Redox-triggered hydrogels revealing switchable stiffness properties and shape-memory functionsElectronic supplementary information (ESI) available. See DOI: 10.1039/c8py00515j

The synthesis, characterization and application of redox-switchable hydrogels are described. The first system includes the crosslinking of terpyridine-functionalized acrylamide copolymer chains by redox-active metal-ion terpyridine complexes (M n / n +1 = Ru 2+/3+ ; Os 2+/3+ ). The redox state of the complexes bridging the hydrogel controls the stiffness of the resulting hydrogels. The Ru 2+ -terpyridine polyacrylamide hydrogel reveals enhanced stiffness ( G ′ = 110 Pa) compared to the Ru 3+ -terpyridine bridged hydrogel that exhibits lower stiffness ( G ′ = 50 Pa). By the cyclic oxidation and reduction of the hydrogel with persulfate and dopamine, respectively, reversible switching of the hydrogel stiffness is demonstrated. Similarly, the Os 3+ -terpyridine-crosslinked hydrogel reveals lower stiffness ( G ′ = 30 Pa) compared to the Os 2+ -terpyridine-bridged hydrogel ( G ′ = 45 Pa). By the reversible oxidation and reduction of the Os 2+/3+ with sodium persulfate and ascorbic acid, the switchable stiffness of the hydrogel is demonstrated. The second system involves metal-ion-crosslinked carboxymethylcellulose hydrogels (M n +1/ n = Fe 3+/2+ ; Ru 3+/2+ ). The reduced metal-ion-crosslinked hydrogels Fe 2+ -carboxymethylcellulose (formed in the presence of ascorbic acid) and the Ru 2+ -carboxymethylcellulose (formed in the presence of dopamine) exhibit lower stiffness values corresponding to 80 Pa and 320 Pa, respectively, while high-stiffness Fe 3+ - and Ru 3+ -carboxymethylcellulose hydrogels (formed in the presence of sodium persulfate) are observed, G ′ = 210 Pa and 460 Pa, respectively. The reversible redox-stimulated switching of the stiffness of the hydrogels is demonstrated. In addition, carboxymethylcellulose chains modified with self-complementary nucleic acid tethers are crosslinked by two cooperative crosslinkers consisting of Fe 3+/2+ -carboxylate and DNA duplexes. The resulting Fe 3+ -carboxymethyl cellulose/duplex nucleic acid-bridged hydrogel exhibits high stiffness, G ′ = 210 Pa, whereas the Fe 2+ -carboxymethylcellulose/duplex DNA reveals substantially lower stiffness, G ′ = 80 Pa. The hydrogel reveals reversible shape-memory properties. Metal-ion terpyridine-crosslinked acrylamide hydrogels or metal-ion-bridged carboxymethylcellulose hydrogels reveal redox-switchable stiffness and shape-memory properties.