Swelling Controlled Release Systems

To fully understand the effect of swelling on drug swelling, it is necessary to initially examine the effect of aqueous fluids on the physical state of the hydrophilic polymers in swelling controlled release systems. According to Alfrey and co-workers (a.96), the diffusion of aqueous fluid into such systems may be three different regions, namely  

Classification of the behaviour of swelling systems is performed according to the relative rates of water ingress, (i.e., polymer hydration), and polymer relaxation and therefore, three scenarios may be defined  

The classification of the various drug delivery systems as described previously will affect drug release, as discussed next  

In case 1 diffusion the rale limiting step is the diffusion of aqueous fluid into the delivery system. Accordingly, the relationship between the mass of fluid absorbed by the drug delivery system and time may be defined using Pick s law (Equation 18). 

D is the diffusion coefficient of aqueous fluid in the polymer matrix. 

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Fig. 2. Schematic representation of the behavior of a one-dimensional swelling controlled release system. The water (W) penetrates the glassy polymer (P) to form a gel. The drug (D) is released through the swollen layer.

Mandal, T.K. 2000. Swelling-controlled release system for the vaginal delivery of miconazole. Eur... 

Podual K, Doyle F, and Peppas NA. Modeling of water transport in and release from glucose-sensitive swelling-controlled release systems based on poly(diethylaminoethyl methacrylate-g-ethylene glycol). Industrial engineering chemistry research 2004 43 7500-7512. 

R. Morita, R. Honda, Y. Takahashi, Development of oral controlled release preparations, a PVA swelling controlled release system (SCRS). 1. Design of SCRS and its release controlling factor. 7 Control Release, 63 297-304, 2004. 

The composition of the structural material and the choice of the fabrication process are important in the preparation of controlled-release systems. Over the past decades, great advances have been made in the engineering of multicomponent, polymer-based, structural materials. These materials were designed to release active substances by different mechanisms (ref. 1) including diffusion, chemical control (polymer degradation) and solvent activation (swelling or osmotic pressure). In some cases, combinations of such mechanisms have been used. Experimental methods and theoretical analysis of mass transport phenomena in these materials have been developed (refs. 2,3). 

Chitosan films exhibit low swelling in water, but membranes with diverse hydrophilic aptitudes can be prepared through the formation of mixtures or semi-interpenetrated and interpenetrated networks of chitosan with highly hydrophilic polymers like poly(vinyl alcohol), poly(vinylpyrrolidone) and gelatin [164]. PEC of chitosan with polyanions of natural origin like alginate, pectin or CMC or with synthetic ones, like poly(acrylic acid), have been investigated as matrices for controlled-release systems [165]. 

Vlachou M, Hani N, Efentakis M, Tarantili PA, Andreopoulos AG. Polymers for use in controlled release systems the effect of surfactants on their swelling properties. J Biomater Appl 2000 15(1) 65—77. 

Soluble as well as insoluble cellulose ethers have been used in hydrophilic polymeric matrices thanks to their hydrophilicity nature and good gel-forming characteristics. Among cellulose ethers, HEC shows the highest swelling ability and hydration rate. However, HPMC enjoys the most common use in hydrophilic matrix applications thanks to its excellent swelling ability, good compressibility and fast hydration characteristics. Mixtures of cellulose ethers with other cellulose ethers or different polymers are used in controlled release systems [28]. 

Tablet Disintegration, Swelling, Drug Release. - In the past few years, studies on polymer matrixes used as controlled release systems (matrix characterization and factors influencing the release process) have been the subject of intense research interest. Different approaches and techniques have been used to understand the release process, in an effort to achieve maximum pharmaceutical effectiveness of the incorporated drug. 

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