Essential Oils as Dermocosmetic Agents, Their Mechanism of Action and Nanolipidic Formulations for Maximized Skincare

Abstract

Essential oils (EOs) are known for their diverse bioactivities, including antioxidant, anti-inflammatory, antibacterial, antifungal, antiviral, skin-barrier repairing and anticancer, and therefore, hold profound potential to be used in cosmetic and skincare products. Owing to these properties, EOs have long been utilized to address a range of dermatological issues, from acne and inflammation to aging and dryness. However, problems associated with EOs beset their practical applications, which include high volatility, oxidation, hydrophobic nature, low bioavailability, skin irritation, chemical transformation and poor stability in air and light. A prospective of nanolipidic formulations, including the nanostructured lipid carriers (NLCs) and solid lipid nanoparticles (SLNs) system for improved skin delivery of these EOs highlights the possibility of their use in topical applications, which offer several advantages such as improved bioavailability and stability, lower toxicity and higher drug content. These nanoformulations protect the EOs from environmental degradation and improve their penetration into deeper skin layers, leading to prolonged therapeutic benefits. The delivery of bioactive agents using a conventional topical preparation exhibits low penetration, frequent applications, poor adherence and prolonged therapy duration, whereas the novel delivery system exhibits improved stability of the drug, enhanced skin penetration, enhanced retention and better therapeutic efficacy. This review provides a comprehensive compendium of information on EOs, which are widely used in skincare, along with their nanolipidic formulations for maximized skincare uses. The mechanism of action of EOs as skin bioactive agents, challenges associated with their use, advances in nanolipidic formulations and their market value as cosmetic skincare products are also explored.

1. Introduction

Cosmeceuticals is one of the fastest growing industries in personal and health care products such as skin, hair, nail and lip care and conditions such as photo-aging, anti-wrinkle crèmes, hyper-pigmentation, hair damage control and anti-dandruff shampoos. Plant-derived cosmeceuticals, including the EOs, are in greater demand than their synthetic counterparts owing to the diverse biological activities associated with them, such as anti-aging, anti-inflammatory, antioxidants, antibacterial, sun-protective, wound healing and anticancer activities [1,2]. EOs are concentrated plant extracts that contain volatile aromatic compounds and are rich in bioactive phytochemicals. EOs, such as those derived from lavender, rosemary, tea tree and chamomile, are shown to possess various biological activities and have been used widely in traditional medicine for centuries [3]. The delivery of these phytoconstituents in nanosized cosmetic formulations is an innovative breakthrough approach, as these natural compounds often face the issues of poor water solubility, reduced skin permeation and instability issues [4,5,6]. Compared to conventional systems, these novel systems have the advantages of controlled and sustained release of phytochemicals, higher stability profiles, and enhanced skin penetrations [7]. Topical and transdermal skin delivery systems have gained much popularity and witnessed a growing shift toward the use of natural ingredients in recent times; profound research is ongoing in these areas. Despite their popularity, several challenges, including the stability, volatility and risk of skin irritation, have hindered the full potential of EOs in skincare formulations [8].

Nanotechnology is the answer to the technological formulation issues often encountered by these phytoconstituents, providing innovative solutions to address these limitations. Nanotechnology can improve the solubility of poorly soluble compounds, facilitate skin permeation and increase stability against light and temperature [9,10]. Nanosizing phytocompounds enhances their applications in skin-based therapy with sustained delivery and enhanced activities. Various nanocarriers such as liposomes, SLNs, transferosomes, ethosomes, NLCs, fullerenes, carbon nanotubes and cyclodextrins are being used in skincare products to achieve enhanced delivery of phytoconstituents, which can further be incorporated into gels, hydrogels, lotions and crèmes for skin and hair care products. These lipid-based nanocarriers enhance the stability of EOs, protect them from environmental degradation and improve their penetration through skin bilayers. The nanocarrier encapsulates the phytocompounds in size ranging from 1 to 100 nm and delivers them in a controlled manner into the layers of the skin [10,11]. Nanolipidic phytoconstituent formulations are emerging trends in skin care products, with the combined advantages of nanotechnology and phytoconstituents (Figure 1).

Nanocosmetology, through the nanolipidic systems, is an upcoming field of research and development. Among these technologies, lipid-based nanocarriers, such as SLNs and NLCs, have shown great promise for the encapsulation and delivery of EOs in dermocosmetic products. Both NLCs and SLNs are composed of biocompatible and biodegradable lipid materials and are capable of encapsulating lipophilic compounds such as EOs. However, there are a few limitations associated with the use of SLNs, including lower drug loading capacity and increased drug expulsion during storage [12]. These limitations can be minimized by using NLC systems; as lipid particles with a controlled nanostructure, NLCs can improve drug loading and firmly incorporate the drug during storage. Owing to their advantages, NLCs have garnered extensive attention and applications in topical drug delivery, as well as oral and parenteral administration of pharmaceuticals, and are now being explored in cosmeceuticals [13,14].

Skin-related problems and dermatological issues increase the demand for therapeutics and their delivery to the skin through topical or transdermal routes. The topical/transdermal delivery of phytoconstituents and drugs is often associated with challenges imposed by the skin, such as hindrance by the upper layer of the epidermis (stratum corneum). Conventional approaches, such as the use of chemical penetration enhancers and physical methods, such as sonophoresis, ionophoresis, electroporation, microneedles, etc., have been used to deliver drugs topically [15]. These conventional methods have their own set of drawbacks. Novel approaches have been explored to overcome the disadvantages of conventional approaches, enhancing the dermal drug disposition [16]. The development of nanocosmeceuticals of phytoconstituents requires a thorough understanding of the mechanism of drug permeation, technological advancement in methodology and physiochemical properties of drugs and carriers. These products promise to deliver the phytoconstituents to deeper layers of the skin, bypassing the barrier function of the skin [17].

The topical drug delivery route has its own set of advantages compared to oral drug delivery, such as patient compliance, avoidance of first-pass metabolism and local delivery of the active ingredients, but the skin barriers such as epidermis, dermis and hypodermis make it difficult for the formulation to permeate [18]. Natural oils have been widely used all over the world for many centuries for their better therapeutic values and fewer side effects. However, their delivery needs a scientific approach to deliver them into the skin in a controlled manner to increase patient compliance and effectiveness. Delivering these oils through a nanocarrier increases their therapeutic value by increasing their bioavailability [19]. Figure 2 depicts the reasons for the restricted use of EO applications in dermocosmetic preparations. These applications of EOs in the fields of food, pharmaceuticals, nutraceuticals and skincare can be considerably improved by delivering them in nanolipidic systems. These systems have the ability to protect the oils from environmental conditions and improve their bioavailability, stability and release profile [20,21].

The present review aims to provide comprehensive insight into natural EOs as dermocosmetic agents, focusing on their mechanism of action, clinical evidence supporting their use and their delivery through nanolipidic systems. The formulation development strategies, production and application of these nanosystems in various skin conditions are discussed as an emerging field of innovation. These novel delivery systems increase drug transport across the skin and enhance solubilization of the phytocompound, drug partitioning and fluidization of skin lipids. Lipid-based delivery systems are regarded as a safe, efficient and attractive delivery strategy for phytoconstituents [22,23]. By exploring the intersection of natural bioactive compounds and nanotechnology, this review highlights the growing potential for EOs to be utilized in advanced dermocosmetic applications that offer safer, more effective and sustainable skincare solutions.

2. Mechanism of Action of EOs as Dermocosmetic Agents

EOs are concentrated aromatic oils, primarily terpenes, extracted from plant sources and are known for their wide applications in perfumes, cosmetics, flavoring and cleaning products. Some common EOs include cedarwood, lemongrass, lavender, chamomile, tea tree, peppermint, eucalyptus, rose, geranium, lemon, myrrh, sandalwood, thyme, cinnamon, frankincense, juniper, jasmine, grapefruit, orange, mandarin and rosemary oil. The medicinal benefits of EOs for numerous skin diseases are widely reported in the literature owing to their antioxidant, anti-inflammatory, antimicrobial, anticancer, modulation of skin barrier function and wound healing properties [24,25]. The skin is often the subject of numerous concerns, including aging, inflammation, psoriasis, microbial infections, damage from environmental stressors and wounds. The realms of natural phytomolecules with potent antioxidant ability counter the phenomenon of cellular aging in humans [26]. Understanding the underlying mechanisms by which these EOs exert these effects is crucial to optimizing their use in skincare formulations. The major mechanisms by which the EOs exert their biological actions include antioxidant, antimicrobial, anti-inflammatory, modulation of skin barrier function and enhancement of wound healing.

2.1. Antiinflammatory Action

EOs have been shown to demonstrate significant anti-inflammatory properties, mainly via the inhibition of pro-inflammatory mediators, including cytokines, prostaglandins and nitric oxide [27]. Figure 3 shows the general mechanism of action and pathways by which EOs exert anti-inflammatory effects on the skin. Inflammation has remained a key factor in the development of a variety of skin-related conditions such as acne, dermatitis and psoriasis; several EOs, including chamomile oil, frankincense oil, curcuma oil, etc., have bioactivities that inhibit the production of inflammatory markers and reduce inflammation [28,29].

Chamomile oil has been reported to be rich in α-bisabolol and chamazulene, which are the compounds that inhibit the production of prostaglandins and leukotrienes, pro-inflammatory markers [30]. Therefore, it soothes irritated and sensitive skin and is used frequently in formulations designed to treat inflammation-related conditions such as dermatitis and eczema. Similarly, frankincense oil contains boswellic acid, which is a potent inhibitor of the enzyme 5-lipoxygenase, a key inflammatory mediator [31,32]. It is also known to decrease the production of leukotrienes, helping alleviate redness, swelling and irritation at the inflammation site. According to a recent study, the essential oil (EO) from Curcuma longa L. effectively reduced skin inflammation by reducing the levels of pro-inflammatory cytokines (TNF-α, IL-6 and IL-1β) at the protein and mRNA levels, controlling the overproduction of oxidative markers, and repairing the histopathological damage in a mouse model of inflammation induced by TPA [33]. Sandalwood oil from the Santalum album tree has also demonstrated anti-inflammatory and antimicrobial activities, which are useful in the treatment of psoriasis, acne and eczema [34]. The EO from Farfugium japonicum was investigated for anti-inflammatory potential, and the results indicated that the tested EO was an effective inhibitor of LPS-induced NO and PEG (2) production in RAW 264.7 cells, proving its anti-inflammatory potential [35]. EOs from the twigs of Cinnamomum cassia Presl (Lauraceae) were found to be antinociceptive and anti-inflammatory in action, corroborating its use in cosmetics formulations [36]. These anti-inflammatory actions of EOs not only help calm irritated skin but also prevent the exacerbation of various skin conditions, such as acne, where inflammation plays a major role.

2.2. UV Protection and Antiphotodamage Action

Several EOs act by protecting against the damaging effects of UV radiation, which is a primary reason for skin cancer and photoaging. These EOs contain polyphenols and flavonoids as their constituents, which act as natural sunscreens that absorb UV radiation and prevent the formation of UV-induced reactive oxygen species (ROS) [37]. Figure 4 shows the mechanism of action by which these EOs can exert sun protection on the skin. Among the various EOs, carrot seed oil, citrus oil, germanium oil, etc., are reported to have high natural sun protection factor (SPF) values [38]. Carrot seed oil is rich in carotenoids and antioxidants, which not only absorb the UV rays but also neutralize the ROS produced thereafter, causing skin protection from DNA damage and prevention of photoaging. Additionally, the citrus oils obtained from lemon and bergamot contain limonene along with other flavonoids, which have shown promise in reducing UV-induced oxidative stress [39].

The antioxidant activity of several antioxidant constituents present in EOs contributes to their UV protection by preventing oxidative damage to skin cells and reducing inflammation. These key antioxidative constituents include terpenoids, such as carnosol and carnosic acid, as found in rosemary oil. Terpenoids provide protection against UV-induced lipid peroxidation. Phenolic compounds such as thymol and carvacrol (present in oregano and thyme EOs) are capable of scavenging free radicals, while the flavonoids present in tea tree oil are known to enhance the antioxidant defense mechanisms [40]. Additionally, certain EOs, such as lavender and sandalwood oil, have constituents such as myrcene and linalool that block harmful UV rays and reduce photodamage. Phytoconstituents, such as bisabolol and chamazulene, present in chamomile oil, are reported to reduce inflammation and erythema caused by prolonged UV exposure to the skin. Similarly, certain EOs, such as argan oil, are rich in vitamin E and essential fatty acids, which can assist in the skin barrier function, reduce moisture loss and minimize the effects of UV radiation on the skin layers [40].

However, it is worth noting here that citrus oils might cause photosensitivity, and careful incorporation of these oils is necessitated by balancing their benefits and risks involved. Geranium oil from the leaves of Pelargonium graveolens and EO from the flowers of Calendula officinalis were obtained in a study using the hydrodistillation method [41]. The sun protection factor (SPF) and antioxidant potential were determined for the oils, and the findings of this study showed that the SPF values of geranium and calendula oil were 6.45 and 8.36, respectively. The chemical constituents of these oils were also determined using gas chromatography–mass spectrometry (GC-MS) analysis. It was observed that both oils had the ability to reduce oxidative stress and could be utilized collectively in various cosmeceutical formulations. These EOs, while protecting from UV-induced oxidative stress, are particularly useful as a natural sunscreen and as antiphotodamaging agents [41].

2.3. Wound-Healing Action

The wound-healing action of some EOs is attributed to their ability to promote skin tissue regeneration by stimulating cell proliferation and collagen synthesis and repairing damaged tissues [42]. Figure 5 depicts the general mechanism of action of EOs involved in wound healing cascades. The antimicrobial properties shown by tea tree oil and lavender oil are due to the presence of terpenes, such as terpinen-4-ol, and contribute to their wound-healing action by disinfecting the wounds. Additionally, the anti-inflammatory effects, antioxidant activity, promotion of collagen synthesis and enhancement of wound contraction and epithelialization are other mechanisms by which EOs induce wound-healing action. EOs with wound-healing properties include helichrysum oil, lavender oil and citronella oil [43]. Helichrysum oil has shown exceptional tissue regenerative properties as it is rich in neryl acetate and stimulates fibroblast activity, promoting collagen formation and accelerating tissue repair. Lavender oil also has calming properties, promoting collagen synthesis and supporting tissue remodeling [44]. It is particularly useful in the treatment of post-inflammatory hyperpigmentation.

Citronella oil from Cympopogon nardus showed activities, including antioxidant, antimicrobial, and wound healing [45]. A study was conducted on EOs obtained from micropropagated lavender on human skin cells and their ability to synthesize procollagen. This EO also served as a preservative in o/w cosmetic emulsion in a concentration of 0.1% for a period of 3 months [46]. The EO from Chrysanthemum boreale Makino flowers induced wound closure in the dorsal side skin of the rat tail, indicating that it can stimulate the proliferation of keratinocytes in human skin via the Akt and ERK1/2 pathways. In human skin, it could aid in wound healing and skin regeneration [47]. A hydrogel formulation consisting of lavender oil and tea tree oil was reported to demonstrate remarkably faster wound closure than the traditional dressings owing to its antimicrobial properties [48]. Similarly, liposomes prepared from peppermint oil showed high wound healing efficacy in vivo owing to their antioxidant effects [49].

2.4. Anti-Aging Action

The mechanism by which EOs act as anti-aging agents involves multiple pathways by which they repair, protect and rejuvenate the skin, thereby helping combat the signs of aging, such as wrinkles, hyperpigmentation and loss of elasticity [50,51]. The bioactive compounds present in several EOs, such as terpenes, phenolic compounds and flavonoids, are responsible for this action. The general mechanism by which EOs exert their anti-aging action is depicted in Figure 6. Collagen and elastin are two critical proteins responsible for maintaining the elasticity and firmness of skin, and the production of these proteins declines as we age [52,53]; EOs such as frankincense, myrrh, rosehip, sandalwood and rosemary oils can help stimulate their production, promoting skin repair and renewal. Another mechanism by which EOs express anti-aging action is by inhibiting matrix metalloproteinases (MMPs), which are the enzymes that break down collagen and elastin in the skin, contributing to aging [54,55]. Oils such as rosemary and tea tree oils are rich in the polyphenols and flavonoids that inhibit MMPs and preserve the structural integrity of the skin.

EOs promote collagen synthesis and skin regeneration through various mechanisms, such as the activation of fibroblasts, growth factor stimulation, improved hydration, in addition to antioxidant and anti-inflammatory effects. The bioactive compounds present in EOs are known to interact with the skin cells and certain biochemical pathways to improve the structural integrity of the skin and promote wound healing. The presence of monoterpenes, sesquiterpenes, phenolic compounds and specific aldehydes, such as geraniol, citronellol, boswellic acid, etc., contribute to the stimulation of collagen in the dermis. Additionally, several EOs are reported to stimulate growth factors, such as transforming growth factor-beta (TGF-β), which plays a crucial role in collagen production and, thereby, skin regeneration [56,57].

The EO from Premna odorata leaves is reported to show mild anti-collagenase potential, suggesting it may be a promising naturally occurring drug that could be used effectively in anti-aging and anti-wrinkle cosmetic formulations [58]. Recent studies revealed the cosmeceutical potential EO of Origanum vulgare L. as an anti-aging agent by conducting various collagenase, elastase and hyaluronidase inhibition assays. EOs could be used as natural skin-aging retardants in cosmetics [59]. Age-defying topical crème formulations containing geranium/calendula EO-entrapped ethanolic lipid vesicles are also reported in the literature. The characterization of prepared nanolipidic vesicles was performed by determining the encapsulation efficiency, size and polydispersity index, and other factors such as spreadability, homogeneity, viscosity, in vitro antioxidant efficacy and in vitro inhibition of collagenase and elastase proteins were also evaluated for crème formulations in comparison to conventional crème. Deep penetrations into the layers of skin through nanovesicles and photoprotective effects of both EOs confirmed the age-defying potential of this crème [60]. The anti-wrinkle activity of coriander EO was found to be highest when compared to cumin, anise and fennel EOs. The reason behind this could be the presence of a high level of oxygenated monoterpenes, with the most abundant constituent being linalool (81.29%). The conventional coriander oil crème and coriander EO-loaded NPs attenuated UV-induced skin photoaging in vivo, establishing its role in the treatment of extrinsic aging [61].

2.5. Antioxidant Action

Many essential oils find their usefulness in skin-related conditions owing to their antioxidant effects. Various environmental factors such as UV radiation, pollution and other environmental stressors lead to the generation of ROS, which ultimately damages cellular components, including proteins, lipids and nucleic acids. Figure 7 depicts the possible mechanism of action by which EOs exert their antioxidant activity. EOs that contain high concentrations of phenolic compounds, flavonoids and terpenoids are particularly useful in this regard, and these active phytocompounds are capable of neutralizing ROS [62,63].

Rosemary oil and lavender oil have shown significant antioxidant activity in previous studies. Rosemary oil is rich in carnosic acid and carnosol, which scavenge free radicals and prevent the skin cells from undergoing lipid peroxidation [64]. Similarly, lavender oil contains antioxidant compounds linalool and linalyl acetate, which can reduce oxidative stress by decreasing ROS levels and supporting cellular repair [65]. The EO of Melaleuca quinquenervia (Cav.) has been reported to have important biological activities both medicinally and cosmetically [66]. The active components, 1,8-cineole, α-pinene and α-terpineol, present in EO were found to be non-toxic and suitable as a skin whitening agent by inhibiting α-melanocyte-stimulating hormone-induced melanin production. The tested EO showed remarkable anti-tyrosinase, anti-melanogenic and antioxidant properties [66].

EOs obtained from Thai plants, including ginger oil, Wan-soa-long leaf oil, lemongrass oil and holy basil oil, also exhibited high antioxidant activities in both DPPH and TBRAS assays for lipid peroxidation, indicating they could be used as natural antioxidants in cosmetic products to prevent signs of skin aging [67]. In one such study, EO from Eucalyptus camaldulensis flowers was investigated for its effects on melanogenesis and antioxidant potential. Downregulation of mitogen-activated protein kinase (MAPK), as well as protein kinase A signaling pathways, was observed for Eucalyptus EO, making it a potential candidate for skin care products [68]. Owing to anti-inflammatory, antifungal, antibacterial and antioxidant activities, EOs from Artemisia absinthium (wormwood, Asteraceae) have also emerged as a promising plant for cosmetic purposes [69].

2.6. Cytotoxic Action

Several EOs have the ability to kill or inhibit the growth of cells, including microbes and cancer cells. The bioactive compounds, such as monoterpenes, sesquiterpenes, phenols, aldehydes and ketones, are responsible for the cytotoxic action of EOs [70]. The antimicrobial action of EOs is primarily attributed to their ability to disrupt the microbial cell membranes and their function, eventually leading to cell death. The EOs are lipophilic in nature, which allows them to interact with the lipoidal cell membranes of bacteria and fungi, altering their permeability and integrity. For instance, tea tree oil contains the monoterpene alcohol terpinen-4-ol, which has been shown to disrupt the cell membranes of S. aureus and C. albicans [71,72]. Similarly, eugenol present in clove oil and thymol present in thyme oil also cause disruption of microbial cell membranes, leading to cytotoxic action against microbes [73,74].

The EOs of Lavandula species such as L. latifolia, L. angustifolia, L. stoechas and L. intermedia have been used as cosmetic and therapeutic agents in the treatment of various skin conditions because of diverse biological activities, such as antibacterial, antifungal, and effective for burn and insect bite and mostly in perfumery industry [75]. In one study, Cinnamon oil showed preservative effectiveness by completely inhibiting bacterial growth when compared to methylparaben and other EOs such as Lavandula officinalis and Melaleuca alternifolia [76]. EOs from Salvia officinalis L. (Lamiaceae) were assessed for their antifungal and anti-inflammatory potential in various assays. The active components 1,8-cineole and camphor were found to be non-toxic on mammalian macrophages and keratinocytes, making it a suitable product to be used in skin care products [77]. EOs of Marrubium vulgare L. grown in Tunisia and the active constituent Marrubiin were found to be natural antioxidants and antifungal agents to treat skin dermatophytic infections [78].

In cancer cells, EOs can induce apoptosis by causing membrane instability and mitochondrial damage. They can also generate ROS within cancer cells, leading to oxidative stress and subsequent damage of cellular components such as DNA, lipids and proteins [79,80]. For instance, carvacrol, present in oregano and thyme oil, has been reported to induce apoptosis in cancer cells by promoting ROS generation and mitochondrial dysfunction [81,82]. The mechanism by which some EOs exert their cytotoxic action is shown in Figure 8. Additionally, EOs (such as citrus oil), which are rich in limonene along with frankincense oil, are shown to disrupt the mitochondrial membrane potential, resulting in the release of cytochrome c and the activation of caspase cascade, which is a group of enzymes involved in the induction of apoptosis [83,84]. Also, the d-limonene present in citrus oils has shown the ability to cause DNA damage in cancer cells, resulting in cell cycle arrest in the G2/M phase, eventually leading to apoptosis.

Several EOs inhibit the growth and proliferation of cancer cells by interfering with signaling pathways that regulate cell division and survival. This mechanism is particularly relevant in cancer therapy, where unchecked cell division is a hallmark of tumor growth. The EOs of lavender and clove have been shown to inhibit the nuclear factor-kappa B (NF-κB) pathway, thereby reducing cancer cell proliferation and promoting cell death [85,86]. The germanium and lemongrass EOs contain compounds, such as citral and geraniol, which inhibit cell proliferation through PI3K/AKT and MAPK signaling pathways. Nanoparticular formulations of hesperidin and EO extracted from orange peels were compared in a study with pure products (non-nanoformulated) for antioxidant, antimicrobial and cytotoxic potential. Findings suggested that the nanointervention played an important role in improving the antimicrobial, antioxidant and cytotoxic efficacy of these oils [87]. In another study, the antioxidant and cytotoxic activities of EO from Magnolia grandiflora (Magnoliaceae) cultivated in Iran were assessed. The oil exhibited encouraging free radical scavenging activity and non-selective weak inhibitory action against A375, MDA-MB 231 and T98 G tumor cell lines, making it a promising candidate for cosmetic formulations [88].

While the cytotoxic properties of EOs offer promising therapeutic applications, particularly in antimicrobial and cancer chemotherapy, exposure to healthy cells must be avoided. The concentration of EOs used, the route of administration and the types of formulation are crucial factors in determining their efficacy and safety. The doses at which EOs are used to selectively target the cancer cells must be optimized to minimize the harm to healthy tissues. One such approach is to incorporate the EOs into nanocarriers that can help control their cytotoxicity, improve their bioavailability and reduce the risk of adverse effects on healthy cells [89].

Table 1. EOs are skincare actives along with their mechanism of action.

Name of the Essential Oil	Source	Mechanism of Action Related to Skincare	Ref.
Citronella oil	Cympopogon nardus	Antioxidant, antimicrobial and wound-healing actions	[45]
Lavender oil	Lavender officinalis	Ability to synthesize procollagen	[75]
Curcumin EO	Curcuma aeruginosa Roxb.	Inhibition of axillary hair growth via 5-α-reductase	[90]
Curcumin EO	Curcuma longa L.	Reduction of skin inflammation by decreasing the levels of pro-inflammatory cytokines (TNF-α, IL-6 and IL-1β) at the protein and mRNA levels, controlling the overproduction of oxidative markers and repairing the histopathological damage	[33]
Geranium EO and Calendula EO	Pelargonium graveolens (leaves) and Calendula officinalis (flowers)	Reduction of oxidative stress	[41]
Sandalwood album oil	Santalum album	Anti-inflammatory and antimicrobial action	[34]
Melaleuca quinquenervia EO	Melaleuca quinquenervia (Cav.)	Skin whitening agent by inhibition of α-melanocyte-stimulating hormone-induced melanin production in addition to anti-tyrosinase, anti-melanogenic and antioxidant action	[66]
Magnoliea Flos EO	Magnoliea flos	Immunosuppressive action, both in vivo and in vitro	[91]
Premna odorata EO	Premna odorata leaves	Anti-aging due to mild anti-collagenase action	[58]
Chrysanthemum boreale EO	Chrysanthemum boreale flowers	Formation of human skin keratinocytes through the Akt and ERK1/2 pathways	[47]
Origanum vulgare EO	Origanum vulgare L.	Anti-aging due to inhibition of collagenase, elastase and hyaluronidase	[59]
Cinnamomum cassia EO	Twigs of Cinnamomum cassia Presl (Lauraceae)	Antinociceptive and anti-inflammatory action	[36]
Mint EO	Mint species	Anti-melanogenic due to the presence of β-caryophyllene	[92]
EOs	Thai plants, including ginger oil, Wan-soa-long leaf oil, lemongrass oil and holy basil oil	Antioxidant action	[67]
Geranium/Calendula EO	Pelargonium graveolens leaves and Calendula officinalis flowers	Antioxidant, sun-protective action, anti-aging action	[60]
Hespiridin and EO from orange peels	Citrus species	Antioxidant-prevention of DNA damage, anticancer inhibition of the proliferation of cancer cell lines, anti-microbial action	[87]
Coriander EO	Coriandrum sativum L.	In vivo UV-induced photoaging prevention by significant reduction in MDA, AP-1 levels, JNK, MMP-1, PEG-2 and COX-2 levels; antiwrinkle, increased skin collagen content; antioxidant, significant improvement in TGFβ, TGFβII and SMAD3 protein expression	[61]
EOs	Magnolia grandiflora (Magnoliaceae)	Moderate radical scavenging action towards free radicals and mild non-selective inhibitory effects against A375, MDA-MB 231 and T98 G tumor cell lines	[88]
EOs	Salvia officinalis L. (Lamiaceae)	1,8-cineole and camphor—non-toxic on mammalian macrophages and keratinocytes	[77]
EOs	Marrubium vulgare L.	Marrubiin, antioxidant and antifungal action	[78]
EOs	Farfugium japonicum	Anti-inflammatory action—inhibition of LPS-induced NO and PEG (2) production in RAW 264.7 cells	[35]
Eucalyptus EO	Eucalyptus camaldulensis flowers	Melanogenesis and antioxidant action—down-regulation of mitogen-activated protein kinase (MAPK) and protein kinase A signaling pathways	[68]

summarizes a few examples of EOs from various aromatic plants, along with their mechanism of action in the prevention and treatment of various skin-related conditions.

Therefore, EOs can be regarded as versatile agents with a broad range of bioactive constituents, which, through their multifaceted mechanisms of action, offer a natural, multifunctional approach to skincare. These mechanisms of action form the basis of the growing interest in the use of EOs as an effective and safer alternative to synthetic skincare ingredients. However, the use of EOs is also associated with numerous challenges, which must be addressed during the development of an effective dermocosmetic formulation.

3. Challenges Associated with the Use of EOs in Dermocosmetics

Despite the numerous benefits EOs offer in dermocosmetics, their applications are often associated with several challenges. These challenges are primarily due to the chemical nature of these oils, leading to skin safety, stability and variability issues. The inconsistent quality and poor skin penetration of EOs are other obstacles that must be addressed using various innovative and emerging approaches to ensure the safe and effective use of EOs in skincare products. Below are a few of those challenges.

3.1. Volatility and Instability

The EOs are chemically highly volatile and unstable, making their integration into dermocosmetic formulations challenging. When exposed to light, heat, moisture and air, EOs can become oxidized, and their active components become degraded, which renders them less effective and reduces their shelf life [93]. Some EOs, such as the citrus oils obtained from lemon and orange, are particularly sensitive to light and can undergo photodegradation, forming ineffective and even harmful byproducts [94]. Similarly, the oxidation of EOs might lead to the formation of reactive compounds, which can cause sensitization or irritation of the skin, apart from compromising the therapeutic value of EOs. Additionally, volatility will lead to quick evaporation of EOs, reducing their concentration and limiting their effectiveness. The volatility and stability issues of EOs can be addressed by adopting appropriate formulation strategies, such as the concomitant use of antioxidants, and by applying encapsulation and protective packaging techniques.

3.2. Allergic Reactions

Some EOs can pose risks of allergic reactions, skin irritation, and sensitization depending upon the type of skin they are used on. As the EOs are used in high concentrations, this can lead to skin safety issues such as dermatitis, itching, redness or burns [95]. Even if the EOs are used in low concentrations, their repeated exposure can also lead to the above-said issues. Additionally, a few EOs, such as that of bergamot and lime, can increase skin sensitivity to UV radiation, causing photosensitive reactions [96]. This can lead to redness, hyperpigmentation and blistering in some cases, limiting their use in daytime formulations. Linalool, limonene and eugenol are known allergens that can cause allergic reactions if applied for prolonged duration. Therefore, proper safety tests must be performed for the formulations during the preclinical and clinical stages. In a previous study, the phototoxic potential of the EOs of orange, lemon and Litsea cubeba, commonly used as cosmetic ingredients, was evaluated. The skin model assay study revealed that the phototoxicity of EOs was dependent on the content of photoactive components and solvents used [97]. In another study, contact allergy and allergic contact dermatitis were reported to be caused by lavender oil. Lavender oil is one of the most commonly used EOs in aromatherapy and a variety of personal care products [98]. Tea tree oil is also commonly present in cosmetics and topical formulations, and contact allergy to tea tree oil was also reported in the literature depending on the product being used [99].

3.3. Variable Chemical Composition of EOs

There are several factors that affect the chemical composition of EOs from a plant species, including the geographic origin, climate, extraction methods and harvest time [100]. This could lead to variability in their quality, efficacy and overall therapeutic properties. For instance, lavender oil obtained from different geographic locations will have different linalool contents, its main active ingredient [101]. This will make it difficult to standardize the dermocosmetic formulations, and ultimately, the efficacy and safety of the final product will be affected. Also, adulterations due to overdemand sometimes affect the quality of EOs, which can further cause skin reactions. Ensuring the consistent quality of EOs is, therefore, challenging, and the associated impurities, forced adulterations and variabilities in the purity of EOs can affect the safety and purity of the developed formulation.

3.4. Poor Skin Penetration

As EOs are lipophilic in nature, their penetration to the skin’s outermost barrier, the stratum corneum, is challenging, especially for EOs of larger molecular size. The conventional formulations, such as cremes and lotions, cannot deliver EOs properly, and their effectiveness is compromised. Although some EOs are reported to enhance skin permeation, their hydrophobic nature limits their ability to penetrate into the deeper skin layers. Therefore, EOs tend to remain on the outer skin surface and will not be effective for conditions such as inflammation, acne, hyperpigmentation, etc., where deeper skin penetration of the active ingredient is warranted [102,103]. This reduced bioavailability of EOs can be addressed by incorporating EOs into nanolipidic carriers or nanoemulsions; however, this increases the overall cost of the developed formulation.

EOs display dual behavior as they are both penetration enhancers and have poor penetration due to their hydrophobic character. Being hydrophobic, EO solubility in water-based formulations is limited, which further necessitates their uniform distribution and effective delivery in certain cosmetic products [104]. In addition, the hydrophobic nature of EOs makes it further difficult to penetrate the hydrophilic outermost stratum corneum layer of skin. On the other hand, the EOs can act as penetration enhancers of other ingredients that are used in conjunction with the EOs. EOs can disrupt the lipids present at the stratum corneum, increase the solubility and partitioning of active ingredients and improve skin hydration. In conclusion, EOs might not penetrate the skin deeply on their own; however, the volatile and non-volatile components present in EOs, especially the terpenes, disrupt the lipid skin barrier, allowing other ingredients to pass through easily, acting as a penetration enhancer [105,106].

EOs are sometimes required to penetrate the deeper layers of skin depending upon their intended application and desired therapeutic effects. For instance, when they are intended to promote collagen and elastin synthesis for anti-aging and wound healing effects by penetrating deep into the dermis and stimulating the fibroblast activity, facilitating tissue repair and regeneration. In addition, several skin conditions such as dermatitis, psoriasis or rosacea are associated with inflammation in the deep skin layers, thereby the EOs are needed in the dermis and demonstrate anti-inflammatory effects by reducing the inflammatory mediators and promoting healing.

However, EOs need not always penetrate deep into the skin layers to be effective and may act on the skin’s outer surface depending on the intended purpose. Their surface-level benefits include moisturizing and hydrating the skin, providing nourishment, repairing the skin barrier with antioxidant activity and protecting the skin cells from oxidative stress caused by UV exposure and environmental pollution. Similarly, some EOs are intended to express antimicrobial effects to treat conditions such as acne, where they can remain on the outer skin layer of the epidermis, and deeper penetration is not required. The penetration of EOs into the deep skin layers is attributed to several factors, which mainly include their molecular size, delivery system used and the purpose of application.

3.5. Regulatory and Safety Issues

The regulatory and safety concerns associated with the use of EOs are crucial, as their excessive use or their use in high concentrations should be limited. There is often misinformation that natural products are always risk free. In some countries, EOs are regulated as cosmetics, while in others, they are regulated as therapeutic agents. This creates further challenges in product development and market expansion internationally. Specific guidelines are provided by regulatory agencies across the world, which cover the important aspects of the use of EOs, including information about their permitted concentrations, labeling requirements and safety testing [107]. These guidelines must be followed by the formulation developers before the product is approved and marketed. Comprehensive safety assessments comprising skin irritation tests and allergenicity and phototoxicity assessments must be performed to meet the regulatory standards.

3.6. Incompatibility Issues

Another very important concern in the development of EO-based formulations is that these EOs might interact with other ingredients present in the formulation, which can affect the safety, efficacy and stability of the product. Some formulations might become rancid and can produce free radicals if the lipids and oils used in the formulation become oxidized by the EOs used [108]. This can lead to skin damage, degradation or reduced efficacy. For instance, citrus oils are reported to have interactions with the UV filters used in sunscreen products and should not be used together in a formulation. Ensuring the compatibility of EOs with the concomitantly used additives of the formulation, such as emulsifiers, preservatives, etc., is crucial in using EOs as dermocosmetic agents.

3.7. Sustainability Issues

The extraction process of EOs is tedious and requires large amounts of plant material, leading to sustainability issues and harmful effects on the environment, especially if a large production of EOs is warranted [109]. Sometimes, the overharvesting of wild species can result in the depletion of natural resources and a loss of biodiversity. Also, the ethical and sustainable sourcing of raw materials can be challenging in order to maintain reliable supply.

Therefore, these challenges must be overcome, and the obstacles must be addressed prior to the appropriate use of EOs. This can be achieved by undertaking innovative or novel formulation techniques, proper quality control and regulatory compliance. The use of emerging novel technologies such as nanotechnology can provide potential answers to many of the above-mentioned challenges so that the safe and effective use of EOs can be ensured [110]. Nanotechnology, specifically nanolipidic formulations, has emerged as an innovative solution to address the issues associated with the use of EOs, enabling their better delivery and improving their therapeutic effectiveness in dermocosmetic formulations.

4. Nanolipidic Formulations of EOs

Nanolipidic systems are ideal systems for the delivery of lipophilic drugs with optimal drug loading capacity and good long-term stability. These lipid-based delivery systems not only improve the stability and bioavailability of EOs but also enhance their controlled release, minimizing their potential adverse effects [111]. Compared to solid lipid nanoparticles (SLNs), nanostructured lipid carriers (NLCs) are more popular owing to their more stable nature. In an NLC formulation, liquid lipid is basically an oil, and if this oil is an EO, then apart from being a matrix, it can also serve as an active ingredient [20]. Oil, which is the vital ingredient in any NLC formulation, should be selected judiciously, as the present literature shows that EOs are complex mixtures of a variety of substances obtained from plant extracts of varying chemical compositions. The EOs from jasmine, lavender, chamomile, clary sage, sandalwood, eucalyptus, peppermint, rosemary and tea tree are some of the most sought after and vigorously used EOs in the cosmetic industry [112]. Other than their use in perfumes, these are now extensively being utilized in cosmetics as preservatives, antioxidants, antimicrobial and anti-aging agents [113].

EOs have a proven role in skin care products in both conventional and novel formulations as a green cosmetic ingredient. The green cosmetic concept is very new wherein EOs are considered ecofriendly alternatives to synthetic preservatives and can be used as natural preservatives instead of parabens, such as methyl paraben, ethyl paraben, propyl paraben, diethylparaben, etc., [114,115]. Along with the preservative properties, EOs are potent antioxidants and have the potential to scavenge free radicals and protect the skin from damage. EOs such as lavender oil, eucalyptus oil and peppermint oil have been reported to have remarkable antioxidant properties and could be an ideal alternative to synthetic antioxidants in skin care products [116]. Increased public awareness towards environmental issues caused by harmful chemicals used in cosmetic preparations and a firm belief in natural products and sustainability have led to a marked increase in the demand for skin care products that create less environmental impact with utmost efficacy. Replacement of synthetic preservatives and antioxidants by potent EOs is the future of cosmetics and demands extensive in vitro and in vivo investigations concerning their use as natural cosmetic preservatives and antioxidants in place of synthetic ones. EOs have the tendency to metabolize rapidly and experience fast excretion from the body, ensuring their safe use over synthetic materials, for instance, penetration enhancers, antioxidants and preservatives [117].

SLNs and NLCs are ideal carriers for dermal drug delivery, and a plethora of studies have already been conducted on them regarding their use in topical formulations [118]. To be more precise, NLCs are a stable lipid drug delivery system holding immense potential as nanocosmeceuticals owing to benefits such as ease of preparation, easy scale-up, enhanced bioavailability, skin targeting, skin hydration, non-toxicity, biocompatibility, improved drug-loading and stability profile [119,120]. NLCs counter some of the limitations of SLNs, such as the use of liquid lipids along with crystalline solid fatty acids to build the core [121]. Herein, the oil-loaded NLCs as potential topical drug delivery systems for skin disorders such as psoriasis, dermatitis, wound healing, bacterial infections and skin cancers are being explored. Nanotechnology has emerged as an effective treatment modality for addressing these skin conditions. Nano skin delivery has multiple benefits over conventional topical skin preparations [122].

Thymol-NLCs were prepared in a study using the sonication method using natural lipids for in vivo anti-inflammatory and antipsoriatic activity [123]. The prepared NLCs were incorporated into a gel and characterized by their rheological properties and pH, which made them suitable for skin application. In an imiquimod-induced psoriasis mouse model, improved healing was observed for the tested nanoformulation compared to the negative control. Sensitive skin is yet another skin condition in a large number of people, and several compounds are known to trigger hypersensitive skin, which often imposes a negative impact on the quality of life (QoL) of the patient [124]. Psoriasis is an inflammatory and auto-immune condition of the skin requiring a long-term treatment similar to any other chronic disease [125]. Large-sized drug molecules cannot cross the stratum corneum layer of the skin, hampering their absorption and leading to discontinuation of the treatment [126]. Silymarin-loaded NLCs have been prepared to increase the therapeutic value of silymarin and decrease its toxicity by using a 2⁽³⁾ full-factorial design characterized on the basis of particle size, photostability and cell line studies. Significantly higher permeability results were observed in the NLC formulation compared to the marketed phytosome formulation [127].

In a similar study, prickly pear seed oil was used to develop NLCs for the topical delivery of vitamin A. The prepared NLCs showed superior attributes supporting the use of prickly pear oil in cosmeceutical applications [128]. For the topical delivery of tea tree oil, nanoemulsion-based nanogel was developed and optimized using a central composite design. The optimized oil nanogel had pH = 5.57 ± 0.05 and flux value = 7.96 μL cm²/h through the skin in 10 h, which were characteristics suitable for topical applications [129]. Ridolfia segetum EO-loaded NLCs with encapsulation efficacy of 100% for topical skin delivery showed a sustained release behavior suitable for prolonged skin delivery [130]. For the controlled delivery of Lippia origanoides EOs, NLCs were studied to evaluate their ability to encapsulate hydroxypropyl-β-cyclodextrin inclusion complexes of EO for follicular accumulation [131]. High efficiency and a better safety profile were observed when liquid crystal NPs composed of isostearyl glyceryl ether and ethoxylated hydrogenated castor oil were developed for the transdermal drug delivery of 4-biphenyl acetic acid [132].

Sesamol extracted from Sesamum indicum seed oil was formulated as an NLC for effective topical administration of sesamol. Higher encapsulation efficiency (˃90%) and prolonged antioxidant activity of sesamol were observed in NLC systems containing sesame oil as an oil phase compared to Miglyol^® 812 [133]. Topical application of rosemary EO-loaded NLCs was assessed for in vitro antibacterial activity and in vivo infected wound healing in an animal model. Findings revealed that this nanosystem was effective in wound healing, as the oil-loaded NLCs increased vascularization, re-epithelialization and fibroblast infiltration and decreased the wound size considerably [134]. Enhanced antifungal activity of clotrimazole-loaded NPs formulated with oils of Rosmarinus or Lavandula was observed in NLC formulations. NPs were found to be effective against topical candidiasis, and a synergy was observed between the drug and natural oils [135]. Codelivery of ferulic acid and Lavandula EO was assessed in an NLC formulation for its beneficial effects in wound healing. NLC-NPs of high encapsulation efficiency (˃85%) and high stability were obtained. Together, the components in a lipidic carrier system promoted cell proliferation and migration and proved to be a promising strategy in wound healing [136].

Crucial processes are involved in wound repair, such as tissue regeneration. In one study, lipidic NPs of eucalyptus and rosemary EOs were assessed for wound repair. NLCs made up of olive oil loaded with eucalyptus oil showed better properties in terms of bioadhesion, wound healing towards fibroblasts and in vitro proliferation enhancement along with other properties, making them ideal for wound repair promotion and synergistic antimicrobial activity [137]. Dose-dependent anti-inflammatory activity was observed in the order Lavandula ˃ Rosmarinus ˃ Origanum when these Mediterranean EOs were developed as an NLC delivery system. The lipidic particles were characterized for physicochemical parameters, and in vitro anti-inflammatory activity was evaluated in Raw 264.7 cells [138]. Punica granatum EO-loaded NLCs exhibited impressive benefits by showing good antimicrobial and antioxidant properties, making it a viable formulation to be used in dental- and skin-related conditions [139]. The newly designed lipoid colloidal carrier of diosmin/EOs of Rosmarinus officinalis, Zingiber officinale and Vitis vinifera combinations showed antioxidant, sun-blocking and anti-photoaging effects when compared to diosmin alone [140]. On the onychomycosis model, the antifungal efficacy of Melaleuca alternifolia EO (tea tree oil) in nanocapsule and nanoemulsion formulations was recently evaluated. The EO-loaded nanocapsule formulation was able to reduce Trichophyton rubrum growth in nail infection models more than the nanoemulsion, emulsion or untreated nail [141]. Enhanced chlorhexidine skin penetration was observed when co-delivered with eucalyptus oil, promoting antisepsis properties; nanoformulation-related work is warranted for this combination [142].

Table 2. Nanolipidic formulations prepared for natural EOs.

EOs	Formulation Type	Size, PDI and Zeta Potential (ZP)	Important Findings	Ref.
Prickly pear seed oil	NLCs	Size = 215–244 nm; PDI < 0.3	NLCs showed superior attributes compared to SLNs	[128]
Tea tree oil	Nanoemulsion-based nanogel	Size = 16.23 ± 0.411 nm; ZP = 36.11 ± 1.234 mV	Nanogel with same amount of tea tree oil as conventional gel revealed broader zone of inhibitions against all selected microbial strains	[129]
Ridolfia segetum (L.) Moris EOs	NLCs	Size = 143 ± 5 nm; PDI = 0.21, ZP = −16.3 ± 0.6 mV	Enhanced permeation profile by NPs towards topical delivery	[130]
Lippia origanoides EOs	NLCs	-	Lipid NPs exhibited promising results for inclusion complexes of this EO	[131]
Sesamum indicum seed oil	NLCs	Homogenous dispersion of particles in a nanometric range	NLCs were able to control the rate of diffusion of sesamol through skin	[133]
Rosemary EO	NLCs	-	Accelerated wound healing by topical application of EO-loaded NLCs	[134]
Rosmarinus officinalis and Lavandula x intermedia oil	NLCs	Size <100 nm; PDI < 0.15	Synergistic treatment of candida skin infection with clotrimazole-loaded EOs–NLCs	[135]
Lavandula EO	NLCs	Size <150 nm; PDI < 0.2	Codelivery of ferulic acid and Lanvandula EO promoted cell proliferation and migration in treatment of wounds	[136]
Eucalyptus and Rosemary EOs	SLNs and NLCs	-	NLCs based on olive oil and eucalyptus oil proved to exert synergic effects in wound repair and healing	[137]
Rosmarinus officinalis L., Lavandula x internedia, Origanum vulgare subsp. Hirtum and Thymus capitatus EOs	NLCs	Size < 200 nm; PDI = 0.126–0.141	A dose-dependent anti-inflammatory activity was observed in the order Lavandula ˃ Rosmarinus ˃ Origanum	[138]
Punica granatum seed oil	NLCs	Size = 102.10 nm; PDI = 0.26	EO-loaded NLCs showed antimicrobial and antioxidant potential ideal for skin care products	[139]
Rosmarinus officinalis, Zingiber officinale, Vitis vinifera	Lipoidal colloidal carrier	Size = 121.1–144.3 nm; ZP = 30 mV	Diosmin in combination with these EOs was found to be effective nano-dermal care product	[140]
Melaleuca alternifolia EO (tea tree oil)	Nanocapsule and nanoemulsion	-	EO-loaded nanocapsule formulation was able to reduce Trichophyton rubrum growth in nail infection models more than nanoemulsion, emulsion and untreated nail	[141]
Nigella sativa oil	Ethosomal vesicles	Spherical-shaped vesicles in nanosized range	Thymoquinone-loaded ethosomal gel exhibited enhanced anti-psoriatic activity compared to plain TQ and marketed formulation	[142]
Rosemary EO	NLCs	Small particle size, low PDI and good stability	Improved topical efficacy of rosemary EO-loaded NLCs for the treatment of cutaneous alterations involving loss of skin hydration and elasticity	[143]
Melaleuca alternifolia EO	Nanoemulsion	NE presented a lower antioxidant capacity and a higher penetration through the stratum corneum	Melaleuca alternifolia EO had the potential to improve the photoaged skin, as EO-NE was able to reach deeper layers of skin	[144]
Lemon EO	Nanoemulsion	Size = 64 ± 60 nm; PDI = 0.255	Higher stability and anti-oxidant capability of LEO-NE than EO alone, making it an efficacious transdermal nanoformulation	[145]

summarizes important developments in the nanolipidic formulations prepared for EOs.

Therefore, nanolipidic formulations such as NLCs, SLNs, liposomes and nanoemulsions offer innovative solutions to achieve effective delivery of EOs in dermocosmetics. These systems can address the challenges of volatility, instability and poor skin penetration of EOs and can provide advanced delivery systems that are more effective, safer and have greater consumer appeal. The applications of nanoformulations in this field are consistently increasing and have great future prospects in the development of advanced next-generation dermocosmetic formulations for personalized and sustainable skincare.

5. Market Value and Regulatory Aspects of EOs

Given the growing demand for natural product-based ingredients and the application of nanotechnology in skincare products, the market value and regulatory aspects of nanocosmeceuticals have become critical topics. As the global cosmeceutical market is expanding rapidly, consumers’ interest has now shifted to products that can provide both cosmetic and therapeutic benefits. There is a great demand for plant-based or natural products owing to the concerns about the toxic effects of synthetic chemicals [146]. Another market growth driver is the application of nanotechnology in cosmetic formulations, as it increases the effectiveness of the formulation and stability, increasing its shelf life and patient compliance.

The US FDA has enforced laws, regulations and guidance for industries for all cosmetic products. Many products of everyday use contain fragrances, and some of these are regulated as cosmetics by the FDA. Depending upon the intended use of the product, regulations are made by the FDA, and if it is to be applied on the body of an individual to make them more attractive, it is regarded as a cosmetic under the law; for instance, perfumes, cologne and aftershaves. For EOs, there is no regulatory definition, except that they are referred to as certain oils extracted from plants [147]. In the US, cosmeceuticals are not a distinct category under the law, and products with cosmetic and drug-like properties must adhere to both cosmetic and drug regulations. The market value of the use of EOs in skin care products is quite significant due to factors such as low cost, easy scale up, certification and validation and simple technology. In recent years, there have been growing trends in the market for sustainable and eco-friendly dermo-cosmeceuticals [148]. The physicochemical properties of the drug and delivery carriers, formulation and process factors, the mechanism of skin distribution, new technical developments, particular constraints and regulatory issues must all be thoroughly understood in order to develop these delivery systems.

The numerous advantages of EOs place them in high demand as green cosmetics are in line with the increased public awareness of the environment. In a previous study, EOs from peppermint, lavender and eucalyptus were used as an oil phase to formulate a naringin-loaded microemulsion, and the results demonstrated better or similar antimicrobial and anti-oxidant properties compared with the synthetic ones. EOs have also been serving as natural preservatives these days in various skin care products [116]. Recent studies on lemongrass EO for potential skincare applications revealed the development of lemongrass-incorporated chitosan bioactive films as a green cosmetic treatment owing to the antimicrobial and anti-oxidant properties of the lemongrass EO and biodegradable behavior of chitosan. The developed biofilm sheet was non-cytotoxic, highly flexible, improved permeability characteristics and showed antimicrobial activity against E. coli and S. aureus [149]. The safety profile of nanocosmetic formulations is a prerequisite in the assessment of nanomaterials in cosmetics, wherein nanoparticles are able to penetrate into the deep layers of skin owing to their small size. Various regulatory bodies, such as the International Cooperation on Cosmetic Regulations (ICCR), are setting up the necessary regulatory guidelines for nanocosmeceutical products [148].

Recent reports showed that the market for EOs is large and competitive, especially in cosmetic skincare products to treat various skin-related conditions such as acne, scars, wrinkles and aging. Proper certification is warranted to ensure the quality of these oils. A report published on dermocosmetic skincare products such as anti-aging, hair and scalp, skincare, babycare, skin brightening and anti-stretch marks has reached a global market size at USD 52.47 billion in the year 2022 and is expected to expand up to 132.66 billion by 2032, a growth of CAGR of 9.71% is expected between 2023–2032 [150]. As per recent reports on the market value of cosmetic oils (almond oil, olive oil, coconut oil and essential oils) in various skincare and hair care products in regions such as North America, Europe, Asia Pacific, Middle East and Africa and Central and South America, an increasing trend is reported, and the global cosmetic oil market is expected to be valued at USD 72.28 billion by the year 2025, and demand is expected to increase further in the coming years [151].

The use of EOs in cosmetics, toiletries, fragrances, body sprays, perfumes and air fresheners is very evident. In yet another market report, the global market size of several EOs (orange, corn mint and eucalyptus) in cosmetics, medical, food and beverages, and spas is currently valued at USD 21.79 billion in the year 2022 and is anticipated to grow several times in the period 2023–2030. The revenue forecast for 2030 has reached USD 40.12 billion, with an expected growth rate of CAGR 7.9% between 2023 and 2030. The major players in the EO market are the US, Canada, Mexico, Germany, UK, France, Italy, Spain, China, India, Taiwan, South Korea, Thailand, Singapore and Australia [152]. The market size of lavender EO was robust in 2021 and is now expected to grow and increase many folds in coming years, owing to its wide application in food and beverages, pharmaceuticals, therapeutics, aromatherapy and personal care products. The demand for other EOs, including tea tree oil, peppermint oil and eucalyptus oil, also increased for aromatherapy during pandemic times [153].

Rapidly increasing global trends towards the use of natural ingredients in cosmetic preparations meant for skin in the treatment of a myriad of skin-related conditions have paved the way for the introduction of nanotechnological interventions. The delivery of natural ingredients such as EOs into the deep layers of the skin is achieved with the help of NLCs, SLNs, liposomes, niosomes, nanoemulsions, etc. The risks and benefits of delivering cosmetics by nanocarriers should be taken into consideration, and extensive research is warranted in this area in terms of toxicities, shortcomings, stability aspects, controlled delivery of actives for better skin penetrations, etc., [154,155]. The market for nanocosmeceuticals of EOs is poised for significant growth, driven by increased consumer demand for natural products. Different countries have their own approaches to the safety, efficacy and labeling of nanomaterials; therefore, the regulatory landscape remains complex.

6. Conclusions and Future Perspectives

EOs are now at the forefront of innovations in dermocosmetic formulations as the interest in natural, sustainable and bioactive ingredients is growing. However, the use of EOs in skincare products is still challenging and is associated with several limitations, including the instability, side effects and variability in their composition. With technological advancements, advanced research is warranted in the future that could render EOs safer, more effective and sustainable ingredients for use in dermocosmetics. The use of nanotechnology, particularly the development of nanolipidic formulations (such as NLCs and SLNs), has already shown great promise in addressing the stability, bioavailability and safety issues of EOs. Future developments in nanotechnology and the development of other lipidic or non-lipidic systems for the enhanced and effective delivery of EOs are warranted.

NLCs have numerous advantages, such as skin penetration enhancement, protection of EOs against degradation, solubilization of EOs and effective partitioning into the deep layers of the skin, which make them ideal nanostructures in skin care formulations. Therefore, NLCs are an upcoming alternative approach to the oral, dermal and transdermal delivery of EOs. EOs are applied directly to the target site of action, with reduced systemic effects and high levels of EO localizations achieved. Lipidic NPs, especially NLCs, have the potential to overcome the restrictions of other colloidal systems, such as emulsions, liposomes, polymeric NPs and SLNs, in encapsulating these EOs. NLCs of EOs have garnered much interest and have been extensively studied for their use in topical applications in various skincare and cosmetic products.

The next frontier lies in the development of nanocarriers that are able to deliver the EOs in specific layers of the skin or cells, and this would allow the EOs to act precisely on the problem sites, such as the inflammation site or the hyperpigmented area. This will reduce their effects on surrounding cells or tissues, making them safer. This could be achieved by the use of stimuli-responsive materials in the nanoformulations, as they can release the EOs when triggered or when they sense fluctuations in the pH or temperature. Further, the use of hybrid nanoparticles is another promising approach for the more controlled delivery of EOs. Hybrid nanoparticles combine multiple nanocarriers, such as liposomes and polymeric nanoparticles, and these systems are able to address the bioavailability issues of EOs more comprehensively and can accommodate multi-functional materials in their core.

The future also lies in the development of eco-friendly and sustainable skincare products by using environmentally benign materials and processes during the synthesis of nanoparticles. Biodegradable nanocarriers should be identified and emphasized by the use of plant-derived lipids or other sustainable materials. Both EOs and their delivery systems should be natural and derived from renewable sources, as this could mitigate the impact of nanoparticle-based formulations on the environment. Also, the extraction and isolation of EOs should be carried out using sustainable practices. Green extraction technologies, such as microwave-assisted extraction and supercritical fluid extraction, must be emphasized. In conclusion, the future of EOs as dermocosmeceutical agents is promising, and numerous opportunities exist for innovation and growth in this field. Advances in nanotechnological methods, sustainable practices, personalized skincare, etc., will pave the way for next-generation EO-based skincare products. However, more extensive and targeted research is warranted to fully realize their potential and to overcome the challenges associated with their use. With continued scientific exploration and technical advancements, nevertheless, EOs have the potential to play a central driving role in natural and personalized dermocosmetic formulations.