I. Classification and Basic Properties of Phospholipids

Phospholipids are the core membrane material of liposomes. Based on their origin, they are divided into two major categories: natural and synthetic. Differences in their physicochemical properties directly affect liposome stability:

• Natural phospholipids: They exhibit excellent biocompatibility and are widely available. Commonly used types include soybean lecithin and egg yolk lecithin. Soybean lecithin contains unsaturated fatty acids, making it cost-effective but prone to oxidation; egg yolk lecithin is primarily composed of saturated fatty acids, offering superior stability at a higher cost.

• Synthetic phospholipids: Their molecular structure can be precisely controlled, resulting in high purity and strong stability (such as hydrogenated soybean phosphatidylcholine HSPC, distearoyl phosphatidylcholine DSPC, dipalmitoyl phosphatidylcholine DPPC, etc.). These are suitable for formulations requiring high stability, though the manufacturing process is complex and costs are relatively high.

The length and saturation of the fatty acid chains, along with the polar head group of phospholipids, determine their phase transition temperature, thereby regulating the physicochemical characteristics of liposomes.

II. Fundamental Impact of Phospholipid Phase Transition Temperature on Liposome Stability

The phase transition temperature of phospholipids is the temperature at which the fatty acid chains transition from an ordered crystalline state to a disordered liquid state. This temperature determines the fluidity of the liposome membrane and is dictated by the phospholipid’s structural composition.

When the temperature reaches the phase transition point, the liposome membrane shifts from a “gel” state to a “liquid crystal” state, increasing membrane fluidity while reducing stability, potentially leading to drug leakage. Conversely, when the temperature falls below the phase transition point, the membrane structure becomes more stable but its permeability decreases, affecting drug release.

During formulation, it is essential to select phospholipids with an appropriate phase transition temperature to balance stability and drug release efficiency.

III. Specific Effects of Phospholipid Composition on Liposome Stability (Five Core Dimensions)

The composition of phospholipids—particularly the degree of fatty acid chain saturation—affects liposome stability through five key dimensions by modulating molecular arrangement, membrane properties, and intermolecular interactions:

1. Influence on Liposome Particle Size and Distribution

The level of phospholipid saturation affects steric hindrance among fatty acid chains, thereby controlling particle size and dispersibility:

Higher saturation (e.g., HSPC) results in greater rigidity of the bilayer, leading to larger particles and poorer dispersion; lower saturation (e.g., egg yolk phosphatidylcholine EPC) yields smaller particles and improved dispersibility; extremely low saturation (e.g., soybean phosphatidylcholine SPC) causes excessive bilayer fluidity, increasing the likelihood of liposome fusion.

2. Impact on Liposome Microstructure

The degree of phospholipid saturation determines the microstructural morphology and membrane integrity of liposomes:

Low saturation (e.g., soybean phosphatidylcholine SPC) leads to irregular membrane shapes; moderate saturation (e.g., egg yolk phosphatidylcholine EPC) produces smooth, well-rounded liposomes with enhanced stability; excessively high saturation (e.g., hydrogenated soybean phosphatidylcholine HSPC) results in overly rigid bilayers, increasing the risk of membrane damage and reduced stability.

3. Effect on Liposome Membrane Properties

The saturation level of phospholipids alters both the surface characteristics and internal microenvironment of the liposome membrane:

High saturation promotes tight packing of phospholipids, reducing surface hydrophobicity and internal polarity, thus minimizing hydrolysis and molecular migration, enhancing stability; low saturation increases membrane fluidity, making it susceptible to oxidation and hydrolysis, which can compromise membrane integrity.

4. Influence on Interactions Between Phospholipids and Sterols, and Molecular Arrangement

The stabilizing role of sterols (such as cholesterol) is affected by phospholipid saturation:

At high saturation, strong intermolecular forces between phospholipids and sterols (including hydrogen bonding) help organize lipid molecules, reinforcing bilayer stability; at low saturation, weaker molecular interactions lead to loose lipid arrangements and diminished stability.

The regulatory effect of cholesterol is temperature-dependent and works synergistically with phospholipids to maintain liposome stability.

5. Direct Impact on Long-Term Liposome Stability

The degree of fatty acid saturation directly determines the long-term storage stability of liposomes:

High levels of saturated fatty acids result in low membrane fluidity and tight packing, reducing hydrolysis and oxidation, thereby extending shelf life; conversely, high levels of unsaturated fatty acids increase susceptibility to oxidative degradation, leading to drug breakdown and formulation aggregation.

IV. Summary

Phospholipid composition influences liposome stability across five core dimensions by regulating phase transition temperatures and membrane fluidity. During formulation, it is crucial to consider application scenarios and drug characteristics, carefully selecting appropriate phospholipid types and optimizing their ratios to strike a balance between stability, encapsulation efficiency, and drug release performance.

Further Reading

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