A biodegradable pH-sensitive hydrogel composed of poly (acrylic acid) (PAA) and poly (L-glutamic acid) (PGA) was synthesized. The swelling behavior of the hydrogel samples was studied. PGA is both biodegradable and pH-sensitive, with the pK(a) at 4.51. The hydrogels showed different swelling ratios (SR) in buffers at different pH values, indicating the pH-sensitivity. The hydrogels exhibited lower swelling ratios in an acidic medium because of the deprotonation of both PAA and PGA. While in a basic medium (pH 8.3), the highest swelling ratios were 5 to 10 times higher than those in the acidic medium. The SR increased sharply when the pH was increased from 4, which was consistent to the pK(a) of PAA and PGA. In neutral medium, the sample with the lowest SR was the one composed of 50% PGA and 50% PAA,of which the SR was only about 30. In contrast, with either more PGA or more PAA, the SR got higher. The SR of the sample with 70% of PAA was 66.3, while the SR of hydrogel composed of 70% PGA was 121.4. The swelling kinetics studies showed that all the hydrogel samples underwent a rapid swelling process in 20 min, followed by gradual decrease of the swelling rate. The samples reached swelling balance at 2 h. Because of the pH-sensitivity of the hydrogels, drugs can be protected at acidic pH, and released at neutral pH. Therefore, die hydrogels may have potential applications as oral drug delivery systems.
