Redox-triggered hydrogels revealing switchable stiffness properties and shape-memory functions

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 Ru²⁺-terpyridine polyacrylamide hydrogel reveals enhanced stiffness (G′ = 110 Pa) compared to the Ru³⁺-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 Os³⁺-terpyridine-crosslinked hydrogel reveals lower stiffness (G′ = 30 Pa) compared to the Os²⁺-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 Fe²⁺-carboxymethylcellulose (formed in the presence of ascorbic acid) and the Ru²⁺-carboxymethylcellulose (formed in the presence of dopamine) exhibit lower stiffness values corresponding to 80 Pa and 320 Pa, respectively, while high-stiffness Fe³⁺- and Ru³⁺-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 Fe³⁺-carboxymethyl cellulose/duplex nucleic acid-bridged hydrogel exhibits high stiffness, G′ = 210 Pa, whereas the Fe²⁺-carboxymethylcellulose/duplex DNA reveals substantially lower stiffness, G′ = 80 Pa. The hydrogel reveals reversible shape-memory properties.