Microspheres concentration

FIGURE 6.5 The measured power of the third harmonic power generated in the bulk of solution containing 0.5- xm-diameter fused silica microspheres as a function of microspheres concentration. Circles are experimentally measured data points the dashed line is a linear fit. 

The consistency (fluidity) of an initial mixture depends on the binder filler weight ratio, all other parameters (binder viscosity, microsphere type, shape, size, density, and mixing conditions) being equal. The mixtures are casting compositions (viscous fluids) at small microsphere concentrations, while they become molding compositions (pastes) at higher concentrations. Thus, the fluidity of a syntactic composition depends primarily on the filler concentration and not on the binder viscosity (Fig. 1)73 ... 

As mentioned above, the binder/filler ratio determines the fluidity and hence the process technology for a given syntactic composition. The solid line in Fig. 1 shows how the apparent density depends on the relative microsphere concentration in syntactic compounds, provided there are no air inclusions. The lower limit of apparent density is at the concentration at which the filler is most densily packed at this point the material has its highest specific strength (strength per unit apparent density). Experience has shown that the closest packing of the spheres within the binder is obtained when the material is mixed and cast or molded under vacuum 8 10). 

Fig. 3. Apparent density (y) of epoxy syntactic foams versus glass microsphere concentration (C). The figures next to the curves stand for the microsphere bulk densities in kg/m3 the maximum microsphere concentration is 67% for molding compositions 11...

If the microsphere concentration exceeds a certain threshold value, called the space factor (K,), the mixture loses its fluidity and turns from a casting composition to a press-molding composition. Each type of microspheres has its own binder/filler ratio... 

An increase in the microsphere concentration that changes the composition from a casting to a molding one is accompanied by qualitative changes in the final material s strength and elastic properties. Zavalina et al.139), for example, demonstrated that... 

Hobaica and Cook142) demonstrated that for microsphere concentrations below 67 vol %, syntactic-foam water absorptivities are virtually independent of the apparent density, but above that concentration the absorptivities rise rapidly due to the loss of binder integrity and the appearance of cavities and ducts. 

As the filler concentration increases so does the initial slope of Wt = f(j/x), see Fig. 12. According to Eq. (5) this may be due either to an increase of the diffusion coefficient or to the increased effective thickness of the specimen due to the addition of the filler that does not absorb water. Initially, there are small increments of the relative content of absorbed water, but the absolute quantity (referred to the binder) is almost independent of the filler concentration however in the next stages the absolute quantity of absorbed water becomes proportional to the microsphere concentration. Thus there are different absorption mechanisms effective in the course of time. 

Fig. 20. Calculated and experimental (dotted and solid curves respectively) values of (1) elastic modulus (E) and (2) ultimate compression strength (crj versus phenolic microsphere concentration (C) in an epoxy syntactic foam at 23 °C 162)...

The signal measured by sensor can be used to quantify the amount of analyte present. The response to the sensor is determined by the in-plane component of the stray fields induced by the magnetized microspheres. Concentrations as low as 3.2 pg/ml have been detected [98], The introduction of tunneling magnetoresistance (TMR) sensing elements and smaller magnetic markers will increase the sensitivity of the method. 

Figure 2. The geometry shown in Figure 1 is refined for dielectric analysis by fixing a microsphere concentric with the macrosphere and contains the elements of molecular structure.

Studies of the pharmacokinetics of this deHvery system in two animal models have been reported in the Hterature. After iajection of these microspheres at three doses, leuproHde concentrations were sustained for over four weeks foUowing an initial burst (116). The results iadicated that linear pharmacokinetic profiles in absorption, distribution, metaboHsm, and excretion were achieved at doses of 3 to 15 mg/kg using the dmg loaded microspheres in once-a-month repeated injections. 

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... 

Polyanion Dry weight ratio chitosan/polyanion NH2/COOH Molar ratio Total polysaccharide concentration, g/1 Solubility microsphere Inlet air /°C... 

The approach to standardization used by Haaijman (53) and others (66,67), in which the fluorophor is incorporated within or bound to the surface of a plastic sphere, is more versatile than the use of inorganic ion>doped spheres, since the standard can be tailored exactly to the specifications required by the analyte species. However, this approach increases the uncertainty of the measurement because the photobleaching characteristics of both the standard and the sample must be considered. The ideal approach is to employ both types of standards. The glass microspheres can be used to calibrate instruments and set instrument operating parameters on a day-to-day basis, and the fluorophor-doped polymer materials can be used to determine the concentration-instrument response function. 

Hydrocortisone microspheres (108,109) and films (110) based on poly(lactic acid) have been investigated. A cage implant technique was used to study the performance of monolithic poly (DL-lactide) films loaded with hydrocortisone acetate (110). Films 1.5 x 0.6 cm were inserted into titanium wire-mesh cages 3.5 x 1.0 cm. The cages were implanted in the backs of rats and the inflammatory exudate was sampled periodically. The white cell concentration in the samples was lower than that of controls at all times during the 21-day test. 

Position in the microsphere Molecular weight Physicochemical properties Concentration... 

Tissue distribution of lipo-preparations. The tissue distribution of lipid microspheres in normal and pathologic animals was studied. Research into liposomes of similar size suggested that lipid microspheres accumulated preferentially in the reticuloendothelial system, inflammatory sites, or certain tumors. The distribution of lipid microspheres to these tissues has been found in our studies (7,2). Interestingly, our study showed that lipid microspheres accumulated, particularly at high concentrations, in damaged vascular walls such as atherosclerotic vascular walls. 

---