Tough Hydrogels Research Articles

Strong and tough fibrous hydrogels reinforced by multiscale hierarchical structures with multimechanisms.

Tough natural materials such as nacre, bone, and silk exhibit multiscale hierarchical structures where distinct toughening mechanisms occur at each level of the hierarchy, ranging from molecular uncoiling to microscale fibrillar sliding to macroscale crack deflection. An open question is whether and how the multiscale design motifs of natural materials can be translated to the development of next-generation biomimetic hydrogels. To address this challenge, we fabricate strong and tough hydrogel with architected multiscale hierarchical structures using a freeze-casting-assisted solution substitution strategy. The underlying multiscale multimechanisms are attributed to the gel's hierarchical structures, including microscale anisotropic honeycomb-structured fiber walls and matrix, with a modulus of 8.96 and 0.73MPa, respectively; hydrogen bond-enhanced fibers with nanocrystalline domains; and cross-linked strong polyvinyl alcohol chains with chain-connecting ionic bonds. This study establishes a blueprint of structure-performance mechanisms in tough hierarchically structured hydrogels and can inspire advanced design strategies for other promising hierarchical materials.

Carbon Nanotubes and Silica@polyaniline Core-Shell Particles Synergistically Enhance the Toughness and Electrical Conductivity in Hydrophobic Associated Hydrogels.

Soft, conductive, and stretchable sensors are highly desirable in many applications, including artificial skin, biomonitoring patches, and so on. Recently, a combination of good electrical and mechanical properties was regarded as the most important evaluation criterion for judging whether hydrogel sensors are suitable for practical applications. Herein, we demonstrate a novel carboxylated carbon nanotube (MWCNT-COOH)-embedded P(AM/LMA)/SiO2@PANI hydrogel. The hydrogel benefits from a double-network structure (hydrogen bond cross-linking and hydrophobic connectivity network) due to the role of MWCNT-COOH and SiO2@PANI as cross-linkers, thus resulting in tough composite hydrogels. The obtained P(AM/LMA)/SiO2@PANI/MWCNT-COOH hydrogels exhibited high tensile strength (1939 kPa), super stretchability (3948.37%), and excellent strain sensitivity (gauge factor = 11.566 at 100-1100% strain). Obviously, MWCNT-COOH not only improved the electrical conductivity but also enhanced the mechanical properties of the hydrogel. Therefore, the integration of MWCNT-COOH and SiO2@PANI-based hydrogel strain sensors will display broad application in sophisticated intelligence, soft robotics, bionic prosthetics, personal health care, and other fields using inexpensive, green, and easily available biomass.

Bionic artificial penile Tunica albuginea

Bionic artificial penile Tunica albuginea

Sensitive piezoresistive pressure sensor based on micropyramid patterned tough hydrogel

Microstructures are decisive factors of hydrogel-based piezoresistive pressure sensors for excellent performances in flexible electronics. However, the desirable microstructures are still difficult to be completely engineered on the surface of hydrogel, owing to limitations concerning to comprehensive mechanical performances of hydrogels. Herein, we prepared a type of piezoresistive pressure sensor by decorating pyramid microarrays on the surface of a tough and ionically conductive polyacrylamide (PAAm)/poly(vinyl alcohol) (PVA) hydrogel. To achieve high sensing behaviors, sizes and topologies of the recessed pyramid microarrays molded by three-dimensional (3D) printing photosensitive resin were investigated for elaborate fabrication of pyramid microarray hydrogel. The ionically conductive PAAm/PVA hydrogel of excellent comprehensive mechanical performances decorated with pyramid microarrays was able to generate large contact resistance variation to respond to external pressure, achieving a maximum sensitivity to 2.27 kPa−1, low detection limit of 9.0 Pa, as well as good recoverability and stability. Such tough and sensitive device based on micropyramid patterned tough hydrogel have potential applications in the fields of flexible electronics.

A universal strategy for preparing tough and smart glassy hydrogels

With a large amount of water as plasticizer, most current hydrogels stay in rubbery states with low rigidity. Inspired by the robust thermoplastics, herein we propose a facile, effective, and universal strategy for preparing tough and smart “glassy hydrogels” by maintaining the hydrogels’ network in glassy states. The key point is to integrate an appropriate amount of rigid hydrophobic polymers within hydrogels’ hydrophilic networks. Based on the composition optimization, we further establish a generally applicable criterion for material preparation: the hydrogels should possess a thermo-softening temperature slightly higher than the service condition. Following this principle, various glassy hydrogels have been readily fabricated, which demonstrate excellent rigidity (∼50.5 MPa), strength (∼8.46 MPa), extensibility (∼2200%), and toughness (∼35.2 MJ.m−3). More interestingly, the hydrogels demonstrate broad-range tunable mechanical performance in response to temperature and additives. Upon heating, the hydrogels manifest evident thermo-softening performance with the stiffness dramatically dropping more than 1000 times, endowing the hydrogels with desirable shape memory properties. When encountering various small-molecules, the hydrogels show over 3 orders’ broad-range tunable stiffness. Collectively, the ease of manipulation and universality of the as-established method, we hope, can become a widely used strategy to inspire novel designs and substantially broaden the application potentials of gels.

Tough, Transparent, and Slippery PVA Hydrogel Led by Syneresis.

Slippery and transparent polyvinyl alcohol (PVA) hydrogels with mechanical robustness exhibit broad applications in artificial biological soft tissues, flexible wearable electronics, and implantable biomedical devices. Most of the current PVA hydrogels, however, are unable to integrate these features, which compromises its performance in biological and engineering applications. To achieve such purpose, herein, a novel tactic is proposed, salting-out-after-syneresis of PVA, to realize a mechanically robust and highly transparent slippery PVA hydrogel. The syneresis of PVA sol is first conducted to form highly dense and transparent PVA polymer networks, then the salting-out effect tunes the aggregation of the polymer chains to rapidly induce the phase separation and crystallization. The resultant hydrogels show the transparency up to 98% in the visible region, the tribological coefficient down to 0.0081, and the excellent mechanical properties with strength, modulus, and toughness of 26.72±1.05, 6.66±0.29MPa, and 55.21±1.62MJm-3 , respectively. To reveal the potentials, PVA contact lens that combine remarkable lubrication, anti-protein adhesion, biocompatibility, and drug-loading functions are demonstrated. This strategy provides a simple and new avenue for developing the mechanically robust, transparent, and hydrated hydrogels, showing the potential in biomedicine and wearable devices.

Controlling the mechanical properties of hydrogels via modulating the side-chain length.

Even though the toughness of hydrogels is usually adjusted by changing the cross-linking density and structure, or the polymer concentration, we have discovered a new strategy to control the toughness via modulating the side-chain length. In this study, this strategy was applied to biocompatible poly(oligo(ethylene glycol) methyl ether methacrylate) with long ethylene-oxide side chains.

Tough, anti-drying and thermoplastic hydrogels consisting of biofriendly resources for a wide linear range and fast response strain sensor

Tough, anti-drying, thermoplastic, and biodegradable hydrogels was developed and successfully applied as a wide linear range and fast response strain sensor.

Strong and tough conductive PVA hydrogels based on the synergistic effect of acetic acid induction and salting-out for flexible solid-state supercapacitors

A synergistic strategy of acetic acid induction and salting-out is proposed for optimizing the microstructure, mechanical properties, and ionic conductivity, thus constructing strong and tough conductive PVA hydrogels for flexible supercapacitors.

Strain sensing multi-stimuli responsive light emitting lanthanide-based tough and stretchable hydrogels with tunable luminescence and fast self-recovery using metal–ligand and hydrophobic interactions

Multi-stimuli responsive lanthanide based tough luminescent hydrogels were engineered using hydrophobic and metal–ligand interactions.

Ultra-stiff and tough hydrogels based on small but strong hydrophobic associations via a low-reactive hydrophilic monomer

Strong hydrophobic associations were formed between hydrophobic moiety-protected carbamate side groups of monomers, whose density and strength were regulated by using a low-reactive hydrophilic monomer to prepare ultra-stiff and tough hydrogels.

Tough hydrogel with high water content and ordered fibrous structures as an artificial human ligament.

Natural biological tissues such as ligaments, due to their anisotropic across scale structure, have high water content, while still maintaining high strength and flexibility. Hydrogels are ideal artificial materials like human ligaments. However, conventional gel materials fail to exhibit high strength or fatigue resistance at high water content in human tissues. To address this challenge, we propose a simple integrated strategy to prepare an anisotropic hierarchical hydrogel architecture for artificial ligaments by combining freeze-casting assisted compression annealing and salting-out treatments. The hybrid polyvinyl alcohol hydrogels are of water content up to 79.5 wt%. Enhanced by the added carbon nanotubes, the hydrogels exhibit high strength of 4.5 MPa and a fatigue threshold of 1467 J m-2, as well as excellent stress sensitivity. The outstanding durability of the artificial ligament provides an all-around solution for biomedical applications.

Mechanical tough and multicolor aggregation-induced emissive polymeric hydrogels for fluorescent patterning.

Aggregation-induced emissive fluorogens (AIEgens) are promising building blocks for fluorescent polymeric hydrogels (FPHs) because intense fluorescence intensities are usually guaranteed by spontaneous aggregates of hydrophobic AIEgens in a hydrophilic polymer network. However, most AIE-active FPHs are single-color fluorescent and cannot display tunable emission colors. Additionally, efforts to produce mechanically strong AIE-active hydrogels have been largely ignored, restricting their potential uses. Herein, we present the synthesis of an AIE-active methyl picolinate-substituted 1,8-naphthalimide monomer (MP-NI) for fabricating mechanical tough and multicolor FPHs. Owing to the introduction of bulky and coordinative methyl picolinate group, these specially designed MP-NI molecules were forced to adopt propeller-shaped conformation that renders them with intense aggregation-induced blue emission. Moreover, the MP-NI moieties grafted in a hydrogel matrix can sensitize red and green fluorescence of Eu3+and Tb3+ via antenna effect. Consequently, multicolor fluorescent hydrogels that sustain a high stress of 1 MPa were obtained by chemically introducing MP-NI moieties into dually cross-linked alginate polymer networks with high-density metal (Ca2+/Tb3+/Eu3+) coordination and hydrogen bonding crosslinks. Their capacity to enable the writing of arbitrary multicolor fluorescent patterns using Eu3+/Tb3+ as inks were finally demonstrated, suggesting their potential uses for smart display and information encryption.

Modulation of double-network hydrogels via seeding calcium carbonate microparticles for the engineering of ultrasensitive wearable sensors

Stretchable and tough double-network hydrogels mediated by CaCO3 microparticles can be used to monitor human motions and mild pain signals at the wound site.

Cartilage-bioinspired, tough and lubricated hydrogel based on nanocomposite enhancement effect.

The maintenance of high load-bearing tissues and joint lubrication is essential for suppressing osteoarthritis. The lubrication of natural joints is mainly attributed to the hydration lubrication mechanism of articular cartilage. Phospholipids on the cartilage surface attract water molecules to form a tough hydrated layer to reduce friction. In this work, inspired by the phosphatidylcholine lipids, we synthesized lubricated nanospheres by grafting hydrophilic polymer brushes and further synthesized a nanocomposite hydrogel. The addition of the lubricated nanospheres enhanced both the mechanical and lubricated properties of the hydrogel. The nanocomposite-lubricated hydrogel exhibited a friction coefficient 81.7% lower than the blank hydrogel because of grafting the polymer brushes. Also, the nanocomposite enhancement helped the hydrogel achieve high mechanical properties with a compressive strength of 6.63 MPa (50%). The nanocomposite hydrogel developed here could be a promising candidate material in bionic articular cartilage substitute materials.

A sweat-pH-enabled strongly adhesive hydrogel for self-powered e-skin applications.

On-skin hydrogel electrodes are poorly conformable in sweaty scenarios due to low electrode-skin adhesion resulting from the sweat film formed on the skin surface, which seriously hinders practical applications. In this study, we fabricated a tough adhesive cellulose-nanofibril/poly(acrylic acid) (CNF/PAA) hydrogel with tight hydrogen-bond (H-bond) networks based on a common monomer and a biomass resource. Furthermore, inherent H-bonded network structures can be disrupted through judicious engineering using excess hydronium ions produced through sweating, which facilitate the transition to protonation and modulate the release of active groups (i.e., hydroxyl and carboxyl groups) accompanied by a pH drop. The lower pH enhances adhesive performance, especially on skin, with a 9.7-fold higher interfacial toughness (453.47 vs. 46.74 J m-2), an 8.6-fold higher shear strength (600.14 vs. 69.71 kPa), and a 10.4-fold higher tensile strength (556.44 vs. 53.67 kPa) observed at pH 4.5 compared to the corresponding values at pH 7.5. Our prepared hydrogel electrode remains conformable on sweaty skin when assembled as a self-powered electronic skin (e-skin) and enables electrophysiological signals to be reliably collected with high signal-to-noise ratios when exercising. The strategy presented here promotes the design of high-performance adhesive hydrogels that may serve to record continuous electrophysiological signals under real-life conditions (beyond sweating) for various intelligent monitoring systems.

Highly stretchable, injectable hydrogels with cyclic endurance and shape-stability in dynamic mechanical environments, by microunit reformation.

Traditional injectable hydrogels have so far found it difficult to accommodate resistance to large deformation and shape-stability under cyclic deformation. Polyampholyte (PA) hydrogels exhibit resistance to large deformation, good fatigue-resistance and rapid self-healing under dynamic forces. The limitations of the preparation process result in non-injectability of polyampholyte (PA) hydrogels. Electrostatic interactions as a medium for resistance to large deformation and shape-stability after cyclic deformation in reformed injectable hydrogels has been explored in this study. The prepared hydrogels (as-prepared PA-N) were dried and smashed into microunits and then mixed with 0.9% NaCl solution to transform them into reformed hydrogels (as-reformed PA-N) via a needle to achieve injectability. The as-reformed PA-N could exhibit 913.6% elongation at break and showed shape-stability under cyclic deformation due to the efficient self-healing abilities of the microunits and the inherited structure of the prepared hydrogels, which are superior to those of current tough injectable hydrogels. Potential applications in elbow cyclic bending and frequent movement of mobile wounds have been proved in this study. Overall, the results showed that the as-reformed PA-N achieved convenient injectability with resistance to large deformation and shape-stability under cyclic deformation at the same time.

Tough hydrogel–elastomer hybrids hydrophobically regulated by an MXene for motion monitoring in harsh environments

The introduction of MXene improves hydrophobic properties and enhance the hydrophobic encapsulation effect of a hydrogel. It can effectively prevent the influence of external moisture on the hydrogel, enabling outstanding sensing stability performance in harsh environments.

3D printed microstructured ultra-sensitive pressure sensors based on microgel-reinforced double network hydrogels for biomechanical applications.

Hydrogel-based wearable flexible pressure sensors have great promise in human health and motion monitoring. However, it remains a great challenge to significantly improve the toughness, sensitivity and stability of hydrogel sensors. Here, we demonstrate the fabrication of hierarchically structured hydrogel sensors by 3D printing microgel-reinforced double network (MRDN) hydrogels to achieve both very high sensitivity and mechanical toughness. Polyelectrolyte microgels are used as building blocks, which are interpenetrated with a second network, to construct super tough hydrogels. The obtained hydrogels show a tensile strength of 1.61 MPa, and a fracture toughness of 5.08 MJ m-3 with high water content. The MRDN hydrogel precursors exhibit reversible gel-sol transitions, and serve as ideal inks for 3D printing microstructured sensor arrays with high fidelity and precision. The microstructured hydrogel sensors show an ultra-high sensitivity of 0.925 kPa-1, more than 50 times that of plain hydrogel sensors. The hydrogel sensors are assembled as an array onto a shoe-pad to monitor foot biomechanics during gaiting. Moreover, a sensor array with a well-arranged spatial distribution of sensor pixels with different microstructures and sensitivities is fabricated to track the trajectory of a crawling tortoise. Such hydrogel sensors have promising application in flexible wearable electronic devices.

Engineering viscoelastic mismatch for temporal morphing of tough supramolecular hydrogels.

Viscoelasticity is a generic characteristic of soft biotissues and polymeric materials, endowing them with unique time- and rate-dependent properties. Here, by spatiotemporally tailoring the viscoelasticity in tough supramolecular hydrogels, we demonstrate reprogrammable morphing of the gels based on differential viscoelastic recovery processes that lead to internal strain mismatch. The spatial heterogeneity of viscoelasticity is encoded through integrating dissimilar hydrogels or by site-specific treatment of a singular hydrogel. The temporal morphing behavior of tough gels, including a fast deformation process and then a slow shape-recovery process, is related to the kinetics of associative interactions and the entropic elasticity of supramolecular networks after pre-stretching and release, which takes place spontaneously in the absence of external stimuli. Such a kinetically driven morphing mechanism resolves the trade-off between the mechanical robustness and shape-changing speed in tough hydrogels with dense entanglements and physical associations, and should be applicable to other viscoelastic materials. A numerical theory for the temporal morphing of tough supramolecular gels has been formulated by dynamic coupling of viscoelastic recovery and mechanics of deformations, which is further implemented to predict the sophisticated morphed structures. Furthermore, magnetic particles are incorporated into the morphed tough hydrogels to devise versatile soft actuators and robots for specific applications.