Stimuli‐Responsive Donor–Acceptor and DNA‐Crosslinked Hydrogels: Application as Shape‐Memory and Self‐Healing Materials

Abstract

Carboxymethyl cellulose (CMC) chains are functionalized with self-complementary nucleic acid tethers and electron donor or electron acceptor functionalities. The polymer chains crosslinked by the self-complementary duplex nucleic acids and the donor-acceptor complexes as bridging units, yield a stiff stimuli-responsive hydrogel. Upon the oxidation of the electron donor units, the donor-acceptor bridging units are separated, leading to a hydrogel of lower stiffness. By the cyclic oxidation and reduction of the donor units, the hydrogel is reversibly transformed across low and high stiffness states. The controlled stiffness properties of the hydrogel are used to develop shape-memory hydrogels. In addition, CMC hydrogels crosslinked by donor-acceptor complexes and K + -stabilized G-quadruplexes reveal stimuliresponsive properties that exhibit dually triggered stiffness functions. While the hydrogel bridged by the two crosslinking motifs reveals high stiffness, the redox-stimulated separation of the donor-acceptor complexes or the crownether-stimulated separation of the G-quadruplex bridges yields two alternative hydrogels exhibiting low stiffness states. The control over the stiffness properties of the dually triggered hydrogel is used to develop shape-memory hydrogels, where the donor-acceptor units or G-quadruplex bridges act as "memories", and to develop triggered self-healing process of the hydrogel.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201803111.hydrogel-to-solid [29] or hydrogel-to-liquid [30] transitions attract growing interest, and different triggers, such as pH, [31] heat, [32] light, [33][34][35][36] or magnetic field [37,38] are used to reversibly transform the hydrogels between the different states. For example, the mixture of trans-azobenzene-functionalized acrylamide polymer chains and β-cyclodextrin-modified acrylamide polymer chains yielded a photoresponsive hydrogel via crosslinking of the chains by trans-azobenzene/β-cyclodextrin hostguest complexes. [33][34][35][36] The photoisomerization of the trans-azobenzene units to cis-azobenzene units, that lack affinity toward β-cyclodextrin, resulted in the separation of the crosslinking units and the transition of the hydrogel into a solution of the polymer chains. Within the broad class of stimuli-responsive supramolecular hydrogels, deoxyribonucleic acid (DNA)-crosslinked hydrogels provide a rich arsenal of functional materials and, specifically, of stimuli-triggered hydrogel matrices. [39][40][41] Besides the possibility to reconfigure nucleic acid duplexes by fuel/antifuel strands, [42] the pH-stimulated transitions of cytosine-rich strands into i-motif structures, [43][44][45][46] the self-assembly of pH-induced formation and separation of C-G·C + or T-A·T triplexes, [47] the K + -ion-stimulated assembly of G-quadruplexes and their separation by crown ethers or cryptate, [48] the cooperative stabilization of duplex nucleic structures by metal-ion bridges [49] (e.g., THg...

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