Crosslinking Mechanism Research Articles

Transglutaminase Activity Is Conserved in Stratified Epithelia and Skin Appendages of Mammals and Birds.

The cross-linking of structural proteins is critical for establishing the mechanical stability of the epithelial compartments of the skin and skin appendages. The introduction of isopeptide bonds between glutamine and lysine residues depends on catalysis by transglutaminases and represents the main protein cross-linking mechanism besides the formation of disulfide bonds. Here, we used a fluorescent labeling protocol to localize the activity of transglutaminases on thin sections of the integument and its appendages in mammals and birds. In human tissues, transglutaminase activity was detected in the granular layer of the epidermis, suprabasal layers of the gingival epithelium, the duct of sweat glands, hair follicles and the nail matrix. In the skin appendages of chickens, transglutaminase activity was present in the claw matrix, the feather follicle sheath, the feather sheath and in differentiating keratinocytes of feather barb ridges. During chicken embryogenesis, active transglutaminase was found in the cornifying epidermis, the periderm and the subperiderm. Transglutaminase activity was also detected in the filiform papillae on the tongue of mice and in conical papillae on the tongue of chickens. In summary, our study reveals that transglutaminase activities are widely distributed in integumentary structures and suggests that transglutamination contributes to the cornification of hard skin appendages such as nails and feathers.

The effect mechanism and properties of poplar wood cross-linking modified with polyols and polycarboxylic acid

ABSTRACT Although cross-linking modification by polycarboxylic acid greatly improves dimensional stability and decay resistance, it causes severe brittleness and rigidity in wood. Introducing polyols into the cross-linking network could improve the flexibility of the modified wood, however, the related research is limited, especially the relationship between the strength and the reaction. In this study, poplar wood was modified by cross-linking with 1,2,3,4-butanetetracarboxylic acid (BTCA). D-sorbitol (SO) and sucrose (SU) used as the polyols were introduced to the modification system to esterify with BTCA. The physical and mechanical properties of the resulting wood were investigated. The effect mechanism and microstructure of the wood were also examined. The findings demonstrated that a cross-linked structure was formed in wood with increased ester bonds and ample polymer filling in the lumens. SU has lower leachability than SO. Although the incorporation of polyols has lowered anti-swelling efficiency, the reduction of modulus of rupture caused by BTCA treatment could be redeemed by reducing the degree of cross-linking between BTCA and wood components, especially SO-BTCA treatment. The addition of polyols has slight impact on the modulus of elasticity of BTCA treated wood.

PH and Thrombin Concentration Are Decisive in Synthesizing Stiff, Stable, and Open-Porous Fibrin-Collagen Hydrogel Blends without Chemical Cross-Linker.

Fibrin-collagen hydrogel blends exhibit high potential for tissue engineering applications. However, it is still unclear whether the underlying cross-linking mechanisms are of chemical or physical nature. It is here hypothesized that chemical cross-linkers play a negligible role and that instead pH and thrombin concentration are decisive for synthetizing blends with high stiffness and hydrolytic stability. Different fibrin-collagen formulations (pure and with additional transglutaminase) are used and the blends' compaction rate, hydrolytic stability, compressive strength, and hydrogel microstructure are investigated. The effect of thrombin concentration on gel compaction is examined and the importance of pH control during synthesis observed. It is revealed that transglutaminase impairs gel stability and it is deduced that fibrin-collagen blends mainly cross-link by mechanical interactions due to physical fibril entanglement as opposed to covalent bonds from chemical cross-linking. High thrombin concentrations and basic pH during synthesis reduce gel compaction and enhance stiffness and long-term stability. Scanning electron microscopy reveals a highly interpenetrating fibrous network with unique, interconnected open-porous microstructures. Endothelial cells proliferate on the blends and form a confluent monolayer. This study reveals the underlying cross-linking mechanisms and presents enhanced fibrin-collagen blends with high stiffness, hydrolytic stability, and large, interconnected pores; findings that offer high potential for advanced tissue engineering applications.

Recent advances in enhanced polymer gels for profile control and water shutoff: A review.

Polymer gels have been effectively employed as a water management material for profile control and water shutoff treatments in low-middle temperature and low-middle salinity reservoirs. However, most polymer gel systems have limitations under high temperature and salinity reservoir conditions, such as short gelation time, poor strength, and long-term instability. Therefore, several researchers have developed enhanced polymer gels to satisfy the water control requirements in high temperature and salinity reservoirs. This work reviews the five main types of enhanced polymer gels that have been developed so far: nano silica-enhanced gel systems, cellulose-enhanced gel systems, graphite-enhanced gel systems, oily sludge-enhanced gel systems, and foam-enhanced polymer gel systems. Further, this article investigates the fundamental properties, strengthening and crosslinking mechanisms, reservoir application conditions, and field applications of several enhanced polymer systems. In this paper, it is found that the addition of strengthening materials can increase the bound water content in the gel network and significantly improve the temperature and salt resistance of polymer gel, so as to cope with the application of profile control and water plugging in high temperature and high salt reservoirs. Moreover, it also offers references and future research directions for enhanced polymer gel systems.

Macromolecular changes of vinyl ester resin after thermal oxidation

AbstractThe thermal aging of filled and unfilled vinyl ester was performed at 180°C under air and was followed at the molecular (Fourier Transform InfraRed Spectroscopy) and macromolecular (Dynamical Mechanical Analysis) scales. Aluminum trihydrate filler was observed to be stable in the investigated conditions, which simplifies the analysis of aging. At the molecular scale, oxidation is visible by the broadening of the carbonyl band, the depletion of hydroxypropyl ether, and the loss of mass. Subglassy relaxation and glass transition (Tg) of unaged samples were also characterized by dynamic mechanical analysis. After aging, a clear crosslinking mechanism was evidenced by an increase in Tg. The possible oxidation mechanisms are discussed.

Cross-linking electrospinning.

Although electrospinning (e-spinning) has witnessed rapid development in recent years, it has also been criticized by environmentalists due to the use of organic solvents. Therefore, aqueous e-spinning (green e-spinning) is considered a more attractive technique. However, considering the poor water resistance and mechanical properties of electrospun (e-spun) nanofibers, cross-linking is a perfect solution. In this review, we systematically discuss the cross-linking e-spinning system for the first time, including cross-linking strategies (in situ, liquid immersion, vapor, and spray cross-linking), cross-linking mechanism (physical and chemical cross-linking) of e-spun nanofibers, and the various applications (e.g., tissue engineering, drug delivery, water treatment, food packaging, and sensors) of cross-linked e-spun nanofibers. Among them, we highlight several cross-linking methods, including UV light cross-linking, electron beam cross-linking, glutaraldehyde (and other commonly used cross-linking agents) chemical cross-linking, thermal cross-linking, and enzymatic cross-linking. Finally, we confirm the significance of cross-linking e-spinning and reveal the problems in the construction of this system.

Recent advances in double network hydrogels based on naturally-derived polymers: synthesis, properties, and biological applications.

Hydrogels composed of naturally-derived biopolymers have garnered significant research interest due to the bioavailability and biocompatibility of starting materials. However, translating these advantages to practical use is challenged by limitations of mechanical properties and stability of the resulting materials. The development of double network (DN) hydrogels has led to greatly enhanced mechanical properties and shows promise toward broadening the applications of conventional synthetic or natural hydrogels. This review highlights recently developed protein-based and polysaccharide-based DN hydrogels. For each biopolymer, we focus on a subset of DN hydrogels centered around a theme related to synthetic design or applications. Network structures and crosslinking mechanisms that endow enhanced mechanical properties and performance to the materials are discussed. Important applications, including tissue engineering, drug delivery, bioadhesives, wound healing, and wearable sensors, that arise from the inherent properties of the natural polymer or its combination with other materials are also emphasized. Finally, we discuss ongoing challenges to stimulate the discovery of new design principles for the future of DN hydrogels based on naturally-derived polymers for biological applications.

Ionizing radiation induces cross-linking of two noncovalently bound collagen mimetic peptide triple helices in the absence of a molecular environment.

Cross-linking is a fundamental molecular process that is highly important for many applications, in particular, to tune the properties of collagen-based biomaterials. Chemical reagents, the action of enzymes or physical factors such as heat or radiation can facilitate collagen cross-linking. Ionizing radiation has the advantages of being fast, efficient and free from potentially toxic reagents. Collagen cross-linking by ionizing radiation is thought to occur via a water-mediated pathway. In the past, synthesized collagen mimetic peptides have proven to be of great value for understanding the influence of the amino acid sequence on the stability of tertiary (fibrous) as well as secondary (triple helical) structures of collagen. Cross-linking of synthetic collagen mimetic peptides is often used for modifying the properties of biomaterials. In this work, for the first time, we apply radiation-induced cross-linking to synthetic collagen mimetic peptides and present an experimental and theoretical study of peptide hexamers consisting of two noncovalently bound triple helices in the absence of a molecular environment, i.e. in the gas phase. Our results show that X-ray photoabsorption of the hydroxylated hexamer leads to ionization and cross-linking of the two triple helices: thus, we found evidence that cross-linking can be achieved by ionizing radiation, without the presence of any reagent or water. We propose a cross-linking mechanism involving the creation of two radicals on hydroxyproline side-chains and their recombination, ultimately leading to covalent bond formation between the triple helices.

Ionotropic Gelation and Chemical Crosslinking as Methods for Fabrication of Modified-Release Gellan Gum-Based Drug Delivery Systems.

Hydrogels have a tridimensional structure. They have the ability to absorb a significant amount of water or other natural or simulated fluids that cause their swelling albeit without losing their structure. Their properties can be exploited for encapsulation and modified targeted drug release. Among the numerous natural polymers suitable for obtaining hydrogels, gellan gum is one gaining much interest. It is a gelling agent with many unique features, and furthermore, it is non-toxic, biocompatible, and biodegradable. Its ability to react with oppositely charged molecules results in the forming of structured physical materials (films, beads, hydrogels, nanoparticles). The properties of obtained hydrogels can be modified by chemical crosslinking, which improves the three-dimensional structure of the gellan hydrogel. In the current review, an overview of gellan gum hydrogels and their properties will be presented as well as the mechanisms of ionotropic gelation or chemical crosslinking. Methods of producing gellan hydrogels and their possible applications related to improved release, bioavailability, and therapeutic activity were described.

Kinetic and chemorheological evaluation on the crosslinking process of peroxide-initiated low-density polyethylene

Peroxide-initiated low-density polyethylene (PLPE) has been extensively used as insulating materials for high-voltage cables, while the lack of comprehensive knowledge on crosslinking process of PLPE seriously limits the development of cable insulation. Here, the crosslinking process and mechanism of PLPE are fully revealed by combining kinetic and chemorheological analysis for the first time. Rheological parameters during crosslinking process of PLPE are obtained by dynamic rheology experiments at different frequencies and temperatures. Crosslinking kinetics of PLPE is explored by rheological conversion method and diffusion-controlled Kamal-Sourour model. Meanwhile, chemorheological behaviors of PLPE are discussed based on percolation theory and Arrhenius law. The results indicate that the crosslinking reaction of PLPE involves complex n-order and autocatalytic reactions. Importantly, the autocatalytic reaction and the gelation formation significantly enhance the reaction rate. Furthermore, physical diffusion of macromolecular chains obviously reduces the reaction rate and prolongs the termination reaction. Based on that, the mutual influences and relationships between the kinetic and chemorheological behaviors are established by combining the apparent activation energy. It is suggested that PLPE crosslinking process is controlled by alternant synergistic action of chemical reaction and physical diffusion. This work can provide a critical theoretical basis for the comprehension and optimization of PLPE crosslinking process.

Cross-Linked Gamma Polyglutamic Acid/Human Hair Keratin Electrospun Nanofibrous Scaffolds with Excellent Biocompatibility and Biodegradability.

Recently, human hair keratin has been widely studied and applied in clinical fields due to its good histocompatibility, biocompatibility, and biodegradability. However, the regenerated keratin from human hair cannot be electrospun alone because of its low molecular weight. Herein, gamma polyglutamic acid (γ-PGA) was first selected to fabricate smooth and uniform γ-PGA/keratin composite scaffolds with excellent biocompatibility and biodegradability by electrospinning technology and a chemical cross-linking method in this study. The effect of electrospinning parameters on the structure and morphology, the mechanism of chemical cross-linking, biocompatibility in vitro cell culture experiments, and biodegradability in phosphate-buffered saline buffer solution and trypsin solution of the γ-PGA/keratin electrospun nanofibrous scaffolds (ENS) was studied. The results show that the cross-linked γ-PGA/keratin ENSs had excellent water stability and biodegradability. The γ-PGA/keratin ENSs showed better biocompatibility in promoting cell adhesion and cell growth compared with the γ-PGA ENSs. It is expected that γ-PGA/keratin ENSs will be easily and significantly used in tissue engineering to repair or regenerate materials.

Combining thermosensitive physical self-assembly and covalent cycloaddition chemistry as simultaneous dual cross-linking mechanisms for the preparation of injectable hydrogels with tuneable properties

Injectable hydrogels are promising materials for the sustained delivery of therapeutics, bioactives and cell therapies. Hydrogels that display suitable processing times and rapid gelling kinetics upon injection are particularly desirable as they can limit burst release of their payload. Therefore, we aimed to develop hydrogels that could be readily processed at ambient temperature, gelled rapidly at physiological temperature, and possessed tuneable properties (e.g., degradation and release profiles). To achieve this, a unique dual cross-linking mechanism was investigated, employing a combination of pentafulvene-maleimide Diels-Alder cycloaddition (DAC) chemistry and thermoresponsive physical cross-linking with a functionalised poly(ethylene glycol-b-propylene glycol-b-ethylene glycol) (PEG-PPG-PEG; Pluronic F-127) copolymer. A combination of temperature dependent nuclear magnetic resonance spectroscopy, dynamic light scattering, and rheological measurements were conducted to interrogate the gelation mechanism. Whereas the reaction between aqueous solutions of maleimide functionalised Pluronic and pentafulvene functionalised PEG star polymer resulted in hydrogels with gelation times (tgel) between 30 and 55 mins at ambient temperature, significantly faster tgel values (5–10 mins) were observed at physiological temperatures (37 °C). The relatively slow gelation at ambient temperature provided good working times and injectability through narrow gauge (261/2 G) needles. While the initial Young’s modulus (E) of the hydrogels was highly dependent on the gelation temperature, the E remained dynamic in response to further temperature changes post-gelation. The hydrogels displayed relatively low swelling (20–30% mass increases) and degradation times (td) up to 16 d, which provided for the sustained release of latex particles (cell mimic) over a period of 7–14 d. With their tuneable gelation kinetics and properties, the thermoresponsive DAC hydrogels have potential for the controlled release of therapeutics.

Characterization of benzoxazine resins for brake pad friction materials manufacturing

Phenolic resins are the most commonly used binders in brake pads for automotive disc brake systems owing to their affordability and thermal properties. However, they also show some limitations related to their crosslinking mechanism. Benzoxazine resins present themselves as possible alternatives for this application by providing enhanced thermal properties as well as other industrially attractive characteristics such as lower moisture absorption and unlimited shelf life. This study investigates the thermal properties of two different benzoxazine resins, with the aim of assessing their capabilities as binder for brake pad and of understanding how to process them in order to actually employ them as such. DSC, TGA, hardness and tribological analyses were carried out on neat resin samples and on friction materials containing them as binder. The presence of several concurring reactions was detected during the crosslinking reaction of benzoxazine resins. The benzoxazine resins showed lower mass loss respect to a phenolic resin in the temperature range of interest for commercial brake pads application. Friction material containing benzoxazine resin binder showed promising tribological results.

Understanding the role of host-guest interactions in enhancing oil recovery through β‑cyclodextrin and adamantane modified copolymer

Adamantane (ADA) and β-Cyclodextrin (β-CD) modified hydrolyzed polyacrylamide (MHPAM) polymers have been confirmed to have excellent performance in enhanced oil recovery (EOR). However, the molecular mechanism of the formation of three-dimensional polymer network structure has been not yet fully understood. In this work, the cross-linked mode of MHPAM polymers and their interaction information was investigated by molecular dynamics (MD) simulations and quantum chemical (QC) calculations. Results suggest that the ADA and β-CD molecules play a pivotal molecular recognition and binding role (like hormones and receptors, antigens and antibodies) in the process of host-guest interaction. The proposed mechanism of MHPAM cross-linking in the experiments was supplemented by our MD simulations. In addition, the strong electrostatic and van der Walls (vdW) component can be explained by the overlap of positive and negative ESP/vdW side. The viscosity of hydrolyzed polyacrylamide (HPAM) was increased via the modification of ADA and β-CD, which can be attributed to the hydrophobic associations of MHPAM. Based on our results, it will allow a more informed selection of these as polymers for use in enhanced oil recovery, enabling the properties of these polymers to be tailored to a particular application.

Synthesis and Crosslinking Mechanism of Colloidal Graphene Oxide Crosslinker for Crosslinking Low-Concentration Hydroxypropyl Guar Gum Fracturing Fluids

Synthesis and Crosslinking Mechanism of Colloidal Graphene Oxide Crosslinker for Crosslinking Low-Concentration Hydroxypropyl Guar Gum Fracturing Fluids

Gelation-based feed-stock technologies for HVOF thermal spray development: Micro-composite powder preparation and HVOF coating microstructure

The current study proposes use of the polysaccharide, sodium alginate, as a gelation aid to ‘solidify’ aqueous suspensions, comprised of micron-sized ceramic-metal particles, into spherical, micro-composite ‘cermet’ feedstock powders. These feedstock powders, with finely divided ceramic and metallic components, can then be used for the preparation of high velocity oxygen fuel coatings and/or laser deposition cladding. Sodium alginate gelation is typically based on ion exchange, and a variety of gelation agents have been examined in the present work. It is shown that aqueous solutions of CaCl2, Ca-acetate, Al-nitrate, methanol, acetic acid, and 5 % HCl acid are all capable of gelling alginate droplets through a simple cross-linking mechanism. Consequently, spraying a sodium alginate containing ceramic-metal particle suspension, with a suitable air atomising nozzle, into an aqueous bath of one of the outlined solutions allows the production of nominally spherical ‘cermet’ feedstock particles. In the present work, TiC-Ni3Al micro-composite cermet particles have been successfully produced, and then analysed both before and after a sintering heat-treatment. Furthermore, rheological assessment of the colloidal suspensions both prior to and during the gelling stage are also described. As a preliminary ‘proof-of-principle’, an initial set of HVOF coatings has also been prepared using the gelled feedstock material, and initial characterisation of these coatings is presented.

Crosslinking Mechanism on a Novel Bacillus cereus Transglutaminase-Mediated Conjugation of Food Proteins.

Until now, Streptoverticillium mobaraense transglutaminase (TG) is the only commercialized TG, but limited information is known about its selection tendency on crosslinking sites at the protein level, restricting its application in the food industry. Here, four recombinant Bacillus TGs were stable in a broad range of pH (5.0-9.0) and temperatures (<50 °C), exhibiting their maximum activity at 50-60 °C and pH 6.0-7.0. Among them, TG of B. cereus (BCETG) demonstrated the maximal specific activity of 177 U/mg. A structural analysis indicated that the Ala147-Ala156 region in the substrate tunnel of BCETG played a vital role in catalytic activity. Furthermore, bovine serum albumin, as well as nearly all protein ingredients in soy protein isolate and whey protein, could be cross-linked by BCETG, and the internal crosslinking paths of three protein substrates were elucidated. This study demonstrated Bacillus TGs are a candidate for protein crosslinking and provided their crosslinking mechanism at the protein level for applications in food processing.

Microfluidic-assisted fiber production: Potentials, limitations, and prospects.

Besides the conventional fiber production methods, microfluidics has emerged as a promising approach for the engineered spinning of fibrous materials and offers excellent potential for fiber manufacturing in a controlled and straightforward manner. This method facilitates low-speed prototype synthesis of fibers for diverse applications while providing superior control over reaction conditions, efficient use of precursor solutions, reagent mixing, and process parameters. This article reviews recent advances in microfluidic technology for the fabrication of fibrous materials with different morphologies and a variety of properties aimed at various applications. First, the basic principles, as well as the latest developments and achievements of microfluidic-based techniques for fiber production, are introduced. Specifically, microfluidic platforms made of glass, polymers, and/or metals, including but not limited to microfluidic chips, capillary-based devices, and three-dimensional printed devices are summarized. Then, fiber production from various materials, such as alginate, gelatin, silk, collagen, and chitosan, using different microfluidic platforms with a broad range of cross-linking agents and mechanisms is described. Therefore, microfluidic spun fibers with diverse diameters ranging from submicrometer scales to hundreds of micrometers and structures, such as cylindrical, hollow, grooved, flat, core-shell, heterogeneous, helical, and peapod-like morphologies, with tunable sizes and mechanical properties are discussed in detail. Subsequently, the practical applications of microfluidic spun fibers are highlighted in sensors for biomedical or optical purposes, scaffolds for culture or encapsulation of cells in tissue engineering, and drug delivery. Finally, different limitations and challenges of the current microfluidic technologies, as well as the future perspectives and concluding remarks, are presented.

Crosslinking Mechanisms of Phenol, Catechol, and Gallol for Synthetic Polyphenols: A Comparative Review

Since the first introduction of a synthetic polyphenol called polydopamine, both it and its derivatives have received significant attention from material scientists owing to their unique functionality. In particular, synthetic polyphenols have been utilized as interfacial engineering tools; many important review papers have been published regarding this topic. However, despite those that have focused on the applicability of synthetic polyphenols, fundamental aspects of crosslinking mechanisms and resultant characteristics have still been overlooked in the community. This review covers the mechanisms for building synthetic polyphenols, which are dependent on the number of hydroxyl groups of each phenolic building block. The inherent physicochemical properties of the developed polyphenolic materials are discussed in depth herein. This review can provide guidelines for selecting appropriate phenolic building blocks when designing relevant polyphenolic biomaterials.

Print-and-Grow within a Novel Support Material for 3D Bioprinting and Post-Printing Tissue Growth.

3D bioprinting holds great promise for tissue engineering, with extrusion bioprinting in suspended hydrogels becoming the leading bioprinting technique in recent years. In this method, living cells are incorporated within bioinks, extruded layer by layer into a granular support material followed by gelation of the bioink through diverse cross-linking mechanisms. This approach offers high fidelity and precise fabrication of complex structures mimicking living tissue properties. However, the transition of cell mass mixed with the bioink into functional native-like tissue requires post-printing cultivation in vitro. An often-overlooked drawback of 3D bioprinting is the nonuniform shrinkage and deformation of printed constructs during the post-printing tissue maturation period, leading to highly variable engineered constructs with unpredictable size and shape. This limitation poses a challenge for the technology to meet applicative requirements. A novel technology of "print-and-grow," involving 3D bioprinting and subsequent cultivation in κ-Carrageenan-based microgels (CarGrow) for days is presented. CarGrow enhances the long-term structural stability of the printed objects by providing mechanical support. Moreover, this technology provides a possibility for live imaging to monitor tissue maturation. The "print-and-grow" method demonstrates accurate bioprinting with high tissue viability and functionality while preserving the construct's shape and size.