Starburst triblock copolymers consisting of 8-arm poly(ethylene glycol) (8-arm PEG) and biodegradable poly(L-lactide) (PLLA) or its enantiomer poly(D-lactide) (PDLA), 8-arm PEG-b-PLLA-b-PEG (Stri-L), and 8-arm PEG-b-PDLA-b-PEG (Stri-D) were synthesized. An aqueous solution of a 1:1 mixture (Stri-Mix) of Stri-L and Stri-D assumed a so] state at room temperature, but instantaneously formed a physically crosslinked hydrogel in response to increasing temperature. The resulting hydrogel exhibited a high-storage modulus (9.8 kPa) at 37 degrees C. Interestingly, once formed at the transition temperature, the hydrogel was stable even after cooling below the transition temperature. The hydrogel formation process was irreversible because of the formation of stable stereocomplexes. In aqueous solution, gradual hydrolytic erosion was observed because of degradation of the hydrogel. The combination of rapid temperature-triggered irreversible hydrogel formation, high-mechanical strength, and degradation behavior render this polymer mixture system suitable for use in injectable biomedical materials, for example, as a drug delivery system for bioactive reagents or a biodegradable scaffold for tissue engineering. (C) 2008 Wiley Periodicals, Inc.