Capsule diameter

Figure 3 presents an example of islet functioning inside a particular capsule. A typical two-phase perifusion profile is noted,similar in quantitative terms to that of unencapsulated islets. Clearly the ratio of the membrane thickness to capsule diameter is an important parameter, with low membrane capsule ratios providing rapid transfer of nutrients and the exodiffusion of insulin. Contrarily, for thick walled capsules of diameter less then approximately one-half millimeter the perifusion response, as measured by the stimulation index and retardation in insulin response to a glucose stimulus, is slower for encapsulated islets relative to free islets. 

Hwang JR, Sefton MV. Effect of capsule diameter on the permeability to horseradish peroxidase of individual HEMA-MMA microcapsules. J Controlled Release 1997 49 217-227. 

Another requirement is that the capsule should have sufficient strength. Mechanics tells us that the strength is determined by the mechanical properties of the (dry) wall polymer and by the ratio of the wall thickness to capsule diameter. We settle on a wall thickness of 3% of the diameter. Because capsule diameter and wall thickness are coupled, the release time turns out to be proportional to the capsule diameter squared. This makes it difficult to use very small capsules. With our dispersing method we do not want very small capsules anyhow the gardener should not inhale them We settle on a diameter of 0.3 mm. 

The capsule membrane appeared to consist of an outer skin, a thin macropo-rous layer and a very thick dense membrane, with an overall (mean) thickness of 90 pm (Fig. 12). The capsule diameter was 900 pm after 7 days. Smaller capsules can be made with a variation of this method in which the needle is held stationary and the hexadecane pumped past the end of the needle the hexa-decane is recirculated and care must be exercised to ensure that capsules are not entrained with the recirculated hexadecane (Fig. 13). The effect of hexadecane flowrate on capsule size (as it leaves the needle) in an early prototype is shown in Fig. 14 capsules shrink to approximately half their initial diameter as solvent is extracted. Note that capsules as small as 300 pm can be produced. As a consequence of the thin skin it is possible to damage the capsules through mishandling forceps with serrated edges or forcing capsules through narrow bore needles are avoided as a result. 

Fig. 14. Effect of hexadecane flowrate on capsule diameter (as formed prior to shrinkage) in the initial prototype small diameter apparatus...

To describe the concentration profiles and release kinetics of fluorescein from single NP-shelled capsules, Munoz Tavera et al. solved a mathematical model that describes unsteady-state transport from multilayered spheres using the Sturm-Liouville approach [92], Several aspects of dye release, such as the asymptotic plateau effect of diffusive release and the effects of capsule diameter and shell thickness distribution on dye release, were captured by the model and confirmed by experimental data. 

Equation (2) shows a linear relationship between the shell thickness and the capsule diameter when the ratio of wJ(Wg+w,) is in the range of 0.50 to 0.95 [83]. 

Capsule diameter (pm) Figure 5.5 Size distributions of microcapsules containing insecticide [28]. 

Shell layer number is connected with capsules stability. In principle it is better to have capsules with as few layers as possible. The only low limit is connected with osmotic pressure rising inside the capsules during the core dissolution process. To avoid pressure values critical to the shells one needs to find new materials, to decrease the capsules diameter or to use some chemical reactions to strengthen capsule shells. 

Table 3.1 Capsule diameter (measured by CLSM) and thickness in the dried state (determined by AFM) of different types of biodegradable shells before and after incubation at 90 °C for 20 min.

Figure 3.5. Microcapsules simultaneously loaded with luminescent semiconductor and magnetic oxide nanoparticles are aligned in a magnetic field. The images were obtained with a confocal laser scanning microscope TCS Leica operating in transmission (left column) and luminescence (right column, excitation wavelength 476 nm) modes, respectively. Capsule diameter is 5.6 fim in all cases. (Reproduced from Ref. 78).

The increasing strain near the hole was illustrated in the photos placed in order. The cracks grew through each hole visually. For low concentration of capsules, cracks led to rupture. For high concentration, big crack arose from capsules. It meant that local concentration of stress depended on overlapping of stress fields near capsules too. Theoretically, such interaction appeared when distance between capsules was less than 5 capsule diameters -Figure 29. 

Small capsule diameters to ensure sufficient diffusion and internal organ tiansplantability (depending on application, < 400 pm for bioartificial pancreas), with the cell centering within the microcapsule... 

The spherical microcapsules (colloids) are formed by interfacial organic synthesis in water, but the capsule diameter is often too high for coating use and filtration of the dispersion to separate large from small particles is required. However, not only mesh size (voids) of the filtration device but also interactions between filtration material and microcapsules can be of importance for an efficient filtration. 

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