Introduction:
Liposomes are tiny spherical vesicles, which have been widely used as delivery systems for a range of bioactive molecules.
They are formed when certain lipid molecules are hydrated in an aqueous environment.
Their structure resembles that of cell membranes, making them biocompatible and capable of interacting with human cells in various ways.
Structural Composition:
Liposomes consist of an aqueous core surrounded by one or multiple concentric lipid bilayers.
These lipid layers are composed primarily of phospholipids, which naturally organize into a bilayer structure when exposed to water due to their amphiphilic nature. Each phospholipid molecule consists of a hydrophilic "head" and two hydrophobic "tails".
The tails associate with each other, forming the internal section of the bilayer, while the hydrophilic heads face outwards, interacting with the surrounding aqueous environment.
Types of Liposomes:
Based on size and the number of lipid bilayers, liposomes can be categorized into:
1. Small Unilamellar Vesicles (SUVs):
These are small vesicles with a single bilayer. They typically range from 20 to 100 nm in diameter.
2. Large Unilamellar Vesicles (LUVs):
These have a single bilayer but are larger than SUVs, with diameters usually ranging from 100 to 1000 nm.
3. Multilamellar Vesicles (MLVs):
These consist of multiple concentric lipid bilayers. They typically resemble an "onion structure" and are larger, usually ranging from 500 nm to several micrometers in diameter.
4. Multivesicular Vesicles (MVVs):
These are large vesicles containing many smaller vesicles inside them.
Preparation of Liposomes
here's a concise summary of the preparation of liposomes for targeted drug delivery:
Lipid Selection: Choose appropriate lipids, like phosphatidylcholine, and possibly cholesterol for stability. Include targeting ligands if specific targeting is desired.
Liposome Formation:
Thin Film Hydration: Dissolve lipids in an organic solvent, create a thin lipid film by evaporating the solvent, and hydrate the film with an aqueous solution (containing the drug).
Other Methods: Use techniques like reverse-phase evaporation, solvent injection, or microfluidic methods, depending on the required liposome characteristics.
Size Reduction and Homogenization: Utilize methods like sonication, extrusion, or high-pressure homogenization to achieve uniform liposome size.
Purification: Remove unencapsulated drugs and solvents through dialysis, centrifugation, or size-exclusion chromatography.
Characterization: Assess liposome size, stability, encapsulation efficiency, and drug release profile.
Functionalization for Targeting: Attach targeting ligands to liposome surface if targeted delivery is required.
Liposomes can be prepared using several methods, including:
1. Thin Film Hydration:
This involves evaporating a solvent from a lipid solution to form a thin film of lipid, which is then hydrated with an aqueous solution.
2. Sonication:
This method uses ultrasonic waves to break down larger liposomal structures into smaller vesicles.
3. Extrusion:
In this method, a liposome suspension is forced through a polycarbonate membrane to control vesicle size.
Characterization of Liposomes
Liposomes are small spherical vesicles composed of one or more phospholipid bilayers surrounding an aqueous core.
They can encapsulate both hydrophilic (in the aqueous core) and hydrophobic (within the lipid bilayer) drugs.
1. Size and Lamellarity:
Liposomes can be unilamellar (single bilayer) or multilamellar (multiple concentric bilayers).
Their size can range from nanometers (nanosomes) to micrometers.
2. Surface Charge:
Based on the lipids used, liposomes can be cationic, anionic, or neutral.
3. Composition:
The specific phospholipids and cholesterol content.
4. Drug Entrapment Efficiency:
The efficiency with which the drug is encapsulated within the liposome.
5. Release Profile:
The rate and mechanism by which the drug is released from the liposome.