Transferosome-Based Drug Delivery Systems: Formulation Approaches, Characterization And Recent Advances In Buccal And Transdermal Delivery

Drug delivery systems are designed not only to transport an active pharmaceutical ingredient to the body but also to control the rate, site and duration of drug exposure. Conventional dosage forms can be limited by poor aqueous solubility, chemical or enzymatic degradation, extensive first-pass metabolism, short biological half-life, variable absorption and inadequate patient adherence. Non-invasive routes such as transdermal and buccal administration can offer useful alternatives for selected drugs.

However, both routes contain physiological barriers that must be addressed by formulation design.

The skin provides an attractive route for systemic and local therapy because it is accessible, relatively large and capable of supporting prolonged administration. The principal barrier is the stratum corneum, which has a highly organized lipid–protein architecture and strongly restricts penetration of many compounds. Conventional passive diffusion is therefore generally most suitable for molecules with favorable molecular size, lipophilicity and potency. Transferosomes were developed to overcome some of these limitations through an ultra-deformable vesicular architecture. Their phospholipid bilayer contains an edge activator, typically a surfactant or bile salt, that introduces controlled flexibility into the membrane.¹–?

The proposed advantage of transferosomes is not simply that they are nanoparticles or vesicles. Their functional identity is associated with deformability. Edge activators alter lipid packing and permit the vesicle to change shape under stress.

Under suitable non-occlusive conditions, the hydration gradient between the formulation and skin can contribute to directed transport. Experimental evidence suggests that vesicle composition, deformability, hydration and interaction with skin lipids jointly influence delivery.¹–?

The buccal mucosa presents a different biological environment. It is moist, continuously exposed to saliva and mechanical movement, and subject to mucus turnover. At the same time, buccal delivery can avoid gastrointestinal degradation and may reduce first-pass hepatic exposure for drugs that are absorbed directly through the mucosa. Transferosomes have therefore been investigated in buccal gels and other mucosal systems. A human pharmacokinetic study of loratadine-loaded transferosomal gel reported favorable bioavailability characteristics and reduced inter-individual variability compared with oral administration, supporting further interest in this route.?

The current literature is extensive but heterogeneous. Studies differ in phospholipid source, edge activator, drug concentration, preparation technique, measurement conditions and definitions of deformability. Recent critical reviews have consequently emphasized standardization, reproducibility, scale-up and the need to connect physicochemical attributes with biological performance.?,? This review addresses these issues by integrating formulation science, analytical characterization and applications in transdermal and buccal drug delivery.

2. CONCEPT, HISTORY AND TERMINOLOGY

Transferosomes are generally described as ultra-deformable lipid vesicles derived conceptually from conventional liposomes. The term is associated with vesicles containing at least one vesicle-forming phospholipid and an edge activator that makes the bilayer highly flexible.?–? The edge activator is usually a single-chain surfactant or related amphiphile. Examples include sodium cholate, sodium deoxycholate, Tween 20, Tween 80, Span 60 and Span 80.?–¹?

The development of transferosomes addressed a fundamental limitation of conventional liposomes: a relatively rigid bilayer may have difficulty passing through narrow intercellular pathways. In an ultradeformable vesicle, membrane components can redistribute during deformation.

The edge activator preferentially stabilizes curved regions and reduces the energetic cost of shape change.?

Terminology in this field requires care. Terms such as transfersome, transferosome, ultradeformable liposome and elastic vesicle are sometimes used interchangeably, while related systems such as ethosomes and transethosomes have different compositions and mechanisms. A transferosome should therefore be characterized by its composition and functional deformability rather than by size alone. Recent literature specifically identifies inconsistent terminology and non-harmonized deformability assays as barriers to comparison among studies.?

3. STRUCTURE AND COMPOSITION

A transferosome consists primarily of a phospholipid bilayer, an edge activator and an aqueous phase. Additional components may be introduced to modify membrane properties, improve stability, enhance drug loading or facilitate incorporation into a final dosage form.

3.1 Phospholipids

Phospholipids provide the structural bilayer. Soy phosphatidylcholine and egg phosphatidylcholine are commonly reported materials. The degree of saturation, fatty-acid chain length, purity and transition characteristics of the phospholipid can affect membrane packing and elasticity. A more ordered bilayer may provide greater structural rigidity, whereas excessive fluidity may increase leakage.

3.2 Edge activators

Edge activators are the defining functional excipients. Commonly reported materials include sodium cholate, sodium deoxycholate and non-ionic surfactants such as Tween and Span derivatives. Their concentration must be optimized because increasing edge activator content may improve deformability up to an optimum, while excessive surfactant can destabilize or solubilize the bilayer.?–¹?

3.3 Cholesterol

Cholesterol may be included to regulate membrane packing and reduce permeability or leakage. Its effect is concentration-dependent. Excessive cholesterol can increase membrane order and reduce flexibility, potentially opposing the intended ultradeformable behavior.

3.4 Alcohol and penetration modifiers
Some transferosomal formulations contain ethanol or another alcohol, especially when the preparation process or drug properties require a cosolvent. Ethanol can modify lipid organization and skin permeability. However, systems with substantial ethanol may overlap conceptually with ethosomal or transethosomal platforms, so the composition should be reported clearly.³,?

3.5 Hydration medium

Water or buffered aqueous media are commonly used for hydration. The pH and ionic strength can influence drug ionization, vesicle surface charge and colloidal behavior. For buccal systems, the formulation pH also becomes important for mucosal tolerability.

4. MECHANISM OF FORMATION AND DRUG ENCAPSULATION

When amphiphilic phospholipids are exposed to an aqueous environment, hydrophobic tails tend to minimize contact with water while hydrophilic head groups interact with the aqueous phase. This spontaneous organization produces bilayer structures that can close into vesicles. Edge activators insert into the phospholipid bilayer and modify membrane curvature and elasticity.

Hydrophilic drugs are primarily associated with the aqueous compartment, while lipophilic drugs can partition into the bilayer. Amphiphilic drugs may distribute between both regions. Entrapment efficiency therefore depends on drug solubility, partition coefficient, ionization state, lipid composition, vesicle concentration and preparation conditions.

A useful formulation principle is that drug loading and deformability may not change in parallel. Increasing lipid concentration may increase the amount of drug associated with vesicles but can also change viscosity and membrane organization. Likewise, increasing surfactant may improve elasticity while reducing entrapment if the bilayer becomes excessively disrupted. Optimization is therefore a multidimensional problem rather than a single-variable adjustment.

5. MECHANISM OF TRANSDERMAL TRANSPORT

The stratum corneum is commonly described as a 'brick-and-mortar' barrier, in which corneocytes are embedded within an organized lipid matrix. Transferosomes are proposed to overcome this barrier through a combination of deformability, hydration-driven movement, lipid interaction and drug release.

The hydration-gradient hypothesis is particularly important. Under non-occlusive application, the skin surface is relatively dry compared with the deeper hydrated tissue. This gradient can act as a driving force for vesicle movement. Highly deformable vesicles may squeeze through pathways smaller than their apparent diameter while maintaining vesicle integrity to some extent.?,?,¹¹

However, intact vesicle penetration should not be assumed for every formulation. Depending on membrane composition and environmental conditions, vesicles may partially disintegrate, exchange lipids with the skin, release drug near the barrier or modify the organization of skin lipids. Current reviews emphasize that the relative contribution of these mechanisms remains formulation-dependent.?,¹¹

Consequently, claims of 'intact vesicle penetration' should be supported by appropriate experimental evidence rather than inferred from enhanced permeation alone. Permeation enhancement can arise from several mechanisms, and fluorescent labeling or particle tracking should be interpreted carefully because labels may dissociate from the vesicle.

6. PREPARATION METHODS

6.1 Thin-film hydration

Thin-film hydration remains one of the most frequently used laboratory approaches. Phospholipid and lipid-soluble components are dissolved in a volatile organic solvent system. The solvent is removed under reduced pressure or controlled evaporation to obtain a thin lipid film. The film is then hydrated with an aqueous phase containing the drug or other water-soluble components. Hydration is followed by agitation, sonication, extrusion or homogenization when smaller and more uniform vesicles are required.?,?

6.2 Ethanol injection

In ethanol injection, lipid components are dissolved in ethanol and injected or added into an aqueous phase under controlled mixing. Rapid dilution changes the solvent environment and promotes vesicle formation. This method can be relatively simple but requires control of ethanol concentration, mixing rate and residual solvent.

6.3 Reverse-phase evaporation

A water-in-oil emulsion is prepared from aqueous and organic phases, followed by removal of the organic solvent. The method can provide useful entrapment of hydrophilic compounds but involves more complex solvent handling.

6.4 Sonication, extrusion and homogenization

These are generally size-reduction or post-formation processing approaches rather than independent vesicle-composition strategies. Sonication can reduce vesicle size but excessive energy may increase temperature or promote leakage. Extrusion through defined membranes can narrow the size distribution. High-pressure homogenization can be useful for scale-up but requires process optimization.

6.5 Mechanical stirring

Magnetic stirring or mechanical agitation may be used during hydration and dispersion. Stirring alone may produce larger or more heterogeneous vesicles than high-energy methods, depending on composition and conditions. The method can nevertheless be useful for preliminary formulation development when the objective is to investigate composition before applying a defined size-reduction step.

6.6 Freeze-drying

Lyophilization can improve storage stability by removing water, but vesicle fusion or leakage may occur during freezing and rehydration. Cryoprotectants such as sugars may be required and must be optimized.

7. FORMULATION VARIABLES AND QUALITY-BY-DESIGN

Transferosome development benefits from systematic identification of critical material attributes (CMAs) and critical process parameters (CPPs). Typical CMAs include phospholipid type and concentration, edge activator type and concentration, drug concentration, cholesterol concentration and aqueous-phase composition. CPPs may include hydration time, mixing speed, temperature, sonication time, extrusion cycles and solvent-removal conditions.

The phospholipid-to-edge-activator ratio is particularly influential. An insufficient amount of edge activator may produce a relatively rigid vesicle, whereas excessive surfactant can disrupt the bilayer. The optimal ratio is drug- and composition-specific and should be established experimentally.

Design-of-experiments approaches can evaluate multiple variables simultaneously and identify interactions that would be missed by one-factor-at-a-time experimentation. Responses can include vesicle size, PDI, zeta potential, entrapment efficiency, deformability and drug release. For a final pharmaceutical dosage form, additional responses such as film thickness, folding endurance, mucoadhesion and ex-vivo permeation may be incorporated.

A useful QbD workflow is: define the quality target product profile; identify critical quality attributes; conduct risk assessment; select experimental factors; perform DoE; model responses; confirm the optimum; and establish a control strategy. This approach is especially valuable when several formulation components interact.

8. CHARACTERIZATION OF TRANSFEROSOMES

8.1 Vesicle size

Dynamic light scattering is widely used to measure hydrodynamic diameter. Results depend on sample concentration, dilution medium, viscosity, temperature and instrument algorithm. Measurements should therefore be reported with sufficient detail for reproducibility.

8.2 Polydispersity index

PDI describes the breadth of the particle-size distribution. Lower PDI generally indicates a more homogeneous dispersion, but there is no universal single cutoff that defines an acceptable transferosome for every route. The value should be interpreted with the measurement quality and formulation purpose.

8.3 Zeta potential

Zeta potential provides information about electrokinetic behavior and can help monitor changes during storage. Highly charged systems may show greater electrostatic repulsion, but steric stabilization and surfactant effects can also contribute to colloidal stability. A modest zeta potential does not automatically mean a formulation is unstable.

8.4 Entrapment efficiency

Entrapment efficiency is usually calculated as:
EE (%) = [(Total drug − Free drug) / Total drug] × 100.
The separation technique must efficiently distinguish vesicle-associated drug from free drug without causing vesicle disruption.

8.5 Morphology

TEM, cryo-TEM, SEM and AFM can provide complementary information. Conventional drying for SEM can collapse or deform soft vesicles, so the resulting image should not automatically be interpreted as the native hydrated structure. Cryo-TEM is better suited for preserving hydrated morphology but may not be available in all laboratories.

8.6 Deformability

Deformability is a central functional attribute. One common approach measures vesicle passage through a membrane with a defined pore size under controlled pressure. Deformability indices are calculated from the amount or volume of dispersion passing through the membrane and the vesicle size before and after the test. Because equations differ among studies, the exact method must be reported.

8.7 Drug release

In-vitro release can be evaluated using dialysis, diffusion cells or other validated systems. Membrane resistance, sink conditions and formulation dilution can substantially affect the observed release profile.

8.8 Ex-vivo permeation

Franz diffusion cells are commonly used for skin studies. Parameters include cumulative amount permeated per unit area, steady-state flux, permeability coefficient and lag time. Drug retained in the skin should also be quantified when local delivery is relevant.

8.9 Stability

Stability should include visual appearance, vesicle size, PDI, zeta potential, drug content, entrapment and leakage. Lipid oxidation should be considered when appropriate. The final dosage form, such as a film, requires separate stability evaluation because drying and storage can change the vesicular state.

9. TRANSDERMAL APPLICATIONS

Transferosomes have been investigated for delivery of analgesics, anti-inflammatory agents, corticosteroids, antihypertensives, antifungals, antioxidants, peptides and other therapeutic agents.?,?,¹²–¹? Their main attraction is the possibility of improving penetration of drugs that otherwise have inadequate passive skin permeability.

NSAIDs are a particularly relevant class because local transdermal delivery may provide high concentrations at the site of inflammation while reducing some gastrointestinal exposure associated with oral administration. Studies have investigated meloxicam, diclofenac, ketoprofen and other drugs using transferosomal gels and related systems. However, enhanced skin permeation should not automatically be equated with improved clinical efficacy; pharmacodynamic and safety outcomes require separate evaluation.

For macromolecules, transferosomes are attractive because deformability may provide an opportunity for non-invasive delivery of peptides and proteins. Nevertheless, the molecular size and stability of such agents make the formulation and biological evaluation more challenging. Claims of successful systemic delivery should be supported by validated bioanalytical and pharmacokinetic data.

10. BUCCAL DRUG DELIVERY

The buccal mucosa is a promising site for systemic and local drug delivery. It is relatively accessible, well vascularized and can provide an alternative to swallowing. Drugs absorbed through the buccal mucosa can potentially avoid gastrointestinal degradation and a portion of hepatic first-pass metabolism. Nevertheless, saliva dilution, swallowing, mucus turnover and mechanical movement reduce formulation residence time.

Transferosomes may address part of the permeability problem through flexible vesicular structures. The formulation must, however, remain at the mucosal site long enough for drug release and permeation. Therefore, mucoadhesive polymers are important components of many buccal transferosomal systems.

A notable example is loratadine-loaded transferosomal gel. In the reported study, optimized vesicles had a submicron size of approximately 380 nm and about 60% loading/entrapment characteristics; incorporation into a mucoadhesive gel improved release, permeation and mucoadhesion compared with a transferosome-free gel. Human pharmacokinetic testing showed bioavailability comparable to the reference oral product, while inter-individual variability in Cmax and AUC was reduced.? This study demonstrates the value of combining vesicle engineering with a dosage-form strategy rather than evaluating transferosomes only as a dispersion.

11. TRANSFEROSOME-LOADED BUCCAL FILMS

Buccal films are thin polymeric dosage forms that can provide unit-dose administration, ease of handling and prolonged mucosal residence. A transferosome-loaded film combines the functions of two delivery technologies: the transferosome serves as the drug carrier and permeability-enhancing vesicle, while the polymeric film provides structural support and retention.

Hydroxypropyl methylcellulose grades, carbomers and other hydrophilic polymers can be used as film-forming materials. Plasticizers such as propylene glycol or glycerol may improve flexibility. The formulation pH should be compatible with buccal tissue, while excessive acidity or alkalinity should be avoided.

The manufacturing process is critical. Solvent casting can involve mixing the vesicular dispersion with a hydrated polymer solution followed by casting and controlled drying. Excessive temperature, prolonged drying or strong mechanical processing may change vesicle size, promote aggregation or alter drug leakage. Consequently, transferosome characteristics should ideally be measured both before incorporation and after reconstitution of the final film.

Recommended film evaluations include appearance, thickness, weight variation, folding endurance, tensile strength, surface pH, swelling index, moisture content, drug content uniformity, mucoadhesive strength, residence time, in-vitro release, ex-vivo permeation and stability. Where the scientific objective is to demonstrate a vesicular mechanism, particle size/PDI and other vesicle characteristics after film hydration should also be assessed.

12. COMPARISON WITH RELATED VESICULAR SYSTEMS

Transferosomes should be distinguished from related systems because formulation composition determines mechanism and performance. Conventional liposomes use phospholipid bilayers but are generally less deformable. Ethosomes contain relatively high concentrations of ethanol and depend strongly on alcohol-mediated changes in skin structure and lipid fluidity. Transethosomes combine deformable vesicles with ethanol. Niosomes are based mainly on non-ionic surfactants and can provide useful encapsulation and stability but have different membrane characteristics.

No single vesicular system is universally superior. Comparative studies should control drug dose, composition, administration conditions, tissue model and analytical methods. A formulation that gives smaller vesicles is not necessarily better if its deformability, drug loading or biological performance is inferior.

13. ADVANTAGES AND LIMITATIONS

Potential advantages include improved penetration, ability to carry hydrophilic and lipophilic drugs, non-invasive administration, potential sustained release and adaptability to gels, films and patches. Transferosomes may be particularly useful when conventional passive diffusion is inadequate.

Limitations include susceptibility of phospholipids to oxidation and hydrolysis, leakage during storage, aggregation, sensitivity to formulation composition, potential surfactant irritation, complex scale-up and lack of standardized deformability testing. Biological barriers also vary substantially between experimental models and human tissues. Recent critical reviews identify inconsistent terminology, heterogeneous formulation strategies and limited clinical translation as major challenges.?,?

14. RECENT ADVANCES AND FUTURE DIRECTIONS

Recent work has increasingly moved toward rational design rather than empirical formulation. Quality-by-design methods, factorial designs and response-surface approaches can help identify optimal combinations of phospholipid, edge activator and processing variables. Hybrid systems, surface modification, polymer coatings and lyophilization are being explored to improve stability and targeting.?,¹?

For mucosal delivery, future systems may combine transferosomes with mucoadhesive polymers, stimuli-responsive materials or rapidly dissolving films. Surface modification could improve residence time, while controlled hydration could regulate release. Integration with microneedles or other physical enhancement technologies is also being explored for transdermal applications.

The most important future requirement is stronger translational evidence. Studies should report standardized critical quality attributes, use validated analytical methods and compare optimized transferosomes with appropriate controls. Ex-vivo permeation should be followed by pharmacokinetic or pharmacodynamic evaluation where systemic or therapeutic benefit is claimed. Long-term safety and irritation studies are also essential.

15. TABLES

Table 1. Major components of transferosomes and their functions

Table 2. Common preparation approaches

Table 3. Critical characterization parameters

Table 4. Transferosomes compared with related vesicular systems

Table 5. Representative literature on transferosome applications

Table 6. Formulation variables and expected effects