Biodegradable poly(aspartic acid) (PASP) hydrogel and PASP/gelatin complex were prepared to evaluate their potential application as pH-sensitive matrices for controlled protein release. Entrapment of myoglobin (Mb) and its release were compared between the two types of carriers. It was found that incorporation of Mb into PASP hydrogel strongly depended on the medium pH and NaCl concentration, and was time-consuming. However, complete entrapment of Mb into PASP/gelatin complex was found within pH ranged from 2.5 to 4.0, which was concomitant with the formation of PASP/gelatin complex. By adjusting Mb feed ratio, Mb entrapment in the complex can be up to 31.54% (by weight) with high loading efficiency (96.2%). Gradual release of Mb from PASP hydrogel was observed within PH 2.0-7.4, while Mb release from PASP/gelatin complex was negligible within pH 2.0-4.2 for 4 days. In addition, pulsatile Mb release can be achieved by combining polyanhydride with pH-sensitive PASP/gelatin complex, while the device composed of polyanhydride and PASP hydrogel is mechanically unstable. PASP/gelatin complex formed by electrostatic interactions is superior to the single-component PASP hydrogel synthesized by chemical cross-linking as pH-sensitive matrices for controlled protein release when entrapment of proteins and pH-sensitivity of protein release are concerned. (c) 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2320-2329, 2006