A family of multi-responsive hydrogel-based actuators capable of rapid and controllable motion in response to any immediate environmental change is herein demonstrated towards the 3D-printing of functionally graded structures that are encoded with anisotropic swelling behavior.

Cross-linked poly(ε-caprolactone) (PCL)-based polyesterurethane (PUR) systems have been synthesized through Diels-Alder reactions by reactive extrusion. The Diels-Alder and retro-Diels-Alder reactions proved to be useful for enhancing the molecular motion of PCL-based systems, and therefore their crystallization ability, in the design of cross-linked semicrystalline polymers with one-way and two-way shape-memory properties. Successive reactions between α,ω-diol PCL (PCL(2) ), furfuryl alcohol, and methylene diphenyl 4,4'-diisocyanate straightforwardly afforded the α,ω-furfuryl PCL-based PUR systems, and subsequent Diels-Alder reactions with N,N-phenylenedimaleimide afforded the thermoreversible cycloadducts. The cross-linking density could be modulated by partially replacing PCL-diol with PCL-tetraol. Interestingly, the resulting PUR systems proved to be semicrystalline cross-linked polymers, the melting temperature of which (close to 45 °C) represented the switching temperature for their shape-memory properties. Qualitative and quantitative measurements demonstrated that these PUR systems exhibited one-way and two-way shape-memory properties depending on their cross-linking density.