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The high water content of hydrogels makes them important as synthetic biomaterials, and tuning the mechanical properties of hydrogels to match those of natural tissues without changing chemistry is usually difficult. In this study, we have developed a series of hydrogels with varied stiffness, strength, and toughness based on a combination of poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), a strong acidic polyelectrolyte, and poly- N -(carboxymethyl)- N , N -dimethyl-2-(methacryloyloxy) ethanaminium) (PCDME), a polyzwitterion with a weak acidic moiety. We demonstrate that modifying the true molar ratio, R , of PCDME to PAMPS results in four unique categories of hydrogels with different swelling ratios and Young's moduli. When R < 1, a negatively charged polyelectrolyte gel (PE) is formed; when 1 < R < 3, a tough and viscoelastic polyelectrolyte complex gel (PEC) is formed; when 3 < R < 6.5, a conventional, elastic interpenetrating network gel (IPN) is formed; and when R > 6.5, a tough and stiff double-network gel (DN) is formed. Both the PEC and DN gels exhibit high toughness and fracture stress, up to 1.8 and 1.5 MPa, respectively. Importantly, the PEC gels exhibit strong recovery properties along with high toughness, distinguishing them from DN gels. Without requiring a change in chemistry, we can tune the mechanical response of hydrogels over a wide spectrum, making this a useful system of soft and hydrated biomaterials.

Hydrogels have promising applications in diverse areas, especially wet environments including tissue engineering, wound dressing, biomedical devices, and underwater soft robotics. Despite strong demands in such applications and great progress in irreversible bonding of robust hydrogels to diverse synthetic and biological surfaces, tough hydrogels with fast, strong, and reversible underwater adhesion are still not available. Herein, a strategy to develop hydrogels demonstrating such characteristics by combining macroscale surface engineering and nanoscale dynamic bonds is proposed. Based on this strategy, excellent underwater adhesion performance of tough hydrogels with dynamic ionic and hydrogen bonds, on diverse substrates, including hard glasses, soft hydrogels, and biological tissues is obtained. The proposed strategy can be generalized to develop other soft materials with underwater adhesion., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Developing nonspecific, fast, and strong adhesives that can glue hydrogels and biotissues substantially promotes the application of hydrogels as biomaterials. Inspired by the ubiquitous adhesiveness of bacteria, it is reported that neutral polyampholyte hydrogels, through their self-adjustable surface, can show rapid, strong, and reversible adhesion to charged hydrogels and biological tissues through the Coulombic interaction., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A series of tough polyion complex hydrogels is synthesized by sequential homopolymerization of cationic and anionic monomers. Owing to the reversible interpolymer ionic bonding, the materials are self-healable under ambient conditions with the aid of saline solution. Furthermore, self-glued bulk hydrogels can be built from their microgels, which is promising for 3D/4D printing and the additive manufacturing of hydrogels., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biological tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mechanical properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mechanical properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These physical hydrogels of supramolecular structure can be tuned to change multiple mechanical properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.

Polyampholyte (PA) hydrogels have attracted great attention as an innovative material having tough, self-healing, and viscoelastic behavior. PA hydrogels are synthesized using one-step radical polymerization of equal amounts of oppositely charged ionic monomers at a very high monomer concentration. They have 50-70 wt% of water at an equilibrium state, much lower than that of conventional hydrogels that usually have a high-water content (>80%). They are strongly viscoelastic and have a high toughness (fracture energy of 1,000-4,000 J/m²), a high modulus (0.01-8 MPa), a high failure strain (150-1,500%), and a failure stress (0.1-2 MPa), together with 100% self-recovery and a high self-healing efficiency behavior. These excellent mechanical performances are comparable to that of rubbers, most tough double-network hydrogels, and soft bio-tissues. The extensive experimental studies show that these multiple mechanical properties are related to the formation of dynamical features of inter- and intra-chain ionic bonds, and this study opens a common strategy to develop tough and self-healing hydrogels. [ABSTRACT FROM AUTHOR]

Abstract: Tough and self‐recoverable hydrogel membranes with micrometer‐scale thickness are promising for biomedical applications, which, however, rarely be realized due to the intrinsic brittleness of hydrogels. In this work, for the first time, by combing noncovalent DN strategy and spin‐coating method, we successfully fabricated thin (thickness: 5–100 µm), yet tough (work of extension at fracture: 105–107 J m−3) and 100% self‐recoverable hydrogel membranes with high water content (62–97 wt%) in large size (≈100 cm2). Amphiphilic triblock copolymers, which form physical gels by self‐assembly, were used for the first network. Linear polymers that physically associate with the hydrophilic midblocks of the first network, were chosen for the second network. The inter‐network associations serve as reversible sacrificial bonds that impart toughness and self‐recovery properties on the hydrogel membranes. The excellent mechanical properties of these obtained tough and thin gel membranes are comparable, or even superior to many biological membranes. The in vitro and in vivo tests show that these hydrogel membranes are biocompatible, and postoperative nonadhesive to neighboring organs. The excellent mechanical and biocompatible properties make these thin hydrogel membranes potentially suitable for use as biological or postoperative antiadhesive membranes. [ABSTRACT FROM AUTHOR]

Tough hydrogels with adjustable stiffness are expected for adapting application as various biomaterials. Oppositely charged polyelectrolytes form tough and self-healing physical polyion-complex (PIC) hydrogels via formation of inter-chain ionic bonds with a wide distribution in bond strength. The strong bonds serve as permanent crosslinking to impart elasticity and the weak bonds as reversible sacrificial bonds to dissipate energy and to self-heal. In this work, we fabricate four PIC hydrogels using four positively charged trimethyl-ammonium monomers with slightly different chemical moieties and a same negatively charged polymer. The obtained PIC hydrogels all show high toughness but large difference in stiffness, extensibility, and self-recovery kinetics. With slight difference in the monomer structure of the polycations, the modulus of the hydrogels varies over two orders in magnitude, from 0.36 to 56 MPa, and the difference in elongation at break is up to five times. The presence of acryloyl moiety and methyl moiety increase the stiffness of the hydrogels. In the temperature range studied, all the four PIC hydrogels exhibit the rheological simple behaviours, following the time-temperature superposition principle. The four samples show quite different dynamic relaxation spectra over wide frequency range, revealing large difference in the strength distribution of dynamic ionic bonds. SEM observation reveals quite different phase separation structure for the four samples, in which the polymer chain stiffness should play an important role. This understanding of structure-properties of the PIC hydrogels will merit the designing of various supramolecular tough hydrogels and therefore broaden the scope of hydrogels for the applications as biomaterials. [ABSTRACT FROM AUTHOR]

Recently, many tough and self-healing hydrogels have been developed based on physical bonds as reversible sacrificial bonds. As breaking and re-forming of physical bonds are time-dependent, these hydrogels are viscoelastic and the deformation rate and temperature pronouncedly influence their fracture behavior. Using a polyampholyte hydrogel as a model system, we observed that the time-temperature superposition principle is obeyed not only for the small strain rheology but also for the large strain hysteresis energy dissipation and the fracture energy below a certain temperature. The three processes possess the same shift factors that obey the equation of Williams, Landel, and Ferry (WLF) time-temperature equivalence. The fracture energy Γ scales with the crack velocity Vc over a wide velocity range as Γ ~ Vc α (α = 0.21). The exponent α of the power law is well-related to the exponent κ of the relaxation modulus G (t) ~ t-κ (κ = 0.26), obeying the prediction α = κ/(1 + κ) from classic viscoelasticity theory. These results show that the fracture energy of the polyampholyte gel is dominated by the bulk viscoelastic energy dissipated around the crack tip. This investigation gives an insight into designing tough and self-healing hydrogels and predicting their fracture behaviors from their dynamic mechanical spectrum. [ABSTRACT FROM AUTHOR]

Tough hydrogels have shown strong potential as structural biomaterials. These hydrogels alone, however, possess limited mechanical properties (such as low modulus) when compared to some load-bearing tissues, e.g., ligaments and tendons. Developing both strong and tough soft materials is still a challenge. To overcome this obstacle, a new material design strategy has been recently introduced by combining tough hydrogels with woven fiber fabric to create fiber reinforced soft composites (FRSCs). The new FRSCs exhibit extremely high toughness and tensile properties, far superior to those of the neat components, indicating a synergistic effect. Here, focus is on understanding the role of energy dissipation of the soft matrix in the synergistic toughening of FRSCs. By selecting a range of soft matrix materials, from tough hydrogels to weak hydrogels and even a commercially available elastomer, the toughness of the matrix is determined to play a critical role in achieving extremely tough FRSCs. This work provides a good guide toward the universal design of soft composites with extraordinary fracture resistance capacity. [ABSTRACT FROM AUTHOR]

Polyampholyte (PA) hydrogels are a new class of tough and self-healing supramolecular hydrogels that have a potential as load-bearing soft materials. Studying on the creep behavior of these hydrogels and understanding the molecular mechanism are important for prediction of lifetime of the materials. In the present work, we study the creep rupture dynamics of the PA hydrogels with and without chemical cross-linking, in a certain observation time window. We have found that above some critical loading stress both physical and lightly chemically cross-linked hydrogels undergo creep rupture while moderately chemically cross-linked hydrogel resists creep flow. To elucidate the molecular mechanism, we have further compared the creep behaviors of the physical and lightly chemically cross-linked samples. The creep rate of the samples decreases with the creep time, following a power law relation, regardless of the loading stress variation. The fracture time of both of these hydrogels exponentially decreases with the increase of the loading stress, following the same master curve at high loading stress region, while the behavior of the two samples becomes different in the low loading stress region. We have explained the delayed fracture dynamics at high loading stress region in terms of a relatively weak strong bond rupture mechanism. [ABSTRACT FROM AUTHOR]

Recently, polyampolytes have been discovered to form hydrogels that possess high toughness, full resilience, and self-healing between two cut surfaces. The self-healing of this class of hydrogels is based on the re-forming of the multiple ionic bonds at the fractured surfaces, in which the mobility of the polymer segments and strength of the ionic bonds play an important role. In this work, we study the effects of healing temperature and chemistry of the polyampholyte hydrogels (chemical cross-linker density and chemical structure of the monomers) on the healing kinetics and healing efficiency. The high healing temperature substantially accelerates the self-healing kinetics. Chemical cross-linking reduces the self-healing efficiency. Monomers with more hydrophobic feature give a low self-healing efficiency. For polyampholyte physical hydrogels with a softening temperature below the room temperature, excellent healing efficiency (∼84% on average and maximum 99%) was observed without any external stimuli. We found a correlation between the self-healing efficiency and the fraction of dynamic bonds in the total bonds for relatively soft samples, which is an evidence that the self-healing is due to the re-forming of dynamic bonds. [ABSTRACT FROM AUTHOR]

Oppositely charged homopolyelectrolytes were found to form strong, tough, and self-healing polyion-complex (PIC) hydrogels, similar to polyampholytes (PA) which have opposite charges randomly distributed on the same polymer chains. The excellent mechanical performances of these two novel hydrogels are the results of dynamic ionic bonds formation between entangled polymer chains. For the PIC system, only interchain bonding occurs, while for the PA system both inter- and intrachain bonding exist. In addition, the ion pairs are expected to form stronger bonding in the PIC system than those in the PA system. In this work, we performed a comparative study of PIC hydrogels with the PA hydrogels. The PIC hydrogels are synthesized by sequential homopolymerization of cationic and anionic monomers at varied formulation, and their swelling and mechanical properties are systematically studied in comparison to the PA hydrogels that were synthesized from random copolymerization of anionic monomers and cationic monomers of the similar formulation. Different from the PA system which only forms tough hydrogels around zero net charge composition without chemical cross-linking, the PIC system forms tough physical hydrogels even at weakly off-balanced charge composition. At the charge-balanced composition, the low entanglement concentration of homocharged polyelectrolyte chains leads to tough PIC hydrogels formation at much lower concentrations than that of PA hydrogels. As a result, the PIC hydrogels are much tougher than the PA hydrogels prepared at the same monomer composition. In similar to PA hydrogels, the PIC hydrogels also exhibit broad dynamic mechanical spectra, indicating the formation of ion complexes with widely ranged bond strength. The PIC hydrogels have strong viscoelasticity in comparison with PA hydrogels. However, the two systems show the similar activation energies of the dynamic mechanical spectra. The SEM microstructural observation shows that the PIC hydrogels have segregated structure while PA hydrogels are more homogeneous. [ABSTRACT FROM AUTHOR]

Recently, we have reported that polyampholytes,synthesized fromfree radical copolymerization of anionic monomer and cationic monomer,form physical hydrogels of high toughness and self-healing. The randomdistribution of the opposite charges forms ionic bonds of a wide distributionof strength. The strong bonds serve as permanent cross-links, impartingelasticity, whereas the weak bonds serves as reversible sacrificialbonds by breaking and reforming to dissipate energy. In this work,we focus on the rupture behaviors of the polyampholyte physical hydrogel,P(NaSS-co-MPTC), copolymerized from sodium p-styrenesulfonate (NaSS) and 3-(methacryloylamino)propyltrimethylammoniumchloride (MPTC). Tensile test and pure shear test were performed atvarious stretch rates in the viscoelastic responses region of thematerial. Tensile test showed yielding, strain softening, and strainhardening, revealing the dually cross-linked feature of the gel. Pureshear test showed crack blunting at the notched tip and a large yieldingzone with butterfly shaped birefringence pattern ahead of the cracktip. After blunting, crack advanced at steady-state velocity witha constant angle. The conditions for the occurrence of crack bluntingand variables governing the crack advancing angle are discussed. Wefound that even for these highly stretchable samples, significantblunting only occurs when the tensile fracture stress σfis larger than modulus Eby a factor of about 2, in consistent with Hui’s theoreticalprediction for elastic materials. The crack advancing angle θwas found to be proportional to σy/Eover a wide stretch rate range, where σyis the yielding stress. In addition, thefracture energy was correlated to small strain modulus by a powerlaw in the viscoelastic response region. This systematic study willmerit revealing the fracture mechanism of tough viscoelastic materialsincluding biological tissues and recently developed tough and highlystretchable hydrogels. [ABSTRACT FROM AUTHOR]

Understanding the energy dissipation mechanism during deformation is essential for the design and application of tough soft materials. We show that, in a class of tough and self-healing polyampholyte hydrogels, a bicontinuous network structure, consisting of a hard network and a soft network, is formed, independently of the chemical details of the hydrogels. Multiscale internal rupture processes, in which the double-network effect plays an important role, are found to be responsible for the large energy dissipation of these hydrogels. [ABSTRACT FROM AUTHOR]

Double-network hydrogels (DN gels)have aroused considerable interestbecause of their excellent mechanical strength and toughness, lowsliding friction, good biocompatibility, as well as wide tunabilityin components. By revisiting DN gels, we provide an ingenious wayto fabricate a kind of strong and tough microgel-reinforced hydrogels(MR gels), that densely cross-linked polyelectrolyte microgels ofpoly(2-acrylamido-2-methylpropanesulfonic sodium) (PNaAMPS) (replacingthe densely cross-linked PNaAMPS macro-network for conventional DNgels) are incorporated into sparsely cross-linked neutral polyacrylamide(PAAm) matrix. The structure of MR gels can be considered as a two-phasecomposite, where the disperse phase is the rigidDNmicrogels, and the continuous phase is the softPAAmmatrix. Similar to DN gels, MR gels show the irreversible energy dissipationin the hysteresis measurement, demonstrating the permanent fractureof the brittle PNaAMPS phase. Thus, the discontinuous brittle phasealso serves as sacrificial bonds. Through quantitativecomparison of the hysteresis curves with DN gels and monitoring themorphology change of the embedded microgels in MR gels during thereal-time stretching process, we conclude that the DN microgels inMR gels show four times higher in fracture efficiency of the sacrificialbonds than bulk DN gels at the same strain, as a result of the stressconcentration around the microgels. [ABSTRACT FROM AUTHOR]

Tough bio-based hydrogels based on the association of physical bonds are promising materials for use in biomedical applications due to their biocompatibility, biomimicry and biodegradability. Understanding the mechanical mechanisms at work during the fracture in such applications is indispensable. Using tough and self-healing bio-based hydrogels attained by the complexation of the negatively charged sodium hyaluronate (HA) and positively charged chitosan as a model system, an investigation of the tensile and fracture behavior over five decades of stretch velocities in physically and chemically crosslinked HA/chitosan hydrogels was systematically performed. In this study, the tensile analysis shows that the both of physically and chemically crosslinked hydrogels at small deformation are viscoelastic at a low stretch velocity, while they exhibit quasi-elastic character at a relatively high stretch velocity. In comparison to the physically and lightly chemically crosslinked HA/chitosan hydrogels exhibiting plastic-like behavior, the highly chemically crosslinked hydrogels demonstrate strong finite chain extensibility before fracture. The fracture analysis shows that the fracture energy of the highly chemically crosslinked hydrogels scales with the crack velocity as a power law relation Γ ∝ V c a (a = 0.15), where the exponent a can be well predicted from the viscoelastic spectrum of the bulk sample. These findings indicate that the viscoelastic energy dissipation around the crack tip dominates the fracture energy of the highly chemically crosslinked hydrogels. On the other hand, the fracture energy of the physically and lightly chemically crosslinked hydrogels is noted to increase with the crack velocity as a complex growth function, which can be effectively predicated by a viscoplastic model. The observed finding shows that the fracture energy of the physically and lightly chemically crosslinked hydrogel is dominated by the unzipping of the polyion complex structure and viscous energy dissipation due to the pulled out chains from the polymer network structure. • The rate-dependent mechanical tests of bio-based hydrogels were systematically performed. • The physical hydrogels show the plastic flow while the highly crosslinked hydrogels exhibit the viscoelastic behaviour. • Viscoelasic and viscoplastic models are proposed to explain their fracture behaviour. [ABSTRACT FROM AUTHOR]