Tough Hydrogels Research Articles

Injectable, tough and adhesive zwitterionic hydrogels for 3D-printed wearable strain sensors

3D printing of high-strength hydrogel remains a great challenge owing to trade-off between strength and printability. Herein, a zwitterionic strategy was proposed through electrostatic reversible interaction to improve the rheology of shear-thinning based on an agarose-based network. Therefore, this mechanism of dual network (DN) ultimately imparts the hydrogel with remarkable strength (with a tensile strength of 0.79 MPa and a compressive strength of 14.18 MPa) and ductility (with a toughness of 766%), while simultaneously possessing multiple functionalities, such as high transparency, excellent biocompatibility, and adhesion. Furthermore, the developed agarose/poly([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium -co-N- hydroxyethylacrylamide) (A/D-H) DN hydrogel exhibited impressive strain sensitivity with a strain gauge factor GF of 4.33 even under large deformations. Various forms of flexible sensors based on this printed gel were successfully utilized in human motion sensing and remote heartbeat monitoring, thereby opening up new perspectives for the further expansion of injectable materials.

Designing anti-dehydration and ion-conductive tough hydrogels as environment-adaptable strain sensors for e-skin

Designing anti-dehydration and ion-conductive tough hydrogels as environment-adaptable strain sensors for e-skin

Ultra-stretchable and conductive polyacrylamide/carboxymethyl chitosan composite hydrogels with low modulus and fast self-recoverability as flexible strain sensors

There is a great demand for the fabrication of soft electronics using hydrogels due to their biomimetic structures and good flexibility. However, conventional hydrogels have poor mechanical properties, which restricts their applications as stretchable sensors. Herein, a facile one-step strategy is proposed to fabricate tough and conductive hydrogels by making use of the graftability of carboxymethyl chitosan without extra conductive matter and crosslinking agent. The obtained polyacrylamide/carboxymethyl chitosan composite hydrogels possess outstanding transmittance and excellent mechanical performances, with tensile breaking stress of 630 kPa, breaking strain of 4560 %, toughness of 8490 kJ/m3. These hydrogels have low modulus of 5–20 kPa, fast recoverability after unloading, high conductivity of ∼0.85 S/m without the addition of other conductive substances and good biocompatibility. The ionic conductivity of the gels originates from the counterions of carboxymethyl chitosan, affording the hydrogels as resistive-type sensors. The resultant hydrogel sensors demonstrate a broad strain window (0.12–1500 %), excellent linear response, high sensitivity with the gauge factor reaching 11.72, and great durability, capable of monitoring diverse human motions. This work provides a new strategy to develop stretchable conductive hydrogels with promising applications in the fields of artificial intelligence and flexible electronics.

Enhancing hydrogel toughness by uniform cross-linking using modified polyhedral oligomeric silsesquioxane

The use of macro cross-linkers is one of the most effective approaches for developing tough hydrogels. However, the presence of uneven cross-linking and the resulting hydrogel inhomogeneity restrict further improvement. Here, we achieve uniform cross-linking by employing polyhedral oligomeric silsesquioxane (POSS)-grafted acrylated polyethylene glycol (PEG) as a cross-linker to enhance the toughness of hydrogels. The nano-sized hard silica core of POSS facilitates energy dissipation, and its dissolved form ensures uniform cross-linking through molecular-level dispersion. The peripheral acrylate groups introduce multiple interacting points, and the physical entanglements of long-chain PEG contribute to enhanced toughness. Incorporating acrylated POSS-PEG into polyacrylamide hydrogel yields enhanced properties such as toughness of up to 6531 kJ m−3 and break elongation up to 9455%, where the length of PEG chains grafted onto POSS is demonstrated to play a crucial role in facilitating energy dissipation and achieving high toughness.

Incorporation of Cell-Adhesive Proteins in 3D-Printed Lipoic Acid-Maleic Acid-Poly(Propylene Glycol)-Based Tough Gel Ink for Cell-Supportive Microenvironment.

In extrusion-based 3D printing, the use of synthetic polymeric hydrogels can facilitate fabrication of cellularized and implanted scaffolds with sufficient mechanical properties to maintain the structural integrity and physical stress within the in vivo conditions. However, synthetic hydrogels face challenges due to their poor properties of cellular adhesion, bioactivity, and biofunctionality. New compositions of hydrogel inks have been designed to address this limitation. A viscous poly(maleate-propylene oxide)-lipoate-poly(ethylene oxide) (MPLE) hydrogel is recently developed that shows high-resolution printability, drug-controlled release, excellent mechanical properties with adhesiveness, and biocompatibility. In this study, the authors demonstrate that the incorporation of cell-adhesive proteins like gelatin and albumin within the MPLE gel allows printing of biologically functional 3D scaffolds with rapid cell spreading (within 7 days) and high cell proliferation (twofold increase) as compared with MPLE gel only. Addition of proteins (10% w/v) supports the formation of interconnected cell clusters (≈1.6-fold increase in cell areas after 7-day) and spreading of cells in the printed scaffolds without additional growth factors. In in vivo studies, the protein-loaded scaffolds showed excellent biocompatibility and increased angiogenesis without inflammatory response after 4-week implantation in mice, thus demonstrating the promise to contribute to the printable tough hydrogel inks for tissue engineering.

Tough, recyclable and biocompatible carrageenan-modified polyvinyl alcohol ionic hydrogel with physical cross-linked for multimodal sensing

Biocompatibility hydrogel conductors are considered as sustainable bio-electronic materials for the application of wearable sensors and implantable devices. However, they mostly face the limitations of mismatched mechanical properties with skin tissue and the difficulty of recycling. In this regard, here, a biocompatible, tough, reusable sensor based on physical crosslinked polyvinyl alcohol (PVA) ionic hydrogel modified with ι-carrageenan (ι-CG) helical network was reported. Through simulating the ion transport and network structure of biological systems, the ionic hydrogels with skin-like mechanical features exhibit large tensile strain of 640 %, robust fracture strength of 800 kPa, soft modulus and high fatigue resistance. Meanwhile, the ionic hydrogel-based sensors possess a high response to strain/pressure over a wide range and could be utilized for multimodal sensing of human activity signals. Benefit from biosafety and temperature reversibility of ι-CG and PVA endow hydrogels with not only biocompatibility, but also meaningfully recyclability. The as-prepared hydrogels could be freely reconstructed into new flexible electronics and safely integrated with the human skin. It could be anticipated that the physically cross-linked ionic hydrogel conductor could expand the options for next-generation bio-based sensors.

Tough silk fibroin hydrogel via polypropylene glycol (PPG) blending for wearable sensors

AbstractRegenerated silk fibroin is a promising bio‐material because of its sustainability, biocompatibility, degradability, and eco‐friendliness. However, silk fibroin hydrogel's intrinsic hard and brittle mechanical properties hinder its application on human skin as a wearable sensor. We used polypropylene glycol (PPG) as a plasticizer to physically blend with a silk fibroin solution, and designed a set of green and environmentally friendly preparation methods without organic solvents to obtain the hydrogel for wearable sensors. PPG molecules will aggregate into clusters to act as a plasticizer. Meanwhile, a large number of hydrogen bonds are formed between the hydroxyl groups in PPG and silk fibroin molecular chains. When the hydrogel is stretched, the hydrogen bond in the silk fibroin chain will release heat in an irreversible form to increase the toughness. The PPG/silk fibroin hydrogel exhibits mechanical strength of 16.6 MPa, breaking elongation of 160%, and toughness of 17.7 MJ/m3. The Ca2+ introduced during the sample preparation process gives the hydrogel ionic conductivity, which provides the possibility of using the hydrogel as a human skin sensing material.

Polysaccharide-Based Double-Network Hydrogels: Polysaccharide Effect, Strengthening Mechanisms, and Applications.

Polysaccharides are carbohydrate polymers that are major components of plants, animals, and microorganisms, with unique properties. Biological hydrogels are polymeric networks that imbibe and retain large amounts of water and are the major components of living organisms. The mechanical properties of hydrogels are critical for their functionality and applications. Since synthetic polymeric double-network (DN) hydrogels possess unique network structures with high and tunable mechanical properties, many natural functional polysaccharides have attracted increased attention due to their rich and convenient sources, unique chemical structure and chain conformation, inherently desirable cytocompatibility, biodegradability and environmental friendliness, diverse bioactivities, and rheological properties, which rationally make them prominent constituents in designing various strong and tough polysaccharide-based DN hydrogels over the past ten years. This review focuses on the latest developments of polysaccharide-based DN hydrogels to comprehend the relationship among the polysaccharide properties, inner strengthening mechanisms, and applications. The aim of this review is to provide an insightful mechanical interpretation of the design strategy of novel polysaccharide-based DN hydrogels and their applications by introducing the correlation between performance and composition. The mechanical behavior of DN hydrogels and the roles of varieties of marine, microbial, plant, and animal polysaccharides are emphatically explained.

Time-dependent mechanical behavior of tough hydrogels under multiaxial stretching

ABSTRACT Understanding the time-dependent mechanical behavior of tough and viscoelastic hydrogels under complex external loading is crucial. In this study, we utilized tough and viscoelastic hydrogels synthesized through the copolymerization of methacrylic acid and methacrylamide as a model system to investigate their mechanical behavior under multiaxial stretching across a wide range of strain rates. Three stretching modes examined were uniaxial, pure shear, and equal biaxial stretching. Our findings show that under equal biaxial stretching, the hydrogels exhibit higher mechanical properties and energy dissipation compared to uniaxial and pure shear stretching, owing to the greater contribution of hydrogen bonds to energy dissipation in the former stretching mode. Additionally, employing the time-elongation separability method during the stretching process, we observed that the relaxation of dynamic hydrogen bonds in the hydrogels only depends on stretching time, independent of the elongation ratio and stretching modes. We anticipate that this study will yield valuable contributions to the design of durable load-bearing soft materials, particularly in dealing with complex deformation and strain rate responses.

Chiral design of tough spring-shaped hydrogels for smart umbrellas

Developing hydrogel artificial muscles to mimic the motion of natural muscles has long attracted scientists from the perspective of materials science for potential applications in soft robotics. However, rational design of hydrogel artificial muscles with large stroke, rapid actuation speed, and high work capacity remains a major challenge. Herein, we reported two kinds of chiral spring-shaped hydrogels that were prepared via consecutive shaping process (e.g., stretching, twisting, folding, coiling, and fixing). By switching the chirality of coil, homochiral muscle and heterochiral muscle were obtained, respectively. Homochiral muscle could rapidly expand to 560% with an average speed of 6.7 % s−1 in response to NIR irradiation, whose maximum work capacity reached 45 J kg−1. On contrary, heterochiral muscle contracted 69% within 1 min under NIR irradiation with a maximum work capacity of 33 J kg−1. Interestingly, the parasol containing homochiral muscles opened autonomously during dehydration process, while the umbrellas containing heterochiral muscle could opened rapidly when water was applied. This work provided an innovative strategy for developing tough hydrogel muscles with opposite chiralities.

A Mechanically Resilient and Tissue-Conformable Hydrogel with Hemostatic and Antibacterial Capabilities for Wound Care.

Hydrogels are used in wound dressings because of their tissue-like softness and biocompatibility. However, the clinical translation of hydrogels remains challenging because of their long-term stability, water swellability, and poor tissue adhesiveness. Here, tannic acid (TA) is introduced into a double network (DN) hydrogel consisting of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) to realize a tough, self-healable, nonswellable, conformally tissue-adhesive, hemostatic, and antibacterial hydrogel. The TA within the DN hydrogel forms a dynamic network, enabling rapid self-healing (within 5min) and offering effective energy dissipation for toughness and viscoelasticity. Furthermore, the hydrophobic moieties of TA provide a water-shielding effect, rendering the hydrogel nonswellable. A simple chemical modification to the hydrogel further strengthens its interfacial adhesion with tissues (shear strength of ≈31kPa). Interestingly, the TA also can serve as an effective hemostatic (blood-clotting index of 58.40±1.5) and antibacterial component, which are required for a successful wound dressing. The antibacterial effects of the hydrogel are tested against Escherichia coli and Staphylococcus aureus. Finally, the hydrogel is prepared in patch form and applied to a mouse model to test in vivo biocompatibility and hemostatic performances.

Nacre-inspired, strong, tough silk fibroin hydrogels based on biomineralization and the layer-by-layer assembly of ordered silk fabric

Hydrogels are attractive materials with structures and functional properties similar to biological tissues and widely used in biomedical engineering. However, traditional synthetic hydrogels possess poor mechanical strength, and their applications are limited. Herein, a multidimensional material design method is developed; it includes the in situ gelation of silk fabric and nacre-inspired layer-by-layer assembly, which is used to prepare silk fibroin (SF) hydrogels. The in situ gelation method of silk fabric introduces a directionally ordered fabric network in a silk substrate, considerably enhancing the strength of hydrogels. Based on the nacre structure, the layer-by-layer assembly method enables silk hydrogels to break through the size limit and increase the thickness, realizing the longitudinal extension of the hydrogels. The application of the combined biomineralization and hot pressing method can effectively reduce interface defects and improve the interaction between organic and inorganic interfaces. The multidimensional material design method helps increase the strength (287.78 MPa), toughness (18.43 MJ m−3), and fracture energy (50.58 kJ m−2) of SF hydrogels; these hydrogels can weigh 2000 times their own weight. Therefore, SF hydrogels designed using the aforementioned combined method can realize the combination of strength and toughness and be used in biological tissue engineering and structural materials.

Direct construction of strong, tough, conductive, and adhesive hydrogel bioelectronics enabled by salt-dissolved cellulose

Long-active mechanical toughness, conductivity, and adhesiveness are essential in the applications of hydrogel bioelectronics. Existing approaches usually integrate different functional materials into one system to endow hydrogels with desired properties, which tends to suffer from poor compatibility, cumbersome operations, and unsustainable processes, seriously limiting their real-world applications. Herein, we report a one-step strategy for directly constructing polyacrylic acid/cellulose (PAA/Cel) ionogels with a combination of strong, tough, conductive, and adhesive properties by adding ZnCl2-dissolved cellulose in a facile, efficient, and environmentally friendly manner. The introduction of salt-dissolved cellulose can simultaneously realize the all-round improvement of the ionogel in terms of mechanics, conductivity and adhesion. Particularly, for the PAA/Cel-20 % ionogel, it has a 3.3-fold higher compressive strength (18.8 MPa vs. 4.4 MPa), a 17.2-fold higher fracture toughness (154.0 kJ m−3 vs. 8.5 kJ m−3), a 5.5-fold higher interfacial toughness (247.3 J m−2 vs. 45.0 J m−2), a 4.0-fold higher shear strength (394.5 kPa vs. 78.6 kPa), and a 29.9-fold higher ionic conductivity (74.1 mS m−1 vs. 2.4 mS m−1) compared to the PAA hydrogel. These improved mechanical and adhesive behaviors are due to the synergistic enhancement effect of the compact hydrogen bonds formed between the carboxyl groups (–COOH) of PAA and the hydroxyl groups (–OH) of Cel, as well as the coordination interactions produced by Zn2+ and –COO− for PAA. The observed increase in ionic conductivity is attributed to the presence of abundant dissociated active ions including Zn2+ and Cl−. Our resulting ionogel enables electrophysiological signals to be reliably recorded with high signal-to-noise ratios when assembled as bioelectronics for use in monitoring body movements. The strategy presented here facilitates the development of high-performance adhesive ionogels and could go beyond for bioelectronics in medical monitoring systems.

A model with contact maps at both polymer chain and network scales for tough hydrogels with chain entanglement, hidden length and unconventional network topology

Tough hydrogels with unconventional polymer network architectures often show superior mechanical properties because of the microstructures at the polymer chain and network scales. In this study, a single-chain based model, or called the network topology with hidden length and entanglement (NeTHE) model for the tough hydrogels with chain entanglement, hidden length and unconventional network topology, is developed by imposing contact maps at both polymer chain and network scales on the Arruda-Boyce model. In particular, the contact map is imposed at the polymer chain scale to characterize both entanglement and hidden length simultaneously, while the contact map is imposed at the polymer network scale to consider different polymeric network topologies. The present NeTHE model is validated well by comparing published experimental results of a highly entangled hydrogel and two soft regular hydrogels with hidden lengths. It is confirmed that the present model is able to characterize the hardening, softening, swelling, and damage of the hydrogels. By simplifications, the NeTHE model is reduced directly to the pseudo-elasticity model [1]. Furthermore, the model is able to cover the exponential or scaling law type of damage behaviors of the hydrogels. Finally, several parameter studies are conducted for the swelling and hysteresis of the hydrogels. It is observed theoretically that the constraints of polymer chains due to entanglements result in the high swelling resistance of highly entangled hydrogels, while the hidden length facilitates the swelling of soft regular hydrogels. It is also found that, regarding the hysteresis of hydrogels with different polymer network topologies, the ideal polymer network possesses the highest damage tolerance when compared with both the random and high-functionality crosslinked polymer networks. Regarding the effects of entanglements and hidden lengths on the hysteresis, the highly entangled hydrogel shows a lower stress–stretch hysteresis due to the energy dissipated partially to harden polymer chains when compared with the soft regular hydrogel of which the energy is dissipated fully to fracture the polymer chains.

Tough, Stretchable, and Thermoresponsive Smart Hydrogels.

Self-healing, thermoresponsive hydrogels with a triple network (TN) were obtained by copolymerizing N-isopropyl acryl amide (NiPAAm) with polyvinyl alkohol (PVA) functionalized with methacrylic acid and N,N'-methylene bis(acryl amide) crosslinker in the presence of low amounts (<1 wt.%) of tannic acid (TA). The final gels were obtained by crystalizing the PVA in a freeze-thaw procedure. XRD, DCS, and SEM imaging indicate that the crystallinity is lower and the size of the PVA crystals is smaller at higher TA concentrations. A gel with 0.5 wt.% TA has an elongation at a break of 880% at a tension of 1.39 MPa. It has the best self-healing efficiency of 81% after cutting and losing the chemical network. Step-sweep strain experiments show that the gel has thixotropic properties, which are related to the TA/PVA part of the triple network. The low amount of TA leaves the gel with good thermal responsiveness (equilibrium swelling ratio of 13.3). Swelling-deswelling loop tests show enhanced dimensional robustness of the hydrogel, with a substantial constancy after two cycles.

A Transparent, Tough and Self-Healable Biopolymeric Composites Hydrogel for Open Wound Management.

Modern healthcare engineering requires a wound dressing solution supported by materials with outstanding features such as high biological compatibility, strong mechanical strength, and higher transparency with effective antibacterial properties. Here, we present a unique hydrogel technology consisting of two negatively charged biopolymers and a positively charged synthetic polymer. The interaction between charged polymers through hydrogen bonds has been created, which are revealed in the simulation by density functional theory and Fourier transform infrared spectra of individual polymers and the hydrogel film. The transparent hydrogel film dressings showed excellent stretchability, a higher water swelling ratio (60%), and strong mechanical strength (∼100 MPa) with self-healing abilities (85-90%). The fabricated hydrogel film showed stable blood clots (within 119 ± 15 s) with rapid hemostasis (<2%) properties and effective antibacterial studies against E. coli and S. aureus bacterial strains. In addition, the obtained hydrogel film also showed excellent cell viability on mouse fibroblast cells. With their enormous amenability to modification, these hydrogel films may serve as promising biomaterials for wound dressing applications.

Chain folding double-network hydrogels leads to ultra-strong, stretchable and tough sensors

Strong, stretchable and tough sensors are in great demand but challenging. Here, we present a simple and generable strategy toward fabricating high-performance sensors by folding the polymer chains of the ionic double-network (DN) hydrogels via dehydration in poor solvent of ethanol. The combination of chain folding and DN network dramatically improved stress buffering and energy dissipation capacity of hydrogels during the unfolding/stretching process. The as-prepared hydrogels exhibited ultra-high strength (1.06 MPa), stretchability (15,400%) and toughness (88 MJ m−3), representing the most tough hydrogels reported as far as we know. By taking these advantages, the hydrogels were demonstrated as strain sensors with extremely broad work window (0.05–8000%). This chain folding DN concept provides an attractive strategy to improve the whole mechanical properties of hydrogels to new limits and expand the scope of applications.

Tough Ion-Conductive Hydrogel with Anti-Dehydration as a Stretchable Strain Sensor for Gesture Recognition

Tough Ion-Conductive Hydrogel with Anti-Dehydration as a Stretchable Strain Sensor for Gesture Recognition

Water-Induced Cellulose Nanofibers/Poly(vinyl alcohol) Hydrogels Regulated by Hydrogen Bonding for In Situ Water Shutoff.

Hydrogels have been widely applied to the water shutoff in oilfields due to their excellent three-dimensional network and thermal and physicochemical stability, and it is still a huge challenge to develop new hydrogels with simple preparation, low cost, and high mechanical performance that can meet the requirements of practical applications. Herein, we devised a simple and universal manufacturing method for regulating the hydrogen bonds between poly(vinyl alcohol) (PVA) and cellulose nanofibers (CNF) via the water-diffusion action, thus fabricating a physically tough PVA-CNF hydrogel for the in situ water shutoff. This method allowed the polymer chains to strengthen the molecular interactions between polymers upon replacing with water (a poor solvent) to regulate the cross-linking structure, characterizing by the nano-crystallinity domains and fibrillar segments, which also accounted for the thermal stability, extraordinary elasticity, high stretchability, and toughness of PVA-CNF hydrogel. Further, the obtained PVA-CNF hydrogel exhibited superb plugging performance, that is, the breakthrough pressure gradient could reach 71.56 MPa·m-1, surpassing all currently reported gelling water shutoff agents. This water-induced in situ hydrogelation made it well suited as a water shutoff agent in oilfields and may provide a promising strategy to fabricate mechanically robust smart materials for the water shutoff projects with low cost, simple processing, and high efficiency.

A self-reinforced tough and multifunctional polyvinyl alcohol fabric composite hydrogel

It is particularly challenging to prepare hydrogels with excellent mechanical properties that are comparable to those of strong biological tissues such as ligaments and tendons. Here, we present a facile method to fabricate tough and multifunctional hydrogels by freeze-thawing the self-reinforced polyvinyl alcohol (PVA) composite prepared from acetal hydrolysis of commercial polyvinyl formal (PVFM) fabric. The resultant hydrogels exhibit simultaneously high tensile strength (18.5 MPa), toughness (14.7 ± 2.9 MJ/m3), tearing resistance (35 N/mm), flexibility and excellent environment adaptability without compromising transparency. In addition, this hydrogel has good ionic conductivity, which can be applied to strain sensors and touch screens, and with proper design, it is successfully applied in flexible actuators. These results indicate this hydrogel has great potential for applications in different fields such as smart wearable devices and bionic actuators. The preparation method proposed in this paper is simple and controllable, and the material properties can be customized to meet specific needs, opening up possibilities for large-scale production of hydogels.