Strong Hydrogels Research Articles

Strong Tough Polyampholyte Hydrogels via the Synergistic Effect of Ionic and Metal–Ligand Bonds

AbstractDespite existing in biological systems, developing synthetic polyampholyte (PA) hydrogels constructed by both ionic and metal–ligand bonds remains challenging. Herein, a simple secondary equilibrium approach is proposed to fabricate strong and tough PA hydrogels via the synergy of ionic and metal–ligand bonds. The original PA gels (constructed by ionic bonds) are first dialyzed in multivalent metal‐ion solutions to reach a swelling equilibrium and then moved to deionized water to dialyze excess free ions to achieve a new equilibrium. Through this approach, the original PA gel network can be optimized and eventually constructed by ionic and metal–ligand bonds, enabling a synergistic reinforcement. By selecting different original PA gel systems and diverse multivalent metal‐ions, the proposed approach is proved to be generalizable to fabricate strong and tough PA gels. Additionally, the hydrogels have stable ion‐conductivity even at the water‐equilibrium state, making them promising as strain sensors. The viscoelastic and elastic contributions to the mechanical properties of the hydrogels by a viscoelastic model are also discussed to further understand the strengthening and toughening mechanisms. The proposed strategy is simple but effective for achieving strong and tough PA‐based hydrogels. This study also provides new insights for PA hydrogels in electrolyte environments.

Enzyme-induced mineralization of hydrogels with amorphous calcium carbonate for fast synthesis of ultrastiff, strong and tough organic\u2013inorganic double networks

Hydrogels with good mechanical properties have great importance in biological and medical applications. Double-network (DN) hydrogels were found to be very tough materials. If one of the two network phases is an inorganic material, the DN hydrogels also become very stiff without losing their toughness. So far, the only example of such an organic–inorganic DN hydrogel is based on calcium phosphate, which takes about a week to be formed as an amorphous inorganic phase by enzyme-induced mineralization. An alternative organic–inorganic DN hydrogel, based on amorphous CaCO3, which can be formed as inorganic phase within hours, was designed in this study. The precipitation of CaCO3 within a hydrogel was induced by urease and a urea/CaCl2 calcification medium. The amorphous character of the CaCO3 was retained by using the previously reported crystallization inhibiting effects of N-(phosphonomethyl)glycine (PMGly). The connection between organic and inorganic phases via reversible bonds was realized by the introduction of ionic groups. The best results were obtained by copolymerization of acrylamide (AAm) and sodium acrylate (SA), which led to water-swollen organic–inorganic DN hydrogels with a high Young’s modulus (455 ± 80 MPa), remarkable tensile strength (3.4 ± 0.7 MPa) and fracture toughness (1.1 ± 0.2 kJ m−2).Graphical The present manuscript describes the method of enzymatic mineralization of hydrogels for the production of ultrastiff and strong composite hydrogels. By forming a double-network structure based on an organic and an inorganic phase, it is possible to improve the mechanical properties of a hydrogel, such as stiffness and strength, by several orders of magnitude. The key to this is the formation of a percolating, amorphous inorganic phase, which is achieved by inhibiting crystallization of precipitated amorphous CaCO3 with N-(phosphonomethyl)glycine and controlling the nanostructure with co polymerized sodium acrylate. This creates ultrastiff, strong and tough organic–inorganic double-network hydrogels.

Soft-fiber-reinforced tough and fatigue resistant hydrogels

Soft-fiber-reinforced tough and fatigue resistant hydrogels

Preparation and characterization of tough and highly resilient nanocomposite hydrogels reinforced by surface‐grafted cellulose nanocrystals

AbstractDespite many strong and tough hydrogels have been fabricated according to the energy dissipating mechanism, they usually lack high resilience due to the presence of large hysteresis. Herein, poly (N‐vinylpyrrolidone) grafted cellulose nanocrystal (CNC‐g‐PVP) was used as special multifunctional physical crosslinkers to fabricate tough and highly resilient nanocomposite hydrogels. CNC‐g‐PVP with varying loading was incorporated into chemically crosslinked polyacrylamide (PAM) networks by in‐situ radical polymerization to give PAM/CNC‐g‐PVP nanocomposite hydrogels. Robust cooperative hydrogen bonds existed between the surface‐grafted PVP chains and the PAM matrix, which could rupture to dissipate energy upon deformation and recover instantly on the removal of stress. This unique energy dissipating mechanism led to excellent mechanical performance of the hydrogels. Their tensile elastic modulus, toughness, and compressive strength are 1.4–1.8, 2.1–3.0, and 1.44–2.73 times of pure PAM hydrogel, respectively. Moreover, the hydrogels exhibit low hysteresis, high resilience (ca. 97%) under cyclic tensile loading‐unloading and good recovery of hysteresis (ca. 90%) under cyclic compressive loading‐unloading.

The race for strong and tough hydrogels

In a recent publication in Nature, He and co-workers reported an elegant strategy for the preparation of structurally organized porous hydrogels that are remarkably strong but extremely tough at the same time. The methodology combines approaches of molecular and structural engineering. In a recent publication in Nature, He and co-workers reported an elegant strategy for the preparation of structurally organized porous hydrogels that are remarkably strong but extremely tough at the same time. The methodology combines approaches of molecular and structural engineering.

Strong fatigue-resistant nanofibrous hydrogels inspired by lobster underbelly

•Hierarchical assembly of welded and high-crystalline bouligand structure in hydrogels •Mechanics-guided design of high fatigue resistance for nanofibrous hydrogels •Fatigue characterization of nanofibers in synthetic hydrogels •High-velocity microparticle impacts on synthetic nanofibrous hydrogels Nanofibrous hydrogels are pervasive in animal and plant bodies and have been widely seen in engineering applications. Electrospinning is one of the most widely used methods for the fabrication of nanofibrous hydrogels. Whereas the biological nanofibrous hydrogels can maintain high strength and high toughness under multiple cycles of mechanical loads, such fatigue-resistant properties have not been achieved in electrospun nanofibrous hydrogels. Here, we report a bioinspired design of strong and fatigue-resistant nanofibrous hydrogels that can closely mimic the bouligand structure of the natural hydrogel in the lobster underbelly. The resultant nanofibrous hydrogel can reach high nominal strength up to 8.4 MPa and high fatigue threshold up to 770 J/m2. We further demonstrate the superior impact resistance of the bioinspired bouligand-type nanofibrous hydrogel with specific penetration energy of 40 kJ/kg. We show that it is critical to weld the interfaces between nanofibers and introduce intrinsically high-energy phases (nanocrystalline domains) into nanofibers. Nanofibrous hydrogels are pervasive in animal and plant bodies and have been widely seen in engineering applications. Electrospinning is one of the most widely used methods for the fabrication of nanofibrous hydrogels. Whereas the biological nanofibrous hydrogels can maintain high strength and high toughness under multiple cycles of mechanical loads, such fatigue-resistant properties have not been achieved in electrospun nanofibrous hydrogels. Here, we report a bioinspired design of strong and fatigue-resistant nanofibrous hydrogels that can closely mimic the bouligand structure of the natural hydrogel in the lobster underbelly. The resultant nanofibrous hydrogel can reach high nominal strength up to 8.4 MPa and high fatigue threshold up to 770 J/m2. We further demonstrate the superior impact resistance of the bioinspired bouligand-type nanofibrous hydrogel with specific penetration energy of 40 kJ/kg. We show that it is critical to weld the interfaces between nanofibers and introduce intrinsically high-energy phases (nanocrystalline domains) into nanofibers.

Strong fish gelatin hydrogels enhanced by carrageenan and potassium sulfate

Gelatin-based hydrogels have multiple advantages because of their biocompatibility, biodegradability, and non-toxic properties. The feasibility of using gelatin-based hydrogels could be greatly improved if they could overcome the weak mechanical property for application. Here, strong fish gelatin hydrogels were prepared by introducing κ-carrageenan to gelatin solution to form structure preliminarily enhanced gelatin hydrogel, followed by immersing in 10 wt% concentration of K2SO4 solution. The modified hydrogels have an extraordinary hardness (21.22 N) and storage modulus (>104 Pa) when carrageenan amount was 0.8 wt%, superior to those of unmodified gelatin hydrogel. The effects of carrageenan concentrations on thermal stability, water distribution and microstructures of hydrogels were systematically evaluated, and the possible strengthening mechanism was also discussed. This article provided a simple method for the preparation of gelatin hydrogels with strong structure and the modified hydrogels can be used in foods, materials and other fields.

Designing a Transparent and Fluorine Containing Hydrogel.

Physical hydrogels are supramolecular materials obtained by self-assembly of small molecules called gelators. Aromatic amino acids and small peptides containing aromatic rings are good candidates as gelators due to their ability to form weak bonds as π-π interactions and hydrogen bonds between NH and CO of the peptide chain. In this paper we show our results in the preparation of a transparent hydrogel that was obtained by self-assembly of a fluorine-containing dipeptide that relies on the additional formation of halogen bonds due to the fluorine atoms contained in the dipeptide. We used Boc-D-F2Phe-L-Oxd-OH (F2Phe = 3,4-difluorophenylalainine; Oxd = 4-methyl-5-carboxy-oxazolidin-2-one) that formed a strong and transparent hydrogel in 0.5% w/w concentration at pH = 4.2. The formation of a hydrogel made of unnatural fluorinated amino acids may be of great interest in the evaluation of patients with parkinsonian syndromes and may be used for controlled release.

Self-Healing and Highly Stretchable Hydrogel for Interfacial Compatible Flexible Paper-Based Micro-Supercapacitor.

Most reported wearable electronic devices lack self-healing chemistry and flexible function to maintain stable energy output while irreversible damages and complex deformations. In this work, we report a dual-dynamic network electrolyte synthesized by micellar elastomers introduced into strong hydrogel matrix. The gel electrolyte is fabricated by physically cross-linking the borax-polyvinyl alcohol (B-PVA) network as tough matrix and poly (ethylene oxide) (PEO)-poly (propylene oxide) (PPO)-poly (ethylene oxide) (Pluronic) to frame elastic network, followed by immersion in potassium chloride solution. Under the action of dynamic borate ester bond and multi-network hydrogen bond, the as-prepared electrolyte exhibits high stretchability (1535%) and good self-healing efficiency. Based on the electrolyte, we assemble the interfacial compatible micro-supercapacitor (MSC) by multi-walled carbon nanotubes (MWCNT) interdigital electrode printed on cellulosic paper by direct ink writing (DIW) technique. Thanks to the large specific area and compressive deformation resistance of cellulosic paper, the MSC with tightly interfacial contact achieves high volumetric capacitance of 801.9 mF cm−3 at the current density of 20 μA cm−2. In the absence of stimulation of the external environment, the self-healing MSC demonstrates an ideal capacity retention (90.43%) after five physical damaged/healing cycles. Our research provides a clean and effective strategy to construct wearable MSC.

Ion Conductive Phytic Acid‐G Quadruplex Hydrogel as Electrolyte for Flexible Electrochromic Device

AbstractSelf‐healing and thermo‐reversible hydrogels derived from pristine biomolecules with potential ionic conductivity represent a promising class of functional and flexible materials for a range of applications such as bioelectronics, energy storage, and wearable and implantable devices. Herein, spontaneous formation of an ion conductive hydrogel through the reversible non‐covalent interaction of pristine biomolecules, phytic acid and guanosine‐5'‐monophosphate in aqueous medium is reported that shows excellent self‐healing and thermo‐reversible properties. The phytic acid biomolecule induces effective cross‐linking of the highly ordered G‐tetrad leading to gelation. The homogeneous and mechanically strong hydrogel is used as a gel electrolyte for flexible electrochromic devices taking advantage of the high ionic conductivity. The biocompatibility, robustness, good conductivity and multifunctional applicability of phytic acid induced G‐quadruplex hydrogel, makes it a prospective material for environmental and bioelectronics applications.

Recent Advances in Design Strategies for Tough and Stretchable Hydrogels.

The development of multifunctional hydrogels with excellent stretchability and toughness is one of the most fascinating subjects in soft matter research. Numerous research efforts have focused on the design of new hydrogel systems with superior mechanical properties because of their potential applications in diverse fields. In this Minireview, we consider the most up-to-date mechanically strong hydrogels and summarize their design strategies based on the formation of double networks and dual physical crosslinking. Based on the synthetic approaches and different toughening mechanisms, double-network hydrogels can be further classified into three different categories, namely chemically crosslinked, hybrid physically-chemically crosslinked, and fully physically crosslinked. In addition to the above-mentioned methods, we also discuss few uniquely designed hydrogels with the intention of guiding the future development of these fascinating materials for superior mechanical performance.

3D Printing Method for Tough Multifunctional Particle-Based Double-Network Hydrogels.

3D printing of hydrogels finds widespread applications in biomedicine and engineering. Artificial cartilages and heart valves, tissue regeneration and soft robots, require high mechanical performance of complex structures. Although many tough hydrogels have been developed, complicated synthesis processes hinder their fabrication in 3D printing. Here, a strategy is proposed to formulate hydrogel inks, which can be printed into various strong and tough particle-based double-network (P-DN) hydrogels of arbitrary shapes without any rheological modifiers. These hydrogel inks consist of microgels and a hydrogel precursor. The microgels are individual highly cross-linked networks. They are prepared by swelling dried microparticles in the hydrogel precursor that consists of monomers, initiators, and cross-linkers. Microgels regulate the rheological properties of the hydrogel ink and enable the direct printing. After printing and curing, the precursor forms a sparsely cross-linked network that integrates the microgels, leading to a P-DN hydrogel. The proposed hydrogel inks allow 3D printing of multifunctional hydrogel structures with high mechanical performance and strong adhesion to diverse materials. This strategy will open new avenues to fabricate multifunctional devices in tissue engineering and soft robotics.

Development of a Mechanically Strong Nondegradable Protein Hydrogel with a Sponge-Like Morphology.

Protein-based hydrogels are important functional materials with many potential applications. However, the relatively small pore size and poor mechanical properties substantially limit their application. Here a superporous bovine serum albumin (BSA) hydrogel is prepared with high porosity and interconnectivity by using BSA and 1,2,7,8-diepoxyoctane (DEO). The equilibrium water contents of hydrogels can reach 76.5%. Moreover, the BSA hydrogels show excellent mechanical properties and excellent deformation recoverability, with a maximum compression modulus of 50 MPa at 75% strain and no residual strain generated after 500 cyclic compression tests. The resulting BSA hydrogel has excellent biocompatibility for cell adherence and is nonbiodegradable for 40 weeks. More importantly, the BSA hydrogel exhibits excellent hemostatic ability, with hemostatic times in a rabbit ear artery and rabbit liver of 33 and 28 s, respectively. Therefore, BSA hydrogels have potential applications as painless nonadherent wound dressings and implant materials for plastic surgery.

Bioinspired Highly Anisotropic, Ultrastrong and Stiff, and Osteoconductive Mineralized Wood Hydrogel Composites for Bone Repair

AbstractAnisotropic hydrogels mimicking the biological tissues with directional functions play essential roles in damage‐tolerance, cell guidance and mass transport. However, conventional synthetic hydrogels often have an isotropic network structure, insufficient mechanical properties and lack of osteoconductivity, which greatly limit their applications for bone repair. Herein, inspired by natural bone and wood, a biomimetic strategy is presented to fabricate highly anisotropic, ultrastrong and stiff, and osteoconductive hydrogel composites via impregnation of biocompatible hydrogels into the delignified wood followed by in situ mineralization of hydroxyapatite (HAp) nanocrystals. The well‐aligned cellulose nanofibrils endow the composites with highly anisotropic structural and mechanical properties. The strong intermolecular bonds of the aligned cellulose fibrils and hydrogel/wood interaction, and the reinforcing nanofillers of HAp enable the composites remarkable tensile strength of 67.8 MPa and elastic modulus of 670 MPa, three orders of magnitude higher than those of conventional alginate hydrogels. More importantly, the biocompatible hydrogel together with aligned HAp nanocrystals could effectively promote osteogenic differentiation in vitro and induce bone formation in vivo. The bone ingrowth into the hydrogel composite scaffold also yields good osteointegration. This study provides a low‐cost, eco‐friendly, feasible, and scalable approach for fabricating anisotropic, strong, stiff, hydrophilic, and osteoconductive hydrogel composites for bone repair.

Injectable In Situ Induced Robust Hydrogel for Photothermal Therapy and Bone Fracture Repair

AbstractMalignant bone tumors are often accompanied by osteolytic destruction and severe pathological fractures. Current therapeutic strategies can largely inhibit tumor proliferation, but the high recurrence rate of tumors and related bone defects remain a significant challenge. This study aims to address these issues by developing a novel near‐infrared (NIR) light‐responsive and a mechanically strong hydrogel that offers excellent photothermal tumor therapy and bone fracture repair capabilities. The as‐prepared hydrogel exhibits good biocompatibility and an ultra‐strong photothermal effect due to the formation of a complex network with up‐conversion lanthanide‐Au hybrid nanoparticles and alginate molecules. A subcutaneous tumor model is used to demonstrate that tumors can be efficiently eradicated via local photothermal treatment, where there is no tumor recurrence within the observation period. Moreover, the injected hydrogel becomes mechanically strong due to in situ Ca2+ crosslinking, which provides a supportive matrix to promote the repair of bone defects via stabilization of the fractured bone structure. The high photothermal effect and robust support offered by this single material demonstrate the potential of using the proposed hydrogel for the simultaneous treatment of bone tumor removal and bone healing.

PH Responsive Strong Polyion Complex Shape Memory Hydrogel with Spontaneous Shape Changing and Information Encryption.

Polyion complex (PIC) hydrogels attract lots of studies because of the relatively definite network and excellent mechanical strength. However, the stability of the PIC hydrogel is poor in salt solutions due to the counter-ion screening effect, which restricts their applications. Besides, novel functions of the PIC hydrogels also need to be explored. In this work, a multifunctional PIC hydrogel is prepared by polymerizing a hydrophobic monomer 2-(diethylamino)ethyl methacrylate in poly(styrene sulfonic acid) aqueous solution with the presence of counter-ion NaCl. Fourier transform infrared (FTIR) spectra, water content, and mechanical properties of the hydrogel are investigated. The introduction of hydrophobic weak electrolyte into the hydrogel brings stable excellent mechanical strength even in NaCl solutions with high concentration and pH modulated softening and strengthening. Surprisingly, the hydrogel swells but is strengthened in HCl, while it shrinks but is softened in NaOH. pH induced shape memory and unique spontaneous shape changing is thus presented benefiting from this synergistic effect. Moreover, information encryption is realized on the PIC hydrogel owing to the transmittance change and the different water absorption capability of the hydrogel at different states. This new kind of PIC hydrogel proposes a new smart material in continuously actuating soft machines and secretive information transformation.

Strong and elastic pea protein hydrogels formed through pH-shifting method

Pea protein has attracted attentions as an alternative for soy protein, but its weaker gelling properties has limited its applications in food formulations. This research reports a heat-induced strong and elastic gel from pea protein isolate (PPI) pre-treated by a modified pH-shifting method. The scanning electron microscopy (SEM) observation and rheological measurement revealed that the formation of three-dimensional (3D) coagulum for pea protein at pH 12 by extending the alkali holding time (1, 24 and 48 h) and such coagulum maintained their structure when adjusting pH back to neutral. Further heating the pH-shifted PPI suspension at 92 °C led to gels of densely cross-linked polymer-like network with thin connective walls and pore size of 1–2 μm. Such uniform polymer-like gel network microstructure obtained in this work might be related to a high level of pea protein unfolding during pH-shifting treatment, which made the protein chain flexible and created more active sites to promote molecular interactions including hydrogen bonds and hydrophobic interactions when the protein was neutralized and heated. Notably the gel mechanical strength was dramatically increased to 21 kPa, accompanied by excellent elasticity and water holding capacity (>95%). The modified method of pH-shifting process has provided a new approach for preparation of pea protein gels with superior strength, thus may enable wider applications of pea protein applications in food formulations where gelling and texturization functions are required.

Cationic Cellulose Nanocrystals-Based Nanocomposite Hydrogels: Achieving 3D Printable Capacitive Sensors with High Transparency and Mechanical Strength.

Hydrogel ionotronics are intriguing soft materials that have been applied in wearable electronics and artificial muscles. These applications often require the hydrogels to be tough, transparent, and 3D printable. Renewable materials like cellulose nanocrystals (CNCs) with tunable surface chemistry provide a means to prepare tough nanocomposite hydrogels. Here, we designed ink for 3D printable sensors with cationic cellulose nanocrystals (CCNCs) and zwitterionic hydrogels. CCNCs were first dispersed in an aqueous solution of monomers to prepare the ink with a reversible physical network. Subsequent photopolymerization and the introduction of Al3+ ion led to strong hydrogels with multiple physical cross-links. When compared to the hydrogels using conventional CNCs, CCNCs formed a stronger physical network in water that greatly reduced the concentration of nanocrystals needed for reinforcing and 3D printing. In addition, the low concentration of nanofillers enhanced the transparency of the hydrogels for wearable electronics. We then assembled the CCNC-reinforced nanocomposite hydrogels with stretchable dielectrics into capacitive sensors for the monitoring of various human activities. 3D printing further enabled a facile design of tactile sensors with enhanced sensitivity. By harnessing the surface chemistry of the nanocrystals, our nanocomposite hydrogels simultaneously achieved good mechanical strength, high transparency, and 3D printability.

Strong tough hydrogels via the synergy of freeze-casting and salting out

Natural load-bearing materials such as tendons have a high water content of about 70 per cent but are still strong and tough, even when used for over one million cycles per year, owing to the hierarchical assembly of anisotropic structures across multiple length scales1. Synthetic hydrogels have been created using methods such as electro-spinning2, extrusion3, compositing4,5, freeze-casting6,7, self-assembly8 and mechanical stretching9,10 for improved mechanical performance. However, in contrast to tendons, many hydrogels with the same high water content do not show high strength, toughness or fatigue resistance. Here we present a strategy to produce a multi-length-scale hierarchical hydrogel architecture using a freezing-assisted salting-out treatment. The produced poly(vinyl alcohol) hydrogels are highly anisotropic, comprising micrometre-scale honeycomb-like pore walls, which in turn comprise interconnected nanofibril meshes. These hydrogels have a water content of 70-95 per cent and properties that compare favourably to those of other tough hydrogels and even natural tendons; for example, an ultimate stress of 23.5±2.7 megapascals, strain levels of 2,900±450 per cent, toughness of 210±13 megajoules per cubic metre, fracture energy of 170±8 kilojoules per square metre and a fatigue threshold of 10.5±1.3 kilojoules per square metre. The presented strategy is generalizable to other polymers, and could expand the applicability of structural hydrogels to conditions involving more demanding mechanical loading.

A robust regenerated cellulose-based dual stimuli-responsive hydrogel as an intelligent switch for controlled drug delivery

Constructing robust hydrogels with biodegradability and dual stimuli-responsive by utilizing natural polymer as raw materials remains a sustaining challenge. Herein, we proposed an interpenetrating strategy in which N-isopropyl acrylamide (NIPAM) and acrylamide (AM) block copolymers were introduced as the second network into the carboxymethyl cellulose single network gel (CMC gel) to construct a dual-network robust hydrogel (CMC/PNIPAM-co-PAM). The dual-network design strategy effectively improves the mechanical strength of CMC gel. The hydrogel suggests intelligent dual stimuli-responsive behavior to pH and temperature. Furthermore, the copolymerization of NIPAM and AM regulated the low critical solution temperature (LCST) of the hybrid hydrogel, making it close to the physiological temperature of the human body. With the aim of evaluating its application in drug delivery, we loaded tetracycline into the dual-network hydrogel and simulated its release process under the pH microenvironment of the small intestine and the physiological temperature to infer its potential application in intestinal inflammation treatments. Moreover, it is proved that the strong hydrogel possesses good cytocompatibility in vitro biocompatibility testing. In addition, the embedding of tetracycline makes the hydrogel excellent antioxidant performance. This dual-stimulus response integrated hydrogel is expected to play a critical role in drug delivery and targeted therapy.