Controlled release reservoir devices

Hollow spheres are a typical stmcture of CPCs, which in particular have a wide number of potential applications, including stationary phases for separation science, biomedical devices, coating additives, controlled-release reservoirs, and encapsulation. In fact, the multifunctionality of hollow spheres of CPs is an important subject in the field of materials science. 

Controlled release can be achieved by a wide range of techniques a simple but important example is illustrated in Eigure 45. In this device, pure dmg is contained in a reservoir surrounded by a membrane. With such a system, the release of dmg is constant as long as a constant concentration of dmg is maintained within the device. Such a constant concentration is maintained if the reservoir contains a saturated solution and sufficient excess of soHd dmg. 

FIGURE 14 In vitro rate of release of testosterone from a PCL capsule (reservoir device), illustrating rate control by drug dissolution when the polymer membrane thickness is small. (From Ref. 68.)... 

We described reservoir capacity as the ability to contain an active ingredient within a matrix. In this sense, we give the term active ingredient the broadest possible meaning. We will show how polyurethanes are used to absorb exudates from deep tissue wounds. The exudates are considered active ingredients. We likened reservoir capacity to a bottle and controlled release to a bottle with a leak. A polyurethane can serve as a controlled release device, and we will illustrate this in a number of applications. 

The Ocusert system illustrated in Figure 12.7 is one example of a diffusion-controlled reservoir device. Another is the steroid-releasing intrauterine device (IUD) shown in Figure 12.9. Inert IUDs of various shapes were widely used for... 

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,... 

Many controlled release devices are not membranes by the conventional definition, since only transient release of an active agent, without permeation occurring between an upstream and a downstream, is typical. Nevertheless, some controlled release units do operate with a concentration driving force to achieve effectively steady state release from the internal reservoir of the device to the external surrounding. Such processes are included here for completeness. 

The hormone-releasing devices in uterus have a closer resemblance to controlled release because they involve the release of a steroid compound by diffusion [82,83], Progesterone, the active ingredient, is dispersed in the inner reservoir, surrounded by ethlene/vinyl acetate copolymer membrane. The release of progesterone from this system is maintained almost constant for about a year [84-86],... 

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.)...

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. 

Diffusion-controlled devices may be designed for continuous release and usually use either a matrix or reservoir construction. In matrix systems, the drug is dispersed randomly throughout a polymer, whereas reservoir devices surround the drug with an intact rate-controlling membrane. Regardless of the method of construction, the system must be safe and biocompatible for biological application. 

Most pulsatile delivery systems are reservoir devices coated with a barrier layer. The barrier dissolves or erodes after a specified lag time, after which the drug is released rapidly from the reservoir core. In general, the lag time prior to drug release can be controlled by the thickness of the coating layer. 

Figure 11. Reservoir-type membrane device for controlled release.

Figure 9.2 Reservoir delivery systems based on rate-limiting polymer membranes. Rate-limiting polymer membranes can be used to produce several different types of drug delivery devices including (a) transdermal delivery systems, (b) planar con-trolled-release systems, and (c) cylindrical controlled-release systems.

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