# Mechanical Properties of DNA Hydrogels: Towards Highly Programmable Biomaterials
## 3 Responsive DNA linkers  
### Nucleic Acid Structural Motifs  
Nucleic acid structural motifs have been used as dynamic linkers to modulate the mechanical properties of pure DNA hydrogels or other polymeric hydrogels upon specific stimuli. These structural motifs are, in general, short sequences of DNA (or RNA) that can adopt various structural conformations, which can be canonical and follow the Watson-Crick base pairing rule, or noncanonical, not following the Watson-Crick base pairing [67; 68]. These motifs, which play a significant role in biology, are sensitive to various environmental parameters such as temperature, pH, and salt concentration [69; 70; 71; 72; 73]. Their capacity to convert from an open (extended) state to a folded state upon specific conditions has allowed researchers to induce change in the hydrogels' mechanical and structural properties (Figure 2a). The two most common noncanonical motifs used are the G-quadruplexes and the i-motifs. The G-quadruplex motifs are four-stranded helical structures that can form in G-rich nucleic acid sequences via Hoogsteen hydrogen-bonding between a multi-G-track (GGG) separated by another nucleotide [74]. The i-motifs, also called C-quadruplexes, are found in C-rich regions and are composed of four short strands maintained together by a noncanonical C:C base pair [72]. Both the G and C-quadruplexes are pH sensitive and exhibit high structural stability at acidic pH and are unfolded at physiological or basic pH (Figure 2a). They naturally became motifs of choice to control the structural and the mechanical properties of DNA-based hydrogels. For example, [PERSON] et al., in 2009 [49] and [PERSON] et al., in 2016 [31] used trivalent DNAs and linkers containing, respectively, one or two i-motifs to control the mechanical properties of DNA hydrogels exhibiting rapid actuation (~100 ms) [75] and a sharp stiffness increase at pH lower than pH 6.0. [PERSON] et al. demonstrated the reversibility of the mechanical properties with a G' ranging from 250 Pa at pH 8.0 and 1000 Pa at pH 5.0 [31]. In another study, [PERSON] et al. used another Hoogsten-type motifs called triplex DNA structures [76] to assemble pH-sensitive hydrogels that can be used to cyclically control the gel shape [15]. genes. A few reversible processes have, however, been developed [80; 81; 82; 83], most notably with regards to DNA hydrogels, such as the Azo-benzene moiety and the synthetic base, 3-cyanovinylcarbazole nucleoside, CNVK. The Azo-benzene moiety undergoes cis-to-trans isomerization when exposed to visible light (>400 nm) and trans-to-cis when exposed to UV light between 300-400 nm. The cis isomer caused a decrease in melting temperature, due to increased steric hindrance, from 24.8 to 15.9 \({}^{\circ}\)C in an 8 bp poly-T dsDNA strand [80] Azo-benzene-modified DNA crosslinkers were used as in acrylamide based gels to induce the release of fluorescein, gold nanoparticles, and horseradish peroxidase with UV-light [38]. The synthetic base CNVK offers rapid conformational switching inducing DNA hybridization with a complementary pyrimidine base, Cytosine or Thymine, within 1 s of UV (366 nm) irradiation [81; 83]. Inversely, a minute of irradiation with 312 nm UV-light will split the base-pairing by inducing conformational switching to the cis state. [PERSON] et al. utilized CNVK to form a pure DNA hydrogel of tetravalent DNAns [84]. The DNAns' sticky ends consisted of 4, 6, or 8 nts with CNVK base at the end. Gel-sol transition was monitored using a steel ball which sinks in the sol state or is suspended in the gel state. The sticky ends of 4 and 6 nt showed a reversible gel-sol transition, while an 8 nt sticky end did not, demonstrating the importance of working within the relevant shift of linker T\({}_{\text{m}}\) induced by these photoswitchable bases.