Liposomes are artificially prepared nanoscale spherical vesicular structures composed of phospholipids, cholesterol, surfactants, and other components. Due to their advantages such as flexible regulation of charge and particle size, as well as surface modification properties, liposomes have shown application potential in numerous fields. This article provides a comprehensive overview of liposomes, including their composition, structure, classification, factors influencing their efficacy, and applications in various industries, aiming to offer a clear understanding of this remarkable nanocarrier system.
Liposomes, a concept first proposed by Bangham et al. in 1965, are lipid-based, nanoscale spherical vesicular structures similar to biological membranes, with diameters ranging from 25 to 1000 nm. These self-assembled structures possess a unique architecture consisting of lipid membranes and an aqueous core, mimicking the structure of cell membranes. This characteristic enables them to deliver both hydrophobic and hydrophilic molecules, while also exhibiting biocompatibility and biodegradability. Liposomes offer several advantages, including enhancing the solubility of encapsulated substances, reducing adverse reactions and toxicity of free drugs, allowing flexible regulation of charge and size, and enabling surface modification (Bangham et al., 1965).
1. Composition of Liposomes
The lipids in animal cell membranes are mainly phospholipids and cholesterol, while those in plant cell membranes are primarily phospholipids and phytosterols. Various lipids and lipid mixtures can be used to prepare liposomes, with phospholipids being the most commonly employed.
Phospholipids consist of a hydrophilic head and two hydrophobic tails. The head is formed by esterification of a phosphate backbone with water-soluble molecules such as choline and serine, making it water-soluble. The parallel tails extending downward are fatty acid chains, each containing 10-24 carbon atoms and 0-6 double bonds, which are insoluble in water. The simplest phospholipid is phosphatidic acid, and the most commonly used phospholipid for liposome preparation is phosphatidylcholine (lecithin). Under electron microscopy, liposomes often appear spherical or elliptical, with diameters ranging from tens of nanometers to tens of micrometers.
2. Structure of Liposomes
Liposomes are unilamellar or multilamellar vesicles formed by the dispersion of phospholipids in water, where each layer is a lipid bilayer, and the layers are separated by water. Lipophilic compounds are localized between the bilayer lipid membranes, polar compounds can be encapsulated in the polar regions of liposomes, amphiphilic compounds are positioned at the phospholipid interface between the aqueous phase and the membrane interior, and water-soluble compounds are encapsulated in the aqueous phase of liposomes.
3. Classification of Liposomes
Based on the structure and the number of bilayer phospholipid membranes they contain, liposomes can be divided into unilamellar liposomes and multilamellar liposomes.
Unilamellar liposomes are further classified into small unilamellar vesicles (SUVs, also known as nanoliposomes) and large unilamellar vesicles (LUVs). Compared with SUVs, LUVs have a higher encapsulation efficiency for water-soluble compounds and a larger encapsulation volume. Most liposome suspensions dispersed by ultrasound are unilamellar liposomes.
Multilamellar liposomes (MLVs) contain vesicles with multiple bilayer membranes, with particle sizes ranging from 1.0 to 5.0 μm. In MLVs, several lipid bilayers separate the water films containing water-soluble compounds, forming heterogeneous aggregates, while lipophilic compounds are dispersed in the several bilayers.
4. Factors Influencing Liposome Efficacy
The physicochemical properties that affect liposome efficacy mainly include phase transition temperature, membrane permeability, membrane fluidity, pH sensitivity, as well as the charge and particle size of liposomes.
① Phase Transition Temperature (Tc)
The temperature at which the hydrophobic chains in the lipid bilayer change from an ordered arrangement to a disordered arrangement, transforming from a "gel-crystalline" state to a "liquid-crystalline" state, is the phase transition temperature.
All phospholipids have a specific Tc value. Generally, the longer the hydrophobic chain or the higher the degree of chain saturation, the higher the phase transition temperature; conversely, shorter chains or lower saturation result in a lower phase transition temperature. Thermosensitive liposomes (TSLs) are prepared based on this property. When the temperature rises to the specific temperature of the target site, the liposomes carrying active substances reach the phase transition temperature and can rapidly release the encapsulated active substances (Allen et al., 1987).
② Membrane Permeability
Liposome membranes are semi-permeable, and the rates at which different ions cross the membrane and molecules diffuse through the membrane vary significantly.
For molecules with high solubility in both aqueous and organic solutions, the barrier effect of the phospholipid membrane is very weak; polar substances (such as glucose) and macromolecular compounds pass through the membrane very slowly; neutral small molecules (such as water and urea) can diffuse quickly, while the behavior of charged ions varies greatly: protons and hydroxyl ions cross the membrane very quickly, while sodium and potassium ions cross the membrane very slowly.
③ Membrane Fluidity
At Tc, the fluidity of the membrane increases. Cholesterol, known as a "fluidity buffer," can regulate membrane fluidity: when added to phospholipids below Tc, it reduces the ordered arrangement of the membrane and increases membrane fluidity; above Tc, adding cholesterol increases the ordered arrangement of the membrane and reduces membrane fluidity.
④ pH Sensitivity
When the pH decreases, certain lipids (such as dipalmitoylphosphatidylcholine) can cause protonation of fatty acid carboxyl groups, leading to the formation of a hexagonal crystalline phase (non-lamellar structure), which is the main mechanism of membrane fusion, thereby releasing the contents.
Based on this property, various types of pH-sensitive liposomes have been prepared using different types of protonatable lipids.
⑤ Charge
Liposomes containing acidic lipids (such as phosphatidic acid, phosphatidylserine) are negatively charged, those containing basic (amine, amino) lipids (such as stearamide, octadecylamine) are positively charged, and liposomes without ions are electrically neutral.
For example, the use of positively charged liposomes to encapsulate ophthalmic antibiotics significantly improves their penetration efficiency.
⑥ Particle Size
The particle size and distribution uniformity of liposomes are related to their encapsulation efficiency and stability. A more uniform particle size distribution is more conducive to the stability and efficacy of liposomes.
According to particle size and lamellar structure, liposomes can be divided into small unilamellar vesicles (particle size 20-100 nm), large unilamellar vesicles (particle size 100-1000 nm), giant unilamellar vesicles (particle size >1000 nm), multilamellar vesicles (particle size >500 nm), and multivesicular vesicles (particle size >1000 nm).
One of the important factors affecting the particle size of liposomes is the preparation method. Common laboratory preparation methods include thin-film hydration, reverse-phase evaporation, and ethanol (ether) injection. The thin-film hydration method is simple to operate, and the structure and size of liposomes can be affected by adjusting hydration conditions such as temperature and pH, making it suitable for laboratory-scale production. However, it has the disadvantage of mostly forming multilamellar vesicle suspensions with low encapsulation efficiency for water-soluble drugs. Reverse-phase evaporation and ethanol (ether) injection methods are suitable for encapsulating water-soluble drugs, mostly forming mixtures of large unilamellar vesicles and multilamellar vesicles, and may have solvent residue issues, affecting the stability and safety of liposomes.
With the deepening of research on liposomes, various new liposome preparation methods have been developed, including microfluidic hydrodynamic focusing, pH jump, hydration in colloidal particle packed beds, and freeze-thaw cycles. Homogenization of liposome suspensions by extrusion through polycarbonate membranes with specific pore sizes and ultrasonic treatment can convert multilamellar vesicles with particle sizes up to 500-5000 nm into small unilamellar vesicles of 20-100 nm or large unilamellar vesicles of 100-1000 nm, obtaining smaller and more uniform liposome particles.
5. Applications of Liposomes
Pharmaceutical Industry
Compared with traditional administration methods, the use of liposomes for drug delivery can slow down the release rate of drugs, reduce toxicity, alleviate allergic reactions and immune responses, and more importantly, can change the distribution of drugs in the body to achieve targeted drug delivery.
In addition to being used as carriers for antitumor drugs, gene therapy vectors, antibacterial and antiviral drugs, etc., they can also be used in the treatment of infectious diseases, autoimmune diseases, and neurodegenerative diseases (Lasic, 1998).
Food Industry
Food-grade liposomes used for encapsulating and delivering nutritional factors and functional ingredients have the characteristics of non-toxicity, amphiphilicity, and biodegradability. They can improve the stability and bioavailability of functional nutrients during storage and digestion, control the targeted and timed release of encapsulated substances, and improve food texture.
For example, in the production of dark chocolate, encapsulating black mulberry extract with a chitosan-coated liposome system can reduce the loss of anthocyanins in anthocyanins and improve the in vitro bioaccessibility of anthocyanins and chocolate.
Cosmetics Industry
With the successive launch of liposome cosmetics with anti-aging, whitening, and moisturizing properties, they are becoming increasingly popular among consumers. Liposome cosmetics can improve the stability of active ingredients both inside and outside the body, reducing skin irritation. They also slowly release active ingredients, extending their duration of action on the skin and allowing them to exert their effects directly and persistently inside and outside cells.