Reservoir devices/systems

Diffusion systems are characterized by the release rate of a drug being dependent on its diffusion through an inert membrane barrier. Usually this barrier is an insoluble polymer. In general, two types or subclasses of diffusional systems are recognized reservoir devices and matrix devices. These will be considered separately. 

Reservoir devices, as the name implies, are characterized by a core of drug, the reservoir, surrounded by a polymeric membrane. The nature of the membrane determines the rate of release of drug from the system. A schematic description of this process is given in Fig. 4, and characteristics of the system are listed in Table 3. 

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... 

As for the non-porous reservoir device, in the microporous system, both ... 

Reservoir/matrix hybrid-type non-degradable polymeric implants are also available. Such systems are designed in an attempt to improve the M t1/2" release kinetics of a matrix system, so that release approximates the zero-order release rate of a reservoir device. Examples of these types of systems include ... 

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 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.

Figure 9.5 Drug release from a cylindrical-reservoir drug delivery system. The cumulative mass of drug released is plotted versus time for cylindrical-reservoir devices with a range of physical characteristics, which are determined principally by the diffusion coefficient for the drug in the polymer and the membrane thickness. In all cases, the overall length of the device, L, and the cross-sectional radius, b, were fixed at 2.7 and 0.5 cm, respectively. In each panel, one of the relevant design parameters was varied (a) hja was varied between 1.2 and 4 with Dj.p = 1x10" and Cl = 20mg/mL (b) Dj.p was varied (for curve A, Dj.p = 5 x 10 cm /s ...

The typical release profiles shown in Figure 32.4 for reservoir and matrix delivery systems may present variations a burst effect due to the presence of some core material too close to the external device surface for matrix devices, or a delayed time to start diffusion due to the diffusion of the core through the encapsulating layer of the reservoir device. Also, the physical state of the core material (dissolved or dispersed) defines the release kinetics. For example, a reservoir system in which the active core is not dissolved results in zero-order kinetics (constant flow), whereas it results in first-order kinetics (exponentially decreasing flow) if the core is dissolved in the encapsulated material. 

The most well known commercial reservoir controlled release systems deliver hormones for contraception from hydrophobic polymers. The Norplant subcutaneous device controls the release of levonorgestrel with silicone rubber, and the Progestasert intrauterine device (lUD) releases progesterone from reservoir devices of ethylene vinyl acetate. In the field of insecticides, reservoir dispensers called BioLure were developed to provide zero-order release ofinsect pheromones to disrupt mating (Smith et al, 1983). The dispenser consists of a slab configuration with a rate-controlling membrane, with constant release described by Eq. 1. 

The polymer will play a passive role if it acts solely as a barrier which controls the rate of drug delivery by diffusion. Indeed, changes in the properties of the polymer are undesirable in this case since thereby the parameters governing the diffusion process will change. Purely diffusion controlled delivery systems generally belong to either one of two types, monolithic devices or reservoir devices. 

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