CNT-MXene ultralight membranes: fabrication, surface nano/microstructure, 2D-3D stacking architecture, ion-transport mechanism, and potential application as interlayers for Li-O2 batteries.

Multiwalled carbon nanotubes (MWCNTs) have shown effectiveness in improving the suitability of MXenes for energy-related applications. However, the ability of individually dispersed MWCNTs to control the structure of MXene-based macrostructures is unclear. Here, the correlation among composition, surface nano- and microstructure, MXenes' stacking order, structural swelling, and Li-ion transport mechanisms and properties in individually dispersed MWCNT-Ti3C2 films was investigated. The compact surface microstructure of MXene film, characterized by prominent wrinkles, is dramatically changed as MWCNTs occupy MXene/MXene edge interfaces. The 2D stacking order is preserved up to 30 wt% MWCNTs despite a significant swelling of ∼400%. Such alignment is completely disrupted at 40 wt%, and a more pronounced surface opening and internal expansion of ∼770% are realized. Both 30 wt% and 40 wt% membranes show stable cycling performance under a significantly higher current density due to faster transport channels. Notably, for the 3D membrane, the overpotential during repeated Li deposition/dissolution reactions is further reduced by ∼50%. Ion-transport mechanisms in the absence and presence of MWCNTs are discussed. Furthermore, ultralight yet continuous hybrid films comprising up to ∼0.027 mg cm-2 Ti3C2 can be prepared using aqueous colloidal dispersions and vacuum filtration for specific applications. The potential application of such ultralight membranes as interlayers for Li-O2 batteries is briefly examined.

Novel MAB phases and insights into their exfoliation into 2D MBenes.

Considering the recent breakthroughs in the synthesis of novel two-dimensional (2D) materials from layered bulk structures, ternary layered transition metal borides, known as MAB phases, have come under scrutiny as a means of obtaining novel 2D transition metal borides, the so-called MBenes. Here, based on a set of phonon calculations, we show the dynamic stability of many Al-containing MAB phases, MAlB (M = Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc), M2AlB2 (Sc, Ti, Zr, Hf, V, Cr, Mo, W, Mn, Tc, Fe, Rh, Ni), M3Al2B2 (M = Sc, T, Zr, Hf, Cr, Mn, Tc, Fe, Ru, Ni), M3AlB4 (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe), and M4AlB6 (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo). By comparing the formation energies of these MAB phases with those of their available competing binary M-B and M-Al, and ternary M-Al-B phases, we find that some of the Sc-, Ti-, V-, Cr-, Mo-, W-, Mn-, Tc-, and Fe-based MAB phases could be favorably synthesized under appropriate experimental conditions. In addition, by examining the strengths of various bonds in MAB phases via crystal orbital Hamilton population and spring constant calculations, we find that the B-B and then M-B bonds are stiffer than the M-Al and Al-B bonds. The different strengths between these bonds imply the etching possibility of Al atoms from MAB phases, consequently forming various 2D MB, M2B3, and M3B4 MBenes. Furthermore, we employ the nudged elastic band method to investigate the possibility of the structural phase transformation of the 2D MB MBenes into graphene-like boron sheets sandwiched between transition metals and find that the energy barrier of the transformation is less than 0.4 eV per atom.

Carbon nanotubes embedded in embryoid bodies direct cardiac differentiation.

We embedded carbon nanotubes (CNTs) in mouse embryoid bodies (EBs) for modulating mechanical and electrical cues of the stem cell niche. The CNTs increased the mechanical integrity and electrical conductivity of the EBs. Measured currents for the unmodified EBs (hereafter, EBs) and the EBs-0.25 mg/mL CNTs were 0.79 and 26.3 mA, respectively, at voltage of 5 V. The EBs had a Young's modulus of 20.9 ± 6.5 kPa, whereas the Young's modulus of the EB-0.1 mg/mL CNTs was 35.2 ± 5.6 kPa. The EB-CNTs also showed lower proliferation and greater differentiation rates compared with the EBs as determined by the expression of pluripotency genes and the analysis of EB sizes. Interestingly, the cardiac differentiation of the EB-CNTs was significantly greater than that of the EBs, as confirmed by high-throughput gene analysis at day 5 of culture. Applying electrical stimulation to the EB-CNTs specifically enhanced the cardiac differentiation and beating activity of the EBs.

Moldable elastomeric polyester-carbon nanotube scaffolds for cardiac tissue engineering.

Polymer biomaterials are used to construct scaffolds in tissue engineering applications to assist in mechanical support, organization, and maturation of tissues. Given the flexibility, electrical conductance, and contractility of native cardiac tissues, it is desirable that polymeric scaffolds for cardiac tissue regeneration exhibit elasticity and high electrical conductivity. Herein, we developed a facile approach to introduce carbon nanotubes (CNTs) into poly(octamethylene maleate (anhydride) 1,2,4-butanetricarboxylate) (124 polymer), and developed an elastomeric scaffold for cardiac tissue engineering that provides electrical conductivity and structural integrity to 124 polymer. 124 polymer-CNT materials were developed by first dispersing CNTs in poly(ethylene glycol) dimethyl ether porogen and mixing with 124 prepolymer for molding into shapes and crosslinking under ultraviolet light. 124 polymers with 0.5% and 0.1% CNT content (wt) exhibited improved conductivity against pristine 124 polymer. With increasing the CNT content, surface moduli of hybrid polymers were increased, while their bulk moduli were decreased. Furthermore, increased swelling of hybrid 124 polymer-CNT materials was observed, suggesting their improved structural support in an aqueous environment. Finally, functional characterization of engineered cardiac tissues using the 124 polymer-CNT scaffolds demonstrated improved excitation threshold in materials with 0.5% CNT content (3.6±0.8V/cm) compared to materials with 0% (5.1±0.8V/cm) and 0.1% (5.0±0.7V/cm), suggesting greater tissue maturity. 124 polymer-CNT materials build on the advantages of 124 polymer elastomer to give a versatile biomaterial for cardiac tissue engineering applications. STATEMENT OF SIGNIFICANCE: Achieving a high elasticity and a high conductivity in a single cardiac tissue engineering material remains a challenge. We report the use of CNTs in making electrically conductive and mechanically strong polymeric scaffolds in cardiac tissue regeneration. CNTs were incorporated in elastomeric polymers in a facile and reproducible approach. Polymer-CNT materials were able to construct complicated scaffold structures by injecting the prepolymer into a mold and crosslinking the prepolymer under ultraviolet light. CNTs enhanced electrical conductivity and structural support of elastomeric polymers. Hybrid polymeric scaffolds containing 0.5wt% CNTs increased the maturation of cardiac tissues fabricated on them compared to pure polymeric scaffolds. The cardiac tissues on hybrid polymer-CNT scaffolds showed earlier beating than those on pure polymer scaffolds. In the future, fabricated polymer-CNT scaffolds could also be used to fabricate other electro-active tissues, such neural and skeletal muscle tissues. In the future, fabricated polymer-CNT scaffolds could also be used to fabricate other electro-active tissues, such as neural and skeletal muscle tissues.

Graphene induces spontaneous cardiac differentiation in embryoid bodies.

Graphene was embedded into the structure of mouse embryoid bodies (EBs) using the hanging drop technique. The inclusion of 0.2 mg per mL graphene in the EBs did not affect the viability of the stem cells. However, the graphene decreased the stem cell proliferation, probably by accelerating cell differentiation. The graphene also enhanced the mechanical properties and electrical conductivity of the EBs. Interestingly, the cardiac differentiation of the EB-graphene was significantly greater than that of the EBs at day 5 of culture, as confirmed by high-throughput gene analysis. Electrical stimulation (voltage, 4 V; frequency, 1 Hz; and duration, 10 ms for 2 continuous days) further enhanced the cardiac differentiation of the EBs, as demonstrated by analyses of the cardiac protein and gene expression and the beating activity of the EBs. Taken together, the results demonstrated that graphene played a major role in directing the cardiac differentiation of EBs, which has potential cell therapy and tissue regeneration applications.

Hybrid hydrogel-aligned carbon nanotube scaffolds to enhance cardiac differentiation of embryoid bodies.

Carbon nanotubes (CNTs) were aligned in gelatin methacryloyl (GelMA) hydrogels using dielectrophoresis approach. Mouse embryoid bodies (EBs) were cultured in the microwells fabricated on the aligned CNT-hydrogel scaffolds. The GelMA-dielectrophoretically aligned CNT hydrogels enhanced the cardiac differentiation of the EBs compared with the pure GelMA and GelMA-random CNT hydrogels. This result was confirmed by Troponin-T immunostaining, the expression of cardiac genes (i.e., Tnnt2, Nkx2-5, and Actc1), and beating analysis of the EBs. The effect on EB properties was significantly enhanced by applying an electrical pulse stimulation (frequency, 1Hz; voltage, 3V; duration, 10ms) to the EBs for two continuous days. Taken together, the fabricated hybrid hydrogel-aligned CNT scaffolds with tunable mechanical and electrical characteristics offer an efficient and controllable platform for electrically induced differentiation and stimulation of stem cells for potential tissue regeneration and cell therapy applications. STATEMENT OF SIGNIFICANCE: Dielectrophoresis approach was used to rapidly align carbon nanotubes (CNTs) in gelatin methacryloyl (GelMA) hydrogels resulting in hybrid GelMA-CNT hydrogels with tunable and anisotropic electrical and mechanical properties. The GelMA-aligned CNT hydrogels may be used to apply accurate and controllable electrical pulses to cell and tissue constructs and thereby regulating their behavior and function. In this work, it was demonstrated that the GelMA hydrogels containing the aligned CNTs had superior performance in cardiac differentiation of stem cells upon applying electrical stimulation in contrast with control gels. Due to broad use of electrical stimulation in tissue engineering and stem cell differentiation, it is envisioned that the GelMA-aligned CNT hydrogels would find wide applications in tissue regeneration and stem cell therapy.

Synthesis and characterization of porous carbon-MoS2 nanohybrid materials: electrocatalytic performance towards selected biomolecules.

Porous carbon nanohybrids are promising materials as high-performance electrodes for both sensing and energy conversion applications. This is mainly due to their high specific surface area and specific physicochemical properties. Here, new porous nanohybrid materials are developed based on exfoliated MoS2 nanopetals and either negatively charged phenylsulfonated carbon nanoparticles or positively charged sulfonamide functionalized carbon nanoparticles. MoS2 nanopetals not only act as a scaffold for carbon nanoparticles to form 3D porous hierarchical architectures but also result in well-separated electrochemical signals for different compounds. The characteristics of the new carbon nanohybrid materials are studied by dynamic light scattering, zeta potential analysis, high resolution X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, infrared spectroscopy and electrochemistry. The new hybrid materials show superior charge transport capability and electrocatalytic activity toward selected biologically relevant compounds compared to earlier reports on porous carbon electrodes.

45S5 Bioglass(®)-MWCNT composite: processing and bioactivity.

Multi-walled carbon nanotube (MWCNT)-Bioglass (BG) matrix composite was fabricated using a facile and scalable aqueous colloidal processing method without using any surfactants followed by spark plasma sintering (SPS) consolidation. The individual MWCNTs were initially uniformly dispersed in water and then entirely immobilized on the BG particles during the colloidal processing, avoiding their common re-agglomeration during the water-removal and drying step, which guaranteed their uniform dispersion within the dense BG matrix after the consolidation process. SPS was used as a fast sintering technique to minimise any damage to the MWCNT structure during the high-temperature consolidation process. The electrical conductivity of BG increased by 8 orders of magnitude with the addition of 6.35 wt% of MWCNTs compared to pure BG. Short-duration tests were used in the present study as a preliminary evaluation to understand the effect of incorporating MWCNTs on osteoblast-like cells. The analysed cell proliferation, viability and phenotype expression of MG-63 cells showed inhibition on 45S5 Bioglass(®)-MWCNT composite surfaces.

Facile and green production of aqueous graphene dispersions for biomedical applications.

We proposed a facile, low cost, and green approach to produce stable aqueous graphene dispersions from graphite by sonication in aqueous bovine serum albumin (BSA) solution for biomedical applications. The production of high-quality graphene was confirmed using microscopy images, Raman spectroscopy, UV-vis spectroscopy, and XPS. In addition, ab initio calculations revealed molecular interactions between graphene and BSA. The processability of aqueous graphene dispersions was demonstrated by fabricating conductive and mechanically robust hydrogel-graphene materials.

Trends in electronic structures and structural properties of MAX phases: a first-principles study on M(2)AlC (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta), M(2)AlN, and hypothetical M(2)AlB phases.

MAX phases are a large family of layered ceramics with many potential structural applications. A set of first-principles calculations was performed for M(2)AlC and M(2)AlN (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta) MAX phases as well as for hypothetical M(2)AlB to investigate trends in their electronic structures, formation energies, and various mechanical properties. Analysis of the calculated data is used to extend the idea that the elastic properties of MAX phases can be controlled according to the valence electron concentration. The valence electron concentrationcan be tuned through the various combinations of transition metal and nonmetal elements.

Facile and rapid generation of 3D chemical gradients within hydrogels for high-throughput drug screening applications.

We propose a novel application of dielectrophoresis (DEP) to make three-dimensional (3D) methacrylated gelatin (GelMA) hydrogels with gradients of micro- and nanoparticles. DEP forces were able to manipulate micro- and nanoparticles of different sizes and materials (i.e., C2C12 myoblasts, polystyrene beads, gold microparticles, and carbon nanotubes) within GelMA hydrogels in a rapid and facile way and create 3D gradients of these particles in a microchamber. Immobilization of drugs, such as fluorescein isothiocyanate-dextran (FITC-dextran) and 6-hydroxydopamine (6-OHDA), on gold microparticles allowed us to investigate the high-throughput release of these drugs from GelMA-gold microparticle gradient systems. The latter gradient constructs were incubated with C2C12 myoblasts for 24h to examine the cell viability through the release of 6-OHDA. The drug was released from the microparticles in a gradient manner, inducing a cell viability gradient. This novel approach to create 3D chemical gradients within hydrogels is scalable to any arbitrary length scale. It is useful for making anisotropic biomimetic materials and high-throughput platforms to investigate cell-microenvironment interactions in a rapid, simple, cost-effective, and reproducible manner.

Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family.

A newly synthesized family of two-dimensional transition metal carbides and nitrides, so-called MXenes, exhibit metallic or semiconducting properties upon appropriate surface functionalization. Owing to their intrinsic ceramic nature, MXenes may be suitable for energy conversion applications at high temperature. Using the Boltzmann theory and first-principles electronic structure calculations, we explore the thermoelectric properties of monolayer and multilayer M2C (M = Sc, Ti, V, Zr, Nb, Mo, Hf, and Ta) and M2N (M = Ti, Zr, and Hf) MXenes functionalized with F, OH, and O groups. From our calculations, it turns out that monolayer and multilayer nanosheets of Mo2C acquire superior power factors to other MXenes upon any type of functionalization. We therefore propose the functionalized Mo2C nanosheets as potential thermoelectric materials of the MXene family. The exceptional thermoelectric properties of the functionalized Mo2C nanosheets are attributed to the peculiar t2g band shapes, which are a combination of flat and dispersive portions. These types of band shapes allow Mo2C to gain a large Seebeck coefficient and simultaneously a good electrical conductivity at low carrier concentrations.

Hybrid hydrogels containing vertically aligned carbon nanotubes with anisotropic electrical conductivity for muscle myofiber fabrication.

Biological scaffolds with tunable electrical and mechanical properties are of great interest in many different fields, such as regenerative medicine, biorobotics, and biosensing. In this study, dielectrophoresis (DEP) was used to vertically align carbon nanotubes (CNTs) within methacrylated gelatin (GelMA) hydrogels in a robust, simple, and rapid manner. GelMA-aligned CNT hydrogels showed anisotropic electrical conductivity and superior mechanical properties compared with pristine GelMA hydrogels and GelMA hydrogels containing randomly distributed CNTs. Skeletal muscle cells grown on vertically aligned CNTs in GelMA hydrogels yielded a higher number of functional myofibers than cells that were cultured on hydrogels with randomly distributed CNTs and horizontally aligned CNTs, as confirmed by the expression of myogenic genes and proteins. In addition, the myogenic gene and protein expression increased more profoundly after applying electrical stimulation along the direction of the aligned CNTs due to the anisotropic conductivity of the hybrid GelMA-vertically aligned CNT hydrogels. We believe that platform could attract great attention in other biomedical applications, such as biosensing, bioelectronics, and creating functional biomedical devices.

Mechanically reliable thermoelectric (TE) nanocomposites by dispersing and embedding TE-nanostructures inside a tetragonal ZrO2 matrix: the concept and experimental demonstration in graphene oxide-3YSZ system.

Novel low-dimensional thermoelectric (TE) materials suffer from poor mechanical reliability, which limits their applications, especially in mechanically harsh environments. Here, we propose a new concept, in which the novel, abundant, thermally stable TE-nanostructures are dispersed and then intimately embedded inside a protective, mechanically reliable tetragonal ZrO2 (TZP) ceramic matrix with a low thermal conductivity. We also demonstrate an experimental proof-of-principle verification of our concept in reduced-graphene oxide (GO)-3 mol% Y2O3-ZrO2 (3YSZ or 3Y-TZP) nanocomposite system. TE characterizations suggest that our protective TZP matrix does not degrade the intrinsic TE property of the reduced GO network. These preliminary results are promising and encouraging to start research on similar TZP-matrix TE-nanocomposites, which contain more effective TE-nanostructures with larger intrinsic power factors. In this regard, we propose a scalable approach for fabrication of similar dense TE-nanocomposites composed of other one-dimensional and/or two-dimensional TE-nanostructures, which involves an aqueous colloidal approach and a subsequent spark plasma sintering. These new TZP-matrix TE-nanocomposites could be used for sustainable clean power generation, especially in mechanically harsh environments with thermal/mechanical shocks and vibrations, where energy availability, reliability and durability are more important than the energy efficiency. Considering the excellent biocompatibility of TZP matrix, they could even be used inside the body to power implanted medical devices.

Microstructure and high-temperature strength of textured and non-textured ZrB2 ceramics.

Zirconium diboride (ZrB2) ceramic possesses a unique combination of nice mechanical performance, high melting point (> 3000 °C) and great high-temperature oxidation resistance (up to 1600 °C), which makes it a promising material system for ever-increasing ultra-high temperature (UHT) applications. However, ZrB2 suffers from poor mechanical performance at UHTs, which could strongly limit its applications at UHT. Here, we successfully demonstrate that texturing is an effective strategy to greatly enhance the flexural strength of monolithic ZrB2, reaching a high value of 810 ± 60 MPa at 1600 °C when loaded in c-axis direction. We thoroughly discuss the strengthening mechanism by in-depth microstructural observations and analysis. Our discovery has technological and scientific implications for other UHT ceramic systems, especially those using ZrB2 as a matrix.

The effect of the interlayer element on the exfoliation of layered Mo2AC (A = Al, Si, P, Ga, Ge, As or In) MAX phases into two-dimensional Mo2C nanosheets.

The experimental exfoliation of layered, ternary transition-metal carbide and nitride compounds, known as MAX phases, into two-dimensional (2D) nanosheets, is a great development in the synthesis of novel low-dimensional inorganic systems. Among the MAX phases, Mo-containing ones might be considered as the source for obtaining Mo2C nanosheets with potentially unique properties, if they could be exfoliated. Here, by using a set of first-principles calculations, we discuss the effect of the interlayer 'A' element on the exfoliation of Mo2AC (A = Al, Si, P, Ga, Ge, As or In) MAX phases into the 2D Mo2C nanosheets. Based on the calculated exfoliation energies and the elastic constants, we propose that Mo2InC with the lowest exfoliation energy and the highest elastic constant anisotropy between C11 and C33 might be a suitable compound for exfoliation into 2D Mo2C nanosheets.

Recent advances in understanding the reinforcing ability and mechanism of carbon nanotubes in ceramic matrix composites.

Since the discovery of carbon nanotubes (CNTs), commonly referred to as ultimate reinforcement, the main purpose for fabricating CNT-ceramic matrix composites has been mainly to improve the fracture toughness and strength of the ceramic matrix materials. However, there have been many studies reporting marginal improvements or even the degradation of mechanical properties. On the other hand, those studies claiming noticeable toughening measured using indentation, which is an indirect/unreliable characterization method, have not demonstrated the responsible mechanisms applicable to the nanoscale, flexible CNTs; instead, those studies proposed those classical methods applicable to microscale fiber/whisker reinforced ceramics without showing any convincing evidence of load transfer to the CNTs. Therefore, the ability of CNTs to directly improve the macroscopic mechanical properties of structural ceramics has been strongly questioned and debated in the last ten years. In order to properly discuss the reinforcing ability (and possible mechanisms) of CNTs in a ceramic host material, there are three fundamental questions to our knowledge at both the nanoscale and macroscale levels that need to be addressed: (1) does the intrinsic load-bearing ability of CNTs change when embedded in a ceramic host matrix?; (2) when there is an intimate atomic-level interface without any chemical reaction with the matrix, could one expect any load transfer to the CNTs along with effective load bearing by them during crack propagation?; and (3) considering their nanometer-scale dimensions, flexibility and radial softness, are the CNTs able to improve the mechanical properties of the host ceramic matrix at the macroscale when individually, intimately and uniformly dispersed? If so, how? Also, what is the effect of CNT concentration in such a defect-free composite system? Here, we briefly review the recent studies addressing the above fundamental questions. In particular, we discuss the new reinforcing mechanism at the nanoscale responsible for unprecedented, simultaneous mechanical improvements and highlight the scalable processing method enabling the fabrication of defect-free CNT-concentered ceramics and CNT-graded composites with unprecedented properties. Finally, possible future directions will be briefly presented.

Dielectrophoretically aligned carbon nanotubes to control electrical and mechanical properties of hydrogels to fabricate contractile muscle myofibers.

Dielectrophoresis is used to align carbon nanotubes (CNTs) within gelatin methacrylate (GelMA) hydrogels in a facile and rapid manner. Aligned GelMA-CNT hydrogels show higher electrical properties compared with pristine and randomly distributed CNTs in GelMA hydrogels. The muscle cells cultured on these materials demonstrate higher maturation compared with cells cultured on pristine and randomly distributed CNTs in GelMA hydrogels.

Unprecedented simultaneous enhancement in strain tolerance, toughness and strength of Al2O3 ceramic by multiwall-type failure of a high loading of carbon nanotubes.

Carbon nanotubes (CNTs) have a remarkable load-bearing ability. Recently, however, multi-walled CNTs (MWCNTs) have been shown to possess dramatically higher load-bearing ability when intimately embedded in an oxide ceramic (Al2O3), because the load could be transferred not to only their outermost walls but also their generally unloaded inner walls via the strong interwall shear resistance originating from residual compressive stresses. This phenomenon is characterized by an uncommon, highly energy-dissipating, multiwall-type failure of individual MWCNTs during hybrid fracture, with no evidence of pullout. Here, we demonstrate that this nanoscale in-MWCNT load-transfer process, at an optimized, high loading of MWCNTs (10 vol%) and in a pore-free and uniform platform, leads to unprecedented, dramatic simultaneous enhancement in strain tolerance (81%), fracture toughness (52.2%), and flexural strength (22%) of the Al2O3 ceramic matrix. The extent of toughening by this mechanism is also the highest ever reported. This unprecedented performance by using a high loading of functional MWCNTs, namely, toughening, strengthening, softening and lightening, simultaneously and at this level, has implications for many functional and structural applications.

Highly concentrated 3D macrostructure of individual carbon nanotubes in a ceramic environment.

A highly concentrated 3D macrostructure of individual multiwalled carbon nanotubes (MWCNTs) is practically realized in a ceramic environment with poreless/intimate interfaces by a scalable aqueous colloidal approach. This concept dramatically improves not only the transport property and network connectivity of the MWCNT 3D macrostructures (a DC-conductivity of nearly 5000 S m(-1) ) but also the strain tolerance of the ceramic environment. Such low-cost and novel MWCNT/ceramic hybrids have many potential functional and structural applications.