Hydrogels have been extensively studied because of their abilities to simulate biological tissues and to swell or collapse reversibly in response to external stimuli. Without adding a coloring agent, the hydrogels exhibit optical transparency when they fully swell in water.

In this talk, I'll first present design and synthesis of novel hydrogel crystals with nanoscale lattice spacings, also known as �Hydrogel Opals�. Hydrogel opals were created by first making monodisperse N-isopropylacrylamide (NIPA) /2-hydroxyethylacrylate (HEAC) hydrogel nanoparticles and then covalently inter-connect the self-assembling nanoparticles in aqueous medium at room temperature using crosslinker divinylsulfone (DVS). The inter-nanoparticle covalent bondings contribute to the stability of crystal structure, while self-assembly provides the crystal structure that diffract light, resulting in different colors of the hydrogel opals. As a result, the novel hydrogel crystals, which contain up to 97 wt% water, display a striking iridescence like precious opals but soft and flexible like �Jello�. The novel nanostructured hydrogels demonstrated the good mechanical strength and reversibly thermo- and compressive-induced stabilities of the hydrogel crystal structures.

Another interesting subtopic is about aqueous synthesis and self-assembly of monodisperse, interpenetrating (IPN) poly(NIPA-co-HEAC)/poly(acrylic acid) (PAA) nanogels in water. Novel IPN nanogels dispersions formed thermo-induced sol-gel transition above the phase transition temperature (Tp), 33oC, of poly(NIPA-co-HEAC) nanogels. More importantly, IPN nanogels self-assembled into giant colloidal crystals (up to 6 mm). Controlled growth of such large crystals will be of significant importance in fundamental studies and practical applications.