Artificial membranes
Lipid associates and model artificial membranes - types, preparation methods, properties and significance. Liposomes as drug carriers.
The formation of lipid aggregates in water is due to the so called hydrophobic effect. The lipid molecules are amphiphilic (a chemical compound possessing both hydrophilic (water-loving, polar) and lipophilic (fat-loving) properties). They consist of two parts – a water-soluble part (hydrophilic polar group) and a water-insoluble part (hydrophobic tail). In water, the hydrophobic tails join together and form a lipid aggregate. The polar groups coat the surface of the lipid aggregate. In this way, the unfavorable contacts of the hydrophobic tails with water are eliminated and the free energy of the system is minimized. The lipids can pack in aggregates of different shapes depending on the lipid molecular structure.
Model artificial membranes:
- 1. Liposomes
Liposomes were discovered in 1961 by Alec Bangham and since then they have become essential, very versatile tools in biology, biochemistry and medicine. Liposomes are the smallest artificial vesicles of spherical shape that can be produced from natural, non-toxic phospholipids and cholesterol. They can be used as drug carriers and loaded with a great variety of molecules such as small drug molecules, proteins, nucleotides and even plasmids. Liposomes are extremely versatile and, due to the variability of their composition, they can be used for a large number of applications. When preparing liposomes there are four stages; 1.Evaporation 2. Hydration 3. Vortexing 4. Extrusion or Ultrasound depending on type of liposome. There are four types of liposomes, 1. SUV - Small Unilamellar Vesicles. 2. LUV - Large Unilamellar Vesicles. 3. MLV - MultiLamellar Vesicles. 4. GUV - Giant Unilamellar Vesicles.
- 2. Lipid monolayers
- 3. Supported lipid bilayers
- 4. Bimolecular lipid membranes (BLM)
- 5. Lipid foam films
- 6. Lyotropic liquid crystalline phases: Lyotropic (phases form in aqueous medium). Liquid (low viscosity like other liquids). Crystals (lipid molecules are oriented like in crystals).
Liposomes as drug carriers: Good when targeting certain cells, example cancer cells. Other advantages: 1. Encapsulation of water-soluble drugs. 2. Encapsulation of water-insoluble drugs.
Drug loading in liposomes
Drug loading can be attained either passively (i.e., the drug is encapsulated during liposome formation) or actively (i.e., after liposome formation). Hydrophobic drugs, for example amphotericin B taxol or annamycin, can be directly combined into liposomes during vesicle formation, and the amount of uptake and retention is governed by drug-lipid interactions.