Cross-linking Strategy Research Articles

Synthesizing Hypercrosslinked Polymers with Deep Eutectic Solvents to Enhance CO2 /N2 Selectivity.

Hypercrosslinked polymers (HCPs) are widely used in ion exchange, water purification, and gas separation. However, HCP synthesis typically requires hazardous halogenated solvents e. g., dichloroethane, dichloromethane and chloroform which are toxic to human health and environment. Herein we hypothesize that the use of halogenated solvents in HCP synthesis can be overcome with deep eutectic solvents (DES) comprising metal halides-FeCl3 , ZnCl2 that can act as both the solvent hydrogen bond donor and catalyst for polymer crosslinking via Friedel Crafts alkylation. We validated our hypothesis by synthesizing HCPs in DESs via internal and external crosslinking strategies. [ChCl][ZnCl2 ]2 and [ChCl][FeCl3 ]2 was more suitable for internal and external hypercrosslinking, respectively. The specific surface areas of HCPs synthesized in DES were 20-60 % lower than those from halogenated solvents, but their CO2 /N2 selectivities were up to 453 % higher (CO2 /N2 selectivity of poly-α,α'-dichloro-p-xylene synthesized in [ChCl][ZnCl2 ]2 via internal crosslinking reached a value of 105). This was attributed to the narrower pore size distributions of HCPs synthesized in DESs.

Poly(4-vinylpyridine) and Poly(methacrylic acid) Particle Architectures for pH-Responsive and Mechanochromic Opal Films.

While stimuli-responsive structural colors are commonly found in nature, mimicking these in artificial materials is challenging. Dynamically switchable and tunable coloration, however, is in high demand in widespread fields of applications, including advanced display and monitoring technologies, smart sensing, and anticounterfeiting. This work reports a scalable protocol for the synthesis of tailor-made core-shell particles and subsequent processing to opal films with iridescent, pH-responsive, and mechanochromic structural color. Novel monodisperse core-shell architectures based on hard polystyrene core particles are synthesized via stepwise emulsion polymerization in a starved-feed mode. The incorporation of 4-vinylpyridine and methacrylic acid as functional comonomers in the soft particle shell facilitates pH-responsive swelling and deswelling. Mechanically stable and well-ordered colloidal crystal films are obtained by the self-assembly of the particles during processing with the powerful melt-shear organization technique. Thereby obtained opal films show Bragg-scattering at the colloidal crystalline structure and exhibit brilliant green-turquoise to blue-violet reflection colors, dependent on the angle of view and illumination. Upon changes in the pH value or mechanical deformation, the reflected wavelength shifts by more than 100 nm, leading to intriguing changes in the visible structural color. Excellent reversibility is achieved by the subsequent application of a convenient UV cross-linking strategy, corroborating the high application potential of these advanced functional materials.

Fabrication and Photothermal Actuation Performances of Electrospun Carbon Nanotube/Liquid Crystal Elastomer Blend Yarn Actuators.

Liquid crystal elastomers (LCEs) are a kind of polymer network that combines the entropic elasticity of polymer networks and the mesogenic unit by means of mild cross-linking. LCEs have been extensively investigated in various fields, including artificial muscles, actuators, and microrobots. However, LCEs are characterized by the poor mechanical properties of the light polymers themselves. In this study, we propose to prepare a carbon nanotube/liquid crystal elastomer (CNT/LCE) composite yarn by electrospinning technology and a two-step cross-linking strategy. The CNT/LCE composite yarn exhibits a reversible shrinkage ratio of nearly 70%, a tensile strength of 16.45 MPa, and a relatively sensitive response speed of ∼3 s, enabling a fast response by photothermal actuation. The research disclosed in this article may provide new insights for the development of artificial muscles and next-generation smart robots.

Advances in Hydrogels for Meniscus Tissue Engineering: A Focus on Biomaterials, Crosslinking, Therapeutic Additives.

Meniscus tissue engineering (MTE) has emerged as a promising strategy for meniscus repair and regeneration. As versatile platforms, hydrogels have gained significant attention in this field, as they possess tunable properties that allow them to mimic native extracellular matrices and provide a suitable microenvironment. Additionally, hydrogels can be minimally invasively injected and can be adjusted to match the shape of the implant site. They can conveniently and effectively deliver bioactive additives and demonstrate good compatibility with other functional materials. These inherent qualities have made hydrogel a promising candidate for therapeutic approaches in meniscus repair and regeneration. This article provides a comprehensive review of the advancements made in the research on hydrogel application for meniscus tissue engineering. Firstly, the biomaterials and crosslinking strategies used in the formation of hydrogels are summarized and analyzed. Subsequently, the role of therapeutic additives, including cells, growth factors, and other active products, in facilitating meniscus repair and regeneration is thoroughly discussed. Furthermore, we summarize the key issues for designing hydrogels used in MTE. Finally, we conclude with the current challenges encountered by hydrogel applications and suggest potential solutions for addressing these challenges in the field of MTE. We hope this review provides a resource for researchers and practitioners interested in this field, thereby facilitating the exploration of new design possibilities.

In situ cross-linking strategy for the synthesis of three-dimensional interconnected polymer/ceramic composite electrolyte

In this work, we propose an alternative approach to develop a polymer-ceramic composite electrolyte through the in-situ PEG-MMA and PEG-DA cross-linking reaction within interconnected microporous LATP ceramic. This synthesis approach results in a PEO-LiTFSI/LATP electrolyte with an ionic conductivity of 0.25 × 10−4 S cm−1 and 4.01 × 10−4 S cm−1 at 30 and 80 °C, respectively. A continuous conductive path through the electrolyte is provided by the polymer phase and the 3D interconnected support. The electrolyte system demonstrates the synergy between the ceramic support and the cross-linked polymer electrolyte complex. The connected domains of the LATP support maximize the interaction with anions, promoting the Li+ transference number enhancement. Impedance and dielectric analyses indicate that the Correlated Barrier Hopping (CBH) model is the most likely conduction mechanism for the composite electrolyte, following the non-Debye type dielectric relaxation. Additionally, the PEO-LiTFSI/LATP composite exhibits excellent anodic stability and it has great potential for use in Li-Ion battery technologies.

A Holistic Review of C = C Crosslinkable Conjugated Molecules in Solution-Processed Organic Electronics: Insights into Stability, Processibility, and Mechanical Properties.

Solution-processable organic conjugated molecules (OCMs) consist of a series of aromatic units linked by σ-bonds, which present a relatively freedom intramolecular motion and intermolecular re-arrangement under external stimulation. The cross-linked strategy provides an effective platform to obtain OCMs network, which allows for outstanding optoelectronic, excellent physicochemical properties, and substantial improvement in device fabrication. An unsaturated double carbon-carbon bond (C = C) is universal segment to construct crosslinkable OCMs. In this review, the authors will set C = C cross-linkable units as an example to summarize the development of cross-linkable OCMs for solution-processable optoelectronic applications. First, this review provides a comprehensive overview of the distinctive chemical, physical, and optoelectronic properties arising from the cross-linking strategies employed in OCMs. Second, the methods for probing the C = C cross-linking reaction are also emphasized based on the perturbations of chemical structure and physicochemical property. Third, a series of model C = C cross-linkable units, including styrene, trifluoroethylene, and unsaturated acid ester, are further discussed to design and prepare novel OCMs. Furthermore, a concise overview of the optoelectronic applications associated with this approach is presented, including light-emitting diodes (LEDs), solar cells (SCs), and field-effect transistors (FETs). Lastly, the authors offer a concluding perspective and outlook for the improvement of OCMs and their optoelectronic application via the cross-linking strategy.

Strategies for decellularization, re-cellularIzation and crosslinking in liver bioengineering.

Liver transplantation is the only definitive treatment for end-stage liver disease and its availability is restricted by organ donor shortages. The development of liver bioengineering provides the probability to create a functional alternative to reduce the gap in organ demand and supply. Decellularized liver scaffolds have been widely applied in bioengineering because they can mimic the native liver microenvironment and retain extracellular matrix (ECM) components. Multiple approaches including chemical, physical and biological methods have been developed for liver decellularization in current studies, but a full set of unified criteria has not yet been established. Each method has its advantages and drawbacks that influence the microstructure and ligand landscape of decellularized liver scaffolds. Optimizing a decellularization method to eliminate cell material while retaining as much of the ECM intact as possible is therefore important for biological scaffold applications. Furthermore, crosslinking strategies can improve the biological performance of scaffolds, including reinforcing biomechanics, delaying degradation in vivo and reducing immune rejection, which can better promote the integration of re-cellularized scaffolds with host tissue and influence the reconstruction process. In this review, we aim to present the different liver decellularization techniques, the crosslinking methods to improve scaffold characteristics with crosslinking and the preparation of soluble ECM.

Study on wear resistance and toughness of calcium ion crosslinked cellulose nanofibers/multi-walled carbon nanotubes/hexagonal boron nitride hybrid reinforced Ni[sbnd]B composite coatings on N80 steel

This study developed a novel NiB/Ca-CNFs/MWCNTs/h-BN coating on N80 steel. The disadvantages of hexagonal boron nitride (h-BN) were improved by creatively combining multi-walled carbon nanotubes (MWCNTs), cellulose nanofibers (CNFs), and calcium ion cross-linking strategies. This led to the creation of Ca-CNFs/MWCNTs/h-BN hybrids, which were then doped into NiB coatings by varying pulse current densities. The results demonstrated that the surface morphology, mechanical properties, and toughness of the composite coating are all influenced by the Ca-CNFs/MWCNTs/h-BN doped at various current densities. Meanwhile, the NiB/Ca-CNFs/MWCNTs/h-BN (4 A/dm2) coating has the greatest microhardness (902.42 Hv) and the smallest friction coefficient (0.487) under reciprocating dry sliding friction conditions of Si3N4 friction balls. In addition, the results of nanoindentation and tensile tests show that the elastic modulus of NiB/Ca-CNFs/MWCNTs/h-BN (4 A/dm2) composite coating is 194.096 MPa and the tensile elongation is 12.2 %.

Dynamic H-Bond crosslinking strategy to prepare impact-hardening protective materials with tunable negative Poisson's ratio

Protective materials with a wide range of practical applications should exhibit a high energy absorption efficiency and be smart and stress-responsive. Herein, a novel smart protective material was formed from an impact-hardening polymer (IHP), which comprised a branched structure, and polyurethane (PU) with quadruple H-bonding. Notably, the quadruple H-bonding was used to enhance the interaction between the two microphases, thus regulating the negative Poisson's ratio. The value of the reversible softness–stiffness switching effect of this material (UPy-IHP-PU) achieved more than 5000, five times stronger than that of a previously reported linear IHP-PU. In addition, the impact-protective ability did not weaken. As the impact velocity increased, the impact energy absorption efficiency of UPy-IHP-PU also increased, even exceeded 70 % under the impact velocity of 5 m/s, thus demonstrating the ability to efficiently reduce the impact force and extend the buffer time against the strike via the impact-hardening phenomenon. Moreover, an adjustable scale of microphase separation can regulate the negative Poisson's ratio to enhance the impact-hardening phenomenon. The excellent energy dissipation ability and smart stress- and rate-responsiveness indicate the application potentials of UPy-IHP-PU as a future wearable impact-protective sensor.

Improving the ohmic polarization of high-temperature proton exchange membrane fuel cells using crosslinked polybenzimidazole membranes containing acidophilic quaternary ammonium groups synthesized by one-step strategy

Ingenious crosslinked network structure in phosphoric-acid-doped polybenzimidazole membranes can mitigate the mutual restriction of proton conductivity and mechanical properties. However, the complicated synthesis of tailored macromolecular crosslinker and the time-consuming post-treatment hinder their practical application as high-temperature proton exchange membranes. Herein, crosslinked polybenzimidazole membranes are synthesized using small molecular crosslinkers containing acidophilic quaternary ammonium groups through a one-step crosslinking strategy. After doping with phosphoric acid, the quaternary ammonium-biphosphate ion-pair coordination and the crosslinked structure result in the improved anhydrous proton conductivity, oxidation stability, and mechanical strength of the formed membranes compared to the sample without the crosslinking structure. A membrane with the optimized degree of crosslinking exhibits an anhydrous conductivity of 72.27 mS/cm at 160 °C, with a tensile strength of 12.14 MPa. Benefiting from the crosslinked structure and high proton conductivity, the accordingly formed membrane electrode assembly possesses a high open-circuit voltage of 1.01 V and animproved ohmic polarization, delivering a peak power density of 0.66 W/cm2 using hydrogen as fuel and air as oxidant.

Ferrous/Ferric Ions Crosslinked Type II Collagen Multifunctional Hydrogel for Advanced Osteoarthritis Treatment.

Osteoarthritis (OA) is a highly prevalent and intricate degenerative joint disease affecting an estimated 500 million individuals worldwide. Collagen-based hydrogels have sparked immense interest in cartilage tissue engineering, but substantial challenges persist in developing biocompatible and robust crosslinking strategies, as well as improving their effectiveness against the multifaceted nature of OA. Herein, a novel discovery wherein the simple incorporation of ferrous/ferric ions enables efficient dynamic crosslinking of type II collagen, leading to the development of injectable, self-healing hydrogels with 3D interconnected porous nanostructures, is unveiled. The ferrous/ferric ions crosslinked type II collagen hydrogels demonstrate exceptional physical properties, such as significantly enhanced mechanical strength, minimal swelling ratios, and remarkable resistance to degradation, while also exhibiting extraordinary biocompatibility and bioactivity, effectively promoting cell proliferation, adhesion, and chondrogenic differentiation. Additionally, the hydrogels reveal potent anti-inflammatory effects by upregulating anti-inflammatory cytokines while downregulating pro-inflammatory cytokines. In a rat model of cartilage defects, these hydrogels exhibit impressive efficacy, substantially accelerating cartilage tissue regeneration through enhanced collagen deposition and increased proteoglycan secretion. The innovative discovery of the multifunctional role of ferrous/ferric ions in endowing type II collagen hydrogels with a myriad of beneficial properties presents exciting prospects for developing advanced biomaterials with potential applications in OA.

Cross-linking strategies for efficient and highly stable perovskite solar cells

This review summarizes recent developments on cross-linking strategies which have shown great potential in enhancing all-round stability of perovskite solar cells (PSCs) and provides guidance for further design of cross-linking molecules or approaches to promote the commercialization of PSCs.

Small molecule-based core and shell cross-linked nanoassemblies: from self-assembly and programmed disassembly to biological applications.

Supramolecular assemblies of stimuli-responsive amphiphilic molecules have been of utmost interest in targeted drug delivery applications, owing to their capability of sequestering drug molecules in one set of conditions and releasing them in another. To minimize undesired disassembly and stabilize noncovalently encapsulated drug molecules, the strategy of core or shell cross-linking has become a fascinating approach to constructing cross-linked polymeric or small molecule-based nanoassemblies. In this article, we discuss the design and synthetic strategies for cross-linked nanoassemblies from small molecule-based amphiphiles, with robust stability and enhanced drug encapsulation capability. We highlight their potential biomedical applications, particularly in drug or gene delivery, and cell imaging. This feature article offers a comprehensive overview of the recent developments in the application of small molecule-based covalently cross-linked nanocarriers for materials and biomedical applications, which may inspire the use of these materials as a potential drug delivery system for future chemotherapeutic applications.

Engineering chitosan into a recyclable and flame-resistant gel electrolyte via a dual cross-linking strategy for flexible supercapacitors

A dual cross-linking strategy was developed using LA and sodium borate to engineer chitosan into flame-resistant and recyclable GPEs with satisfactory mechanical properties and high ionic conductivity for application in flexible SCs.

Double crosslinking decellularized bovine pericardium of dialdehyde chondroitin sulfate and zwitterionic copolymer for bioprosthetic heart valves with enhanced antithrombogenic, anti-inflammatory and anti-calcification properties.

Due to the increasing aging population and the advancements in transcatheter aortic valve replacement (TAVR), the use of bioprosthetic heart valves (BHVs) in patients diagnosed with valvular disease has increased substantially. Commercially available glutaraldehyde (GA) cross-linked biological valves suffer from reduced durability due to a combination of factors, including the high cell toxicity of GA, subacute thrombus, inflammation and calcification. In this study, oxidized chondroitin sulfate (OCS), a natural polysaccharide derivative, was used to replace GA to cross-link decellularized bovine pericardium (DBP), carrying out the first crosslinking of DBP to obtain OCS-BP. Subsequently, the zwitterion radical copolymerization system was introduced in situ to perform double cross-linking to obtain double crosslinked BHVs with biomimetic modification (P(APM/MPC)-OCS-BP). P(APM/MPC)-OCS-BP presented enhanced mechanical properties, collagen stability and enzymatic degradation resistance due to double crosslinking. The ex vivo AV-shunt assay and coagulation factors test suggested that P(APM/MPC)-OCS-BP exhibited excellent anticoagulant and antithrombotic properties due to the introduction of P(APM/MPC). P(APM/MPC)-OCS-BP also showed good HUVEC-cytocompatibility due to the substantial reduction of its residual aldehyde group. The subcutaneous implantation also demonstrated that P(APM/MPC)-OCS-BP showed a weak inflammatory response due to the anti-inflammatory effect of OCS. Finally, in vivo and in vitro results revealed that P(APM/MPC)-OCS-BP exhibited an excellent anti-calcification property. In a word, this simple cooperative crosslinking strategy provides a novel solution to obtain BHVs with good mechanical properties, and HUVEC-cytocompatibility, anti-coagulation, anti-inflammatory and anti-calcification properties. It might be a promising alternative to GA-fixed BP and exhibited good prospects in clinical applications.

Enzymatic functionalization of decellularized tilapia skin scaffolds with enhanced skin regeneration.

The decellularized tilapia skin (dTS) has gained significant attention as a promising material for tissue regeneration due to its ability to provide unique structural and functional components that support cell growth, adhesion, and proliferation. However, the clinical application of dTS is limited by its low mechanical strength and rapid biodegradability. Herein, we prepare a novel RGD (arginine-glycine-aspartic acid) functionalized dTS scaffold (dTS/RGD) by using transglutaminase (TGase) crosslinking. The developed dTS/RGD scaffold possesses excellent properties, including a medium porosity of ∼59.2%, a suitable degradation rate of approximately 80% over a period of two weeks, and appropriate mechanical strength with a maximum tensile stress of ∼46.36 MPa which is much higher than that of dTS (∼32.23 MPa). These properties make the dTS/RGD scaffold ideal for promoting cell adhesion and proliferation, thereby accelerating skin wound healing in a full-thickness skin defect model. Such an enzymatic cross-linking strategy provides a favorable microenvironment for wound healing and holds great potential for application in skin regeneration engineering.

One-step fabrication of an injectable antibacterial collagen hydrogel with in situ synthesized silver nanoparticles for accelerated diabetic wound healing

Developing antibacterial collagen-based wound dressings that target multidrug-resistant bacteria holds promise for healing diabetic wounds prone to infection, yet traditional approaches pose persistent challenges of cytotoxicity and uneconomical steps. To address these issues, we develop a biocompatible, eco-friendly, one-step cross-linking and reduction strategy, fabricating injectable antibacterial collagen hydrogels containing in situ-generated silver nanoparticles for diabetic wound care. By exposing the collagen-silver ion (Ag+) mixture to visible light, collagen-silver nanoparticle (Col I/AgNPs) hydrogels were one step created, eliminating the need for additional cross-linkers and toxic reducing agents. The synergistic interaction between collagen and silver ions was the first time investigated, revealing a pivotal redox reaction with a dual role that simultaneously initiated collagen cross-linking and reduced Ag+ to AgNPs. The resulting biocompatible Col I/AgNPs hydrogel was found to have multifunctional properties, exhibiting excellent injectability, enhanced biological activity, and outstanding broad-spectrum antimicrobial ability. Furthermore, animal experiments utilizing a diabetic full-thickness wound model demonstrated their excellent in vivo healing efficacies, capable of establishing efficient antibacterial barriers and serving as bioactive regenerative scaffolds. Our study proposes an innovative one-step approach to create injectable antibacterial collagen-silver nanoparticle hydrogels, highlighting their potential as advanced dressings for diabetic wound treatment and other biomedical applications.

High-performance flexible reduced graphene oxide/polyimide nanocomposite aerogels fabricated by double crosslinking strategy for piezoresistive sensor application

Flexible conductive polyimide aerogels used for piezoresistive pressure sensors in harsh environments are often limited by brittleness, structural collapse, low elastic recovery, low sensitivity, and narrow pressure detection range. Herein, we report a simple and green method of ice-templating unidirectional solidification combined with freeze-drying and thermal imidization to obtain a flexible reduced graphene oxide/polyimide (rGO/PI) nanocomposite aerogels with a double crosslinked structure, which achieve a significant transformation of PI aerogels from brittleness to high flexibility. The strong layered structure and dense interlaminar porous network are established by the covalent crosslinking of 1,3,5-triaminophenoxybenzene (TAB) and the hydrogen bonding crosslinking of graphene oxide (GO), imparting the double crosslinked rGO/PI aerogels with unique “porous honeycomb” structure and excellent mechanical properties. Due to the synergistic effect of TAB, rGO, PI, and unidirectional freezing process, the double crosslinked rGO/PI nanocomposite aerogel shows low density (25 mg/cm3), high compression cycle stability (3000 cycles), wide pressure detection range (0–85.1 kPa), extremely short response time (about 129 ms) and excellent environmental resistance (maintaining high structural stability and pressure response even at high temperatures of 180 °C and low temperatures of −50 °C), suitable for detecting various motion signals. It is proved that the elastic double crosslinked rGO/PI nanocomposite aerogels have potential applications serving as candidate materials for piezoresistive sensors and wearable electronic protective equipment in the fields of national defense, military industry, aerospace, and other fields in extreme environments.

High-strength ionic hydrogel constructed by metal-free physical crosslinking strategy for enhanced uranium extraction from seawater

Uranium is one of the most important elements in the nuclear industry, and efficient extraction of uranium from the ocean is conducive to the sustainable development of nuclear energy. In this work, rigid perylene tetracarboxylate anion (PTC4−) was introduced by physical (non-covalent) crosslinking strategy to obtain metal-free ionic hydrogel PAOIB-PTC. PTC4− not only increased the rigid structure content of ionic hydrogel, but also formed dynamic macrocycle AO-PTC with amidoxime (AO) through hydrogen bonds. Unlike reported heterogeneous additives, PTC4− did not undergo inhomogeneity or shedding. Therefore, PAOIB-PTC exhibited high strength (7.34 MPa), excellent solvent resistance and salinity tolerance. A large number of hydrophilic groups and multistage pore structure endowed PAOIB-PTC with high hydrophilicity, which facilitated U(VI) mass transfer and led to rapid adsorption rates (88.6 mg g−1 h−1, c0 = 250 ppm). Since the metal-free component enabled more adsorption sites to be utilized, the maximum adsorption capacity of PAOIB-PTC was as high as 1008.03 mg g−1, which exceeded most reported hydrogel-based uranium adsorbents. Experiments and DFT calculations proved that PAOIB-PTC possessed high adsorption selectivity through uranyl recognition via hydrogen bonding and synergistic coordination by supramolecular structure AO-PTC. The low-concentration uranium capture capability and excellent recycling performance demonstrated the practical application potential of PAOIB-PTC. This work provides an idea for the design of metal-free ionic hydrogels with high-strength, and the established PAOIB-PTC is expected to be a candidate material for enhanced uranium extraction from seawater.

Ultraflexible, cost-effective and scalable polymer-based phase change composites via chemical cross-linking for wearable thermal management

Phase change materials (PCMs) offer great potential for realizing zero-energy thermal management due to superior thermal storage and stable phase-change temperatures. However, liquid leakage and solid rigidity of PCMs are long-standing challenges for PCM-based wearable thermal regulation. Here, we report a facile and cost-effective chemical cross-linking strategy to develop ultraflexible polymer-based phase change composites with a dual 3D crosslinked network of olefin block copolymers (OBC) and styrene-ethylene-butylene-styrene (SEBS) in paraffin wax (PW). The C-C bond-enhanced OBC-SEBS networks synergistically improve the mechanical, thermal, and leakage-proof properties of PW@OBC-SEBS. Notably, the proposed peroxide-initiated chemical cross-linking method overcomes the limitations of conventional physical blending methods and thus can be applicable across diverse polymer matrices. We further demonstrate a portable and flexible PW@OBC-SEBS module that maintains a comfortable temperature range of 39–42 °C for personal thermotherapy. Our work provides a promising route to fabricate scalable polymer-based phase change composite for wearable thermal management.