Tough Hydrogels for Load-Bearing Applications

Tough hydrogels have emerged as a promising class of materials to target load-bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high-performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state-of-the-art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high-performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high-performance hydrogels are compared with existing materials, and promising future opportunities are discussed.

Tough Hydrogels with Different Toughening Mechanisms and Applications

Load-bearing biological tissues, such as cartilage and muscles, exhibit several crucial properties, including high elasticity, strength, and recoverability. These characteristics enable these tissues to endure significant mechanical stresses and swiftly recover after deformation, contributing to their exceptional durability and functionality. In contrast, while hydrogels are highly biocompatible and hold promise as synthetic biomaterials, their inherent network structure often limits their ability to simultaneously possess a diverse range of superior mechanical properties. As a result, the applications of hydrogels are significantly constrained. This article delves into the design mechanisms and mechanical properties of various tough hydrogels and investigates their applications in tissue engineering, flexible electronics, and other fields. The objective is to provide insights into the fabrication and application of hydrogels with combined high strength, stretchability, toughness, and fast recovery as well as their future development directions and challenges.

In-situ Spontaneous Fabrication of Tough and Stretchable Polyurethane-Polyethyleneimine Hydrogels Selectively Triggered by CO2

We develop CO₂ -triggered in-situ hydrogels from waterborne poly(ε-caprolactone)-based polyurethane (PU) dispersion and aqueous polyethyleneimine (PEI) solution without any other chemicals and apparatus (e.g., UV light). In our approach, non-toxic CO₂ in air is used as a selective trigger for the hydrogel formation. CO₂ adsorption onto PEI results in the formation of ammonium cations in PEI and the subsequent multiple ionic crosslinking between PU and PEI chains. Besides the amount of CO₂ in air, the rate of hydrogel formation can be controlled by NaHCO₃ in the PU-PEI mixture, which serves as a CO₂ supplier. The PU hydrogels exhibit tough and stretchable properties with high tensile strength (2.05 MPa) and elongation at break (438.24%), as well as biocompatibility and biodegradability. In addition, the PU hydrogels exhibit high adhesion strength on skin and injectability due to the in-situ formation. We believe that these PU hydrogels have the ideal features for various future applications, such as tissue adhesion barriers, wound dressing, artificial skin, and injectable fillers. This article is protected by copyright. All rights reserved.

Biomimetic Gradient Hydrogel Actuators with Ultrafast Thermo-Responsiveness and High Strength

Most current hydrogel actuators suffer from either poor mechanical properties or limited responsiveness. Also, the widely used thermo-responsive poly-(N-isopropylacrylamide) (PNIPAM) homopolymer hydrogels have a slow response rate. Thus, it remains a challenge to fabricate thermo-responsive hydrogel actuators with both excellent mechanical and responsive properties. Herein, ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogels containing multivalent vinyl functionalized silica nanoparticles (VSNPs) are fabricated. The ultrafast thermo-responsiveness is due to the mobile polymer chains grafted from the surfaces of the VSNPs, which can facilitate hydrophobic aggregation, inducing the phase transition and generating water transport channels for quick water expulsion. In addition, the copolymerization of NIPAM with acrylic acid (AA) decreases the transition temperature of the thermo-responsive PNIPAM-based hydrogels, contributing to ultrafast thermo-responsive shrinking behavior with a large volume change of as high as 72.5%. Moreover, inspired by nature, intelligent hydrogel actuators with gradient structure can be facilely prepared through self-healing between the ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogel layers and high-strength VSNPs-PAA-Fe³⁺ multibond network (MBN) hydrogel layers. The obtained well-integrated gradient hydrogel actuators show ultrafast thermo-responsive performance within only 9 s in 60 °C water, as well as high strength, and can be used for more practical applications as intelligent soft actuators or artificial robots.

Mechanically tuneable physical nanocomposite hydrogels from polyelectrolyte complex templated silica nanoparticles for anionic therapeutic delivery

Hydrogels have shown great promise for drug delivery and tissue engineering but can be limited in practical applications by poor mechanical performance. The incorporation of polymer grafted silica nanoparticles as chemical or physical crosslinkers in in situ polymerised nanocomposite hydrogels has been widely researched to enhance their mechanical properties. Despite the enhanced mechanical stiffness, tensile strength, and self-healing properties, there remains a need for the development of simpler and modular approaches to obtain nanocomposite hydrogels. Herein, we report a facile protocol for the polyelectrolyte complex (PEC) templated synthesis of organic-inorganic hybrid poly(ethylenimine) functionalised silica nanoparticles (PEI-SiNPs) and their use as multifunctional electrostatic crosslinkers with hyaluronic acid (HA) to form nanocomposite hydrogels. Upon mixing, electrostatic interactions between cationic PEI-SiNPs and anionic HA resulted in the formation of a coacervate nanocomposite hydrogel with enhanced mechanical stiffness that can be tuned by varying the ratios of PEI-SiNPs and HA present. The reversible electrostatic interactions within the hydrogel networks also enabled self-healing and thixotropic properties. The excess positive charge present within the PEI-SiNPs facilitated high loading and retarded the release of the anionic anti-cancer drug methotrexate from the nanocomposite hydrogel. Furthermore, the electrostatic complexation of PEI-SiNP and HA was found to mitigate haemotoxicity concerns associated with the use of high molecular weight PEI. The method presented herein offers a simpler and more versatile strategy for the fabrication of coacervate nanocomposite hydrogels with tuneable mechanical stiffness and self-healing properties for drug delivery applications.

Super Tough and Intelligent Multibond Network Physical Hydrogels Facilitated by Ti3C2Tx MXene Nanosheets

Stretchable and conductive hydrogels have emerged as promising candidates for intelligent and flexible electronic devices. Herein, based on a multibond network (MBN) design rationale, super tough and highly stretchable nanocomposite physical hydrogels are prepared, where 2D Ti₃C₂T_x MXene nanosheets serve as multifunctional cross-linkers and effective stress transfer centers. Further MXene-poly(acrylic acid) (PAA)-Fe³⁺ MBN physical hydrogels fabricated through controlled permeation of Fe³⁺ exhibit prominent and well-balanced mechanical properties (e.g., the tensile strength can reach 10.4 MPa and elongation at break can be as high as 3080%), attributed to the dual cross-linking network with dense Fe³⁺-mediated coordination cross-links between MXene nanosheets and PAA chains and sparse carboxy-Fe³⁺ cross-links between PAA chains. Moreover, both conductive MXene nanosheets and numerous ions endow the hydrogels with superior conductivity (up to 3.8 S m^-1), strain sensitivity (high gauge factor of 10.09), and self-healing performance, showing great prospect as intelligent flexible electronics.

Equipment-free photothermal effect promoted self-healing and self-recovery of hydrogels

The self-healing and self-recovery of the hydrogel materials can be promoted under sunlight without the assistance of electrical equipment by adding a light-to-heat conversion substance during the synthetic process, which will greatly extend the service life of the hydrogels even for the elastomer materials in the off-grid areas.

How can multi-bond network hydrogels dissipate energy more effectively: an investigation on the relationship between network structure and properties

Constructing a multi-bond network (MBN), which involves hierarchical dynamic bonds with different bond association energies, is an effective method for achieving super tough hydrogels. In this work, a small amount of poly(vinyl alcohol) (PVA) is introduced into a loosely chemically crosslinked poly(acrylic acid) (PAA) network. The hydrophilic PVA chains can physically interact and form hydrogen bonds with the PAA chains. After a freeze-thaw process, PVA could partially crystallize and the generated microcrystals could become new crosslinking points of the hydrogels. Meanwhile, the hydrogen bonds between PAA and PVA, which connect to the microcrystal "core" through PVA chains, could also become new crosslinking points of the hydrogels. The obtained ternary-crosslinked hydrogels (T-gel 10%) exhibit toughness as high as 8 times that in pure PAA hydrogels. When the PVA content exceeds 15 wt%, PVA chains will run through the whole PAA network. Thus the PVA chains will be crosslinked by microcrystals through freeze-thaw treatment, leading to a double network structure, resulting in a brittle hydrogel. The step-increased modulus of the hydrogels with different PVA contents clearly demonstrates the change in the network structure of the hydrogels. Successively, Fe3+ is introduced into the MBN hydrogels as a third cross-linking point. The obtained quaternary-crosslinked hydrogels (Q-gel 10%-Fe5) (50 wt% water content) exhibit significantly improved mechanical properties: tensile strength as high as 6.83 MPa with a fracture energy of 29.9 MJ m-3. This work provides clear insight into the relationship between network structure and mechanical properties in super tough MBN hydrogels.

Coordination-Driven Hierarchical Assembly of Hybrid Nanostructures Based on 2D Materials

2D materials have received tremendous scientific and engineering interests due to their remarkable properties and broad-ranging applications such as energy storage and conversion, catalysis, biomedicine, electronics, and so forth. To further enhance their performance and endow them with new functions, 2D materials are proposed to hybridize with other nanostructured building blocks, resulting in hybrid nanostructures with various morphologies and structures. The properties and functions of these hybrid nanostructures depend strongly on the interfacial interactions between 2D materials and other building blocks. Covalent and coordination bonds are two strong interactions that hold high potential in constructing these robust hybrid nanostructures based on 2D materials. However, most 2D materials are chemically inert, posing problems for the covalent assembly with other building blocks. There are usually coordination atoms in most of 2D materials and their derivatives, thus coordination interaction as a strong interfacial interaction has attracted much attention. In this review, recent progress on the coordination-driven hierarchical assembly based on 2D materials is summarized, focusing on the synthesis approaches, various architectures, and structure-property relationship. Furthermore, insights into the present challenges and future research directions are also presented.

Self-healing and shape memory metallopolymers: state-of-the-art and future perspectives

Recent achievements and problems associated with the use of metallopolymers as self-healing and shape memory materials are presented and evaluated.

Homogeneous and Real Super Tough Multi-Bond Network Hydrogels Created through a Controllable Metal Ion Permeation Strategy

Poly(acrylic acid) (PAA) hydrogels with a multi-bond network composed of sparse chemical cross-links and carboxyl-Fe³⁺ coordination are prepared through a controllable permeation strategy utilizing ferric citrate (FeCA). The existing strategies that directly soak PAA hydrogels in Fe³⁺ solutions usually induce an inhomogeneous network with densely cross-linked shells and uncertain water content of the hydrogels, which brings about ambiguity when investigating strengthening mechanisms because water content significantly affects the mechanical properties of hydrogels. Herein, the controllable permeation of Fe³⁺ into PAA networks based on the competition between citric acid (CA)-Fe³⁺ chelation and PAA-Fe³⁺ coordination guarantees sustained release of Fe³⁺, facilitating homogeneous distribution of ionic cross-links and a certain water content. The obtained hydrogels exhibit excellent and balanced mechanical properties (high tensile strength of 3.28 to 6.95 MPa with large elongations at break of 1400 to 780% when water content decreases from 80 to 50 wt %). The real robust tensile strength of this hydrogel originates from the effective energy dissipation of the homogeneous PAA-Fe³⁺ cross-links, and the high water content ensures a large elongation at break. Furthermore, the hydrogel also has pH-responsive and shape-memory properties.

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

Super-tough, anti-fatigue, self-healable, anti-fogging, and UV shielding hybrid hydrogel prepared via simultaneous dual in situ sol–gel technique and radical polymerization

A hydrogel crosslinked by hierarchical inorganic hybrid crosslinks via simultaneous in situ sol–gel technique and radical polymerization exhibits excellent mechanical performance.

Highly tough, anti-fatigue and rapidly self-recoverable hydrogels reinforced with core-shell inorganic-organic hybrid latex particles

The introduction of SiO₂ particles as crosslinking points into hydrogels has been recognized as a suitable way for toughening hydrogels, due to their versatile functionalization and large specific surface area. However, chemically linked SiO₂ nanocomposite hydrogels often exhibited negligible fatigue resistance and poor self-recoverable properties due to the irreversible cleavage of covalent bonds. Here, we proposed a novel strategy to improve stretchability, fatigue resistance and self-recoverable properties of hydrogels by using SiO₂-g-poly(butyl acrylate) core-shell inorganic-organic hybrid latex particles as hydrophobic crosslinking centers for hydrophobic association. The obtained hydrogel could distribute the surrounding applied stress by disentanglement of the hybrid latex particles from hydrophobic segments. Based on this strategy, the formulated hydrogels showed an excellent tensile strength of 1.48 MPa, superior stretchability of 2511% and remarkable toughness of 12.62 MJ m^-3. Moreover, the hydrogels owned extraordinary anti-fatigue, rapid self-recovery and puncture resistance properties. Therefore, this strategy provided a novel pathway for developing advanced soft materials with potential applications in biomedical engineering, such as tendons, muscles, cartilages, etc.

Facile one-pot synthesis and self-healing properties of tetrazole-based metallopolymers in the presence of iron salts

Self-healing MPs were prepared with tetrazole group for coordinating with FeCl₃·6H₂O by one-pot method. This simple and efficient synthesis will provide a green route for preparing excellent self-healing materials.

Enhancing the self-recovery and mechanical property of hydrogels by macromolecular microspheres with thermal and redox initiation systems

A redox initiation system was used to efficiently enhance the mechanical behavior of macromolecular microsphere hydrogels.

Highly Stretchable and Notch-Insensitive Hydrogel Based on Polyacrylamide and Milk Protein

Protein-based hydrogels have received attention for biomedical applications and tissue engineering because they are biocompatible and abundant. However, the poor mechanical properties of these hydrogels remain a hurdle for practical use. We have developed a highly stretchable and notch-insensitive hydrogel by integrating casein micelles into polyacrylamide (PAAm) networks. In the casein-PAAm hybrid gels, casein micelles and polyacrylamide chains synergistically enhance the mechanical properties. Casein-PAAm hybrid gels are highly stretchable, stretching to more than 35 times their initial length under uniaxial tension. The hybrid gels are notch-insensitive and tough with a fracture energy of approximately 3000 J/m². A new mechanism of energy dissipation that includes friction between casein micelles and plastic deformation of casein micelles was suggested.