Diffusion-controlled delivery systems

Matrix Diffusion. Historically, the most popular diffusion-controlled delivery system has been the matrix system, such as tablet and granules, where the drug is uniformly dissolved or dispersed, because of its low cost and ease of fabrication. However, the inherent drawback of the matrix system is its first-order release behavior with continuously diminishing release rate. This is a result of the increasing diffusional resistance and decreasing area at the penetrating diffusion front as matrix diffusion proceeds. 

Mathematically, the rate of drug delivery in diffusion-controlled delivery systems can be described by Pick s laws. Pick s first law of diffusion is expressed as ... 

Another critical consideration in protein delivery from hydrogel systems is the potential for protein denaturation in the device. For diffusion-controlled delivery systems, where water is the main transporting medium, the protein solution stability governs the type of device. Extended releasing times can be achieved with reservoir systems (Fig. 1) for highly stable proteins (Langer, 1990). Alternatively, dehydrated delivery systems... 

DIFFUSION-CONTROLLED DELIVERY SYSTEMS 2.1. Reservoir Systems... 

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. 

Transdermal delivery is a noninvasive intravenous infusion of drug to maintain efficacious drug levels in the body for predictable and extended duration of activity. Diffusion-controlled transdermal systems are designed to deliver the therapeutic agent at a controlled rate from the device to and through the skin into the systemic circulation. This route of administration avoids unwanted presystemic metabolism (first-pass effect) in the GI tract and the liver. Patient satisfaction has been realized through decreased... 

Lee, P. Diffusion-controlled matrix systems, in Kydonieus, A. (ed.), Treatise on Controlled Drug Delivery Fundamentals, Optimization, Applications. New York Marcel Dekker, 1992, pp. 155—198. 

Diffusion-Controlled Delivery of Proteins from Hydrogels and Other Hydrophilic Systems... 

Introduction Implantable Controlled Delivery Systems for Chemotherapy Drug Transport after Release from the Implant Application of Diffusion-Elimination Models to Intracranial BCNU Delivery Systems Limitations and Extensions of the Diffusion-Elimination Model New Approaches to Drug Delivery Suggested by Modeling Conclusion References... 

Because of their relative ease of production and cost compared with other methods of sustained or controlled delivery, dissolution and diffusion-controlled systems have classically been of primary importance in oral delivery of medication. Dissolution systems have been some of the oldest and most successful oral systems in early attempts to market sustaining products. 

Novel drug delivery systems can also have a profound effect on pharmacokinetics, even if they do not involve the use of prodrags in the classical sense. Novel polymers have permitted the development of membranes with controlled diffusion rates. For example. 

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