A new method was used for the production of fast-responding bulk hydrogels with microstructure (BHMs) with a high swelling ratio. These BHMs were synthesized first by the formation of poly(N-isopropylacrylamide-co-acrylic acid) (NIPAAm-AA) microgel particles and then by the crosslinking of the particles with N-isopropylacrylamide monomer. The polymer obtained had the desired microstructure but was bulk (monolithic), so it could be used in a variety of applications. The NIPAAm-AA microgel particles were characterized with transmission electron microscopy, and the formed BHMs were characterized with scanning electron microscopy. Compared with conventional bulk hydrogels, the BHMs had very high swelling ratios and much faster swelling rates attributable to the collaboration of the ionized microgel particles and bulk hydrogels. An increase in the microgel particles embedded in the BHMs provided faster hydrogel swelling. The number of ionic acrylic acid groups in the hydrogels affected their swelling behavior. (C) 2002 John Wiley & Sons, Inc.