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 pKa 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 pKa 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

the hydrogels may have potential applications as oral drug delivery systems.