Reservoir and transdermal delivery systems

The total amount of material leaving the membrane, can be determined by calculating the flux at the external surface and integrating with respect to time  

When the initial concentration within the membrane, Cp o, and the concentration at the external surface of the membrane, Cp,2, are both equal to zero. Equation 9-4 can be simplified considerably  

The cumulative mass of drug released is plotted versus time for planar reservoir devices with a range of physical characteristics, which are determined principally by the diffusion coefficient for the drug in the polymer and the membrane thickness. The cumulative mass released on the y-axis is scaled by Mq = Ac iL and each of the separate curves represents the normalized mass released at a particular value of Dyp/ l (in min ). If Di p is equal to 1 x 10 cm /s, then these curves correspond to thicknesses (L) of 20, 40, 60, and 120 /xm (from left to right). 

In situations where the concentrations at each surface of the polymer, Cpj and Cp 2, are maintained for a sufficient period, the drug diffusion will achieve steady state, which can be identified by examining the limit of Equation 9-3 as too  

At steady state, the rate of drug release from the membrane is  

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