Extrusion-based 3D printing of soft active materials.

Active materials are capable of responding to external stimuli, as observed in both natural and synthetic systems, from sensitive plants to temperature-responsive hydrogels. Extrusion-based 3D printing of soft active materials facilitates the fabrication of intricate geometries with spatially programmed compositions and architectures at various scales, further enhancing the functionality of materials. This Feature Article summarizes recent advances in extrusion-based 3D printing of active materials in both non-living (i.e., synthetic) and living systems. It highlights emerging ink formulations and architectural designs that enable programmable properties, with a focus on complex shape morphing and controllable light-emitting patterns. The article also spotlights strategies for engineering living materials that can produce genetically encoded material responses and react to a variety of environmental stimuli. Lastly, it discusses the challenges and prospects for advancements in both synthetic and living composite materials from the perspectives of chemistry, modeling, and integration.

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions.

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

Hysteresis-Free, Elastic, and Tough Hydrogel with Stretch-Rate Independence and High Stability in Physiological Conditions.

Many existing synthetic hydrogels are inappropriate for repetitive motions because of large hysteresis, and their mechanical properties in warm and saline physiological conditions remain understudied. In this study, a stretch-rate-independent, hysteresis-free, elastic, and tough nanocomposite hydrogel that can maintain its mechanical properties in phosphate-buffered saline of 37 °C similar to warm and saline conditions of the human body is developed. The strength, stiffness, and toughness of the hydrogel are simultaneously reinforced by biomimetic silica nanoparticles with a surface of embedded circular polyamine chains. Such distinctive surfaces form robust interfacial interactions by local topological folding/entanglement with the polymer chains of the matrix. Load transfer from the soft polymer matrix to stiff nanoparticles, along with the elastic sliding/unfolding/disentanglement of polymer chains, overcomes the traditional trade-off between strength/stiffness and toughness and allows for hysteresis-free, strain-rate-independent, and elastic behavior. This robust reinforcement is sustained in warm phosphate-buffered saline. These properties demonstrate the application potential of the developed hydrogel as a soft, elastic, and tough bio-strain sensor that can detect dynamic motions across various deformation speeds and ranges. The findings provide a simple yet effective approach to developing practical hydrogels with a desirable combination of strength/stiffness and toughness, in a fully swollen and equilibrated state.

Trend of Developing Aqueous Liquid and Gel Electrolytes for Sustainable, Safe, and High-Performance Li-Ion Batteries.

Current lithium-ion batteries (LIBs) rely on organic liquid electrolytes that pose significant risks due to their flammability and toxicity. The potential for environmental pollution and explosions resulting from battery damage or fracture is a critical concern. Water-based (aqueous) electrolytes have been receiving attention as an alternative to organic electrolytes. However, a narrow electrochemical-stability window, water decomposition, and the consequent low battery operating voltage and energy density hinder the practical use of aqueous electrolytes. Therefore, developing novel aqueous electrolytes for sustainable, safe, high-performance LIBs remains challenging. This Review first commences by summarizing the roles and requirements of electrolytes-separators and then delineates the progression of aqueous electrolytes for LIBs, encompassing aqueous liquid and gel electrolyte development trends along with detailed principles of the electrolytes. These aqueous electrolytes are progressed based on strategies using superconcentrated salts, concentrated diluents, polymer additives, polymer networks, and artificial passivation layers, which are used for suppressing water decomposition and widening the electrochemical stability window of water of the electrolytes. In addition, this Review discusses potential strategies for the implementation of aqueous Li-metal batteries with improved electrolyte-electrode interfaces. A comprehensive understanding of each strategy in the aqueous system will assist in the design of an aqueous electrolyte and the development of sustainable and safe high-performance batteries.

Specimen Geometry Effect on Experimental Tensile Mechanical Properties of Tough Hydrogels.

Synthetic tough hydrogels have received attention because they could mimic the mechanical properties of natural hydrogels, such as muscle, ligament, tendon, and cartilage. Many recent studies suggest various approaches to enhance the mechanical properties of tough hydrogels. However, directly comparing each hydrogel property in different reports is challenging because various testing specimen shapes/sizes were employed, affecting the experimental mechanical property values. This study demonstrates how the specimen geometry-the lengths and width of the reduced section-of a tough double-network hydrogel causes differences in experimental tensile mechanical values. In particular, the elastic modulus was systemically compared using eleven specimens of different shapes and sizes that were tensile tested, including a rectangle, ASTM D412-C and D412-D, JIS K6251-7, and seven customized dumbbell shapes with various lengths and widths of the reduced section. Unlike the rectangular specimen, which showed an inconsistent measurement of mechanical properties due to a local load concentration near the grip, dumbbell-shaped specimens exhibited a stable fracture at the reduced section. The dumbbell-shaped specimen with a shorter gauge length resulted in a smaller elastic modulus. Moreover, a relationship between the specimen dimension and measured elastic modulus value was derived, which allowed for the prediction of the experimental elastic modulus of dumbbell-shaped tough hydrogels with different dimensions. This study conveys a message that reminds the apparent experimental dependence of specimen geometry on the stress-strain measurement and the need to standardize the measurement of of numerous tough hydrogels for a fair comparison.

Superstrong, superstiff, and conductive alginate hydrogels.

For the practical use of synthetic hydrogels as artificial biological tissues, flexible electronics, and conductive membranes, achieving requirements for specific mechanical properties is one of the most prominent issues. Here, we demonstrate superstrong, superstiff, and conductive alginate hydrogels with densely interconnecting networks implemented via simple reconstructing processes, consisting of anisotropic densification of pre-gel and a subsequent ionic crosslinking with rehydration. The reconstructed hydrogel exhibits broad ranges of exceptional tensile strengths (8-57 MPa) and elastic moduli (94-1,290 MPa) depending on crosslinking ions. This hydrogel can hold sufficient cations (e.g., Li+) within its gel matrix without compromising the mechanical performance and exhibits high ionic conductivity enough to be utilized as a gel electrolyte membrane. Further, this strategy can be applied to prepare mechanically outstanding, ionic-/electrical-conductive hydrogels by incorporating conducting polymer within the hydrogel matrix. Such hydrogels are easily laminated with strong interfacial adhesion by superficial de- and re-crosslinking processes, and the resulting layered hydrogel can act as a stable gel electrolyte membrane for an aqueous supercapacitor.

Bioinspired Design and Fabrication of Polymer Composite Films Consisting of a Strong and Stiff Organic Matrix and Microsized Inorganic Platelets.

Intensive studies on nacre-inspired composites with exceptional mechanical properties based on an organic/inorganic hierarchical layered structure have been conducted; however, integrating high strength, stiffness, and toughness for engineering materials still remains a challenge. We herein report the design and fabrication of polymer composites through a hydrogel-film casting method that allow for building uniformly layered organic/inorganic microstructure. Alginate (Alg) was used for an organic matrix, whose mechanical properties were controlled by Ca2+ cross-linking toward the simultaneously strong, stiff, and tough resultant composite. Alumina (Alu) microplatelets were used for horizontally aligned inorganic phase, and their alignment and interactions with the organic matrix were improved by polyvinylpyrrolidone (PVP) coating on the platelet. The composite film exhibits well-balanced elastic and plastic deformation under tensile stress, leading to high stiffness and toughness, which have not been generally achieved in microplatelet-based composite films developed in previous studies. The synergistic effect of Ca2+ cross-linking and PVP-coated Alu platelets on the mechanical properties improved polymer-platelet interfacial interactions, and platelet alignment is clearly demonstrated through mechanical tests and Fourier transform infrared and X-ray diffraction analyses. We further demonstrate that the reinforcing effect of the Alu platelet and PVP-coated platelet on the mechanical properties is dependent on humidity. Such effects are maximized at highly dry conditions, which is consistent with the model estimation. Furthermore, a thick bulk composite was produced by laminating thin films and showed high mechanical properties under flexural stress. Our design and fabrication strategies combined with the understanding of their mechanism yield an alternative approach to produce engineered composite materials.

A Hydrogel-Film Casting to Fabricate Platelet-Reinforced Polymer Composite Films Exhibiting Superior Mechanical Properties.

The fabrication of mechanically superior polymer composite films with controllable shapes on various scales is difficult. Despite recent research on polymer composites consisting of organic matrices and inorganic materials with layered structures, these films suffer from complex preparations and limited mechanical properties that do not have even integration of high strength, stiffness, and toughness. Herein, a hydrogel-film casting approach to achieve fabrication of simultaneously strong, stiff, and tough polymer composite films with well-defined microstructure, inspired from a layer-by-layer structure of nacre is reported. Ca2+ -crosslinked alginate hydrogels incorporated with platelet-like alumina particles are dried to form composite films composed of horizontally aligned alumina platelets and alginate matrix with uniformly layered microstructure. Alumina platelets are evenly distributed parallel without precipitations and contribute to synergistic enhancements of strength, stiffness and toughness in the resultant film. Consequentially, Ca2+ -crosslinked alginate/alumina (Ca2+ -Alg/Alu) films show exceptional tensile strength (267 MPa), modulus (17.9 GPa), and toughness (3.60 MJ m-3 ). Furthermore, the hydrogel-film casting allows facile preparation of polymer composite films with controllable shapes and various scales. The results suggest an alternative approach to design and prepare polymer composites with the layer-by-layer structure for superior mechanical properties.