SUPER ABSORBENT HYDROGELS: INTERPENETRATING NETWORKS OF POLY(ACRYLAMIDE-co-ACRYLIC ACID) AND POLY (VINYL ALCOHOL): SWELLING BEHAVIOR AND STRUCTURAL PARAMETERS^*

Delivery of bupivacaine included in poly(acrylamide‐co‐monomethyl itaconate) hydrogels as a function of the pH swelling medium

“…At high ionic strength values, the carboxylic groups of the copolymer are protected by the cations, avoiding the repulsive forces. 10,29 Thus the swelling of A/MMI hydrogels in phosphate buffer medium at pH 7.5 was favored by the very low ionic strength, whereas in simulated gastric fluid at pH 1.5, when the ionic strength was very much higher, the swelling was not favored.…”

“…At pH 7.5, the swelling process is favored due to the side groups that are ionized increasing the repulsion forces among them and, therefore, the expansion process, which involves a larger W t . 10,29 When pH decreased, with the corresponding acidification of the medium, H ϩ prevented the ionization of the side groups of the hydrogels, and as a consequence, they tended to interact among them, decreasing the expansion and, therefore, the W t . In this sense, swelling studies of these copolymers in saline solution (NaCl 0.9 wt %), 22 whose pH was 6.0, showed W ϱ values of 78 Ϯ 1 wt % for 75A/25MMI and 76 Ϯ 2 wt % for 60A/40MMI, which means W ϱ was lower than that at pH 7.5 as a consequence of a lower ionization degree of the side groups.…”

“…The pK a of Bp (pK a ϭ 8.16) was larger than pH of the medium, and the drug is a cation; 37 therefore, the drug could establish interactions with the ionized side groups of the hydrogels, which prevented Bp release. 10,29 On the other hand, in acid pH (1.5), Bp was much more ionized because the pH was very much lower than the pK a of the drug. In this case, the side groups of the hydrogels were not ionized because there was a large amount of H ϩ in the medium; thus, the side groups of the hydrogels tended to interact among them, and these interactions gained importance to the detriment of the drug-polymer interactions.…”

“…Therefore, the Bp release was favored at pH 1.5. 10,20 For drug-release experiments, the n value of eq. (2) ( Table II) in simulated gastric fluid; thus, a Fickian behavior of the desorption process could be considered for all Bp loads used in the experiments.…”

“…Different polymeric and copolymeric pH-sensitive hydrogels have been synthesized with monomers of methacrylic acid, 8 N-isopropylacrylamide, 9 and acrylamide (A). 10 Because of these characteristics, pH-sensitive hydrogels have been studied to develop drug-delivery systems. 11 They are mainly interesting for oral administration because they can swell at acidic pH, 12 and thus, drug release takes place in the stomach; they can also swell at neutral or basic pH, 13 in which case drug would be released in the intestine.…”