Solid organic microspheres

Inexpensive, finely ground minerals like barium sulfate (barytes), dolomite, limestone (whiting), clays, and silica are widely used to provide bulk and reduce the cost of friction material formulations. These materials also act as friction modifiers and alter the performance of the end product. Other less commonly used fillers are hollow and solid organic and inorganic microspheres and fly ash. 

This is one of the earliest methods of microsphere manufacture. The polymer and drug must be soluble in an organic solvent, frequently methylene chloride. The solution containing the polymer and the drug may be dispersed in an aqueous phase to form droplets. Continuous mixing and elevated temperatures may be employed to evaporate the more volatile organic solvent and leave the solid polymer-drug particles suspended in an aqueous medium. The particles are finally filtered from the suspension. Fig. 3 shows polylactic acid particles prepared in this manner. ... 

Polymer nanoparticles with diameters of 50-500 nm are now widely used. As with microspheres and microcapsules, one can differentiate between solid polymeric spheres (nanoparticles) and those spheres with thin polymeric walls (nanocapsules). The locus of polymerisation is not an emulsion droplet as in microencapsulation, hut a micelle. The process involves the soluhilisation of a water-soluhle monomer such as acrylamide along with the dmg or other agent such as antigen to he encapsulated. An organic liquid such as n-hexane serves as the outer phase. Polymerisation is induced hy irradiation (y-rays. X-rays, UV light), exposure to visible light or heating with an initiator. 

Another approach to extend the action of proteins is to develop biodegradable polymeric microspheres. Owing to their excellent biocompatibility, the biodegradable polyesters, poly (lactic acid) (PEA) and poly(lactic-co-glycolic acid) (PEG A), are the most frequently used biomaterials to achieve sustained action [204, 205]. Polymeric microspheres are commonly prepared by the solvent extraction/evapora-tion methods [206]. In brief, protein in a solid or liquid form is mixed with a polymer (dissolved in an organic solvent, e.g., dichloromethane) to prepare a solid-... 

Figure 2. Solvent evaporation method for production of polylactide microspheres. The polymer (PLGA or PLA) is dissolved in an organic solvent (e.g., methylene chloride or ethyl acetate). The aqueous or solid dmg is then added, and the solution is mixed by sonication or homogenization to form the primary emulsion (solid or water in-oil), This emulsion is then transferred to water containing an emulsifying agent [e.g., poly(vinyl alcohol)]. Mixing of the primary emulsion in the water phase produces the microspheres, resulting in a secondary emulsion (solid or water in-oil-in-water). The final emulsion is diluted with excess water to facilitate removal of the organic solvent in the oil phase. The microspheres are then dried. (Reproduced om Cleland, 1995, cop)Tight Plenum Press.)...

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