Two-step Crosslinking Strategy Research Articles

Mechanically strengthened polybenzimidazole membrane via a two-step crosslinking strategy for high-temperature proton exchange membrane fuel cell

To simultaneously increase the mechanical strength and proton conductivity of Phosphoric acid-doped polybenzimidazole (PA-PBI), a facile two-step crosslinking approach is proposed to prepare the imidazole-rich crosslinked polybenzimidazole membranes for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) in this work. Unlike the classic crosslinking approaches that usually enhance mechanical strength of membrane but lead to a decrease of proton conductivity, the mechanical strength of two-step crosslinked membrane improves from 3.93 MPa to 9.62 MPa and proton conductivity increases from 91 mS cm−1 to 183 mS cm−1 at 200 °C compared with pristine OPBI membrane. The promotion should be ascribed to the introduction of PA-affinity sites on crosslinkers and the formation of homogeneous crosslinking networks. Meanwhile, due to the crosslinked networks, the oxidative and dimensional stability of crosslinked membranes are also promoted. In addition, the two-step cross-linking method can avoid the self-reaction between cross-linking agents, thereby obtaining a more uniform cross-linking network, which is of great significance for the use of fuel cells. When the membranes are used in a fuel cells, they exhibited a maximum power density of 448.8 mW cm−2 at 160 °C (H2/Air, without humidification.).

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.

Novel Lipase Reactor based on Discontinuous Interfaces in Hydrogel-Organogel Hybrid Gel: A Preliminary Exploration.

According to the "interfacial activation" mechanism, constructing a sufficient interface is the key strategy for lipase-catalytic system designing. Based on the "infinite interface in finite three-dimensional space" logic, in the current study, poly(N,N-dimethylacrylamide) (PDMA)-polybutyl methacrylate (PBMA) hybrid gels were prepared by a two-step crosslinking strategy, subsequently constructed as lipase-interfacial catalytic systems. The results confirm that the PDMA-PBMA hybrid gels with "networks in pores" structures could swell both the aqueous phase and organic phase. The balance between water swelling and isooctane swelling, hybrid gel space (height control), and the lipase entry manner significantly affect the interface construction and consequently the catalytic efficiency. The enzyme-substrate contact rate affected by swelling leads to three catalytic stages. Considering the spatial barrier and distribution of lipases, a potential high-performance lipase reactor can be assembled from small-size, lamellar-like, and porous hybrid gels. The reactors also show good time storage and low temperature tolerance.

3D printing of tough hydrogels based on metal coordination with a two-step crosslinking strategy.

We demonstrate the self-supporting 3D printing of complex hydrogel structures based on simultaneous crosslinking reactions while printing. The printing strategy is based on the Schiff base reaction and metal coordination with a two-step crosslinking process. The printing ink was first prepared by dispersing oxidized sodium alginate (OSA) and adipic dihydrazide (ADH) in poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) polymer solutions, and was mixed and printed into 3D structures with an extrusion-based coaxial printing platform. Because of the rapid chemical crosslinking reaction between the aldehyde group in OSA and the hydrazide group in ADH, the printed structures can be solidified quickly, and are further crosslinked by forming carboxyl-Fe3+ coordination complexes to enhance their mechanical properties. The dynamic time-sweep rheological properties of the gel composed of different proportions of OSA and ADH were systematically investigated for the characteristic gelation time, and compression tests were carried out to measure the mechanical properties of the gel composed of OSA and ADH. Combining the gelation time and mechanical properties of the gel, the weight ratio of OSA and ADH was selected as 1 : 0.44 for an optimized setting in the subsequent printing. To evaluate the printability of inks, the material formula and printing parameters were systematically varied. The ink exhibited a wide printing range, self-supporting properties, and good printability. Tensile tests of the printed single fiber crosslinked by Fe3+ show that its strength and toughness are tunable. Complex 3D structures such as pyramids, cylinders, and noses were constructed to demonstrate the printability of the ink. This printing method provides a facile approach for tough hydrogel fabrication without changing the rheological properties of the ink or sacrificing the ultimate mechanical properties of the printed materials.

Synthesis of granulated H4Mn5O12/chitosan with improved stability by a novel cross-linking strategy for lithium adsorption from aqueous solutions

Adsorption is one of the most promising methods for lithium recovery from aqueous solutions. Herein, eco-friendly chitosan (CTS) was used to prepare granulated H4Mn5O12 (HMO) ion-sieve for lithium adsorption from aqueous solutions. It was found that the hydroxyl and amino groups in CTS can react with the epoxy group and alkyl chloride in epichlorohydrin (ECH) under acidic and alkaline conditions, respectively. This unique property resulted in the adequate cross-linking of CTS and the uniform distribution of HMO in the CTS matrix by a novel two-step cross-linking strategy with ECH as the cross-linking agent. Especially when the superfine hydrogen-type rather than lithium-type ion-sieve was coated by this method at a mass ratio of CTS/HMO = 1:1, the material exhibited high thermal and chemical stability. The dissolution loss of Mn was no more than 1.15% in 0.25 mol·L−1 HCl even after 24 h continuous stirring, and it still maintained good thermal stability at 433 K. When the material was used for Li+ (25.78 mg·L−1) recovery from geothermal water which is characterized by high temperature, the adsorption capacity reached 11.4 mg·g−1 with a recovery rate of 88.42% at a solid-to-liquid ratio of 4 g·L−1. Because chitosan is widely applied for the preparation of various adsorption materials, the cross-linking strategy developed in this work provides a new idea for the preparation of chitosan-based composite materials with high stability and adsorption properties.

A Dual-sensitive Hydrogel Based on Poly(Lactide-co-Glycolide)-Polyethylene Glycol-Poly(Lactide-co-Glycolide) Block Copolymers for 3D Printing.

The thermo-sensitive hydrogel formed by triblock copolymers of polyethylene glycols and aliphatic polyesters serves as a promising candidate for bioink due to its excellent biodegradability and biocompatibility. However, the thermo-crosslinking alone cannot achieve a robust hydrogel to support the 3D printed constructs without collapse. Herein, a photo-crosslinkable group was introduced into the triblock copolymers to achieve a dual-sensitive hydrogel. A triblock copolymer poly(lactide-co-glycolide)-polyethylene glycol-poly(lactide-co-glycolide) decorated with acrylate group in the chain end was prepared. The obtained aqueous solutions of the copolymers could transform into hydrogels with excellent shear thinning properties and rapid elastic recovery properties spontaneously on the increase of temperature. The resulted thermogels also allowed for photo-crosslinking by exposure to ultraviolet radiation, with storage modulus dramatically increased to stable the printed constructs. Through a two-step crosslinking strategy, complicated tissue-like constructs with high shape fidelity can be printed using the dual-sensitive inks. Moreover, the mechanical strength, swelling ratio, and printability of the hydrogels can be tuned by varying the substitution rate of the acrylate group without compromising the inks’ extrudability. We expect that the dual-sensitive hydrogels may be used as bioinks to print large constructs for applications in tissue engineering.

Role of temperature on bio-printability of gelatin methacryloyl bioink in two-step cross-linking strategy for tissue engineering applications

Additive manufacturing has shown promising results in reconstructing three-dimensional (3D) living tissues for various applications, including tissue engineering, regenerative medicine, drug discovery, and high-throughput drug screening. In extrusion-based bioprinters, stable formation of filaments and high-fidelity deposition of bioinks are the primary challenges in fabrication of physiologically relevant tissue constructs. Among various bioinks, gelatin methacryloyl (GelMA) is known as a photocurable and physicochemically tunable hydrogel with a demonstrated biocompatibility and tunable biodegradation properties. The two-step crosslinking of GelMA (reversible thermal gelation and permanent photo-crosslinking) has attracted researchers to make complex tissue constructs. Despite promising results in filament formation and printability of this hydrogel, the effect of temperature on physicochemical properties, cytocompatibility, and biodegradation of the hydrogel are to be investigated. This work studies the effect of thermoreversible, physical crosslinking on printability of GelMA. The results of 3D printing of GelMA at different temperatures followed by irreversible chemical photo-crosslinking show that the decrease in temperature improves the filament formation and shape fidelity of the deposited hydrogel, particularly at the temperatures around 15 °C. Time dependant mechanical testing of the printed samples revealed that decreasing the extruding temperature increases the elastic properties of the extruded filaments. Furthermore, our novel approach in minimizing the slippage effect during rheological study enabled to measure changes in linear and non-linear viscoelastic properties of the printed samples at different temperatures. A considerable increase in storage modulus of the extruded samples printed at lower temperatures confirms their higher solid behavior. Scanning electron microscopy revealed a remarkable decrease in porosity of the extruded hydrogels by decreasing the temperature. Chemical analysis by Fourier-transform infrared spectroscopy and circular dichroism showed a direct relationship between the coil-helix transition in hydrogel macromers and its physical alterations. Finally, biodegradation and cytocompatibility of the extruded hydrogels decreased at lower extruding temperatures.

3D Freeform Printing of Nanocomposite Hydrogels through in situ Precipitation in Reactive Viscous Fluid.

Composite hydrogels have gained great attention as three-dimensional (3D) printing biomaterials because of their enhanced intrinsic mechanical strength and bioactivity compared to pure hydrogels. In most conventional printing methods for composite hydrogels, particles are preloaded in ink before printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the printing process. Herein, we introduced a "printing in liquid" technique coupled with a hybridization process, which allows 3D freeform printing of nanoparticle-reinforced composite hydrogels. A viscoplastic matrix for this printing system provides not only support for printed hydrogel filaments but also chemical reactants to induce various reactions in printed objects for in situ modification. Nanocomposite hydrogel scaffolds were successfully fabricated through this 3D freeform printing of hyaluronic acid (HAc)-alginate (Alg) hydrogel inks through a two-step crosslinking strategy. The first ionic crosslinking of Alg provided structural stability during printing, while the secondary crosslinking of photo-curable HAc improved the mechanical and physiological stability of the nanocomposite hydrogels. For in situ precipitation during 3D printing, phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the viscoplastic matrix. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability, as well as biological performance compared to pure HAc. Moreover, the multi-material printing of composites with different calcium phosphate contents was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries through the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.

3D Freeform Printing of Nanocomposite Hydrogels through in situ Precipitation in Reactive Viscous Fluid

Composite hydrogels have gained great attention as three-dimensional (3D) printing biomaterials because of their enhanced intrinsic mechanical strength and bioactivity compared to pure hydrogels. In most conventional printing methods for composite hydrogels, particles are preloaded in ink before printing, which often reduces the printability of composite ink with little mechanical improvement due to poor particle-hydrogel interaction of physical mixing. In contrast, the in situ incorporation of nanoparticles into a hydrogel during 3D printing achieves uniform distribution of particles with remarkable mechanical reinforcement, while precursors dissolved in inks do not influence the printing process. Herein, we introduced a “printing in liquid” technique coupled with a hybridization process, which allows 3D freeform printing of nanoparticle-reinforced composite hydrogels. A viscoplastic matrix for this printing system provides not only support for printed hydrogel filaments but also chemical reactants to induce various reactions in printed objects for in situ modification. Nanocomposite hydrogel scaffolds were successfully fabricated through this 3D freeform printing of hyaluronic acid (HAc)-alginate (Alg) hydrogel inks through a two-step crosslinking strategy. The first ionic crosslinking of Alg provided structural stability during printing, while the secondary crosslinking of photo-curable HAc improved the mechanical and physiological stability of the nanocomposite hydrogels. For in situ precipitation during 3D printing, phosphate ions were dissolved in the hydrogel ink and calcium ions were added to the viscoplastic matrix. The composite hydrogels demonstrated a significant improvement in mechanical strength, biostability, as well as biological performance compared to pure HAc. Moreover, the multi-material printing of composites with different calcium phosphate contents was achieved by adjusting the ionic concentration of inks. Our method greatly accelerates the 3D printing of various functional or hybridized materials with complex geometries through the design and modification of printing materials coupled with in situ post-printing functionalization and hybridization in reactive viscoplastic matrices.

Creating an Interface: Rendering a Double-Network Hydrogel Lubricious via Spontaneous Delamination.

Hydrogels engineered with specific surface chemistries and architectures have foundmyriadapplications in electronics, biofouling, biolubrication, and biomedical devices. Free-radical polymerization is frequently employed to construct covalently bonded networks in hydrogels, and any inhibition of the radical reactions by oxygen at the surface of the reaction mixture is generally undesirable. The internal stress caused by the resulting gradient in the cross-linking density during polymerization can give rise to a physical deformation of the surface, resulting in wrinkles, creases, or cracks. However, this oxygen-inhibition effect can be positively utilized to create finely controlled surface structures. We describe a two-step cross-linking strategy for the fabrication of a P(AAm-AMPS)/alginate double-network hydrogel in the presence of air, which enables greater independent control over surface chemistry and functionality than homogeneously processed conventional double-network hydrogels. An alginate-rich "skin" layer is spontaneously delaminated due to the mechanical instability and osmotic mismatch between the swollen double-network hydrogel matrix and the rigid "skin" layer. Removal of the "skin" layer results in a lubricious surface with coefficients of friction as low as 0.02 against glass in aqueous solutions. The proposed strategy can be generalized to develop soft functional materials with unique structures and properties and precise control over the surface chemistry.

Highly efficient flame-retardant and low-smoke-toxicity poly(vinyl alcohol)/alginate/ montmorillonite composite aerogels by two-step crosslinking strategy

A highly efficient flame-retardant and ultra-low-smoke-toxicity biodegradable material, poly(vinyl alcohol) (PVA)/alginate/montmorillonite (MMT) composite aerogel, was fabricated by a new environment-friendly two-step crosslinking strategy using borate and calcium ions. Compressive and specific moduli of the crosslinked PVA/alginate/MMT (P4A4M4/BA/Ca) aerogel increased to 7.2- and 1.9-folds those of the non-crosslinked aerogel, respectively, and the limited oxygen index value increased to 40.0%. Cone calorimeter tests revealed that the total heat release and peak heat release rate values of the P4A4M4/BA/Ca composite aerogel distinctly decreased. Remarkably, the total smoke release value of the P4A4M4/BA/Ca aerogel was considerably lower than those of non-crosslinked PVA composite aerogels, indicating its superior smoke suppression ability and high fire hazardous safety. The flame-retardancy mechanism of the crosslinked P4A4M4/BA/Ca composite aerogels involved a combination of the gaseous phase and condensed phase flame retardancy. The high-performance PVA/alginate/MMT biodegradable composite aerogels with good sustainability is a promising alternative to conventional flame-retardant foams.

3D Bioprinting of Low-Concentration Cell-Laden Gelatin Methacrylate (GelMA) Bioinks with a Two-Step Cross-linking Strategy.

Methacrylated gelatin (GelMA) has been widely used as a tissue-engineered scaffold material, but only low-concentration GelMA hydrogels were found to be promising cell-laden bioinks with excellent cell viability. In this work, we reported a strategy for precise deposition of 5% (w/v) cell-laden GelMA bioinks into controlled microarchitectures with high cell viability using extrusion-based three-dimensional (3D) bioprinting. By adding gelatin into GelMA bioinks, a two-step cross-linking combining the rapid and reversible thermo-cross-linking of gelatin with irreversible photo-cross-linking of GelMA was achieved. The GelMA/gelatin bioinks showed significant advantages in processability because the tunable rheology and the rapid thermo-cross-linking of bioinks improved the shape fidelity after bioprinting. Here, the rheology, mechanical properties, and swelling of GelMA/gelatin bioinks with different concentration ratios were carefully characterized to obtain the optimized bioprinting setup. We successfully printed the 5% (w/v) GelMA with 8% (w/v) gelatin into 3D structures, which had the similar geometrical resolution as that of the structures printed by 30% (w/v) GelMA bioinks. Moreover, the cell viability of 5/8% (w/v) GelMA/gelatin bioinks was demonstrated by in vitro culture and cell printing of bone marrow stem cells (BMSCs). Larger BMSC spreading area was found on 5/8% (w/v) GelMA/gelatin scaffolds, and the BMSC viability after the printing of 5/8% (w/v) GelMA/gelatin cell-laden bioinks was more than 90%, which was very close to the viability of printing pure 5% (w/v) GelMA cell-laden bioinks. Therefore, this printing strategy of GelMA/gelatin bioinks may extensively extend the applications of GelMA hydrogels for tissue engineering, organ printing, or drug delivery.

Mechanically dynamic PDMS substrates to investigate changing cell environments.

Mechanics of the extracellular matrix (ECM) play a pivotal role in governing cell behavior, such as cell spreading and differentiation. ECM mechanics have been recapitulated primarily in elastic hydrogels, including with dynamic properties to mimic complex behaviors (e.g., fibrosis); however, these dynamic hydrogels fail to introduce the viscoelastic nature of many tissues. Here, we developed a two-step crosslinking strategy to first form (via platinum-catalyzed crosslinking) networks of polydimethylsiloxane (PDMS) and then to increase PDMS crosslinking (via thiol-ene click reaction) in a temporally-controlled manner. This photoinitiated reaction increased the compressive modulus of PDMS up to 10-fold within minutes and was conducted under cytocompatible conditions. With stiffening, cells displayed increased spreading, changing from ∼1300 to 1900μm2 and from ∼2700 to 4600μm2 for fibroblasts and mesenchymal stem cells, respectively. In addition, higher myofibroblast activation (from ∼2 to 20%) for cardiac fibroblasts was observed with increasing PDMS substrate stiffness. These results indicate a cellular response to changes in PDMS substrate mechanics, along with a demonstration of a mechanically dynamic and photoresponsive PDMS substrate platform to model the dynamic behavior of ECM.

Poly(styrene sulfonic acid)/poly(vinyl alcohol) copolymers with semi-interpenetrating networks as highly sulfonated proton-conducting membranes

Poly(styrene sulfonic acid)/poly(vinyl alcohol) proton-conducting membranes with semi-interpenetrating networks (semi-IPNs) were prepared using a modified two-step crosslinking strategy. We previously employed sulfosuccinic acid (SSA) and glutaraldehyde (GA) as crosslinking agents to form a dense hydrophobic layer at the outer membrane surface. Although the proton conductivity of the resulting membrane increased with the content of SSA, the methanol permeability also increased. In this study it was found that the introduction of a sufonating agent, with a high molecular weight, i.e. poly(styrene sulfonic acid) (PSSA), at a PSSA/poly(vinyl alcohol) (g g −1) ratio greater than 0.72, increased the density of the tangled IPN structures that effectively impede the membrane's permeability to MeOH, while enhancing its proton conductivity. The synthesized semi-IPN membranes exhibited high proton conductivities (up to 5.88 × 10 −2 S cm −1 at room temperature, i.e. greater than those of Nafion membranes) and high resistances to MeOH permeation (ca. 1 × 10 −7 cm 2 S −1, that is approximately one order of magnitude lower than that of Nafion membranes).