Liposomes are microscopic carriers that have become a highly utilized tool in modern medicine and scientific research. These spherical lipid structures are designed to encapsulate a payload, such as a drug or genetic material. Encapsulation protects the payload from the body’s environment until it reaches a specific target site. The ability to create these carriers with specific properties, including size, stability, and composition, makes them valuable for drug delivery systems.
The Essential Building Blocks
The fundamental ingredient for constructing a liposome is the phospholipid, which forms the bilayer structure of the vesicle membrane. Phospholipids are amphiphilic, meaning each molecule has a hydrophilic (water-loving) head group and two hydrophobic (water-fearing) fatty acid tails. When placed in an aqueous environment, these molecules spontaneously organize through self-assembly to minimize contact between the tails and water.
This self-assembly creates a spherical bilayer. The hydrophobic tails point inward toward each other, forming the membrane core, while the hydrophilic heads face outward and inward toward the aqueous spaces. Secondary components are often added to modify the liposome’s characteristics, with cholesterol being the most common additive. Cholesterol is inserted into the lipid bilayer to modulate membrane fluidity, which helps prevent leakage and increases structural stability. Specialized lipids can also be incorporated to give the liposome a surface charge or attach targeting elements for site-specific delivery.
Fundamental Methods of Vesicle Formation
The initial step in forming liposomes involves establishing the basic lipid structure. This process is often categorized by the physical forces used to achieve self-assembly.
Thin-Film Hydration
The Thin-Film Hydration method begins by dissolving the lipids in an organic solvent, such as chloroform. The solvent is then evaporated, leaving behind a thin, dry film of lipids coated on the inside surface of a flask. This film is hydrated by adding an aqueous buffer solution, causing the lipid layers to swell and peel off under gentle agitation. This initial hydration primarily yields large, unstable multilamellar vesicles (MLVs) with multiple concentric lipid layers. The temperature must be maintained above the lipids’ phase transition temperature to ensure the bilayer is fluid enough for self-assembly.
Detergent Dialysis
The Detergent Dialysis method is useful for preparing liposomes containing molecules sensitive to organic solvents. In this approach, lipids are mixed with a detergent, which solubilizes them into small, mixed micelles. The detergent is then slowly removed, often through dialysis or gel chromatography. This removal causes the lipids to aggregate and reorganize into liposome structures.
Sonication
Sonication applies high-frequency sound energy to an MLV suspension. This breaks down the larger, multi-layered vesicles into smaller, single-layered ones. The intense energy input generates cavitation bubbles that collapse, homogenizing the mixture. While sonication rapidly creates small, unilamellar vesicles, it can lead to phospholipid degradation and produces a population with a wide variation in size.
Post-Formation Processing and Standardization
Once the initial population of liposomes is formed, secondary processing refines the mixture, ensuring uniformity and purity for biological application. The most common technique for size standardization is Extrusion. This involves repeatedly forcing the liposome suspension under high pressure through polycarbonate membranes with defined pore diameters. This mechanical sizing shears larger vesicles down to a diameter close to the filter’s pore size, yielding a highly uniform population.
A uniform size distribution is necessary for controlling the liposome’s behavior in the body, including its circulation time and biodistribution. Following sizing, the preparation must be purified to remove residual unencapsulated material, organic solvent, or detergent. Techniques like centrifugation, size-exclusion chromatography, or ultrafiltration separate the large liposomes from smaller, free-floating molecules.
For pharmaceutical products, the final steps involve Sterilization and appropriate storage. Sterilization is typically achieved through sterile filtration, requiring liposomes to be small enough (under 200 nanometers) to pass through a 0.22 micrometer filter. For long-term preservation, aqueous liposomal suspensions are often freeze-dried. Cryoprotectants like sucrose are added during freeze-drying to prevent the physical degradation of the vesicles.
How Preparation Influences Function
The choices made during preparation directly dictate the functional attributes governing a liposome’s performance as a carrier. One important metric is Encapsulation Efficiency, which measures the percentage of payload successfully trapped within the liposome. Efficiency is influenced by lipid concentration, the preparation method used, and the liposome’s size. Larger vesicles often demonstrate higher encapsulation rates for hydrophilic compounds.
The selection of lipid components and post-formation processing steps affect the liposome’s Stability and shelf life. Incorporating cholesterol increases membrane rigidity, which minimizes drug leakage and slows vesicle degradation. The final size of the liposome, determined by extrusion, affects its targeting and circulation profile in the body. Liposomes generally need to be smaller than 200 nanometers to evade rapid clearance by the immune system and accumulate effectively in target tissues. By controlling components, size distribution, and stability, researchers engineer the liposome to control the payload release rate at the desired site.