Lack of water in the skin can lead to a series of skin problems. If the skin is too dehydrated, it will become rough and scaly, causing defects in the skin barrier, making it easier for harmful substances from the outside to enter the skin, and serious skin diseases will occur, and the use of moisturizing products can improve this situation. At the same time, moisturizing is often considered to be the first step in fighting skin aging, helping to maintain the appearance and elasticity of the skin and strengthening its barrier role against harmful environmental factors. This paper mainly introduces the skin moisturizing mechanism and the plant extracts with moisturizing effect. Plant extracts are mainly described from polysaccharides, oils, saponins, flavonoids and polyphenols, in order to provide reference for future research on plant-based natural moisturizing cosmetics.

Skin moisturizing mechanism

The skin is composed of epidermis, dermis and subcutaneous tissue. The epidermis is a homeostasis and self-renewing tissue, and moisture plays a key role in the orderly differentiation, desquamation and eventual formation of the skin, as well as giving the skin plasticity and flexibility. The stratum corneum is the outermost layer of the skin epidermis, and although the water content of the stratum corneum is only about 15%, it plays a key role in skin barrier and moisturizing function. The dermis is not only the main water storage site of the skin, but also the metabolic exchange site of nutrients.

Hydration of natural moisturizing factors. Natural moisturizing factor is a general term for small molecular compounds that can be hydrated by osmotic pressure in the stratum corneum. The natural moisturizing factor has a strong hygroscopic property and can absorb water to act as a moisturizer for the skin. Natural moisturizing factor comes from filaggrin, which can be produced by complete hydrolysis of filaggrin. The content of natural moisturizing factor decreased significantly with age, and the decrease in its content may be exacerbated by age-related decline in barrier function, and the decrease in content also reflects the cumulative effect of photoaging damage.

Water storage in the dermis. The dermis is the thickest of the three layers of the skin, and its water content accounts for about 70% of the entire skin. The collagen fibers of the dermis can store a lot of water. In addition, mucosaccharides such as hyaluronic acid and chondroitin sulfate in the dermis matrix have strong hydration ability. Hyaluronic acid can fix water in the tissue, thereby changing the volume and compressibility of the dermis, affecting cell proliferation and differentiation, and tissue repair. Water in the dermis comes from subcutaneous tissues and capillaries. When the dermis loses fullness, especially the reduction of hyaluronic acid, it will lead to the disordered arrangement, thickening and fragmentation of collagen fibers, so that the elastic fibers of the whole skin are in a disordered state, and this phenomenon will change irreversibly with the growth of age.

Skin moisturizing method

Endogenous moisturizing. The moisture of the skin is related to many hydration components in the epidermis and dermis. The outermost layer of the skin, the stratum corneum, acts as a barrier between the external environment and the body, preventing water loss and foreign body intrusion. The stratum corneum is essential for maintaining the moisture content of the skin's surface. Water retention in the stratum corneum mainly comes from the natural moisturizing factors in keratinocytes, aquaporin AQP3 transport water molecules and the orderly arrangement of interkeratinocyte lipids to form a barrier against water loss through the epidermis. Abnormal filagin, which is completely hydrolyzed to produce natural moisturizing factors, is closely related to transdermal water loss and skin dryness. Aquaporin AQP3 also plays a key role in the permeability barrier of the epidermis and the relative stability of the skin's internal environment, and its imbalance can lead to a variety of skin diseases. In addition, ceramides are also important components of the skin epidermal structure and play an important role in maintaining the relative stability of the internal environment.

The dermis plays the role of nutrition, structure, vascular support and wound repair to the epidermis. The collagen fibers in the dermis can not only store a lot of water, but also have high toughness and strong tensile resistance, which can give the skin toughness and tension and resist external mechanical damage. The hyaluronic acid in the dermis constitutes the main protective barrier for the underlying tissue to defend against the external environment, which not only binds with water to prevent water loss, but also has the effect of removing free radicals.

Extrinsic moisturizing. Depending on the sealing and moisturizing properties of the exogenous moisturizers, their hydrating hydration mechanisms are also different: the sealing ingredients produce a thin layer of oil on the skin, preventing water from evaporating; Moisturizing ingredients hydrate the skin to absorb and retain water. The hydration ability of different moisturizers is different, and the impact on the skin hydration degree is also different.

Water-soluble moisturizers in moisturizers can temporarily increase skin hydration by absorption into the epidermis. Among them, hyaluronic acid is considered to be an ideal natural moisturizing factor and is widely used in cosmetics. Lipids and other fat-soluble components in moisturizers can form a film on the surface of the skin to reduce transdermal water loss, and lipids and other components can penetrate the skin, pass through the stratum corneum and mix with the surface sebum, thereby relieving skin dryness, providing skin protection and physiological lipids.

Several plant extracts with moisturizing properties

In plants, there are sugar groups in the structure of water-soluble components such as polysaccharides and glycosides, which have good hydration, water absorption and water retention properties through hydrogen bonding. The phenolic hydroxyl group structure of flavonoids and polyphenols also binds water through hydrogen bonding, which makes them have the ability to absorb and retain water. Vegetable oils retain moisture primarily by forming a thin layer of oil on the skin, preventing it from drying out.

polysaccharide

Polysaccharides are widely found in plants. The monosaccharide units in plant polysaccharides are connected to each other to form branching or linear structures, but the main chain is usually dextran, fructan, xylan, mannan or galactan, etc., or a polymer of two or more monosaccharides. The branched chain structure of polysaccharides showed great diversity. There are a lot of hydroxyl groups in the structure of plant polysaccharides, which have good hydration ability through hydrogen bonding. In addition, polar groups such as carboxyl groups of plant polysaccharides can also be bonded to water molecules by hydrogen bonding. Therefore, plant polysaccharides mostly have the properties of hygroscopicity and water retention. Plant polysaccharides have been widely used in various cosmetics as moisturizers. The moisture retention ability of plant polysaccharides is mainly affected by the structure, composition and molecular weight of the glycosyl. The polysaccharides of many plant extracts can improve the moisturizing effect of the skin and reduce water loss even at low concentrations.

β-glucan is a kind of macromolecular cell wall polysaccharide widely found in plants and fungi, which is composed of glucoside groups connected by β-glucoside bond. Beta-glucan is an effective moisturizing ingredient in cosmetic formulations, which can relieve dry skin and atopic diseases, reduce itching caused by bacterial infections, and other effects. Different sources of β-glucan have different glucoside bond and its proportion and different three-dimensional structure, so that β-glucan has different properties and biological activities, such as promoting wound healing, anti-inflammatory and anti-allergy. Oat β-glucan is formed by the glucoside linkage of β-1,4 and 1,3. The mixed chain structure of oat β-glucan breaks down the uniform structure of β-D-glucan, which makes oat β-glucan have good water solubility and moisture retention. The high molecular weight oat β-glucan has high viscosity.

Among higher plants, aloe extract is widely used as moisturizing material in cosmetics. Aloe vera extract is found in many moisturizing cosmetics. The main active component of aloe extract is aloe polysaccharide (Table 1). In addition to its good moisturizing effect, aloe polysaccharide has certain viscosity and gel properties, and also has a good effect on skin wound healing and dry skin repair. In addition, aloe polysaccharides also have a certain effect on atopic dermatitis. Polysaccharide in cactus extract is composed of neutral polysaccharide, acidic polysaccharide and acidic glycoprotein. Cactus polysaccharide can not only absorb and maintain skin moisture through hydration, but also has anti-inflammatory and wound healing effects. Most of the water-soluble plant polysaccharides, such as Astragalus polysaccharides, glycyrrhiza polysaccharides, Dendrobium polysaccharides and fire ginseng polysaccharides, have good hydration and water absorption characteristics, have good moisture and film formation, and have good effects in maintaining skin moisture and improving the moisture of the stratum corneum, and also have antioxidant activities.

Table 1 Plant polysaccharides with moisturizing effect

There are also a lot of polysaccharides in seaweed, and the types of polysaccharides in different algae are also different. Many seaweed polysaccharides have good hydration hygroscopicity and water retention ability. Compared with hyaluronic acid, the low molecular weight polysaccharides of some macroalgae show high ability to absorb and retain water. Alginate, a polysaccharide from brown algae (Table 2), is used in cosmetics not only as a humectant, but also as a stabilizer and gelling agent. Alginic acid can quickly absorb water, and can also form a protective moisturizing complex through ion-ion interaction, which has good moisture absorption and hydration properties, thereby helping to increase skin moisture and improve skin elasticity. In the presence of bivalent cations, alginate has good gel properties and ion chelation. In the mask prepared by alginate, alginate forms a flexible film with better skin absorption. In addition, algal extracts, such as rhizocladus longiflorus extract, have a moisturizing effect similar to hyaluronic acid, and the moisturizing effect is good and lasting; Fucose gum improves skin elasticity through hydration. The hydroxy group and water formed intermolecular hydrogen bond to promote the absorption of water, and the moisturizing effect of polysaccharides increased with the increase of sulfate content. Ulva polysaccharide not only has moisturizing properties, but also has antioxidant and anticoagulant activities.

Table 2. Algal polysaccharides with moisturizing effect

oil

Vegetable oil can form a water lipid film on the surface of the skin, which not only has a moisturizing effect, but also makes the skin waterproof. Vegetable oils strengthen the skin barrier and affect the reconstruction of the stratum corneum, preventing the loss of moisture in the skin and normalizing the sebaceous glands. Vegetable oils also have an important impact on the normal appearance and function of the skin. Vegetable oils are used in a wide range of cosmetics, including creams, lotions, hair conditioners, pomades, facial masks and lipsticks.

Unsaturated fatty acids are the main components of vegetable oils. In addition to unsaturated fatty acids, wheat germ oil is also rich in vitamins (Table 3), and wheat germ oil is added to cosmetic formulas to form a closed layer on the skin, preventing water loss, increasing moisturizing effect, and also has the effect of promoting skin regeneration on dry and rough skin. In addition, the combination of wheat germ oil and some plant extracts has a better synergistic effect and is very effective in preventing dry skin. Many vegetable oils not only have a moisturizing effect, are easily absorbed by the skin, and have good spreadability, but also have a variety of biological activities. Coconut oil is a mild moisturizer; Coconut oil can increase the expression levels of aquaporins AQP3, integrin and filagmin, and play a role in keratinocyte differentiation and skin barrier, thus changing the skin barrier function. The combination of coconut oil and olive oil can slow down the loss of water through the epidermis through sealing, and has a better role in protecting the skin. As an emollient, coconut oil is also one of the indispensable oils in handmade soaps and some detergents. Sea buckthorn oil not only has a moisturizing effect in the cosmetic formula, but also has the effect of skin repair and regeneration. It can also protect, regenerate and soften the cuticle, relieve inflammation, and ensure the stability of the structure between skin cells. Sea buckthorn oil is suitable for dry, rough, flaky, itchy or irritated skin. Shampoos and conditioners with sea buckthorn oil can promote the regeneration of damaged hair and restore the elasticity and smoothness of hair. In addition, raspberry oil is a good cosmetic base oil, which not only promotes skin hydration and moisture retention, avoiding epidermal water evaporation, but also strengthens the lipid barrier of the epidermis, improves sebaceous glands, and is suitable for some skin diseases such as psoriasis and rosacea. Grapeseed oil contains a large amount of omega-6 fatty acids, which can regenerate the lipid barrier of the epidermis for dry skin and prevent excessive water loss; For oily and seborrheic skin can improve acne and sebaceous gland normalization.

Table 3 Vegetable oils with moisturizing properties

saponin

Saponins are widely distributed in higher plants with rich structural and functional diversity. According to the different structure of aglycones, saponins can be divided into triterpenoid saponins and steroid saponins. Triterpenoid saponins are mainly found in dicotyledonous plants, while steroid saponins are mainly found in monocotyledonous plants. Most saponins are the combination of hydrophobic glycosides and hydrophilic glycosides, which makes saponins have high amphosexual compatibility, and have the properties of moisturizing, foaming and emulsifying.

The main active ingredient in the extract of Asiaticose sinensis is the pentacyclic triterpene saponins such as hydroxyasiaticoside. Centella sinensis extract is widely used in cosmetics, which has the functions of moisturizing, promoting skin wound healing, anti-inflammatory and antioxidant. The emulsion of Asiaticum asiatica extract combined with hyaluronic acid and glycerin produces a long-lasting hydrating and moisturizing effect on the skin while improving skin barrier function. The main active component of ginseng extract is tetracyclic triterpenoid saponins ginsenosides. Ginsenosides not only have a good moisture absorption and moisturizing effect on the skin, but also stimulate skin regeneration and wound healing by increasing the proliferation and migration of keratinocytes, repair the skin barrier, and have whitening and anti-wrinkle effects. In addition, the main components of extracts of many plants such as glycyrrhiza glabra, viola tricolor, Tribulus terrestris and Quilan are saponins. These saponins combine with water molecules through hydrogen bonding, absorb water and prevent water loss, and help to play a moisturizing role on the skin; It can also repair the skin barrier and maintain the function of the stratum corneum, with obvious anti-inflammatory and antibacterial properties.

flavone

Flavonoids usually have multiple phenolic hydroxyl structures, and in many plants, flavonoids combine with sugar groups to form flavonoid glycosides. The flavonoids of plants not only have certain hygroscopicity, but also have a variety of biological activities. Adding flavonoids to cosmetics helps restore the elasticity and shine of dry, sagging skin. The flavonoid quercetin is widely found in plants. Chamomile extract containing quercetin can enhance the moisturizing effect of the skin, repair the dry skin cracking water shortage and balance the oil secretion, and the effect of soothing sensitive skin is more obvious. Quercetin promotes the expression of tight junction protein in a dose-dependent manner, thereby enhancing the tight ability of cell connections, maintaining the integrity of the skin barrier and regulating water homeostatic balance, preventing skin dehydration and reducing skin sensitivity. The combination of quercetin and essential oil can produce a synergistic effect and promote the moisturizing effect on the skin. Quercetin also has antioxidant, anti-inflammatory, anti-allergic and anti-aging effects. The combined antioxidant/anti-inflammatory effects of quercetin are also beneficial for wound healing. Many plant extracts rich in flavonoids, such as mother chrysanthemum, pear pear cactus and bamboo extracts in addition to moisturizing effect, but also have antioxidant, anti-aging and anti-inflammatory biological activities.

polyphenol

Polyphenols are also widely found in plants. Polyphenols, which contain hydrophilic phenolic hydroxyl group, bind with water through hydrogen bond to reduce skin moisture loss and show a certain moisturizing effect. The main component of tea extract is tea polyphenols, and tea polyphenols are typical polyphenols with moisturizing effect. Tea polyphenols not only have the functions of moisturizing, sunscreen, inhibiting tyrosinase and anti-photoaging, but also promote the differentiation of keratinocytes and the formation of skin barrier, and promote wound healing. In addition, tea polyphenols can also shift the moisture rich skin tissue fluid and reduce the viscosity of the cell space. Tea polyphenols have better hygroscopicity and water retention ability when the humidity is relatively low. Even when used in cosmetic formulations, it works better in a relatively dry environment. Proanthocyanidins are a kind of polyphenol polymer, whose phenolic hydroxyl group has strong hydrophilicity, can absorb water, maintain the water content of the stratum corneum, and produce moisturizing effect. The water-soluble polyphenol compounds in plant extracts, the more phenolic hydroxyl groups in the polyphenol structure, the stronger the hydration ability, the more obvious the moisturizing effect.

In addition, the composition of many plant extracts is more complex, and there are many types of natural products. For example, the main components of the extract of chestnut tree and hypericum perforatum are saponins, polyphenols and flavonoids, which not only have protection