A series of novel pH-sensitive triblock copolymers composed of poly(beta-amino ester)-poly(ethylene glycol)-poly(beta-amino ester) (PAE-PEG-PAE) were synthesized by conjugating poly(beta-amino ester) to poly(ethylene glycol). The resulting polymers were characterized by H-1 and C-13 NMR in CDCl3 and get permeation chromatography in tetrahydrofuran. The concentrated polymer solutions (30 wt%) exhibited a gel-to-sol transition in the pH range 6.4-7.8. The gel window spanned physiological conditions (37 degrees C, pH 7.4). After injection into a rat, the copolymer solution (30 wt%) changed to a gel in a short time. This copolymer hydrogel showed bioadhesive properties and in vitro release of lidocaine was controllable. (C) 2009 Elsevier Ltd. All rights reserved.