Microcapsule distribution

Microcapsules can have a wide range of geometries and stmctures. Figure 1 illustrates three possible capsule stmctures. Parameters used to characteri2e microcapsules include particle size, size distribution, geometry, actives content, storage stabiHty, and core material release rate. 

Fig. 1. Schematic diagrams of several possible capsule stmctures (a) continuous core/sheU microcapsule in which a single continuous sheU surrounds a continuous region of core material (b) multinuclear microcapsule in which a number of small domains of core material are distributed uniformly throughout a matrix of sheU material and (c) continuous core capsule with two different sheUs.

The pore size and distribution in the porous particles play essential roles in NPS synthesis. For example, only hollow capsules are obtained when MS spheres with only small mesopores (<3 nm) are used as the templates [69]. This suggests that the PE has difficulty infiltrating mesopores in this size range, and is primarily restricted to the surface of the spheres. The density and homogeneity of the pores in the sacrificial particles is also important to prepare intact NPSs. In a separate study, employing CaC03 microparticles with radial channel-like pore structures (surface area 8.8 m2 g 1) as sacrificial templates resulted in PE microcapsules that collapse when dried, which is in stark contrast to the free-standing NPSs described above [64]. 

Particle size distribution, on a population basis, presented a predominantly unimodal distribution, with a mean size of 26.53 pm for 1 1 ratio microcapsules and 50.29 pm for 2 1 ratio systems. On a population basis the number of aggregates is small, although some of those produced from the 2 1 core wall systems were 200-300 pm. 

Syntactic foamed plastics (from the Greek ovvxa C, to put together) or spheroplastics are a special kind of gas filled polymeric material. They consist of a polymer matrix, called the binder, and a filler of hollow spherical particles, called microspheres, microcapsules, or microballoons, distributed within the binder. Expoxy and phenolic resins, polyesters, silicones, polyurethanes, and several other polymers and oligomers are used as binders, while the fillers have been made of glass, carbon, metal, ceramics, polymers, and resins. The foamed plastic is formed by the microcapsular method, i.e. the gas-filled particles are inserted into the polymer binder1,2). 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

Lamprecht, A. Schafer, U.F. Lehr, C.M. Visualization and quantification of polymer distribution in microcapsules by confocal laser scanning microscopy (CLSM). Int. J. Pharm. 2000, 196 (2), 223-226. 

Each of these can be related to the manufacture and rate of drug release from the systems. The following discussion presents methods of manufacture of coated or encapsulated systems, referred to as microcapsules, and matrix systems containing homogeneously distributed drug, referred to as micromatrices. 

Nevertheless, monolithic microspheres can be made to release drug at an approximately constant rate. " The core loading of these microspheres may be increased to create structures similar to those of reservoir microcapsules. An optimum combination of particle sizes (a size distribution), may be prepared to achieve a constant rate of drug release. Preparing microspheres with an erodible polymer in such a way that maximum erosion occurs in conjunction with minimum diffusion may establish a constant release rate. Although the principles described here appear simple, they are difficult to utilize because of their dependence on a number of factors, each of which can complicate the process. 

The properties of the microcapsule in relation to use, size distribution, porosity and permeability of the wall... 

Several studies have utilized CLSM techniques to study the distribution and release of biomolecules incorporated in microcapsules and microspheres and to measure the encapsulation efficiency (9,10). Lipophilic fluorophores have been utilized to locate oil-rich regions within mixed-phase microspheres and to examine the distribution of polymeric components with microcapsules. Encapsulated oil could be differentiated from other components, and other fluorescent markers allowed visualization of polymer distribution in the capsule wall (11). The technique has also been used to explore the... 

Using the so-called two-step process [15, 16], polymer nanoparticles are first synthesized via emulsion polymerization. The size of the resulting nanoparticles can be tuned by a simple process parameter and covers a range of about 30-400 nm. In a second step these nanoparticles are used to coat microbubbles in a controlled bubble formation process. The nanoparticles migrate to the surface of the bubbles (this is related to the interface activity of hydrophobic nanoparticles in general) and build a monolayer around the bubbles. Consequently, the size of the nanoparticles determines the shell thickness of the final microcapsules. Additionally, a carefully chosen nanoparticle concentration regime results in a certain microcapsule size distribution. In principle, particle sizes in the range of 0.5-10 jum can be adjusted and the microcapsule size distributions are ex-... 

The release rates that are achievable from a single microcapsule are generally 0 order, 1/2 order, or 1st order. 0 order occurs when the core is a pure material and releases through the wall of a reservoir microcapsule as a pure material. The 1/2-order release generally occurs with matrix particles. Ist-order release occurs when the core material is actually a solution. As the solute material releases from the capsule the concentration of solute material in the solvent decreases and a Ist-order release is achieved. Please note that these types of release rates occur from a given single microcapsule. A mixture of microcapsules will include a distribution of capsules varying in size and wall thickness. The effect, therefore, is to produce a release rate different from O , 1/2 , or 1 because of the ensemble of microcapsules. It is therefore very desirable to examine carefully on... 

The morphology of the resulting solid material depends both on the material structure (crystalline or amorphous, composite or pure, etc.) and on the RESS parameters (temperature, pressure drop, distance of impact of the jet against the surface, dimensions of the atomization vessel, nozzle geometry, etc.)[ l It is to be noticed that the initial investigations consisted of pure substrate atomization in order to obtain very line particles (typically of 0.5-20 m diameter) with narrow diameter distribution however, the most recent publications are related to mixture processing in order to obtain microcapsules or microspheres of an active ingredient inside a carrier. 

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