Radiation polymerization and crosslinking of (N-isopropylacrylamide) in solution and in solid state Alejandra Ortega, Emilio Bucio, Guillermina Burillo ( ) Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, 04510 México D.F. E-mail: burillo@nucleares.unam.mx Received: 8 August 2006 / Revised version: 4 September 2006 / Accepted: 2 October 2006 Published online: 16 October 2006 – © Springer-Verlag 2006 Summary N-isopropylacrylamide (NIPAAm) hydrogels were synthesized by means of gamma irradiation, with three different methods: (a) irradiation of NIPAAm aqueous solutions; (b) irradiation of NIPAAm aqueous solution in presence of crosslinking agent N, N´-methylenebisacrylamide (BIS); and (c) in solid state, in form of disks compacted at different pressures prior to irradiation. Hydrogels of different crosslinking morphology were generated under different conditions. The hydrogels were characterized with regard to their gel content, swelling behavior in dependence on the temperature, and network properties. Morphology of the samples was characterized by SEM. Introduction Hydrogels are three dimensional networks of crosslinked hydrophilic polymers swollen in water. The hydrophilic gels based on both natural and synthetic polymers have continued to be of interest for encapsulation of cells and most recently such hydrogels have been attractive to the new field of “tissue engineering” as matrices for repairing and regenerating a wide variety of tissues and organs [1], and in a variety of industrial applications, such as bio-medical devices [2, 3]. Radiation methods are specially attractive to synthesize such gels because no additives are needed, the process is easily controlled and the economy is competitive when compared to other conventional methods [2, 4-8]. Gel formation of different polymers in solid state [9], compacted at different pressures before irradiation; have been also studied to obtain the lower dose of incipient gel and higher radiochemical yields of crosslinking [10-14]. Recently, considerable research attention has been focused on hydrogels that are able to alter their volume and properties in response to environmental stimuli such as pH, temperature, ionic strength and others [5, 15, 16], this phenomenon provides stimuli- sensitive hydrogels “smart hydrogels” suitable for applications in medical [17], agricultural, electrical, and many other industry field [18]. Thermoresponsive hydrogels undergo discontinuous volume phase transition behavior, they are in a highly swollen state at temperatures below a critical temperature, and above this Polymer Bulletin 58, 565–573 (2007) DOI 10.1007/s00289-006-0690-4