As mentioned above, the presented classification system for time-controlled drug delivery devices should not be viewed very strictly. There are various overlaps and other types of release rate-controlling mechanisms, including, for example, systems in which the drug is chemically bound to a matrix former (which is not necessarily biodegradable), coated solid dosage forms from which drug release occurs only upon crack formation within the surrounding membrane, and microchip-based drug delivery systems.
If the drug is covalently bound to an insoluble matrix former (e.g., polymer) via hydrolysable bondings, the latter are more or less rapidly cleaved upon water penetration into the device. The rate of this chemical reaction together with the rate of diffusion of water and drug through the polymeric network determine the overall drug-release rate (2).
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