Gradients, drug release from polymer

For diffusion-mediated release, the amount of drug released from the polymer is proportional to the concentration gradient of the drug in the polymer. By performing a mass balance for drug within a differential volume element in the polymer, the concentration of drug within the polymer as a function of position and time can be described as above (see Equation 10-4) ... 

C. Ferrero, D. MassueUe, D. Jeannerat and E. Doelker, Towards elucidation of the drug release mechanism from compressed hydrophihc matrices made of cellulose ethers. I. Pulse-fleld-gradient spin-echo NMR study of sodium salicylate diffusMty in swollen hydrogels with respect to polymer matrix physical structure,/. Control. Release, 128 (1) 71-79, 2008. 

The concept of these systems comprises a coaxial fiber. In this fiber, a drug is dispersed or dissolved in a core polymer. The release of the drug from these coaxial fibers is proportional to the concentration gradient in the fiber. If the drug is present in a concentration that exceeds solubility in the membrane, on the adjacent surface the saturated concentration is established. This stationary concentration is responsible for the gradient. 

On contact with water on the other hand, DC1 precipitates because of its low water solubility, especially in the less polar Pol. 6 and 7 extraction subsequently becomes more dissolution controlled. Because of higher solubilities in the polymer and the additional osmotic driving force the formed drug distribution gradients are less sharp than for the OX/acetone system (Figure 11). This results in some release delay for Pol. 6, but not for the more polar Pol. 8, from which DC1 does not as readily precipitate, even on contact with water (Figures 6, 7 Table II). 

Fig. 3 Release of drug from various shapes of pol5mer membrane permeation-controlled drug-delivery systems (A) sphere-type, (B) cylinder-type, and (C) sheet-type. In (D), the drug concentration gradients across the rate-controlling polymeric membrane and hydrodynamic diffusion layer exist in series. Both the polymer membrane, which is either porous or non-porous, and the diffusion layer have a controlled thickness and h, respectively).

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