrophilic ointment USP and noted high concentrations of EGCG in the skin accompanied by low systemic influence [31]. However, in studies performed by [PERSON] et al., 1% of EGCG oil-in-water emulsion revealed permeation potential into the deep region of the human stratum corneum when compared to gel formulation [32]. This finding emphasizes the importance of topical vehicles being used in formulation studies. It could also be concluded that when systemic activity of EGCG is unattainable, its topical formulations could reveal an excellent potential for moisturizing skin [33]. Many efforts have been made to increase the systemic bioavailability of EGCG through topical applications. [PERSON] et al. noticed that transdermal delivery of EGCG allows the achievement of higher plasma levels of this compound than drinking tea [34]. It was found that the bioavailability of EGCG can be increased through the application of some nanoparticles. Specifically, microspheres containing EGCG were studied in terms of their stability and skin permeation properties, among which were niosomes, wherein catechins were incorporated with surfactant and cholesterol to form niosomal gels presenting prolonged stability [35]. Other interesting particles used in EGCG formulations were ethosomes and transethosomes. These modern lipid nanovesicles prepared from phospholipids, and containing high concentrations of alcohol, water, and penetration enhancers, revealed better drug penetration properties than classic liposomes and were more elastic. Ethosomal and  
Figure 1: Various formulations of green tea extracts (GTE) and (\(-\))-epigallocatechin-3-O-gallate (EGCG) in pharmaceutical and cosmetic applications. transethosomal formulations of EGCG were found to improve its stability and increased its intake through the skin [36; 37]. (\(-\))-Epigallocatechin-3-gallate is known for its photo protecting ability due to strong UVB absorption, which subsequently leads to its photodegradation. In this way, all catechins reveal potential antiphotocarcinogenic activity. Thus, they can hamper DNA damage and prevent neoplastic skin changes [38]. Therefore, it is necessary to improve EGCG stability upon light irradiation to maintain its antioxidant features when applied to the skin. Many studies have focused on the photostability of EGCG in topical formulations.