Multiple Crosslinking Research Articles

PVA/gelatin/β-CD-based rapid self-healing supramolecular dual-network conductive hydrogel as bidirectional strain sensor

Hydrogel-based flexible sensors have exhibited great application prospect in the fields of e-skin, movement monitoring and human-computer interaction. Nevertheless, it is still a difficult problem to construct the biomass-based hydrogel sensors with considerable mechanical property, biocompatibility, self-healing property, sensitivity, self-adhesion and fatigue resistance. Herein, a gelatin-based dual-network (DN) hydrogel sensor with multiple dynamic crosslinking was successfully fabricated, in which the dynamic Schiff base bonds between gelatin and an aldehyde-containing supramolecular crosslinker (PCD-Fc-CHO) formed by the host-guest complexing of β-cyclodextrin (β-CD) to ferrocene (Fc) were used to construct the first network of the hydrogel, while the reversible borate ester bonds of PVA with borax were utilized to establish the second network. Meantime, carbon nanotubes (CNTs) were added to endow the gelatin-based DN hydrogel with good conductivity and improved mechanical property. The prepared gelatin-based DN hydrogel possesses favorable ductility (1200%), biocompatibility, double self-healing (HE = 96%), high sensitivity (GF = 6.4), self-adhesion (repeatable adhesion), dual stimuli-responsiveness (pH and oxidation), plasticity and fatigue resistance. The DN hydrogel-based strain sensor can not only monitor various human body (e.g. fingers, knees, speech, etc.) movements and vital signs (pulse), but also monitor in vitro the breathing movements of pig lungs. Interestingly, the self-healed gelatin-based DN hydrogel sensor can also be reused, and its monitoring ability is basically the same as the original hydrogel. This work puts forward a new strategy for the construction of multiple dynamically cross-linked biomass-based self-healing hydrogels as multifunctional wearable flexible sensors.

Reinforcing DNA Supramolecular Hydrogel with Polymeric Multiple-Unit Linker

Reinforcing DNA Supramolecular Hydrogel with Polymeric Multiple-Unit Linker

Halogen-free and phosphorus-free flame retardants endow epoxy resin with high flame retardancy through crosslinking strategy

Epoxy resin (EPs) has been widely used in many fields in recent years, such as electronics, adhesives, coatings, and so on, which mainly benefiting from its excellent mechanical and chemical properties, low price and easy preparation. However, conventional EPs tend to be flammable, which significantly prevents their applications especially in high flame-resistance required areas. In this work, we introduce nitrile groups and the benzoxazine ring into the flame-retardant, followed by a simple heat treatment for a multiple cross-linking reaction in EPs. The resultant halogen/phosphorus-free and environmentally friendly network not only suppress the migration of the functional flame retardants from the substrate, but also shows much enhanced flame-retardant property, including the UL-94 rate, total heat release and reduced peak heat release rate. As a result, the thermosets can pass the UL-94 V-0 rate and reach a LOI value at 32.7% at a very low addition amount (10 wt%) of this cross-linked flame retardant.

Injectable photosensitizing supramolecular hydrogels: A robust physically cross-linked system based on polyvinyl alcohol/chitosan/tannic acid with self-healing and antioxidant properties

The development of injectable and photosensitizing supramolecular hydrogels has proved to be a challenging and interesting task. In this study, N, N′-di-(L-alanine)-3,4,9,10-perylene tetracarboxylic diimide (PDI-Ala), as a photosensitizing agent, was incorporated into single (PVA-CS-PDI/TA) and double cross-linked (PVA-CS-PDI/TA/Fe (III)) hydrogels formed by poly (vinyl alcohol) (PVA), chitosan (CS), and tannic acid (TA) through a freeze-thawing method based on non-covalent interaction (hydrogen bonding and metal coordination). TA molecules act as cross-linkers to connect polymer chains through multiple hydrogen bonds and cross-link them using coordinate bonds in the presence of Fe (III) ions. The double cross-linked hydrogels had a higher singlet oxygen quantum yield than single cross-linked hydrogels under 660 nm laser irradiation, indicating those that could be more efficient in photodynamic therapy (PDT). Supramolecular hydrogels with interconnected porous morphologies exhibited mechanical strength, injectability, self-healing and shear-thinning capabilities, antioxidant and antibacterial properties. Additionally, the supramolecular photosensitizing hydrogels showed excellent biocompatibility without showing evident cytotoxicity and hemolysis in vitro. This study presents a straightforward and cost-effective approach to prepare multifunctional physical cross-linking supramolecular hydrogels, appropriate for large-scale preparation, which shows great potential in biomedical applications such as PDT and wound dressings.

Anisotropic Hybrid Hydrogels Constructed via the Noncovalent Assembly for Biomimetic Tissue Scaffold

AbstractAnisotropic structure is key for exploring the biomimetic functions of anisotropic hydrogels. However, the anisotropic hydrogel study should not be limited to its architecture design but must include the understanding and improvement of the internal interaction among their components. Herein, a noncovalent mediated assembly strategy is proposed to simultaneously improve the chitin chain mobility and enhance the interfacial interaction, for achieving anisotropic chitin/2D material (molybdenum disulfide and brushite as example) hydrogels via mechanical deformation. Tannic acid (TA) is used to i) introduce the dynamic noncovalent crosslinking structure among the chitin chains for affording considerable molecular mobility to allow chitin chains alignment under mechanical deformation; ii) enhance chitin–2D interfacial interaction for benefiting 2D materials orientation under the chitin chains driving. The design concept achieves multiple noncovalent assembly crosslinks (chitin–chitin, chitin–TA, and chitin–TA–2D) and biomimetic anisotropic nanofibrous morphology, leading to the superior mechanical performance. The anisotropic chitin–TA/brushite hydrogel effectively accelerates bone regeneration by promoting cell osteogenic differentiation and directional migration, showing potential in tissue engineering. It is anticipated that the noncovalent mediated assembly concept could be used to fabricate other polymer based composite anisotropic hydrogels for diverse applications.

Effect of Cross-Linkers on Mussel- and Elastin-Inspired Adhesives on Physiological Substrates.

Surgical adhesives can be useful in wound closure because they reduce the risk of infection and pain associated with sutures and staples. However, there are no commercially available surgical adhesives for soft tissue wound closure. To be effective, soft tissue adhesives must be soft and flexible, strongly cohesive and adhesive, biocompatible, and effective in a moist environment. To address these criteria, we draw inspiration from the elasticity and resilience of elastin proteins and the adhesive of marine mussels. We used an elastin-like polypeptide (ELP) for the backbone of our adhesive material due to its elasticity and biocompatibility. A mussel-inspired adhesive molecule, l-3,4-dihydroxyphenylalanine (DOPA), was incorporated into the adhesive to confer wet-setting adhesion. In this study, an ELP named YKV was designed to include tyrosine residues and lysine residues, which contain amine groups. A modified version of YKV, named mYKV, was created through enzymatic conversion of tyrosine residues into DOPA. The ELPs were combined with iron(III) nitrate, sodium periodate, and/or tris(hydroxymethyl)phosphine (THP) cross-linkers to investigate the effect of DOPA- and amine-based cross-linking on adhesion strength and cure time on porcine skin in a warm, humid environment. Incorporation of DOPA into the ELP increased adhesive strength by 2.5 times and reduced failure rates. Iron cross-linkers improved adhesion in the presence of DOPA. THP increased adhesion for all proteins tested even in the absence of DOPA. Using multiple cross-linkers in a single formulation did not significantly improve adhesion. The adhesives with the highest performance (iron nitrate mixed with mYKV and THP mixed with YKV or mYKV) on porcine skin had 10-18 times higher adhesion than a commercial sealant and reached appreciable adhesive strength within 10 min.

The modification of collagen with biosustainable POSS graft oxidized sodium alginate composite

As a kind of renewable biological resource, leather collagen is the raw material of leather industry. The chrome tanning modified collagen has high humidity and heat resistance, but there are some defects such as environmental pollution, harm to human health and shortage of chromium resources, so it is urgent to research chromium-free tanning to achieve clean modified colloidal. In the chromium-free tanning system, the modification of skin fibrin by bio-based material has the significance of environmental protection. Sodium alginate can be used as crosslinking agent to stabilize collagen. In this study, the polyhedral oligomeric silsesquioxane (POSS) grafted oxidized sodium alginate polymer composite (POSS-OSA-MAA) containing aldehyde group was prepared by two steps. Firstly, POSS grafted sodium alginate polymer composite (POSS-SAG-MAA) was prepared by graft polymerization with POSS, sodium alginate (SAG) and methacrylic acid (MAA) as raw materials. Then POSS-OSA-MAA was obtained by oxidation of POSS-SAG-MAA with sodium periodate. The aldehyde group highest concentration in POSS-OSA-MAA was 3.26 mmol•g−1, and the tanned leather shrinkage temperature was 73.1 °C. The X-ray photoelectron spectroscopy showed that POSS-OSA-MAA could form Schiff base structure with amino-containing substances and form multiple points crosslinking, and POSS-OSA-MAA could improve the shrinkage temperature and thermal stability of skin collagen.

Janus Cross-links in Supramolecular Networks.

Thermosets composed of cross-linked polymers demonstrate enhanced thermal, solvent, chemical, and dimensional stability as compared to their non-cross-linked counterparts. However, these often-desirable material properties typically come at the expense of reprocessability, recyclability, and healability. One solution to this challenge comes from the construction of polymers that are reversibly cross-linked. We relied on lessons from Nature to present supramolecular polymer networks comprised of cooperative Janus-faced hydrogen bonded cross-links. A triazine-based guanine-cytosine base (GCB) with two complementary faces capable of self-assembly through three hydrogen bonding sites was incorporated into poly(butyl acrylate) to create a reprocessable and recyclable network. Rheological experiments and dynamic mechanical analysis (DMA) were employed to investigate the flow behavior of copolymers with randomly distributed GCB units of varying incorporation. Our studies revealed that the cooperativity of multiple hydrogen bonding faces yields excellent network integrity evidenced by a rubbery plateau that spanned the widest temperature range yet reported for any supramolecular network. To verify that each Janus-faced motif engages in multiple cross-links, we studied the effects of local concentration of the incorporated GCB units within the polymer chain. Mechanical strength improved by colocalizing the GCB within a block copolymer morphology. This enhanced performance revealed that the number of effective cross-links in the network increased with the local concentration of hydrogen bonding units. Overall, this study demonstrates that cooperative noncovalent interactions introduced through Janus-faced hydrogen bonding moieties confers excellent network stability and predictable viscoelastic flow behavior in supramolecular networks.

A triple-diazonium reagent for virus crosslinking and the synthesis of an azo-linked molecular cage.

The first bench-stable triple-diazonium reagent (TDA-1) was rationally designed and synthesized for coupling and crosslinking. The three reactive sites of TDA-1 can react with phenol-containing molecules as well as plant viruses in aqueous buffers efficiently. In addition, a new-type azo-linked cage was constructed by the direct reaction of TDA-1 with a triple-phenol molecule and was characterized by X-ray crystallography.

Hybridly double-crosslinked carbon nanotube networks with combined strength and toughness via cooperative energy dissipation.

Although chemical crosslinking has been extensively explored to enhance the mechanical properties of network-type materials for structural and energy (electrochemical, thermal, etc.) applications, loading-induced energy dissipations usually occur through a single channel that either leads to network brittleness or low strength/stiffness. In this work, we apply coarse-grained molecular dynamics simulations to explore the potential of hybridly double-crosslinked carbon nanotube (CNT) networks as a light weight functional material with combined strength and toughness. While increasing the crosslinking density or strong crosslink composition may, in general, enhance the strength and toughness, further increasing the two parameters would surprisingly lead to deteriorated strength and toughness. We find that double-crosslinked networks can nicely achieve cooperative energy dissipation with minimal structural damage. In particular, the weak crosslinks serve as "sacrificial bonds" to dissipate elastic energies from external loading, while the strong crosslinks act as "structure holders" and break at a much later stage during the tensile test. Therefore, the combination of more than one type of crosslinking with hybrid potential energy landscapes and breaking time scales can prevent premature simultaneous breaking of multiple strong crosslinks. By deploying intermediate amounts of weak and strong crosslinks, we observe an outstanding density-normalized strength of 227-2130 kPa m3 kg-1 as compared to many structural materials and advanced nanocomposites. The crosslinking strategies developed here would pave new avenues for the rational design of functional network materials beyond CNTs, such as hydrogels, nanofibers, and nanocomposites.

Polymer–solvent interactions as a tool to engineer material properties

Materials with multiple reversible cross-linkers will reassemble during exposure to solvent vapours altering mechanical properties even after drying.

Robust and Highly Stretchable Chitosan Nanofiber/Alumina-Coated Silica/Carboxylated Poly (Vinyl Alcohol)/Borax Composite Hydrogels Constructed by Multiple Crosslinking.

We investigated the mechanical and structural properties of composite hydrogels composed of chitosan nanofiber (ChsNF), positively charged alumina-coated silica (ac-SiO2) nanoparticles, carboxylated poly (vinyl alcohol) (cPVA), and borax. ChsNF/cPVA/borax hydrogels without ac-SiO2 exhibited high Young’s modulus but poor elongation, whereas cPVA/ac-SiO2/borax hydrogels without ChsNF had moderate Young’s modulus but high elongation. ChsNF/ac-SiO2/cPVA/borax hydrogels using both ChsNF and ac-SiO2 as reinforcement agents exhibited high extensibility (930%) and high Young′s modulus beyond 1 MPa at a high ac-SiO2 concentration. The network was formed by multiple crosslinking such as the complexation between borate and cPVA, the ionic complexation between ac-SiO2 and cPVA, and the hydrogen bond between ChsNF and cPVA. Structural analysis by synchrotron small-angle X-ray scattering revealed that the nanostructural inhomogeneity in ChsNF/ac-SiO2/cPVA/borax hydrogel was suppressed compared to those of the ChsNF/cPVA/borax and cPVA/ac-SiO2/borax hydrogels.

Multiple Crosslinking Hyaluronic Acid Hydrogels with Improved Strength and 3D Printability.

Hyaluronic acid (HA) hydrogel is preferred for biomedicine applications, as it possesses biodegradability, biocompatibility, and cell-regulated capacity as well as high hydration nature similar to the native extracellular matrix. However, HA hydrogel fabricated via a 3D printing technique often faces poor printing properties. In this study, maleiated sodium hyaluronate (MHA) with a high substituted degree of the acrylate group (i.e., 2.27) and thiolated sodium hyaluronate (SHHA) were synthesized. By blending these modified HAs, the MHA/SHHA hydrogels were prepared via pre-crosslinking through thiol-acrylate Michael addition and subsequently covalent crosslinking using thiol-acrylate and acrylate-acrylate photopolymerization mechanisms. Rheological properties, swelling behaviors, and mechanical properties can be modulated by altering the molar ratio of the thiol group and acrylate group. The results showed that the MHA/SHHA hydrogel precursors have rapidly gelling capacity and improved compressive strength. Based on these results, high-resolution hydrogel scaffolds with good structural stability were prepared by extrusion-based 3D printing. This HA hydrogel is cytocompatible and capable of supporting adherence of L929 cells, indicating its great potential for tissue engineering scaffolds.

3D Bio-Printability of Hybrid Pre-Crosslinked Hydrogels.

Maintaining shape fidelity of 3D bio-printed scaffolds with soft biomaterials is an ongoing challenge. Here, a rheological investigation focusing on identifying useful physical and mechanical properties directly related to the geometric fidelity of 3D bio-printed scaffolds is presented. To ensure during- and post-printing shape fidelity of the scaffolds, various percentages of Carboxymethyl Cellulose (CMC) (viscosity enhancer) and different calcium salts (CaCl2 and CaSO4, physical cross-linkers) were mixed into alginate before extrusion to realize shape fidelity. The overall solid content of Alginate-Carboxymethyl Cellulose (CMC) was limited to 6%. A set of rheological tests, e.g., flow curves, amplitude tests, and three interval thixotropic tests, were performed to identify and compare the shear-thinning capacity, gelation points, and recovery rate of various compositions. The geometrical fidelity of the fabricated scaffolds was defined by printability and collapse tests. The effect of using multiple cross-linkers simultaneously was assessed. Various large-scale scaffolds were fabricated (up to 5.0 cm) using a pre-crosslinked hybrid. Scaffolds were assessed for the ability to support the growth of Escherichia coli using the Most Probable Number technique to quantify bacteria immediately after inoculation and 24 h later. This pre-crosslinking-based rheological property controlling technique can open a new avenue for 3D bio-fabrication of scaffolds, ensuring proper geometry.

Effects of silica and clay nanoparticles on the mechanical properties of poly(vinyl alcohol) nanocomposite hydrogels

We investigated the effects of silica(SiO 2 ) and clay nanoparticles on the tensile properties of poly(vinyl alcohol) (PVA) hydrogels containing borax. Clay/PVA/borax hydrogels possessed comparatively high tensile strength even at low concentrations of clay, but the mechanical performance drastically became worse at high concentrations. On the contrary, the tensile strength of SiO 2 /PVA/borax hydrogels increased with increasing SiO 2 concentrations under high elongation. In these hydrogels, multiple crosslinking such as physical interactions between clay and PVA, complexations between PVA (or SiO 2 ) and borate, microcrystals of PVA produced a tough polymer network. Besides, the synergistic effects of clay and SiO 2 nanoparticles on the mechanical performance of the PVA/borax hydrogels were confirmed, i.e., both extensibility and modulus were largely improved compared to clay/PVA/borax or SiO 2 /PVA/borax hydrogels. Synchrotron small-angle X-ray scattering analysis revealed that the addition of both clay and SiO 2 nanoparticles suppressed inhomogeneities in the composite gel. • Clay/PVA/borax gels possessed high tensile strength at low clay concentrations. • SiO 2 /PVA/borax gels possessed high elongation even at high SiO 2 concentrations. • The addition of SiO 2 and clay to PVA/borax gels caused high elongation and strength. • Combined use of nanoparticles and borax was effective for production of tough polymer gels.

Multi‐Network Poly(β‐cyclodextrin)/PVA/Gelatin/Carbon Nanotubes Composite Hydrogels Constructed by Multiple Dynamic Crosslinking as Flexible Electronic Devices

AbstractMultifunctional hydrogels with good stretchability and self‐healing properties have attracted considerable attention in the field of flexible electronic devices. However, hydrogels constructed by traditional chemical crosslinking usually have poor mechanical properties and lack self‐healing, adhesion, and biocompatibility, which cannot meet the requirements of flexible wearable devices. This work introduced multiple dynamic crosslinking, including borate ester bond, Schiff‐base, and host–guest interaction, into polymer networks to fabricate triple‐network gelatin/polyvinyl alcohol/carbon nanotube composite hydrogels with good self‐healing ability (healing efficiency of 95.0%), mechanical strength (54.6 kPa), stretchability (778%), biocompatibility, conductivity, adhesion, and remodeling properties. Carbon nanotubes can serve as the filler to improve the electrical conductivity of the triple‐network hydrogels. The resulting hydrogel can be made into a strain sensor with high strain sensitivity (gauge factor up to 16.0). More importantly, the strain sensor can be used to monitor various human and organ movements. Owing to its good self‐healing ability, the self‐healed hydrogel sensor also can detect human movements with almost the same electrical signal strength as the original one. This study gives novel insights for the design of multifunctional self‐healing hydrogels that have great potential in various application fields such as electronic skins, soft robots, and wearable devices.

One-Step Synthesis of Multifunctional Chitosan Hydrogel for Full-Thickness Wound Closure and Healing.

Multifunctional hydrogel as a sealant or wound dressing with high adhesiveness and excellent antibacterial activity is highly desirable in clinical applications. In this contribution, one-step synthetic hydrogel based on quaternized chitosan (QCS), tannic acid (TA), and ferric iron (Fe(III)) is developed for skin incision closure and Staphylococcus aureus (S. aureus)-infected wound healing. In this hydrogel system, the ionic bonds and hydrogen bonds between QCS and TA form the main backbone of hydrogel, the metal coordination bonds between TA and Fe(III) (catechol-Fe) endow hydrogel with excellent adhesiveness and (near-infrared light) NIR-responsive photothermal property, and these multiple dynamic physical crosslinks enable QCS/TA/Fe hydrogel with flexible self-healing ability and injectability. Moreover, QCS/TA/Fe hydrogel possesses superior antioxidant, anti-inflammatory, hemostasis, and biocompatibility. Also, it is safe for vital organs. The data from the mouse skin incision model and infected full-thickness skin wound model presented the high wound closure effectiveness and acceleration of the wound healing process by this multifunctional hydrogel, highlighting its great potential in wound management.

Semi–interpenetrating networks based on epoxy resin and oligophosphonate: Comparative effect of three hardeners on the thermal and fire properties

Traditional materials are being constantly replaced by synthetic polymers, most of which are highly flammable. Epoxy resins are versatile and among the most important class of polymers, due to their multiple crosslinking capacity endowed by the oxirane ring, their applications ranging from adhesives to aeronautics. This study investigates the comparative effect of three hardeners (aromatic, cycloaliphatic, aliphatic) and the addition of an oligophosphonate on the thermal behavior and fire performance of bisphenol A diglycidyl ether semi–interpenetrating polymer networks. Networks with epoxy resin and 2% phosphorus loading were prepared. Evolved gases analyses, thermal and fire experiments were used to propose the action mode of the hardeners and oligophosphonate in the fire performance enhancement of the epoxy resin. Non–isothermal decomposition kinetics studies were conducted. The oligophosphonate in the epoxy resins promoted a significant reduction in the peak of heat release rate values (33 to 55%) in microscale combustion calorimeter experiments. A UL 94–V0 classification was achieved for the network with oligophosphonate and aromatic curing agent. The obtained results recommend the semi–interpenetrating network cured with aromatic hardener as a potential matrix for flame retardant composites or coatings.

Actin crosslinker competition and sorting drive emergent GUV size-dependent actin network architecture

The proteins that make up the actin cytoskeleton can self-assemble into a variety of structures. In vitro experiments and coarse-grained simulations have shown that the actin crosslinking proteins α-actinin and fascin segregate into distinct domains in single actin bundles with a molecular size-dependent competition-based mechanism. Here, by encapsulating actin, α-actinin, and fascin in giant unilamellar vesicles (GUVs), we show that physical confinement can cause these proteins to form much more complex structures, including rings and asters at GUV peripheries and centers; the prevalence of different structures depends on GUV size. Strikingly, we found that α-actinin and fascin self-sort into separate domains in the aster structures with actin bundles whose apparent stiffness depends on the ratio of the relative concentrations of α-actinin and fascin. The observed boundary-imposed effect on protein sorting may be a general mechanism for creating emergent structures in biopolymer networks with multiple crosslinkers.

An in situ catechol functionalized ε-polylysine/polyacrylamide hydrogel formed by hydrogen bonding recombination with high mechanical property for hemostasis

In situ hydrogel has attracted widely attention in hemostasis due to its ability to match irregular defects, but its application is limited by insufficient mechanical strength and long gelation time. Although some specifical in situ chemically cross-linked hydrogels could be fast formed and exhibit high mechanical strength, they unable to absorb blood. Hence their applications were further limited in emergency hemostasis usage. In this study, a robust hydrogel formed by hydration of powders was developed using multiple hydrogen bonds crosslinking. Here, catechol groups modified ε-polylysine (PL-CAT) and polyacrylamide (PAAM) were used to construct the PL-CAT/PAAM hydrogel. This hydrogel could be formed within 7 s to adhere and seal bleeding sites. The catechol groups endowed the hydrogel outstanding adhesive strength, which was 3.5 times of fibrin glue. Besides, the mechanical performance of in-situ PL-CAT/PAAM hydrogel was explored and the results showed that the hydrogel exhibited high compressive strength (0.47 MPa at 85% strain). Most importantly, the blood loss of wound treated with PL-CAT/PAAM hydrogel powders was 1/7 of untreated group, indicating the hydrogel's excellent hemostatic effect. And the cytotoxicity studies indicated that the PL-CAT/PAAM hydrogel had low toxicity. To summarize, this hydrogel could be a potential hemostatic material in emergency situations.