Membrane phospho- lipid bilayer

Polymeric microencapsulates and lipid microencapsulates have extensive potential applications in food, cosmetics and pharmaceutics [1-5]. Microencapsulates can protect and conserve an active component until its release is desired and stimulated. Polymeric microencapsulates consist of a (biocompatible) polymer matrix in which an active component is encapsulated. Most frequently poly(lactic add) (PLA) or poly (lactic-co-glycolic acid) (PLGA) is used as the polymer [6,7], but alternatives have been investigated [8, 9]. Lipid microencapsulates, lipid vesicles and liposomes are composed of a (phospho-)lipid bilayer membrane that encapsulates an aqueous volume, thus mimicking a cell structure. 

Liposomes are microstructures composed of one or more concentric spheres of (phospho)lipid bilayer, separated by water or aqueous buffer compartments. Those particles can encapsulate and dehver both hydrophilic and lipophilic substances. Water soluble substances can be entrapped in the central aqueous core, lipid soluble substances in the membrane and peptide and small proteins at the hquid aqueous interface. The size of such a particle can differ from 20 nm to 10 pm. Liposomes are in general made synthetically e.g. by the lipid hydration method. Liposomal medicines are on the market for the treatment of systemic fungal infections, tumours and for vaccination. 

Interesting and practically relevant materials for stud5dng the behavior of smart polymers attached to lipid membranes are liposomes, self-assembled 50-200 nm vesicles that have one or more (phospho)lipid bilayers which encapsulate a fraction of the solvent. Liposomes are stable in aqueous suspension because of the repulsive forces that appear when two liposomes approach each other. Liposomes are widely used for drug delivery and in cosmetics (96). 

The biochemical structure of a membrane is that of a lipid bilayer composed of phospho- and sphingolipids, as well as cholesterol. These lipids are amphipathic in nature, that is, they each have a polar and a nonpolar end. In water the nonpolar (hydrophobic, lipophilic) ends will seek to avoid the polar solvent and aggregate into a bilayer with the polar (hydrophilic, lipophobic) ends oriented towards the outside of the bilayer. As this structure extends in all directions the exposed nonpolar regions will close up and form a sphere (or ellipsoid) with water trapped inside and excluded outside. See Figures 2a and 2b. 

Due to their bilayer (phospho-) lipid membrane, the liposomes are often considered biomimetic products, even though the complexity of living cells membranes is hardly achievable by their simple structure, hi order to approach that complexity, several preparation methods are used to tune the hposomes physicochemical and colloidal characteristics (mechanical agitation, solvent evaporation, solvent injection, detergent solubilization, etc.). 

---