Microspheres precipitation

Yttrium silicate cerium phosphors of superior properties were synthesized by Marsh et al. (2002). A precipitate was obtained by adding excess ammonia to a sol obtained from a mixture of tetraethyl orthosilicate (TEOS) in ethanol and yttrium nitrate in concentrated HNO3 solution. The precipitate was stirred in 2-propanol with cerium nitrate as the precursor for the dopant (see above for syntheses with dopant salt addition), dried and fired at 1600°C/2 h. Apparently the particles were not monodisperse (as in the ammonia-mediated synthesis in the so-called StOber method see Gel Microspheres Precipitated through pH Control ). 

Figure 12 Fluorescence (left) and transmission (right) images of poly(L-lactic acid)-pyrene microspheres precipitated at pyrene concentrations in CO2 of (top) 0.0013 wt % and (bottom) >0.002 wt %. (From Ref. 101.)...

In the last review, several variants of the sol-gel process for the preparation of oxide powders were discussed, such as thermal crystallization of gels, mechanical dispersion of the sols, gel microspheres precipitated through pH control, and sol emulsification processes. 

In the method based on gel microspheres precipitated through pH control, mainly spherical Si02 particles of different sizes are prepared, depending on the experimental conditions. The procedure is based on the Stober method established in 1968 [13]. 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

PCA [Precipitation with a compressed anti-solvent] A process for making a solid with unusual morphology by spraying a solution of it into a supercritical fluid. The process resembles spray drying into a supercritical fluid. Used for making microspheres, microporous fibers, and hollow microporous fibers. 

Microspheres are particles ranging between 1 and 100 pm. They are typically formed from degradable polymeric materials such as albumin, polysaccharides, or poly(a-hydroxy acids) by precipitation or phase-separation emulsion techniques [6, 332]. The relatively large diameters of microspheres make their extravasation into the tumor mass difficult and the uptake of microspheres by the RES is very rapid. 

Few works on proteins encapsulation have been presented. A recent study by Young et al. [60] deals with the encapsulation of lysozyme in biodegradable polymer microspheres. A 1-10 pm lysozyme particle suspension in a polymer solution was sprayed into a CO2 vapour phase through a capillary nozzle. The droplets solidified after falling into the liquid phase. By delaying the precipitation in the vapour phase, the larger microparticles obtained were able to encapsulate the suspended lysozyme. The final capsules were in the range of 5 - 70 pm. This work is a nice example of protein encapsulation for microparticle delivery systems. 

MIP beads or microspheres are also widely used for sensing purposes [166]. They are prepared by precipitation polymerization and then they are embedded in a dedicated matrix, which is immobilized on the transducer surface. Moreover, the MIP beads are used to serve as stationary phases in HPLC [167] and for catalytic purposes. Other systems, such as self-assembled monolayers, SAMs [168], sol-gel matrices [169] and preformed polymers [170] have also been utilized for fabrication of MIP constructs. 

Dispersion polymerization is defined as a type of precipitation polymerization by which polymeric microspheres are formed in the presence of a suitable steric stabilizer from an initially homogeneous reaction mixture. Under favorable circumstances, this polymerization can yield, in a batch process, monodisperse, or nearly monodisperse, latex particles with a relatively large diameter (up to 15 pm) [103]. The solvent selected as the reaction medium is a good solvent for both the monomer and the steric stabilizer, but a non-solvent for the polymer being formed and therefore a selective solvent for the graft copolymer. This restriction on the choice of solvent means that these reactions can be carried out... 

The production of polymeric HYAFF microspheres loaded with the pharmaceutical is currently performed by solvent emulsion precipitation [4], This process requires the preparation of an emulsion of two immiscible liquids. The polymer and the co-precipitated pharmaceutical can be inactivated or degraded due to the temperature that is required for solvents removing. Moreover, a complete separation of the residual solvents cannot be achieved and a relevant percentage of liquid is retained within the final product. Finally, liquid solvent and antisolvent cannot be recovered. 

---