Ultrafine hydrogel fibers that were responsive to both temperature and pH signals were prepared through the electrospinning of poly(N-isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) mixtures in dimethylformamide. Both the diameters (700 nm to 1.2 mum) and packing of the fibers could be controlled through changes in the polymer compositions and PNIPAAm molecular weights. These fibers were rendered water-insoluble by the addition of either Na2HPO4 or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na2HPO4 swelled to 30-120 times in water; this was significantly higher than the swelling of those crosslinked with PVA. The PVA-crosslinked hydrogel fibers, however, exhibited faster swelling kinetics; that is, they reached equilibrium swelling in less than 5 min at 25 degreesC. They were also more stable after I week of water exposure; that is, they lost less mass and retained their fibrous form better. All the hydrogel fibers showed a drastic increase in the swelling between pH 4 and 5. The PVA-crosslinked hydrogel fibers exhibited distinct temperature-responsive phase-transition behavior of PNIPAAm, whereas the Na2HPO4-crosslinked hydrogel fibers showed altered two-stage phase transitions that reflected side-chain modification of PNIPAAm. (C) 2004 Wiley Periodicals, Inc.