What Are Multilamellar Vesicles and Their Applications?

Multilamellar vesicles, or MLVs, are a specific class of liposomes, which are microscopic, spherical sacs made of lipid molecules. These structures are synthetically created and serve as mimics of natural cell membranes. When lipid molecules are introduced into a water-based environment, they arrange themselves into these vesicular structures. The defining feature of MLVs is their composition of multiple, concentric lipid layers, which gives them unique properties harnessed for various scientific and commercial purposes.

Structural Characteristics

The structure of a multilamellar vesicle is compared to an onion, consisting of multiple concentric lipid bilayers separated by layers of water. Each bilayer is a sheet of lipid molecules arranged with their water-attracting (hydrophilic) heads facing the aqueous environment and their water-repelling (lipophilic) tails facing inward. In an MLV, this bilayer repeats, creating a series of alternating lipid and water compartments.

This layered architecture is the feature that distinguishes MLVs from other types of liposomes. Unilamellar vesicles consist of only a single lipid bilayer and can be categorized by size into small unilamellar vesicles (SUVs) and large unilamellar vesicles (LUVs). Both lack the complex, layered interior of an MLV, and their size is smaller than their multilamellar counterparts, which range from a few hundred nanometers up to several micrometers in diameter.

The presence of numerous bilayers contributes to the physical stability of multilamellar vesicles. This robust structure makes them less prone to leakage or rupture compared to single-layered vesicles. The composition of the lipids can also be adjusted by adding molecules like cholesterol, which integrates into the bilayers to enhance their structural integrity and control their permeability.

Methods of Preparation

The most established technique for producing multilamellar vesicles is the thin-film hydration method. This process begins with the dissolution of lipids in an organic solvent like chloroform or ethanol. This ensures that the lipid molecules are completely mixed, which is important when using a combination of different lipid types to achieve specific properties.

Once a clear lipid solution is obtained, the organic solvent is removed under reduced pressure, leaving behind a thin, dry film of lipid coated on the inner surface of the flask. The quality and evenness of this film can influence the characteristics of the final vesicles. Any residual solvent is removed by drying the film under a vacuum for an extended period.

The final step is hydration. An aqueous solution is added to the flask containing the lipid film, which is then agitated. This agitation causes the lipid sheets to swell and peel away from the glass, where they self-assemble into large, multilamellar vesicles as the bilayers fold to shield their exposed edges from the water. The result is a milky-white suspension of newly formed MLVs.

Role in Drug Delivery Systems

The multi-layered structure of MLVs makes them effective vehicles for transporting therapeutic compounds. Their architecture allows for the encapsulation of different types of drugs simultaneously. Hydrophilic, or water-soluble, drugs can be trapped within the numerous aqueous compartments between the lipid bilayers. In contrast, lipophilic, or fat-soluble, drugs can be incorporated directly into the lipid bilayers themselves.

This dual-carrying capacity results in a high encapsulation efficiency, meaning a significant portion of the drug is successfully loaded into the vesicles. The large internal volume and extensive membrane surface area provided by the multiple layers contribute to this high capacity. This efficiency allows for the delivery of a potent dose of medication within a relatively small volume of the formulation.

The layered structure is also well-suited for achieving sustained or controlled release of a drug. For a drug to be released from an MLV, it must sequentially pass through each concentric bilayer. This creates a natural barrier that slows the drug’s diffusion out of the vesicle. This slow-release mechanism can reduce the need for frequent dosing and improve patient adherence.

Applications Beyond Medicine

The utility of multilamellar vesicles extends into the cosmetics industry, where they are used to enhance the performance of skincare products. Active ingredients, such as vitamins, antioxidants, and moisturizers, can be encapsulated within MLVs. This encapsulation protects sensitive compounds from degradation due to air and light exposure, preserving their potency. The vesicle structure can also facilitate the delivery of these ingredients deeper into the layers of the skin.

In the food industry, MLVs serve as micro-encapsulation systems for nutrients, flavors, and antimicrobial agents. They can be used to protect sensitive compounds like omega-3 fatty acids from oxidation, which would otherwise lead to spoilage. Encapsulating flavors allows for their controlled release during cooking or consumption, providing a more impactful sensory experience.

Beyond commercial uses, multilamellar vesicles are tools in basic scientific research. Their structure provides a model system for studying the properties and functions of biological membranes. Scientists can use MLVs to investigate how different molecules interact with and move across cell membranes without the complexity of a living cell. This research helps to build a fundamental understanding of cellular biology and membrane transport processes.