Polyaspartamides, which are termed to a variety of amide derivatives of poly(aspartic acid), one of the poly(amino acid)s or polypeptides, have been intensively investigated as biodegradable and biocompatible polymers with a broad range of potential biomedical applications as well as eco-friendly industrial uses. By discovering that polymers containing amidine or guanidine functionality have been shown to be reversibly responsive to carbon dioxide (CO 2 ), we have developed two polyaspartamide systems: novel CO 2 -responsive hydrogel and amphiphilic polyaspartamide derivative containing L-arginine. In this work, poly(2-hydroxyethyl aspartamide) derivative was modified with L-arginine unit (PHEA-Larg), before cross-linked by hexamethylene diisocyanate in the presence of dibutyltin dilaurate catalyst to provide a hydrogel having not only good gel strength, but reversible CO 2 absorption characteristics. On the other hand, amphiphilic polyaspartamide derivative containing hydrophobic long alkyl moiety (octyl) and L-arginine unit was synthesized, and the CO 2 -responsive solubility and molecular self-assembly behaviors of the systems were investigated. These new polyaspartamide systems have potential in several applications including CO 2 capture, CO 2 -responsive and switchable surface, sensor, smart hydrogel for controlled drug delivery system, etc.