Key parameters affecting the initial release (burst) and encapsulation efficiency of peptide-containing poly(lactide-co-glycolide) microparticles.

The objective of this study was to identify key variables affecting the initial release (burst) and the encapsulation of leuprolide acetate-containing poly(lactide-co-glycolide) (PLGA) microparticles, which were prepared by the cosolvent evaporation method. Adjusting parameters, which affected the PLGA precipitation kinetics, provided efficient ways to increase the encapsulation efficiency and to control the initial release. Addition of 0.05M NaCl to the external aqueous phase increased the encapsulation efficiency and the initial release; in contrast, NaCl at high concentration (0.5M) delayed polymer precipitation and resulted in non-porous microparticles with a low initial release. The presence of ethanol in the external phase led to porous microparticles with an increased initial release but a decreased encapsulation efficiency. The initial release also decreased with decreasing volume of the external phase and homogenization speed, as well as with covering the preparation apparatus; however, these variations had no significant effect on the encapsulation efficiency. Scale-up of the laboratory size by a factor of 5 and 25 showed insignificant influence on the encapsulation efficiency, particle size, and drug release.

Rigid lipid membranes and nanometer clefts: motifs for the creation of molecular landscapes.

Amphiphilic lipids associate in water spontaneously to form micelles, vesicles, monolayers, or biological membranes. These aggregates are soft and their shape can be changed easily. They behave like complex fluids because they are merely held together by weak, nondirected forces. The most important characteristic of these monolayers is their ability to dissolve hydrophobic molecules in the form of freely movable monomers. The fluid molecular layers are not suitable to anchor the components of chain reactions. However, if the alkyl chains are replaced by rigid skeletons or if the head groups are connected through intermolecular interactions, the aggregates become rigid and their fluid solvent character is lost. The construction of chiral surfaces by synkinesis (synthesis of noncovalent compounds) and of enzyme-type surface clefts of defined size can now be carried out by using rigid lipid membranes. Monolayers and nanometer pores on solid substrates attain sharp edges, and upright nanometer columns on smooth surfaces no longer dissipate. Five examples illustrate the advantages of using rigid molecular assemblies: 1) Cationic domains of rigid edge amphiphiles in fluid membranes act as manipulable ion channels. 2) Spherical micelles, micellar helical fibers, and vesicular tubes can be dried and stored as stable material. Molecular landscapes form on smooth surfaces. 3) alpha,omega-Diamide bolaamphiphiles form rigid nanometer-thick walls on smooth surfaces and these barriers cannot be penetrated by amines. Around steroids and porphyrins, they form rigid nanometer clefts whose walls and water-filled centers can be functionalized. 4) The structure of rigid oligophenylene- and quinone monolayers on electrodes can be changed drastically and reversibly by changing the potential. 5) 10(10) Porphyrin cones on a 1-cm2 gold electrode can be controlled individually by AFM- and STM-tips and investigated by electrochemical, photochemical, and mechanical means. In summary, rigid monolayers and bilayers allow the formation of a great variety of membrane structures that cannot be obtained from classical fluid alkyl amphiphiles.