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As one of the most important anisotropic intelligent materials, bi-layer stimuli-responsive actuating hydrogels have proven their wide potential in soft robots, artificial muscles, biosensors, and drug delivery. However, they can commonly provide a simple one-actuating process under one external stimulus, which severely limits their further application. Herein, we have developed a new anisotropic hydrogel actuator by local ionic crosslinking on the poly(acrylic acid) (PAA) hydrogel layer of the bi-layer hydrogel for sequential two-stage bending under a single stimulus. Under pH = 13, ionic-crosslinked PAA networks undergo shrinking (-COO−/Fe3+ complexation) and swelling (water absorption) processes. As a combination of Fe3+ crosslinked PAA hydrogel (PAA@Fe3+) with non-swelling poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate) (PZ) hydrogel, the as-prepared PZ-PAA@Fe3+ bi-layer hydrogel exhibits distinct fast and large-amplitude bidirectional bending behavior. Such sequential two-stage actuation, including bending orientation, angle, and velocity, can be controlled by pH, temperature, hydrogel thickness, and Fe3+ concentration. Furthermore, hand-patterning Fe3+ to crosslink with PAA enables us to achieve various complex 2D and 3D shape transformations. Our work provides a new bi-layer hydrogel system that performs sequential two-stage bending without switching external stimuli, which will inspire the design of programmable and versatile hydrogel-based actuators.

Periosteum, a membrane covering the surface of the bone, plays an essential role in maintaining the function of bone tissue—and especially in providing nourishment and vascularization during the bone regeneration process. Currently, most artificial periostea have relatively weak mechanical strength and a rapid degradation rate, and they lack integrated angiogenesis and osteogenesis functions. In this study, a bi-layer, biomimetic, artificial periosteum composed of a methacrylated gelatin–nano-hydroxyapatite (GelMA-nHA) cambium layer and a poly (N-acryloyl 2-lycine) (PACG) -GelMA-Mg 2+ fibrous layer was fabricated via 3D printing. The GelMA-nHA layer is shown to undertake the function of improving osteogenic differentiation of rat bone marrow mesenchymal stem cells with the sustainable release of Ca 2+ from nHA nanoparticles. The hydrogen-bonding-strengthened P(ACG-GelMA-L)-Mg 2+ hydrogel layer serves to protect the inner defect site and prolong degradation time (60 days) to match new bone regeneration. Furthermore, the released magnesium ion exhibits a prominent effect in regulating the polarization phenotype of macrophage cells into the M2 phenotype and thus promotes the angiogenesis of the human umbilical vein endothelial cells in vitro. This bi-layer artificial periosteum was implanted into a critical-sized cranial bone defect in rats, and the 12-week post-operative outcomes demonstrate optimal new bone regeneration. Graphic abstract