Michael Fadeev

Switchable Bifunctional Stimuli-Triggered Poly-N-Isopropylacrylamide/DNA Hydrogels†

DNA-tethered poly-N-isopropylacrylamide copolymer chains, pNIPAM, that include nucleic acid tethers have been synthesized. They are capable of inducing pH-stimulated crosslinking of the chains by i-motif structures or to be bridged by Ag(+) ions to form duplexes. The solutions of pNIPAM chains undergo crosslinking at pH 5.2 or in the presence of Ag(+) ions to form hydrogels. The hydrogels reveal switchable hydrogel-to-solution transitions by the reversible crosslinking of the chains at pH 5.2 and the separation of the crosslinking units at pH 7.5, or by the Ag(+) ion-stimulated crosslinking of the chains and the reverse dissolution of the hydrogel by the cysteamine-induced elimination of the Ag(+) ions. The DNA-crosslinked hydrogels are thermosensitive and undergo reversible temperature-controlled hydrogel-to-solid transitions. The solid pNIPAM matrices are protected against the OH(-) or cysteamine-stimulated dissociation to the respective polymer solutions.

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 (Mn/n+1 = Ru2+/3+; Os2+/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 Os2+/3+ with sodium persulfate and ascorbic acid, the switchable stiffness of the hydrogel is demonstrated. The second system involves metal-ion-crosslinked carboxymethylcellulose hydrogels (Mn+1/n = Fe3+/2+; Ru3+/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 Fe3+/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.