Drug Delivery Systems Utilizing Essential Oils and Their Compounds—A Promising Approach to Fight Pathogens

Abstract

Essential oils (EOs) and their compounds are becoming a growing interest in medical sciences. Despite their potential as antimicrobial, anxiolytic, cytotoxic, and immunosuppressive drugs, their chemical characteristics make them difficult to use in direct treatment. This article intends to summarize the current body of knowledge regarding drug delivery systems that can overcome obstacles, such as low water solubility, volatility, oxidation potential, photodegradation, and thermal instability of EO compounds. Various materials like zeolites, alginate, chitosan, cellulose nanomaterials, zein, poly (D,L-lactic-co-glycolic) acid, liposomes, nanoemulsions, and their modifications can help to mitigate these problems, but their utilization in medical settings is still lacking. The biggest issue in the utilization of natural compounds seems to be the very low number of clinical trials, which seriously impedes their usage despite favorable outcomes in/of in vitro experiments.

1. Introduction

Essential oils (EOs) are naturally occurring chemicals produced by plants. They can be found in virtually all plant organs, and their composition is highly variable in quality and quantity [1,2,3]. Typically, they are produced by the plant to protect it from bacterial and fungal infections, and this role can be used to explain their antimicrobial properties, which have been demonstrated in numerous studies [4,5,6]. An important aspect of EO research is the utilization of either plant oils or only the substance of its chemical constituents. This distinction is vital since plant oils can differ substantially in their chemical composition, which may lead to confounding results. The differences arise from using various extraction methods and depend on the plant parts, plant morphology, genetics, and conditions in which the plant grew [1,2,3]. Hence, the importance of the usage of single chemical compounds instead of plant oils to make the results more replicable and conclusive has been stressed in the current article.

The antimicrobial action of EOs results from various mechanisms that may not be well understood in some cases. The general trend is that Gram-negative bacteria seem more resistant to EOs than Gram-positive ones. This is probably due to the difference between their cell wall and outer cell membrane, which, in the case of Gram-positive bacteria, enables the EOs’ compounds (EOCs) to penetrate the cell wall more effectively [4]. In the case of Gram-negative bacteria, their outer membrane may act protectively against the EO mode of action. Despite this, studies on Gram-negative bacteria like Escherichia coli demonstrated that they are not immune to them. However, most of the research on this topic concerns food spoilage prevention and not clinical settings [4]. EOCs usually show various mechanisms of antibacterial action; the most common one is the interference with cell membrane permeability or modification of its properties. It causes the bacterial cell components (like ions or adenosine-5′-triphosphate) to spill out. Other common effects of EOCs include protein denaturation, inhibition of cell walls, and fatty acid synthesis [4]. This multivariate mode of action slows down the potential bacterial tolerance and improves bacteriostatic or bactericidal activity thanks to targeting various microbial cell elements. Despite this, compared to traditional antibiotics, their minimum inhibitory/bactericidal concentration (MIC/MBC) values are substantially higher, which may hinder their broad utilization in modern medicine, albeit their MIC values still fall in the range of effective concentrations [7].

Additionally, EOs are not broadly used as antimicrobials in medical settings due to their properties, which include vulnerability to environmental conditions (light, temperature, oxygen) and hydrophobic structure, making it more challenging to create formulas easily absorbed and utilized by the organism. The selected physicochemical and biological properties of the most common EOCs are summarized in Table 1. Moreover, despite them being considered safe substances, relatively few studies assess their cytotoxicity [6,8,9,10]. The new trend that emerged to counter these issues is the utilization of EOs in combination with different carriers or other antimicrobials.

Several studies assessed the synergism of EOCs with other substances. One of such compositions is adding silver nanoparticles (SNP) in the context of bactericidal properties. Ghosh et al. [11] have shown that cinnamaldehyde, in combination with SNP, has a high synergistic effect against Bacillus cereus and Clostridium perfringens. Additionally, the synergism of both SNP and cinnamaldehyde was within the range of safe chronic exposure to these compounds, which is important since SNPs are known to be highly toxic to water ecosystems, and their cytotoxicity on humans is not fully known [12]. Other studies combined EOs with various antibiotics, which produced varying results. So far, the studies have reported synergism [13,14,15,16,17], indifference, and antagonisms [8]. This combination may be critical due to the growing antibiotic tolerance, where the multivariate mode of action with EOCs can be helpful. Aside from purely antimicrobial effects, there is still a need for more cytotoxic studies of the combinations before they can be utilized in a regular treatment [15].

As mentioned before, the chemical properties of EOs lead to severe problems in their utilization as drugs. One of the most significant challenges is the delivery method since their lipophilic structure seriously hinders their ability to penetrate all tissues or be injected into the circulatory system. The problem of delivering EOs is currently being researched, and these substances are being used to combat antibiotic resistance in microbes or as anticancer agents. This work intends to summarize current research into drug delivery systems (zeolites, alginate, chitosans, cellulose nanomaterials, zein, poly (D,L-lactic-co-glycolic) acid [PLGA], liposomes, and nanoemulsions) loaded with EOs with focus on antimicrobial medical applications.

Table 1. Properties of the most common essential oil compounds.

Compound Name	Density [g/cm3]	Vapor Pressure [mm/Hg]	Solubility in Water	Boiling Point	Selected Biological Properties	Plant Sources	References
Geraniol	0.870–0.885	0.03	100 mg/L at 25 °C	230 °C	AM, AI	Cymbopogon spp. Rosa spp.	[18,19]
Farnesol	0.884–0.889	0.0000394	Insoluble in water	110–113 °C	AM, AI	Cymbopogon spp.	[20,21,22]
Linalyl acetate	0.895	0.11	Insoluble in water	221 °C	AM, AI, AO	Lavandula spp. Citrus bergamia	[23,24]
Carvacrol	0.974–0.979	-	Insoluble in water	237–238 °C	AM, AO	Origanum spp. Thymus spp.	[25,26]
α-Pinene	0.8592 at 20 °C/4 °C	4.75 at 25 °C	2.49 mg/L at 25 °C	156 °C	AM	Pinus spp. Picea spp.	[27,28]
α-Terpineol	0.930–0.936	0.0423 at 24 °C	7100 mg/L at 25 °C	218–221 °C	AO, AC, AH	Origanum vulgare Ocimum canum	[29,30]
1,8-Cineole	0.921–0.924	1.90 at 25 °C	Insoluble in water	176–177 °C	AM, AI, AO	Eucalyptus spp.	[31,32]
Eugenol	1.064–1.070	0.0221 at 25 °C	Insoluble in water	252–253 °C	AM, AI, AO	Syzygium spp.	[33,34,35]
Cinnamaldehyde	1.048–1.052	0.0289 at 25 °C	Insoluble in water	248.0 °C	AM	Cinnamomum spp.	[36,37,38]
p-Cymene	0.853–0.855	1.50 at 25 °C	Insoluble in water	176.0–178.0 °C	AM, AI	Cuminum cyminum Thymus spp.	[39,40]
Vanillin	1.06	0.000118 at 25 °C	Insoluble in water	285 °C	AM, AI, AO	Vanilla spp.	[41,42]
Menthol	0.9131	0.06	420 mg/L at 25 °C	214.6 °C	AM	Mentha spp.	[43,44]
trans-Anethole	0.983–0.988	0.05	Insoluble in water	234.0 °C	AM, AI, AO	Pimpinella spp. Foeniculum spp.	[45,46]

AM—antimicrobial; AI—anti-inflammatory; AO—antioxidant; AC—anticancer; AH—antihypertensive.

Table 1. Properties of the most common essential oil compounds.

Compound Name	Density [g/cm3]	Vapor Pressure [mm/Hg]	Solubility in Water	Boiling Point	Selected Biological Properties	Plant Sources	References
Geraniol	0.870–0.885	0.03	100 mg/L at 25 °C	230 °C	AM, AI	Cymbopogon spp. Rosa spp.	[18,19]
Farnesol	0.884–0.889	0.0000394	Insoluble in water	110–113 °C	AM, AI	Cymbopogon spp.	[20,21,22]
Linalyl acetate	0.895	0.11	Insoluble in water	221 °C	AM, AI, AO	Lavandula spp. Citrus bergamia	[23,24]
Carvacrol	0.974–0.979	-	Insoluble in water	237–238 °C	AM, AO	Origanum spp. Thymus spp.	[25,26]
α-Pinene	0.8592 at 20 °C/4 °C	4.75 at 25 °C	2.49 mg/L at 25 °C	156 °C	AM	Pinus spp. Picea spp.	[27,28]
α-Terpineol	0.930–0.936	0.0423 at 24 °C	7100 mg/L at 25 °C	218–221 °C	AO, AC, AH	Origanum vulgare Ocimum canum	[29,30]
1,8-Cineole	0.921–0.924	1.90 at 25 °C	Insoluble in water	176–177 °C	AM, AI, AO	Eucalyptus spp.	[31,32]
Eugenol	1.064–1.070	0.0221 at 25 °C	Insoluble in water	252–253 °C	AM, AI, AO	Syzygium spp.	[33,34,35]
Cinnamaldehyde	1.048–1.052	0.0289 at 25 °C	Insoluble in water	248.0 °C	AM	Cinnamomum spp.	[36,37,38]
p-Cymene	0.853–0.855	1.50 at 25 °C	Insoluble in water	176.0–178.0 °C	AM, AI	Cuminum cyminum Thymus spp.	[39,40]
Vanillin	1.06	0.000118 at 25 °C	Insoluble in water	285 °C	AM, AI, AO	Vanilla spp.	[41,42]
Menthol	0.9131	0.06	420 mg/L at 25 °C	214.6 °C	AM	Mentha spp.	[43,44]
trans-Anethole	0.983–0.988	0.05	Insoluble in water	234.0 °C	AM, AI, AO	Pimpinella spp. Foeniculum spp.	[45,46]

AM—antimicrobial; AI—anti-inflammatory; AO—antioxidant; AC—anticancer; AH—antihypertensive.

2. Problems Arising from the Chemical Properties of EOCs

One of the most significant challenges in employing EOs in medicine has been overcoming their volatility and vulnerability to light, oxygen, temperature, and water insolubility. Their summary is shown in Figure 1. Due to being structurally similar, the properties of various EOCs broadly overlap. It can be viewed as a disadvantage in their application, but on the other hand, their similarity might facilitate their usage once the effective formula is found.

2.1. Volatility

Volatility is defined here as the quality describing how easily a given substance takes a vapor form. This characteristic is based on the boiling point and vapor pressure resulting from the substance’s molecular weight and intermolecular interactions. Most EOs consist of terpenes, sesquiterpenes, alcohols, and ketones with relatively low mass; hence, their vapor pressure values are high. Table 1 provides a more detailed summary of this parameter. The high volatility of EOs is the reason for their pungent smell, which finds their application in the fragrance industry. Despite having boiling temperatures significantly higher than needed for medical applications, the high volatility poses a crucial problem in using them in biomedical settings. The problem can also arise during the storage of these compounds, which can affect their composition and complicate adjusting a proper dose. At the same time, high volatility has been used as an advantage in some contexts. For instance, eucalyptol is widely used in inhalation and has been shown to help with respiratory tract diseases [48]. Additionally, it has been used as a fragrance that was speculated to improve patient well-being and reduce stress in care facilities [49].

2.2. Water Insolubility

The main components of EOs are usually terpenes, sesquiterpenes, phenols, alcohols, esters, ketones, and various oxides. Most are water-insoluble due to their long hydrophobic chains and aromatic rings (see Table 1 for more details). This characteristic has proven difficult to utilize in biomedical contexts since adding them directly to the blood system is impossible. The low water solubility is also tied to a low absorption rate from the digestive tract [5,50]. Nevertheless, some EOCs tend to form micelles (e.g., triterpenoids and monoterpenes). Their lipophilic nature enables them to penetrate easily through the epithelial cell membrane, which may explain why the majority of reported effects come from the digestive system [7].

2.3. Thermal Instability

Most EOCs have relatively high boiling temperatures, circulating at over 100 °C (for reference, see Table 1). While it is not a problem within the human body, this may be significant in drug manufacturing processes, such as zeolites [51,52]. However, the temperature may crucially influence the composition. Monoterpenic carbons like β-myrcene, β-pinene, sabinene, or γ-terpinene have been shown to decrease in concentration in higher temperatures, whereas the concentrations of p-cymene increase. Additionally, temperature can influence oxygen solubility, which, in turn, can modify the composition of an EO through oxidation processes [47].

2.4. Light Susceptibility

Like thermal instability, the degradation of some EOs under the influence of light is more of a problem during the manufacturing process than a biomedical one. Due to this characteristic, caution is advised regarding the storage methods of EOs. Studies have shown that plant exposition to different light spectra can change their chemical composition [9]. The degrading effect of light can occur both in the UV and visible spectrum, with some compounds like geranial, terpinolene, and γ-terpinene being demonstrated as susceptible to this type of degradation [47].

2.5. Susceptibility to Oxidation

EOs contain compounds that are readily oxidized by the oxygen in the atmosphere. The products of oxidation can differ from the original compound. Aside from lower therapeutic potential, the oxidized forms can have different allergenic potential that may be an important medical aspect [9]. Additionally, oxidation can have an important influence on their composition, which is important both in their production and during their storage as free oxygen radicals can degrade more complex compounds like α-terpinene, limonene, β-phellandrene, β-caryophyllene, and citral can be oxidized into compounds such as limonene oxide, α-terpineol, and geranic acid [47].

3. Search Strategy

This article utilized the PubMed search engine to determine the scope of this review. The literature from 1 January 2009 to 3 October 2024 was analyzed. As shown in Figure 2, each of the delivery systems was searched using the following formulas: “alginate AND essential oils”, “cellulose AND essential oils”, “chitosan AND essential oils”, “liposome AND essential oils”, “nanoemulsion AND essential oils”, “PLGA AND essential oils”, “zein AND essential oils”, and “zeolite AND essential oils”. The subsequent search was followed by adding the formula “AND medical application” (e.g., “alginate AND essential oils AND medical application”) to exclude nonmedical applications like the food industry. A code later used to create the flow chart was added to differentiate between studies or prevent duplicates from messing up the results.

4. Search Results

Based on the performed study, 2299 papers concerning delivery systems and EOs were found. However, after adding the formula “AND medical application,” the number of studies drastically dropped. The carrier with the highest results was chitosan; however, only approximately 16% of studies were related to the medical field. The carrier with the highest proportion of medical application studies was liposome, with over 31% related to medical application. Conversely, the carrier with the lowest number of articles was zeolite, with the proportions of medical studies being 12.5%. The detailed results of the search are shown in Figure 2 and Figure 3.

Despite the original search not retrieving any results in the narrowest set of criteria, the selected studies have been described in Chapter 5 that were assessed as insightful for the subject of the current paper.

5. Drug Delivery Systems

5.1. Zeolites

Zeolites are a type of inorganic material that is commonly used as adsorbent due to their porous structure. Their main constituents are silicon, aluminum, and oxygen [51,52]. Aside from purely industrial applications like catalysts and adsorbents, they have also been used as drug delivery systems. Their uses included modulation of vitamin absorption, antimicrobial action, as carriers of anticancer and non-steroidal anti-inflammatory drugs, hemostasis, wound healing, and anti-diarrheal properties. They have also been proven to carry organic substances into the cells [53]. Generally speaking, they work based on their increased ion exchange capacity in zeolites with low Si/Al ratios. Such zeolites also tend to be more hydrophilic, which can help with EOCs’ water-insolubility. However, it is worth emphasizing that zeolites are quite susceptible to degradation in mild acid pH or the presence of water. Of more industrial and synthetic importance is the possible degradation of EOCs under high temperatures [54].

Much research focuses on the encapsulation of metal ions inside the zeolites to assess their antimicrobial properties. The results have shown that the best results are achieved using silver and zinc ions. Their zeolite composites were also more effective than sole ions [55]. Current studies indicate that various chemicals constituting major EOCs can be encapsulated in synthetic zeolites [54]. So far, there have been studies demonstrating a successful zeolite encapsulation of cinnamaldehyde, D-limonene, vanillin [56], eucalyptol [57], linalool [58], and numerous EOs [52,59,60,61,62]. To encapsulate an EO into zeolite, the former has to be in a high-concentration solution, and the latter is to be dried at about 160 °C. The drying process is crucial as it removes water molecules that reduce the adsorption surface, as the process needs to be separate from adding the EO solution since EOs degrade in these temperatures. The following process involves adding zeolites to EO’s solution and removing the solvent, such as ethanol. A couple of studies assessed the effect of such encapsulation and reported higher stability and gaseous phase retention. The addition of cyclodextrins has been shown to reduce the release of EOs from zeolites and increase their antioxidant properties [63,64].

The primary characteristics that influence the rate of adsorption of EOs in zeolites are the molecular size, structure, and the presence of functional groups. These characteristics explain various results obtained in studies analyzing different EOs [65]. The main issues that may arise are zeolite toxicity and the antimicrobial effect in vivo. Zeolite toxicity can vary substantially. Some zeolites, like erionite, scolecite, and offretite, were proven to be cytotoxic due to their fibrous structure, and this characteristic was used in possible applications as anticancer drugs [66]. Current studies indicate that zeolite toxicity is mainly influenced by its shape (spherical ones are less toxic) and aluminum content [67]. A couple of studies show that the toxic potential of zeolites depends on the cell type [54]. Studies dealing with the antimicrobial effect of zeolite-encapsulated EOs need to consider possible difficulties regarding proper effect assessment. Namely, measuring the actual bactericidal effect in vivo is not easy. Despite the encapsulation, EOs are difficult to assess in the classical agar plates approach due to their insolubility and volatility [68]. Few studies have researched the effect of EOs encapsulated in zeolites on bacteria directly. The possible antimicrobial effect is usually extrapolated based on the bactericidal properties of a given EO and not assessed directly [59]. An example of a study that directly showed the antimicrobial effect is thymol encapsulated in zeolite 4A [69]. Using the microdilution method, the authors obtained results showing that the composite has bactericidal activity against Staphylococcus aureus, Candida albicans, and E. coli and bacteriostatic effect against Pseudomonas aeruginosa. Binay et al. [70] assessed the antimicrobial effect of thymol encapsulated in zeolite X and added zinc ions to the composite structure. While the thymol-only zeolite had no antimicrobial effect against S. aureus, its combination with zinc showed superior bactericidal activity. The authors developed a hierarchical zeolite model that was more effective than more “classical” single porous plane zeolites. This study further proves strong bactericidal properties from adding metal ions in zeolites [55]. Some studies utilized zeolite EOs composites in food preservation; though it reaches out of the scope of this paper, this research has added further evidence for the practical application of this drug delivery system [71].

Further studies should focus on directly assessing bactericidal zeolite-EOCs. An additional combination of those composites in hierarchical structures with metal ions can also be a promising field of research.

5.2. Liposomes

Liposomes are spherical vesicles comprising one or several enclosing phospholipid bilayers, usually in an internal aqueous phase. Their main advantage is the ability to carry both hydrophilic and hydrophobic substances. The former is dissolved in the internal aqueous compartment, whereas the latter is in the lipid bilayer—Eos—as predominantly hydrophobic molecules are transported in the lipid bilayer [72]. So far, the liposomes used to encapsulate EOs include phosphatidylcholine, cholesterol, and lecithin. A couple of studies have shown that encapsulating EOs in liposomes helps extend their antimicrobial, antioxidant, and cytotoxic properties [72,73,74]. Despite the improved antimicrobial and antioxidant activities, studies have reported a couple of problems that hinder the medicinal application of this drug delivery system. Liposomes are unstable concerning pH and were also shown to have poor loading capacity and low physical stability. Additionally, the manufacturing process of liposomes has quite high costs. Despite these drawbacks, liposomes are experimentally researched to prevent food spoilage [75]. It has also been insinuated that double coating or other carrier techniques can help overcome these difficulties [72], a topic that is currently being researched experimentally [74].

There are many reports in the available literature on the use of liposomes loaded with EOs in medical applications. For example, Khatibi et al. [76] tested the antimicrobial activity of Zataria multiflora EO loaded into nanoliposomes against enterohaemorrhagic E. coli. Not only were the authors able to demonstrate lower MIC values of this method compared to the EO or carrier alone, but they also showed that it had a positive effect on stx2 gene expression. In turn, Ellboudy et al. [77] demonstrated that cinnamon EO loaded into liposomes showed potent antibacterial and antibiofilm properties against S. aureus, as evidenced by transmission electron microscopy. The authors reported the MIC values of 25 µg/mL compared to 50 µg/mL when the EOs were used alone. The results are consistent with other experiments like [78,79].

5.3. Nanoemulsions

Emulsions are formed by the dispersion of two immiscible phases: one phase (dispersed phase) is spread as droplets into another phase (continuous phase) [72]. Emulsions are created using various carrier materials like alginate, gum Arabic, lipids like soybean oil, or surfactant solutions like Surfynol, Tween, Span, Brij, or lecithin. Due to the low-cost carrier materials, producing EO emulsions is relatively cheap; they can be produced in larger quantities and are generally safe for organisms [72,80]. Current studies have demonstrated their improved antimicrobial and cytotoxic effects compared to pure EOs [81,82,83]. This can stem from the fact that their properties enable them to disperse better, especially in food matrices [83].

Yingngam et al. [84] described a two-step emulsification and dry spraying technique that enabled them to cover a textile surface with an EO, which was proven to reduce its volatility and irritation potential. At the same time, they showed poor loading capacity, their water solubility is quite low, and they have been shown to undergo lipid oxidation. It may be important medically since lipid oxidation can induce an inflammatory response, whose effect in medical applications is unknown so far [80].

A number of studies on the biological effects of nanoemulsions loaded with EOs/EOCs and their medical application can be found in the available literature. For example, El-Sherbiny et al. [85] showed that nanoemulsion of cinnamon EO could be effectively used against colistin-resistant Klebsiella pneumoniae. Aside from improved antibacterial activity, this study noted the significant downregulation of the mcr-1 gene expression, a fact that can be crucial in combating colistin resistance. What is more, Fan et al. [86] used sophorolipids for nanoemulsion stabilization, which, when loaded with eugenol, showed excellent antimicrobial properties against E. coli and B. cereus. In turn, Lin et al. [87] conducted an animal trial on mice that used nanoemulsion loaded with cinnamon EO in the treatment of Candida vaginitis. Aside from antifungal properties, the formula also showed anti-inflammatory action by curbing the inflammatory cytokines. The results of this study are important to this topic as pure cinnamon EO cannot be readily used due to its hydrophobicity, hence the crucial role of the drug delivery system. Moreover, antifungal properties against various pathogenic species were also demonstrated in the case of the nanoemulsion loaded with clove EO [88]. Another example of research can be found in the work of Miastkowska et al. [89], who conducted tests for a possible burn-wound dressing consisting of nanoemulsion loaded with Lavandula angustifolia EO. Their study showed mitigation of inflammation in the in vitro model. It is also worth emphasizing that the authors did not conduct an antimicrobial assay; as such, it is unknown whether the concentrations determined as safe would also exhibit potential to prevent skin sepsis. However, such an application may not be out of the scope, as Kakadia and Conway [90] showed that nanoemulsion loaded with eucalyptus, olive Eos, and triclosan could be topical antiseptics.

5.4. Chitosan

Chitosan is a naturally occurring polysaccharide composed of randomly distributed β-(1→4)-linked D-glucosamine and N-acetyl-D-glucosamine. In neutral pH, the polysaccharide is water-insoluble. However, even a slight decrease in pH causes protonation of the amine group, making it water-soluble. Chitosan is not toxic and is known to be highly mucoadhesive. From an economic standpoint, it is easy to produce and is readily biodegradable.

Additionally, chitosan has been shown to have antimicrobial properties that are possibly linked to the disruption of bacterial cell wall permeability [80]. Moreover, no significant drawbacks to its utilization have been found so far. It has a wide variety of industrial uses (aside from purely chemical to recently approved bio packaging and wound dressing), but in this paper, we will focus only on its potential as a nanocarrier [91]. Chitosan has been utilized as a nanocarrier for many EOs. A couple of studies assessed the antimicrobial effects of this drug delivery method. Dupuis et al. [80] summarized the research on this topic, referencing studies that show the antimicrobial potential of chitosan-loaded EOs against Staphylococcus epidermidis, S. aureus, B. cereus, Listeria monocytogenes, E. coli, Salmonella Typhi, Shigella dysenteriae, and Xanthomonas campestris. Since then, a couple of additional studies have been conducted. Nur Fatin Nazurah et al. [92] demonstrated curry leaf EO encapsulation and its release mechanism. Jiang et al. [93] showed that Eucommia ulmoides seed EO encapsulated in chitosan had better antimicrobial and antioxidant properties in vitro. Demirhan et al. [94] demonstrated that St. John’s Wort EO encapsulated in a hydrogel had worse antimicrobial properties than the hydrogel loaded with only chitosan. However, when applied in a wound healing study, the combination of St. John’s Wort EO and chitosan in the hydrogel effectively reduced wound size. Apart from bactericidal properties, some studies demonstrated that this delivery method can also be a viable strategy against fungi in vitro and in vivo using an animal model [95,96,97].

Despite the apparent lack of significant disadvantages of this drug delivery system, in vivo human studies are still lacking. Much research has been conducted to establish chitosan-encapsulated EOs in the food and agriculture industry; however, most studies were performed in vitro only. Current in vivo studies have only been conducted on farm animals, which does not give us a clear picture of the effect on humans.

5.5. Cellulose Nanomaterials

Cellulose is one of the most abundant polymers quickly produced using plants or bacteria. Aside from its low cost, it is also readily biodegradable and non-toxic. Numerous studies have demonstrated that cellulose can carry various substances, including EOs. Studies have shown that cellulose nanomaterial structure could protect EOs from ultraviolet exposure and help in controlled release [98]. The release system can be easily modified using pH changes [98].

Additionally, there are reports concerning hemostatic and mucoadhesive characteristics of cellulose [80,99,100,101]. These properties have been so far utilized in various wound dressings [102]. Kumineck Junior et al. [102] used bacterial cellulose in combination with various EOs and showed that clove, lemongrass, and ginger composites could maintain their antimicrobial properties. However, some tested EOs did not exhibit antimicrobial properties (which may be due to different morphologies of the samples), and the EOs/bacterial cellulose combination results were comparable to or lower than the EOs’ results alone. It may be due to the different amounts of EOs incorporated into the cellulose structure. Such changes may result in different concentrations or dispersion methods of the substances, thus influencing the results. Whereas some studies demonstrated good antimicrobial properties of cellulose nanomaterials combined with EOs, it is worth emphasizing that cellulose appears to have no antimicrobial effect [101,102]. This is a serious problem since inadequate filling of the matrix with EO can lead to poor results. Hence, we stress the importance of adequate dosing and environmental conditions in future studies and applications [102].

5.6. Zein

Zein is a naturally occurring protein that is found in corn. It has been widely used in various industrial applications, usually as a coating agent for various substances. Recently, more studies have started revealing the actual properties of zein, which has inspired possible new applications in the pharmaceutical industry [80]. Zein has been widely studied in the food industry as a possible way to increase the shelf life of various products. Combined with EOs, it demonstrated important antimicrobial properties that are used to prevent food spoilage and possible food-borne disease outbreaks [80,103]. Some studies combined zein with mesoporous silicate, achieving higher local concentration and decreasing volatility [104,105]. Studies have demonstrated that zein-encapsulated EOs have improved stability and modified release profiles [80,106]. Worth noting is the fact that a study performed by Bilenler et al. [107] has found that zein-encapsulated thyme EO had substantially lower antimicrobial activity than the EO alone. The authors speculated whether the low water solubility of zein might be one of the factors that led to these results and stressed that other antimicrobial assay methods could be better for an adequate determination of zein’s properties. In contrast, a study that conducted a thorough assay of lemon grass EO encapsulated in zein-caseinate showed that the encapsulation resulted in increased cell membrane permeability, as evidenced by the analysis of intracellular enzymes’ leakage [108].

Unfortunately, there is a severe lack of studies in the biomedical field; the available literature concentrates on food industry applications, which seriously hampers the possible use in medicine.

5.7. Alginate

Alginate is a naturally occurring polysaccharide composed of unbranched chains of b-D-Mannuronate and a-L-Glucuronate residues linked by a b-(1-4) glycosidic covalent bond. It is produced from brown algae, mostly Macrocystis pyrifera, Laminaria hyperborea, and Ascophyllum nodosum [109]. Most of its applications use alginate as an aerogel, such as tissue scaffolds, wound dressings, carriers of antibiotics, and metal cations [109].

So far, numerous studies have shown the encapsulation of EOs into alginate aerogel. Aside from successful encapsulation, the authors also assessed their antimicrobial [110,111,112,113,114], antioxidation [114], and physical [113,115] properties. Current research indicates that such composites have the same antimicrobial properties as pure EOs. It corroborates neatly with an observation that although they are non-toxic, alginate aerogels do not have any antimicrobial properties. While the scope of antibacterial properties is quite broad, ranging in E. coli, L. monocytogenes, B. cereus, and S. aureus, the majority of research in the field is focused on the food industry, looking for applications of EO-infused alginate aerogels as food preservatives. However, due to their hemostatic and mucoadhesive properties, Nqoro et al. [110] showed that its application in wound dressing as a gel could be viably used. Their study assessed the antimicrobial, blood clotting, and cytotoxic properties of EOs, EOs in alginate aerogel, and their combination with Fe₃O_4. The results showed that EOs combined with alginate had minimum inhibitory concentration values comparable to EOs alone. Additionally, they showed that while rose with Fe₃O₄ combination had decreased antimicrobial activity, the same parameters were better in lavender with Fe₃O₄ combination in alginate. The latter was a promising wound dressing based on the rest of the tested parameters (spreadability, cytotoxicity, and blood clotting) [110].

5.8. Poly(D,L-lactic-co-glycolic) Acid (PLGA)

PLGA is a copolymer that is composed of lactic and glycolic acid. The endogenous nature of these substances and the fact that the organism easily metabolizes the polymer itself contribute to its very low toxicity and safe biodegradability profile. There have been studies that assessed its release profile, which are summarized in Dupuis et al. [80]. Since then, studies have researched its possible applications outside the food industry. PLGA has been used, among other things, to increase the shelf life of drugs at their final site of action, as such compounds include anastrozole (an aromatase inhibitor), a compound administered to treat breast cancer in postmenopausal women. In 2006, Zidan et al. [116] published the first paper showing the encapsulation of this drug through the use of biodegradable microparticles based on lactic-co-glycolic acid copolymers (PLGA-poly[lactic-co-glycolic acid]).

PLGA has been studied extensively as a polymeric carrier for biodegradable microspheres. PLGA microparticles have proven to be an effective way to deliver drugs belonging to various classes, including non-steroidal anticancer drugs—including anastrozole—and non-steroidal anti-inflammatory drugs, peptides, and steroid hormones [117]. In addition, another paper documented that electrospun PLGA ultrafine fibers loaded with fusidic acid with antimicrobial activity have found their application in wound healing as a wound dressing material [118]. PLGA was selected as a Food and Drug Administration-approved biodegradable and biocompatible copolymer compound. PLGA with different ratios of glycolic acid to lactic acid produces fibers with suitable mechanical properties and a wide range of diameters and degradation rates. Due to its hydrophobicity, PLGA is electrospun from organic solvents. The developed electrospun fibrous mats were used to test in vitro interactions with the following three strains of bacteria found on wounds: P. aeruginosa; methicillin-resistant S. aureus (MRSA) reference strain; and MRSA clinical isolate. This study investigated the effect of wound bacteria on the structural integrity and function of the ultrathin fibrous mat, mainly in terms of matrix degradation and drug release properties, as well as the effect of mat properties containing fatty acids on in vitro antimicrobial activity. This study confirmed that biodegradable polymeric ultrathin fibrous wound dressing materials, in addition to providing structural support for wound healing and antimicrobial reservoir function for wound infection control, are actively involved in dynamic interaction with the wound environment [119].

Two studies conducted by Folle et al. [119,120] used thyme EO encapsulated in PLGA to treat skin inflammation and acne. Their results indicate that such a strategy may be viable. In one of the works, the authors showed that Cutibacterium acnes viability decreased after 2 h of application, while the healthy skin microbiota was unaffected [121]. The second work has also shown that this formulation could promote wound healing, probably due to its antimicrobial and anti-inflammatory effects [119,120]. However, it is worth emphasizing that those studies were conducted on relatively few participants (12). The participants had no skin conditions, and the authors stressed the need to evaluate the long-term effects of this application method. However, there are severe limitations to the use of PLGA nanosystems. Various authors have noticed that their loading capacity is quite low, and their production cost is high and difficult to scale up [80].

6. Conclusions

EOs and their constituents are of growing interest in some fields, especially the food and veterinary industry, but their application in medical settings is still lacking. Given the sheer diversity of EOs, this might be a substantial concern, as they could theoretically be used in treating various conditions. Even the methods whose results are equal to that of an EO alone can have possible medical applications by delivering the EO to a tissue that would otherwise be unreachable without the carrier.

Despite their low production costs and mucoadhesive and hemostatic properties that can become useful in wound dressings, alginate and cellulose do not have antimicrobial properties on their own, which can limit their effectiveness. Chitosan might prove to be their substitute, but there have been no clinical studies on it so far. PLGA can be another promising alternative to the aforementioned drug delivery systems. It can provide adequate scaffolding for wound dressings and may also deliver various EOs. However, its high production costs and low loading capacity need to be addressed. Other more economical drug delivery systems like zein and nanoemulsions suffer from low water solubility in the case of the former and oxidation potential and low loading capacity in the latter. Nevertheless, zein has been shown to have a high loading capacity, whereas nanoemulsions have been shown to be generally stable. Zeolites are promising due to their possible modification to accommodate various EOs and bactericidal properties; however, they are susceptible to pH changes and water presence. A similar problem is present in the case of the liposomes, which, aside from the mentioned susceptibility, are also unstable and have low loading capacity despite their synergistic effects when loaded with EOs. A summary of the significant advantages and disadvantages of different delivery systems loaded with EOs can be seen in Figure 4.

The studies we assessed in this review are mostly limited in scope. The discrepancies in methods used can be partially responsible for the differences in results from the in vitro studies. Clinical studies are limited regarding patient numbers and the range of conditions. Hence, it is not easy to make firm conclusions regarding the efficacy and safety of EOs combined with a given drug delivery system. The lack of research in this area seriously hinders the full potential of EO utilization as a possible antimicrobial drug. Nevertheless, this review shows that various drug delivery systems can potentially mitigate the most common problems regarding the EOs.