Weakly cross-linked heteropolymer gels that memorize molecular pairs have been designed and synthesized. The polymer consists of a main monomer component responsible for volume phase transition, methacrylic acid that adsorbs one divalent ion as a pair, and cross-links. The memory of pairing of methacrylic acids within the gels was encoded in the primary sequence of main monomers, methacrylic acids and cross-links within the gels, which was achieved by “imprinting”, namely, by synthesizing gels while methacrylic monomers were paired prior to polymerization. The control gels, where methacrylic monomers were randomly distributed, showed frustration in forming pairs, whereas such frustration was completely diminished in the imprinted gels allowing the memory of pair formation.

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-isopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calcium ion, and the NIPA component allows the polymers to swell and shrink reversibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to calcium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templates in methylsulfoxide and dioxane media, respectively. The adsorption capacity of the imprinted gels was compared with that of the nonimprinted gels, and the effects of the templates, the solvents, and the amount of methacrylic monomers used in the synthesis and the medium temperature over the Ca2+ adsorption capacity of the gels from aqueous solutions were evaluated. The analysis of the adsorption revealed that (a) the adsorption can be described by the Langmuir isotherms; (b) there is an approximately linear relationship between saturation and methacrylic monomer concentration; (c) the affinity depends on the degree of gel swelling or shrinkage that can be switched on and off by temperature; (d) in the shrunken state, the affinity depends approximately linearly on the MAA concentration in the imprinted gels, whereas in the nonimprinted gels it is proportional to the square of MAA concentration; (e) the imprinted gels adsorb more than the nonimprinted gels when MAA concentration is less than that of permanent cross linkers. The success of imprinting of CaMAA2 and PbMAA2 complex is evidence for memory of such complex onto the weakly cross-linked gel.

Imprinted gels incorporating two different breakable cross-linkers, a PbMAA2 complex and a disulfide (S−S) bond, were prepared by radical polymerization. After the lead ions were removed by washing, these gels showed a high affinity for calcium ions. Breakage and subsequent reconnection of the S−S bonds in the absence of Ca2+ decreases the Ca2+ binding affinity of the gel. This indicates that random reconnection of the S−S bonds produces a frustration in the adsorption of Ca2+ by the carboxyl groups. However, if the S−S bonds were reconnected in the presence of Ca2+ and the Ca2+ was subsequently removed (the post-imprinting technique), the resulting gels showed a higher binding affinity for Ca2+. This indicates that the post-imprinting technique creates a more favorable conformation for Ca2+ binding in the polymer network. We interpret our data to mean that “memory” of target-binding sites was encoded effectively into the polymer network by the initial imprinting technique and then enhanced by the post-imprinting technique.

We present mechanically strong and self-healable clay hydrogels containing 2-8 w/v % ds-DNA together with a synthetic biocompatible polymer, poly( N, N-dimethylacrylamide). Clay nanoparticles in the hydrogels act like a chemical cross-linker and promote their elastic behavior, whereas DNA contributes to their viscoelastic energy dissipation. The extent of mechanical hysteresis during cyclic tensile tests reveals that the strength of intermolecular bonds in DNA/clay hydrogels is in the range of the strength of hydrogen bonds. The hydrogels exhibit a high stretchability (up to 1500%) and a tensile strength between 20 and 150 kPa. They have the ability to self-heal, which is induced by heating the damaged gel samples above the melting temperature of ds-DNA. When comparing the mechanical properties of the hydrogels before and after healing, the healing efficiency is greater than 100%. We also demonstrate that ds-DNA molecules entrapped in the gel network undergoes thermal denaturation/renaturation cycles, leading to a further improvement in the mechanical properties of the hydrogels.