Microspheres volume

Usually glass microspheres crush during the loading step therefore, a damage allowable constitutive model of the microsphere inclusions is required. If an internal stress and time dependent evolution parameter (t)dp, 0 is introduced to represent the volume fraction of the damaged microspheres out of the total microsphere volume, the deformation gradient of the inclusions could be expressed as... 

The number of microencapsulated commercial oral formulations available and the volume of these formulations sold annuaUy is comparatively smaU. This may reflect the difficulty of developing new dmg formulations and bringing them successfully to market or the fact that existing microencapsulation techniques have had difficulty economically producing mictocapsules that meet the strict performance requirements of the pharmaceutical industry. One appHcation that is a particularly active area of development is mictocapsules or microspheres for oral deUvery of vaccines (45,46). 

We varied the gel-forming parameters, the content of Kaolin clay, the crystal size, and the thickness-to-diameter ratio in a series of experiments. A dual structure began to appear at about 35-40% Kaolin. If the clay content was increased to too high a level, the microspheres became too weak and began to have poor attrition resistance. The cost of the various grades also affected our choice. Figure 9 shows a comparison of pore volume distribution for a typical synthetic versus a clay modified catalyst. 

Kaolin had little or no cracking activity, and catalyst activity as tested in the laboratory was directly related to silica-alumina gel content. However, the catalyst performed much better in commercial tests than anticipated from laboratory testing. Undoubtedly, this open structure encountered much less severe conditions at the outer surface of the microsphere during regenerations and made internal catalytic surfaces more readily available. This first of the so-called "semisynthetics" was called Nalco 783, and the matrix is still used in many forms some 28 years later.(7,13) Today it is estimated that some 200,000 tons/yr. of kaolin clay is used for cracking catalyst manufacture as reported by Georgia Kaolin Corporation.(24) Figure 10 shows the pore volume distribution for Nalco 783 and two other commercial semisynthetics from that period. 

The requirement of 3 mg/day of carmustine over 20 days means a load of up to 60 mg in a device. Most formulations would suggest no higher than a 10% load of drug in the polymer matrix, so that the entire drug depot would be approximately 0.6 mL in volume. This is a very modest size disk, of about 0.1 x 2.5 x 2.5 cm. Or this amount of drug can be packaged into numerous injectible microspheres of 300 ttm... 

B) on the resultant microsphere diameter did not significantly affect Jms. However, the number and location of the impellers had a significant impact on 4ns. As a result, to simplify the system, Maa and Hsu always used double impellers (wimp = 2), with the lower one placed close to the bottom of the tank and the other located in the center of the total emulsion volume. Finally, Maa and Hsu determined that the volumes of the organic and aqueous phases, in the range they were concerned with, played only a minor role in affecting 4ms- Thus, by the omission of D/T, B, and and v. Equation (42) was simplified considerably to yield... 

The assay was carried out using a Varian gas chromatograph (model 5000 LC) under the following experimental condition. The oven injector and flame ionization detector temperatures were 125°C and 225°C respectively. A Porapak column was used, the eluent was N2 at a flow rate of 30 ml/min and the injected volume 2 pi. Various concentrations of purified methylene chloride in purified methanol were injected (both solvents were distilled to discard any impurity which might interfere with the sensitive assay). Calibration curves were linear in the range 50-500 ppm (the limit of detection was 10 ppm). Methylene chloride detection in the microspheres was performed by dissolving various amounts (20-200 mg) of microspheres in 220 ml of purified methanol prior to the injection. 

All Eudragit microspheres (RS RL 1 1) had a theoretical content of9.1% w/w nifedipine and were prepared using 25 g of Eudragit mixture and 2.5 g of nifedipine dissolved in 80 ml initial methylene chloride volume phase. aAs in table 1. 

PNIPAM microsphere gels with diameter of 100-200 jim were prepared by emulsion polymerization [21]. The gel containing 12 mole % benzo[18]crown-6 was immersed in water and the diameter change of the gel was measured during heating at a rate of 0.3 °C/min. The gel was swollen below 25 °C. In the absence of metal ions, it started to shrink at 26 °C and showed a sharp volume change at 28.4 °C. Finally, the volume decreased by as much as 10 times the original volume. 

Lower apparent densities can be obtained by allowing air intrusions into the binder (dotted line in Fig. 1). If the concentration of filler exceeds a critical value (67 vol %), the quantity of binder falls below that of the free volume between the microspheres. The binder then no longer covers all microspheres, the homogeneity of the system is disturbed, and defects (cavities) occur. This can formally be regarded as the appearance of open pores in the structure, the consequence of which is the deterioration of all macroscopic properties of the material. 

The space factor is given by the volume the microspheres occupy when packed the closest8,78), i.e.,... 

The density of a syntactic foam is determined by the relative proportions (volume fractions) of microspheres, resin matrix, and dispersed air. The theoretical lower limit of the density in a two-phase system (without dispersed air) is determined by the close packing rules for spheres. Various packing possibilities in systems containing uniform-size spheres are summarized below 8> 8S) ... 

Figure 6 is a graphic representation of foam structures in which the microspheres are dispersed randomly (a) and uniformly in close packing (b). In both structures, the two phases fill completely the whole volume (no dispersed air voids) and the density of the product is thus calculated from the relative proportions of the two. Measured density values often differ from the calculated ones, due to the existence of some isolated or interconnected, irregularly shaped voids as shown in Fig. 6c. The voids are usually an incidental part of the composite, as it is not easy to avoid their formation. Nevertheless, voids are often introduced intentionally to reduce the density below the minimum possible in a close-packed two-phase structure. In such three-phase systems the resin matrix is mainly a binding material, holding the structure of the microspheres together.

The principle of composition of three-phase syntactic foams can be represented by a ternary phase diagram (Fig. 7). Point A on the diagram denotes a composite of the following volume fractions resin 0.15, microspheres 0.60, voids 0.2586>. The pure void free syntactic foam has a two-phase composition which falls along the... 

The two-phase composite of microspheres and voids only (i.e. packed spheres without any binding resin) has also a minimum limit. Cubic packing (volume fraction 0.53) is the lower limit, below which the structure is not selfsupporting any more, and some resin is required to fill in between the spheres. S). 

Another two-phase composite is chemically or physically blown foam, composed of polymer and voids only (i.e. conventional foamed or cellular polymer). Its compositions lie along the polymer-void border of Fig. 7, and it, too, is limited by the maximum volume fraction of voids allowed, while still maintaining the definition of a foam. The limits mentioned define the allowed compositions for syntactic foams and determine the area within the diagram where they are located. One limiting case is point B which represents the composition of microspheres (0.74), polymer (0.11), and voids (0.15). The microspheres, in this case, are arranged in a hexagonal close packing 85). 

Fig. 9. Size distribution of the volume fraction of open cells in a carbonized syntactic foam. For a phenolic syntactic foam with Kerasphere carbon microspheres, specimen apparent densities are (1) 130, (2) 220, and (3) 390 kg/m3 391...

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