Size of microcapsules

Microencapsulation is a process in which a pure active ingredient or a mixture of ingredients is coated with or entrapped within a protecting material or system (see also Chapter 24). As a result, useful and otherwise unusual properties may be conferred to the microencapsulated ingredient(s), or unusefiil properties may be eliminated from the original ingredient(s) (Shahidi and Han 1993). The particle size of microcapsules formed by either encapsulation or entrapment can vary between... 

Zhuo, L. Chen, S. Effects of catalyst and core materials on the morphology and particle size of microcapsules. International Journal of Polymeric Materials (2004), 53(5), 385—393. 

Measurement of Size of Microcapsules. We measured the size of poly(L-lysine-a/r-terephthalic acid) microcapsules in various buffer solutions by the usage of a particle sizer (Malvern System 3601). About 15000 microcapsules are dispersed in 1 ml of suspension medium. 

These observations Aat Ae membrane becomes transparent and Aat the size of microcapsules increases in alkaline medium suggest that poly(L-lysine-a/t-terephthalic acid) microcapsule membranes have a gel-like structure and undergo a volume transition at a pH value between 4 and 6, as usually seen in ionic hydrogels. This pH-driven volume transition is considered to be due to the change of Ae charge density in Ae microcapsule membrane. 

EC microcapsules containing Rosemary oil or limonene were obtained by phase separation method. According to this procedure, EC microcapsules without oil could also be produced. This could be explained due to EC inter cial activity, which stabilizes the formed emulsion. Surfactant-fiee multiple emulsions using EC as a polymeric emulsifier have already been reported by Melzer and collaborators [8]. From the scanning electron micrographs shown in Fig. 1 it is observed that EC microc g)sules had regular spherical sh, the size of microcapsules varied and that the surface was porous. 

This work described the cotton treatment with EC microcapsules prepared by phase separation method. The size range of EC microcapsules depended on the stirring speed employed in encapsulation. Reducing the stirrer speed increased the size of microcapsules. The oil presence in EC microcapsule has been proved by vibrational spectroscopic analysis after microcapsules dissolution in acetone or after sonication in cyclohexane. The obtained EC microcapsules were bonded to cotton fabrics through the... 

Although a variety of alternative microencapsulation techniques is available (for details of sol-gel techniques, see Chapter 8), no single method is suitable for encapsulating different types of core material. Ultimately, the best method will depend upon the type of core material, the required particle size, the permeability of the shell wall, and the different properties of the microcapsule, and consequently the process must be custom-tailored in order to provide a satisfactory outcome. An overview of the size of microcapsules obtained by different techniques is provided in Table 1.2. 

Table 9.1 Stability and particle size of microcapsules prepared from different concentration of...

In addition to the nature of surfactants, other factors such as the ratio of core to wall material components, the surfactant quantity, the feeding order and preparation time also affect the stability and particle size of microcapsules. 

Recently, many synthetic polymers such as urea/formalin resin, melamine/formalin resin, polyester, and polyurethane have been widely used as the wall material for the microcapsule, though the gelatin microcapsule is still used. Microcapsules using a synthetic polymer wall have several advantages over those using a gelatin wall (1) the preparation process is simple, (2) the size of the microcapsules is well balanced, (3) the microcapsule concentration can be increased twofold or more and (4) the microcapsules have a high resistance to water and many chemicals. Synthetic microcapsules are prepared by interfacial polymerization or in situ polymerization. 

Model Membranes and Their Characteristics Liposome preparation and size characterization, 171, 193 preparation of microcapsules from human erythrocytes use in transport experiments of glutathione and its S-conjugate, 171, 217 planar lipid-protein membranes strategies of formation and of detecting dependencies of ion transport functions on membrane conditions, 171,... 

Colloidosomes are a recent class of microcapsules and thus far have only been applied to catalysis and drug delivery in a few cases. The term colloidosomes was coined by Anthony Dinsmore and colleagues in 2002 to refer to capsules where the shells are composed of close-packed layers of monodisperse colloidal particles (usually micron-sized polymer beads) that have been linked together by sintering. 

Dobetti and Pantaleo (38) investigated the influence of hydrodynamic parameters per se on the efficiency of a coacervation process for microcapsule formation. They based their work on that of Armenante and Kirwan (39) who described the size of the smallest eddies or vortices generated in a turbulent regime on a microscopic scale in the vicinity of the agitation source, i.e., microeddies, as... 

Fig. 4—In vitro release profile of microcapsules (particle size ( ), 540 pm core polymer ratio, 1 2) RSPM A,S 100 n,L 100.

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. 

The release from microcap systems is more complicated. First the drug has to dissolve in dissolution fluid which has diffused into the tablets via pores and then between the plates of poly (DL-lactic acid) forming the walls of the microcapsules. This drug solution then has to diffuse out of the tablet via the same route. The effect of compression on the release has more significance in the simple matrix tablets than the microcap systems, because of the above mechanism of release. Higher compressions reduce the size of the pores between the poly(DL-lactic acid) plates, which extends the release. 

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. 

The micromanipulation technique has been used to measure the mechanical strength of microcapsules of different size, shell thickness and shell composition (Sun and Zhang, 2001,2002 Xue and Zhang, 2008 Zhao and Zhang, 2004 Zhang et al., 1999). For example, the mechanical... 

It was determined that the ideal stilt material should have a size 1.5-2.5 times the diameter of the microcapsules or microcapsule clusters. Since the microcapsules are often agglomerated into clusters of a maximum size of 10 pm, the optimal stilt diameter should vary from 15 to 25 pm, with an average of 20 pm.204 In addition, a round stilt material is more efficient than a polygonal material which can fracture the microcapsules by contact. It also should possess a certain rigidity to provide protection to the microcapsules during normal manipulation of the carbonless paper, but not be so... 

The term microcapsule is defined, as a spherical particle with the size varying between 50 nm and 2 mm containing a core substance. Microspheres are, in a strict sense, spherically empty particles. However, the terms microcapsules and microspheres are often used synonymously. In addition, some related terms are used as well. For example, microbeads and beads are used alternatively. Spheres and spherical particles are also employed for a large size and rigid morphology. Due to attractive properties and wider applications of microcapsules and microspheres, a survey of their applications in controlled drug release formulations is appropriate. 

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