Synthesis and characterization of pH-sensitive dextran hydrogels as a potential colon-specific drug delivery system

pH-Sensitive dextran hydrogels were prepared by activation of dextran (T-70 ) with 4-nitrophenyl chloroformate, followed by conjugation of the activate d dextran with 4-aminobutyric acid and cross-linking with 1,10-diaminodecan e. The cross-linking efficiencies determined by mechanical measurements wer e in the range of 52-63%. Incorporation of carboxylpropyl groups in dextran hydrogels led to a higher equilibrium and faster swelling under high pH co nditions. The swelling reversibility of hydrogels was also observed after r epeated changes in buffers between pH 2.0 and 7.4. The slow rates of swelli ng and deswelling in response to changes in pH were attributed to the hydro philic nature of dextran and formation of hydrogen bonds between the hydrox yl groups of dextran with water molecules. The pronounced effect of carboxy lic acid content on degradation of hydrogels was observed after 4 h of incu bation with dextranase and the influence significantly decreased after expo sure to the enzyme for 8 h. The mechanism of bulk degradation of hydrogels under high swelling extent was substantiated using Coomassie blue protein a ssay. The release rate of bovine serum albumin from hydrogrels was primaril y determined by the swelling extent. The release rate was further enhanced by addition of dextranase in buffer solutions.