Leaching controlled release polymers

It is the combination of slow solution rate and the formation of viscous surface layers that makes hydrophilic polymers useful in controlling the release rate of soluble dmgs which are perhaps irritant to the stomach or which cause nausea on rapid release (see section 8.5). Choice of appropriate polymer molecular weight controls both the rate of dissolution and the viscosity of its resulting solution. A balance between rate of polymer solution and viscosity of the solution layer must be achieved in controlled-release systems. If the polymer solution rate is too slow, then soluble dmg is leached out with little retardation. 

A gas foaming/particulate leaching process can be employed to fabricate interconnected open pore structures of PLGA for controlled release of DNA [113,199]. This process employs carbon dioxide to process a mixture of polymer and porogen, in order to fuse adjacent polymer particles into an interconnected structure. The DNA can be lyophilized with the microspheres [113] or encapsulated within the microspheres [200,201]. Lyophilization of DNA with the microspheres can provide large quantities of incorporated DNA, with relatively rapid release kinetics. Incorporation of DNA into the microspheres provides for a more sustained release relative to the lyophilization method [200], with the release kinetics dependent on the polymer molecular weight and microsphere size [201]. DNA can be incorporated into polymer microspheres using several approaches [201-205]. Subcutaneous implantation of scaffolds results in transfected cells observed within the scaffold... 

Controlled release (CR) is a method by which active chemicals are provided to specific plant species at preset rates and times. Polymers are mainly used to control the delivery rates, mobilities, and periods of effectiveness of the chemicals. The main benefit of the CR method is that if fewer chemicals are used for the protected plants over the predetermined period, then the effect on the other plant species is less, while leaching, volatihzation, and degradation are reduced. The macromo-lecular character of polymers is the key to reduction of chemical loss throughout the production. 

Release of tetracycUne hydrochloride from PCL fibers was evaluated as a means of controlled administration to periodontal pockets (69). Only small amounts of the drug were released rapidly in vitro or in vivo, and poly(ethylene-co-vinyl acetate) gave superior results. Because Fickian diffusion of an ionic hydrochloride salt in a UpophiUc polymer is unlikely, and because PCL and EVA have essentially identical Fickian permeabilities, we attribute this result to leaching of the charged salt by a mechanism similar to release of proteins from EVA (73). Poly-e-caprolactone pellets have been found unsuitable for the release of methylene blue, another ionic species (74,75). In this case, blending PCL with polyvinyl alcohol (75% hydrolyzed) increased the release rate. 

PEG-free composites. In polymer-free (I)-silica composites, diffusion model is only valid for the initial period. The course of further liberation can be described by zero-order kinetics. Because the diffusion rate of (I) is lower than its dissolutitMi rate, when a saturated solution of (I) is formed the drug is able to form crystalline precipitates in the pores. Therefore, liberation is only controlled by the diffusitm constant and is directly proportional to the leaching time. From a pharmaceutical point of view, it is desirable that a drug should have such a pattern of releasing behavior, Bottcher (1998) said. 

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