Chap. 10The controlled delivery of drugs can be effectively obtained using systems based on hydrogels. Tablets, to be orally administered, represent the simplest and the most traditional dosage systems based on hydrogel. Their formulation and preparation require to mix and to compress, in proper ratios, various excipients, including a swellable polymer and a drug. Carriers for controlled release systems are usually cross-linked polymers able to form hydrogels that show peculiar release mechanisms, where both diffusion and tablet swelling play important roles. When a dry swellable hydrogel-based matrix is immersed in a physiological fluid, this starts to penetrate inside the polymeric hydrophilic matrix. When a certain solvent concentration is reached, the polymeric chains unfold due to a glass–rubber transition, and a gel-like layer is formed. In the swollen region, the drug molecules can easily diffuse toward the outer dissolution medium, once they are dissolved. The polymer network became extremely hydrated where the swollen matrix is in contact with the outer medium, and processes like chain disentanglement take place, “eroding” the matrix. This chapter is focused on the analysis of the state of the art about the uses of carriers for controlled release systems composed by hydrogel-based matrices. This analysis has been performed studying in deep both the experimental and the modeling techniques which have been investigated over the years to characterize all the phenomena involved during the drug release.
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