Digital Light Processing 3D Printing of Tough Supramolecular Hydrogels with Sophisticated Architectures as Impact-Absorption Elements

Poly(methyl methacrylate) Nanosphere-Based Photocrosslinked Hydrogels with Ultralong Phosphorescence Lifetimes for High-Precision 3D Printing

Hydrogel-based afterglow materials offer significant potential for broadening the application field of organic room-temperature phosphorescence (RTP) materials owing to their tissue-mimetic flexibility and superior biocompatibility. However, achieving a colorful and efficient RTP in a water-rich hydrogel environment remains challenging. Here, we present a general strategy to fabricate colorful and efficient RTP hydrogels by incorporating compact and hydrophobic nanospheres loaded with chromophores, synthesized via emulsion polymerization, into photocrosslinked hydrogels with oxygen barrier properties. The resultant hydrogel demonstrates a remarkably high water content of 94.6% and a maximum phosphorescence lifetime of up to 1697.0 ms, both significantly surpassing the relevant values of organic RTP hydrogels reported in prior studies. Furthermore, 3D RTP hydrogels with complex geometries and high precision are fabricated using digital light processing (DLP) 3D printing technology. This approach connects the RTP hydrogel and 3D printing fields for the first time, opening up substantial potential for advancing the applications of RTP materials.

Hierarchical Engineering of Amphiphilic Peptides Nanofibrous Crosslinkers toward Mechanically Robust, Functionally Customable, and Sustainable Supramolecular Hydrogels

Hierarchical architectures spanning multiple length scales are ubiquitous in biological tissues, conferring them with both mechanical robustness and dynamic functionalities via structural reorganization under loads. The design of hierarchical architectures within synthetic hydrogels to concurrently achieve mechanical reinforcement and functional integration remains challenging. Here, a biomimetic hierarchical engineering approach is reported to develop mechanically robust and function-customizable supramolecular hydrogels by utilizing strong yet dynamic fibrous nanoarchitectures of amphiphilic peptides as crosslinkers. This design, on one hand, resolves the strength-toughness trade-off in hydrogel design through energy-dissipative mechanisms involving dynamic detachment and reinsertion of peptides within their assembled nanostructures upon loading. On the other hand, the amphiphilicity and sequence programmability of peptides allow spatially orthogonal integration of multiple dynamic functionalities across distinct structural domains, including lipophilic fluorophore encapsulation, photopatterning capability, and anisotropic contraction. Capitalizing on its ultralow hysteresis and rapid recovery properties, the hydrogel's effectiveness is demonstrated as high-sensitivity strain sensors. Moreover, the fully noncovalent crosslinking strategy permits closed-loop recycling and reprocessing via reversible crosslinker disassembly-reassembly processes. Through systematic extension of this principle across diverse peptide systems, a generalized platform is demonstrated for creating advanced soft materials that synergistically integrate traditionally incompatible attributes of mechanical robustness, customable dynamic functionality, and environmental sustainability.

Photothermal Nanotrigger Enables Rapid Gelation and Spatially-Programmable Printability Under Broad-Spectrum Light

Controllable and versatile manufacturing of nanocomposite hydrogels is highly desirable for attaining preferential performance and pushing their applications. However, the conventional thermal-initiated method suffers from sluggish and uncontrollable bulky gelation, whereas the photo-initiated method is controllable yet is limited by a special wavelength of light and is ineffective for hydrogel precursors containing light-absorbing conductive/magnetic nanomaterials. Herein, these limitations are fundamentally overcome, and a new concept of photothermal nanotrigger (PTNT) is discovered that leverages the classic photothermal effect of light-absorbing nanomaterials to transform uncontrollable thermal initiation into a new photo-controlled initiation. The design of PTNT is universal and can be extended for arbitrary photothermal nanomaterials from 2D nanosheets to 1D nanotubes and 0D nanoparticles, exhibiting high gelation efficiency and superior compatibility to broad-spectrum light from visible to ultraviolet and near-infrared. Intriguingly, the PTNT approach can be imparted with excellent spatially-programmable printability by introducing viscous and low thermally-conductive glycerol to confine the photothermal-generated heat and eliminate the diffusion of the formed free radicals to the unexposed region, enabling the manufacturing of high-resolution and heterogenous architecture. Moreover, in addition to the basic initiation function, the PTNT can parasitize the hydrogel network to find many additional applications in the fields of flexible electronics, light/magnetic-manipulated soft robotics, and beyond.

Digital light processing 3D printing of flexible devices: actuators, sensors and energy devices

Flexible devices are increasingly crucial in various aspects of our lives, including healthcare devices and human-machine interface systems, revolutionizing human life. As technology evolves rapidly, there is a high demand for innovative manufacturing methods that enable rapid prototyping of custom and multifunctional flexible devices with high quality. Recently, digital light processing (DLP) 3D printing has emerged as a promising manufacturing approach due to its capabilities of creating intricate customized structures, high fabrication speed, low-cost technology and widespread adoption. This review provides a state-of-the-art overview of the recent advances in the creation of flexible devices using DLP printing, with a focus on soft actuators, flexible sensors and flexible energy devices. We emphasize how DLP printing and the development of DLP printable materials enhance the structural design, sensitivity, mechanical performance, and overall functionality of these devices. Finally, we discuss the challenges and perspectives associated with DLP-printed flexible devices. We anticipate that the continued advancements in DLP printing will foster the development of smarter flexible devices, shortening the design-to-manufacturing cycles.

Development of Hydrogels Fabricated via Stereolithography for Bioengineering Applications

The architectures of hydrogels fabricated with stereolithography (SLA) 3D printing systems have played various roles in bioengineering applications. Typically, the SLA systems successively illuminated light to a layer of photo-crosslinkable hydrogel precursors for the fabrication of hydrogels. These SLA systems can be classified into point-scanning types and digital micromirror device (DMD) types. The point-scanning types form layers of hydrogels by scanning the precursors with a focused light, while DMD types illuminate 2D light patterns to the precursors to form each hydrogel layer at once. Overall, SLA systems were cost-effective and allowed the fabrication of hydrogels with good shape fidelity and uniform mechanical properties. As a result, hydrogel constructs fabricated with the SLA 3D printing systems were used to regenerate tissues and develop lab-on-a-chip devices and native tissue-like models.

Reduced Thermal Damage Achieved by High-Conductivity Hydrogel in RF Energy Tissue Welding

Radiofrequency (RF) tissue welding is an innovative tissue anastomosis technique that utilizes bioimpedance to convert electrical energy into thermal energy, enabling the connection and reconstruction of tissues via the denaturation and crosslinking of proteins. However, the high temperatures generated in this process often lead to excessive thermal damage to tissues, thereby adversely impacting cellular activity and impeding tissue repair in practical applications. In this study, we developed a polyacrylamide/alginate (PAAm/Alg) hydrogel with high ionic conductivity (16.8 ± 1.2 S/m) achieved by introducing Ca2+ for the purpose of reducing thermal damage in RF tissue welding. The PAAm/Alg-Ca2+0.5M hydrogel possessed excellent mechanical properties with a stress of 315.6 ± 14.1 kPa and an elongation of 382.7 ± 89.0%. Additionally, the hydrogel exhibited a high water content (83.7 ± 0.3%) and excellent stability of swelling property in water. In addition, the hydrogel extract showed good biocompatibility with no significant adverse effects on cell activity in the cytotoxicity test. At last, we conducted ex vivo experiments to investigate the effectiveness of the hydrogel as a cooling agent during RF tissue welding. The result showed that the maximum temperature was effectively reduced from 137.9 ± 4.7 to 101.8 ± 2.5 °C, while the strength of the anastomotic stoma (12.0 ± 3.2 kPa) was not affected by the intervention of this hydrogel. Histological analysis also revealed that the anastomotic structure of the tissue with hydrogel intervention was more intact than that of the control. Thus, the PAAm/Alg-Ca2+0.5M hydrogel has been demonstrated to function effectively as a cooling agent, offering a new strategy for thermal damage control in RF tissue welding.

Muscle-like hydrogels with fast isochoric responses and their applications as soft robots: a minireview

Hydrogels with abundant water and responsiveness to external stimuli have emerged as promising candidates for artificial muscles and garnered significant interest for applications as soft actuators and robots. However, most hydrogels possess amorphous structures and exhibit slow, isotropic responses to external stimuli. These features are far inferior to real muscles, which have ordered structures and endow living organisms with programmable deformations and motions through fast, anisotropic responses in complex environments. In recent years, this issue has been addressed by a conceptual new strategy to develop muscle-like hydrogels with highly oriented nanosheets. These hydrogels exhibit fast, isochoric responses based on temperature-mediated electrostatic repulsion between charged nanosheets rather than water diffusion, which significantly advances the development of soft actuators and robots. This minireview summarizes the recent progress in muscle-like hydrogels and their applications as soft actuators and robots. We first introduce the synthesis of muscle-like hydrogels with monodomain structures and the unique mechanism for rapid and isochoric deformations. Then, the developments of hydrogels with complex ordered structures and hydrogel-based soft robots are discussed. The morphing mechanisms and motion kinematics of the hydrogel actuators and robots are highlighted. Finally, concluding remarks are given to discuss future opportunities and challenges in this field.

Salt-welding strategy for the design of repairable impact-resistant and wear-resistant hydrogels

Self-healing hydrogels can autonomously repair damage, enhancing their performance stability and broadening their applications as soft devices. Although the incorporation of dynamic interactions enhances self-healing capabilities, it simultaneously weakens the hydrogels' strength. External stimuli such as heating, while accelerating the healing process, may also lead to dehydration. Developing a stable repair strategy that combines rapid healing and high mechanical strength is challenging. Here, we introduce "salt-welding" for high-strength hydrogels with rapid room temperature self-healing. This is achieved through dynamic borate ester bonds in a salt-responsive poly(methacrylamide) hydrogel. The process involves "salt-fusion" to convert fractures into a viscous liquid for swift healing, followed by "salt-concretion" to toughen the hydrogel. The hydrogels achieve a posthealing strength of 23 megapascals in 95 minutes at room temperature, with near 100% healing efficiency. Leveraging their tunable mechanical strength and rapid healing rate, the hydrogel can be tailored for applications as a reparable wear-resistant material and damping device.

3D Biofabrication of Microporous Hydrogels for Tissue Engineering

Microporous hydrogels have been utilized in an unprecedented manner in the last few decades, combining materials science, biology, and medicine. Their microporous structure makes them suitable for wide applications, especially as cell carriers in tissue engineering and regenerative medicine. Microporous hydrogel scaffolds provide spatial and platform support for cell growth and proliferation, which can promote cell growth, migration, and differentiation, influencing tissue repair and regeneration. This review gives an overview of recent developments in the fabrication techniques and applications of microporous hydrogels. The fabrication of microporous hydrogels can be classified into two distinct categories: fabrication of non-injectable microporous hydrogels including freeze-drying microporous method, two-phase sacrificial strategy, 3D biofabrication technology, etc., and fabrication of injectable microporous hydrogels mainly including microgel assembly. Then, the biomedical applications of microporous hydrogels in cell carriers for tissue engineering, including but not limited to bone regeneration, nerve regeneration, vascular regeneration, and muscle regeneration are emphasized. Additionally, the ongoing and foreseeable applications and current limitations of microporous hydrogels in biomedical engineering are illustrated. Through stimulating innovative ideas, the present review paves new avenues for expanding the application of microporous hydrogels in tissue engineering.

One-Step Digital Light Processing 3D Printing of Robust, Conductive, Shape-Memory Hydrogel for Customizing High-Performance Soft Devices

Mechanically robust and electrically conductive hydrogels hold significant promise for flexible device applications. However, conventional fabrication methods such as casting or injection molding meet challenges in delivering hydrogel objects with complex geometric structures and multicustomized functionalities. Herein, a 3D printable hydrogel with excellent mechanical properties and electrical conductivity is implemented via a facile one-step preparation strategy. With vat polymerization 3D printing technology, the hydrogel can be solidified based on a hybrid double-network mechanism involving in situ chemical and physical dual cross-linking. The hydrogel consists of two chemical networks including covalently cross-linked poly(acrylamide-co-acrylic acid) and chitosan, and zirconium ions are induced to form physically cross-linked metal-coordination bonds across both chemical networks. The 3D-printed hydrogel exhibits multiple excellent functionalities including enhanced mechanical properties (680% stretchability, 15.1 MJ/m3 toughness, and 7.30 MPa tensile strength), rapid printing speed (0.7-3 s/100 μm), high transparency (91%), favorable ionic conductivity (0.75 S/m), large strain gauge factor (≥7), and fast solvent transfer induced phase separation (in ∼3 s), which enable the development of high-performance flexible wearable sensors, shape memory actuators, and soft pneumatic robotics. The 3D printable multifunctional hydrogel provides a novel path for customizing next-generation intelligent soft devices.

Sustainable and Recyclable Acrylate Resins for Liquid-Crystal Display 3D Printing Based on Lipoic Acid

The development of renewable vinyl-based photopolymer resins offers a promising solution to reducing the environmental impact associated with 3D printed materials. This study introduces a bifunctional lipoate cross-linker containing a dynamic disulfide bond, which is combined with acrylic monomers (n-butyl acrylate) and conventional photoinitiators to develop photopolymer resins that are compatible with commercial stereolithography 3D printing. The incorporation of disulfide bonds within the polymer network's backbone imparts the 3D printed objects with self-healing capabilities and complete degradability. Remarkably, the degraded resin can be fully recycled and reused for high-resolution reprinting of complex structures while preserving mechanical properties that are comparable to the original material. This proof-of-concept study not only presents a sustainable strategy for advancing acrylate-based 3D printing materials, but also introduces a novel approach for fabricating fully recyclable 3D-printed structures. This method paves the way for reducing the environmental impact while enhancing material reusability, offering significant potential for the development of eco-friendly additive manufacturing.

Robust liquid crystal semi-interpenetrating polymer network with superior energy-dissipation performance

Liquid crystal networks (LCN) have attracted surging interest as extraordinary energy-dissipation materials owning to their unique dissipation mechanism based on the re-orientation of mesogens. However, how to integrate high Young's modulus, good dissipation efficiency and wide effective damping temperature range in energy-dissipation LCN remains a challenge. Here, we report a strategy to resolve this challenge by fabricating robust energy-dissipation liquid crystal semi-interpenetrating polymer network (LC-semi-IPN) consisting crystalline LC polymers (c-LCP). LC-semi-IPN demonstrates a superior synergistic performance in both mechanical and energy-dissipation properties, surpassing all currently reported LCNs. The crystallinity of c-LCP endows LC-semi-IPN with a substantial leap in Young's modulus (1800% higher than single network). The chain reptation of c-LCP also promotes an enhanced dissipation efficiency of LC-semi-IPN by 200%. Moreover, its effective damping temperature reaches up to 130 °C, which is the widest reported for LCNs. By leveraging its exceptional synergistic performance, LC-semi-IPN can be further utilized as a functional architected structure with exceptional energy-dissipation density and deformation-resistance.

Advances in materials and technologies for digital light processing 3D printing

Digital light processing (DLP) is a projection-based vat photopolymerization 3D printing technique that attracts increasing attention due to its high resolution and accuracy. The projection-based layer-by-layer deposition in DLP uses precise light control to cure photopolymer resin quickly, providing a smooth surface finish due to the uniform layer curing process. Additionally, the extensive material selection in DLP 3D printing, notably including existing photopolymerizable materials, presents a significant advantage compared with other 3D printing techniques with limited material choices. Studies in DLP can be categorized into two main domains: material-level and system-level innovation. Regarding material-level innovations, the development of photocurable resins with tailored rheological, photocuring, mechanical, and functional properties is crucial for expanding the application prospects of DLP technology. In this review, we comprehensively review the state-of-the-art advancements in DLP 3D printing, focusing on material innovations centered on functional materials, particularly various smart materials for 4D printing, in addition to piezoelectric ceramics and their composites with their applications in DLP. Additionally, we discuss the development of recyclable DLP resins to promote sustainable manufacturing practices. The state-of-the-art system-level innovations are also delineated, including recent progress in multi-materials DLP, grayscale DLP, AI-assisted DLP, and other related developments. We also highlight the current challenges and propose potential directions for future development. Exciting areas such as the creation of photocurable materials with stimuli-responsive functionality, ceramic DLP, recyclable DLP, and AI-enhanced DLP are still in their nascent stages. By exploring concepts like AI-assisted DLP recycling technology, the integration of these aspects can unlock significant opportunities for applications driven by DLP technology. Through this review, we aim to stimulate further interest and encourage active collaborations in advancing DLP resin materials and systems, fostering innovations in this dynamic field.

Low-Hysteresis and Tough Ionogels via Low-Energy-Dissipating Cross-Linking

Low-hysteresis merits can help polymeric gel materials survive from consecutive loading cycles and promote life span in many burgeoning areas. However, it is a big challenge to design low-hysteresis and tough polymeric gel materials, especially for ionogels. This can be attributed to the fact that higher viscosities of ionic liquids (ILs) would increase chain friction of polymeric gels and eventually dissipate large amounts of energy under deformation. Herein, a chemical design of ionogels is proposed to achieve low-hysteresis characteristics in both mechanical and electric aspects via hierarchical aggregates formed by supramolecular self-assembly of quadruple H-bonds in a soft IL-rich polymeric matrix. These self-assembled nanoaggregates not only can greatly reinforce the polymeric matrix and enhance resilience, but also exhibit low-energy-dissipating features under stress conditions, simultaneously benefiting for low-hysteresis properties. These aggregates can also promote toughness and subsequent anti-fatigue properties in response to external cyclic mechanical stimuli. More importantly, these ionogels are presented as a model system to elucidate the underlying mechanism of the low hysteresis and fatigue resistance. Based on these findings, it is further demonstrated that the supramolecular low-hysteresis strategy is universal.

Highly Transparent, Mechanically Robust, and Conductive Eutectogel Based on Oligoethylene Glycol and Deep Eutectic Solvent for Reliable Human Motions Sensing

Recently, eutectogels have emerged as ideal candidates for flexible wearable strain sensors. However, the development of eutectogels with robust mechanical strength, high stretchability, excellent transparency, and desirable conductivity remains a challenge. Herein, a covalently cross-linked eutectogel was prepared by exploiting the high solubility of oligoethylene glycol in a polymerizable deep eutectic solvent (DES) form of acrylic acid (AA) and choline chloride (ChCl). The resulting eutectogel exhibited high transparency (90%), robust mechanical strength (up to 1.5 MPa), high stretchability (up to 962%), and desirable ionic conductivity (up to 1.22 mS cm-1). The resistive strain sensor fabricated from the eutectogel exhibits desirable linear sensitivity (GF: 1.66), wide response range (1-200%), and reliable stability (over 1000 cycles), enabling accurate monitoring of human motions (fingers, wrists, and footsteps). We believe that our DES-based eutectogel has great potential for applications in wearable strain sensors with high sensitivity and reliability.

Tough Supramolecular Hydrogels Crafted via Lignin-Induced Self-Assembly

Supramolecular hydrogels are typically assembled through weak non-covalent interactions, posing a significant challenge in achieving ultra strength. Developing a higher strength based on molecular/nanoscale engineering concepts is a potential improvement strategy. Herein, a super-tough supramolecular hydrogel is assembled by gradually diffusing lignosulfonate sodium (LS) into a polyvinyl alcohol (PVA) solution. Both simulations and analytical results indicate that the assembly and subsequent enhancement of the crosslinked network are primarily attributed to LS-induced formation and gradual densification of strong crystalline domains within the hydrogel. The optimized hydrogel exhibits impressive mechanical properties with tensile strength of ≈20 MPa, Young's modulus of ≈14 MPa, and toughness of ≈50 MJ m⁻3, making it the strongest lignin-PVA/polymer hydrogel known so far. Moreover, LS provides the supramolecular hydrogel with excellent low-temperature stability (<-60 °C), antibacterial, and UV-blocking capability (≈100%). Interestingly, the diffusion ability of LS is demonstrated for self-restructuring damaged supramolecular hydrogel, achieving 3D patterning on hydrogel surfaces, and enhancing the local strength of the freeze-thaw PVA hydrogel. The goal is to foster a versatile hydrogel platform by combining eco-friendly LS with biocompatible PVA, paving the way for innovation and interdisciplinarity in biomedicine, engineering materials, and forestry science.

Sustainable Plastics with High Performance and Convenient Processibility

Designing and making sustainable plastics is especially urgent to reduce their ecological and environmental impacts. However, it remains challenging to construct plastics with simultaneous high sustainability and outstanding comprehensive performance. Here, a composite strategy of in situ polymerizing a petroleum-based monomer with the presence of an industrialized bio-derived polymer in a quasi-solvent-free system is introduced, affording the plastic with excellent mechanical robustness, impressive thermal and solvent stability, as well as low energy, consumes during production, processing, and recycling. Particularly, the plastic can be easily processed into diverse shapes through 3D printing, injection molding, etc. during polymerization and further reprocessed into other complex structures via eco-friendly hydrosetting. In addition, the plastic is mechanically robust with Young's modulus of up to 3.7 GPa and tensile breaking strength of up to 150.2 MPa, superior to many commercially available plastics and other sustainable plastics. It is revealed that hierarchical hydrogen bonds in plastic predominate the well-balanced sustainability and performance. This work provides a new path for fabricating high-performance sustainable plastic toward practical applications, contributing to the circular economy.

Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are commonly based on multifunctional monomers, resulting in the formation of thermosets that usually cannot be reprocessed or recycled. Some efforts are made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, a new strategy is presented for DLP 3D printing of thermoplastics based on a sequential construction of two linear polymers with contrasting (stiff and flexible) mechanical properties. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid, which forms the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible linear polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1 GPa to 0.7 MPa, while the strain at break increased from 4% to 574%. Furthermore, these printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, the potential of the new material in applications ranging from soft robotics to electronics is demonstrated.

Hard- and Soft-Coded Strain Stiffening in Metamaterials via Out-of-Plane Buckling Using Highly Entangled Active Hydrogel Elements

Metamaterials show elaborate mechanical behavior such as strain stiffening, which stems from their unit cell design. However, the stiffening response is typically programmed in the design step and cannot be adapted postmanufacturing. Here, we show hydrogel metamaterials with highly programmable strain-stiffening responses by exploiting the out-of-plane buckling of integrated pH-switchable hydrogel actuators. The stiffening upon reaching a certain extension stems from the initially buckled active hydrogel beams. At low strain, the beams do not contribute to the mechanical response under tension until they straighten with a resulting step-function increase in stiffness. In the hydrogel actuator design, the acrylic acid concentration hard-codes the configuration of the metamaterial and range of possible stiffening onsets, while the pH soft-codes the exact stiffening onset point after fabrication. The utilization of out-of-plane buckling to realize subsequent stiffening without the need to deform the passive structure extends the application of hydrogel actuators in mechanical metamaterials. Our concept of out-of-plane buckled active elements that stiffen only under tension enables strain-stiffening metamaterials with high programmability before and after fabrication.

Ligand Dissociation of Metal-Complex Photocatalysts toward pH-Photomanipulation in Dynamic Covalent Hydrogels for Printing Reprocessable and Recyclable Devices

Dynamic covalent hydrogels are gaining attention for their potential in smart materials, soft devices, electronics, and more thanks to their impressive mechanical properties, biomimetic structures, and dynamic behavior. However, a significant challenge lies in designing precise and efficient dynamic photochemistry for their preparation, allowing for complex structures and control over the dynamic process. Herein, we propose a general and straightforward orthogonal dynamic covalent photochemistry strategy for preparing high-performance printable dynamic covalent hydrogels, thereby broadening their advanced applications. This photochemical strategy uses a bifunctional photocatalyst to initiate radical polymerization and release ligands through a rapid light-mediated dissociation mechanism. This process leads to a controlled increase in system pH from mildly acidic to alkaline conditions within one hundred seconds, which in turn triggers the pH-sensitive model reactions of boronic acid/diol complexation and Knoevenagel condensation. The orthogonal photochemistry enables the formation of interpenetrated and conjoined networks, significantly enhancing the mechanical properties of the hydrogels. The reversible bonds formed during the process, i.e., boronic ester and unsaturated ketone bonds, confer excellent self-healing, reprocessable, and recyclable properties on the hydrogels through photochemical pH variations. Furthermore, this rapid, controlled fabrication process and dynamic behavior are highly compatible with printing techniques, enabling the design of adaptive and recyclable sensors with different structures. These advancements are promising for various material science, medicine, and engineering applications.

Recent Progress in 3D Printing of Polymer Materials as Soft Actuators and Robots

With inspiration from natural systems, various soft actuators and robots have been explored in recent years with versatile applications in biomedical and engineering fields. Soft active materials with rich stimulus-responsive characteristics have been an ideal candidate to devise these soft machines by using different manufacturing technologies. Among these technologies, three-dimensional (3D) printing shows advantages in fabricating constructs with multiple materials and sophisticated architectures. In this Review, we aim to provide an overview of recent progress on 3D printing of soft materials, robotics performances, and representative applications. Typical 3D printing techniques are briefly introduced, followed by state-of-the-art advances in 3D printing of hydrogels, shape memory polymers, liquid crystalline elastomers, and their hybrids as soft actuators and robots. From the perspective of material properties, the commonly used printing techniques and action-generation principles for typical printed constructs are discussed. Actuation performances, locomotive behaviors, and representative applications of printed soft materials are summarized. The relationship between printing structures and action performances of soft actuators and robots is also briefly discussed. Finally, the advantages and limitations of each soft material are compared, and the remaining challenges and future directions in this field are prospected.

Photo-steered rapid and multimodal locomotion of 3D-printed tough hydrogel robots

Hydrogels are an ideal material to develop soft robots. However, it remains a grand challenge to develop miniaturized hydrogel robots with mechanical robustness, rapid actuation, and multi-gait motions. Reported here is a facile strategy to fabricate hydrogel-based soft robots by three-dimensional (3D) printing of responsive and nonresponsive tough gels for programmed morphing and locomotion upon stimulations. Highly viscoelastic poly(acrylic acid-co-acrylamide) and poly(acrylic acid-co-N-isopropyl acrylamide) aqueous solutions, as well as their mixtures, are printed with multiple nozzles into 3D constructs followed by incubation in a solution of zirconium ions to form robust carboxyl-Zr4+ coordination complexes, to produce tough metallo-supramolecular hydrogel fibers. Gold nanorods are incorporated into ink to afford printed gels with response to light. Owing to the mechanical excellence and small diameter of gel fibers, the printed hydrogel robots exhibit high robustness, fast response, and agile motions when remotely steered by dynamic light. The design of printed constructs and steering with spatiotemporal light allow for multimodal motions with programmable trajectories of the gel robots. The hydrogel robots can walk, turn, flip, and transport cargos upon light stimulations. Such printed hydrogels with good mechanical performances, fast response, and agile locomotion may open opportunities for soft robots in biomedical and engineering fields.

Photocuring 3D Printing of Hydrogels: Techniques, Materials, and Applications in Tissue Engineering and Flexible Devices

Photocuring 3D printing of hydrogels, with sophisticated, delicate structures and biocompatibility, attracts significant attention by researchers and possesses promising application in the fields of tissue engineering and flexible devices. After years of development, photocuring 3D printing technologies and hydrogel inks make great progress. Herein, the techniques of photocuring 3D printing of hydrogels, including direct ink writing (DIW), stereolithography (SLA), digital light processing (DLP), continuous liquid interface production (CLIP), volumetric additive manufacturing (VAM), and two photon polymerization (TPP) are reviewed. Further, the raw materials for hydrogel inks (photocurable polymers, monomers, photoinitiators, and additives) and applications in tissue engineering and flexible devices are also reviewed. At last, the current challenges and future perspectives of photocuring 3D printing of hydrogels are discussed.

Masking Strategy Constructed Metal Coordination Hydrogels with Improved Mechanical Properties for Flexible Electronic Sensors

Metal coordination hydrogels (MC-HGs) that introduce dynamically coordinate bonds together with metal ionic conduction have attracted considerable attention in flexible electronics. However, the traditional soaking method alleged to have technical scalability faces the challenge of forming MC-HGs with a "core-shell" structure, which undoubtedly reduces the whole mechanical properties and ionic stimulation responsiveness required for flexible electronics materials. Herein, a novel strategy referred to as "masking" has been proposed based on the theory of the valence bond and coordination chemistry. By regulating the masking agents and their concentrations as well as pairing mode with the metal ions, the whole mechanical properties of the resulting composites (MC-HGsMasking) show nearly double the values of their traditional soaking samples (MC-HGsSoaking). For example, the fracture stress and toughness of Fe-HGsMasking(SA, 5.0 g/L) are 1.55 MPa and 2.14 MJ/m3, almost twice those of Fe-HGsSoaking (0.83 MPa and 0.93 MJ/m3, respectively). Microstructure characterization combined with finite element analysis, molecular dynamics, and first-principles simulations demonstrates that the masking strategy first facilitating interfacial permeation of metal complexes and then effective coordination with functional ligands (carboxylates) of the hydrogels is the mechanism to strengthen the mechanical properties of composites MC-HGsMasking, which has the potential to break through the limitations of current MC-HGs in flexible electronic sensor applications.

Insights into Translational and Biomedical Applications of Hydrogels as Versatile Drug Delivery Systems

Hydrogels are a network of crosslinked polymers which can hold a huge amount of water in their matrix. These might be soft, flexible, and porous resembling living tissues. The incorporation of different biocompatible materials and nanostructures into the hydrogels has led to emergence of multifunctional hydrogels with advanced properties. There are broad applications of hydrogels such as tissue culture, drug delivery, tissue engineering, implantation, water purification, and dressings. Besides these, it can be utilized in the field of medical surgery, in biosensors, targeted drug delivery, and drug release. Similarly, hyaluronic acid hydrogels have vast applications in biomedicines such as cell delivery, drug delivery, molecule delivery, micropatterning in cellular biology for tissue engineering, diagnosis and screening of diseases, tissue repair and stem cell microencapsulation in case of inflammation, angiogenesis, and other biological developmental processes. The properties like swellability, de-swellability, biodegradability, biocompatibility, and inert nature of the hydrogels in contact with body fluids, blood, and tissues make its tremendous application in the field of modern biomedicines nowadays. Various modifications in hydrogel formulations have widened their therapeutic applicability. These include 3D printing, conjugation, thiolation, multiple anchoring, and reduction. Various hydrogel formulations are also capable of dual drug delivery, dental surgery, medicinal implants, bone diseases, and gene and stem cells delivery. The presented review summarizes the unique properties of hydrogels along with their methods of preparation and significant biomedical applications as well as different types of commercial products available in the market and the regulatory guidance.

Poly(Ionic Liquid) Double-Network Elastomers with High-Impact Resistance Enhanced by Cation-π Interactions

With the continuous development of impact protection materials, lightweight, high-impact resistance, flexibility, and controllable toughness are required. Here, tough and impact-resistant poly(ionic liquid) (PIL)/poly(hydroxyethyl acrylate) (PHEA) double-network (DN) elastomers are constructed via multiple cross-linking of polymer networks and cation-π interactions of PIL chains. Benefiting from the strong noncovalent cohesion achieved by the cation-π interactions in PIL chains, the prepared PIL DN elastomers exhibit extraordinary compressive strength (95.24 ± 2.49 MPa) and toughness (55.98 ± 0.66 MJ m-3 ) under high-velocity impact load (5000 s-1 ). The synthesized PIL DN elastomer combines strength and flexibility to protect fragile items from impact. This strategy provides a new research idea in the field of the next generation of safety and protective materials.

Hydrogel-Based Macroscopic Click Chemistry

The click reaction has found good utility across various fields due to the characteristics of high efficiency, atom economy, simple and mild reaction conditions. Click chemistry is usually utilized for connecting components of microscopic level, while it is still unable for joining macroscopic building blocks. Materials consisting of macroscopic building blocks realize the flexible fabrication of three-dimensional structures at macroscopic level, exerting significance on parallel manufactures. In this work, we reported macroscopic click chemistry utilizing hydrogel as macroscopic building blocks. Hydrogels G1 and G2 were prepared by incorporating M1 (N,N'-dimethyl-1,2-ethanediamine) and P1 (alkyne functionalized polyethylene glycol) respectively, where polymer chains formed through diffusion-induced amino-yne click reaction entangled different hydrogel networks together. Additionally, chain-like aggregates and complicated 3D structures such as tetrahedron and quadrangular pyramid were constructed based on the adhesion of the hydrogel blocks. The approach enables us to find more possibilities in the delicate designation of 3D aggregations as well as large-scale manufacturing.

3D-Printed Hydrogel for Diverse Applications: A Review

Hydrogels have emerged as a versatile and promising class of materials in the field of 3D printing, offering unique properties suitable for various applications. This review delves into the intersection of hydrogels and 3D printing, exploring current research, technological advancements, and future directions. It starts with an overview of hydrogel basics, including composition and properties, and details various hydrogel materials used in 3D printing. The review explores diverse 3D printing methods for hydrogels, discussing their advantages and limitations. It emphasizes the integration of 3D-printed hydrogels in biomedical engineering, showcasing its role in tissue engineering, regenerative medicine, and drug delivery. Beyond healthcare, it also examines their applications in the food, cosmetics, and electronics industries. Challenges like resolution limitations and scalability are addressed. The review predicts future trends in material development, printing techniques, and novel applications.

Mucosa-Inspired Electro-Responsive Lubricating Supramolecular-Covalent Hydrogel

Enabling the living capability of secreting liquids dynamically triggered by external stimuli while maintaining the bulk frame is a significant challenge for mucosa-inspired hydrogels. A mucosa-inspired electro-responsive hydrogel is developed in this study using the synergy between electro-responsive silk fibroin supramolecular non-covalent networks and covalent polyacrylamide and polyvinyl alcohol polymer networks. The formed supramolecular-covalent hydrogel exhibits a partial gel-sol transition upon the application of an electric field, and the liquid layer on the hydrogel surface near the cathode is used to mimic the mucus-secreting capability to regulate lubrication. The electro-responsive lubricating process can operate under a safe voltage and exhibits good reversibility. It is also a universal strategy to construct an electro-responsive hydrogel by introducing an electro-responsive supramolecular network into the polymer network. This mucosa-inspired electro-responsive supramolecular-covalent hydrogel offers a promising method for designing soft actuators or robots that can regulate lubrication using an electric strategy.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Visible light-induced 3D bioprinted injectable scaffold for minimally invasive tissue regeneration

Pre-formed hydrogel scaffolds have emerged as favorable vehicles for tissue regeneration, promoting minimally invasive treatment of native tissue. However, due to the high degree of swelling and inherently poor mechanical properties, development of complex structural hydrogel scaffolds at different dimensional scales has been a continuous challenge. Herein, we take a novel approach at the intersections of engineering design and bio-ink chemistry to develop injectable pre-formed structural hydrogel scaffolds fabricated via visible light (VL) induced digital light processing (DLP). In this study, we first determined the minimum concentration of poly(ethylene glycol) diacrylate (PEGDA) to be added to the gelatin methacrylate (GelMA) bio-ink in order to achieve scalable and high printing-fidelity with desired cell adhesion, viability, spreading, and osteogenic differentiation characteristics. Despite the advantages of hybrid GelMA-PEGDA bio-ink in improving scalability and printing-fidelity, compressibility, shape-recovery, and injectability of the 3D bioprinted scaffolds were compromised. To restore these needed characteristics for minimally invasive tissue regeneration applications, we performed topological optimization to design highly compressible and injectable pre-formed (i.e., 3D bioprinted) microarchitectural scaffolds. The designed injectable pre-formed microarchitectural scaffolds showed a great capacity to retain the viability of the encapsulated cells (>72 % after 10 cycles of injection). Lastly, ex ovo chicken chorioallantoic membrane (CAM) studies revealed that the optimized injectable pre-formed hybrid hydrogel scaffold is biocompatible and supports angiogenic growth.

An effective DLP 3D printing strategy of high strength and toughness cellulose hydrogel towards strain sensing

Photocurable 3D printing technology has outperformed extrusion-based 3D printing technology in material adaptability, resolution, and printing rate, yet is still limited by the insecure preparation and selection of photoinitiators and thus less reported. In this work, we developed a printable hydrogel that can effectively facilitate various solid or hollow structures and even lattice structures. The chemical and physical dual-crosslinking strategy combined with cellulose nanofibers (CNF) significantly improved the strength and toughness of photocurable 3D printed hydrogels. In this study, the tensile breaking strength, Young's modulus, and toughness of poly(acrylamide-co-acrylic acid)_D/cellulose nanofiber (PAM-co-PAA)_D/CNF hydrogels were 375 %, 203 % and 544 % higher than those of the traditional single chemical crosslinked (PAM-co-PAA)_S hydrogels, respectively. Notably, its outstanding compressive elasticity enabled it to recover under 90 % strain compression (about 4.12 MPa). Resultantly, the proposed hydrogel can be utilized as a flexible strain sensor to monitor the motions of human movements, such as the bending of fingers, wrists, and arms, and even the vibration of a speaking throat. The output of electrical signals can still be collected through strain even under the condition of energy shortage. In addition, photocurable 3D printing technology can provide customized services for hydrogel-based e-skin, such as hydrogel-based bracelets, fingerstall, and finger joint sleeves.

A volatile microemulsion method of preparing water-soluble photo-absorbers for 3D printing of high-resolution, high-water-content hydrogel structures

Digital light processing (DLP)-based three-dimensional (3D) printing is an ideal tool to manufacture hydrogel structures in complex 3D forms. Using DLP to print hydrogel structures with high resolution requires the addition of water-soluble photo-absorbers to absorb excess light and confine photopolymerization to the desired area. However, the current photo-absorbers for hydrogel printing are neither efficient to absorb the excess light nor water-soluble. Herein, we report a volatile microemulsion template method that converts a wide range of commercial non-water-soluble photo-absorbers including Sudan orange G, quercetin, and many others to water-soluble nanoparticles with solubility above 1.0 g mL^-1. After using these water-soluble photo-absorber nanoparticles, the highest lateral and vertical resolutions of printing high-water-content (70-80 wt%) hydrogels can be improved to 5 μm and 20 μm, respectively. Moreover, the quercetin nanoparticle can be easily washed out so that we achieve colorless and transparent printed hydrogel structures with excellent mechanical deformability and biocompatibility as well as thermally controllable variations on transparency and actuation. The proposed methods pave a new efficient way to develop water-soluble photo-absorbers, which helps to greatly improve the printing resolution of the high-water-content hydrogel structure and would be beneficial to extend the application scope of hydrogels.

Intrinsic Anti-Freezing and Unique Phosphorescence of Glassy Hydrogels with Ultrahigh Stiffness and Toughness at Low Temperatures

Most hydrogels become frozen at subzero temperatures, leading to degraded properties and limited applications. Cryoprotectants are massively employed to improve anti-freezing property of hydrogels; however, there are accompanied disadvantages, such as varied networks, reduced mechanical properties, and the risk of cryoprotectant leakage in aqueous conditions. Reported here is the glassy hydrogel having intrinsic anti-freezing capacity and excellent optical and mechanical properties at ultra-low temperatures. Supramolecular hydrogel of poly(acrylamide-co-methacrylic acid) with moderate water content (≈50 wt.%) and dense hydrogen-bond associations is in a glassy state at room temperature. Since hydrogen bonds become strengthened as the temperature decreases, this gel becomes stronger and stiffer, yet still ductile, with Young's modulus of 900 MPa, tensile strength of 30 MPa, and breaking strain of 35% at -45 °C. This gel retains high transparency even in liquid nitrogen. It also exhibits unique phosphorescence due to presence of carbonyl clusters, which is further enhanced at subzero temperatures. Further investigations elucidate that the intrinsic anti-freezing property is related to a fact that most water molecules are tightly bound and confined in the glassy matrix and become non-freezable. This correlation, as validated in several systems, provides a roadmap to develop intrinsic anti-freezing hydrogels for widespread applications at extreme conditions.

Intrinsic and Extrinsic Tunability of Double-Network Hydrogel Strength and Lubricity

Double-network (DN) hydrogels are promising materials for tissue engineering due to their biocompatibility, high strength, and toughness, but understanding of their microstructure-property relationships still remains limited. This work investigates a DN hydrogel comprising a physically crosslinked agarose, as the first network, and a chemically crosslinked copolymer with a varying ratio of acrylamide and acrylic acid, as the second network. The charge, intrinsic to most DN hydrogels, introduces a responsive behavior to chemical and electrical stimuli. The DN strengthens agarose hydrogels, but the strengthening decreases with the swelling ratio resulting from increasing acrylic acid content or reducing salt concentration. Through careful imaging by atomic force microscopy, the heterogenous surface structure and properties arising from the DN are resolved, while the lubrication mechanisms are elucidated by studying the heterogeneous frictional response to extrinsic stimuli. This method reveals the action of the first (agarose) network (forming grain boundaries), copolymer-rich and poor regions (in grains), charge and swelling in providing lubrication. Friction arises from the shear of the polymeric network, whereas hydrodynamic lift and viscoelastic deformation become more significant at higher sliding velocities. We identify the copolymer-rich phase as the main source of the stimulus-responsive behavior. Salt concentration enhances effective charge density and reduces viscoelastic deformation, while electric bias swells the gel and improves lubrication. This work also demonstrates the dynamic control of interfacial properties like hydrogel friction and adhesion, which has implications for other areas of study like soft robotics and tissue replacements.

Bistable Joints Enable the Morphing of Hydrogel Sheets with Multistable Configurations

Joints, as a flexing element to connect different parts, are widespread in natural systems. Various joints exist in the body and play crucial roles to execute gestures and gaits. These scenarios have inspired the design of mechanical joints with passive, hard materials, which usually need an external power supply to drive the transformations. The incorporation of soft and active joints provides a modular strategy to devise soft actuators and robots. However, transformations of responsive joints under external stimuli are usually in uni-mode with a pre-determined direction. Here, hydrogel joints capable of folding and twisting transformation in bi-mode are reported, which enable the composite hydrogel to form multiple configurations under constant conditions. These joints have an in-plane gradient structure and comprise stiff, passive gel as the frame and soft, active gel as the actuating unit. Under external stimuli, the response mismatch between different gels leads to out-of-plane folding or twisting deformation with the feature of bistability. These joints can be modularly integrated with other gels to afford complex deformations and multistable configurations. This approach favors selective control of hydrogel's architectures and versatile design of hydrogel devices, as demonstrated by proof-of-concept examples. It shall also merit the development of metamaterials, soft actuators, and robots, etc.

DLP 3D printed hydrogels with hierarchical structures post-programmed by lyophilization and ionic locking

Porous hydrogels have been intensively used in energy conversion and storage, catalysis, separation, and biomedical applications. Controlling the porosity of these materials over multiple length scales brings about new functionalities and higher efficiency but is a challenge using the current manufacturing methods. Herein we developed a post-programming method to lock the lyophilized pores of 3D printed hydrogels as an experimental platform towards hierarchically structured pores. 3D printing endows the hydrogels with arbitrary 3D geometries and controllable pores at the millimeter length scale. Lyophilization and ionic crosslinking of the as-printed hydrogel networks are conducted as a post-programming process which results in pores at micrometer length scales beyond the printing resolution. Utilizing this combined manufacturing technology, 3D hydrogel lattices with tunable porosities and mechanical properties can be created, which are further exploited for efficient solar vapor generation.