A liposome is a microscopic, spherical vesicle formed from a fatty substance. This structure encloses an aqueous compartment, separating it from its external environment. Scientists utilize liposomes for various purposes, including the delivery of therapeutic compounds or nutrients to specific cells within the body. They encapsulate and protect their contents until they reach their intended destination.
Assembly of the Spherical Bilayer
The fundamental building blocks of a liposome are phospholipid molecules. Each phospholipid possesses a dual nature, featuring a hydrophilic, or “water-loving,” head group and two hydrophobic, or “water-fearing,” fatty acid tails. One can visualize this structure much like a buoy with two dangling ropes, where the buoy represents the water-attracting head and the ropes are the water-repelling tails. This design dictates their behavior when introduced to an aqueous solution.
When phospholipids are dispersed in water, their amphipathic nature drives them to spontaneously organize. The hydrophobic tails, seeking to escape contact with water, cluster together, while the hydrophilic heads orient towards the surrounding water molecules. This arrangement results in a double layer, known as a lipid bilayer, where the tails face inwards and the heads face outwards towards the aqueous environment on both sides.
This flat sheet of lipid bilayer then undergoes a remarkable transformation, curving in on itself to form a closed, hollow sphere. This self-sealing process is driven by the hydrophobic effect, as it minimizes the exposure of the lipid tails to water. The resulting spherical structure creates an internal aqueous compartment, isolated from the external solution, allowing for the encapsulation of water-soluble substances.
Variations in Liposome Architecture
Liposomes exhibit different architectural forms depending on their preparation. Unilamellar vesicles (UVs) are characterized by a single phospholipid bilayer forming their membrane. These single-layered structures represent the simplest form of liposome.
Unilamellar vesicles are further distinguished by size. Small unilamellar vesicles (SUVs) range in diameter from 20 to 100 nanometers. Large unilamellar vesicles (LUVs) are larger, with diameters exceeding 100 nanometers and often reaching several hundred nanometers. Both SUVs and LUVs maintain a single bilayer membrane surrounding their internal aqueous core.
Multilamellar vesicles (MLVs) are another architectural type. Unlike unilamellar vesicles, MLVs are composed of multiple, concentric phospholipid bilayers, resembling the layers of an onion. These layers are separated by thin films of water, creating multiple internal aqueous compartments nested within one another.
Key Structural Characteristics
The size of a liposome is a controllable characteristic, ranging from tens of nanometers for small unilamellar vesicles up to several micrometers for larger types. This dimension can be tailored during the manufacturing process, influencing how the liposome behaves within biological systems. For instance, smaller liposomes demonstrate longer circulation times within the bloodstream, as they are less readily cleared by the body’s immune system.
The surface of a liposome can carry a specific electrical charge, determined by the types of phospholipids incorporated into its bilayer. Liposomes can be engineered to be neutrally, negatively, or positively charged. This surface charge plays a role in how the liposome interacts with other biological components, such as cell membranes or proteins, influencing its targeting capabilities and cellular uptake.
The fluidity and permeability of the liposome membrane are directly influenced by its lipid composition. Incorporating different types of phospholipids, such as those with saturated or unsaturated fatty acid tails, affects the packing density of the bilayer. The addition of cholesterol, for example, can increase the rigidity of the membrane, making it less permeable to encapsulated substances. A more rigid membrane acts as a barrier, retaining its internal contents and controlling their release.