A new class of hydrogels utilizing DNA (DNA quadruplex gel) has been constructed by directly and symmetrically coupling deoxynucleotide phosphoramidite monomers to the ends of polyethylene glycols (PEGs) in liquid phase, and using the resulting DNA‐PEG‐DNA triblock copolymers as macromonomers. Elongation of merely four deoxyguanosine residues on PEG, which produces typically ≈10 grams of desired DNA‐PEG conjugates in one synthesis, resulted in intelligent and biodegradable hydrogels utilizing DNA quadruplex formation, which are responsive to various input signals such as Na+, K+, and complementary DNA strand. Gelation of DNA quadruplex gels takes place within a few seconds upon the addition of a trigger, enabling free formation just like Ca+‐alginate hydrogels or possible application as an injectable polymer (IP) gel. The obtained hydrogels show good thermal stability and rheological properties, and even display self‐healing ability.

A new pH-responsive hydrogel biomaterial, that is composed of solely two popular biocompatible materials, oligodeoxynucleotides (ODN) and polyethylene glycol (PEG) have been prepared. Merely five deoxycytidine residues were elongated to the ends of linear or 4-arm PEG in ×1000 larger scale than conventional systems by using liquid-phase DNA synthesis technique, and applied them as a macromonomer for the preparation of hydrogels. The syntheses of the conjugates are simply elongating ODN onto the ends of PEG as a semisolid phase substrate using standard phosphoramidite chemistry. The resulting dC5-PEG conjugates gave quite stable and stiff hydrogels triggered by the formation of a unique DNA quadruplex, i-motif. Introduction of only one chemical linkage between two linear conjugates resulted in unexpectedly high thermal stabilities for the melting temperatures of i-motifs themselves. Nonlinearly improved rheological properties compared to the original linear conjugates were also observed, probably because of topological entanglement between macromonomers of fused circles.

Application of Na+-responsive DNA quadruplex hydrogels, which utilize G-quadruplexes as crosslinking points of poly(ethylene glycol) (PEG) network as cell culture substrate, has been examined. PEG-oligodeoxynucleotide (ODN) conjugate, in which four deoxyguanosine (dG4) residues are tethered to both ends of PEG, was prepared by modified high-efficiency liquid phase (HELP) synthesis of oligonucleotides and used as the macromonomer. When mixed with equal volume of cell culture media, the solution of PEG-ODN turned into stiff hydrogel (G-quadruplex hydrogel) as the result of G-quadruplex formation by the dG4 segments in the presence of Na+. PEG-ODN itself did not show cytotoxicity and the resulting hydrogel was stable enough under cell culture conditions. However, L929 fibroblast cells cultured in G-quadruplex hydrogel remained spherical for a week, yet alive, without proliferation. The cells gradually sedimented through the gel day by day, probably due to the reversible nature of G-quadruplex formation and the resulting slow rearrangement of the macromonomers. Once they reached the bottom glass surface, the cells started to spread and proliferate.

Biodegradable, intelligent, and self-healing hydrogel beads showing peroxidase activity has been prepared by adding hemin to "DNA quadruplex hydrogel" beads prepared by assembling dG 4 -PEG-dG 4 triblock copolymers through G quadruplex formation between the dG 4 portion. Efficient binding of hemin to parallel G-quadruplexes in hydrogels was confirmed by observing hyperchromicity of hemin absorbance. The enzymatic activity of resulting hemin peroxidase was visualized both with fluorogenic and chromogenic substrates. These observations serve as the first direct evidence of the presence of G-quadruplexes in DNA quadruplex hydrogels utilizing G-quadruplexes.