Drug-depletion zone

Fig. 13 The controlled release of drug molecules from a (membrane-matrix) hybrid-type drug delivery system in which solid drug is homogeneously dispersed in a polymer matrix, which is then encapsulated inside a polymeric membrane, where D, P, and h are the diffusivity, permeability, and thickness, respectively, and the subscripts p, m, and d denote the drug depletion zone in the polymer matrix, polymer coating membrane, and diffusion layer, respectively.

In the case of a high dissolution rate of excipient and/or high matrix loading, strong initial swelling, rapid depletion of matrix excipient and consequently depletion of the elastic matrix pores occur. This should lead to curved release plots. It is noteworthy that swelling maxima have no influence on release profiles obviously, therefore, excipient is released from zones of the matrix already drug depleted. 

Figure 4.7 A matrix-type implant in which a drug is dissolved. The initial diffusion of drug molecules leaves a drug-depleted polymeric zone with a length h, which increases with time. This event leads to an increase in diffusional distance over time...

In Fig. 22.3, Cg is drug concentration in the reservoir or matrix compartment, C, is solubility of drug in the polymer phase, is the concentration in the diffusitm layer, h is the thickness of the membrane, hj is thickness of the di sion layer, and hf + dhf indicates the changing thickness of the depletion zone of matrix. 

Equation (22.15) indicates that after the depletion zone is large enough, the cumulative amount of drug released (0 is proportiiHial to the square root of time (i. ... 

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