Equation 8.35 is similar to equation (8.33) except that the effective volume V of the hydrated matrix is used as this is not a fixed quantity. W0 is the dose of the drug in the matrix. If the drug completely dissolves on hydration of the matrix, an analogue of equation (8.34) is used:

In the initial stages of the process the rate of movement of water into the matrix may be important in determining release characteristics. When a homogeneous barrier wall is present diffusion through the walls has to take place and equations in section 8.5 apply.

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