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Mechanically adaptive hydrogels with reversible cross-links can change their mechanical characteristics to adapt to the external environment. However, inevitable swelling/shrinkage occurs with the mechanical property change, which impedes the applications of these hydrogels. In this study, mechanical adaptivity with high dimensional stability is achieved in alginate-based polyelectrolyte hydrogels by introducing an opposite swelling mechanism. The dually crosslinked alginate-polystyrene sulfonate (Alg-PSS) hydrogels are constructed through the copolymerization of alginate-methacrylate (Alg-MA) and sodium p-styrene sulfonate (NaSS), as well as Ca2+ crosslinking. In the Alg-PSS hydrogel network, the reversible Ca2+ -carboxylate and Ca2+ -sulfonate cross-links can be disrupted by Na+ and soften the hydrogels. Moreover, the PSS chains crosslinked in the hydrogel network undergo the coil-globule transition in concentrated NaCl solutions to suppress hydrogel swelling during softening. The optimized Alg-PSS hydrogel (Alg5 -PSS0.75 -MBAA2.5 ) shows a dramatic tensile modulus change from 191.3 kPa in deionized water (DIW) to 15.1 kPa in 2.0 mol L-1 NaCl solution with a negligible volume increase ratio of only 0.6%. The Alg-PSS hydrogels may find applications in artificial valves or soft robotics, where high dimensional stability and invariable volume are required for smart hydrogels.

A sol–gel transition and the self-healing of the resultant hydrogels are achieved utilizing a metallo-supramolecule composed of terpyridine-terminated low-molecular-weight three-armed poly(ethylene glycol) (trisPEGtpy450) in water at ambient temperature. The gelation of trisPEGtpy450 with cobalt ions occurs by aerobic oxidation (mild conditions), and therefore the solution can be moulded into any shape hydrogel. The solation of the cured hydrogels occurs spontaneously upon the addition of the reductant, and the solution is then re-gelled by exposure to air due to the consumption of the reductant. Both a sol–gel transition by aeration and self-healing of the resultant hydrogels without any stimuli are observed when replacing some of the cobalt ions with nickel ions.

Surface polyion complex (sPIC) gels were prepared with a nonionic hydrogel interior core, composed of poly(N-vinylformamide and poly(N-vinylacetamide), and a chemically bounded polyion complex layer on the outer surface, composed of poly(vinylamine) and poly(acrylic acid). The gels were investigated as controlled drug release models based on electrostatic interactions depending on pH. Methylene blue and allura red were employed as cationic and anionic drug models, respectively, and resulted in the selective adsorption depending on pH conditions. Monovalent and multivalent anionic drug models, allura red and 1,3,6-naphthalenetrisulfonate were observed for their releasing behavior from the sPIC gel. The results indicated that the multivalent anionic drug effectively controlled release depending on pH conditions. We further investigated sPIC gels regarding their ability to control the release of ionic molecules as a function of pH-driven changes in electrostatic interactions. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42081.

Porous poly(N-isopropylacrylarfide) (PNI-PAAm) hydrogels were successfully prepared in the presence of nanosized silica particles with network morphology and subsequent silica extraction by HF treatment. The nanosized pores induced very rapid deswelling of the hydrogels, accompanied by volume phase transitions from swelling to deswelling states at approximately 30 °C. Deswelling, which was analyzed by a quick change in temperature of the aqueous media from 20 to 40 °C, was facilitated by increased silica particle content. The deswelling rate constant of the hydrogel prepared using silica nanoparticles with a network morphology (NT gel) was about 1 900-fold greater than that of nonporous hydrogels and 13-fold larger than that of porous hydrogels perforated using spherical silica nanoparticles (SP gel). The releasing rate of the low-molecular weight chromophore from the NT gel was higher than that from the SP gel. In the chromophore-labeled macromolecules, the network morphology was effective for increasing the releasing rate based on the transfer inside the hydrogel, as well as for increasing the amount released based on the surface pore morphology.