Ultrafine Hydrogel Fibers with Dual Temperature- and pH-Responsive Swelling Behaviors HONG CHEN, YOU-LO HSIEH Fiber and Polymer Science, University of California, Davis, California 95616 Received 18 June 2004; accepted 4 August 2004 DOI: 10.1002/pola.20461 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: 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 m) 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 Na 2 HPO 4 or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na 2 HPO 4 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 °C. They were also more stable after 1 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 Na 2 HPO 4 -crosslinked hydrogel fibers showed altered two- stage phase transitions that reflected side-chain modification of PNIPAAm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6331– 6339, 2004 Keywords: electrospinning; fibers; hydrogels; pH-responsive; temperature-respon- sive; swelling INTRODUCTION Hydrogels are polymer networks that swell dra- matically in the presence of abundant water. Those hydrogels that exhibit dramatic swelling transitions in response to environmental signals, such as the pH, temperature, ionic strength, sol- vent composition, and magnetic fields, have been called smart hydrogels. Their swelling behaviors are usually reversible, and this makes these smart hydrogels highly desirable for applications in many areas, including biotechnology, chemis- try, medicine, agriculture, and consumer prod- ucts. 1–3 The swelling and deswelling of smart hydro- gels are diffusion-controlled processes, 4 with dif- fusion coefficients of 0.8  10 -7 to 8  10 -7 cm 2 /s. Therefore, a 1-mm-thick gel would take hours to reach equilibrium in response to a stimulus. Sev- eral chemical and physical approaches have been reported to improve the response rate of smart hydrogels. Adding linear poly(N-isopropylacryl- amide) (PNIPAAm) grafts to the existing three- dimensional network of a PNIPAAm hydrogel has been shown to increase the swelling rate signifi- cantly. 5,6 The temperature-triggered collapse of a 2-mm-thick grafted hydrogel disc (15 mm in di- ameter) occurred in less than 20 min. The more Correspondence to: Y.-L. Hsieh (E-mail: ylhsieh@ucdavis. edu) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 6331– 6339 (2004) © 2004 Wiley Periodicals, Inc. 6331