Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Asymmetric toughening in the lap shear of metamaterial structural adhesives

Metamaterial structural adhesives (MSAs), whose properties primarily rely on structural design, offer promising advantages over traditional adhesives, including asymmetric, switchable, and programmable adhesion. However, the effects of thick backing structures on the adhesion properties remain largely underexplored. Herein, we investigate a series of MSAs featuring a thin adhesive layer and an asymmetric thick beam structure terminated with a film. We conduct lap shear tests on the MSAs with varying terminated film thickness (t) and beam tilting angle (θ) while maintaining an identical adhesive layer. For MSAs with a thick terminated film (t = 2 mm), the effective adhesion energy is double that of solid samples without compromising shear strength, consistent with the theoretical predictions based on the crack trapping mechanism. Conversely, for MSAs with a thin terminated film (t = 0.5 mm), the maximum shear strength and effective adhesion energy are ∼2.8 times and ∼18.6 times those of solid samples, respectively, deviating significantly from the theoretical predictions due to new crack initiations. We further explore adhesion asymmetry by tuning the beam tilting angle (θ). For MSAs with highly tilted beams (θ = 70.3°), we achieve a maximum adhesion strength asymmetry factor of τ2/τ1 ∼ 2.2 for a thick terminated film (t = 2 mm), and a maximum adhesion energy asymmetry factor of Γ1/Γ2 ∼ 5.3 for a thin terminated film (t = 0.5 mm). Our work provides useful insights for designing metamaterial structural adhesives suitable for robotic grippers, wall-climbing robots, and wearable devices, particularly those requiring asymmetric, switchable, and stimuli-responsive adhesion, and adhesives on rough surfaces or in underwater conditions.

Highly Sensitive, Degradable, and Rapid Self-Healing Hydrogel Sensor with Semi-Interpenetrating Network for Recognition of Micro-Expressions

Flexible conductive hydrogels have revolutionized the lives and are widely applied in health monitoring and wearable electronics as a new generation of sensing materials. However, the inherent low mechanical strength, sensitivity, and lack of rapid self-healing capacity results in their short life, poor detection accuracy, and environmental pollution. Inspired by the molecular structure of bone and its chemical characteristics, a novel fully physically cross-linked conductive hydrogel is fabricated by the introduction of nanohydroxyapatite (HAp) as the dynamic junction points. In detail, the dynamically cross-linked network, including multiple physical interactions, provides it with rapid self-healing ability and excellent mechanical properties (elongation at break (>1200%), tensile strength (174kPa), and resilience (92.61%)). Besides, the ions (Cl-, Li+, Ca2+) that move freely within the system impart outstanding electrical conductivity (2.46 ± 0.15 S m-1), high sensitivity (gauge factor, GF>8), good antifreeze (-40.2 °C), and humidity properties. The assembled sensor can be employed to sensitively detect various large human motions and subtle changes in behavior (facial expressions, speech recognition). Meanwhile, the hydrogel sensor can also degrade in phosphate-buffered saline solution without causing any environmental pollution. Therefore, the designed hydrogels may become a promising candidate material in the future potential applications for smart wearable sensors and electronic skin.

Strong Acid Enabled Comprehensive Training of Poly (Sodium Acrylate) Hydrogel Networks

The design of admirable hydrogel networks is of both practical and fundamental importance for diverse applications of hydrogels. Herein a general strategy of acid-assisted training is designed to enable multiple improvements of conventional poly (sodium acrylate) networks for hydrogels. Hydrophobic homogeneous crosslinked poly (sodium acrylate) hydrogels are prepared to verify the strategy. The multiple improvements of poly (sodium acrylate) networks are simply achieved by immersing the hydrogel networks into 4 M H2SO4 solutions. The introduced acids would induce transformation of poly (sodium acrylate) into poly (acrylic acid) at hydrogel surface, which constructs dynamic hydrogen bonding interactions to tighten the network. The acid-containing poly (sodium acrylate) hydrogels newly generate anti-swelling and self-healing performance, and show mechanical improvement. The internal poly (sodium acrylate) of the pristine acid-containing hydrogels is further fully transformed via acid-infiltration after following cyclic stretch/release training to significantly improve the mechanical performance. The Young's modulus, stress, and toughness of the fully-trained hydrogels are 187.6 times, 35.6 times, and 5.4 times enhanced, respectively. The polymeric networks retain isotropic in fully-trained hydrogels to ensure superior stretchability of 8.6. The acid-assisted training performance of the hydrogels can be reversibly recovered by NaOH neutralization. The acid-assisted training strategy here is general for poly (sodium acrylate) hydrogels.

Green, tough, and heat-resistant: A GDL-induced strategy for starch-alginate hydrogels

Hydrogels fabricated by non-covalent interaction garnered significant attention for their eco-friendly and robust mechanical attributes, and are often used in food, medicine and other fields. Although starch-alginate hydrogels exhibit high adhesion and are environmentally sustainable, their applications are limited due to their low elasticity and hardness. Addressing this challenge, we introduce a solvent-induced strategy using glucolactone (GDL) to fabricate hydrogels with enhanced strength and thermal resilience. Utilizing corn starch with varying amylose contents, sodium alginate and calcium carbonate to prepare a double network structure. This GDL-induced hydrogel outperforms most previous starch-based hydrogels in mechanical robustness and thermal stability. Typical starch-alginate hydrogel had a homogeneous network structure and exhibited a high tensile stress of 407.57 KPa, and a high enthalpy value of 1857.67 J/g. This investigation furnishes a facile yet effective method for the synthesis of hydrogels with superior mechanical and thermal properties, thereby broadening the design landscape for starch-based hydrogels.

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

New insights into pure zwitterionic hydrogels with high strength and high toughness

Zwitterionic hydrogels are electrically neutral materials with both cationic and anionic groups that impart excellent anti-fouling properties and ion channel orientations. However, pure zwitterionic hydrogels generally exhibit low strength and toughness. In this study, it has been discovered that polymerizable zwitterionic monomers in aqueous solution exhibit a unique liquid-liquid phase separation phenomenon at a high monomer concentration of ≥50 wt%, resulting in pure and commercial zwitterionic hydrogels with high compressive strength (6.5 MPa) and high toughness (2.12 kJ m-2). This phase separation and the corresponding aggregations might be caused by strong dipole-dipole interactions among residual zwitterionic monomers under the lack of free-water condition. The synergistic effect of liquid-liquid phase separation and polymer entanglement enhances the mechanical strength, toughness, self-recovery, and anti-freezing properties of pure polyzwitterionic hydrogels. Moreover, the high fracture energy of highly elongated yet tough polyzwitterionic hydrogels facilitates the development of high crack propagation resistance, which supports an expanded role in tissue engineering, soft flexible devices, and electronics applications with improved durability. A wide range of applications for the proposed polyzwitterionic hydrogels is demonstrated by the development and testing of a strain sensor and a triboelectric nanogenerator device. Our findings provide novel insights into the network structure of pure polyzwitterionic hydrogels.

Versatile, modular, and customizable magnetic solid-droplet systems

Magnetic miniature robotic systems have attracted broad research interest because of their precise maneuverability in confined spaces and adaptability to diverse environments, holding significant promise for applications in both industrial infrastructures and biomedical fields. However, the predominant construction methodology involves the preprogramming of magnetic components into the system's structure. While this approach allows for intricate shape transformations, it exhibits limited flexibility in terms of reconfiguration and presents challenges when adapting to diverse materials, combining, and decoupling multiple functionalities. Here, we propose a construction strategy that facilitates the on-demand assembly of magnetic components, integrating ferrofluid droplets with the system's structural body. This approach enables the creation of complex solid-droplet robotic systems across a spectrum of length scales, ranging from 0.8 mm to 1.5 cm. It offers a diverse selection of materials and structural configurations, akin to assembling components like building blocks, thus allowing for the seamless integration of various functionalities. Moreover, it incorporates decoupling mechanisms to enable selective control over multiple functions, leveraging the fluidity, fission/fusion, and magneto-responsiveness properties inherent in the ferrofluid. Various solid-droplet systems have validated the feasibility of this strategy. This study advances the complexity and functionality achievable in small-scale magnetic robots, augmenting their potential for future biomedical and other applications.

Adhesive Zwitterionic Poly(ionic liquid) with Unprecedented Organic Solvent Resistance

The resistance of adhesives to organic solvents is of paramount importance in diverse industries. Unfortunately, many currently available adhesives exhibit either weak intermolecular chain interactions, resulting in insufficient resistance to organic solvents, or possess a permanent covalent crosslinked network, impeding recyclability. This study introduces an innovative approach to address this challenge by formulating zwitterionic poly(ionic liquid) (ZPIL) derivatives with robust dipole-dipole interactions, incorporating sulfonic anions and imidazolium cations. Due to its unique dynamic and electrostatic self-crosslinking structure, the ZPIL exhibits significant adhesion to various substrates and demonstrates excellent recyclability even after multiple adhesion tests. Significantly, ZPIL exhibits exceptional adhesion stability across diverse nonpolar and polar organic solvents, including ionic liquids, distinguishing itself from nonionic polymers and conventional poly(ionic liquid)s. Its adhesive performance remains minimally affected even after prolonged exposure to soaking conditions. The study presents a promising solution for the design of highly organic solvent-resistant materials for plastics, coatings, and adhesives.

Laser-Based Selective Material Processing for Next-Generation Additive Manufacturing

The connection between laser-based material processing and additive manufacturing is quite deeply rooted. In fact, the spark that started the field of additive manufacturing is the idea that two intersecting laser beams can selectively solidify a vat of resin. Ever since, laser has been accompanying the field of additive manufacturing, with its repertoire expanded from processing only photopolymer resin to virtually any material, allowing liberating customizability. As a result, additive manufacturing is expected to take an even more prominent role in the global supply chain in years to come. Herein, an overview of laser-based selective material processing is presented from various aspects: the physics of laser-material interactions, the materials currently used in additive manufacturing processes, the system configurations that enable laser-based additive manufacturing, and various functional applications of next-generation additive manufacturing. Additionally, current challenges and prospects of laser-based additive manufacturing are discussed.

Mechanically Robust Lubricating Hydrogels Beyond the Natural Cartilage as Compliant Artificial Joint Coating

Natural cartilage exhibits superior lubricity as well as an ultra-long service lifetime, which is related to its surface hydration, load-bearing, and deformation recovery feature. Until now, it is of great challenge to develop reliable cartilage lubricating materials or coatings with persistent robustness. Inspired by the unique biochemical structure and mechanics of natural cartilage, the study reports a novel cartilage-hydrogel composed of top composite lubrication layer and bottom mechanical load-bearing layer, by covalently manufacturing thick polyelectrolyte brush phase through sub-surface of tough hydrogel matrix with multi-level crystallization phase. Due to multiple network dissipation mechanisms of matrix, this hydrogel can achieve a high compression modulus of 11.8 MPa, a reversible creep recovery (creep strain: ≈2%), along with excellent anti-swelling feature in physiological medium (v/v0 < 5%). Using low-viscosity PBS as lubricant, this hydrogel demonstrates persistent lubricity (average COF: ≈0.027) under a high contact pressure of 2.06 MPa with encountering 100k reciprocating sliding cycles, negligible wear and a deformation recovery of collapse pit in testing area. The extraordinary lubrication performance of this hydrogel is comparable to but beyond the natural animal cartilage, and can be used as compliant coating for implantable articular material of UHMWPE to present, offering more robust lubricity than current commercial system.

Chemically Programmed Hydrogels for Spatiotemporal Modulation of the Cardiac Pathological Microenvironment

After myocardial infarction (MI), sustained ischemic events induce pathological microenvironments characterized by ischemia-hypoxia, oxidative stress, inflammatory responses, matrix remodeling, and fibrous scarring. Conventional clinical therapies lack spatially targeted and temporally responsive modulation of the infarct microenvironment, leading to limited myocardial repair. Engineered hydrogels have a chemically programmed toolbox for minimally invasive localization of the pathological microenvironment and personalized responsive modulation over different pathological periods. Chemically programmed strategies for crosslinking interactions, interfacial binding, and topological microstructures in hydrogels enable minimally invasive implantation and in situ integration tailored to the myocardium. This enhances substance exchange and signal interactions within the infarcted microenvironment. Programmed responsive polymer networks, intelligent micro/nanoplatforms, and biological therapeutic cues contribute to the formation of microenvironment-modulated hydrogels with precise targeting, spatiotemporal control, and on-demand feedback. Therefore, this review summarizes the features of the MI microenvironment and chemically programmed schemes for hydrogels to conform, integrate, and modulate the cardiac pathological microenvironment. Chemically programmed strategies for oxygen-generating, antioxidant, anti-inflammatory, provascular, and electrointegrated hydrogels to stimulate iterative and translational cardiac tissue engineering are discussed.

Unique stick-slip crack dynamics of double-network hydrogels under pure-shear loading

In this work, we have found that a prenotched double-network (DN) hydrogel, when subjected to tensile loading in a pure-shear geometry, exhibits intriguing stick-slip crack dynamics. These dynamics synchronize with the oscillation of the damage (yielding) zone at the crack tip. Through manipulation of the loading rate and the predamage level of the brittle network in DN gels, we have clarified that this phenomenon stems from the significant amount of energy dissipation required to form the damage zone at the crack tip, as well as a kinetic contrast between the rapid crack extension through the yielding zone (slip process) and the slow formation of a new yielding zone controlled by the external loading rate (stick process).

Stretchable, Self-Healing, and Bioactive Hydrogel with High-Functionality N,N-bis(acryloyl)cystamine Dynamically Bonded Ag@polydopamine Crosslinkers for Wearable Sensors

Hydrogels present attractive opportunities as flexible sensors due to their soft nature and tunable physicochemical properties. Despite significant advances, practical application of hydrogel-based sensor is limited by the lack of general routes to fabricate materials with combination of mechanical, conductive, and biological properties. Here, a multi-functional hydrogel sensor is reported by in situ polymerizing of acrylamide (AM) with N,N'-bis(acryloyl)cystamine (BA) dynamic crosslinked silver-modified polydopamine (PDA) nanoparticles, namely PAM/BA-Ag@PDA. Compared with traditional polyacrylamide (PAM) hydrogel, the BA-Ag@PDA nanoparticles provide both high-functionality crosslinks and multiple interactions within PAM networks, thereby endowing the optimized PAM/BA-Ag@PDA hydrogel with significantly enhanced tensile/compressive strength (349.80 kPa at 383.57% tensile strain, 263.08 kPa at 90% compressive strain), lower hysteresis (5.2%), improved conductivity (2.51 S m-1) and excellent near-infrared (NIR) light-triggered self-healing ability. As a strain sensor, the PAM/BA-Ag@PDA hydrogel shows a good sensitivity (gauge factor of 1.86), rapid response time (138 ms), and high stability. Owing to abundant reactive groups in PDA, the PAM/BA-Ag@PDA hydrogel exhibits inherent tissue adhesiveness and antioxidant, along with a synergistic antibacterial effect by PDA and Ag. Toward practical applications, the PAM/BA-Ag@PDA hydrogel can conformally adhere to skin and monitor subtle activities and large-scale movements with excellent reliability, demonstrating its promising applications as wearable sensors for healthcare.

A Triple Crosslinked Binder with Hierarchical Stress Dissipation and High Ionic Conductivity for Advanced Silicon Anodes in Lithium-ion Batteries

Silicon (Si) is a promising anode material for high-energy-density lithium-ion batteries, but the significant volume change of Si particles during alloying/dealloying with lithium (Li) undermines the mechanical integrity of Si anode, causing electrode fracture, delamination and rapid capacity decay. Herein, a robust triple crosslinked network (TCN) binder with high ionic conductivity and hierarchical stress dissipation is reported for Si anodes, which is prepared by in situ chemical crosslinking polyacrylic acid (PAA) and melamine (MA). The triple interactions of hydrogen bonds, electrostatic interactions, and covalent amide bonds enhance the adhesion of binder to Si and synergistically promote stress dissipation within Si anodes, thus strengthening the dynamic structural stability of Si anodes during cycling. Moreover, the rapid coupling/decoupling of Li+ with the TCN binder enables an impressive Li+ transference number of 0.63 and high ionic conductivity of 1.2 × 10-4 S cm-1. Consequently, the Si-TCN anode delivers specific capacity of 2268 mAh g-1 with a high mass loading of 2 mg cm-2, high-rate performance of 1673 mAh g-1 at 5 A g-1, and stable cycling for 250 cycles at 1 A g-1, thus showing great prospects for high-energy-density Si-based batteries.

A strategy for tough and fatigue-resistant hydrogels via loose cross-linking and dense dehydration-induced entanglements

Outstanding overall mechanical properties are essential for the successful utilization of hydrogels in advanced applications such as human-machine interfaces and soft robotics. However, conventional hydrogels suffer from fracture toughness-stiffness conflict and fatigue threshold-stiffness conflict, limiting their applicability. Simultaneously enhancing the fracture toughness, fatigue threshold, and stiffness of hydrogels, especially within a homogeneous single network structure, has proven to be a formidable challenge. In this work, we overcome this challenge through the design of a loosely cross-linked hydrogel with slight dehydration. Experimental results reveal that the slightly-dehydrated, loosely cross-linked polyacrylamide hydrogel, with an original/current water content of 87%/70%, exhibits improved mechanical properties, which is primarily attributed to the synergy between the long-chain structure and the dense dehydration-induced entanglements. Importantly, the creation of these microstructures does not require intricate design or processing. This simple approach holds significant potential for hydrogel applications where excellent anti-fracture and fatigue-resistant properties are necessary.

Ultrasound-guided Hydrogel Injection Provides Better Therapeutic Effects After Hand Tendon Surgery Than Intraoperative Injection: A Randomized Controlled Trial

BACKGROUND

Hydrogels are used to provide a barrier against peritendinous adhesion formation, but when implanted intraoperatively, they degrade rapidly and aggravate early inflammatory pain. It is uncertain whether clinical efficacy can be improved by avoiding the inflammatory phase when hydrogels are delivered during adhesion formation.

QUESTIONS/PURPOSES

(1) Compared with intraoperative hydrogel application, does ultrasound-guided postoperative application result in better total active motion (TAM) at 12 months after tendon injury? (2) Does ultrasound-guided postoperative application of hydrogels result in lower pain, better function, and better satisfaction?

METHODS

This open-label, prospective, single-center, randomized controlled trial was conducted by reparative and reconstructive surgeons at the National Orthopedics Clinical Medical Center, Shanghai, People's Republic of China. Between May 2021 and December 2022, 53% (168 of 317) of patients who met our inclusion criteria were recruited, and 47% (149 of 317) of patients were excluded because of the exclusion criteria. Finally, 84 patients were randomized to the postoperative group to receive ultrasound-guided carboxymethyl chitosan (CMC) hydrogel delayed injection, and 84 patients were randomized to the intraoperative group to receive CMC hydrogel intraoperative application. Another 8% (7 of 84) of patients in the postoperative group and 10% (8 of 84) of patients in the intraoperative group were lost before the minimum study follow-up time of 1 year or had incomplete datasets, leaving 91% (153 of 168) of patients with data for analysis. Data on outcome events were analyzed according to the intention-to-treat principle, which included all patients who underwent randomization. Follow-up visits were completed at 3 weeks, 6 weeks, 3 months, 6 months, and 12 months after tendon repair. The primary outcome was TAM (ie, the sum of the degrees of active metacarpophalangeal joint, proximal interphalangeal joint, and distal interphalangeal joint flexion less the degrees from full extension; minimum clinically important difference [MCID] 20°) at 12 months. Secondary outcomes included pain (measured with a VAS; range 0 to 10, a higher score indicating worse pain; MCID 0.6), Michigan Hand Outcomes Questionnaire activities of daily living (MHQ-ADL) score (range 0 to 100, a higher score indicating better outcomes; MCID 10.1), and MHQ satisfaction (MHQ-SAT) score (range 0 to 100, a higher score indicating better outcomes; MCID 33.0).

RESULTS

At 12 months, the ultrasound-guided postoperative injection group had improved TAM (intraoperative 189° [95% CI 179° to 199°] versus postoperative 209° [95% CI 199° to 219°], mean difference 20° [95% CI 6° to 35°]; p = 0.006; the mean difference in the primary outcome fulfilled the MCID value at all time points). At 6 weeks, we found no clinically important difference in VAS pain scores among groups (intraoperative mean ± SD 2.0 ± 1.0 versus postoperative 1.7 ± 1.0, mean difference 0.3 [95% CI 0.1 to 0.7]; p = 0.02); however, at 3 weeks, the VAS pain scores showed clinically important difference among groups (3.6 ± 1.4 versus 2.9 ± 1.2, mean difference 0.7 [95% CI 0.3 to 1.1]; p = 0.001). At 3 months, the ultrasound-guided postoperative injection group had higher MHQ-ADL scores (intraoperative 62 ± 10 versus postoperative 75 ± 10, mean difference 13 [95% CI 11 to 17]; p < 0.001), and the mean difference of MHQ-ADL scores reached the MCID value at all time points. At 3 months, there was no clinically important difference in MHQ-SAT scores between groups (intraoperative 62 ± 8 versus postoperative 70 ± 8, mean difference 8 [95% CI 6 to 11]; p < 0.001).

CONCLUSION

Compared with intraoperative CMC hydrogel injection, postoperative ultrasound-guided injection improved the TAM and function of the affected limb, showed a short-term pain control effect, and did not increase the risk of complications. Clinical trials are needed to confirm the safety and efficacy of ultrasound-guided postoperative injection of CMC hydrogels and to determine the most effective dose and the health and economic benefits of treatment.

LEVEL OF EVIDENCE

Level I, therapeutic study.

Tough Bonding of PVA Hydrogel-on-Textured Titanium Alloy with Varying Texture Densities in Swollen State

Cartilage defects in large joints are a common occurrence in numerous degenerative diseases, especially in osteoarthritis. The hydrogel-on-metal composite has emerged as a potential candidate material, as hydrogels, to some extent, replicate the composition of human articular cartilage consisting of collagen fibers and proteoglycans. However, achieving tough bonding between the hydrogel and titanium alloy remains a significant challenge due to the swelling of the hydrogel in a liquid medium. This swelling results in reduced interfacial toughness between the hydrogel and titanium alloy, limiting its potential clinical applications. Herein, our approach aimed to achieve durable bonding between a hydrogel and a titanium alloy composite in a swollen state by modifying the surface texture of the titanium alloy. Various textures, including circular and triangular patterns, with dimple densities ranging from 10 to 40%, were created on the surface of the titanium alloy. Subsequently, poly(vinyl alcohol) (PVA) hydrogel was deposited onto the textured titanium alloy using a casting-drying method. Our findings revealed that PVA hydrogel on the textured titanium alloy with a 30% texture density exhibited the highest interfacial toughness in the swollen state, measuring at 1300 J m-2 after reaching equilibrium swelling in deionized water, which is a more than 2-fold increase compared to the hydrogel on a smooth substrate. Furthermore, we conducted an analysis of the morphologies of the detached hydrogel from the textured titanium alloy after various swelling durations. The results indicated that interfacial toughness could be enhanced through mechanical interlocking, facilitated by the expanded volume of the hydrogel protrusions as the swelling time increased. Collectively, our study demonstrates the feasibility of achieving tough bonding between a hydrogel and a metal substrate in a liquid environment. This research opens up promising avenues for designing soft/hard heterogeneous materials with strong adhesive properties.

Biomimetic and Wound Microenvironment-Modulating PEGylated Glycopolypeptide Hydrogels for Arterial Massive Hemorrhage and Wound Prohealing

Despite great progress in the hydrogel hemostats and dressings, they generally lack resistant vascular bursting pressure and intrinsic bioactivity to meet arterial massive hemorrhage and proheal wounds. To address the problems, we design a kind of biomimetic and wound microenvironment-modulating PEGylated glycopolypeptide hydrogels that can be easily injected and gelled in ∼10 s. Those glycopolypeptide hydrogels have suitable tissue adhesion of ∼20 kPa, high resistant bursting pressure of ∼150 mmHg, large microporosity of ∼15 μm, and excellent biocompatibility with ∼1% hemolysis ratio and negligible inflammation. They performed better hemostasis in rat liver and rat and rabbit femoral artery bleeding models than Fibrin glue, Gauze, and other hydrogels, achieving fast arterial hemostasis of <20 s and lower blood loss of 5-13%. As confirmed by in vivo wound healing, immunofluorescent imaging, and immunohistochemical and histological analyses, the mannose-modified hydrogels could highly boost the polarization of anti-inflammatory M2 phenotype and downregulate pro-inflammatory tumor necrosis factor-α to relieve inflammation, achieving complete full-thickness healing with thick dermis, dense hair follicles, and 90% collagen deposition. Importantly, this study provides a versatile strategy to construct biomimetic glycopolypeptide hydrogels that can not only resist vascular bursting pressure for arterial massive hemorrhage but also modulate inflammatory microenvironment for wound prohealing.

Fabrication and characterization of sodium alginate-silicon nitride-PVA composite biomaterials with damping properties

Silicon nitride is utilized clinically as a bioceramic for spinal fusion cages, owing to its high strength, osteoconductivity, and antibacterial effects. Nevertheless, silicon nitride exhibits suboptimal damping properties, a critical factor in mitigating traumatic bone injuries and fractures. In fact, there is a scarcity of spinal implants that simultaneously demonstrate proficient damping performance and support osteogenesis. In our study, we fabricated a novel sodium alginate-silicon nitride/poly(vinyl alcohol) (SA-SiN/PVA) composite scaffold, enabling enhanced energy absorption and rapid elastic recovery under quasi-static and impact loading scenarios. Furthermore, the study demonstrated that the incorporation of physical and chemical cross-linking significantly improved stiffness and recoverable energy dissipation. Concerning the interaction between cells and materials, our findings suggest that the addition of silicon nitride stimulated osteogenic differentiation while inhibiting Staphylococcus aureus growth. Collectively, the amalgamation of ceramics and tough hydrogels facilitates the development of advanced composites for spinal implants, manifesting superior damping, osteogenic potential, and antibacterial properties. This approach holds broader implications for applications in bone tissue engineering.

Toughening Hydrogels with Fibrillar Connected Double Networks

Biological tissues, such as tendons or cartilage, possess high strength and toughness with very low plastic deformations. In contrast, current strategies to prepare tough hydrogels commonly utilize energy dissipation mechanisms based on physical bonds that lead to irreversible large plastic deformations, thus limiting their load-bearing applications. This article reports a strategy to toughen hydrogels using fibrillar connected double networks (fc-DN), which consist of two distinct but chemically interconnected polymer networks, that is, a polyacrylamide network and an acrylated agarose fibril network. The fc-DN design allows efficient stress transfer between the two networks and high fibril alignment during deformation, both contributing to high strength and toughness, while the chemical crosslinking ensures low plastic deformations after undergoing high strains. The mechanical properties of the fc-DN network can be readily tuned to reach an ultimate tensile strength of 8 MPa and a toughness of above 55 MJ m-3, which is 3 and 3.5 times more than that of fibrillar double network hydrogels without chemical connections, respectively. The application potential of the fc-DN hydrogel is demonstrated as load-bearing damping material for a jointed robotic lander. The fc-DN design provides a new toughening mechanism for hydrogels that can be used for soft robotics or bioelectronic applications.

Sterically Hindered Oleogel-Based Underwater Adhesive Enabled by Mesh-Tailoring Strategy

Underwater adhesives hold significant relevance in daily life and numerous industrial applications. Despite considerable efforts, developing scalable, high-performance underwater adhesives through a simple and effective method remains a formidable challenge. This study presents a novel mesh-tailoring strategy for in situ, rapid, and ultrastrong oleogel-based underwater adhesives (OUA), which comprises a highly crosslinked polyurethane network with a matching mesh size (≈2.22 nm) that precisely entraps bio-based epoxidized soybean oil (ESO) molecules (≈2.31 nm) by steric hindrance effect. This oleogel exhibits unprecedented robust mechanical properties (≈35 MPa) and maintains stability under extreme conditions, including high temperatures (100 °C), high pressures (30 MPa), and immersion in various solvents (water, ethanol, or ESO). In particular, this oleogel displays high hydrophobicity, rapid curing, and strong interface affinity, resulting in ultrahigh underwater adhesion strength (up to 2.13 MPa) and exceptional substrate universality. Moreover, the remarkable environmental adaptability and stability of OUA enable its use in harsh aqueous environments, including acidic/alkaline, saline, and extreme temperature solutions. The comprehensive capabilities of the OUA underscore its potential for building underwater structures, repairing leaky containers, and sealing broken submarine pipelines. This research establishes the foundation for the designing of next-generation underwater adhesives and offers fresh perspectives for exploring oleogel-based materials.

Surface Topology of Redox- and Thermoresponsive Nanogel Droplets

Hydrogels are usually depicted as a homogenous polymer block with a distinct surface. While defects in the polymer structure are looked into frequently, structural irregularities on the hydrogel surface are often neglected. In this work, thin hydrogel layers of ≈100 nm thickness (nanogels) are synthesized and characterized for their structural irregularities, as they represent the surface of macrogels. The nanogels contain a main-chain responsiveness (thermo responsive) and a responsiveness in the cross-linking points (redox responsive). By combining data from ellipsometry using box-model and two-segment-model analysis, as well as atomic force microscopy, a more defined model of the nanogel surface can be developed. Starting with a more densely cross-linked network at the silica wafer surface, the density of cross-linking gradually decreases toward the hydrogel-solvent interface. Thermo-responsive behavior of the main chain affects the entire network equally as all chain segments change solubility. Cross-linker-based redox-responsiveness, on the other hand, is only governed by the inner, more cross-linked layers of the network. Such dual responsive nanogels hence allow for developing a more detailed model of a hydrogel surface from free radical polymerization. It provides a better understanding of structural defects in hydrogels and how they are affected by responsive functionalities.

Robust and Lubricating Interface Semi-Interpenetrating Network on Inert Polymer Substrates Enabled by Subsurface-Initiated Polymerization

Lubricating hydrogel coatings on inert rubber and plastic surfaces significantly reduce friction and wear, thus enhancing material durability and lifespan. However, achieving optimal hydration lubrication typically requires a porous polymer network, which unfortunately reduces their mechanical strength and limits their applicability where robust durability and wear-resistance are essential. In the research, a hydrogel coating with remarkable wear resistance and surface stability is developed by forming a semi-interpenetrating polymer network with polymer substrate at the interface. By employing a good solvent swelling method, monomers, and photoinitiators are embedded within the substrates' subsurface, followed by in situ polymerization under ultraviolet light, creating a robust semi-interpenetrating and entangled network structure. This approach, offering a thicker energy-dissipating layer, outperforms traditional surface modifications in wear resistance while preserving anti-fatigue, hydrophilicity, oleophobicity, and other properties. Adaptable to various rubber and plastic substrates by using suitable solvents, this method provides an efficient solution for creating durable, lubricating surfaces, broadening the potential applications in multiple industries.

An Organic-Inorganic Hydrogel with Exceptional Mechanical Properties via Anion-Induced Synergistic Toughening for Accelerating Osteogenic Differentiation

Mineralized bio-tissues achieve exceptional mechanical properties through the assembly of rigid inorganic minerals and soft organic matrices, providing abundant inspiration for synthetic materials. Hydrogels, serving as an ideal candidate to mimic the organic matrix in bio-tissues, can be strengthened by the direct introduction of minerals. However, this enhancement often comes at the expense of toughness due to interfacial mismatch. This study reveals that extreme toughening of hydrogels can be realized through simultaneous in situ mineralization and salting-out, without the need for special chemical modification or additional reinforcements. The key to this strategy lies in harnessing the kosmotropic and precipitation behavior of specific anions as they penetrate a hydrogel system containing both anion-sensitive polymers and multivalent cations. The resulting mineralized hydrogels demonstrate significant improvements in fracture stress, fracture energy, and fatigue threshold due to a multiscale energy dissipation mechanism, with optimal values reaching 12 MPa, 49 kJ m-2, and 2.98 kJ m-2. This simple strategy also proves to be generalizable to other anions, resulting in tough hydrogels with osteoconductivity for promoting in vitro mineralization of human adipose-derived mesenchymal stem cells. This work introduces a universal route to toughen hydrogels without compromising other parameters, holding promise for biological applications demanding integrated mechanical properties.

Advances in Hydrogels of Drug Delivery Systems for the Local Treatment of Brain Tumors

The management of brain tumors presents numerous challenges, despite the employment of multimodal therapies including surgical intervention, radiotherapy, chemotherapy, and immunotherapy. Owing to the distinct location of brain tumors and the presence of the blood-brain barrier (BBB), these tumors exhibit considerable heterogeneity and invasiveness at the histological level. Recent advancements in hydrogel research for the local treatment of brain tumors have sought to overcome the primary challenge of delivering therapeutics past the BBB, thereby ensuring efficient accumulation within brain tumor tissues. This article elaborates on various hydrogel-based delivery vectors, examining their efficacy in the local treatment of brain tumors. Additionally, it reviews the fundamental principles involved in designing intelligent hydrogels that can circumvent the BBB and penetrate larger tumor areas, thereby facilitating precise, controlled drug release. Hydrogel-based drug delivery systems (DDSs) are posited to offer a groundbreaking approach to addressing the challenges and limitations inherent in traditional oncological therapies, which are significantly impeded by the unique structural and pathological characteristics of brain tumors.

Small Ionic-Liquid-Based Molecule Drives Strong Adhesives

Nature has inspired scientists to fabricate adhesive materials for applications in many burgeoning areas. However, it is still a significant challenge to develop small-molecule adhesives with high-strength, low-temperature and recyclable properties, although these merits are of great interest in various aspects. Herein, we report a series of strong adhesives based on low-molecular-weight molecular solids driven by the terminal modification of ionic liquids (ILs) and subsequent supramolecular self-assembly. The emergence of high strength and liquid-to-solid transitions for these supramolecular aggregates relies on modifying IL with a high melting point motif and enriching the types of noncovalent interactions in the original ILs. Using this strategy, we demonstrate that our IL-based molecular solids can efficiently obtain a high adhesion strength (up to 8.95 MPa). Importantly, we elucidate the mechanism underlying the reversible and strong adhesion enabled by monomer-to-polymer transitions. These fundamental findings provide guidance for the design of high-performance supramolecular adhesive materials.

Continuous and Controllable Preparation of Sodium Alginate Hydrogel Tubes Guided by the Soft Cap Inspired by the Apical Growth of the Plant

Hydrogel tubes made of sodium alginate (SA) have potential applications in drug delivery, soft robots, biomimetic blood vessels, tissue stents, and other fields. However, the continuous preparation of hollow SA hydrogel tubes with good stability and size control remains a huge challenge for chemists, material scientists, and medical practitioners. Inspired by the plant apical growth strategy, a new method named soft cap-guided growth was proposed to produce SA hydrogel tubes. Due to the introduction of inert low gravity substances, such as air and heptane, into the extrusion needle in front of calcium chloride solution to form a soft cap, the SA hydrogel tubes with controllable sizes were fabricated rapidly and continuously without using a template through a negative gravitropism mechanism. The SA hydrogel tubes had good tensile strength, high burst pressure, and good cell compatibility. In addition, hydrogel tubes with complex patterns were conveniently created by controlling the motion path of a soft cap, such as a rotating SA bath or magnetic force. Our research provided a simple innovative technique to steer the growth of hydrogel tubes, which made it possible to mass produce hydrogel tubes with controllable sizes and programmable patterns.

Investigating Temperature-Dependent Microscopic Deformation in Tough and Self-Healing Hydrogel Using Time-Resolved USAXS

Tough and self-healing hydrogels are typically sensitive to loading rates or temperatures due to the dynamic nature of noncovalent bonds. Understanding the structure evolution under varying loading conditions can provide valuable insights for developing new tough soft materials. In this study, polyampholyte (PA) hydrogel with a hierarchical structure is used as a model system. The evolution of the microscopic structure during loading is investigated under varied loading temperatures. By combining ultra-small angle X-ray scattering (USAXS) and Mooney-Rivlin analysis, it is elucidated that the deformation of bicontinuous hard/soft phase networks is closely correlated with the relaxation dynamics or strength of noncovalent bonds. At high loading temperatures, the gel is soft and ductile, and large affine deformation of the phase-separated networks is observed, correlated with the fast relaxation dynamics of noncovalent bonds. At low loading temperatures, the gel is stiff, and nonaffine deformation occurs from the onset of loading due to the substantial breaking of noncovalent bonds and limited chain mobility as well as weak adaptation of phase deformation to external stretch. This work provides an in-depth understanding of the relationship between structure and performance of tough and self-healing hydrogels.

Triple-Mechanism Enhanced Flexible SiO2 Nanofiber Composite Hydrogel with High Stiffness and Toughness for Cartilaginous Ligaments

Hydrogels are widely used in tissue engineering, soft robotics and wearable electronics. However, it is difficult to achieve both the required toughness and stiffness, which severely hampers their application as load-bearing materials. This study presents a strategy to develop a hard and tough composite hydrogel. Herein, flexible SiO2 nanofibers (SNF) are dispersed homogeneously in a polyvinyl alcohol (PVA) matrix using the synergistic effect of freeze-drying and annealing through the phase separation, the modulation of macromolecular chain movement and the promotion of macromolecular crystallization. When the stress is applied, the strong molecular interaction between PVA and SNF effectively disperses the load damage to the substrate. Freeze-dried and annealed-flexible SiO2 nanofibers/polyvinyl alcohol (FDA-SNF/PVA) reaches a preferred balance between enhanced stiffness (13.71 ± 0.28 MPa) and toughness (9.9 ± 0.4 MJ m-3). Besides, FDA-SNF/PVA hydrogel has a high tensile strength of 7.84 ± 0.10 MPa, super elasticity (no plastic deformation under 100 cycles of stretching), fast deformation recovery ability and excellent mechanical properties that are superior to the other tough PVA hydrogels, providing an effective way to optimize the mechanical properties of hydrogels for potential applications in artificial tendons and ligaments.

Multifunctional Biodegradable Conductive Hydrogel Regulating Microenvironment for Stem Cell Therapy Enhances the Nerve Tissue Repair

The nerve guidance conduits incorporated with stem cells, which can differentiate into the Schwann cells (SCs) to facilitate myelination, shows great promise for repairing the severe peripheral nerve injury. The innovation of advanced hydrogel materials encapsulating stem cells, is highly demanded for generating supportive scaffolds and adaptive microenvironment for nerve regeneration. Herein, this work demonstrates a novel strategy in regulating regenerative microenvironment for peripheral nerve repair with a biodegradable conductive hydrogel scaffold, which can offer multifunctional capabilities in immune regulation, enhancing angiogenesis, driving SCs differentiation, and promoting axon regrowth. The biodegradable conductive hydrogel is constructed by incorporation of polydopamine-modified silicon phosphorus (SiP@PDA) nanosheets into a mixture of methacryloyl gelatin and decellularized extracellular matrix (GelMA/ECM). The biomimetic electrical microenvironment performs an efficacious strategy to facilitate macrophage polarization toward a pro-healing phenotype (M2), meanwhile the conductive hydrogel supports vascularization in regenerated tissue through sustained Si element release. Furthermore, the MSCs 3D-cultured in GelMA/ECM-SiP@PDA conductive hydrogel exhibits significantly increased expression of genes associated with SC-like cell differentiation, thus facilitating the myelination and axonal regeneration. Collectively, both the in vitro and in vivo studies demonstrates that the rationally designed biodegradable multifunctional hydrogel significantly enhances nerve tissues repair.

Stepwisely Assembled Multicomponent Fiber with High Water Content and Superior Mechanical Properties for Artificial Ligament

The ligament, which connects bones at the joints, has both high water content and excellent mechanical properties in living organisms. However, it is still challenging to fabricate fibrous materials that possess high water content and ligament-like mechanical characteristics simultaneously. Herein, the design and preparation of a ligament-mimicking multicomponent fiber is reported through stepwise assembly of polysaccharide, calcium, and dopamine. In simulated body fluid, the resulting fiber has a water content of 40 wt%, while demonstrating strength of ≈120 MPa, a Young's modulus of ≈3 GPa, and a toughness of ≈25 MJ m-3. Additionally, the multicomponent fiber exhibits excellent creep and fatigue resistance, as well as biocompatibility to support cell growth in vitro. These findings suggest that the fiber has potential for engineering high-performance artificial ligament.

Near-infrared Light-Induced Polymerizations: Mechanisms and Applications

Photopolymerizations have garnered significant attention in polymer science due to their low polymerization temperature, high production efficiency, environmental friendliness, and spatial controllability. Despite these merits, the poor penetration and severe chemical damage from ultraviolet/visible (UV/Vis) light resources pose significant barriers to their success in conventional photopolymerizations. A recent breakthrough involving the utilization of near-infrared (NIR) laser with long wavelength has been exploited for diverse applications. With the combination of a NIR photosensitizer (PS), NIR-induced photopolymerizations have been successfully developed to alleviate the challenges in conventional methods. The enhancement of penetration depth and safety of NIR-induced photopolymerizations can contribute significantly to improving the efficiency of polymerization for production of intricate structures across various scales. In this concept, the typical types of PSs and polymerization mechanisms (PMs) within the NIR-induced photopolymerization systems have been classified in detail. Additionally, the applications of various polymers achieved by NIR-induced photopolymerizations are summarized. Furthermore, research directions and future challenges of this field are also discussed comprehensively.

Dual-network carboxymethyl chitosan conductive hydrogels for multifunctional sensors and high-performance triboelectric nanogenerators

With the development of technology, there is a growing demand for wearable electronics that can fulfill different application scenarios. Hydrogel-based sensors are considered ideal candidates for realizing multifunctional wearable flexible devices. However, there are great challenges in preparing hydrogel-based sensors with both superior mechanical and electrical properties. Herein, we report a composite conductive hydrogel prepared by using a dynamically crosslinked carboxymethyl chitosan network and a covalently crosslinked polymer network, and carboxylated carbon nanotubes as conductive filler. The carboxymethyl chitosan-based hydrogels had excellent mechanical properties and strength (tensile strength of 475.4 kPa, and compressive strength of 1.9 MPa) and ultra-high conductivity (0.19 S·cm-1). Based on the above characteristics, the hydrogel could accurately identify the movement signals of the human body and different writing signals, and achieve encrypted transmission of signals, broadening the application scenarios. In addition, a triboelectric nanogenerator (TENG) was fabricated based on the hydrogel, which had an outstanding output performance with open-circuit voltage of 336 V, short-circuit current of 18 μA, transferred charge of 52 nC and maximum power density of 340 mW·m-2, and could power small devices. This work is expected to provide new ideas for the development of self-powered, multi-functional wearable, and flexible polysaccharide-based devices.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Materials and Structural Designs toward Motion Artifact-Free Bioelectronics

Bioelectronics encompassing electronic components and circuits for accessing human information play a vital role in real-time and continuous monitoring of biophysiological signals of electrophysiology, mechanical physiology, and electrochemical physiology. However, mechanical noise, particularly motion artifacts, poses a significant challenge in accurately detecting and analyzing target signals. While software-based "postprocessing" methods and signal filtering techniques have been widely employed, challenges such as signal distortion, major requirement of accurate models for classification, power consumption, and data delay inevitably persist. This review presents an overview of noise reduction strategies in bioelectronics, focusing on reducing motion artifacts and improving the signal-to-noise ratio through hardware-based approaches such as "preprocessing". One of the main stress-avoiding strategies is reducing elastic mechanical energies applied to bioelectronics to prevent stress-induced motion artifacts. Various approaches including strain-compliance, strain-resistance, and stress-damping techniques using unique materials and structures have been explored. Future research should optimize materials and structure designs, establish stable processes and measurement methods, and develop techniques for selectively separating and processing overlapping noises. Ultimately, these advancements will contribute to the development of more reliable and effective bioelectronics for healthcare monitoring and diagnostics.

Thermally Conductive Elastomer Composites with High Toughness, Softness, and Resilience Enabled by Regulating Interfacial Structure and Dynamics

The emerging applications of thermally conductive elastomer composites in modern electronic devices for heat dissipation require them to maintain both high toughness and resilience under thermomechanical stresses. However, such a combination of thermal conductivity and desired mechanical characteristics is extremely challenging to achieve in elastomer composites. Here this long-standing mismatch is resolved via regulating interfacial structure and dynamics response. This regulation is realized both by tuning the molecular weight of the dangling chains in the polymer networks and by silane grafting of the fillers, thereby creating a broad dynamic-gradient interfacial region comprising of entanglements. These entanglements can provide the slipping topological constraint that allows for tension equalization between and along the chains, while also tightening into rigid knots to prevent chain disentanglement upon stretching. Combined with ultrahigh loading of aluminum-fillers (90 wt%), this design provides a low Young's modulus (350.0 kPa), high fracture toughness (831.5 J m-2), excellent resilience (79%) and enhanced thermal conductivity (3.20 W m-1 k-1). This work presents a generalizable preparation strategy toward engineering soft, tough, and resilient high-filled elastomer composites, suitable for complex environments, such as automotive electronics, and wearable devices.

Dynamic Transformation of High-Architectural Nanocrystal Superlattices upon Solvent Molecule Exposure

The cluster-based body-centered-cubic superlattice (cBCC SL) represents one of the most complicated structures among reported nanocrystal assemblies, comprised of 72 truncated tetrahedral quantum dots per unit cell. Our previous report revealed that truncated tetrahedral quantum dots within cBCC SLs possessed highly controlled translational and orientational order owing to an unusual energetic landscape based on the balancing of entropic and enthalpic contributions during the assembly process. However, the cBCC SL's structural transformability and mechanical properties, uniquely originating from such complicated nanostructures, have yet to be investigated. Herein, we report that cBCC SLs can undergo dynamic transformation to face-centered-cubic SLs in response to post-assembly molecular exposure. We monitored the dynamic transformation process using in situ synchrotron-based small-angle X-ray scattering, revealing a dynamic transformation involving multiple steps underpinned by interactions between incoming molecules and TTQDs' surface ligands. Furthermore, our mechanistic study demonstrated that the precise configuration of TTQDs' ligand molecules in cBCC SLs was key to their high structural transformability and unique jelly-like soft mechanical properties. While ligand molecular configurations in nanocrystal SLs are often considered minor features, our findings emphasize their significance in controlling weak van der Waals interactions between nanocrystals within assembled SLs, leading to previously unremarked superstructural transformability and unique mechanical properties. Our findings promote a facile route toward further creation of soft materials, nanorobotics, and out-of-equilibrium assemblies based on nanocrystal building blocks.

Carbodiimide-Driven Toughening of Interpenetrated Polymer Networks

Recent work has demonstrated that temporary crosslinks in polymer networks generated by chemical "fuels" afford materials with large, transient changes in their mechanical properties. This can be accomplished in carboxylic-acid-functionalized polymer hydrogels using carbodiimides, which generate anhydride crosslinks with lifetimes on the order of minutes to hours. Here, the impact of the polymer network architecture on the mechanical properties of transiently crosslinked materials was explored. Single networks (SNs) were compared to interpenetrated networks (IPNs). Notably, semi-IPN precursors that give IPNs on treatment with carbodiimide give much higher fracture energies (i.e., resistance to fracture) and superior resistance to compressive strain compared to other network architectures. A precursor semi-IPN material featuring acrylic acid in only the free polymer chains yields, on treatment with carbodiimide, an IPN with a fracture energy of 2400 J/m2, a fourfold increase compared to an analogous semi-IPN precursor that yields a SN. This resistance to fracture enables the formation of macroscopic complex cut patterns, even at high strain, underscoring the pivotal role of polymer architecture in mechanical performance.

Experimental observation of near-wall effects during the puncture of soft solids

Performing conventional mechanical characterization techniques on soft materials can be challenging due to issues such as limited sample volumes and clamping difficulties. Deep indentation and puncture is a promising alternative as it is an information-rich measurement with the potential to be performed in a high-throughput manner. Despite its promise, the method lacks standardized protocols, and open questions remain about its possible limitations. Addressing these shortcomings is vital to ensure consistent methodology, measurements, and interpretation across samples and labs. To fill this gap, we examine the role of finite sample dimensions (and by extension, volume) on measured forces to determine the sample geometry needed to perform and unambiguously interpret puncture tests. Through measurements of puncture on a well-characterized elastomer using systematically varied sample dimensions, we show that the apparent mechanical response of a material is in fact sensitive to near-wall effects, and that additional properties, such as the sliding friction coefficient, can only be extracted in the larger dimension case where such effects are negligible.

Tailoring Network Topology in Mechanically Robust Hydrogels for 3D Printing and Injection

Tissue engineering and regenerative medicine are confronted with a persistent challenge: the urgent demand for robust, load-bearing, and biocompatible scaffolds that can effectively endure substantial deformation. Given that inadequate mechanical performance is typically rooted in structural deficiencies─specifically, the absence of energy dissipation mechanisms and network uniformity─a crucial step toward solving this problem is generating synthetic approaches that enable exquisite control over network architecture. This work systematically explores structure-property relationships in poly(ethylene glycol)-based hydrogels constructed utilizing thiol-yne chemistry. We systematically vary polymer concentration, constituent molar mass, and cross-linking protocols to understand the impact of architecture on hydrogel mechanical properties. The network architecture was resolved within the molecular model of Rubinstein-Panyukov to obtain the densities of chemical cross-links and entanglements. We employed both nucleophilic and radical pathways, uncovering notable differences in mechanical response, which highlight a remarkable degree of versatility achievable by tuning readily accessible parameters. Our approach yielded hydrogels with good cell viability and remarkably robust tensile and compression profiles. Finally, the hydrogels are shown to be amenable to advanced processing techniques by demonstrating injection- and extrusion-based 3D printing. Tuning the mechanism and network regularity during the cell-compatible formation of hydrogels is an emerging strategy to control the properties and processability of hydrogel biomaterials by making simple and rational design choices.

N-type semiconducting hydrogel

Hydrogels are an attractive category of biointerfacing materials with adjustable mechanical properties, diverse biochemical functions, and good ionic conductivity. Despite these advantages, their application in electronics has been restricted because of their lack of semiconducting properties, and they have traditionally only served as insulators or conductors. We developed single- and multiple-network hydrogels based on a water-soluble n-type semiconducting polymer, endowing conventional hydrogels with semiconducting capabilities. These hydrogels show good electron mobilities and high on/off ratios, enabling the fabrication of complementary logic circuits and signal amplifiers with low power consumption and high gains. We demonstrate that hydrogel electronics with good bioadhesive and biocompatible interface can sense and amplify electrophysiological signals with enhanced signal-to-noise ratios.

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.

Black Phosphorus-Based Dynamic Self-Healing Hydrogel to Integrate Demineralized Bone Matrix and Noggin-Targeting siRNA for Synergistic Osteogenesis

Although a demineralized bone matrix (DBM) is often used as an alternative to an autologous bone graft, its clinical application is still hampered by easy dispersion of DBM particles and insufficient osteoinductivity in the defect site. Herein, we designed a self-healing hydrogel for DBM that can rapidly restore its structural integrity after damage based on amino-rich black phosphorus (BP) nanosheets and aldehyde-functionalized hyaluronic acid (AHA). Given the increased expression of bone morphogenetic protein (BMP) antagonists by DBM stimulation, the osteogenic potency of DBM in the hydrogel carrier was further enhanced by abrogating the BMP antagonism. The BP/AHA hydrogel provided dynamic polymer-nanosheet networks that combine injectability, modability, and physical stability with high DBM loading, where the BP nanosheets served as osteogenic cross-linkers to promote biomineralization and deliver siRNA to suppress undesirable expression of BMP antagonist noggin by DBM. As a result, the BP/AHA hydrogel integrated with DBM and noggin-targeting siRNA synergistically promoted osteogenic differentiation of mesenchymal stem cells by enhancing BMP/Smad signaling. This work demonstrates a promising strategy to improve the efficacy of bone regeneration using bone graft.

Thermo-growing ion clusters enabled healing strengthening and tough adhesion for highly reliable skin electronics

Self-healing and self-adhesion capacities are essential for many modern applications such as skin-interfaced electronics for improving longevity and reliability. However, the self-healing efficiency and adhesive toughness of most synthetic polymers are limited to their original network, making reliability under dynamic deformation still challenging. Herein, inspired by the growth of living organisms, a highly stretchable supramolecular elastomer based on thermo-responsive ion clusters and a dynamic polysulfide backbone was developed. Attributed to the synergic growth of ion clusters and dynamic exchange of disulfide bonds, the elastomer exhibited unique healing strengthening (healing efficiency >200%) and thermo-enhanced tough adhesion (interfacial toughness >500 J m-2) performances. To prove its practical application in highly reliable skin electronics, we further composited the elastomer with a zwitterion to prepare a highly conductive ionic elastomer and applied it in wearable strain sensing and long-term electrophysiological detection. This work provides a new avenue to realize high reliability in skin interfaced electronics.

Bioresorbable, wireless, passive sensors for continuous pH measurements and early detection of gastric leakage

Continuous monitoring of biomarkers at locations adjacent to targeted internal organs can provide actionable information about postoperative status beyond conventional diagnostic methods. As an example, changes in pH in the intra-abdominal space after gastric surgeries can serve as direct indicators of potentially life-threatening leakage events, in contrast to symptomatic reactions that may delay treatment. Here, we report a bioresorbable, wireless, passive sensor that addresses this clinical need, designed to locally monitor pH for early detection of gastric leakage. A pH-responsive hydrogel serves as a transducer that couples to a mechanically optimized inductor-capacitor circuit for wireless readout. This platform enables real-time monitoring of pH with fast response time (within 1 hour) over a clinically relevant period (up to 7 days) and timely detection of simulated gastric leaks in animal models. These concepts have broad potential applications for temporary sensing of relevant biomarkers during critical risk periods following diverse types of surgeries.

Modeling the Temperature-Dependent Size Change of Polydisperse Nano-objects using a Deep Generative Model

Modern microscopy techniques can be used to investigate soft nano-objects at the nanometer scale. However, time-consuming microscopy measurements combined with low numbers of observable polydisperse objects often limit the statistics. We propose a method for identifying the most representative objects from their respective point clouds. These point cloud data are obtained, for example, through the localization of single emitters in super-resolution fluorescence microscopy. External stimuli, such as temperature, can cause changes in the shape and properties of adaptive objects. Due to the demanding and time-consuming nature of super-resolution microscopy experiments, only a limited number of temperature steps can be performed. Therefore, we propose a deep generative model that learns the underlying point distribution of temperature-dependent microgels, enabling the reliable generation of unlimited samples with an arbitrary number of localizations. Our method greatly cuts down the data collection effort across diverse experimental conditions, proving invaluable for soft condensed matter studies.

Structural heterogeneity in tetra-armed gels revealed by computer simulation: Evidence from a graph theory assisted characterization

Designing homogeneous networks is considered one typical strategy for solving the problem of strength and toughness conflict of polymer network materials. Experimentalists have proposed the hypothesis of obtaining a structurally homogeneous hydrogel by crosslinking tetra-armed polymers, whose homogeneity was claimed to be verified by scattering characterization and other methods. Nevertheless, it is highly desirable to further evaluate this issue from other perspectives. In this study, a coarse-grained molecular dynamics simulation coupled with a stochastic reaction model is applied to reveal the topological structure of a polymer network synthesized by tetra-armed monomers as precursors. Two different scenarios, distinguished by whether internal cross-linking is allowed, are considered. We introduce the Dijkstra algorithm from graph theory to precisely characterize the network structure. The microscopic features of the network structure, e.g., loop size, dispersity, and size distribution, are obtained via the Dijkstra algorithm. By comparing the two reaction scenarios, Scenario II exhibits an overall more idealized structure. Our results demonstrate the feasibility of the Dijkstra algorithm for precisely characterizing the polymer network structure. We expect this work will provide a new insight for the evaluation and description of gel networks and further help to reveal the dynamic process of network formation.

Continuous production of ultratough semiconducting polymer fibers with high electronic performance

Conjugated polymers have demonstrated promising optoelectronic properties, but their brittleness and poor mechanical characteristics have hindered their fabrication into durable fibers and textiles. Here, we report a universal approach to continuously producing highly strong, ultratough conjugated polymer fibers using a flow-enhanced crystallization (FLEX) method. These fibers exhibit one order of magnitude higher tensile strength (>200 megapascals) and toughness (>80 megajoules per cubic meter) than traditional semiconducting polymer fibers and films, outperforming many synthetic fibers, ready for scalable production. These fibers also exhibit unique strain-enhanced electronic properties and exceptional performance when used as stretchable conductors, thermoelectrics, transistors, and sensors. This work not only highlights the influence of fluid mechanical effects on the crystallization and mechanical properties of conjugated polymers but also opens up exciting possibilities for integrating these functional fibers into wearable electronics.

Towards high performance and durable soft tactile actuators

Soft actuators are gaining significant attention due to their ability to provide realistic tactile sensations in various applications. However, their soft nature makes them vulnerable to damage from external factors, limiting actuation stability and device lifespan. The susceptibility to damage becomes higher with these actuators often in direct contact with their surroundings to generate tactile feedback. Upon onset of damage, the stability or repeatability of the device will be undermined. Eventually, when complete failure occurs, these actuators are disposed of, accumulating waste and driving the consumption of natural resources. This emphasizes the need to enhance the durability of soft tactile actuators for continued operation. This review presents the principles of tactile feedback of actuators, followed by a discussion of the mechanisms, advancements, and challenges faced by soft tactile actuators to realize high actuation performance, categorized by their driving stimuli. Diverse approaches to achieve durability are evaluated, including self-healing, damage resistance, self-cleaning, and temperature stability for soft actuators. In these sections, current challenges and potential material designs are identified, paving the way for developing durable soft tactile actuators.

Using bacterial cellulose to bridge covalent and physical crosslinks in hydrogels for fabricating multimodal sensors

Network optimization is vital for the polysaccharide based hydrogels with multiple crosslinks. In this study, we developed a 'two-step' strategy to activate synergistic effect of chemical and physical crosslinks using a poly (vinyl alcohol) (PVA)/bacterial cellulose (BC) hydrogel as a template. The BC nanofibers, on the one hand, acted as nucleating agents, participating in the crystallization of PVA, and on the other hand, were also involved in the formation of boronic ester bond, anchored with the PVA chains via chemical bonding. Therefore, the existence of BC nanofibers, as 'bridge', linked the crystalline regions and amorphous parts of PVA together, associating the two characteristic crosslinks, which was conducive to load transfer. The mechanical properties of resultant hydrogels, including the tensile elongation and strength, as well as fracture toughness, were significantly improved. Moreover, the dually cross-linked hydrogels possessed ionic conductivity, which was sensitive to the tensile deformation and environmental temperature. This study clarifies a unique role of BC nanofibers in hydrogels, and proposes an effective approach to construct multiple networks in the nanocellulose reinforced PVA hydrogels.