Poly(MAA-co-AN) hydrogels with improved mechanical properties for theophylline controlled delivery

A hydrophobically improved copolymer hydrogel was constructed and developed based on methacrylic acid (MAA) and acrylonitrile (AN) monomers. The swelling investigations indicated that the hydrogels were highly sensitive to the pH and ionic strength of the surrounding environment. The swelling ratios of the gels in stimulated intestinal fluids (SIF, pH 7.4) were higher than those in stimulated gastric fluids (SGF, pH 1.4), and followed a non-Fickian and a Fickian diffusion mechanism, respectively. The results match well with scanning electron microscopy observations showing that mesh sizes in hydrogels in SGF are larger and more patulous than in SIF. It is believed that the phenomena are concerned with the electrostatic repulsion originating from the formation and dissociation of carboxyl groups, as well as a charge screening effect of the cations leading to the reduction of osmotic pressure. A dynamic mechanical analyzer revealed that the addition of hydrophobic AN units notably improved the compression mechanical properties of the control sample. With an increase in AN concentrations, the ability for the copolymers to resist compression deformation enhanced in distilled water and the compression strains have almost the same values in SIF environments. Differential scanning calorimetry was used to analyze the state of water and to determine the amounts of freezing and non-freezing water. The controlled release examination based on the theophylline model drug showed that the release rate in distilled water was faster than that in SIF or SGF. The introduction of the hydrophobic AN decreased the theophylline release rates in both SIF and SGF, and the release rate variation in SIF was notably larger than that in SGF. The drug release behavior of the AN-modified P(MAA-co-AN) copolymer hydrogel deviates from the Fickian diffusion control mechanism. This hydrogel is expected to be used as an excellent material in oral drug controlled release. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.