Chemical and photochemical DNA "gears" reversibly control stiffness, shape-memory, self-healing and controlled release properties of polyacrylamide hydrogels.

A new class of stimuli-responsive DNA-based polyacrylamide hydrogels is described. They consist of glucosamine-boronate ester-crosslinked polyacrylamide chains being cooperatively bridged by stimuli-responsive nucleic acids. The triggered closure and dissociation of the stimuli-responsive units lead to switchable stiffness properties of the hydrogel. One hydrogel includes glucosamine-boronate esters and K-ion-stabilized G-quadruplex units as cooperative crosslinkers. The hydrogel bridged by the two motifs reveals high stiffness, whereas the separation of the G-quadruplex bridges by 18-crown-6-ether yields a low stiffness hydrogel. By cyclic treatment of the hydrogel with K-ions and 18-crown-6-ether, it is reversibly cycled between high and low stiffness states. The second system involves a photo-responsive hydrogel that reveals light-induced switchable stiffness functions. The polyacrylamide chains are cooperatively crosslinked by glucosamine-boronate esters and duplex nucleic acid bridges stabilized by -azobenzene intercalator units. The resulting hydrogel reveals high stiffness. Photoisomerization of the -azobenzene units to the -azobenzene states results in the separation of the duplex nucleic acid bridges and the formation of a low stiffness hydrogel. The control over the stiffness properties of the hydrogel matrices by means of K-ions/crown ether or photoisomerizable -azobenzene/-azobenzene units is used to develop shape-memory, self-healing, and controlled drug-release hydrogel materials.