Aqueous compatible boron nitride nanosheets for high-performance hydrogels

Boron nitride microfiber reinforced polyacrylic acid hydrogels with excellent self-adhesion, fast pH response, and strain sensitivity

Hydrogels are widely utilized in the sensor field, but their inadequate adhesion presents a significant obstacle. Herein, a new multifunctional BNMFs/PAA composite hydrogel was prepared via the incorporation of one-dimensional porous boron nitride microfibers (BNMFs) and polyacrylic acid (PAA) hydrogels. BNMFs, as a reinforcing filler, play a very important role in enhancing the properties of the composite hydrogels. In particular, the porous micrometer structure plays a unique role in improving the adhesion properties of PAA hydrogels. The steric hindrance and the rich hydroxyl functional groups coming from BNMFs are key factors for the excellent adhesion of the composite hydrogels. The composite hydrogels show strong adhesion to various substrate materials. For iron plates and biological tissues, the adhesion energy can reach 1377 J m-2 and 317 J m-2, respectively. In addition, the developed BNMFs/PAA composite hydrogels exhibit excellent mechanical properties. The fracture strain of the composite hydrogels is increased by 2.4 times compared to pure PAA hydrogels. The hydrogen bonds formed between BNMFs and PAA are conducive to the mechanical properties of the BNMFs/PAA composite hydrogels. Meanwhile, BNMFs as fillers play a role in carrying and dissipating force. Furthermore, the BNMFs/PAA composite hydrogels have excellent strain and pH response characteristics. This is because the crosslinking network of the composite hydrogels becomes loose after the addition of BNMFs, resulting in rapid ion transport pathways. Therefore, the developed BNMFs/PAA composite hydrogels will have broad application prospects in the fields of motion monitoring, intelligent skin and biological adhesives.

Hydrogel-Reactive-Microenvironment Powering Reconfiguration of Polymer Architectures

Reconfiguration of architected structures has great significance for achieving new topologies and functions of engineering materials. Existing reconfigurable strategies have been reported, including approaches based on heat, mechanical instability, swelling, origami/kirigami designs, and electromagnetic actuation. However, these approaches mainly involve physical interactions between the host materials and the relevant stimuli. Herein, a novel, easy-manipulated, and controllable reconfiguration strategy for polymer architectures is proposed by using a chemical reaction of host material within a hydrogel reactive microenvironment. 3D printed polycaprolactone (PCL) lattices transformed in an aqueous polyacrylamide (PAAm) hydrogel precursor solution, in which ultraviolet (UV) light triggered heterogeneous grafting polymerization between PCL and AAm. In situ microscopy shows that PCL beams go through volumetric expansion and cooperative buckling, resulting in transformation of PCL lattices into sinusoidal patterns. The transformation process can be tuned easily and patterned through the adjustment of the PCL beam diameter, unit cell width, and UV light on-off state. Controlling domain formation is achieved by using UV masks. This framework enables the design, fabrication, and programming of architected materials and inspires the development of novel 4D printing approaches.

Boron Nitride's Morphological Role in the Design of Injectable Hyaluronic Acid Based Hybrid Artificial Synovial Fluid

The treatment process of osteoarthritis (OA) is challenging as it affects not only cartilage but also subchondral bone, ligament attachment capsules, synovium, and surrounding muscle tissue. Therefore, the search for preventive treatment or methods to slow the onset of the condition. Hexagonal boron nitride (hBN) has a graphite-like lamellar structure and is thought to facilitate cartilage movement for biomedical applications, just like in bearing systems. Hyaluronic acid (HA) is one of the natural polymers that can be used to transport boron nitride and maintain its presence in joints for a long time. In this study, hybrid hydrogels were formulated by using boron nitride nanoparticles and nanosheets. The rheological properties of the hydrogels were evaluated according to the structural differences of hBN. Characterizations have shown that hybrid hydrogels can be produced in injectable form, and the rheological properties are strongly related to the structural properties of the added particle. It has been determined that hBN added to the hydrogel structure reduces the dynamic viscosity of the zero-shear point and the deformation rate of the hydrogel and also changes the viscoelastic properties of the hydrogel depending on boron nitride's structural differences. The suggested mechanism is the hybrid hydrogel that exhibits lower viscosity as the layers detach from each other or disperses the agglomerates under applied shear stress. hBN, which has been proposed as a new strategy for joint injections, is thought to be a promising candidate for the treatment of OA due to its lamellar structures.

Rapid preparation of N-CNTs/P(AA-co-AM) composite hydrogel via frontal polymerization and its mechanical and conductive properties

Deep eutectic solvent (DES) was prepared by using acrylic acid (AA) and acrylamide (AM) as hydrogen bonding donors (HBD) and choline chloride (ChCl) as hydrogen bonding receptors (HBA). Nitrogen-doped carbon nanotubes (N-CNTs) were dispersed in DES as fillers, and N-CNTs/P(AA-co-AM) composite hydrogels were prepared by FP. The interaction mode between the hydrogel and N-CNTs was characterized by Fourier infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The mechanical properties, pH response and electrical conductivity of the composite hydrogels were studied. The results showed that the mechanical properties of the hydrogel were significantly enhanced with the increase of N-CNT content. The tensile strength and compressive strength of the FP4 composite hydrogel reached 5.42 MPa and 4.29 MPa, respectively. Due to the dissociation of carboxyl groups in AA in an alkaline environment the composite hydrogel showed excellent pH response performance. The conductivity of the hydrogel was also found to be improved with the content of N-CNTs. When the content of N-CNTs is 1.0 wt%, the conductivity of the hydrogel was 4.2 times higher than that of the hydrogel without N-CNTs, and connecting it to a circuit can make an LED lamp emit bright light. In this study, a simple and green method was proposed to prepare composite hydrogels with excellent mechanical properties and electrical conductivity by FP of DES in less than 5 min.

This study provides a rapid and low-energy method for the preparation of nanocomposite hydrogels with excellent mechanical properties and electrical conductivity.

The Synthesis and Characterization of h-BN Nanosheets with High Yield and Crystallinity

Nowadays, boron nitride (BN) has attracted a great deal of attention due to its physical and chemical properties, such as high-temperature resistance, oxidation resistance, heat conduction, electrical insulation, and neutron absorption. The unique lamellar, reticular, and tubular morphologies and physicochemical properties of BN make it attractive in the fields of adsorption, catalysis, hydrogen storage, thermal conduction, insulation, dielectric substrate of electronic devices, radiation protection, polymer composites, medicine, etc. Based on this, we propose a novel method to produce boron nitride nanosheets (BNNSs) by a two-step method. The structure and morphology of the prepared BNNSs were characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, XRD, FTIR, etc. The results showed that the prepared BNNSs had high crystallinity and the stripping efficiency of h-BN as well as the performance and yield of BNNSs had been improved, and the cost and environmental pollution of BNNS preparation had been reduced accordingly.

Boron nitride-based nanocomposite hydrogels: preparation, properties and applications

Hexagonal boron nitride (h-BN) nanostructures are well-known for their good chemical stability, thermal conductivity and high elastic modulus. BN can be used as a filler in hydrogels to significantly improve their mechanical and thermal properties, to reinforce their biocompatibility and to provide self-healing capacity. Moreover, in contrast with their carbon equivalents, BN nanocomposites are transparent and electrically insulating. Herein, we present an overview of BN-based nanocomposite hydrogels. First, the properties of h-BN are described, as well as common exfoliation and functionalization techniques employed to obtain BN nanosheets. Then, methods for preparing BN-nanocomposite hydrogels are explained, followed by a specific overview of the relationship between the composition and structure of the nanocomposites and the functional properties. Finally, the main properties of these materials are discussed in view of the thermal, mechanical, and self-healing properties, along with the potential applications in tissue engineering, thermal management, drug delivery and water treatment.

Pristine Titanium Carbide MXene Hydrogel Matrix

The hydrogel matrix normally forms via covalent or noncovalent interactions that make the matrix resistant to hydration and disassembly. Herein this type of chemical transition is demonstrated in titanium carbide MXene (Ti₃C₂T_x), in which the exchange of intercalated Li⁺ with hydrated protons triggers significantly suppressed hydration in stacked Ti₃C₂T_x. Based on this intercalation chemistry behavior, pristine Ti₃C₂T_x hydrogel matrices with an arbitrary microstructures are fabricated by freezing-induced preassembly and a subsequent specially designed thawing process in protic acids. The absence of extrinsic components maximizes the materials performance of the resultant pristine Ti₃C₂T_x hydrogel, which produces a compressive modulus of 2.4 MPa and conductivity of 220.3 ± 16.8 S/m at 5 wt % solid content. The anisotropic Ti₃C₂T_x hydrogel also delivers a promising performance in solar steam generation by facilitating rapid water transport inside vertical microchannels.

Self-healing stimuli-responsive cellulose nanocrystal hydrogels

A facile and universal approach to prepare cellulose nanocrystal reinforced functional hydrogels is proposed. An organic solvent-free and eco-friendly method was adopted, where both the modification and polymerization were conducted in an aqueous solution. Cellulose nanocrystal (CNC) and sodium alginate (SA) were first oxidized under mild conditions to introduce aldehyde groups. Subsequently, amine-containing vinyl functionalized monomers were introduced to the surface of CNC or backbone of oxidized SA via a dynamic Schiff-base reaction. The bio-based hydrogels were then prepared via a one-pot in-situ polymerization, where the functional CNC and SA served as novel macro-cross-linkers that contributed to the structural integrity and mechanical stability of the hydrogels. The hydrogels displayed uniform chemical and macroscopic structures and could self-heal within several hours at room temperature. The design of specific monomers will allow the introduction of stimuli-responsive properties to the functional hydrogels and a chemically robust thermally-triggered actuator was demonstrated. Due to its flexible design and practical approach, the hydrogels could find potential uses in agricultural and pharmaceutical products.

Aerogel Perfusion-Prepared h-BN/CNF Composite Film with Multiple Thermally Conductive Pathways and High Thermal Conductivity

Hexagonal boron nitride (h-BN)-based heat-spreading materials have drawn considerable attention in electronic diaphragm and packaging fields because of their high thermal conductivity and desired electrical insulation properties. However, the traditional approach to fabricate thermally conductive composites usually suffers from low thermal conductivity, and cannot meet the requirement of thermal management. In this work, novel h-BN/cellulose-nano fiber (CNF) composite films with excellent thermal conductivity in through plane and electrical insulation properties are fabricated via an innovative process, i.e., the perfusion of h-BN into porous three dimensional (3D) CNF aerogel skeleton to form the h-BN thermally conductive pathways by filling the CNF aerogel voids. When at an h-BN loading of 9.51 vol %, the thermal conductivity of h-BN/CNF aerogel perfusion composite film is 1.488 W·m⁻¹·K⁻¹ at through plane, an increase by 260.3%. The volume resistivity is 3.83 × 10¹⁴ Ω·cm, superior to that of synthetic polymer materials (about 10⁹~10¹³ Ω·cm). Therefore, the resulting h-BN/CNF film is very promising to replace the traditional synthetic polymer materials for a broad spectrum of applications, including the field of electronics.

Fabrication of Poly(acrylic acid)/Boron Nitride Composite Hydrogels with Excellent Mechanical Properties and Rapid Self-Healing Through Hierarchically Physical Interactions

Many living tissues possess excellent mechanical properties and self-healing ability. To mimic these living tissues, a series of novel composite hydrogels, poly(acrylic acid)/surface-modified boron nitride nanosheets (PAA/BNNS-NH2) were fabricated simply through hierarchically physical interactions: molecular-scale metal coordination interaction between -COOH of PAA and Fe3+ and nanoscale H-bond between -COOH of PAA and -NH2 of BNNS-NH2. The composite hydrogels exhibit both excellent mechanical properties (including enhanced fracture stress, elongation, toughness, Young's modulus, and dissipated energy) and rapid healing ability without any external stimulus. Especially, the B0.5P70 (the hydrogel with BNNS concentration of 0.5 mg mL- 1, the water content of 70 wt%) exhibits a fracture stress of ~ 1311 kPa and toughness of ~ 4.7 MJ m- 3, almost ~ 3 times and ~ 8 times to B0P70, respectively. The excellent properties, combined with the simple preparation method, endow these composite hydrogels with potential applications.

Facile scalable fabrication of ultra-thin freestanding SiO2-based hybrid nanosheets with multifunctional properties

Two-dimensional (2D) nanomaterials with unique features like a large surface-to-volume ratio and the quantum confinement effect have attracted great attention for applications in energy storage, catalysis, sensing, membranes, etc. Silica (SiO₂)-based nanosheets, as members of the 2D material family, are extremely intriguing because of their unique electronic insulation, bio-compatibility and profound chemical and thermal stability. However, there is still a lack of available approaches for fabricating SiO₂ nanosheets in a simple, large-scale and cost-effective fashion. In the present research, we have proposed a facile and mass fabrication method for ultra-thin freestanding SiO₂-based hybrid nanosheets (SS) with a uniform thickness by crashing hollow microcapsules through ultrasonication treatment. The morphology, composition, and application of the hybrid nanosheets are investigated in detail. The experimental results demonstrate that SS nanosheets with an inorganic-organic hybrid structure display a Janus-type composition with double bonds residing on one side and hydroxyl groups on the other. Additionally, the SS nanosheets could be easily modified by introducing various functional components such as aluminium hydroxide (AH). The as-prepared SS nanosheets and AH modified nanosheets (SS-AH) could considerably enhance the thermal stability of silicone rubber with remarkably increased thermal decomposition temperatures and residues compared with the reference samples. SS and SS-AH sheets are highly superior in usage as polymer thermal stability fillers because of the following aspects: the hybrid nature of SS and SS-AH is advantageous to facilitate the filler-polymer interaction, so these particles could be readily dispersed into silicone without any hydrophobicity modification; these fillers could improve the thermal stability of elastomers at a much lower filler loading (<8%) than the previously reported filler system (e.g. >20 wt%). Furthermore, the nanosheets are also proved to be efficient in usage as emulsifiers for the immiscible oil-water system with a higher efficiency and emulsion stability than the commonly used emulsifiers. Consequently, the hybrid nanosheets fabricated in this work will not only enrich the family of ultra-thin 2D materials but also attract more interest in potential applications in functional nanocomposites and solid emulsifiers.

A water-based green approach to large-scale production of aqueous compatible graphene nanoplatelets

The unique properties of graphene are highly desired for printing electronics, coatings, energy storage, separation membranes, biomedicine, and composites. However, the high efficiency exfoliation of graphene into single- or few-layered nanoplates remains a grand challenge and becomes the bottleneck in essential studies and applications of graphene. Here, we report a scalable and green method to exfoliate graphene nanoplatelets (GNPs) from nature graphite in pure water without using any chemicals or surfactants. The essence of this strategy lies in the facile liquid exfoliation route with the assistance of vapor pretreatment for the preparation of edge hydroxylated graphene. The produced graphene consisted primarily of fewer than ten atomic layers. Such the water soluble graphene can be stored in the form of dispersion (~0.55 g L⁻¹) or filter cake for more than 6 months without the risk of re-stacking. This method paves the way for the environmentally friendly and cost-effective production of graphene-based materials.

Biocompatible Hydroxylated Boron Nitride Nanosheets/Poly(vinyl alcohol) Interpenetrating Hydrogels with Enhanced Mechanical and Thermal Responses

Poly(vinyl alcohol) (PVA) hydrogels with tissue-like viscoelasticity, excellent biocompatibility, and high hydrophilicity have been considered as promising cartilage replacement materials. However, lack of sufficient mechanical properties is a critical barrier to their use as load-bearing cartilage substitutes. Herein, we report hydroxylated boron nitride nanosheets (OH-BNNS)/PVA interpenetrating hydrogels by cyclically freezing/thawing the aqueous mixture of PVA and highly hydrophilic OH-BNNS (up to 0.6 mg/mL, two times the highest reported so far). Encouragingly, the resulting OH-BNNS/PVA hydrogels exhibit controllable reinforcements in both mechanical and thermal responses by simply varying the OH-BNNS contents. Impressive 45, 43, and 63% increases in compressive, tensile strengths and Young's modulus, respectively, can be obtained even with only 0.12 wt% (OH-BNNS:PVA) OH-BNNS addition. Meanwhile, exciting improvements in the thermal diffusivity (15%) and conductivity (5%) can also be successfully achieved. These enhancements are attributed to the synergistic effect of intrinsic superior properties of the as-prepared OH-BNNS and strong hydrogen bonding interactions between the OH-BNNS and PVA chains. In addition, excellent cytocompatibility of the composite hydrogels was verified by cell proliferation and live/dead viability assays. These biocompatible OH-BNNS/PVA hydrogels are promising in addressing the mechanical failure and locally overheating issues as cartilage substitutes and may also have broad utility for biomedical applications, such as drug delivery, tissue engineering, biosensors, and actuators.

Boron Nitride Nanosheet-Anchored Pd-Fe Core-Shell Nanoparticles as Highly Efficient Catalysts for Suzuki-Miyaura Coupling Reactions

Boron nitride nanosheets (BNNS) were used to anchor bimetallic Pd-Fe nanoparticles for Suzuki-Miyaura coupling catalysts. The bimetallic nanoparticles were found to be core-shell in structure, and their formation was likely facilitated by their interactions with the BNNS. The Pd-Fe/BNNS catalysts were highly effective in representative Suzuki-Miyaura reactions, with performances matching or exceeding those of the state-of-the-art methods. Specifically, the superior catalytic activities were characterized by generally shortened reaction times, minimal Pd usage, excellent reusability of the catalysts and high or nearly quantitative conversion yields in a benign solvent system without the need for any special conditions, such as ligands and surfactants or inert gas protection. The obvious advantages of the Pd-Fe/BNNS over similar catalysts based on other supports, such as reduced graphene oxide (rGO), suggest that BNNS may be developed into a versatile platform for many other important catalytic reactions.

Cosolvents as Liquid Surfactants for Boron Nitride Nanosheet (BNNS) Dispersions

Despite a range of promising applications, liquid-phase exfoliation of boron nitride nanosheets (BNNSs) is limited, both by low yield in common solvents as well as the disadvantages of using dissolved surfactants. One recently reported approach is the use of cosolvent systems to increase the as-obtained concentration of BNNS; the role of these solvents in aiding exfoliation and/or aiding colloidal stability of BNNSs is difficult to distinguish. In this paper, we have investigated the use of a t-butanol/water cosolvent to disperse BNNSs. We utilize solvent-exchange experiments to demonstrate that the t-butanol is in fact essential to colloidal stability; we then utilized molecular dynamics simulations to explore the mechanism of t-butanol/BNNS interactions. Taken together, the experimental and simulation results show that the key to the success of t-butanol (as compared to the other alcohols of higher or lower molecular weight) lies in its ability to act as a "liquid dispersant" which allows it to favorably interact with both water and BNNSs. Additionally, we show that the stable dispersions of BNNS in water/t-butanol systems may be freeze-dried to yield nonaggregated, redispersible BNNS powders, which would be useful in an array of industrial processes.

High-Yield Production of Boron Nitride Nanosheets and Its Uses as a Catalyst Support for Hydrogenation of Nitroaromatics

Single- or few-layered h-BN nanosheets (BNNSs) are analogous to graphene and possess unique properties. However, their technological applications were severely hindered by the low production efficiency of BNNSs. We reported here a study in which BNNSs were efficiently produced by exfoliating bulk h-BN powder in thionyl chloride without using any dispersion agents. The BNNSs yield was as high as 20%, and it could be doubled through the second round of exfoliation of the h-BN precipitate. Microscopic results revealed that the BNNSs generally consisted of 3-20 layers. Pd nanoparticles were successfully immobilized and uniformly distributed on BNNS surfaces through the deposition-precipitation method. The resultant Pd-BNNS catalyst exhibited high catalytic activity and recyclability for the hydrogenation of nitro aromatics, demonstrating that BNNSs served as a promising platform to fabricate heterogeneous catalysts.