## 1 Introduction  
The skin can be damaged by numerous factors including ultraviolet radiation, smoking, hormonal imbalance, ethanol ingestion, air pollution, and inflammation [1, 2]. Cosmetics are most relied upon for skin damage prevention because they are more convenient and economical, and have fewer side effects than pharmaceuticals or cosmetic surgery [3]. Thus, this has increased the demand for effective cosmeceutical products containing natural bioactive compounds [4].  
The need for natural bioactive compound-containing cosmeceutical products might be met by a certain category of plants containing compounds with useful bioactivities in the human skin. Asian countries, like Korea, Japan, and China, have always used traditional medicines for various diseases, and have been at the forefront of cosmeceutical product production from natural resources [5]. By actively searching for long-lived trees native to Asia that may contain compounds with skin-beneficial effects, we found _Ginkgo biloba_ L. (GB), _Cinnamomum camphora_ ([PERSON]. (CC),and _Cryptomeria japonica_ ([PERSON].) [PERSON] (CJ). The living fossil GB has continuously gained popularity as a dietary supplement [6] and complementary medicine [7]. Extracts of its leaves are rich in ginkgolides, flavonoids, biflavones, terpenoids, and polyphenols [8; 9], and have been used for treating age-related diseases [10]. GB is also used in anti-aging cosmeties due to its antioxidant, anti-lipid peroxidation, and anti-inflammatory activities [11]. The evergreen tree CC, commonly known as the camphor tree, is used as a medicinal plant [12], as it exhibits a number of biological activities, such as antioxidant, anti-inflammatory [13], and antifungal [14], owing to its bioactive compounds such as terpenids and phenylpropanoids. CJ, commonly called the Japanese cedar, is known to have strong antimicrobial [15], anti-inflammatory [16], and antifungal [17] activities. It also has skin-whitening and antioxidant activities based on anti-tyrosine, radical scavenging, and superoxide dismutase assay results [18]. We hypothesized that a mixture of the leaf extracts of these three trees could synergistically exert skin-beneficial effects. The skin-related bioactivities of tree extracts are likely to be associated with polyphenols. Because their extraction yields and compositions can vary greatly depending on a number of factors, including solvent type and concentration, temperature, time, and extraction method, the extraction conditions need to be carefully optimized, as reported by [PERSON] et al. [19].  
The versatile green solvents, deep eutectic solvents (DESs), have recently gained much attention due to their low toxicity, high extractability, and good biodegradability. Their potential usage in various fields, such as the cosmetics industry, has been suggested [20; 21]. Therefore, the current study aims at applying DESs as safe and efficient extraction mediums that could yield maximized skin-related bioactivities from a mixture of the three long-lived trees. We therefore synthesized a number of DESs using components that are permissible according to the European Commission's Inventory of Cosmetic Ingredients (2006) [22], and compared their extraction efficiency for a selected marker compound. Then, we applied a simplex-centroid mixture design to locate the optimal combinations of CC, CJ, and GB that would maximally allow beneficial skin-related bioactivities, including antioxidant, anti-tyrosine, and anti-elastase activities. This study suggests the potential of a mixture of the three extracts in DESs as a new natural bioactive compound-containing material for cosmeceutical product development.