Chemically controlled polymer release

The composition of the structural material and the choice of the fabrication process are important in the preparation of controlled-release systems. Over the past decades, great advances have been made in the engineering of multicomponent, polymer-based, structural materials. These materials were designed to release active substances by different mechanisms (ref. 1) including diffusion, chemical control (polymer degradation) and solvent activation (swelling or osmotic pressure). In some cases, combinations of such mechanisms have been used. Experimental methods and theoretical analysis of mass transport phenomena in these materials have been developed (refs. 2,3). 

Microstructure (see also Stereochemistry and Tacticity) 114,115.128.138,139 Miscibility (see also Compatibility) 12, 53, 68 Model networks 163 Modification of a polymer, chemical 154 Mold release 71, 74 Molecular weight, control 147. 154... 

With continuous development of systems for controlled drug release, new materials are being used whose influence on peptide stability must be carefully examined. Thus, the model hexapeptide Val-Tyr-Pro-Asn-Gly-Ala (Fig. 6.30) embedded in poly (vinyl alcohol) and poly(vinyl pyrrolidone) matrices had rates of deamidation that increased with increasing water content or water activity, and, hence, with decreasing glass transition temperature (Tg). However, the degradation behavior in the two polymers differed so that chemical reactivity could not be predicted from water content, water activity, or T% alone. Furthermore, the hexapeptide was less stable in such hydrated polymeric matrices than in aqueous buffer or lyophilized polymer-free powders [132],... 

Muscles contract and expand in response to electrical, thermal, and chemical stimuli. Certain polymers, such as synthetic polypeptides, are known to change shape on application of electric current, temperature, and chemical environment. For instance, selected bioelastic smart materials expand in salt solutions and may be used in desalination efforts and as salt concentration sensors. Polypeptides and other polymeric materials are being studied in tissue reconstruction, as adhesive barriers to prevent adhesion growth between surgically operated tissues, and in controlled drug release, where the material is designed to behave in a predetermined matter according to a specific chemical environment. 

In concluding this section, the author wishes to note that prior to the studies outlined above, two different research groups [79, 80] had attempted to control the volume of a gel or gel-like polymer membrane enzymatically. However, their purpose was not to convert the energy of enzyme reactions into mechanical work but to control the release of chemicals through the gel porosity. The phase... 

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],... 

The CYPHER stent employs two nonerodible polymers polyethylene-co-vinyl acetate (PEVA) and poly-n-butyl methacrylate (PBMA), The combination of sirolimus and these two polymers constitutes the basecoat formulation that is applied to a stent treated with paryleneC. In addition, a drug-free topcoat of PBMA polymer is applied to control the release kinetics of sirolimus (59), making this a diffusion-controlled reservoir device. The chemical structure of the polymers used in the CYPHER stent is shown in Figure 4,... 

There are a variety of different polymer systems available to deliver a predictable and reproducible release of chemical compounds (13,14). Two of the most versatile systems, adaptable to sensor design, release incorporated reagent by chemically-controlled or diffusion-controlled processes. Figure 1 shows examples of these two systems. 

Chemically-Controlled Systems. In these systems, the polymer matrix contains chemically-labile bonds. On exposure to water or enzymes the bonds hydrolyze, erode the three dimensional structure of the polymer and release the incorporated reagent into the surrounding medium. Depending on the polymer used, the erosion products may act as interferences, such as by altering the pH of the solution. Examples of these systems are polyglycolic acid (PGA) and a polyglycolic acid - polylactic acid (PGA/PLA) copolymer. PGA hydrolyzes to hydroxyacetic acid, and PGA/PLA hydrolyzes to lactic acid and hydroxyacetic acid. Other chemically-controlled systems are based on polyorthoesters, polycaprolactones, polyaminoacids, and polyanhydrides. 

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