Microspheres formation

For the preparation of spray-dried polyelectrolyte complexes, the polyanion was dissolved in dilute NH4HCO3 solution and mixed with the chitosan carbamate solution just before spray-drying. The excess NH4HCO3 decomposed thermally between 60 and 107 °C on the other hand, the carbamate function released carbon dioxide under the effect of the temperature at which the spray-drier was operated, thus regenerating chitosan at the moment of the polyelectrolyte microsphere formation (Fig. 5). 

The coprecipitation technique was based on the dropwise addition of a synthetic polymer solution, in a solvent mixture, into an aqueous protein solution under magnetic stirring. The progressive interaction between the water insoluble polymer and the protein gave rise to the microsphere formation. The glycolipid was then added as an aqueous dispersion to the nanoparticle suspension. No sedimentation was observed after several weeks of storage at room temperature. 

Short heat treatments were found to result in a loss of blowing gas. It is probable that in this case gas evolution precedes polymerization and the formation of a film to keep the gas within the particle. By contrast, for longer treatment times polymerization precedes gas evolution and microspheres are formed. Data on the reaction kinetics during microspheres formation are given in36). 

Microsphere Formation. Because the microspheres were fabricated using a batch process, we monitored the viscosity and pH of the catalyst slurry as it aged. Figure 2 shows that the viscosity of the slurry was dependent on both the age of the slurry and the additive type. The reference formula was stable for 3 h, but the CP-alumina and pseudoboehmite formulations thickened or gelled in the same time period. A typical batch starting at pH 3.0 increased to about pH 3.3 before the onset of thickening (about 100 cP). For CP formulations, the onset of thickening may be related to the median particle size of the powder. 

Thomasin, C. Merkle, H.P. Gander, B. Drug micro-encapsulation by PLA/PLGA coacervation in the light of thermodynamics. 2. Parameters determining microsphere formation. J. Pharm. Sci. 1998, 87 (3), 269-275. 

A variety of methods have been used for microsphere formation including non-degradable polystyrene, poly(methyl methacrylate) (PMMA), and polyacrylamide (PAAm) microspheres, as well as PLLA, PGA, PGLA, polycyanoacrylate, starch, and protein microspheres, which would be more likely to be fully biodegradable [42, 110]. Measurable amounts of polystyrene [107] and PAAm microspheres [110] persist in the tissue for many weeks after administration. It seems likely, however, that PGA [108] and protein microspheres [110] may be degraded in vivo in a period of days. 

Microsphere formation and encapsulation by the Rapid Expansion of Supercritical Solutions (RESS) technique... 

Figure 9 Diagrammatic representation of microsphere formation. After dissolution of the polymer and drug in admixture with CO2, a process of rapid expansion takes place. Adjusting the parameters can lead to nanoparticle formation.

Rather conventional means for the manufacturing of hollow microspheres with diameters between 1 and 1000 pm have been developed [11.9]. Methods include spray drying and dripping as well as emulsion or suspension techniques. The microspheres feature low effective and bulk densities coupled with high specific surfaces. Typical wall thicknesses are in the range 1-10% of the diameter. Potential wall materials include glass, ceramic and mixed oxides, silicates and aluminosilicates, polymers and polycondensates, and metals. Surface phenomena, which may be modified by chemical reactions, additives, and/or post-treatments, play an important role for microsphere formation, properties, and stability. Fig. 11.12 is the photomicrograph of a calcined hollow microsphere [11.9]. 

Almost complete solvent removal and rigid microspheres formation... 

Schematic diagram of microsphere formation by the single emulsion solvent evaporation...

Almost complete solvent evaporation and microsphere formation... 

A biomolecule aided solvothermal synthesis procedure was used to obtain cobalt sulfide hollow spheres.In a typical method L-cysteine, anhydrous cobalt chloride and different surfactants were dissolved in ethylenediamine. Cysteine was incorporated as a sulfur souree and also to facilitate the microsphere formation. The reaetion mixture was autoclaved at 180 °C for different times after stirring for 15 minutes. The resulting products were washed with deionised water and ethanol several times and placed in a vacuum oven at 60 °C for 4 hours. The hollow... 

A description of the gelation process has been given by Chatterjee etal. [170], which has certain commonalities in the later steps of the process, i.e. hollow microsphere formation, with the model proposed by Liu and Wilcox [167]. However, the initial steps were different, as shown below ... 

Lammel, A., Schwab, M., Slotta, U., Winter, G., and Scheibel, T. (2008). Processing conditions for spider silk microsphere formation. ChemSus-Chem 5,413 16. 

Microsphere formation was not only observed for lysozyme and a-lactalbumin other oppositely charged protein couples (lysozyme/ovalbumin, ovalbumin/avidin and lysozyme/BSA) showed a similar behaviour. AU these systems showed colocalisation of the protein molecules within the microspheres and were sensitive to the ionic strength. An analogy to the isoprotic point (see Fig. 2) of two weakly charged polyelectrolytes was found the pH where optimal complexation occurred was 5(p/protein+ + P protein ) Charge Compensation was not observed in all systems, especially when there was a difference in protein size. In this case, the smaller protein molecule was in excess within the microsphere [93]. 

Fig. 2.10 Schematic representation of air-filled microsphere formation in an ultrasonic field. Partially denatured protein molecules adsorb at the ultrasonically-generated bubble solution interface. Superoxide radicals generated during acoustic cavitation leads to inter-molecular cross-linking of proteins resulting in the formation of stable protein-sheUed microbubbles [74]...

HCl was used to promote the polymerization rate and to control the microsphere formation by changing the dispersion medium pH (79). The average size of the particles decreased higher degradation rate was observed with decreasing HCl concentration. 

More recently, succinic acid deamidated wheat gluten microspheres for encapsulation of fish oil via O/W/0 doubleemulsion followed by heat-polymerization of emulsified WG was reported (Liao et al., 2012). During microspheres formation, dramatically elevated intermolecular a regation caused the formation of a compact network structure of succinic acid deamidated WG molecules, in which fish oil was encapsulated. Heat-pol5mierization at the same time increased interactions (hydrogen bonds and hydrophobic interactions) between the two components. 

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