Diffusion-controlled polymer release

Rhine, W., et ai, A new approach to achieve zero-order release kinetics from diffusion-controlled polymer matrix systems, in R.W. Baker, Controlled Release of Bioactive Materials. New York Academic Press, 1980, pp. 177-187. 

Tongwen, X., Binghn, H., 1998. Mechanism of sustained drug release in diffusion-controlled polymer matrix-apphcation of percolation theory. International Journal of Pharmaceutics 170, 139-149. 

Table HI. Some Polymers Used in Diffusion-Controlled Drug Release Formulations...

Matrix diffusion-controlled system in which the drug is homogeneously dispersed in a hydrophilic polymer diffusion from the matrix controls release rate Nitrodur (nitroglycerin key). 

JA Hunt, HN Joshi, VJ Stella, EM Topp. Diffusion and drug release in polymer films prepared from ester derivatives of hyaluronic acid. J Controlled Release 12 159-169, 1990. 

Buccal dosage forms can be of the reservoir or the matrix type. Formulations of the reservoir type are surrounded by a polymeric membrane, which controls the release rate. Reservoir systems present a constant release profile provided (1) that the polymeric membrane is rate limiting, and (2) that an excess amoimt of drug is present in the reservoir. Condition (1) may be achieved with a thicker membrane (i.e., rate controlling) and lower diffusivity in which case the rate of drug release is directly proportional to the polymer solubility and membrane diffusivity, and inversely proportional to membrane thickness. Condition (2) may be achieved, if the intrinsic thermodynamic activity of the drug is very low and the device has a thick hydrodynamic diffusion layer. In this case the release rate of the drug is directly proportional to solution solubility and solution diffusivity, and inversely proportional to the thickness of the hydrodynamic diffusion layer. 

A typical reservoir system consists of a core (the reservoir) and a coating membrane (the diffusion barrier). The core contains the active ingredients and excipients, whereas the membrane is made primarily of rate-controlling polymer(s). The governing release mechanism is diffusion from the reservoir across the membrane to the bulk solution, and the one-dimensional release rate is described by Eqs. (4.4), (4.17), and (4.22).10,14 In addition,... 

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