Complex lipids are present in all living tissues where they play important structural and functional roles. An earlier review published on the topic by Hanahan and Thompson (1963) is a good starting point for beginners to learn about the role of complex lipids in various species and tissues. Alibardi (2003) has published on the role of complex lipids along with the role of other chemical entities during the evolutionary process while species were adapting to land. Human skin is a layered structure and its main function is to protect our internal vital organs. The stratum corneum being the outermost layer, exposed to environmental aggressions, is composed of lipids and other structural elements, including dead cells. The stratum corneum also regulates the entry of external ingredients in to the skin, for example, any cosmetic ingredient for nourishing needs or for hydration purposes. Some of the complex lipids already in cosmetic use may include sphingolipids, choline, ceramides, sophora lipids, phosphatidic acid, glycolipids, and phospholipids. Sphingolipids are functionally important for the skin as barrier components. A few more common types of complex lipids available are listed in the following table along with their reported functions (Table 22.2).

Essential oil or plant oil is composed of water-insoluble compounds containing volatile aroma components. They are used as fragrance, flavor, or as pharmacological actives. Their use in cosmetics is restricted though due to their capacity for skin irritation and side effects associated with the reactive nature of some of the components, which are present in the plant oils. Some of the components are specifically forbidden or limited by regulations within the European Union. A list generally known as the “EU list of 26 Allergens” is available for guidance (http://​ec.​europa.​eu/​enterprise/​sectors/​chemicals/​files/​legislation/​allergenic_​subst_​en.​pdf).

The chemistry of essential oils is diverse and not the same as that of other commonly known oils, for example cooking oils from plants that are composed of triglycerides. Essential oils may contain one or more of terpenes; hydroxyl compounds such as alcohols and phenols; carboxylates such as esters, lactones, and coumarins; ethers; oxides; and carbonyl compounds such as aldehydes and ketones. Many references, reviews, and book chapters have been written on the topic (Baser et al. 2007; Oberdieck 1972; Littlejohn 1940). With such a diverse chemistry it is not surprising that essential oils are used for many purposes from aroma therapy to pharmacological actives and anything in between (Miyazawa 2011). Sometimes they are also prepared and used as a dilute version called waters, for example rose water, white lily flower water, lemongrass oil, citrus peel water, and many others. More than 700 plant oils and waters are available for commercial use in cosmetics. The most common uses for essential oils are undoubtedly fragrance related. However, small uses as antimicrobial, emollient, skin protecting, etc. are also common in cosmetics. For example, lemongrass essential oils are reported to be effective as anti-allergenic and anti-inflammatory (Mizushina et al. 2013).