Multiple Crosslinking Research Articles

PVA/sodium alginate multi-network aerogel fibers, incorporated with PEG and ZnO, exhibit enhanced temperature regulation, antibacterial, thermal conductivity, and thermal stability

This paper presents a novel approach for the fabrication of polyvinyl alcohol (PVA)/sodium alginate (SA) aerogel fibers with a multilayered network structure using wet spinning and freeze-thaw cycling techniques. The multiple cross-linking networks regulate the pore structure, leading to the formation of stable and tunable multilevel pore architectures. PEG and nano-ZnO were successfully loaded onto the PVA/SA modified aerogel fibers (MAFs) using vacuum impregnation. MAFs exhibited excellent thermal stability at 70 °C without leakage after 24 h of heating. Furthermore, MAFs demonstrated excellent temperature regulation performance, with a latent heat of 121.4 J/g, which accounts for approximately 83 % of PEG. After modification, the thermal conductivity of MAFs was significantly improved, and they exhibited excellent antibacterial properties. Therefore, MAFs are expected to be widely used in intelligent temperature-regulating textiles.

Development of foam-based support material for coaxial bioprinting of ionically crosslinking bioinks

In this study, a foam-based method is developed for three-dimensional coaxial bioprinting of ionically crosslinking bioinks. This method introduces the crosslinker to the bioink in calcium chloride-albumin foam which eliminates the need for multiple crosslinking steps and offers an excellent control over the crosslinking rate and the diameter of the hollow fibers. The effects of the foam and alginate flow rates were investigated on the outer diameter and the wall thickness of the hollow fibers. Various structures were 3D printed and characterized by printability number and the method showed an excellent layer adhesion among printed layers. The effects of foam composition and the alginate concentration on the mechanical properties were assessed through breaking strain and filament collapse tests to determine the optimum composition for hollow fiber fabrication. The hollow fiber composed of 2% (w/v) sodium alginate that is crosslinked with a foam made of 1.07% (w/v) albumin and 1.07% (w/v) calcium chloride showed superior mechanical properties. Furthermore, the viability of co-incubation with Neuro-2a cells over seven days was investigated and no significant negative effect of the used concentrations of albumin and calcium chloride was observed on the viability of the cells.

Zr, N-Co-Doped Carbon Quantum Dot Crosslinking Agents for Use in Fracturing Fluids

To further improve the performance of the fracturing fluid, the Zr, N-co-doped carbon quantum dot nano-crosslinking agent was synthesized by the hydrothermal method to solve the problems of easy agglomeration, instability, and gel breaking after crosslinking of the conventional polyacrylamide fracturing fluid crosslinking agent. The crosslinking agent has good stability and multiple crosslinking sites, as well as can be crosslinked with anionic polyacrylamide (HPAM) solution to form a high-strength hydrogel fracturing fluid. Moreover, it can be used as a nano-fluorescent tracer to test the flowback rate of fracturing fluid. The fracturing fluid composed of 0.3 wt % HPAM and 0.8 wt % crosslinking agent was named as Zr-CDG. At 180 °C, the viscosity of Zr-CDG remains above 180 mPa·s after shearing for 2 h, indicating that Zr-CDG has excellent temperature tolerance and shearing resistance. The residue is only 169 mg/L after gel breaking, which means low damage to the formation. Finally, the backflow process of fracturing fluid is simulated by core displacement. According to the good luminescence of the carbon quantum dot nano-crosslinking agent, the flowback rate of the fracturing fluid is calculated. It shows that the carbon quantum dot nano-crosslinking agent also has the role of fracturing fluid tracer. The results show that the properties of the prepared carbon quantum dot nano-crosslinking agent are more stable, and the performance of the crosslinked fracturing fluid system is greatly improved, which points out a direction for the application of carbon quantum dots.

Robust Hydrogel with Significant Swelling Resistance via the Synergy of Hydrogen Bond Regulation and Ionic Cross-linking

The development of high-performance hydrogels has received much attention, but the simple and cost-effective preparation of hydrogels with high mechanical strength and excellent swelling resistance remains a challenge. In this study, a facile and green strategy was proposed to construct a polyvinyl alcohol/chitosan-based (PVA/CS-based) hydrogel with high mechanical strength and superior swelling resistance. By introducing tannic acid (TA), a natural polyphenol, and forming hydrogen bonding cross-linking with the polymer chains through solvent regulation, as well as by ionic cross-linking and salting-out effects further induced through sodium citrate solution immersion, a strong and homogeneous multiple physical cross-linking network was formed, which endowed the resulting hydrogel with excellent mechanical and swelling resistance performance. For the hydrogel with PVA, CS, and TA in the mass ratio of 5:1:5, the tensile strength was up to 8.22 MPa, and the tensile strength retention was up to 96% of the original value after swelling equilibrium. In addition, our hydrogel can be attached in situ to various substrates and enabled wet adhesion. The easy scale-up and energy-saving features made our method highly promising for the preparation of high-performance hydrogels for many advanced applications.

Nano-enabled DNA supramolecular sealant for soft tissue surgical applications

Bioadhesives or sealants have received considerable attention for suture-less wound sealing. However, most adhesive biomaterials involve complex molecular designs which lack dynamic mechanical properties, inherent hemostasis, as well as the capability of preventing postsurgical tissue adhesion. Herein, a nano-enabled supramolecular hydrogel sealant based on complementary DNA duplexes combined with multiple physicochemical crosslinks is designed and demonstrated. This supramolecular sealant exhibits excellent dynamic reversibility, low swelling, and wet adhesive properties. It can achieve rapid sealing of damaged tissues, blood coagulation, as well as hemostasis. Furthermore, DNA molecules endow the sealant with promising capabilities of preventing protein absorption, cell adhesion and postsurgical tissue adhesion. The in vivo evaluation based on the gastric perforation repair model shows that the sealant can improve granulation tissue growth, collagen deposition, and vascularization and facilitate gastric perforation closure. The nano-enabled DNA supramolecular hydrogel sealant represents a promising alternative for gastric perforation repair and has large clinical potential pertaining to suture-less repair of soft tissues. Data availabilityThe data reported in this manuscript are available upon request.

Novel Uracil-Functionalized Poly(ionic liquid) Hydrogel: Highly Stretchable and Sensitive as a Direct Wearable Ionic Skin for Human Motion Detection.

Conductive hydrogel-based ionic skins have attracted immense attention due to their great application prospects in wearable electronic devices. However, simultaneously achieving a combination of a single hydrogel system and excellent comprehensive performance (i.e., mechanical durability, electrical sensitivity, broad-spectrum antibacterial activity, and biocompatibility) remains a challenge. Thus, a novel poly(ionic liquid) hydrogel consisting of poly(acrylamide-co-lauryl methacrylate-co-methyl-uracil-imidazolium chloride-co-2-acryloylamino-2-methyl-1-propane sulfonic acid) (AAm-LMA-MUI-AMPS) was prepared by a micellar copolymerization method. Herein, MUI serves as a supramolecular crosslinker and conductive and bacteriostatic components. Owing to the multiple supramolecular crosslinks and hydrophobic association in the network, the hydrogel exhibits excellent mechanical properties (624 kPa of breaking stress and 1243 kPa of compression stress), skin-like modulus (46.2 kPa), stretchability (1803%), and mechanical durability (200 cycles under 500% strain can be completely recovered). Moreover, with the coordinated combination of each monomer, the hydrogel exhibits the unique advantage of high conductivity (up to 59.34 mS/cm). Hence, the hydrogel was further assembled as an ionic skin sensor, which exhibited a gauge factor (GF) of 10.74 and 7.27 with and without LiCl over a broad strain range (1-1000%), respectively. Furthermore, the hydrogel sensor could monitor human movement in different strain ranges, including body movement and vocal cord vibration. In addition, the antibacterial activity and biocompatibility of the hydrogel sensor were investigated. These findings present a new strategy for the design of new-generation wearable devices with multiple functions.

Low-intensity mixing process of high molecular weight polymer chains leads to elastomers of long network strands and high fatigue threshold.

Many polymer networks are prepared by crosslinking polymer chains. The polymer chains and crosslinkers are commonly mixed in internal mixers or roll mills. These intense processes break the polymer chains, lower viscosity, and ease mixing. The resulting polymer networks have short chains and a fatigue threshold of ∼100 J m-2. Here, we show that a low-intensity process, a combination of kneading and annealing, preserves long chains, leading to a network of polybutadiene to achieve a fatigue threshold of 440 J m-2. In a network, each chain has multiple crosslinks, which divides the chain into multiple strands. At the ends of the chain are two dangling strands that do not bear the load. The larger the number of crosslinks per chain, the lower the fraction of dangling strands. High fatigue threshold requires long strands, as well as a low fraction of dangling strands. Once intense mixing cuts chains short, each short chain can only have a few crosslinks; the strands are short and the fraction of dangling strands is high-both lower the fatigue threshold. By contrast, a low-intensity mixing process preserves long chains, which can have many crosslinks; the strands are long and the fraction of dangling strands is low-both increase the fatigue threshold. It is hoped that this work will aid the development of fatigue-resistant elastomers.

Self-healing liquid metal hydrogel for human-computer interaction and infrared camouflage.

Due to their mechanical flexibility, conductive hydrogels have been widely investigated in the fields of flexible electronics and soft robots, but their non-negligible disadvantages, such as poor toughness and limited self-healing, severally restrict their practical application. Herein, gallium indium alloy (EGaIn) is utilized to initiate the polymerization and simultaneously serve as flexible fillers to construct a super-stretchable and self-healing liquid metal/polyvinyl alcohol/p(acrylamide-co-octadecyl methacrylate) (liquid metal/PVA/P(AAm-co-SMA)) double network hydrogel (LM hydrogel). The synergistic effect of the rigid PVA microcrystal network and the ductile P(AAm-co-SMA) hydrophobic network, together with the ionic coordination and hydrogen bonds between polymer networks (multiple physical cross-links), endow the LM hydrogel with excellent super-stretchability (2000%), toughness (3.00 MJ m-3), notch resistance, and self-healing property (healing efficiency > 99% at 25 °C after 24 h). The LM hydrogel exhibits sensitive strain sensing behavior, allowing human-computer interaction to achieve motion recognition and health monitoring. Significantly, owing to the excellent photothermal effect and low infrared emissivity of EGaIn, the LM hydrogel reveals great potential in infrared camouflage. The work of self-healing conductive liquid metal hydrogels will promote the research and practical application of hydrogels and liquid metal in intelligent devices and military fields.

Tough self-healing polyurethane elastomers based on interpenetrating networks containing multiple hydrogen bond networks, flexible blocks, metal coordination and covalent cross-linking

Currently, it is a challenge to prepare elastomers with high mechanical properties and high self-healing efficiency. In the present work, by mixing a polyurethane (PU) prepolymer with a PU elastomer containing 2-ureido-4[1H]-pyrimidinone (UPy) groups, we obtained a new PU elastomer based on interpenetrating networks. In this condition, four different structures coexist in the interpenetrating polymer networks: covalent cross-linking network, metal-ion coordination network, multiple hydrogen bond network and flexible blocks. The experimental results demonstrated that the presence of metal-ion coordination network, multiple hydrogen bond network and flexible blocks is beneficial for the improvement in healing efficiency, while the coexistence of covalent cross-linking network, metal-ion coordination network and multiple hydrogen bond network benefits to improve the mechanical properties.

Near-Infrared-Responsive Choline-Calix[4]arene-Gold Nanostructures for Potential Photothermal Cancer Treatment

The development of novel chemical approaches for the fabrication of gold nanostructures with localized surface plasmon resonance (LSPR) falling in the near-infrared (NIR) region is one challenging topic in nanomaterials science. Due to their optical and photothermal properties triggered by light excitation in the therapeutic window (λmax = 650–1300 nm), gold-based nanostructures are appealing candidates in anticancer nanomedicine. Here, we report a novel method to prepare water-dispersible gold nanostructures with NIR-LSPR (λmax = 600–1000 nm) properties. The gold nanostructures were achieved in a single step by an unconventional method using NADH as a reducing agent and an amphiphilic choline-calix[4]arene derivative (CholCalix) forming micelles as a template. The CholCalix-Au nanostructures were characterized by UV–visible spectrophotometry, Raman spectroscopy, and atomic force microscopy. Agglomeration of the nanostructures due to multiple crosslinking interactions was observed and supported by modeling simulation. Effective anticancer photothermal-induced effect of the CholCalix-AuNPs was demonstrated on human breast cancer cells irradiated with biofriendly light at 808 nm.

An Eco-Friendly Antheraea Pernyi Silk Gland Protein/Sodium Alginate Multiple Network Hydrogel as Potential Drug Release Systems

To improve the versatility of the sodium alginate-loaded bio-hydrogels, Antheraea pernyi silk gland protein/sodium alginate drug-loaded hydrogels were prepared by using an eco-friendly multiple network cross-link technology. Fourier transform infrared (FT-IR) spectroscopy and UV-Vis spectrophotometer were used separately to evaluate the chemical structure and drug release behavior of drug-loaded hydrogels. The antibacterial drug carrier gels were evaluated by using inhibition zone test against the S. aureus and E. coli. The CCK-8 assay was used to assess the biocompatibility of drug loaded hydrogels. The FT-IR results showed that there was a strong interaction within the drug loaded hydrogels, and the ASGP was beneficial to enhance the interaction within the drug loaded hydrogels. UV-Vis spectrophotometer results indicated the cumulative release reached 80% within 400 min. Antibacterial bio-hydrogels had a good antibacterial property, especially the antibacterial bio-hydrogels with bacitracin exhibits superior to other antibacterial agents. The drug-loaded bio-hydrogels exhibited the adhesion and proliferation of RSC96 cells and perfected biocompatibility. This provides a new idea for further research and development of tissue-friendly drug-loaded biomaterials.

Pectin-based bioinks for 3D models of neural tissue produced by a pH-controlled kinetics.

Introduction: In the view of 3D-bioprinting with cell models representative of neural cells, we produced inks to mimic the basic viscoelastic properties of brain tissue. Moving from the concept that rheology provides useful information to predict ink printability, this study improves and expands the potential of the previously published 3D-reactive printing approach by introducing pH as a key parameter to be controlled, together with printing time. Methods: The viscoelastic properties, printability, and microstructure of pectin gels crosslinked with CaCO3 were investigated and their composition was optimized (i.e., by including cell culture medium, HEPES buffer, and collagen). Different cell models representative of the major brain cell populations (i.e., neurons, astrocytes, microglial cells, and oligodendrocytes) were considered. Results and Discussion: The outcomes of this study propose a highly controllable method to optimize the printability of internally crosslinked polysaccharides, without the need for additives or post-printing treatments. By introducing pH as a further parameter to be controlled, it is possible to have multiple (pH-dependent) crosslinking kinetics, without varying hydrogel composition. In addition, the results indicate that not only cells survive and proliferate following 3D-bioprinting, but they can also interact and reorganize hydrogel microstructure. Taken together, the results suggest that pectin-based hydrogels could be successfully applied for neural cell culture.

Construction of Carboxymethyl Chitosan Hydrogel with Multiple Cross-linking Networks for Electronic Devices at Low Temperature.

On the basis of the original hydrogen bonding interaction and physical entanglement, covalent cross-linking and ionic cross-linking were additionally introduced to construct a carboxymethyl chitosan/allyl glycidyl ether conductive hydrogel (CCH) through a one pot method by a graft reaction, an addition reaction, and simple immersion, successively. The multiple cross-linking networks significantly increased the strength of CCHs and endowed them with ionic conductivity and an antifreezing property at -40 °C, which showed stable, durable, and reversible sensitivity to finger bending activity at subzero temperature. The CCHs could even be assembled into a triboelectric nanogenerator (TENG) to provide electric energy, which demonstrated stability against temperature variation, multiple drawing, long-term storage, or large quantities of contact-separation motion cycles. CCH-TENG can also be used as a tactile sensor within the pressure range from 0.4 kPa to higher than 8000 kPa. This work provided a simple route to fabricate antifreezing conductive hydrogels based on carboxymethyl chitosan and to find potential applications in soft sensor devices under a low temperature environment.

Two dominant timescales of cytoskeletal crosslinking in the viscoelastic response of the cytoplasm

Cells precisely regulate their frequency-dependent viscoelastic properties in response to chemical and mechanical cues. We use optical trap-based active microrheology using intracellular probes to measure the cytoplasmic mechanical response of fibroblast and macrophage cells over a broad frequency range ($\ensuremath{\sim}0.02--350$ Hz). Both cell types show similar frequency-dependent behavior, suggesting that the mechanisms that control the cell's mechanical response are general to many cell types. At frequencies above 1 Hz, the cytoplasmic mechanical behavior shows a broad distribution of relaxation timescales consistent with power-law mechanics. At low frequencies ($<1$ Hz), cells exhibit fluidlike behavior with distinct relaxation timescales, similar to that observed in reconstituted networks of transiently crosslinked actin filaments. The response across all frequencies can be captured by a mathematical model combining a power-law term with two crosslinker-unbinding terms. The two unbinding rates required to describe the low-frequency response suggest that the viscoelastic relaxation of the cytoplasm is governed either by multiple dominant crosslinkers or by a single crosslinker with multiple states.

3D Printed Multifunctional Self‐Adhesive and Conductive Polyacrylamide/Chitosan/Sodium Carboxymethyl Cellulose/CNT Hydrogels as Flexible Sensors

Abstract3D printing of hydrogels with improved mechanical properties will play an important role in many fields in the future. Polyacrylamide with controllable reaction conditions and chitosan with increased mechanical strength are chosen to prepare hybrid hydrogels with high mechanical properties (elongation >2000%). The addition of sodium carboxymethyl cellulose enables this hydrogel system to have excellent rheological properties for 3D printing. The samples prepared by 3D printing technology have larger elongation (>1000%) and higher elastic modulus (141.99 kPa). Carbon nanotube‐added composite hydrogels can be used to fabricate flexible electronic devices with diverse functions and structures. The prepared sensor can detect the signals of human movement (joint movement, breathing, drinking water), and has a sensitive signal response in the range of 12–67 °C. In addition, this sensor can also be extended to the application of NH3 gas signal sensing. Due to the stable performance and long service life of conductive multifunctional hydrogels, the application potential of hydrogel sensors will be further increased. In conclusion, this simple‐prepared 3D‐printable high‐mechanical‐performance hydrogel with multiple network crosslinks has a favorable competitive advantage in future flexible material applications.

Ultrastretchable Composite Organohydrogels with Dual Cross-Links Enabling Multimodal Sensing.

Building multiple cross-links or networks is a favorable way of diversifying applications of the hydrogels, which is also available for the organohydrogels prepared via the solvent replacement way. However, the situations become more complicated for organohydrogels due to the presence of replaced solvents. Therefore, the correlations between the multiple cross-links and final performance need to be better understood for the organohydrogels, which is vital for tailoring their inherent properties to expand final application scenarios. Polyacrylamide (PAM)/poly(vinyl alcohol) (PVA)/MXene composite organohydrogels with dual cross-links, namely, the covalently cross-linked PAM chains as the primary network and the physically cross-linked PVA/PAM chains with MXene particles as the secondary cross-links, were developed here for the study. The occurrence of the secondary cross-links plays multiple roles as sacrificial units endowing the system with ultrastretchability with an excellent strain-resistance effect and as temperature-sensitive units endowing the system with thermosensation ability with an outstanding temperature coefficient of resistance. Thus, the optimized sample can be used as a strain sensor with excellent environmental tolerance for detecting human motion as a pressure sensor to probe compression with weak deformation and as a thermal sensor to capture environmental temperature changes. This work provides valuable information on developing organohydrogels with superior performance for multimodal sensors.

Multiply cross-linked poly(vinyl alcohol)/cellulose nanofiber composite ionic conductive hydrogels for strain sensors

Building multiple chemical crosslinks is an effective strategy to improve mechanical properties and to diversify final application of polysaccharide nanoparticles reinforced poly(vinyl alcohol) (PVA) physical hydrogels. In this work, PVA/cellulose nanofibers (CNFs) were used as composite substrate to fabricate ionic conductive hydrogels for strain sensor. Three types of characteristic crosslinks, including chemical crosslinking via boronic ester covalent bonds only, and with additional metal coordination bonding, as well as coexistence of physical crosslinks via PVA crystallites and aforementioned two kinds of chemical crosslinks, were constructed. The sample with triple crosslinks has superior mechanical strength and resistance to fatigue, and the polydopamine/Fe3+ ratio act as key to tune final performance because double-network structure prefers to form as Fe3+ is superfluous, while dual-crosslink one forms in the case of insufficient Fe3+. As-optimized ionic conductive hydrogel is suitable as strain sensor for probing human motions. This work provides an interesting insight into the network structure and property regulation for PVA/CNF composite hydrogels with multiple crosslinks.

Synergistic improvement of CO2/CH4 separation performance of phenolphthalein-based polyimide membranes by thermal decomposition and thermal-oxidative crosslinking

Thermal oxidative crosslinking results in the generation of oxygen bridges among polymer chains and helps regulate the membrane microstructure. Simultaneously, any strategy to increase the crosslinking sites of polyimide (PI) can improve the crosslinking efficiency and allow precise control of the pore structure of membranes. Phenolphthalein-based PIs are potentially suitable precursors for thermal oxidative crosslinking of membranes, since thermal treatment will result in the degradation of lactone ring leading to the generation of a pair of phenyl crosslinking sites. In this work, three phenolphthalein-based diamines with different structures were designed and synthesized. The three diamines were subsequently reacted with 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) to prepare phenolphthalein-based PIs. The original membranes were post-treated by thermal oxidation crosslinking at 400 °C and 425 °C, respectively; the lactone ring underwent decomposition, and the imide ring opened at the same time, resulting in the generation of multiple crosslinking sites and correspondingly improving the crosslinking efficiency. The effect of the side groups (−CH3, –CF3 or unsubstituted) of phenolphthalein-based diamine and thermal treatment temperatures on the gas separation performance of phenolphthalein-based PI membranes were investigated. Wide-angle X-ray diffractometry (WAXD) and wide-angle X-ray scattering (WAXS) analysis demonstrated that the π-π stacking distance were improved. BTDA-FPP-425 had a CO2/CH4 selectivity of 37.68. This thermal degradation along with thermal oxidative crosslinking strategy provides a promising approach for fabrication of CO2/CH4 separation membrane.

Biomimetic development of a strong, mildew-resistant soy protein adhesive via mineral–organic system and phenol-amine synergy

Plant-derived protein adhesives have attracted extensive research interest in recent years as effective alternatives to nonrenewable and nonbiodegradable formaldehyde-based adhesives. However, owing to their insufficient bonding strength and poor mildew resistance, it has been challenging for biopolymer adhesives to achieve strong and durable adhesion performance. Inspired by insect cuticles , we report a green and versatile strategy for the fabrication of a strong, mildew-resistant, antibacterial soy protein (SP)-based adhesive via phenol-amine synergy and biomineralization reinforcement. Gallic acid (GA), as a phenolic glue molecule, was co-assembled with hydroxyapatite (HAP) through Ca 2+ –phenolic coordination bonds to prepare GA-functionalized HAP (GA@HAP) nanoparticles. Moreover, ε -polylysine (PL) with abundant amino groups was used for grafting to the phenolic GA@HAP nanoparticles through a Schiff base reaction. Owing to multiple cross-linking and the inorganic–organic hybrid system, the wet shear strength of the SP/PL/GA@HAP adhesive increased considerably to 1.09 MPa, 127% higher than the unmodified SP adhesive. The cationic PL polymer endowed the protein adhesive with desirable antifungal and antibacterial properties , synergistically with the phenolic components in GA@HAP. In addition, the resultant adhesive exhibited enhanced flame retardation, thermal stability, and water resistance. This simple and versatile strategy could lead to advances in the development of high-performance biopolymer materials for biological and engineering applications including adhesives, hydrogels, and films. • A mineral–organic hybrid network was constructed in a biopolymer system. • Hydroxyapatite nanoparticles were functionalized with phenolic glue molecules. • Multifunctional soy protein adhesive was fabricated via phenol-amine chemistry. • The wet shear strength of the prepared adhesive was significantly improved by 127%. • Biopolymer adhesive showed enhanced anti-mildew and antibacterial properties.

Bioinspired adhesive and self-healing bacterial cellulose hydrogels formed by a multiple dynamic crosslinking strategy for sealing hemostasis

Bioinspired adhesive and self-healing bacterial cellulose hydrogels formed by a multiple dynamic crosslinking strategy for sealing hemostasis