Strong Hydrogels Research Articles

MoO3-x quantum dots-based hydrogel with excellent light-triggered self-healing efficiency and pressure sensitive photoluminescence for accurate remote force measurement

Multi-responsive functional materials with simultaneous photothermal and photoluminescent properties have broad applicational prospects in the field of flexible electronics, devices and remote force detection and monitor. Here, a feasible method is developed by introducing MoO3-x quantum dots (MoO3-x QDs) and blue fluorescent carbon dots (B-CDs) into a strong hydrogel matrix to fabricate a novel composite hydrogel with concurrent photothermal and photoluminescent properties. Besides rendering the peculiar multifunctionalities, we found that the addition of photothermal MoO3-x QDs enhances the mechanical properties and self-healing properties of the composite hydrogel. The temperature of the MoO3-x-CDs-PVA (Polyvinyl Alcohol) hydrogel can rise by 30 °C within 1 min after 808 nm infrared laser irradiation and the self-healing efficiency could double after 40 s of irradiation. Moreover, there is a good linear relationship between the fluorescence intensity of the composite hydrogel and the external force, which can be used to monitor the force within a certain range. The monitoring range and sensitivity can be further improved by adjusting the infrared laser to for small force detection. Finally, the novel composite hydrogel is successfully applied to fracture monitoring in fracturing and force monitoring at different locations in the fluid model in the field of petroleum engineering.

Super solvent of cellulose with extra high solubility for tunable cellulose structure with versatile application

Low-temperature two-step concentrated H2SO4 was discovered to be a solvent with high cellulose solubility [>300 g/L (17 wt%)], fast cellulose dissolution, high regeneration yield (>0.92 g/g), and cellulose being mouldable during regeneration. The superior performance was enabled by the much better compatibility of cellulose with lower concentrated H2SO4 at low temperature, compared with that of high concentrated H2SO4. The regenerated cellulose was characterized by mostly unchanged composition and highly tunable degree of polymerization (DP). The H2SO4 starting content, cotton fibre temperature, dissolution temperature, regeneration temperature, regeneration bath and storage time were factors determining the DP of regenerated cellulose, which could be equivalent to 4–90 % of the original cotton. These advantages of the solvent enabled versatile application in fabrication of extra strong cellulose hydrogels, manufacture of strong cellulose fibres, preparation of various homogenous composites which would be prepared with much more difficulty by using other solvents, and facile manufacture of cellooligosaccharides.

Stability of hydrogel adhesion enabled by siloxane bonds

Forming anchoring sites by siloxane bonds at the hydrogel-substrate bonding interface is one of prevailing ways to achieve strong hydrogel adhesion. Considering the hydrolysis of siloxane bonds at the existence of water, we investigate the stability of siloxane bonds enabled hydrogel adhesion in various aqueous environments experimentally and theoretically. Samples of polyacrylamide (PAAm) hydrogel bonded glass substrate by siloxane bonds are prepared and immersed in various aqueous environments, including purified water, acid solution, alkaline solution, and phosphate buffered solution (PBS). We find that the PAAm hydrogel spontaneously detaches from the glass substrate in the alkaline solution for around 1 h and in the PBS for 36 days, but still firmly adheres to the glass substrate in the purified water and acid solution for 36 days. To explain the experimental findings, we incorporate the kinetic theory of bond dissociation with the Lake-Thomas model and get the evolution of hydrogel adhesion with time. By changing the activation energy of siloxane bond hydrolysis, the time required for the catastrophic decrease of hydrogel adhesion can be predicted. This work reveals the failure mechanism of siloxane bonds enabled strong hydrogel adhesion, and might inspire studies on the stability of hydrogel adhesion of other types under the combined effects of mechanical and chemical loads.

Synergistic Reinforcement of Cellulose Microfibers from Pineapple Leaf and Ionic Cross-Linking on the Properties of Hydrogels.

Hydrogels contain a large amount of water; thus, they are jelly-like, soft, and fragile. Although hydrogels’ stiffness and strength can be improved by introducing another network to form a double or interpenetrating network, these mechanical properties are still not enough as many applications demand even stiffer and stronger hydrogels. Different methods of reinforcing hydrogels have been proposed and published. In this research, cellulose microfiber isolated from pineapple leaf was used as the reinforcement for hydrogels. The reinforcing efficiency of the fiber was studied for both single and double networks through the compression test. Other properties such as morphology and swelling behavior of the reinforced hydrogels were also studied. A synergistic effect of the second network and the fiber on the reinforcement was observed. The improvement due to the effect of fiber loading of only 0.6 wt % was found to be as high as 150%. This is greater than that observed in some nanofiller systems. Thus, the fiber can be used as a green reinforcement for similar hydrogel systems.

Strong Interfaces Enable Efficient Load Transfer for Strong, Tough, and Impact-Resistant Hydrogel Composites.

Many biological hydrogels are mechanically robust to bear quasi-static and impact loads. In contrast, the mechanical properties of synthetic hydrogels against impact loads remain substantially unexplored, albeit their mechanical robustness under quasi-static loads has been extensively developed. Here, we report on the design and synthesis of strong, tough, and impact-resistant hydrogel composites by reinforcing Ca-alginate/polyacrylamide hydrogels with glass fabrics and conferring strong interfaces between the hydrogel matrix and the fibers. The fabric enables high elastic modulus, the hydrogel matrix enables large dissipation, and the strong interfaces enable efficient load transfer for synergistic strengthening and toughening, which is manifested by digital image correlation analyses. Under quasi-static loads, the hydrogel composite exhibits an elastic modulus of 35 MPa and a toughness of 206.7 kJ/m2. Under impact loads, a piece of 7.7 g sample bears the impact of energy of 7.4 J and resists more than 100 cycles of consecutive impact of 600 mJ. As a proof-of-concept, a hydrogel composite as a safeguard to protect fragile glasses from impact is demonstrated. Because impact phenomena are universal, it is expected that the study on the impact of hydrogels will draw increasing attention.

A strong, ultrastretchable, antifreezing and high sensitive strain sensor based on ionic conductive fiber reinforced organohydrogel

Strain sensors based on ionic conductive hydrogels are highly appreciated for their intrinsic biocompatibility, flexibility as well as soft characteristics. However, simultaneous realization of excellent mechanical properties, ionic conductivity, and low-temperature tolerance remains a grand challenge. Herein, we developed a novel fiber reinforced organohydrogel through facilely combining the polyacrylamide grafted gelatin (PAM/GE) hydrogel with polyurethane (PU) electrospun fiber membrane in glycerol-water binary solvent system. The fiber reinforced organohydrogel exhibited intriguing integrated characteristics, including outstanding mechanical performance (stress up to 3.09 MPa, strain up to 614%), freeze tolerance (−40 °C) and high ionic conductive (1.51 S m−1) at room temperature, attributed to the synergistic effects of PU electrospun fiber membrane and PAM/GE hydrogel. It is worth mentioning that, the presence of glycerol and NaCl endowed the fiber reinforced organohydrogel with outstanding ionic conductivity (0.89 S m−1) even at −40 °C, allowing for practical applications in harsh conditions. When exploited as a wearable strain sensor, the as-prepared fiber reinforced organohydrogel showed excellent sensitivity, stability and repeatability for the detection of human motions. This work provides new insights into design of strong, stretchable, and freezing-tolerant ionic conductive hydrogels, paving the way to developing multifunctional sensors.

Progress in the mechanical enhancement of hydrogels: Fabrication strategies and underlying mechanisms

AbstractHydrogels have become ideal materials in the nascent applications of tissue engineering, soft robots, drug delivery, and so forth. However, compared with biological tissues, the inherent heterogeneous microstructure and low density of polymer chains make hydrogels mechanically weak, severely limiting their use as structural materials. In recent decades, in order to meet the mechanical requirements of load‐bearing biomaterials, significant research effort have been devoted to improving the mechanical parameters of hydrogels. To achieve this goal, fiber/fabric reinforced hydrogels, double network hydrogels, supramolecular‐interaction‐based hydrogels, hydrogels with well‐aligned microstructures, and solvent induced robust hydrogels have been investigated. In this review, the fabrication strategies, the relationships between the structure and the mechanical properties of the resulting hydrogel, and the underlying enhancement mechanisms in various classes of hydrogel have been summarized. Here, the mechanisms behind these strategies rely on creating the mechanically effective networks, which are achieved by introducing a rigidly reinforced phase, synergistic network with distinctive features, sacrificial bonds, oriented hierarchical structures, or increased supramolecular interactions. Despite significant achievements toward strong and tough hydrogels, considerable important challenges remain, such as simultaneously achieving high strength, toughness, and high water content of hydrogels. It is believed that the already proposed strategies will push the development of hydrogels.

Mechanically strong and on-demand dissoluble chitosan hydrogels for wound dressing applications

Physical hydrogels of pure chitosan preserve the natural properties of chitosan, and therefore are ideal candidates for wound dressing applications, however, their poor mechanical strength severely limits their application. In addition, chitosan dressings tend to adhere to the wounds, making their change difficult. Here physical hydrogels of pure chitosan were prepared by first dissolving chitosan in an alkaline aqueous solution, followed by thermal gelling and solvent change. The gels exhibited better mechanical properties than the gels prepared previously. They were soluble in diluted acetic acid, making them on-demand dissoluble. When used as wound dressing and changed by mechanical debridement, the gels achieved a faster wound closure than gauze. An even faster wound closure was achieved when they were changed by dissolution with acetic acid solution. Besides a faster healing, the on-demand dissoluble dressing could also reduce the formation of scars and reestablish the normal anatomy and function of the skin.

Strong hydrogel adhesive comes painlessly off sensitive skin

Strong hydrogel adhesive comes painlessly off sensitive skin

A skin-inspired biomimetic strategy to fabricate cellulose enhanced antibacterial hydrogels as strain sensors

With the development of wearable devices, the fabrication of strong, tough, antibacterial, and conductive hydrogels for sensor applications is necessary but remains challenging. Here, a skin-inspired biomimetic strategy integrated with in-situ reduction has been proposed. The self-assembly of cellulose to generate a cellulose skeleton was essential to realize the biomimetic structural design. Furthermore, in-situ generation of silver nanoparticles on the skeleton was easily achieved by a heating process. This process not only offered the excellent antibacterial property to hydrogels, but also improved the mechanical properties of hydrogels due to the elimination of negative effect of silver nanoparticles aggregation. The highest tensile strength and toughness could reach 2.0 MPa and 11.95 MJ/m3, respectively. Moreover, a high detection range (up to 1300%) and sensitivity (gauge factor = 4.4) was observed as the strain sensors. This study provides a new horizon to fabricate strong, tough and functional hydrogels for various applications in the future.

Strong, tough, ionic conductive, and freezing-tolerant all-natural hydrogel enabled by cellulose-bentonite coordination interactions

Ionic conductive hydrogels prepared from naturally abundant cellulose are ideal candidates for constructing flexible electronics from the perspective of commercialization and environmental sustainability. However, cellulosic hydrogels featuring both high mechanical strength and ionic conductivity remain extremely challenging to achieve because the ionic charge carriers tend to destroy the hydrogen-bonding network among cellulose. Here we propose a supramolecular engineering strategy to boost the mechanical performance and ionic conductivity of cellulosic hydrogels by incorporating bentonite (BT) via the strong cellulose-BT coordination interaction and the ion regulation capability of the nanoconfined cellulose-BT intercalated nanostructure. A strong (compressive strength up to 3.2 MPa), tough (fracture energy up to 0.45 MJ m−3), yet highly ionic conductive and freezing tolerant (high ionic conductivities of 89.9 and 25.8 mS cm−1 at 25 and −20 °C, respectively) all-natural cellulose-BT hydrogel is successfully realized. These findings open up new perspectives for the design of cellulosic hydrogels and beyond.

Energy Landscape of the Sugar Conformation Controls the Sol-to-Gel Transition in Self-Assembled Bola Glycolipid Hydrogels

Self-assembled fibrillar network hydrogels and organogels are commonly obtained through a crystallization process as fibers upon being induced by external stimuli such as temperature or pH. The gel-to-sol-to-gel transition is generally readily reversible, and the rate of change of the stimulus determines the fiber homogeneity and eventual elastic properties of the gels. However, recent work shows that in some specific cases, fibrillation occurs for a given molecular conformation and the sol-to-gel transition depends on the relative energetic stability of one conformation over the other and not on the rate of change of the stimuli. We observe such a phenomenon on a class of bolaform glycolipids, sophorosides, similar to the well-known sophorolipid biosurfactants but composed of two symmetric sophorose units. A combination of oscillatory rheology, small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy, and in situ rheology coupled with SAXS using synchrotron radiation shows that below 14 °C, twisted nanofibers are the thermodynamic phase. Between 14 and about 33 °C, nanofibers coexist with micelles and a strong hydrogel forms, the sol-to-gel transition being readily reversible in this temperature range. However, above the annealing temperature of about 40 °C, the micelle morphology becomes kinetically trapped for hours, even upon cooling, whichever the rate, to 4 °C. A combination of solution and solid-state nuclear magnetic resonance spectroscopy studies suggests two different conformations of the 1″, 1′, and 2′ carbon stereocenters of sophorose, precisely at the β(1,2) glycosidic bond, for which several combinations of the dihedral angles are known to provide at least three energetic minima of comparable magnitude, with each corresponding to a given sophorose conformation.

Strong and Elastic Hydrogels from Dual-Crosslinked Composites Composed of Glycol Chitosan and Amino-Functionalized Bioactive Glass Nanoparticles.

Mesoporous bioactive glass (BG) nanoparticles (NPs) with a high specific surface area were prepared. The surfaces of BG NPs were further modified using an amino-containing compound or synthesized precursors to produce three kinds of amino-functionalized bioactive glass (ABG) NPs via devised synthetic routes. The achieved ABG NPs possessed various spacer lengths with free amino groups anchored at the end of the spacer. These ABG NPs were then combined with glycol chitosan (GCH) to construct single- or dual-crosslinked ABG/GCH composite hydrogels using genipin (GN) alone as a single crosslinker or a combination of GN and poly(ethylene glycol) diglycidyl ether (PEGDE) as dual crosslinkers. The spacer length of ABG NPs was found to impose significant effects on the strength and elasticity of GN-crosslinked ABG/GCH hydrogels. After being dually crosslinked with GN and PEGDE, the elastic modulus of some dual-crosslinked ABG/GCH hydrogels reached around 6.9 kPa or higher with their yielding strains larger than 60%, indicative of their strong and elastic features. The optimally achieved ABG/GCH hydrogels were injectable with tunable gelation time, and also able to support the growth of seeded MC3T3-E1 cells and specific matrix deposition. These results suggest that the dual-crosslinked ABG/GCH hydrogels have the potential for some applications in tissue engineering.

Synthesis and Characterization of Chitosan-Acrylic Acid Based Hydrogels and Investigation the Properties of Bilayered Design with Incorporated Alginate Beads

The paper presents the synthesis of hydrogels via free-radical polymerization, based on Chitosan (CS) grafted with Acrylic acid (AA), using a two-step procedure. Free-radical polymerization has given strong hydrogels with compact structure, dominant elastic behavior, and long linear viscoelastic region. The results of rheological studies have shown that obtained hydrogels have significantly improved mechanical properties in comparison to chitosan hydrogels obtained by other sustainable methods. A step forward in the investigation of the potential application of chitosan hydrogels in wound dressing systems has been made by preparation of the bilayer design by embedding a layer of active compound-loaded alginate beads into the contact surface between two conjoined units of CS/AA hydrogels. Wild garlic (Allium ursinum L.) dried extract was used as an active compound because of its antimicrobial activity and green properties. This system has demonstrated pH-dependent release of extract and higher shear elastic modulus values than ordinary disc gels. A conducted study has given preliminary results for the possible application of bilayer chitosan-based hydrogels in wound dressing systems and represents the first step towards extrapolating the proposed design across other application fields.

An anti-freezing and strong wood-derived hydrogel for high-performance electronic skin and wearable sensing

Inspired by the highly aligned, well-ordered microstructure, and anisotropic mechanical properties of biological soft tissues, artificial electronic skins (e-skins) have been widely reported in recent years. However, challenges remain in achieving green, high mechanical properties, and multifunctionality simultaneously. In this work, we present a smart e-skin system with an anisotropic structure that couples a cellulose scaffold (CS) derived from natural wood with a polyvinyl alcohol (PVA)/MXene nanosheets (PM) network through a facile freeze-thawing process, featuring high toughness and excellent conductivity. With the addition of a biocompatible cryo-protectant, the smart e-skin exhibit ambient stability, anti-freezing properties, and moisturizing capability. The strong cross-linking and bonding between the 3D hierarchical CS and PM polymer network enhancing the mechanical strength of the e-skin in a specific direction, and the tensile strength along the longitudinal direction reached 12.01 MPa. The wood-derived hydrogel e-skin is 75 times and 23 times stronger than the isotropic cellulose hydrogel and pure PVA hydrogel. The soft PM polymer network endows the rigid cellulose skeleton with outstanding flexibility. Importantly, the e-skin with remarkable electromechanical sensing can realize the real-time monitoring of various human motions. In addition, the e-skin can also realize the private information transmission and even object recognition in aquatic environments. This study provides a facile strategy for developing next-generation wearable devices and multifunctional e-skin systems with bionic characteristics.

Highly strong and sensitive bilayer hydrogel actuators enhanced by cross-oriented nanocellulose networks

Temperature-responsive bilayer hydrogel actuator with anisotropic structure in mutually perpendicular directions was successfully prepared, where poly (N-isopropyl acrylamide)/cellulose nanofiber (PNIPAM/CNF) double network hydrogel was applied as the top layer, and poly (acrylamide-co-acrylic acid)/cellulose nanofiber (PAM-AA/CNF) double network hydrogel served as the bottom layer. The cross-oriented structure of the two hydrogel layers contributed to the superior actuation ability and mechanical performance of the bilayer hydrogel actuator. Consequently, the hydrogel actuator could quickly lift a weight with 70 times of their own weight with a high bending speed of 10.21 (°/s). Furthermore, the tensile strength and elongation at break of the hydrogel actuator reached 729.5 ± 21.5 KPa and (406.5 ± 11.3) % respectively. After further cross-linking by Fe3+, the tensile strength of the actuator reached 3.86 MPa. With excellent mechanical performance and actuation ability, the resulting bilayer hydrogel showed a promising application potential as artificial soft tissue materials and intelligent actuators.

Dramatically enhancing mechanical properties of hydrogels by drying reactive polymers at elevated temperatures to introduce strong physical and chemical crosslinks

Mechanically strong and tough hydrogels have a wide range of practical applications, and many preparation methods have been developed in recent two decades. However, facile and convenient methods for preparing hydrogels with excellent mechanical properties are still lacking. In this work, we report a strategy for preparing hydrogels with very high mechanical strengths by utilizing the strong physical and chemical crosslinks formed by drying reactive polymers at elevated temperatures. The obtained polyvinyl alcohol-polyacrylic acid (PVA-PAA) hydrogels show very high tensile strengths and elastic moduli up to 28.9 MPa and 26.6 MPa, which are several tens and hundreds of times those of PVA-PAA hydrogels prepared with freezing-thawing method, respectively. And the mechanical properties of the hydrogels can be adjusted by controlling drying temperature and PAA content. The crosslinking nature of the hydrogels are studied and proved with swelling and rheology measurements as well as structural characterizations. The drying at elevated temperatures enables the occurrence of esterification reaction and promotes the crystallization of PVA chains and the formation of stronger hydrogen bonds between PVA and PAA chains. This work provides a simple and facile method to prepare high-strength hydrogels for wide potential applications.

Dynamic and structural studies on synergetic energy dissipation mechanisms of single-, double-, and triple-network hydrogels sequentially crosslinked by multiple non-covalent interactions

A combination of multiple non-covalent interactions has been used to fabricate strong and tough hydrogels. However, mechanisms behind toughening remain unclear. In this study, a series of single-network (SN), double-network (DN), and triple-network (TN) hydrogels sequentially crosslinked by chemical bonds, freeze-thawed polyvinyl alcohol (PVA) crystallites, and ion coordination of alginate chains are systematically prepared. The contributions of hydrogen bonding, entanglements, crystallite crosslinking, and ion coordination to the dynamics of the hydrogels are comparatively studied. Dynamic mechanical analysis (DMA) reveals the apparent activation energy of hydrogen bonding from 41.1 kJ/mol to 50.6, 56.8, and 65.5 kJ/mol for SN gels with increasing PVA concentration (from 0 to 2, 6, 10 wt/vol%, respectively), whereas the corresponding apparent activation energy of chain entanglements increases from 148.3 to 160.2, 163.5, and 167.0 kJ/mol. PVA crystallites formed upon freeze-thawing act as physical crosslinks to further improve the strength and toughness of gels, as well as the activation energy (251.7 kJ/mol). Subsequent ion coordination with alginate further enhances the activation energy to 304.7 kJ/mol (Cu2+), 309.3 kJ/mol (Fe3+). Structural evolution studies on pre-cracked hydrogels reveal that PVA crystallites render crack blunting upon stretching. Polarized optical microscopy and scanning electron microscopy reveal the rearrangement and reconstruction of internal structures during crack propagation, including orientation and alignment of PVA crystallites. This study provides first insights to synergetic energy dissipation by non-covalent interactions of multi-network hydrogels.

Remote-controllable, tough, ultrastretchable, and magneto-sensitive nanocomposite hydrogels with homogeneous nanoparticle dispersion as biomedical actuators, and their tuned structure, properties, and performances

The development and property control of an intelligently magneto-sensitive (MS) and mechanically strong hydrogel practically usable for diverse biotechnological applications are challenging. In this work, we demonstrate the strategies to fabricate tough, ultrastretchable, and biocompatible MS nanocomposite hydrogels (MSNCHs) with homogeneous nanoparticle dispersion and to tune their network structure and properties for obtaining the optimal MSNCH. The MSNCHs were synthesized by the in-situ thermal polymerization of N,N-dimethylacrylamide (DMAAm), laponite nanoparticles (NPs), and Fe 3 O 4 magnetic NPs (MNPs), and their structural, thermal, swelling, and mechanical properties were examined according to the weight fraction of Fe 3 O 4 MNPs, w F . In addition, their controllable magnetorheological (MR) properties were investigated for various w F s and applied magnetic field (MF) strengths, H s. The correlation between their w F - and H -dependent structure and properties was analyzed, and the magneto-responsive performances in the air and water were also observed. Results showed that their MR properties and performance increased with increasing H , owing to the more robust filler network formed by the better-built alignment of Fe 3 O 4 MNPs, and the dominant one of the reinforcing and interruption effects of Fe 3 O 4 MNP content on the network structure determined the properties and performance of the MSNCH. The MSNCH possessing the homogenous nanoparticle dispersion and the structure, properties, and performances controlled while maintaining its high toughness, ultrastretchability, and biocompatibility holds great potential as a remote-controllable soft actuator for biomedical and pharmaceutical technologies. • Development of a tough, ultra-stretchable, biocompatible, and magneto-sensitive nanocomposite hydrogel (MSNCH). • Homogeneous dispersion of Fe 3 O 4 magnetic nanoparticles (MNPs) in the MSNCH matrix without a dispersant agent. • The control of the properties and performances of the MSNCH by tuning its network structure with MNP content and MF strength. • The reinforcing and interruption effect of MNPs on the MSNCH network structure. • MSNCH system as a promising remote-controllable soft actuator for biomedical and pharmaceutical technologies.

Strong and Reversible Covalent Double Network Hydrogel Based on Force-Coupled Enzymatic Reactions.

Biological load-bearing tissues are strong, tough, and recoverable under periodic mechanical loads. However, such features have rarely been achieved simultaneously in the same synthetic hydrogels. Here, we use a force-coupled enzymatic reaction to tune a strong covalent peptide linkage to a reversible bond. Based on this concept we engineered double network hydrogels that combine high mechanical strength and reversible mechanical recovery in the same hydrogels. Specifically, we found that a peptide ligase, sortase A, can promote the proteolysis of peptides under force. The peptide bond can be re-ligated by the same enzyme in the absence of force. This allows the sacrificial network in the double-network hydrogels to be ruptured and rebuilt reversibly. Our results demonstrate a general approach for precisely controlling the mechanical and dynamic properties of hydrogels at the molecular level.