3D printable and mechanically tunable hydrogels achieved through hydrophobic and ionic interactions.

Thermal stiffening materials are commonly applied in the aerospace and automotive industries, among others, since their dimensional stabilities and stiffness characteristics improve at high temperatures. In this study, temperature-triggered modulus-tunable hydrogels were prepared by combining Pluronic F-127 with charged polymers. Pluronic F-127, a triblock copolymer micelle, provided three-dimensional printing capabilities of fine resolution with high viscosity, while hydrophobic and ionic interactions among polymer networks provided thermal stiffening. The hydrogel ink's printability was demonstrated by successfully creating complex 3D structures. A calcium ion carrying a hydrophobic propionate and carboxylate group in polymer chains was used to form additional physical crosslinking at high temperature, ultimately leading to the thermal stiffening effect without volume change. The thermal stiffening behavior was found to be fully reversible and repeatable. Finally, to demonstrate the versatility of this work, graphene oxide was added to produce a light-controllable modulus based on its photothermal properties.