Theory of electrical conductivity and dielectric permittivity of highly aligned graphene-based nanocomposites

Identification of key pathways in zirconia/dental pulp stem cell composite scaffold-mediated macrophage polarization through transcriptome sequencing

Seed cells and scaffold materials are essential components of tissue engineering. In this study, we investigated the key pathway of the zirconia/dental pulp stem cell composite scaffold in regulating macrophage polarization by transcriptome sequencing. We established N-rGO/ZrO2 composite scaffold and confirmed its structure using various analytical techniques, including SEM, TEM, FTIR, Raman spectra, XPS, and XRD. DPSCs were seeded onto N-rGO/ZrO2 composite scaffold material, and their proliferation, adhesion, and osteogenic differentiation were evaluated by CCK-8, immunofluorescence staining, ALP staining, and alizarin red staining. We then co-cultured DPSCs combined with N-rGO/ZrO2 as composite material with THP-1 cells in a transwell system to investigate the effect of the composite on macrophage polarization. The levels of pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes were assessed by RT-qPCR and western blot. Through bulk RNA sequencing, we detected the transcriptional characteristics of macrophages under the regulation of the composite materials, and identified the differential genes using the DEseq2 package. We also analyzed the cellular and molecular functions of differentially expressed genes (DEGs) in THP-1 cells with DPSCs combined with N-rGO/ZrO2 treatment using GO enrichment analysis and KEGG pathway enrichment analysis. Our results showed that N-rGO/ZrO2 composite scaffold promoted the proliferation, adhesion, and osteogenic differentiation of DPSCs. Moreover, N-rGO/ZrO2 composite scaffold combined with DPSCs regulated macrophage migration, polarization, and glycolysis. Mechanistically, the combination of N-rGO/ZrO2 composite materials and DPSCs regulated macrophage polarization by activating the TNF signaling pathway. This finding provides a new approach to the clinical preservation of maxillofacial bone defect repair.

Analytical Modeling of Wave Absorption Performance in Gradient Graphene/Polymer Nanocomposites

Due to the low impedance matching caused by the high dielectric permittivity of graphene, the strong absorption of electromagnetic waves by graphene/polymer nanocomposites is challenging. In this paper, an analytical model for microwave absorption based on Maxwell's equation and the effective medium theory, considering the interface effect, was constructed to explore the effect of the gradient distribution of graphene in the polymer matrix on its microwave absorption performance. The outcome indicated that the impedance of the composites matched well with the air, and its attenuation ability for electromagnetic waves was obviously improved as the graphene concentration was distributed in a gradient form. For instance, when the thickness of the material is 10 mm, based on the optimal concentration of the homogeneous composites being 0.7 wt%, the graphene concentration range of the gradient composites is set to 0.7-0.9 wt% and distributed in three gradient forms of linear, parabolic, and 0.5 power. The results show that the microwave absorption performance is significantly improved compared with the homogeneous composites. Among them, the effective bandwidth on the 0.5 power distribution is 5.2 GHz, 0.5 GHz higher than that of the homogeneous composites. The minimum reflection loss (RL) is as low as -54.7 dB, which is 26.26 dB lower than that of the homogeneous composites. This paper contributes to the design and application of gradient absorbing structures.

Orientation of graphene nanosheets in suspension under an electric field: theoretical model and molecular dynamic simulations

Orientation regulation of nanoparticles in a suspension by an electric field is a powerful tool to tune its mechanical, thermal, optical, electrical properties etc. However, how molecular modification can affect the orientation of two-dimensional nanoparticles is still unclear. In this paper, the influence of molecular modification on the orientation of graphene nanosheets (GNS) in water was investigated through theoretical analyses and molecular dynamics (MD) simulations. Firstly, a new orientation angle model was proposed, which considers hydration effects, dipole moments and resistance torque. Then, MD simulations were conducted to investigate the effects of position, direction, type, and number of functional groups on the orientation of GNS. The trend observed in MD simulations is consistent with the proposed theoretical model. The results reveal that, under the combined influence of the dipole moment and hydration effects, the modification with hydrophilic functional groups can reduce the orientation angle from 21.31° to 8.34°, while the modification with hydrophobic functional groups increases it to 26.43°. Among the hydrophilic functional groups, orientation of hydroxylated GNS is the best. With an increase in the number of hydroxyl groups, orientation angle is decreased from 12.61° to 8.34°. This work can provide valuable guidance for the design of high-performance suspensions and composites, such as thermal smart materials with adjustable thermal conductivity and intelligent devices with tailored capabilities.

Recent advances in 2D graphene reinforced metal matrix composites

The unique combination of excellent mechanical and functional properties makes graphene an ideal component for high-performance 'smart' composites, which are sensitive to thermal, optical, electrical and mechanical excitations, hence being potential in application of a range of sensors. It has confirmed that the addition of graphene into metal matrix can significantly enhance the mechanical property and deliver surprising functional properties. Thus, graphene reinforced metal matrix composites (GMMCs) have long been regarded as potential prospects of nanotechnology applications. Recently, researchers mainly focused on: (i) solving the interfacial issues and realizing controllable alignment of graphene in metal matrix to achieve optimal performance; (ii) reasonable designing of the microstructures basing on usage requirement and then fabricating via efficient technique. Thus, it is necessary to figure out key roles of microstructure in fabrication process, mechanical and multi-functional properties. This review consists of four parts: (i) fabrication process. The fabrication processes are firstly divided into three kinds basing on the different bonding nature between graphene and metal matrix. (ii) Mechanical property. The microstructural characteristics of metal matrix accompanying by the incorporation of graphene and their vital effects on mechanical properties of GMMCs are systematically summarized. (iii) Functional property. The crucial effects of microstructure on electrical and thermal properties are summarized. (iv) Prospect applications and future challenges. Application and challenges basing on the research status are discussed to provide useful directions for future exploration in related fields. All these four parts are discussed with a focus on key role of microstructure characteristics, which is instructive for the microstructures design and fabrication process optimization during academic researches and potential commercial applications.

The influence of reduced graphene oxide on stem cells: a perspective in peripheral nerve regeneration

Graphene and its derivatives are fascinating materials for their extraordinary electrochemical and mechanical properties. In recent decades, many researchers explored their applications in tissue engineering and regenerative medicine. Reduced graphene oxide (rGO) possesses remarkable structural and functional resemblance to graphene, although some residual oxygen-containing groups and defects exist in the structure. Such structure holds great potential since the remnant-oxygenated groups can further be functionalized or modified. Moreover, oxygen-containing groups can improve the dispersion of rGO in organic or aqueous media. Therefore, it is preferable to utilize rGO in the production of composite materials. The rGO composite scaffolds provide favorable extracellular microenvironment and affect the cellular behavior of cultured cells in the peripheral nerve regeneration. On the one hand, rGO impacts on Schwann cells and neurons which are major components of peripheral nerves. On the other hand, rGO-incorporated composite scaffolds promote the neurogenic differentiation of several stem cells, including embryonic stem cells, mesenchymal stem cells, adipose-derived stem cells and neural stem cells. This review will briefly introduce the production and major properties of rGO, and its potential in modulating the cellular behaviors of specific stem cells. Finally, we present its emerging roles in the production of composite scaffolds for nerve tissue engineering.

Recent Progress in Graphene/Polymer Nanocomposites

Nanocomposites, multiphase solid materials with at least one nanoscaled component, have been attracting ever-increasing attention because of their unique properties. Graphene is an ideal filler for high-performance multifunctional nanocomposites in light of its superior mechanical, electrical, thermal, and optical properties. However, the 2D nature of graphene usually gives rise to highly anisotropic features, which brings new opportunities to tailor nanocomposites by making full use of its excellent in-plane properties. Here, recent progress on graphene/polymer nanocomposites is summarized with emphasis on strengthening/toughening, electrical conduction, thermal transportation, and photothermal energy conversion. The influence of the graphene configuration, including layer number, defects, and lateral size, on its intrinsic properties and the properties of graphene/polymer nanocomposites is systematically analyzed. Meanwhile, the role of the interfacial interaction between graphene and polymer in affecting the properties of nanocomposites is also explored. The correlation between the graphene distribution in the matrix and the properties of the nanocomposite is discussed in detail. The key challenges and possible solutions are also addressed. This review may provide a constructive guidance for preparing high-performance graphene/polymer nanocomposite in the future.

Calculating the Electrical Conductivity of Graphene Nanoplatelet Polymer Composites by a Monte Carlo Method

Electrical conductivity is one of several outstanding features of graphene–polymer nanocomposites, but calculations of this property require the intricate features of the underlying conduction processes to be accounted for. To this end, a novel Monte Carlo method was developed. We first established a randomly distributed graphene nanoplatelet (GNP) network. Then, based on the tunneling effect, the contact conductance between the GNPs was calculated. Coated surfaces (CSs) were next set up to calculate the current flow from the GNPs to the polymer. Using the equipotential approximation, the potentials of the GNPs and CSs met Kirchhoff’s current law, and, based on Laplace equation, the potential of the CSs was obtained from the potential of the GNP by the walk-on-spheres (WoS) method. As such, the potentials of all GNPs were obtained, and the electrical conductivity of the GNP polymer composites was calculated. The barrier heights, polymer conductivity, diameter and thickness of the GNP determining the electrical conductivity of composites were studied in this model. The calculated conductivity and percolation threshold were shown to agree with experimental data.

Electromechanical Behaviors of Graphene Reinforced Polymer Composites: A Review

Graphene (including its derivatives)-reinforced polymer composites (GRPCs) have been drawing tremendous attention from academic and industrial communities for developing smart materials and structures. Such interest stems from the excellent combination of the mechanical and electrical properties of these composites while keeping the beneficial intrinsic attributes of the polymers, including flexibility, easy processability, low cost and good biological and chemical compatibility. The electromechanical performances of these GRPCs are of great importance for the design and optimization of engineering structures and components. Extensive work has been devoted to this topic. This paper reviews the recent studies on the electromechanical behaviors of GRPCs. First the methods and techniques to manufacture graphene and GRPCs are introduced, in which the pros and cons of each method are discussed. Then the experimental examination and theoretical modeling on the electromechanical behaviors of the nanocomposites are presented and discussed.

Multifunctional Polymer Nanocomposites Reinforced by Aligned Carbon Nanomaterials

Carbon nanomaterials such as carbon black (CB), carbon nanotubes (CNTs), and graphene have demonstrated significant potential as fillers to improve the electrical, thermal, and mechanical properties of polymers and their fiber-reinforced polymer composites. The level of improvement has been found to depend significantly on the degree of alignment of carbon nanomaterials. Due to the very small scale and complex interactions of carbon nanomaterials with polymers and structural fibers, alignment in a given direction has been a major challenge. Over the past decade, considerable effort has been devoted to developing effective strategies to align carbon nanomaterials in polymer matrices. However, significant technological challenges remain, and there is still a lack of understanding of the alignment mechanisms and their effects on the properties of polymers and composites. This paper reviews in situ alignment techniques including shear deformation, mechanical stretching, electrospinning, and application of an external magnetic or electric field, and ex situ techniques including using vertically grown CNTs or graphene. This review particularly focuses on physical mechanisms underpinning the magnetic or electric field-induced alignment and theoretical analyses that describe the different motions occurring and the major parameters controlling alignment. Moreover, this review highlights the recent research findings of the effects of alignment on the properties of polymer nanocomposites. The outlook towards the challenges and opportunities in this field are also discussed in this review.

Effects of Graphene Nanoplatelet Size and Surface Area on the AC Electrical Conductivity and Dielectric Constant of Epoxy Nanocomposites

Epoxy nanocomposites reinforced with various grades of multilayer graphene nanoplatelets (GNPs) are manufactured and tested. The effects of size, surface area, and concentration of GNP, as well as alternating current (AC) frequency on the electrical and dielectric properties of epoxy nanocomposites are experimentally investigated. GNPs with larger size and surface area are always beneficial to increase the electrical conductivity of the composites. However, their effects on the dielectric constant are highly dependent on GNP concentration and AC frequency. At lower GNP concentration, the dielectric constant increases proportionally with the increase in GNP size, while decreasing as the AC frequency increases. At higher GNP concentration in epoxy, the dielectric constant first increases with the increase of the GNP size, but decreases thereafter. This trend is also observed for varying the processed GNP surface area on the dielectric constant. Moreover, the variations of the electrical conductivity and dielectric constant with the GNP concentration and AC frequency are then correlated with the measured interfiller spacing and GNP diameter.