Monolithic controlled release device

An important cla of controlled release devices have a reservoir containing the agent to be released which is surrounded by an appropriate polymeric membrane. Clearly, this configuration is nwre difficult to fabricate than the simpler monolithic or matrix ones however, it offers some unique opportunities to regulate the rate and pattern of release. 

The simplest configuration of a controlled release device is where a drug is either dissolved in high concentration or suspended as particles in a monolithic polymer such as a cylindrical polymer fiber. The release of the drug from it may occur via ... 

The main use of PVC is for intravenous bags. However, PVC has been used in the controlled release of volatile insecticides, herbicides, pheromones, and perfumes by diffusion through a PVC membrane of multilaminated stripes. A monolithic matrix device of PVC can be prepared by mixing PVC particles with a suitable plasticizer and an active agent, followed by heating of the mixture in a mold. A solid PVC matrix is obtained from the subsequent cooling. 

Controlled release formulations are a recent innovation in which the pesticide is incorporated into a carrier, generally a polymeric material (Scher, 1999). The rate of release of the pesticide is determined by the properties of the polymer itself as well as environmental factors. There are mainly two types of CR formulations reservoir devices and monolithic devices. As shown in Figure 2.1, in the reservoir device, the toxicant is enclosed in capsules of thin polymeric material to become microcapsules (1-100 pm in diameter), e.g., Penncap-M microcapsules (methyl parathion). In the monolithic device, the toxicant is uniformly... 

Figure 2.1 Reservoir and monolithic diffusion-controlled devices. (From Lewis, D.H. and Cowsar, D.R., in Controlled Release Pesticides, Scher, H.B., Ed., ACS Symposium Series 53, American Chemical Society, Washington, D.C., 1977, p. 1. With permission.)...

Rod-shaped monolithic hydrogel devices with progesterine have been studied in the development of controlled release system for contraceptive application. It was found that progesterine release depends on the initial drug load, the degree of cross-linking, and the water content of the hydrogel. The first-order release was observed. 

Controlled Drug Release—Because the degradation products of Type III bioerosion are small, water soluble molecules, the principal application of polymers undergoing such degradation is for the systemic administration of therapeutic agents from subcutaneous, intramuscular or intraperitoneal implantation sites. Application of Type III bioerosion to controlled drug release was first described in 1970 (32) and has since then been extensively investigated. The various types of devices currently under development can be classified into (a) diffusional and (b) monolithic (7). 

Monolithic Devices—In these systems the drug is homogeneously dispersed within a bioerodible polymer matrix, and release of the drug can be controlled either by diffusion or by polymer erosion. If erosion of the matrix is very much slower than drug diffusion, then release kinetics follow the Higuchi model (37) and drug release rate decreases exponentially with time, following t dependence over a major portion of the release rate. 

In developing such devices, two fundamentally different approaches are possible. In one, mechanism of drug release is by diffusion from a reservoir through a rate-limiting bioerodible pol3nner membrane, and in the other, drug release is controlled by matrix erosion. However, to achieve zero order drug delivery from monolithic erosional devices the erosion process must be confined to the surface of the solid device. ... 

Initial work in this area sought to produce monolithic systems with release being degradation-, dissolution-, or diffusion-controlled. Monolithic systems are characterized by having a more or less uniform dispersion of the therapeutic agent within the polymer or lipid matrix. In this device design, the active agent is essentially immobilized in the matrix... 

The reducing agent, NH3, is adsorbed onto the SCR and then consumed by NOx reduction reactions. The amount of ammonia fed to the reactor must be controlled depending on the operating conditions in order to maintain a high level of NH3 adsorption and NOx conversion of the SCR on the one hand. On the other hand, NH3 emissions from the aftertreatment system into the environment are not desired and have to be avoided. In order to reduce as much as possible the amount of released ammonia, the addition of another catalytic device downstream the SCR one can be an efficient and reliable solution. Furthermore, to maintain the system as compact as possible, one solution is to add an ammonia oxidation functionality directly in the rear part of an SCR monolith (Fig. 18.1a). In this way, an NH3 slip catalyst (ASC) is added after the SCR to oxidize NH3 leaving the SCR brick [2]. 

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