Strong and Ultra-tough Ionic Hydrogel Based on Hyperbranched Macro-Cross-linker: Influence of Topological Structure on Properties

Ink formulation of functional nanowires with hyperbranched stabilizers for versatile printing of flexible electronics

Functional nanowire ink formulations require elaborate control over their composition, rheological properties, and fluidic properties to optimize their printing processes. They also require harsh post-fabrication treatments to maximize the performance of the resulting printed flexible devices, making it challenging to uniformly deposit nanowire-based architectures and ensure device reproducibility and scalability. Here, we propose a strategy for developing silver nanowire (AgNW) ink formulations, where hyperbranched molecules (HPMs) are employed as both dispersant and stabilizer for nanowires. The three-dimensional architecture with functional groups on the periphery of HPMs enables the preparation of thixotropic HPMs-AgNW inks with solid contents of up to 20 wt.% in both aqueous and organic solvents using a low amount of HPMs (AgNW and HPMs weight ratio = 1:0.001). The HPMs-AgNW inks can be printed into patterns with a resolution of 20 μm on various flexible substrates without needing harsh post-treatments. We obtain bar-coated transparent electrodes (sheet resistance of 17.1 Ω sq-1 at 94.7% transmittance), slot-die-coated flexible conductive patterns, screen-printed conductive lines (conductivity exceeding 6.2 × 104 S cm-1), and 3D printed stretchable wires. Importantly, this HPMs-stabilized formulation strategy is general for various functional nanowires, enabling the integration of a diverse set of nanowire-based wearable electronic systems.

Ultrastrong eutectogels engineered via integrated mechanical training in molecular and structural engineering

Ultrastrong gels possess generally ultrahigh modulus and strength yet exhibit limited stretchability owing to hardening and embrittlement accompanied by reinforcement. This dilemma is overcome here by using hyperhysteresis-mediated mechanical training that hyperhysteresis allows structural retardation to prevent the structural recovery of network after training, resulting in simply single pre-stretching training. This training strategy introduces deep eutectic solvent into polyvinyl alcohol hydrogels to achieve hyperhysteresis via hydrogen bonding nanocrystals on molecular engineering, performs single pre-stretching training to produce hierarchical nanofibrils on structural engineering, and fabricates chemically cross-linked second network to enable stretchability. The resultant eutectogels display exceptional mechanical performances with enormous fracture strength (85.2 MPa), Young's modulus (98 MPa) and work of rupture (130.6 MJ m-3), which compare favorably to those of previous gels. The presented strategy is generalizable to other solvents and polymer for engineering ultrastrong organogels, and further inspires advanced fabrication technologies for force-induced self-reinforcement materials.

Mechanically Robust, Durable, and Multifunctional Hyper-Crosslinked Elastomer Based on Metal-Organic-Cluster Crosslinker: The Role of Topological Structure

Polymer materials formed by conventional metal-ligand bonds have very low branch functionality, the crosslinker of such polymer usually consists of 2-4 polymer chains and a single metal ion. Thus, these materials are weak, soft, humidity-sensitive, and unable to withstand their shape under long-term service. In this work, a new hyperbranched metal-organic cluster (MOC) crosslinker containing up to 16 vinyl groups is prepared by a straightforward coordination reaction. Compared with the current typical synthesis of metal-organic cages (MOCs) or metal-organic-polyhedra (MOP) crosslinkers with complex operations and low yield, the preparation of the MOC is simple and gram-scale. Thus, MOC can serve as a high-connectivity crosslinker to construct hyper-crosslinked polymer networks. The as-prepared elastomer exhibits mechanical robustness, creep-resistance, and humidity-stability. Besides, the elastomer possesses self-healing and recyclability at mild condition as well as fluorescence stability. These impressive comprehensive properties are proven to originate from the hyper-crosslinked topological structure and microphase-separated morphology. The MOC-driven hyper-crosslinked elastomers provide a new solution for the construction of mechanically robust, durable, and multifunctional polymers.

3D printing of stiff, tough, and ROS-scavenging nanocomposite hydrogel scaffold for in situ corneal repair

Despite significant advancements in hydrogels in recent years, their application in corneal repair remains limited by several challenges, including unfitted curvatures, inferior mechanical properties, and insufficient reactive oxygen species (ROS)-scavenging activities. To address these issues, this study introduces a 3D-printed corneal scaffold with nanocomposite hydrogel consisting of gelatin methacrylate (GelMA), poly (ethylene glycol) diacrylate (PEGDA), Laponite, and dopamine. GelMA and PEGDA act as matrix materials with photo-crosslinking abilities. As a two-dimensional nanoclay, Laponite enhances the rheological properties of the hydrogel, making it suitable for 3D printing. Dopamine self-polymerizes into polydopamine (PDA), providing the hydrogel with ROS-scavenging activity. The incorporation of Laponite and the synergistic effect of PDA endow the hydrogel with good mechanical properties. In vitro investigations demonstrated the cytocompatibility of GelMA-PEGDA-Laponite-dopamine (GPLD) hydrogel and its ROS-scavenging activity. Furthermore, in vivo experiments using a rabbit model of lamellar keratoplasty showed accelerated corneal re-epithelialization and complete stromal repair after the implantation of the 3D-printed scaffold. Overall, due to its high bioactivity and simple preparation, the 3D-printed scaffold using GPLD hydrogel offers an alternative for corneal repair with potential for clinical translation. STATEMENT OF SIGNIFICANCE: The clinical application of hydrogel corneal scaffolds has been constrained by their inadequate mechanical properties and the complex microenvironment created by elevated levels of ROS post-transplantation. In this study, we developed a kind of nanocomposite hydrogel by integrating Laponite and dopamine into GelMA and PEGDA. This advanced hydrogel was utilized to 3D print a corneal scaffold with high mechanical strength and ROS-scavenging abilities. When applied to a rabbit model of lamellar keratoplasty, the 3D-printed scaffold enabled complete re-epithelialization of the cornea within one week. Three months after surgery, the corneal stroma was fully repaired, and regeneration of corneal nerve fibers was also observed. This 3D-printed scaffold demonstrated exceptional efficacy in repairing corneal defects with potential for clinical translation.

Construction of gradient ionogels by self-floatable hyperbranched organosilicon crosslinkers for multi-sensing and wirelessly monitoring physiological signals

Monitoring complex human movements requires the simultaneous detection of strain and pressure, which poses a challenge due to the difficulty in integrating high stretchability and compressive ability into a single material. Herein, a series of hyperbranched polysiloxane crosslinkers (HPSis) with self-floating abilities are designed and synthesized. Taking advantage of the self-floating capabilities of HPSis, ionogels with gradient composition distribution and conductivities are constructed by in situ one-step photopolymerization, and possess satisfactory stretchability, high compressibility and excellent resilience. The gradient-ionogel-based strain sensor exhibits extraordinary pressure sensitivity (19.33 kPa-1), high strain sensitivity (GF reaches 2.5) and temperature sensing ability, enabling the monitoring of the angles and direction of joint movements, transmitting Morse code and wirelessly detecting bioelectrical signals. This study may inspire the design of development of multi-function flexible electronics.

Halometallate Ionic Liquid Dynamically Regulates Zwitterionic Hydrogels by Synergistic Multiple‐Bond Networks

Improving the compatibility between high concentration metallic ions and zwitterions to controllable construction of coordination bonds is critical and extremely challenging. Here, a facile and effective strategy to fabricate multifunctional hydrogels by randomly copolymerizing halometallate ionic liquids (ILs) and zwitterions through electron beam irradiation is reported. Introducing metal ions into ILs can balance charges and establish moderate and stable cross‐linked networks with zwitterions. The synergistic effect of coordination bonds and multiple interactions with varying strengths endows hydrogel with outstanding stretchability, compressive strength, rapid response, advanced self‐healing ability, and excellent frost resistance. The multifunctional sensor assembled from hydrogels can timely, accurately, and stably monitor human movement, write anti‐counterfeiting and remotely transmit Morse code signals. Multiple hydrogel sensors are also assembled into a flexible sensor array to track the tactile trajectory and detect spatial distribution of force. Moreover, the obtained hydrogel displays high temperature sensitivity with resistance temperature coefficient of −3.85% °C−1 at 25–40 °C, which can detect tiny temperature changes (0.1 °C). Interestingly, the processed hydrogel can effectively modulate the transmissivity through salt triggering to achieve patterning. Considering the structural designability of halometallate ILs, this work provides new insights for the development of multifunctional hydrogels.

Carbon nanotube reinforced ionic liquid dual network conductive hydrogels: Leveraging the potential of biomacromolecule sodium alginate for flexible strain sensors

The rapid evolution of multifunctional wearable smart devices has significantly expanded their applications in human-computer interaction and motion health monitoring. Central to these devices are flexible sensors, which require high stretchability, durability, self-adhesion, and sensitivity. Biomacromolecules have attracted attention in sensor design for their biocompatibility, biodegradability, and unique mechanical properties. This study employs a "one-pot" method to integrate ionic liquids and multi-walled carbon nanotubes into a dual-network hydrogel framework, utilizing tannic acid, sodium alginate, acrylamide, and 2-acrylamido-2-methylpropane sulfonic acid. Tannic acid and sodium alginate, natural biomacromolecules, form a robust physical cross-linking network, while P(AM-AMPS) creates a chemical cross-linking network. Ionic liquids enhance carbon nanotube dispersion, resulting in a hybrid hydrogel with remarkable tensile strength (0.12 MPa), adhesive properties (0.039 MPa), and sensing performance (GF 0.12 for 40 %-100 % strain, GF 0.24 for 100 %-250 % strain). This hydrogel effectively monitors large joint movements (fingers, wrists, knees) and subtle biological activities like swallowing and vocalization. Integrating natural biomacromolecules into this composite hydrogel sensor not only enhances the functionality and biocompatibility of flexible wearable devices but also paves the way for innovations in biomedicine and bioelectronics.

Boosting uranium extraction from Seawater by micro-redox reactors anchored in a seaweed-like adsorbent

Efficient extraction of uranium from seawater is expected to provide virtually infinite fuel sources to power nuclear reactors and thus enable sustainable development of nuclear energy. The extraction efficiency for uranium greatly depends on the availability of active adsorption sites on the adsorbents. Maximization of the utilization rate of the binding sites in the adsorbent is vital for improving adsorption capacity. Herein, micro-redox reactors functioned by Cu(I)/Cu(II) conversion are constructed internally in an adsorbent bearing both amidoxime and carboxyl groups to induce active regeneration of the inactivated binding sites to enhance uranium capture. This adsorbent has high adsorption capacity (962.40 mg-U/g-Ads), superior anti-fouling ability as well as excellent uranium uptake (14.62 mg-U/g-Ads) in natural seawater after 56 days, placing it at the top of high-performance sorbent materials for uranium harvest from seawater.

Jellyfish-Inspired Polyurea Ionogel with Mechanical Robustness, Self-Healing, and Fluorescence Enabled by Hyperbranched Cluster Aggregates

Ionogels are promising for soft iontronics, with their network structure playing a pivotal role in determining their performance and potential applications. However, simultaneously achieving mechanical toughness, low hysteresis, self-healing, and fluorescence using existing network structures is challenging. Drawing inspiration from jellyfish, we propose a novel hierarchical crosslinking network structure design for in situ formation of hyperbranched cluster aggregates (HCA) to fabricate polyurea ionogels to overcome these challenges. Leveraging the disparate reactivity of isocyanate groups, we induce the in situ formation of HCA through competing reactions, enhancing toughness and imparting the clustering-triggered emission of ionogel. This synergy between supramolecular interactions in the network and plasticizing effect in ionic liquid leads to reduced hysteresis of the ionogel. Furthermore, the incorporation of NCO-terminated prepolymer with dynamic oxime-urethane bonds (NPU) enables self-healing and enhances stretchability. Our investigations highlight the significant influence of HCA on ionogel performance, showcasing mechanical robustness including high strength (3.5 MPa), exceptional toughness (5.5 MJ m-3), resistance to puncture, and low hysteresis, self-healing, as well as fluorescence, surpassing conventional dynamic crosslinking approaches. This network design strategy is versatile and can meet the various demands of flexible electronics applications.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Ultrastretchable, Ultralow Hysteresis, High-Toughness Hydrogel Strain Sensor for Pressure Recognition with Deep Learning

Hydrogel, as a promising material for a wide range of applications, has demonstrated considerable potential for use in flexible wearable devices and engineering technologies. However, simultaneously realizing the ultrastretchability, low hysteresis, and high toughness of hydrogels is still a great challenge. Here, we present a dual physically cross-linked polyacrylamide (PAM)/sodium hyaluronate (HA)/montmorillonite (MMT) hydrogel. The introduction of HA increases the degree of chain entanglement, and the addition of MMT acts as a stress dissipation center and cross-linking agent, resulting in a hydrogel with high toughness and low hysteretic properties. This hydrogel synthesized by a simple strategy exhibited ultrahigh stretchability (3165%), high breaking stress (228 kPa), high toughness (4.149 MJ/m3), and ultralow hysteresis (≈2% at 100% of strain). The fabricated hydrogel flexible strain sensors possessed fast response and recovery times (62.5:75 ms), a wide strain detection range (2000%), a strain detection limit of 1%, and excellent cycling stability over 500 cycles. Furthermore, the hydrogel flexible strain sensor can be used for human motion monitoring, gesture recognition, and pressure recognition assisted by deep learning algorithms, showing enormous promise for applications.

Self-Adhesive, Stretchable, and Thermosensitive Iontronic Hydrogels for Highly Sensitive Neuromorphic Sensing-Synaptic Systems

Artificial sensory afferent nerves that emulate receptor nanochannel perception and synaptic ionic information processing in chemical environments are highly desirable for bioelectronics. However, challenges persist in achieving life-like nanoscale conformal contact, agile multimodal sensing response, and synaptic feedback with ions. Here, a precisely tuned phase transition poly(N-isopropylacrylamide) (PNIPAM) hydrogel is introduced through the water molecule reservoir strategy. The resulting hydrogel with strongly cross-linked networks exhibits excellent mechanical performance (∼2000% elongation) and robust adhesive strength. Importantly, the hydrogel's enhanced ionic conductance and heterogeneous structure of the temperature-sensitive component enable highly sensitive strain information perception (GFmax = 7.94, response time ∼ 87 ms), temperature information perception (TCRmax = -1.974%/°C, response time ∼ 270 ms), and low energy consumption synaptic plasticity (42.2 fJ/spike). As a demonstration, a neuromorphic sensing-synaptic system is constructed integrating iontronic strain/temperature sensors with fiber synapses for real-time information sensing, discrimination, and feedback. This work holds enormous potential in bioinspired robotics and bioelectronics.

Intelligent anti-impact elastomers by precisely tailoring the topology of modular polymer networks

High-performance elastomers are essential in daily life and various industrial sectors such as personal protection, soft electronics, and vibration control. Nevertheless, despite massive efforts, concurrently achieving ultrahigh flexibility and remarkable impact resistance continues to be elusive. Herein, we report an innovative modular construction strategy that employs a topology-tailoring polymer network consisting of stereoscopic (epoxy-oligosiloxane nanoclusters) and linear (amino-terminated polyurea) building blocks as independent modules to develop intelligent anti-impact elastomers via an epoxy-amine mechanism. By precisely tailoring the topology of building blocks, the elastomers demonstrate high flexibility and toughness, remarkable impact responsiveness and ultrahigh energy dissipation. Their anti-impact ability surpasses those of most common soft and rigid materials such as steel, plastic, rubber, foam, or even polyborosiloxane. Moreover, the elastomers are well-qualified for use in flexible display technologies, owing to their high transparency (>92% transmittance), exceptional fold-resistance (no creasing after 10 000 bends), and good thermal stability (no discoloration at 100 °C). Furthermore, the elastomers exhibit excellent versatility, enabling them to be combined with either soft or rigid materials to generate composites with ultrahigh puncture and ballistic resistance. This study offers a promising framework for the design and fabrication of intelligent anti-impact elastomers and provides valuable insights into the development of next-generation protective materials.

Metal Ion-Induced Acid Hydrolysis Strategy for the One-Step Synthesis of Tough and Highly Transparent Hydrolyzed Polyacrylamide Hydrogels

Polyacrylamide (PAM) hydrogel is hard to enhance through coordination bonds because amide groups rarely coordinate with metal ions strongly in an aqueous solution. It is known that the aqueous solution of ZrOCl2.8H2O can be strongly acidic depending on its concentration. Consequently, through a facile one-step metal ion-induced acid hydrolysis strategy (MIAHS), tough and highly transparent hydrolyzed PAM physical hydrogels are prepared by using ZrOCl2.8H2O in this work. The formation of the partially hydrolyzed PAM physical hydrogels elucidates that the side reaction of imidization during common acid hydrolysis of PAM can be perfectly overcome because the structure of the Zr(IV) ion and its interaction with amide groups promote selective acidic hydrolysis from amide to carboxyl groups. Compared to most coordination cross-linked hydrogels, which need at least two-step fabrication, the hydrolyzed PAM hydrogel via MIAHS can be obtained by one-step synthesis due to the weak interaction between amide groups and Zr(IV). The obtained PAM hydrogel cross-linked by hydrogen bonds and coordination bond between Zr(IV) and carboxyl is a multibond network (MBN) and can achieve hierarchical energy dissipation, which exhibits excellent mechanical properties (tensile strength of 3.15 MPa, elongation at break of 890%, and toughness of 17.0 MJ m-3), high transparence (transmittance of 95%), and outstanding conductivity (5.6 S m-1) at water content of 80 wt %. The high gauge factor (from 2.24 to 12.8 as the strain increases from 0 to 400%) endows the hydrolyzed PAM hydrogels with promising application as strain sensors. Furthermore, in addition to ZrOCl2.8H2O, the fact that various hydrolyzable compounds of Ti(IV), Zr(IV) Hf(IV), and Sn(IV) can also fabricate tough hydrolyzed PAM hydrogels verifies the universality of MIAHS. Therefore, the simple, efficient, and universal MIAHS will shed new light on preparing functional PAM-based hydrogels.