Highly stretchable,ionic conductive and self-recoverable zwitterionic polyelectrolyte-based hydrogels by introducing multiple supramolecular sacrificial bonds in double network

Zwitterionic hydrogels have been explored for applications in electrochemical devices very recently due to their high water retention ability and interesting electrochemical properties. The use of zwitterionic hydrogels in devices requires them tough and recoverable or healable from fatigue damage. Herein, a double network zwitterionic hydrogel contains a reversible noncovalent interaction crosslinked polyvinyl alcohol (PVA) first network, together with a covalent/noncovalent hybrid crosslinked acrylamide and sulfobetaine methacrylate copolymer (P(AM-co-SBMA)) second network, was fabricated by a simple two-steps methods of copolymerization and freezing/thawing. The reversible hydrogen bonds, crystalline domain, and electrostatic interactions in the double networks work as sacrificial bonds to dissipate energy and toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties with the assistance of the elastic covalent network. The optimal hydrogels are highly stretchable (fracture strain 970%), tough (fracture toughness 693 kJ m⁻³), rapidly recoverable (65% toughness recovery and 75% stiffness recovery after resting 5 min at room temperature) and with widely tunable mechanical properties by multibond crosslinking. Meanwhile, the zwitterionic counterions of SBMA moieties endow the tough and recoverable hydrogels extremely high intrinsic ionic conductivities (7.49 S m⁻¹) at room temperature. This work not only provides a simple strategy for fabricating tough and recoverable zwitterionic hydrogels but also demonstrates multifunctional properties of the zwitterionic hydrogels, which possess a great potential to fulfill flexible devices applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47783.     

Reversibly highly stretchable and self-healable zwitterion-containing polyelectrolyte hydrogel with high ionic conductivity for high-performance flexible and cold-resistant supercapacitor

It remains challenging to develop stretchable and self-healable polymer electrolytes with improved ion-conductive nature for high-performance multifunctional flexible supercapacitors. Herein, a P(AM-SBMA-AMPS)-SiO₂ zwitterion-containing polyelectrolyte hydrogel is fabricated via copolymerization of acrylamide (AM), sulfobetaine methacrylate (SBMA) zwitterionic monomer, and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) anionic monomer grafted from the surface of vinyl silica nanoparticles (VSNPs). The hydrogen bonding among polymer chains and the high-density dynamic ionic interactions between SBMA and AMPS work as reversible “sacrificial bonds” to toughen hydrogel, while the VSNPs function as multifunctional crosslinkers and stress transfer centers, which makes these hydrogels tough (fracture energy 2.7 MJ m⁻³), stretchable (fracture strain 4,016%), and self-healable (fracture strain of healable sample 775%). More importantly, this zwitterion-containing polyelectrolyte hydrogel exhibits high ionic conductivities (3.4 S m⁻¹) owing to the highly hydration capacity of the zwitterionic polyelectrolyte copolymer which produced efficient ion migration channels for ion transport. Accordingly, a flexible supercapacitor based on this multifunctional hydrogel as electrolyte demonstrates a high electric double-layer capacitive capacitance of 60.6 F g⁻¹ at 0.5 A g⁻¹ and excellent capacitance retention of ~98% over 1,000 cycles as well as encouraging electrochemical properties at subzero temperature. This work provides new insights into the synthesis of highly conductive and multifunctional polyelectrolyte hydrogels for high-performance flexible supercapacitors. © 2020 Wiley Periodicals, Inc.     

Ultrastretchable,Super Tough,and Rapidly Recoverable Nanocomposite Double‐Network Hydrogels by Dual Physically Hydrogen Bond and Vinyl‐Functionalized Silica Nanoparticles Macro‐Crosslinking

It remains a challenge to develop tough hydrogels with recoverable or healable properties after damage. Herein, a new nanocomposite double‐network hydrogel (NC‐DN) consisting of first agar network and a homogeneous vinyl‐functionalized silica nanoparticles (VSNPs) macro‐crosslinked polyacrylamide (PAM) second network is reported. VSNPs are prepared via sol‐gel process using vinyltriethoxysilane as a silicon source. Then, Agar/PAM‐SiO₂ NC‐DN hydrogels are fabricated by dual physically hydrogen bonds and VSNPs macro‐crosslinking. Under deformation, the reversible hydrogen bonds in agar network and PAM nanocomposite network successively break to dissipate energy and then recombine to recover the network, while VSNPs in the second network could effectively transfer stress to the network chains grafted on their surfaces and maintain the gel network. As a result, the optimal NC‐DN hydrogels exhibit ultrastretchable (fracture strain 7822%), super tough (fracture toughness 18.22 MJ m^‐3, tensile strength 431 kPa), rapidly recoverable (≈92% toughness recovery after 5 min resting at room temperature), and self‐healable (can be stretched to 1331% after healing) properties. The newly designed Agar/PAM‐SiO₂ NC‐DN hydrogels with tunable network structure and mechanical properties by multi‐bond crosslinking provide a new avenue to better understand the fundamental structure‐property relationship of DN hydrogels and broaden the current hydrogel research and applications.     

Mechanical and Rheological Behavior of Hybrid Cross‐Linked Polyacrylamide/Cationic Micelle Hydrogels

In this work, a hybrid cross‐linked polyacrylamide (PAM)/cationic micelle hydrogel is fabricated by introducing the cationic micelles into the chemically cross‐linked PAM network. The cationic micelles act as the physical cross‐linking points through the strong electrostatic interaction with anionic initiator potassium persulfate. Thereafter, in situ free radical polymerization is initiated thermally from the cationic micelle surface to form the hybrid cross‐linked network. The synergistic effect between chemical and physical cross‐link endows the hydrogel with excellent mechanical and recoverable properties. The resulting hydrogel exhibits tensile stress of 481 kPa and fracture toughness of 1.65 MJ m⁻³. It is found that the chemical cross‐linking can inhibit the hysteresis of the hybrid hydrogel, exhibiting good elasticity in the tensile loading–unloading test. Moreover, dynamic rheological measurements show that the hybrid hydrogels possess fewer defects of network and exhibit excellent self‐recovery behavior. Thus, this investigation provides a different view for the design of new high elastic and tough hydrogels containing hybrid physical and chemical cross‐linking networks.     

Self-assembly and metal ions-assisted one step fabrication of recoverable gelatin hydrogel with high mechanical strength

ABSTRACT

Gelatin hydrogel has been widely applied in bio-applications due to their good biocompatibility and high water content. However, poor mechanical properties of gelatin hydrogel greatly limit their application. Here we present a facile one-step soaking method to fabricate a recoverable gelatin hydrogel with high mechanical property, which is based on hydrogen bonds and metal ionic interaction. The mechanical properties of gelatin hydrogels can be tuned with different metal ions, temperatures and soaking times. Especially, gelatin-Fe³⁺ hydrogel can reach to 65 MPa compression stress with the compressive strain over 99% and possess good fatigue resistance under cyclic loadings. Besides, hydrogels crosslinked with metal ions show better antibacterial ability against Escherichia coli and Staphylococcus aureus. This work suggested an alternative for the design of tough gelatin-based hydrogels with desirable properties, which may hold promising for potential bio-applications under physiological conditions.     

Site‐Specific Oxidation‐Induced Stiffening and Shape Morphing of Soft Tough Hydrogels

Living biological tissues are made of structures with properly defined mechanical properties (toughness and stiffness) toward specific biological functions. Herein, a chemical manipulation strategy is developed to locally vary the oxidation state of Fe ions from divalent to trivalent in the tough hydrogels. The resultant trivalent ionically cross‐linked networks become less flexible and lead to a significant enhancement of the stiffness of the tough hydrogels. The mechanical strengthening of Fe²⁺/Ca²⁺‐alginate/polyacrylamide tough hydrogels is demonstrated by the oxidation with ammonium persulfate (APS). Moreover, by applying surface patterning, the mechanical properties of the tough hydrogels are spatially stiffened and thus can serve as anisotropic elements to guide the shape morphing of tough hydrogels into complex 3D structures. This method opens up a simple strategy not only to dynamically vary the mechanics of tough hydrogels, both in bulk and locally from prefabricated soft tough hydrogels, but also toward their shape morphing behaviors on demand.     

Highly Mechanical and Fatigue‐Resistant Double Network Hydrogels by Dual Physically Hydrophobic Association and Ionic Crosslinking

Double network (DN) hydrogels with high strength and toughness are considered as promising soft materials. Herein, a dual physically cross‐linked hydrophobic association polyacrylamide (HPAAm)/alginate‐Ca²⁺ DN hydrogel is reported, consisting of a HPAAm network and a Ca²⁺ cross‐linked alginate network. The HPAAm/alginate‐Ca²⁺ DN hydrogel exhibits excellent mechanical properties with the fracture stress of 1.16 MPa (3.0 and 1.7 times higher than that of HPAAm hydrogel and HPAAm/alginate hydrogel, respectively), fracture strain of 2604%, elastic modulus of 71.79 kPa, and toughness of 14.20 MJ m^?3. HPAAm/alginate‐Ca²⁺ DN hydrogels also demonstrate self‐recovery, notch‐insensitivity, and fatigue resistance properties without any external stimuli at room temperature through reversible physical bonds consisting of hydrophobic association and ionic crosslinking. As a result, the dual physical crosslinking would offer an avenue to design DN hydrogels with desirable properties for broadening current applications of soft materials.     

抗冻水凝胶的制备及其在柔性电子领域的应用

水凝胶具有优异的柔韧性、离子运输性和可调的机械性,在柔性电子领域具有广阔的应用前景,然而,水凝胶电子器件在严寒气候下容易冻结失效,严重限制了其在低温环境下的应用潜力,通过向水凝胶中引入低温防护剂可以赋予水凝胶抗冻性能,拓宽水凝胶电子器件的工作温度。该文从溶质离子、离子液体、有机溶剂以及抗冻蛋白改性水凝胶4个方面,综述了近年来抗冻水凝胶的制备方法和抗冻机理,阐述了抗冻水凝胶在超级电容器、传感器和电池等柔性电子领域的应用进展,归纳了抗冻水凝胶电子材料面临的问题与挑战,并展望了抗冻水凝胶电子材料的发展趋势,指出以天然可再生资源为原料开发具有优异机械性能、电化学性能、生物无毒性、生物相容性和生物可降解的抗冻水凝胶成为下一步研究重点,同时设计优化柔性电子装置、提高器件安全可靠性和输出稳定性也将成为重要的研究方向之一。抗冻水凝胶的制备及其应用研究将促进柔性电子功能材料领域的快速发展。     

抗冻水凝胶的制备及其在柔性电子领域的应用

水凝胶具有优异的柔韧性、离子运输性和可调的机械性,在柔性电子领域具有广阔的应用前景.然而,水凝胶电子器件在严寒气候下容易冻结失效,严重限制了其在低温环境下的应用潜力.向水凝胶中引入低温防护剂可以赋予水凝胶抗冻性能,拓宽水凝胶电子器件的工作温度.该文从溶质离子、离子液体、有机溶剂以及抗冻蛋白改性水凝胶4个方面,综述了近年...     

A Facile Strategy for Preparing Tough,Self‐Healing Double‐Network Hyaluronic Acid Hydrogels Inspired by Mussel Cuticles

Compared with hydrogel‐like biological tissues such as cartilage, muscles, and blood vessels, current hyaluronic acid hydrogels often suffer from poor toughness and limited self‐healing properties. Herein, a facile and generalizable strategy inspired by mussel cuticles is presented to fabricate tough and self‐healing double‐network hyaluronic acid hydrogels. These hydrogels are composed of ductile, reversible Fe³⁺‐catechol interaction primary networks, and secondarily formed brittle, irreversible covalent networks. Based on this design strategy, the hyaluronic acid hydrogels are demonstrated to exhibit reinforced mechanical strength while maintaining a rapid self‐healing property. In addition, by simply regulating pH or UV irradiation time, the mechanical properties of the hydrogels can be regulated conveniently through variations between the primary and secondary networks.     

Ultratough and recoverable ionogels based on multiple interpolymer hydrogen bonding as durable electrolytes for flexible solid-state supercapacitor

The emerging application of ionogels in flexible devices require it enough durable under repeated mechanical deformation while maintaining their superior electrochemical properties. In this work, ultratough and recoverable ionogels, where ionic liquids are confined in chemically and interpolymer hydrogen-bonding hybrid crosslinked network, were fabricated by in situ copolymerization of acrylic acid and 1-vinylimidazole monomer within 1-buty-3-methylimidazolium chloride ionic liquid. The reversible hydrogen bonds between imidazole and carboxylic acid groups of polymer chains in the network work as reversible “sacrificial bonds” to toughen ionogel, which makes the ionogels tough (tensile strength 1.62 MPa, toughness 8.7 MJ m⁻³), stretchable (elongation at break 1090%), and recoverable (91% recovery resting for 30 min, at 534 kPa stress and 500% strain). Moreover, the hydrogen-bonded ionogels exhibit high ionic conductivity of 2.3 S m⁻¹ at 80°C to 3.2 S m⁻¹ at 150°C. Furthermore, the ionogel-based flexible electrical double-layer capacitor can be operated up to 1.5 V with a capacitance of 341.47 F g⁻¹ at 0.5 A·g⁻¹ and exhibits excellent capacitance retention after 1000 cycles as well as superior electrochemical performance over a wide range of temperature. This work provides new insights into the synthesis of tough and recoverable ionogels for high-performance flexible supercapacitors.     

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

Despite 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.     

Highly stretchable,tough, and self‐recoverable and self‐healable dual physically crosslinked hydrogels with synergistic “soft and hard” networks

Strength, toughness and self‐recoverability are among the most important properties of hydrogels for tissue‐engineering applications. Yet, it remains a challenge to achieve these desired properties from the synthesis of a single‐polymer hydrogel. Here, we report our one‐pot, a monomer‐polymerization approach to addressing the challenge by creating dual physically crosslinked hybrid networks, in particular, synergistic “soft and hard” polyacrylic acid‐Fe³⁺ hydrogels (SHPAAc‐Fe³⁺). Favorable mechanical properties achieved from such SHPAAc‐Fe³⁺ hydrogels included high tensile strength (about 1.08 MPa), large elongation at break (about 38 times), excellent work of extension (about 19 MJ m^?3), and full self‐recoverability (100% recovery of initial properties within 15 min at 50°C and within 60 min in ambient conditions, respectively). In addition, the hydrogels exhibited good self‐healing capabilities at ambient conditions (about 40% tensile strength recovery without any external stimuli). This work demonstrates that dual physical crosslinking combining hydrophobic interaction and ionic association can be achieved in single‐polymer hydrogels with significantly improved mechanical performance but without sacrificing favorable properties. POLYM. ENG. SCI., 59:145–154, 2019. © 2018 Society of Plastics Engineers     

导电水凝胶的制备及应用研究进展

导电水凝胶是一类将亲水性基质和导电介质有机结合的新型水凝胶,具有较高的柔韧性、可调的力学性能和优异的电化学性能,在柔性电子设备等领域具有广阔的应用前景。本文综述了导电水凝胶材料的研究前沿和动态,介绍了导电水凝胶的分类及制备方法,讨论了导电水凝胶的结构设计与性能,重点阐述了导电水凝胶材料的应用研究进展,归纳了导电水凝胶材料面临的问题与挑战,并展望了导电水凝胶材料的发展趋势,指出采用天然可再生资源为原料开发具有高导电性、力学性能稳定、耐极端温度、生物相容性和生物可降解的导电水凝胶将成为下一步研究重点,同时优化柔性电子装置、提高器件输出稳定性也将成为重要的研究方向之一。导电水凝胶的制备及应用研究将促进柔性电子功能材料领域的快速发展。     

Highly Stretchable,Transparent, and Bio‐Friendly Strain Sensor Based on Self‐Recovery Ionic‐Covalent Hydrogels for Human Motion Monitoring

Stretchable, flexible, and strain‐sensitive hydrogels have gained tremendous attention due to their potential application in health monitoring devices and artificial intelligence. Nevertheless, it is still a huge challenge to develop an integrated strain sensor with excellent mechanical properties, broad sensing range, high transparency, biocompatibility, and self‐recovery. Herein, a simple paradigm of stretchable strain sensor based on multifunctional hydrogels is prepared by constructing synergistic effects among polyacrylamide (PAM), biocompatible macromolecule sodium alginate (SA), and Ca ion in covalently and ionically crosslinked networks. Under large deformation, the dynamic SA‐Ca²⁺ bonds effectively dissipate energy, serving as sacrificial bonds, while the PAM chains bridge the crack and stabilize the network, endowing hydrogels with outstanding mechanical performances, for instance, high stretchability and compressibility, as well as excellent self‐recovery performance. The hydrogel is assembled to be a transparent and wearable strain sensor, which has good sensitivity and very wide sensing range (0–1700%), and can precisely detect dynamic strains, including both low and high strains (20–800% strain). It also exhibits fast response time (800 ms) and long‐time stability (200 cycles). The sensor can monitor and distinguish complicated human motions, opening up a new route for broad potential applications of eco‐friendly flexible strain‐sensing devices.     

Preparation and characterization of PAM/SA tough hydrogels reinforced by IPN technique based on covalent/ionic crosslinking

In order to fabricate tough hydrogels with superior formability, polyacrylamide/sodium alginate (PAM/SA) interpenetrating polymer network (IPN) hydrogels were produced with ionically crosslinked SA interpenetrated in covalently crosslinked PAM. TGA results show that the heat resistance of PAM/SA IPN hydrogel is improved as compared to that of the individual component. Swelling studies indicate that increasing either chemical crosslinker content or ionic crosslinking via adding more N,N′‐methylenebisacrylamide (MBA) or SA results in lower ESR. It is concluded by tensile test that loosely crosslinked PAM coupled with tightly crosslinked SA improve mechanical strength for hydrogels based on covalent/ionic crosslinking. PAM/SA hydrogels via “one‐pot” method can form different complex shapes with mechanical properties comparable to conventional double network (DN) gels. The fracture strength of PAM_0.05/SA₂₀ reaches level of MPa, approaching 2.0 MPa. The work strives to provide method to tune mechanical and physical properties for hydrogels, which is hopefully to guide the design of hydrogel material with desirable properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41342.     

A responsive supramolecular hydrogel with robust mechanical properties for actuator and strain sensor

Responsive hydrogels hold great promise for applications such as biological tissue engineering, controlled drug release, soft actuators, and intelligent sensors. However, the design and construction of robust responsive hydrogels using a simple method remains a significant challenge. Herein, a non-covalently crosslinked responsive hydrogel was constructed by introducing carboxyl-Zr⁴⁺ metal coordination to the hydrophobic association network of P(AA-co-LMA) hydrogel through a facile one-pot polymerization method. The incorporation of multiple reversible interactions, including hydrogen bonding, metal coordination, and hydrophobic association, resulted in a responsive hydrogel with exceptional mechanical strength (≈2.92 MPa), outstanding flexibility (elongation>1000%), and rapid response to pH alterations. Furthermore, the hydrogel also presented good ionic conductivity due to the abundant movable ions, as well as high sensitivity and stability. As application demonstrations, the supermolecular hydrogel had been successfully used in actuating and strain sensing. This work establishes an effective design strategy for creating tough and multifunctional responsive hydrogel.     

Agar/PAAc-Fe<Superscript>3+</Superscript> hydrogels with pH-sensitivity and high toughness using dual physical cross-linking

As promising structural materials, various tough hydrogels have been developed recently by incorporating various kinds of bonds. An important challenge is to use dual physical cross-linking to develop both toughness and self-recovery in a single material. Here we report smart, strain-responsive hydrogels composed of a fully physically linked agarose/poly(acrylic acid)-ferric ion (agar/PAAc-Fe³⁺) double network (DN) with high toughness and pH-sensitivity. These hydrogels were fabricated in a one-pot reaction to generate dual physical cross-linking through, first, a hydrogen-bonded cross-linked agarose network, and, second, a physically linked PAAc-Fe³⁺ network via Fe³⁺ coordination interactions. The DN hydrogels possessed high toughness, with breaking strain of 1130%, fast self-recovery properties in ambient conditions (100% recovery in 30 min) and self-healing properties (the healed hydrogels can be manually stretched up to 700% of their original length after self-healing for 60 h from the cut-off state). In addition, the hydrogels exhibited pH-sensitivity due to the dissociation of ionic coordinate bonds between –COO^? ions of the PAAc chains and Fe³⁺ ions. Double-layer hydrogel strips with two different concentrations of PAAc formed a “C”-shaped material when initially immersed in pH 7 solution and then soaked in a pH 3 solution. These characteristics make the hydrogels attractive candidates for tissue engineering, soft actuators and flexible electronics.     

A transparent Laponite polymer nanocomposite hydrogel synthesis via in-situ copolymerization of two ionic monomers

The poor stability of clay dispersion in the presence of ionic species presents a challenge for preparing ionic clay polymer nanocomposite (CPN) transparent hydrogel with desired strength. The transparent and tough ionic hydrogels are highly demanded as potential material options for contact lens or ophthalmic implants. Here we reported an ionic CPN hydrogel with combined high transparency and mechanical properties synthesized via in-situ copolymerization of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and acrylic acid (AA) in Laponite dispersion. The ionic transparent CPN can have strength of 189 kPa and a strain of 1780% which is likely a result of uniformly dispersed Laponite platelets in the structure. A synergistic interaction between acylamino and sulfonic acid functional groups was found to play a key role in the stable dispersion of Laponite. This work provided a new way to prepare transparent ionic CPN hydrogels.     

High strength and toughness of double physically cross-linked hydrogels composed of polyvinyl alcohol and calcium alginate

The weak mechanical properties of hydrogels, especially physically cross-linked hydrogels are usually a major factor to hinder their application. To solve this problem, in this work, we prepared a high strength and toughness of double physically cross-linked (PDN) hydrogels composed of crystalline domain cross-linked polyvinyl alcohol (PVA) and Ca²⁺-cross-linked alginate (Alg). With a further annealing treatment, the noncovalent cross-linked network via the formed crystalline promote the as-prepared PDN PVA/Alg hydrogel to exhibit well mechanical properties with the tensile strength of ~1.94 MPa, elongation at break of ~607% and Young's modulus of ~0.45 MPa (above 70 wt% of water content). By analyzing the mechanism of improving the hydrogel mechanical properties, it is found that annealing can effectively improve the crystallinity of PVA in the hydrogel, and then greatly improve the mechanical properties of the hydrogel. This provides a general method for improving the mechanical properties of PVA PDN hydrogels. In addition, the PDN PVA/Alg hydrogel was also proved to have good ionic conductivity of 1.70 S m⁻¹. These desirable properties make the prepared physically cross-linked hydrogels promising materials for medical and biosensing fields.