The spatial concentration profile of a drug

Equation (4.2) reveals that the fraction of drug released is linearly related to the square root of time. However, (4.2) cannot be applied throughout the release process since the assumptions used for its derivation are not obviously valid for the entire release course. Additional theoretical evidence for the time limitations in the applicability of (4.2) has been obtained [10] from an exact solution of Fick s second law of diffusion for thin films of thickness S under perfect sink conditions, uniform initial drug concentration with cq cs, and assuming constant diffusion coefficient of drug T in the polymeric film. In fact, the short-time approximation of the exact solution is 

This arbitrary recommendation does not rely on strict theoretical and experimental findings and is based only on the fact that completely different physical conditions have been postulated for the derivation of the equivalent (4.2) and (4.3), while the underlying mechanism in both situations is classical diffusion. In this context, a linear plot of the cumulative amount of drug released q (t) or the fraction of drug released q (f) /f/,Xj (utilizing data up to 60% of the release curve) vs. the square root of time is routinely used in the literature as an indicator for diffusion-controlled drug release from a plethora of delivery systems. 

Fickian diffusional release form a thin polymer film. Equation (4.3) gives the short-time approximation of the fractional drug released from a thin film of thickness S. 

Case II release from a thin polymer film. The fractional drug release q (t) /qoo follows zero-order kinetics [65,66] according to 

Case II radial release from a cylinder. The following equation describes the fractional drug released, q(t) /qoo, when case II drug transport with radial release from a cylinder of radius p is considered [66]  

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