Plant Extracts as Skin Care and Therapeutic Agents

Abstract

1. Introduction

The skin consists of the epidermis and the dermis, below which lies subcutaneous tissue. The five-layer epidermis consists of keratinocytes—cells taking part in keratinization, melanocytes—pigment cells, Langerhans cells, mastocytes and Merkel cells. The dermis is composed of connective tissue and consists of a papillary layer and a reticular layer. It contains fibroblasts, which are responsible for the production of collagen, elastin and glycosaminoglycans (GAG), as well as numerous blood vessels, nerve endings and appendages, including hair follicles and sweat and sebaceous glands (Figure 1). The skin performs multiple complex functions; it takes part in metabolic and homeostatic processes and is responsible for the excretion, selective absorption and storage of substances. In addition, it protects against biological (e.g., microbes), physical (e.g., UV radiation) and chemical factors [1,2].

Botanical ingredients are one of the main sources of materials that are used in the cosmetics and pharmaceutical industries. Recent years have seen increasing interest in dermocosmetics and cosmeceuticals produced from plant materials, and thus, there has been greater interest in plant-based products with skin care properties. Plant materials can be applied topically for skin care purposes, as well as for the treatment of numerous skin diseases [2] (Figure 2). Their advantage is that they are gentle but effective, safe and non-toxic, without side effects. Cosmetics fortified with bioactive compounds are ideally suited to the needs of the skin and are more environmentally friendly than conventional cosmetics. A group of natural ingredients widely used in cosmetics is plant extracts, which are a rich source of biologically active substances significantly affecting human skin. They may exhibit a wide range of properties, both medicinal (in certain skin disorders, including inflammatory disorders such as acne, psoriasis or atopic dermatitis) and for use in skin care (e.g., antioxidant, antibacterial, astringent, moisturizing, regenerating, cleansing, smoothing or lightening) [3,4]. Plant extracts are obtained via extraction from various parts of raw plants, e.g., using an appropriately chosen solvent, such as water, ethyl alcohol, glycerine, glycols or vegetable oil. Plant extracts are obtained from whole plants or parts of plants (fruits, leaves, roots, bark, stems, branches, seeds or flowers). The composition and properties of plant extracts, which can be found in the formulas of natural cosmetics, depend on a variety of factors, including cultivation and harvest conditions, how and to what extent the material is broken up, or drying and extraction methods. Extracts from whole plants as well as individual chemical substances contained in them are used in cosmetics. Active plant substances are divided into primary and secondary metabolites. The former are basic substances that are essential to the plant for life, constituting building materials and energy sources. They include sugars, fats, proteins, amino acids and enzymes. Secondary metabolites include terpenes, steroids, saponins, tannins, alkaloids, volatile oils, resins, vitamins and phenolics [1,4].

The aim of the present paper is to describe plants as bioactive cosmetic and therapeutic substances. This review focuses on recent studies on the potential uses of plants and their constituents as photoprotective, anti-inflammatory, regenerative, wound-healing, anti-aging, depigmenting, aromatic and coloring agents.

2. Plants as Photoprotective Agents against Ultraviolet-Radiation-Induced Inflammation and Skin Damage

Ultraviolet (UV) radiation is a physical inflammatory, mutagenic and carcinogenic reagent, as well as a strong enhancer of reactive oxygen species (ROS) production. The biological effects of UV radiation on the skin may be the result of early reactions (erythema or sunburn) or long-term reactions (changes related to skin damage at the molecular and biochemical level). The first response of the skin to UV radiation is the activation of inflammation. UVB irradiation of keratinocytes leads to increased synthesis of pro-inflammatory cytokines in the epidermis, e.g., TNF-α (tumour necrosis factor α) and interleukins IL-1, IL-6, IL-8 and IL-10, which then influence immune cell activity. Another important mediator of inflammation induced by UV radiation is cyclooxygenase-2 (COX-2). COX-2 is an enzyme that is responsible for the synthesis of prostaglandins (PG) from arachidic acid; these play an important role in the regulation of the inflammatory reaction of skin exposed to UVB radiation [5,6,7,8]. Moreover, skin cells exposed to UV radiation respond by activating a cascade of signaling pathways. Disruptions in the activation of these pathways induced by UV radiation lead to disturbances of the homeostasis of the skin, changes in gene expression or the regulation of cytokine secretion, or a loss of control over the cell cycle, which in turn can lead to carcinogenesis [9]. Key signaling pathways activated by UV radiation include transcription factor NFκB (nuclear factor of kappa in B cells) and MAPKs (mitogen-activated protein kinases), including p38 kinases (p38 mitogen-activated protein kinases), JNK (Jun N-terminal kinase) and ERK 1/2 (extracellular signal-regulated kinase 1/2). The p38 kinase is activated by a number of pro-inflammatory cytokines or stress factors. Studies suggest that the p38 kinase is involved in the activation of inflammation induced by UVB radiation through the regulation of COX-2 activity, the production of IL-6, IL-8 and TNFα and the synthesis of nitric oxide (iNOS). Studies have shown that the JNK serine-threonine kinase pathway is more strongly activated by UVA radiation than by UVB radiation in human keratinocytes. The type and dosage of UV radiation have also been shown to determine the activation of ERK1/2 [10,11,12,13]. The exposure of human keratinocytes to UV radiation results in ROS accumulation. Oxidative stress may modulate various signaling cascades in human skin cells and mediate MAPK activity, and it may also be associated with elevated levels of activator protein 1 (AP-1) and NFκB in keratinocytes. Prolonged and intense exposure to UV radiation contributes not only to premature skin aging but also to melanoma and nonmelanoma skin cancers (cutaneous malignant melanoma, basal cell carcinoma or squamous cell carcinoma) [2,14].

Selected plant extracts and single compounds with antioxidant, anti-inflammatory and immunomodulatory effects play an important role in the photoprotection of the skin. Phytochemicals have shown the ability to act as free radical scavengers, radical chain reaction inhibitors, metal chelators, oxidative enzyme inhibitors and antioxidant enzyme cofactors. Some studies have reported that plant extracts promote endogenous antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX), which protect the skin against increasing ROS levels under oxidative stress. Moreover, plant materials can modulate the expression and activation of a wide variety of cytokines, such as TNF-α IL-1β, IL-6 and IL-8. Botanicals have also shown the ability to regulate the expression of various pro-inflammatory genes and inhibit the activity of pro-inflammatory enzymes such as inducible nitric oxide synthase (iNOS), COX-2 and lipoxygenase (LOX) [15,16,17].

Plant extracts and natural compounds from plants have been reported in the earlier literature to possess photoprotective properties. These include phytochemicals such as ferulic, caffeic, cinnamic, rosmarinic acid, quercetin, apigenin, rutin, luteolin, chrysin, hesperidin, dihydromyricetin, chrysanthemin, curcumin, genistein, resveratrol, carnosic, ursolic, ellagic, asiatic acid, zerumbone, astaxanthin, β-carotene, lycopene, zeaxantin, lutein and L-ergothioneine, as well as extracts from plants such as Opuntia humifusa [18], Camellia sinensis [19], Punica granatum [20], Hibiscus furcatus, Atalantia ceylanica, Mollugo cerviana, Leucas zeylanica, Ophiorrhiza mungos, Olax zeylanica [21] Silybum marianum [22], Polypodium leucotomos [23], Vaccinium myrtillus [24], Lonicera caerulea [25], Thymus vulgaris [26], Opuntia ficus-indica [27], Morinda citrifolia [28], Galinsoga parviflora, Galinsoga quadriradiata [29], Coffea arabica [30], Amaranthus cruentus, Moringa oleifera, Malaxis acuminata, Schinus terebinthifolius [31], Schinopsis brasiliensis [32], Crataegus pentagyna [33], Sambucus nigra, Helichrysum arenarium, Crataegus monogyna [34], Capnophyllum peregrinum [35], Dalbergia monetaria [36], Baccharis antioquensis [37], Juglans regia [38], Dimorphandra gardneriana and Lippia microphylla [39].

Some plants that are effective UV filters may be potential sunscreen ingredients [40]. These include plant extracts such as Astragalus gombiformis with an SPF value of 38 [41], Sloanea calva with an SPF value of 35.4 [42], Hylocereus polyrhizus with an SPF value of 35.02 [43] or Rosa centifolia with SPF values of 32 [44]. Moreover, plant extracts, through their synergistic effects with some physical or chemical UV filters (e.g., benzophenone-3 (BP-3), octyl methoxycinnamate (OMC) or titanium dioxide (TiO₂)), may also play a role as cosmetic components that enhance the SPF of sunscreen formulations [40]. This effect has been shown for extracts from Sanionia uncinata [45,46], Vitis vinifera [47], Nephelium lappaceum [48], Psidium guajava [49], Campomanesia adamantium and Campomanesia xanthocarpa [50], as well as moss extracts from Leucobryum spp. and Holomitriopsis laevifolia [51].

Table 1 presents the results of research from the last five years on the protective effects of plant-derived products on UVB-mediated damage, with potential applications in photoprotective products.

3. Plants as Regenerative and Wound-Healing Agents

The process of the regeneration and healing of the skin involves interactions between many types of cells, including endothelial cells, inflammatory cells, keratinocytes and fibroblasts. It consists of stages such as coagulation (haemostasis, fibrin clot formation and activation of the clotting cascade by platelets), inflammation (neutrophil and monocyte migration, phagocytosis of bacteria and the release of proteolytic enzymes to debride the wound), proliferation (angiogenesis by endothelial cells, granulation tissue formation by fibroblasts and reepithelialization by keratinocytes) and tissue maturation (collagen/ECM remodeling by fibroblasts) [82,83,84] (Figure 3). An important step in tissue formation, repair and the maintenance of good skin conditions is proper cell proliferation and migration processes. These depend on many factors, such as biochemical communication, adhesion strength and mechanical flexibility, as well as organization of the cellular cytoskeleton [85,86,87]. Numerous regulators take part in keratinocyte migration and proliferation, including epidermal growth factor (EGF), insulin-like growth factor 1 (IGF-1), fibroblast growth factor (FGF), granulocyte-macrophage colony-stimulating factor (GM-CSF), angiopoietin-related growth factor (AGF), vascular endothelial growth factor (VEGF), transforming growth factor β (TGF-β), connective tissue growth factor (CTGF), platelet-derived growth factor (PDGF) and platelet derived-endothelial cell growth factor (PD-ECGF). In addition, cytokines (e.g., IL-1, IL-6 and TNF-α), neuropeptides (G protein-coupled receptor (GCRP), vasoactive intestinal peptide (VIP) and substance P (SP)), MMPs and extracellular macromolecules also play various roles in the regulation of skin cell motility and proliferation [88,89].

A wound is an injury involving a breach of the integrity of the skin. A chronic wound may lead to complications, such as bacterial infections. Bacterial infections also delay the wound-healing process, prolonging inflammation. The surface of human skin is colonized by commensal bacteria with low virulence, such as coagulase-negative staphylococci and non-pathogenic corynebacteria and cutibacteria, but also by opportunistic pathogenic microbes (such as Candida spp., Malassezia spp. or Staphylococcus aureus) and bacteria with high pathogenic potential (e.g., Streptococcus pyogenes). The skin of hospitalized patients who have undergone antibiotic treatment may be colonized by Gram-negative non-fermenting bacteria (Pseudomonas aeruginosa or Acinetobacter baumannii) or yeasts, including the opportunistic pathogen Candida auris. The choice of treatment for skin and wound infections depends on various factors (e.g., the severity of the disease or host factors), but plants and drugs of natural origin can undoubtedly have broad applications alongside topical synthetic antibiotics and antiseptic agents [92,93,94].

Botanicals have been used topically for decades for skin regeneration and the treatment of dermatological problems, such as chronic diabetic wounds, ulcers, bedsores, burns and non-healing wounds. Numerous plants and drugs of natural origin support the normal repair systems of the skin and therefore show great therapeutic potential in skin regeneration and wound treatment by various mechanisms. These include effects on keratinocyte migration and proliferation rates, modulation of the release of various growth factors, cytokines, chemokines or neuropeptides by skin cells, increasing the formation of capillary vessels and increasing fibroblast activity. Another important group of raw materials comprises plants with astringent and antimicrobial properties, which contribute to wound contraction and increase the rate of epithelialization [83,84,95]. The scientific literature points to the important effects of plants (e.g., Achiella millefolium [96], Aloe vera [97], Althaea officinalis [98], Calendula officinalis [99], Curcuma longa [100], Eucalyptus globulus [101], Simmondsia chinensis [102], Pinus sylvestris [103] and Camellia sinensis [104]) and phytochemicals (e.g., triterpenes, alkaloids and flavonoids) on tissues and their potential to amplify skin regeneration and accelerate the process of wound repair and healing [84,95].

Table 2 cites some original research carried out in the last five years on selected plants and their constituents, as well as formulations based on raw materials of plant origin exhibiting wound-healing activity and potential applications in regeneration and skin treatment.

4. Plants as Anti-Aging Agents

Preventing and combating signs of skin aging (dry skin, loss of firmness and elasticity or wrinkles) is an age-old challenge. The skin is the organ on which these processes are most noticeable, hence the great interest in age-related changes at the level of the epidermis, dermis and subcutaneous tissue. At the level of the epidermis, changes observed with age include (1) thinning of all layers of the epidermis and flattening of the dermo-epidermal junction; (2) disturbances of the production of natural moisturizing factor (NMF), leading to dryness and increased peeling of the epidermis; (3) a reduction in the level of epidermal lipids (mainly ceramides); and (4) oxidation of lipids of intercellular cement, leading to increased transepidermal water loss (TEWL) [144,145]. Disturbed production of lipids binding the corneocytes of the stratum corneum not only causes skin dryness but also disrupts the process of the exfoliation of keratinized epidermal cells. This is linked to the malfunction of enzymes, enabling exfoliation when the water content in the epidermis is low. For example, a deficiency of linoleic acid, a component of ceramide 1, with an important role in the cohesiveness of cement, is associated with dry skin symptoms [146,147]. Major age-related changes in the dermis include (1) a reduced number and activity of fibroblasts, which are cells that are responsible for the synthesis of collagen fibers, elastin fibers and hyaluronic acid; (2) degradation of collagen fibers, progressive collagen cross-linking and a reduction in skin resilience and resistance to stretching; (3) changes in the structure of elastin fibers, which clump together in an amorphous mass (elastosis), loss of elasticity and wrinkle formation; and (4) a reduction in hyaluronic acid, with insufficiently moisturized and resilient skin [144,145].

Over the centuries, the search for new substances to slow down the aging process and restore the skin’s young appearance has not diminished. Bioactive substances with anti-aging properties include moisturizers, which influence the hydrolipid barrier and minimize destructive lesions occurring in the stratum corneum. The skin may be hydrated through the external supply of water from moisturizing agents or via the application of agents forming an occlusive lipid film to slow down water loss from the skin. An important group of anti-aging agents comprises bioactive substances, which take part in the synthesis and metabolism of skin components (e.g., proteins and essential unsaturated fatty acids) and also exhibit collagenase, elastase and hyaluronidase inhibitory activity [1,144]. Collagenase is an enzyme belonging to the family of matrix metalloproteinases (MMP), which can degrade collagen, the fibrous component of the extracellular matrix (ECM) and the major structural protein in human skin. Elastase is a proteolytic enzyme involved in the degradation of elastin, a protein responsible for skin elasticity. Hyaluronidase is an enzyme (an endoglycosidase) responsible for the hydrolysis of hyaluronic acid, a skin glycosaminoglycan, which is a major component of ECM [148,149].

Botanicals that support the health, texture and integrity of the skin are widely used in cosmetic formulations for dry and mature skin. Plant extracts and natural products are recommended because they increase skin hydration, reduce TEWL, display skin-barrier-reinforcing properties, inhibit the degradation of skin components and help to maintain the integrity of the skin’s structure. These are promising approaches to preventing skin aging using products derived from plants. Plants can be a very interesting source of ingredients with potential anti-aging properties, as confirmed by the results of in vitro studies. However, further research is needed to confirm the efficacy of plant-derived materials in vivo, as the most important factor determining the effectiveness of active ingredients of natural origin is their bioavailability. In some studies, plants have been shown to exert notable in vivo anti-aging properties. According to the literature, skin parameters associated with skin aging, such as skin hydration (measured with a corneometer and tewameter), skin elasticity (measured with a cutometer and elastometer) or facial wrinkles (measured with a skin visiometer and camera for skin analysis) have been evaluated following the application of cosmetic formulations based on various plant extracts, alone or in combination [119,150,151,152]. Table 3 cites research from the last five years on selected plant species and their extracts with potential uses as agents preventing and slowing down skin aging.

5. Plants as Anti-Tyrosinase Agents

Tyrosinase is an enzyme that is widely distributed in the cells of animals, plants and microorganisms. It is a key enzyme in the biosynthesis of melanin, responsible for the catalysis of the first two synthesis reactions, i.e., the hydroxylation of tyrosine to DOPA and the oxidation of DOPA to dopaquinone. At the stage of dopaquinone formation, the eumelanin and pheomelanin pathways are separated. When thiol compounds (cysteine and glutathione) are present, they attach to dopaquinone, and the biosynthesis pathway is redirected toward pheomelanin. When the L-tyrosine concentration is low and that of cysteine is high, cysteine attaches to dopaquinone, and cysteinyldopa isomers are formed [170,171]. In the absence of thiol compounds, highly reactive dopaquinone easily undergoes intracellular cyclization, oxidation and transformation to dopachrome [170,171,172,173]. In the presence of TYRP2 (tyrosinase-related protein 2, also called dopachrome tautomerase—DCT) or metal cations (Cu²⁺, Zn²⁺, Fe²⁺, Co²⁺ or Ni²⁺), dopachrome may be converted to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) [170,173]. In the absence of DCT, dopachrome is converted to 5,6-dihydroxyindole (DHI) by nonenzymatic decarboxylation [170]. TYRP1 (tyrosinase-related protein 1) causes the oxidation of DHICA to indole-5,6-quinone-2-carboxylic acid, and TYR causes the oxidation of DHI to indole-5,6-quinone. The polymerization of the resulting monomers (indole and quinone) leads to the formation of eumelanin [171,173] (Figure 4).

As a metalloenzyme, tyrosinase has two copper atoms in its active site, determining its catalytic function. Substances belonging to the group of tyrosinase inhibitors inhibit melanin synthesis by interacting with copper ions in the active site of tyrosinase, thereby reducing the activity of the enzyme [175,176].

In recent years, anti-tyrosinase agents have attracted the attention of researchers searching for substances that can whiten the skin and also treat skin pigmentation disorders. Ongoing research indicates that many plant extracts and plant-derived chemicals are strong tyrosinase inhibitors and prevent the overproduction of melanin in the epidermal layers. At the same time, importantly, they inhibit melanogenesis without exerting cytotoxic or mutagenic effects on melanocytes [175,177,178,179]. Constituents of plant extracts with depigmenting properties resulting from the inhibition of tyrosinase activity include arbutin (found in, e.g., Pyrus pyrifolia peel (3.35 mg/g) [180], Origanum majorana herbs (51.3 mg/g) [181], Arctostaphylos uva–ursi leaves (6.4%) [182], Vaccinium vitis idaeae leaves (46.78 mg/g) [183] or Bergenia crassifolia leaves (22.59%) [184]), coumaric acid (present in, e.g., Artocapus altilis fruits (11.85 mg/100 g) [185,186]), ellagic acid (occurs in, e.g., Juglans regia leaves (16.25%), Castanea sativa stem bark (2.75%) or Eucalyptus camaldulensis leaves (0.28%) [187]), aloesin (isolated from the Aloe vera leaves (64 mg/L) [188]), baicalein (present in Scutellaria baicalensis roots (16.61 mg/g) [189,190]) and glabridin (found in Glycyrrhiza glabra roots (22.87 mg/g) [191]).

Table 4 presents research from the last five years on various plant species tested for anti-tyrosinase activity with potential uses in products for depigmenting or lightening the skin.

6. Plants as Aromatic Agents

Over the centuries, the aromatic applications of plant extracts have gained importance. Plant essential oils, considered to be those with an oil content above 0.01% of the fresh weight of the plant, are of particular importance. Some plant materials may contain even 20% essential oils (EOs) [256,257,258]. EOs are mainly obtained from plants of the Apiaceae, Asteraceae, Lamiaceae, Lauraceae, Myrtaceae, Rutaceae, Verbenaceae and Geraniaceae families [257,259] (Table 5). EOs can be found in all parts of the plant, i.e., the flowers (rose, lavender, jasmine or ylang-ylang), leaves (eucalyptus, peppermint, geranium, rosemary or tea tree), herbs (basil, hyssop and lemon balm), roots (ginger and vetiver), wood (cedarwood, camphor and sandalwood), bark (cinnamon and myrtle), seeds (anise, cumin, cardamom and fennel) and fruits (pepper, nutmeg and juniper). They are obtained from raw plant materials via distillation (water, steam or dry distillation), extraction (microwave, ultrasound, solvent extraction, maceration or enfleurage) or mechanical or cold pressing. EOs are mixtures of volatile substances, mostly colorless or light yellow, with an intense odor and an oily consistency, and they are soluble in liquid fats, alcohol, ether or chloroform. The biological activity and fragrance of EOs are determined according to their chemical composition. Their composition depends on numerous factors, including the origin of the plant materials or the conditions of plant growth. EOs are not chemically homogeneous. They may contain up to several hundred chemical compounds, including terpene hydrocarbons and their oxygen derivatives, alcohols, aldehydes, ketones, organic acids, esters and ethers [256,257,259,260]. Some compounds of EOs have a characteristic aroma, e.g., bisabolol, with a sweet floral odor; geraniol, with a fresh, sweet and rose-like odor; linalyl acetate, with a floral, sweet citrus odor; citronellol, with a strong floral, rose-like and sweet odor; limonene, with a strong orange odor; linalool with a floral, grassy, pleasant and citrus odor; myrcene, with a pleasant floral odor; terpineol, with a sweet, lilac odor; α-pinene, with a fresh, camphor, sweet and pine odor; or β-phellandrene, with a mint, turpentine odor [260].

Cosmetic aromatherapy utilizes EOs for skin, body, face and hair products. EOs are added to skincare and bath cosmetics or massage preparations as substances providing fragrance and as active ingredients. Smell is an important criterion in purchasing cosmetic products. A wide range of essential oils is available, and their marketing potential is enormous. Fragrance composition is an important element of the formulation of new cosmetic preparations. Fragrances also play an important role in masking unpleasant aromas from fatty acids, oils and surfactants used in cosmetic formulations [256,258,260].

EOs and their constituents, in addition to their aromatic effects, are also used in modern cosmetics and dermocosmetics as absorption promoters and preservatives [258]. The absorption of active substances by the skin can also be increased by EOs, such as eucalyptus, peppermint or terpentine oil, as well as by components of essential oils, such as menthol, limonene, carvacrol, linalool, α-pinene or terpineol [258,259]. Due to their antimicrobial action, EOs can act as natural preservatives to prolong the durability of cosmetics, e.g., essential oils from lavender (Lavandula angustifolia) [261], thyme (Thymus vulgaris) [263], peppermint (Mentha piperita) [264], cajuput (Melaleuca cajuputi), cinnamon (Cinnamomum zeylanicum) [271], clove (Syzygium aromaticum) [275], eucalyptus (Eucalyptus globulus) [273], sage (Salvia officinalis) [277] and tea tree (Melaleuca alternifolia) [274]. EO constituents performing this function include phenols, aldehydes, alcohols, ketones and esters [258,259].

The use of EOs may have side effects, such as allergic reactions, irritation or temporary sensitivity to UV radiation. An allergic reaction or skin irritation may occur following the use of cinnamon, clove or lemon grass oil, and oils with a photosensitizing effect include citrus oils (e.g., bergamot, lime, bitter orange, lemon or grapefruit), as well as EOs present in angelica root (Angelica archangelica), rue (Ruta graveolens), parsley leaf (Petroselinum crispum) and marigold (Tagetes minuta). Constituents of EOs that may trigger allergic reactions include benzyl alcohol, cinnamyl alcohol, eugenol, hydroxycitronellal, isoeugenol, benzyl salicylate, cinnamaldehyde, coumarin, geraniol, anisyl alcohol, benzyl cinnamate, farnesol, linalol, benzyl benzoate, citronellol or limonene [258,259,260]. EO safety in the cosmetic industry is monitored in a variety of ways, e.g., by the International Fragrance Association (IFRA) and the International Organization for Standardization (ISO) [260].

7. Plants as Colorants and Dye Agents

The history of the human use of pigments dates back to prehistoric times. Dye plants that are known to have been used in various periods include dyer’s madder (Rubia tinctorum), true indigo (Indigofera tinctoria), dyer’s woad (Isatis tinctoria), dyer’s weed (Reseda luteola) and logwood (Haematoxylum campechianum) [278]. Dyes that are currently used in cosmetics were once used in various branches of industry. It is believed that dyes were originally used for ornamental purposes. In ancient Egypt, mainly the skin and hair were dyed, e.g., using henna (a pigment obtained from the shrub Lawsonia inermis). In modern cosmetology, plant pigments are added to cosmetic products to give them an aesthetic appearance. Like aroma, color plays an important role in marketing cosmetics and pharmaceutical products [278,279,280]. In addition, colorants and dyes are used as beauty enhancers, masking imperfections or correcting minor skin defects. Apart from color cosmetics (e.g., fluids, lip pencils, lipstick, rouge or eyeshadow), plant pigments are also a component of skin care cosmetics with protective and antioxidant properties, with the ability to strengthen blood vessels and improve the condition of skin [281,282].

Plant dyes, which are varied in terms of chemical structure, are a group of compounds that are present in plant parts such as flowers, fruits and leaves. Plant pigments include quinones, polyphenols, chlorophylls, carotenoids and betalains [279,281,282,283,284] (Table 6).

Quinones are compounds whose color ranges from yellow to orange to red to brown. Quinones, which include benzoquinones, naphthoquinones and anthraquinones, are a large group of pigments. Anthraquinones are anthracene derivatives that are widespread in the plant world. They can be found among plants of the Polygonaceae, Rubiaceae, Rhamnaceae, Scrophulariaceae, Liliaceae, Hypericaceae and Fabaceae families. In traditional dyeing, hypericin, a red dye obtained from St John’s wort (Hypericum perforatum), was used as well. Natural fibers were also dyed using rhamnotoxin—a red pigment obtained from the bark of alder buckthorn—as well as with alkannin, from the rhizomes and roots of dyer’s alkanet (Alkanna tinctoria). This dye has been used since ancient times in color cosmetics, such as lipsticks. Another source of alkannin, which is a naphthoquinone derivative, is the root of common bugloss (Anchusa officinalis) [279,281,282].

A wealth of flavonoids can be found in plants of the Apiaceae, Asteraceae, Betulaceae, Polygonaceae, Brassicaceae, Ericaceae, Fabaceae, Hypericaceae, Primulaceae, Lamiaceae, Rosaceae, Rubiaceae, Rutaceae and Scrophulariaceae families. Apart from their role in skin care, flavonoids are used in cosmetics as natural plant dyes, including flavonols (intense yellow), flavones (light yellow and cream-colored), chalcones (light yellow) and aurones (intense yellow) [279,281,282].

Anthocyanins are widespread plant dyes, the most common of which include red pelargonidin (geranium and dahlia), blue-to-red peonidin (elderberry and peony) and cyanidin (cornflower, chokeberry, cranberry and cherry), purple malvidin (mallow and grapes), petunidin (petunia) and delphinidin (grape, elderberry and cranberry). Tannins are broadly distributed in the plant kingdom and are generally classified into two types: hydrolysable tannins (e.g., gallotannins and ellagitannins) and condensed tannins (catechins and leucoanthocyanidins). Plants supplying brown, gray or sometimes rust-colored tannin dyes include the species Uncaria gambir, Galla chinensis (Chinese gallnut), Acacia catechu, Schinopsis balansae, Pteropcarpus marspinum, Eucalyptus rostrata, Quercus infectoria, Quercus robur, Quercus sessilis, Potentilla erecta, Alchemilla vulgaris, Sanguisorba officinalis and Polygonum bistorta [279,281,282,285].

Chlorophylls are a pigment that is present in all green plants (in the stems, leaves, flowers, fruits or seeds), e.g., Urtica dioica, Medicago sativa, spinach, lettuce and broccoli. Among the known plant chlorophylls, two have significance as dyes: chlorophyll a (blue-green) and chlorophyll b (yellow-green). Chemically, chlorophyll is an ester (magnesium porphyrin composed of four pyrrole rings) with two alcohols (phytol and methanol) [280,282].

Carotenoids are polyene dyes, i.e., they have a conjugated double-bond system. Plant sources of carotenoids include Crocus sativus, from which the stigma, containing the yellow carotenoid pigment crocin, is used; Bixa orellana, whose fruits supply the yellow-orange carotenoid pigment bixin (annato, orlean); and Calendula officinalis, whose flowers contain α- and β-carotene, lutein, lycopene and violaxanthin [281,282,286].

Betalains are found in plants of the order Caryophyllales. Sources of betalain pigments include beet root (Beta vulgaris), the fruits of the prickly pear (Opuntia ficus-indica) or cacti of the Hylocereus genus and the flowers of numerous species of the Amaranthaceae family [281,282].

Natural colorants and dyes of plant origin have the important advantages of being nontoxic, safe, without side effects, non-carcinogenic, environmentally friendly (biodegradable and compatible with the environment) and economical. For these reasons, they are becoming an object of consumer interest with broad applications in the cosmetic industry. Plant dyes can be an alternative to synthetic dyes, which involve the use of petrochemical-based materials, and due to their allergic, toxic, mutagenic, genotoxic and carcinogenic effects, they are responsible for various health and skin problems [280,283,287].

8. Future Perspectives and Challenges

In the European Union, before cosmetic products can be sold to customers, they must be evaluated for safety in accordance with Regulation (EC) No. 1223/2009 of the European Parliament and of the Council, and in the United States, the safety of cosmetics is regulated by the Food and Drug Administration (FDA), mainly through the Federal Food, Drug, and Cosmetics Act (FD&C Act) and the Fair Packaging and Labeling Act (FPLA). The global cosmetics industry (encompassing products for the face, eyes, hair, nails, mouth and body, which may be used externally for cleansing, beautifying or altering one’s appearance) is continually growing, together with consumer awareness regarding health care, including hygiene and skin care [288]. Among the entire range of cosmetics, plant-based products have seen tremendous growth of about 15–20 per cent over the past five years. This review presents a wide assortment of plants with various applications in cosmetic preparations that have been reported in the last five years. It is also important to consider certain aspects of the use of plants and bioactive compounds of plant origin in cosmetics and the associated challenges.

First, attention should be paid to the ability of active ingredients of natural origin to penetrate the first skin barrier, as the bioavailability of bioactive compounds is an important factor determining their effectiveness. One promising solution for the future is the development of delivery systems for bioactive ingredients that facilitate penetration, through improved encapsulation and targeted delivery. A related issue is the fact that the effects of these agents have not been conclusively demonstrated in all cases. For example, although some natural agents appear to have promising sun-protection effects, when they are added to sunscreens, this effect has been shown to be poor and to ensure only a modest or low increase in SPF (e.g., lycopene [289] and Cucumis sativus extract [290]). Therefore, in vitro research into the biological activity of plants must also be supported by in vivo studies. Even when preliminary studies show promising effects, confirmation in clinical trials is needed.

Second, it is important to consider the mechanism of action and the safety of plant-derived bioactive ingredients. A good example is bergamot oil. The use of methoxypsoralens from the Citrus bergamia essential oil following sun exposure has been shown to increase photosensitivity, causing further damage rather than providing photoprotection, despite its stimulating effect on tyrosinase activity [291]. Other adverse effects, such as acute toxicity, skin and eye irritation or skin sensitization, may occur following the topical application of materials of plant origin. This is why it is essential to conduct research not only on the effectiveness of these substances but on their safety as well, prior to including them in a cosmetic formulation.

In addition, discussions about ingredients of plant origin and their biological activity should take into account their chemical structure. One example is the role of flavonoids and their effect on melanogenesis in relation to the chemical structure of this complex group of compounds. For example, hesperetin [292] and genistein [293] have been shown to stimulate melanogenesis, whereas compounds such as epicatechin (EGCG) [294] or baicalein [189] act as inhibitors of melanin formation. It is interesting to compare the two structurally similar compounds apigenin and luteolin. One additional hydroxyl group in luteolin results in different cellular functions: apigenin stimulates melanin synthesis [295], whereas luteolin inhibits it [296]. This suggests that the characteristic chemical structure of individual bioactive compounds leads to differences in how they regulate melanogenesis. Conflicting reports in the scientific literature regarding quercetin may also be puzzling, as some data suggest that it stimulates melanogenesis [297], whereas other data indicate an inhibitory effect against melanogenesis [298]. This demonstrates that there is still a need for in-depth research leading to a better understanding of these plant-derived molecules.

Another important consideration is how the plant material to be used as a cosmetic component is obtained (e.g., the extraction/separation technique, temperature or type of solvent used). Some of the active compounds present in plants (e.g., polyphenols, essential oils or vitamins) have low stability, and their sensitivity to light and heat limits their use in cosmetics. Research in this area is aimed at the development of more stable derivatives or the encapsulation of active substances in liposomes, which protects them from degradation.

The implementation of new solutions for obtaining and preparing plant-derived materials and including them in a cosmetic product is associated with the issue of intellectual property. The mechanisms of the legal protection of innovations, such as patents, are also worthy of attention. Naturally, not all research results can be patented. In the context of plant-based cosmetic materials, no plant or substance extracted from it can be protected by the patent system; however, a complex or mixture of plant extracts or isolated molecules, if it meets the criteria of novelty, inventive activity and industrial application, is patentable [299,300]. Patents involving pharmaceutical and cosmetic applications may refer to the ingredients, formulation, product type, use of pharmaceutical carrier systems or cosmetic production/manufacturing methods [300]. In patents filed in the National Institute of Industrial Property (INPI), types of applications of plant extracts in cosmetics include multifunctional product innovation (e.g., the use of a plant extracts for the treatment of gynoid lipodystrophy and acne), extraction processes used to isolate active ingredients with potential applications in cosmetics and the use of extracts with anti-aging, skin/hair pigmentation and conditioning or photoprotection properties [299,301]. An analysis of patents related to cosmetics containing plant ingredients reveals a high proportion of innovations involving the use of species from the Fabaceae, Asteraceae, Rosaceae, Lamiaceae, Poaceae, Rutaceae, Lilliacae and Apiaceae families [301]. Examples of plants described in patents for cosmetic applications include Pothomorphe umbellata root extract for anti-aging activity and the treatment of cell damage caused by exposure to UV rays; Glycyrrhiza glabra and Shophora flavecens roots for the treatment of skin hyperpigmentation; the Artemisia plant species for whitening the skin and delaying aging; or the Pueraria plant species for rejuvenation, lightening the skin and treating skin inflammation [299,301].

9. Conclusions

Plants and their constituents can be used to maintain the physiological balance of human skin. Ongoing research provides valuable information on the chemical composition and pharmacological properties of botanicals. Moreover, studies have confirmed their effectiveness and have demonstrated new potential applications of plant materials in products for topical use as skin care and therapeutic agents with multifaceted effects. Natural products of plant origin can be used as a safe and efficacious alternative to synthetic products. This is reflected in growing consumer interest in natural cosmetics and the market trend expressed by the development and increasing number of products based on plant-derived ingredients.

Funding Statement

This project was financed under the program of the Minister of Education and Science ‘Regional Initiative of Excellence’ in the years 2019–2023, project no. 024/RID/2018/19, with an amount of funding of 11,999,000.00 PLN.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The author declares no conflict of interest.

Footnotes

References