Release of the active agent

Methods were described for the incorporation of proteins in the form of noncovalent complexes with polycationic reagents, into sustained release systems where the polycation stabilizes the protein against inactivation while it resides in the delivery device, and retards release of the protein from the delivery device [469,470]. A variety of polycations have been used, including simple polyamino acids such as polylysine or polyarginine, protamine and chitosan. The end result was the release of the active agent with retention of biological activity, with a high cumulative field and over a sustained period of time. 

Two crucial aspects of the design oi bioactivc polyphosphazenes have been carefully developed. One involves Ihe hydrophilicily or hydrophobicily of the polymer, and ihe other is the stahiliiy of the polymer or tactical substiiuenl linkages lhal allow release of the active agent or ensure its potency io he retained in the bound form,... 

Microencapsulation can be used to provide a temporary barrier between a chemical species and its surrounding environment see also Section 14.3). This permits controlled (slow) release of the active agents following application. Depending on the product and the situation, an active ingredient such as a pesticide may need to be released slowly at low concentration, or slowly at high concentrations. Such controlled release can both reduce the number of crop applications that are required and also help prevent over use and subsequent run-off. The barrier can be provided by a polymer film, in the case of suspensions [867], or a liquid membrane, in the case of single or multiple emulsions [865], Microemulsions have also been used [234,865],... 

If the release of the active agent should either be spontaneous or slowly over a very long time, the matrix-based capsules cannot fulfill the requirements of the ap-phcation. For these cases, a core-shell-encapsulation is preferred. The suitable range of shell materials is very broad, since most gelling systems can be used. The... 

In microcapsules the release of the active agent is controlled by Fickian diffusion through the micropores in the capsule walls. However, the polymer phase in the microcapsule walls are often not homogeneous and have cracks and pores. 

The kinetics of release of the active agent into the liquid free from any microorganisms is shown in Figure 5.2 for a wide range of values of the dimensionless numbers R. Other dimensionless numbers are also used D-t/L for time, and the ratio C/Cj of the concentration of the agent in the food at any time as a fraction of the initial concentration in the linen. 

The other example, a polymer-bound fungicide, is found in the copolymerization of pentachlorophenyl acrylate with vinyl acetate and ethyl acrylate. This pentachlorophenol-based product could have use as an anti-fouling agent in marine coatings. In this case it was necessary to copolymerize with ethyl acrylate. The homopolymer was found to be too hydrophobic to allow decomposition of the polymer and allow release of the active agent in sufficiently high concentrations to have the appropriate biocide effect. 

Controlled release of the active agent either by diffusion from a matrix or hydrolytic/enzymatic cleavage from the polymer carrier. This allows a sustained and more steady delivery of the active agent within the body or from patches applied externally. 

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