A novel type of microgel-crosslinked hydrogels (MCG), which are constructed with poly(N-isopropylacryl-amide-co-acrylic acid) (P(NIPAM-co-AAc)) microgels as building blocks, have been successfully prepared by using functionalized P(NIPAM-co-AAc) microgels as crosslinkers to crosslink the P(NIPAM-co-AAc) networks. The prepared MCG hydrogels are featured with both fast response rate and large volume change ratio to environmental temperature stimuli. The cooperative function of all the fast shrinking of microgels, the fast collapse of the grafted dangling chains on the microgel surfaces and the 3-D interconnected water transportation channels between the microgels and polymeric networks inside the MCG hydrogels make the hydrogels respond fast to the environmental temperature change. The response rate and the volume change ratio of the hydrogel increase with decreasing the functional P(NIPAM-co-AAc) microgel concentration in the hydrogel preparation, which means that the volume-phase transition property of the hydrogel is tunable by simply controlling the microgel content in the hydrogel. The proposed MCG hydrogels with improved thermo-responsive phase transition property are attractive for developing efficient stimuli-responsive smart sensors, actuators, bioseparation absorbants, and so on. (C) 2010 Elsevier Inc. All rights reserved.