Ethosomes as Advanced Vesicular Nanocarriers for Transdermal Drug Delivery: Formulation Principles, Mechanistic Insights, Characterization Strategies, and Therapeutic Applications

The skin represents one of the largest and most versatile routes for both topical and systemic drug administration. However, effective drug delivery through the skin is primarily restricted by the stratum corneum, the outermost layer, which functions as a strong protective barrier and significantly limits drug penetration and bioavailability following topical application. Consequently, extensive research has focused on developing suitable carrier systems capable of overcoming this natural barrier to improve systemic drug delivery. Transdermal drug delivery systems offer a non-invasive alternative for drug administration and provide several advantages, including controlled drug release, reduced dosing frequency, improved patient compliance, and avoidance of first-pass metabolism. These benefits have encouraged the exploration of advanced vesicular carriers for enhanced skin permeation. The introduction of liposomes marked a significant milestone in drug delivery research and led to the development of various vesicular drug delivery systems. In 1992, Cevc and Blume introduced transferosomes, also known as elastic or deformable liposomes, which demonstrated improved flexibility and penetration ability. Later, pioneering research by Elka Touitou and colleagues resulted in the development of ethosomes, a novel lipid-based vesicular carrier system. Limitations associated with conventional liposomes, such as larger particle size, low drug entrapment efficiency, and unfavorable zeta potential, encouraged the development of modified lipid carriers. Ethosomes are composed mainly of phospholipids, ethanol, and water, with ethanol present in relatively high concentrations.

2. Skin Structure and Barrier Function

2.1 Anatomy of the Skin

The skin is the largest organ of the human body and acts as a protective barrier against physical, chemical, and microbial insults. In transdermal drug delivery, particularly with vesicular carriers such as ethosomes, understanding skin structure is essential because drug permeation is governed by its layered organization.

The skin is composed of three main layers: epidermis, dermis, and hypodermis.

Epidermis

The epidermis is the outermost layer and serves as the primary barrier to percutaneous absorption. It is a stratified, keratinized epithelium mainly composed of keratinocytes. The most important sublayer for drug delivery is the stratum corneum (SC), which is approximately 10–20 µm thick. The stratum corneum follows the “brick and mortar” model, where corneocytes (bricks) are embedded within a lipid matrix (mortar) composed primarily of ceramides, cholesterol, and free fatty acids. This highly organized lipid structure restricts the penetration of most drugs and represents the rate-limiting step in transdermal delivery. Beneath the stratum corneum lies the viable epidermis, which contains metabolically active cells but offers comparatively less resistance to drug diffusion. The epidermis is a vascular; therefore, drugs must reach the dermis for systemic absorption.

Dermis

The dermis is a connective tissue layer located below the epidermis and is rich in collagen, elastin fibers, blood vessels, and lymphatics. It provides structural support and plays a crucial role in systemic drug absorption. Once a drug penetrates into the dermal microcirculation, it can enter systemic circulation.

Hypodermis

The hypodermis (subcutaneous tissue) consists mainly of adipose tissue and connective tissue. Although not directly involved in the barrier function, it contributes to insulation and cushioning.

2.2 Relevance to Ethosomes

The stratum corneum is the major barrier limiting drug permeation. Ethosomes, composed of phospholipids and high ethanol content, enhance drug delivery by increasing lipid fluidity and disrupting the tightly packed intercellular lipids of the stratum corneum. This facilitates deeper penetration into the epidermal and dermal layers.

2.3 Barrier Function of the Skin

The skin acts as a selective permeability barrier. Drug permeation across the skin occurs mainly through three pathways: Intercellular pathway – diffusion between lipid bilayers ,Transcellular pathway – diffusion through corneocytes , Appendageal pathway – via hair follicles and sweat glands. Among these, the intercellular route is the predominant pathway for most drugs. The highly ordered lipid organization of the stratum corneum results in Low permeability to hydrophilic drugs, Restricted diffusion of macromolecules and Limited penetration of ionic compounds. Therefore, overcoming the stratum corneum barrier is the primary challenge in transdermal drug delivery system development.

Relevance to Ethosomal Systems

The rigid and hydrophobic structure of the stratum corneum limits the effectiveness of conventional vesicular systems such as liposomes. Ethosomes, due to their high ethanol content, disrupt the lipid packing of the stratum corneum and enhance vesicular deformability, enabling deeper penetration. Thus, understanding the detailed structure and barrier properties of the skin is essential for rational design and optimization of advanced nanocarrier systems like ethosomes.

 Conventional Vesicular Systems and Their Limitations

Traditional lipid vesicles such as Liposomal Drug Delivery System have been extensively investigated for dermal drug delivery. However, conventional liposomes show Poor penetration through intact stratum corneum,   Drug accumulation mainly in upper skin layers and Limited deformability. Transfersomes were later introduced to improve deformability, but stability issues remain a concern. These limitations led to the development of ethosomes.

3. Types of Ethosomal Systems

Ethosomal systems are generally categorized into three types based on their composition: classical ethosomes, binary ethosomes, and transethosomes.

1. Classical Ethosomes

Classical ethosomes are modified lipid vesicles composed of phospholipids, a relatively high concentration of ethanol (up to 45% w/w), and water. Compared to conventional liposomes, these vesicles exhibit smaller particle size, negative zeta potential, and improved drug entrapment efficiency.Due to the presence of ethanol, classical ethosomes demonstrate enhanced flexibility and superior skin permeation capability. They also show improved stability profiles in comparison with traditional liposomal systems. Drugs incorporated into classical ethosomes have ranged widely in molecular weight, from small molecules (~130 Da) to macromolecules up to approximately 24 kDa.

2. Binary Ethosomes

Binary ethosomes were later developed as a modification of classical ethosomes. In this system, an additional alcohol—commonly propylene glycol or isopropyl alcohol—is incorporated along with ethanol.The inclusion of a second alcohol further enhances vesicle flexibility and may improve drug permeation through synergistic effects on skin lipid disruption. This modification aims to optimize the physicochemical characteristics and transdermal performance of the vesicular system.

3. Transethosomes

Transethosomes represent an advanced generation of ethosomal carriers. In addition to the basic components of classical ethosomes, these systems contain an edge activator (surfactant) or a penetration enhancer.They were designed to combine the advantages of ethosomes and deformable liposomes (transfersomes) within a single formulation. The presence of surfactants increases vesicle deformability, allowing enhanced passage through the stratum corneum. Transethosomes have demonstrated improved entrapment efficiency, flexibility, and permeation compared to earlier ethosomal systems. They are capable of incorporating drugs across a broad molecular weight range, including larger biomolecules (up to approximately 200–325 kDa).

Comparison of Different Types of Ethosomal Systems

4. Ethosomal  Composition :

The vesicular carriers known as ethosomes are made of hydroalcoholic or hydro/alcoholic/glycolic phospholipids, which contain a considerable quantity of alcohols or alcohols in combination. [19-25]. The many types of additives utilized in the preparation of ethosomes are shown in Table 2

Table 2.Difference between various ethosomes for transdermal drug delivery

5. Mechanism Of Drug Penetration:

A)Ethanol effect: Ethanol works to improve the penetration of the skin. Its boosting action through absorption has a well-established mechanism. By penetrating intercellular lipids, ethanol increases the fluidity of cell membrane lipids while decreasing their density.

B)Ethosome effect: An increase in skin permeability is the result of ethosome ethanol's greater lipid fluidity in cell membranes. Ethosomes thus penetrate the deep skin layer relatively rapidly, where they interact with skin lipids to release medications into the deep skin layer

Figure No.3: Drug Penetration through Ethosomes

6.Method Of Preparation of Ethosomes:

1.Cold method: For ethosome preparation, the cold approach is one of the most popular techniques. First, phospholipid is vigorously stirred in ethanol at room temperature to dissolve it. Next, polyols such as propylene glycol are added gradually while being stirred frequently, and the mixture is heated to 30ºC in a water bath. The water is then heated to 30 degrees Celsius in a different vessel, and the two combinations are combined. The mixture is then stirred for five minutes in a covered vessel. The ethosomal formulation can have its size reduced to the necessary degree by employing the sonication technique.(28-29)                         

2.Hot method: In the hot process, phospholipid is added to water and heated on a water bath to 40ºC until an aqueous phase, or colloidal solution, is formed. In a separate vessel, ethanol and propylene glycol are properly combined and heated to 40ºC (Organic phase). Under continuous stirring, the organic phase is introduced to the aqueous phase. A desired degree of ethosomal formulation size reduction can be achieved by employing the sonication technique.(29-30)                         

3.Classic mechanical dispersion method: Using a round-bottom flask, this approach dissolves phospholipid in an organic solvent or a combination of organic solvents. To produce a thin layer of lipids on the RBF surface, the organic solvent is removed using a rotating vacuum evaporator. By keeping the contents under vacuum for the entire night, traces of the solvent are extracted from the lipid film that has formed. The drug's hydro-ethanolic solution is used to hydrate the lipid layer by spinning the flask at the proper temperature. Cool the resultant ethosomal suspension at room temperature (31-32)

4. The ethanol injection–sonication method: This procedure involves injecting the organic phase, which contains the phospholipid dissolved in ethanol, into the aqueous phase using a 200-flow syringe system at a rate of 38μl per minute. An ultrasonic probe is then used to homogenize the mixture for five minutes(.32-33)

7. Characterization Of Ethosomes:

1. Vesicle shape: Ethosomes are easily visible with scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
2. Size and zeta potential: The ethosomes' particle size can be ascertained using photon correlation spectroscopy (PCS) and dynamic light scattering (DLS). The formulation's zeta potential can be determined using a zeta meter.
3. Transition temperature: Differential scanning calorimetry (DSC) can be used to determine the transition temperature of vesicular lipid systems.
4. Drug entrapment: The technique of ultracentrifugation can be used to determine the entrapment effectiveness of ethosomes. [
5. Drug content: The ethosomes drug content can be evaluated with a UV spectrophotometer. An alternative approach for quantifying this is a modified high performance liquid chromatographic technique. 
6. Surface tension measurement: Using a Du Nouy ring tensiometer and the ring method, one can determine the drug's surface tension activity in aqueous solution.
7. Stability studies: The size and structure of the vesicles can be evaluated over time to ascertain their stability. DLS measures the mean size, while TEM detects changes in structure.
8. Skin permeation studies: Confocal laser scanning microscopy (CLSM) can be used to assess the ethosomal preparation's capacity to pierce the skin's layers.(34-38)

8. Advantages  and Disadvantages of Ethosomes Drug Delivery

Advantages

1. Better medication penetration than liposomes for topical and transdermal administration.
2. Proteins and peptides are macromolecules that are easily transported through the skin.
3. Patient compliance is improved when ethosomal medicine is administered in a semi-solid (gel or cream) form.
4. It is made with non-toxic, skin- and body-friendly ingredients.
5. The ethosomal delivery is non-invasive, passive, and ready for quick commercialization.
6. Ethosomal drug delivery systems are broadly applicable in cosmetic, pharmaceutical, and medical settings.
7. Easy to produce without requiring the complex technological expenditure needed to produce ethosomes.
8. Better stability and solubility: By encapsulating medications, ethosomes can
9. comparison to traditional vesicles.
10. Ethosomes are comparatively smaller than other types of vesicles.
11. Enhancement of pharmacokinetic effect: Ethosomes lengthen the time it takes for the body to circulate.
12. Ethosomes can be coupled with site-specific ligands to accomplish active targeting because of their flexibility.
13. A simpler approach to drug administration than phosphophoresis, iontophoresis, and other intricate techniques.
14. Low risk profile: Because the ethosomal components' toxicity profiles are well-established in the scientific literature, there is no risk of large-scale drug development using this technique.
15. Products with proprietary technology have a high market appeal. Ethosomes are comparatively easy to produce and don't require complex technical inputs.

Disadvantages

1. It is possible for ethosomes with weak shells to group together and cause precipitation.
2. The medicine should be sufficiently soluble in both aqueous and lipophilic conditions to enter the systemic circulation and reach the cutaneous microcirculation.
3. Ethosomal administration is typically intended to provide gradual, sustained drug delivery rather than a quick bolus-style drug input.
4. Medication that needs elevated blood levels cannot be given; only strong medications can be used. (daily dose -10mg or less)
5. The product is lost when ethosomes move from the organic to the aqueous layer (44-

9.Factors Influencing Ethosomal Performance

The efficiency of ethosomal formulations is influenced by multiple formulation and environmental factors.

1.Ethanol Concentration

Ethanol concentration plays a crucial role in determining vesicle characteristics. The optimal range is typically 20–45%. Ethanol increases membrane fluidity, enhances drug solubility, and improves skin penetration. However, excessive ethanol may destabilize vesicles, cause drug leakage, and increase the risk of skin irritation.

2.Phospholipid Composition

The type and concentration of phospholipids significantly affect vesicle properties. Saturated lipids increase membrane rigidity, whereas unsaturated lipids enhance flexibility. Higher phospholipid concentration improves entrapment efficiency but may also increase vesicle size.

3.Drug Solubility

The physicochemical properties of the drug strongly influence ethosomal performance. Lipophilic drugs generally show higher encapsulation efficiency, while hydrophilic drugs may require optimized preparation techniques to improve loading capacity and stability.

4.Vesicle Size

Vesicle size directly affects penetration and bioavailability. Smaller vesicles improve dermal penetration and enhance drug bioavailability. However, extremely small vesicles may exhibit reduced stability and increased aggregation risk.

5.Storage Temperature

Storage conditions significantly influence formulation stability. High temperatures accelerate ethanol evaporation and promote vesicle aggregation. Refrigerated storage at 4–8°C is recommended to maintain vesicle integrity and prolong shelf life.

6.Presence of Stabilizers

Incorporation of stabilizers such as cholesterol, antioxidants, and cryoprotectants enhances vesicle stability and shelf life. These agents protect phospholipids from oxidation, reduce leakage, and improve overall formulation robustness.

7.Stability Considerations

Ethosomal formulations face several stability challenges that must be carefully addressed.

7.1 Ethanol Evaporation- Ethanol evaporation reduces penetration efficiency, increases vesicle rigidity, and may cause drug leakage.

7.2 Vesicle Aggregation- Aggregation may occur due to low zeta potential, high storage temperature, or poor formulation optimization, leading to reduced stability and efficacy.

7.3 Phospholipid Oxidation- Unsaturated phospholipids are susceptible to oxidative degradation, resulting in structural instability and reduced therapeutic efficiency.

7.4 Hydrolysis- Hydrolytic degradation of phospholipids may occur under improper storage conditions, affecting vesicle integrity.

8.Stability Enhancement Strategies-

Stability can be improved through refrigerated storage, incorporation of antioxidants such as tocopherol, nitrogen flushing, incorporation into gel systems, and lyophilization with cryoprotectants.

11. Therapeutic Applications

Ethosomes have demonstrated versatility across various therapeutic fields.

11.1 Antifungal Drug Delivery- Ethosomes enhance dermal penetration of azole antifungal agents, improving the treatment of candidiasis, dermatophytosis, and tinea infections while reducing systemic toxicity.

11.2 Anti-inflammatory and Analgesic Drugs- Enhanced dermal absorption of NSAIDs reduces gastrointestinal side effects and provides sustained pain relief.

11.3 Anticancer Therapy- Ethosomal delivery of chemotherapeutic agents enhances drug localization in melanoma treatment and reduces systemic exposure.

11.4 Antiviral Therapy- Ethosomes show potential in delivering antiviral agents for herpes simplex virus and other cutaneous viral infections.

11.5 Psoriasis Management- Improved penetration of anti-psoriatic drugs enhances dermal retention and reduces dosing frequency.

11.6 Scar Treatment- Ethosomes improve localization of corticosteroids and anti-fibrotic agents, enhancing scar remodeling and healing outcomes.

12. Ethosomal Gels and Hybrid Systems

To improve patient compliance and formulation stability, ethosomes are incorporated into semisolid systems.

12.1 Ethosomal Gels- Ethosomes are incorporated into gel bases such as Carbopol, Hydroxypropyl methylcellulose (HPMC), and Poloxamer. These gels increase viscosity, prolong residence time, improve stability, and enhance ease of application.

12.2 Thermoresponsive Gels- Thermoresponsive gels remain liquid at room temperature and convert to gel at body temperature, improving patient convenience and retention at the application site.

12.3 Transethosomes- Transethosomes are hybrid vesicular systems containing ethanol and edge activators (surfactants). These systems exhibit superior deformability and enhanced deep tissue penetration compared to conventional ethosomes.

13. Comparison with Other Vesicular Systems