Methacrylated gelatin (GelMA) has been widely used as a tissue-engineered scaffold material, but only low-concentration GelMA hydrogels were found to be promising cell-laden bioinks with excellent cell viability. In this work, we reported a strategy for precise deposition of 5% (w/v) cell-laden GelMA bioinks into controlled microarchitectures with high cell viability using extrusion-based three-dimensional (3D) bioprinting. By adding gelatin into GelMA bioinks, a two-step cross-linking combining the rapid and reversible thermo-cross-linking of gelatin with irreversible photo-cross-linking of GelMA was achieved. The GelMA/gelatin bioinks showed significant advantages in processability because the tunable rheology and the rapid thermo-cross-linking of bioinks improved the shape fidelity after bioprinting. Here, the rheology, mechanical properties, and swelling of GelMA/gelatin bioinks with different concentration ratios were carefully characterized to obtain the optimized bioprinting setup. We successfully printed the 5% (w/v) GelMA with 8% (w/v) gelatin into 3D structures, which had the similar geometrical resolution as that of the structures printed by 30% (w/v) GelMA bioinks. Moreover, the cell viability of 5/8% (w/v) GelMA/gelatin bioinks was demonstrated by in vitro culture and cell printing of bone marrow stem cells (BMSCs). Larger BMSC spreading area was found on 5/8% (w/v) GelMA/gelatin scaffolds, and the BMSC viability after the printing of 5/8% (w/v) GelMA/gelatin cell-laden bioinks was more than 90%, which was very close to the viability of printing pure 5% (w/v) GelMA cell-laden bioinks. Therefore, this printing strategy of GelMA/gelatin bioinks may extensively extend the applications of GelMA hydrogels for tissue engineering, organ printing, or drug delivery.

Soft ionic conductors, such as hydrogels and ionogels, have enabled stretchable and transparent ionotronics, but they suffer from key limitations inherent to the liquid components, which may leak and evaporate. Here, novel liquid‐free ionic conductive elastomers (ICE) that are copolymer networks hosting lithium cations and associated anions via lithium bonds and hydrogen bonds are demonstrated, such that they are intrinsically immune from leakage and evaporation. The ICEs show extraordinary mechanical versatility including excellent stretchability, high strength and toughness, self‐healing, quick self‐recovery, and 3D‐printability. More intriguingly, the ICEs can defeat the conflict of strength versus toughness—a compromise well recognized in mechanics and material science—and simultaneously overcome the conflict between ionic conductivity and mechanical properties, which is common for ionogels. Several liquid‐free ionotronics based on the ICE are further developed, including resistive force sensors, multifunctional ionic skins, and triboelectric nanogenerators (TENGs), which are not subject to limitations of previous gel‐based devices, such as leakage, evaporation, and weak hydrogel–elastomer interfaces. Also, the 3D printability of the ICEs is demonstrated by printing a series of structures with fine features. The findings offer promise for a variety of ionotronics requiring environmental stability and durability.

It is challenging to develop hydrogels with a combination of excellent mechanical properties, versatile functions, and good processability. Here we report a physical hydrogel of poly­(acrylamide-co-acrylic acid) (P­(AAm-co-AAc)) cross-linked by carboxyl–Fe3+ coordination complexes that possesses high stiffness and toughness, fatigue resistance, and stimulation-triggered healing along with shape memory and processing abilities. The copolymers have randomly dispersed AAm and AAc repeat units, making the physical cross-links with different strength. The strong coordination bonds and their associations serve as permanent cross-links, imparting the elasticity, whereas the weak ones reversibly rupture and re-form, dissipating the energy. Furthermore, a stick–slip instability is observed during the tearing test, which should be associated with the specific nature of metal-coordination bonds that are strong yet fragile. Because of the dynamic nature of coordination bonds, both tensile and tearing mechanical properties are rate dependent. By tuning the bond strength via pH, the gels show distinct mechanical properties, shape memory ability, and even reversible sol–gel transition. The system also shows good processability; the copolymer solutions can be processed into tough gels with different structures by three-dimensional printing technology. These versatile, tough, yet processable hydrogels should be a promising material as structural elements in various applications.

3D printing technology has been widely explored for the rapid design and fabrication of hydrogels, as required by complicated soft structures and devices. Here, a new 3D printing method is presented based on the rheology modifier of Carbomer for direct ink writing of various functional hydrogels. Carbomer is shown to be highly efficient in providing ideal rheological behaviors for multifunctional hydrogel inks, including double network hydrogels, magnetic hydrogels, temperature-sensitive hydrogels, and biogels, with a low dosage (at least 0.5% w/v) recorded. Besides the excellent printing performance, mechanical behaviors, and biocompatibility, the 3D printed multifunctional hydrogels enable various soft devices, including loadable webs, soft robots, 4D printed leaves, and hydrogel Petri dishes. Moreover, with its unprecedented capability, the Carbomer-based 3D printing method opens new avenues for bioprinting manufacturing and integrated hydrogel devices.

Shape-morphing hydrogels have emerging applications in biomedical devices, soft robotics, and so on. However, successful applications require a combination of excellent mechanical properties and fast responding speed, which are usually a trade-off in hydrogel-based devices. Here, a facile approach to fabricate 3D gel constructs by extrusion-based printing of tough physical hydrogels, which show programmable deformations with high response speed and large output force, is described. Highly viscoelastic poly(acrylic acid-coacrylamide) (P(AAc-co-AAm)) and poly(acrylic acid-co-N-isopropyl acrylamide) (P(AAc-co-NIPAm)) solutions or their mixtures are printed into 3D constructs by using multiple nozzles, which are then transferred into FeCl 3 solution to gel the structures by forming robust carboxyl-Fe 3+ coordination complexes. The printed gel fibers containing poly(N-isopropyl acrylamide) segment exhibit considerable volume contraction in concentrated saline solution, whereas the P(AAc-co-AAm) ones do not contract. The mismatch in responsiveness of the gel fibers affords the integrated 3D gel constructs the shapemorphing ability. Because of the small diameter of gel fibers, the printed gel structures deform and recover with a fast speed. A four-armed gripper is designed to clamp plastic balls with considerable holding force, as large as 115 times the weight of the gripper. This strategy should be applicable to other tough hydrogels and broaden their applications.