Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically cross-linked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between poly(vinyl alcohol) (PVA) and tannic acid (TA) leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the "permanent" cross-link and the weaker H-bonding between PVA chains as the "temporary" cross-link. The reversible breakage and formation of the weaker H-bonding imparts the PVA-TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60 °C water in a few seconds or at 125 °C in about 2.5 min, respectively.

Convenient patterning and precisely programmable shape deformations are crucial for the practical applications of shape deformable hydrogels. Here, a facile and versatile computer‐assisted ion inkjet printing technique is described that enables the direct printing of batched, very complicated patterns, especially those with well‐defined, programmable variation in cross‐linking densities, on one or both surfaces of a large‐sized hydrogel sample. A mechanically strong hydrogel containing poly(sodium acrylate) is first prepared, and then digital patterns are printed onto the hydrogel surfaces by using a commercial inkjet printer and an aqueous ferric solution. The complexation between the polyelectrolyte and ferric ions increases the cross‐linking density of the printed regions, and hence the gel sample can undergo shape deformation upon swelling/deswelling. The deformation rates and degrees of the hydrogels can be conveniently adjusted by changing the printing times or the different/gradient grayscale distribution of designed patterns. By printing appropriate patterns on one or both surfaces of the hydrogel sheets, many complex 3D shapes are obtained from shape deformations upon swelling/deswelling, such as cylindrical shell and forsythia flower (patterns on one surface), ding (patterns on both surfaces), blooming flower (different/gradient grayscale distributive patterns on one surface), and non‐Euclidean plates (different/gradient grayscale distributive patterns on both surfaces).

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Shape memory hydrogels (SMHs) have a wide range of potential practical applications. However, the mechanically weak and soft nature of most SMHs strongly impedes their applications. Here, we report a novel kind of thermal-responsive SMH with high tensile strength and high elastic moduli. Organogels are first prepared by the copolymerization of a hydrophilic monomer N-vinylpyrrolidone (NVP) and a hydrophobic monomer acryloxy acetophenone (AAP) in N, N'-dimethylformamide (DMF) solutions, and then, poly(vinylpyrrolidone- co-acryloxy acetophenone) [poly(NVP- co-AAP)] hydrogels are obtained by solvent exchange with water. Because of the strong and reversible hydrophobic association and π-π stacking of acetophenone groups, the poly(NVP- co-AAP) hydrogels exhibit tensile strengths up to 8.41 ± 0.83 MPa and Young's moduli up to 94.2 ± 1.3 MPa, which are more than 1 or 3 orders of magnitude higher than those of the organogels, respectively. The poly(NVP- co-AAP) hydrogels exhibit good shape memory behaviors, with a complete fixation ratio and a recovery ratio of 74-89%, as well as very fast shape-fixing and recovering rates (in seconds). These rigid and strong hydrogels are demonstrated to be an ideal shape memory material for surgical fixation devices to wrap around and support various shapes of limbs.

The homogeneous PNIPAM/GO hydrogels could undergo complex shape deformations (e.g., imitating the postures of human) under local NIR irradiation.

Anisotropic poly(vinyl alcohol) hydrogels with significantly enhanced mechanical properties are prepared by a freezing–thawing under drawing (FTD) method.

A novel type of polymeric material with tunable mechanical properties is fabricated from polymers and small molecules that can form hydrogen-bonded intermolecular complexes (IMCs). In this work, poly(vinyl alcohol) (PVA)-glycerol hydrogels are first prepared, and then they are dried to form IMCs. The tensile strengths and moduli of IMCs decrease dramatically with increasing glycerol content, while the elongations increase gradually. The mechanical properties are comparable with or even superior to those of common engineering plastics and rubbers. The IMCs with high glycerol content also show excellent flexibility and cold-resistance at subzero temperatures. Cyclic tensile and stress relaxation tests prove that there is an effective energy dissipation mechanism in IMCs and dynamic mechanical analysis confirms their physical crosslinking nature. FTIR and NMR characterizations prove the existence of hydrogen bonding between glycerol and PVA chains, which suppresses the crystallization of PVA from X-ray diffraction measurement. These PVA-glycerol IMCs may find potential applications in barrier films, biomedical packaging, etc., in the future.

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