Biohybrid Carbon Nanotube/Agarose Fibers for Neural Tissue Engineering

We report a novel approach for producing carbon nanotube fibers (CNF) composed with the polysaccharide agarose. Current attempts to make CNF's require the use of a polymer or precipitating agent in the coagulating bath that may have negative effects in biomedical applications. We show that by taking advantage of the gelation properties of agarose one can substitute the bath with distilled water or ethanol and hence reduce the complexity associated with alternating the bath components or the use of organic solvents. We also demonstrate that these CNF can be chemically functionalized to express biological moieties through available free hydroxyl groups in agarose. We corroborate that agarose CNF are not only conductive and nontoxic, but their functionalization can facilitate cell attachment and response both in vitro and in vivo. Our findings suggest that agarose/CNT hybrid materials are excellent candidates for applications involving neural tissue engineering and biointerfacing with the nervous system.     

Catalytic and Electron Conducting Carbon Nanotube–Reinforced Lysozyme Crystals

Novel reinforced cross‐linked lysozyme crystals containing homogeneous dispersions of single‐walled carbon nanotubes bundles (SWCNTs) are produced and characterized. The incorporation of SWCNTs inside lysozyme crystals gives rise to reinforced composite materials with tunable mechanical strength and electronic conductivity, while preserving the crystal quality and morphology. These reinforced crystals show increased catalytic activity at higher temperatures, being active even above the denaturation temperature. The electron transport through the crystals is linked to the content and distribution of SWCNT bundles inside the crystals. The electron conduction through the crystals is isotropic and very efficient, presenting high conductivity values up to 600 nS at very low (0.05 wt%) SWCNT concentration. To obtain these crystals, a new protocol based on the in situ crystallization of lysozyme in composite SWCNT–peptide hydrogels is developed. These peptide hydrogels are able to homogeneously disperse bundles of hydrophobic SWCNTs allowing first, the crystallization of the enzyme lysozyme and second, transferring the new properties of the inorganic component to the crystals. Taken together, these composite crystals represent an example of the versatility of proteins as biological substrates in the generation of novel functional materials, opening the door to use them in catalysis and bioelectronics at macroscale.     

Silk Hydrogels as Soft Substrates for Neural Tissue Engineering

There is great need for soft biomaterials that match the stiffness of human tissues for tissue engineering and regeneration. Hydrogels are frequently employed for extracellular matrix functionalization and to provide appropriate mechanical cues. It is challenging, however, to achieve structural integrity and retain bioactive molecules in hydrogels for complex tissue formation that may take months to develop. This work aims to investigate mechanical and biochemical characteristics of silk hydrogels for soft tissue engineering, specifically for the nervous system. The stiffness of 1 to 8% silk hydrogels, measured by atomic force microscopy, is 4 to 33 kPa. The structural integrity of silk gels is maintained throughout embryonic chick dorsal root ganglion (cDRG) explant culture over 4 days whereas fibrin and collagen gels decrease in mass over time. Neurite extension of cDRGs cultured on 2 and 4% silk hydrogels exhibit greater growth than softer or stiffer gels. Silk hydrogels release <5% of neurotrophin‐3 (NT‐3) over 2 weeks and 11‐day old gels show maintenance of growth factor bioactivity. Finally, fibronectin‐ and NT‐3‐functionalized silk gels elicit increased axonal bundling suggesting their use in bridging nerve injuries. These results support silk hydrogels as soft and sustainable biomaterials for neural tissue engineering.     

Flexible Multifunctionalized Carbon Nanotubes‐Based Hybrid Nanowires

In this work, flexible multifunctionalized carbon nanotube (CNT)‐based hybrid nanowires are synthesized through surface modification processes. The good dispersability of the hybrid nanowire in polar solvents facilitates directly making fine patterns with a minimum width of 40 μm for applications of flexible and stretchable circuits (FSCs). The hybrid nanowire possesses a flexible and highly conductive structure which demonstrates stable electro‐mechanical properties on polydimethylsiloxane (PDMS) substrates under large structural deformation. FSCs fabricated from the hybrid nanowires show a constant resistance of 0.096 Ω □^?1 (equivalent of a resistivity 0.96 Ω μm) under repeated bending cycles. The FSCs also have a low and stable sheet resistance of 0.4 Ω □^?1 for strains up to 30%, which is almost four orders of magnitude lower than that of pure CNT samples (1316 Ω □^?1). Further improved stretchability and electro‐mechanical properties (0.1 Ω □^?1, at the strain of 100%) are achieved with a prestrain PDMS substrate. Repeated deformation tests demonstrate the high reliability of FSCs. The observed stable and reliable electro‐mechanical performance of FSCs suggests the potential use of the material in wearable and portable electronics.     

Carbon Nanotube–Graphitic Carbon Nitride Hybrid Films for Flavoenzyme‐Catalyzed Photoelectrochemical Cells

In green plants, solar‐powered electrons are transferred through sophistically arranged photosystems and are subsequently channelled into the Calvin cycle to generate chemical energy. Inspired by the natural photosynthetic scheme, a photoelectrochemical cell (PEC) is constructed configured with protonated graphitic carbon nitride (p‐g‐C₃N₄) and carbon nanotube hybrid (CNT/p‐g‐C₃N₄) film cathode, and FeOOH‐deposited bismuth vanadate (FeOOH/BiVO₄) photoanode for the production of industrially useful chiral alkanes using an old yellow enzyme homologue from Thermus scotoductus (TsOYE). In the biocatalytic PEC platform, photoexcited electrons provided by the FeOOH/BiVO₄ photoanode are transferred to the robust and self‐standing CNT/p‐g‐C₃N₄ hybrid film that electrocatalytically reduces flavin mononucleotide (FMN) mediator. The p‐g‐C₃N₄ promotes a two‐electron reduction of FMN coupled with an accelerated electron transfer by the conductive CNT network. The reduced FMN subsequently delivers the electrons to TsOYE for the highly enantioselective conversion of ketoisophorone to (R)‐levodione. Under light illumination (>420 nm) and external bias, (R)‐levodione is synthesized with the enantiomeric excess value of above 83%, not influenced by the scale of applied bias, simultaneously exhibiting stable and high current efficiency. The results suggest that the biocatalytic PEC made up of economical materials can selectively synthesize high‐value organic chemicals using water as an electron donor.     

Open‐Ended,N‐Doped Carbon Nanotube–Graphene Hybrid Nanostructures as High‐Performance Catalyst Support

A hierarchical N‐doped carbon nanotube‐graphene hybrid nanostructure (NCNT‐GHN), in which the graphene layers are distributed inside the CNT inner cavities, was designed to efficiently support noble metal (e.g., PtRu) nanoparticles. Well‐dispersed PtRu nanoparticles with diameters of 2–4 nm were immobilized onto these NCNT‐GHN supports by a low‐temperature chemical reduction method without any pretreatment. Compared to conventional CNTs and commercial catalysts. a much better catalytic performance was achieved by a synergistic effect of the hierarchical structure (graphene‐CNT hybrid) and electronic modulation (N‐doping) during the methanol electrooxidation reaction. Improved single‐cell performances with long‐term stability are also demonstrated using NCNT‐GHN as catalyst support.     

Highly Conductive Carbon Nanotube‐Graphene Hybrid Yarn

An efficient procedure for the fabrication of highly conductive carbon nanotube/graphene hybrid yarns has been developed. To start, arrays of vertically aligned multi‐walled carbon nanotubes (MWNT) are converted into indefinitely long MWNT sheets by drawing. Graphene flakes are then deposited onto the MWNT sheets by electrospinning to form a composite structure that is transformed into yarn filaments by twisting. The process is scalable for yarn fabrication on an industrial scale. Prepared materials are characterized by electron microscopy, electrical, mechanical, and electrochemical measurements. It is found that the electrical conductivity of the composite MWNT‐graphene yarns is over 900 S/cm. This value is 400% and 1250% higher than electrical conductivity of pristine MWNT yarns or graphene paper, respectively. The increase in conductivity is asssociated with the increase of the density of states near the Fermi level by a factor of 100 and a decrease in the hopping distance by an order of magnitude induced by grapene flakes. It is found also that the MWNT‐graphene yarn has a strong electrochemical response with specific capacitance in excess of 111 Fg^?1. This value is 425% higher than the capacitance of pristine MWNT yarn. Such substantial improvements of key properties of the hybrid material can be associated with the synergy of MWNT and graphene layers in the yarn structure. Prepared hybrid yarns can benefit such applications as high‐performance supercapacitors, batteries, high current capable cables, and artificial muscles.     

Tailoring the Electrical Properties of Carbon Nanotube–Polymer Composites

Advances in functionality and reliability of nanocomposite materials require careful formulation of processing methods to ultimately realize the desired properties. An extensive study of how the variation in fabrication process would affect the mechanism of conductivity and thus the final electrical properties of the carbon nanotube–polymer composite is presented. Some of the most widely implemented procedures are addressed, such as ultrasonication, melt shear mixing, and addition of surfactants. It is hoped that this study could provide a systematic guide to selecting and designing the downstream processing of carbon nanocomposites. Finally, this guide is used to demonstrate the fabrication and performance of a stretchable (pliable) conductor that can reversibly undergo uniaxial strain of over 100%, and other key applications are discussed.     

Iron–Nitrogen‐Doped Vertically Aligned Carbon Nanotube Electrocatalyst for the Oxygen Reduction Reaction

A highly active iron–nitrogen‐doped carbon nanotube catalyst for the oxygen reduction reaction (ORR) is produced by employing vertically aligned carbon nanotubes (VA‐CNT) with a high specific surface area and iron(II) phthalocyanine (FePc) molecules. Pyrolyzing the composite easily transforms the adsorbed FePc molecules into a large number of iron coordinated nitrogen functionalized nanographene (Fe–N–C) structures, which serve as ORR active sites on the individual VA‐CNT surfaces. The catalyst exhibits a high ORR activity, with onset and half‐wave potentials of 0.97 and 0.79 V, respectively, versus reversible hydrogen electrode, a high selectivity of above 3.92 electron transfer number, and a high electrochemical durability, with a 17 mV negative shift of E _1/2 after 10 000 cycles in an oxygen‐saturated 0.5 m H₂SO₄ solution. The catalyst demonstrates one of the highest ORR performances in previously reported any‐nanotube‐based catalysts in acid media. The excellent ORR performance can be attributed to the formation of a greater number of catalytically active Fe–N–C centers and their dense immobilization on individual tubes, in addition to more efficient mass transport due to the mesoporous nature of the VA‐CNTs.     

Carbon Nanotube Coatings on Bioglass‐Based Tissue Engineering Scaffolds

The coating of highly porous Bioglass^® based 3D scaffolds with multi‐walled carbon nanotubes (CNT) was investigated. Foam like Bioglass^® scaffolds were fabricated by the replica technique and electrophoretic deposition was used to deposit homogeneous layers of CNT throughout the scaffold pore structure. The optimal experimental conditions were determined to be: applied voltage 15 V and deposition time 20 minutes, utilizing a concentrated aqueous suspension of CNT with addition of a surfactant and iodine. The scaffold pore structure remained invariant after the CNT coating, as assessed by SEM. The incorporation of CNTs induced a nanostructured internal surface of the pores which is thought to be beneficial for osteoblast cell attachment and proliferation. Bioactivity of the scaffolds was assessed by immersion studies in simulated body fluid (SBF) for periods of up to 2 weeks and the subsequent determination of hydroxyapatite (HA) formation. The presence of CNTs can enhance the bioactive behaviour of the scaffolds since CNTs can serve as template for the ordered formation of a nanostructured HA layers, which does not occur on uncoated Bioglass^® surfaces.     

3D Microstructured Carbon Nanotube Electrodes for Trapping and Recording Electrogenic Cells

Electrogenic cells such as cardiomyocytes and neurons rely mainly on electrical signals for intercellular communication. Microelectrode arrays (MEAs) have been developed for long‐term recording of cell signals and stimulation of electrogenic cells under low‐cell‐stress conditions, providing new insights in the behavior of electrogenic cells and the operation of the brain. To date, MEAs are relying on flat or needle‐shaped electrode surfaces, mainly due to limitations in the lithographic processes. This paper relies on a previously reported elasto‐capillary aggregation process to create 3D carbon nanotube (CNT) MEAs. This study shows that CNTs aggregate in well‐shaped structures of similar size as cardiomyocytes are particularly interesting for MEA applications. This is because i) CNT microwells of the right diameter preferentially trap individual cardiomyocytes, which facilitates single cell recording without the need for clamping cells or signal deconvolution, and ii) once the cells are trapped inside of the CNT wells, this 3D CNT structure is used as an electrode surrounding the cell, which increases the cell–electrode contact area. As a result, this study finds that the recorded output voltages increase significantly (more than 200%). This fabrication process paves the way for future study of complex interactions between electrogenic cells and 3D recording electrodes.     

All‐Carbon Nanotube‐Based Flexible Field‐Emission Devices: From Cathode to Anode

The fabrication of a flexible field‐emission device (FED) using single‐walled carbon nanotube (SWNT) network films as the conducting electrodes (anode and cathode) and thin multi‐walled CNT/TEOS hybrid films as the emitters is reported. P‐type doping with gold ions and passivation with tetraethylorthosilicate (TEOS) made the SWNT network film highly conductive and environmentally stable, and hence a good alternative to conventional indium tin oxide electrodes. CNT/TEOS hybrid emitters showed high current density, low turn‐on field, and long‐term emission stability, compared with CNT emitters; these characteristics can be attributed to the TEOS sol, acting both as a protective layer surrounding the nanotube tip, and as an adhesive layer enhancing the nanotube adhesion to the substrate. All‐CNT‐based flexible FEDs fabricated by this approach showed high flexibility in field emission characteristics and extremely bright electron emission patterns.     

Ternary Hybrids of Amorphous Nickel Hydroxide–Carbon Nanotube‐Conducting Polymer for Supercapacitors with High Energy Density,Excellent Rate Capability,and Long Cycle Life

The utilization of Ni(OH)₂ as a pseudocapacitive material for high performance supercapacitors is hindered by its low electrical conductivity and short cycle life. A coaxial ternary hybrid material comprising of amorphous Ni(OH)₂ deposited on multiwalled carbon nanotubes wrapped with conductive polymer (poly (3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate)) is demonstrated. A thin layer of disordered amorphous Ni(OH)₂ is deposited by an effective “coordinating etching and precipitating” method, resulting in an ultrahigh specific capacitance of 3262 F g^?1 at 5 mV s^?1 and excellent rate capability (71.9% capacitance retention at 100 mV s^?1). More importantly, the polymer layer prevents the degradation of the nanostructure and dis­solution of Ni ion during repeated charge–discharge cycling for 30 000 cycles, a phenomenon which often plagues Ni(OH)₂ nanomaterials. Using the ternary Ni(OH)₂ hybrid and the reduced graphene oxide/carbon nanotube hybrid as the positive and negative electrodes, respectively, the assembled asymmetric supercapacitors exhibit high energy density of 58.5 W h kg^?1 at the power density of 780 W kg^?1 as well as long cycle life (86% capacitance retention after 30 000 cycles). The ternary hybrid architecture design for amorphous Ni(OH)₂ can be regarded as a general approach to obtain pseudocapacitive materials for supercapacitors with both high energy density, excellent rate capability, and long cycle life.     

Ultrahigh-Areal Capacitance Flexible Supercapacitors Based on Laser Assisted Construction of Hierarchical Aligned Carbon Nanotubes

Electrochemical energy storage is a key technology for a clean and sustainable energy supply. In this respect, supercapacitors (SC) have recently received considerable attention due to their excellent performance, including high-power density and long-cycle life. However, the poor binding strength between the active materials and substrate, the low active material loading, and small specific capacitance hinder the overall performance improvement of the device. In this study, an ultrahigh-areal capacitance flexible SC based on the Al micro grid-based hierarchical vertically aligned carbon nanotubes (VACNTs) is studied. Interestingly, the Al micro grid-based VACNTs exhibit ultrahigh loading (13 mg cm⁻²), and the as-fabricated VACNTs electrode display outstanding electrochemical performance, including an impressive areal capacitance of 1,300 mF cm⁻² at the current density of 13 mA cm⁻² and excellent stability with a retention ratio of 90% after 20,000 cycles at the current density of 130 mA cm⁻². Furthermore, the hierarchical VACNT electrodes show excellent mechanical flexibility when assembled into quasi-solid-state SC using Na₂SO₄-PVA gel as the electrolyte. The capacitance of this device is hardly changed bending different angles, even 180°. This study demonstrates the tremendous potential of Al micro grid-based hierarchical VACNTs as electrodes for high-performance flexible and wearable energy storage devices.     

Photoconductive Hybrid Films via Directional Self‐Assembly of C60 on Aligned Carbon Nanotubes

Hybrid nanostructured materials can exhibit different properties than their constituent components, and can enable decoupled engineering of energy conversion and transport functions. Novel means of building hybrid assemblies of crystalline C₆₀ and carbon nanotubes (CNTs) are presented, wherein aligned CNT films direct the crystallization and orientation of C₆₀ rods from solution. In these hybrid films, the C₆₀ rods are oriented parallel to the direction of the CNTs throughout the thickness of the film. High‐resolution imaging shows that the crystals incorporate CNTs during growth, yet grazing‐incidence X‐ray diffraction (GIXD) shows that the crystal structure of the C₆₀ rods is not perturbed by the CNTs. Growth kinetics of the C₆₀ rods are enhanced 8‐fold on CNTs compared to bare Si, emphasizing the importance of the aligned, porous morphology of the CNT films as well as the selective surface interactions between C₆₀ and CNTs. Finally, it is shown how hybrid C₆₀–CNT films can be integrated electrically and employed as UV detectors with a high photoconductive gain and a responsivity of 10⁵ A W^?1 at low biases (± 0.5 V). The finding that CNTs can induce rapid, directional crystallization of molecules from solution may have broader implications to the science and applications of crystal growth, such as for inorganic nanocrystals, proteins, and synthetic polymers.     

High Electromechanical Response of Ionic Polymer Actuators with Controlled‐Morphology Aligned Carbon Nanotube/Nafion Nanocomposite Electrodes

Recent advances in fabricating controlled‐morphology vertically aligned carbon nanotubes (VA‐CNTs) with ultrahigh volume fraction create unique opportunities for markedly improving the electromechanical performance of ionic polymer conductor network composite (IPCNC) actuators. Continuous paths through inter‐VA‐CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast device actuation speed (>10% strain/second). One critical issue in developing advanced actuator materials is how to suppress the strain that does not contribute to the actuation (unwanted strain) thereby reducing actuation efficiency. Here, experiments demonstrate that the VA‐CNTs give an anisotropic elastic response in the composite electrodes, which suppresses the unwanted strain and markedly enhances the actuation strain (>8% strain under 4 V). The results reported here suggest pathways for optimizing the electrode morphology in IPCNCs using ultrahigh volume fraction VA‐CNTs to further enhanced performance.     

Biomimetic Scaffolds for Tissue Engineering

Biomimetic scaffolds mimic important features of the extracellular matrix (ECM) architecture and can be finely controlled at the nano‐ or microscale for tissue engineering. Rational design of biomimetic scaffolds is based on consideration of the ECM as a natural scaffold; the ECM provides cells with a variety of physical, chemical, and biological cues that affect cell growth and function. There are a number of approaches available to create 3D biomimetic scaffolds with control over their physical and mechanical properties, cell adhesion, and the temporal and spatial release of growth factors. Here, an overview of some biological features of the natural ECM is presented and a variety of original engineering methods that are currently used to produce synthetic polymer‐based scaffolds in pre‐fabricated form before implantation, to modify their surfaces with biochemical ligands, to incorporate growth factors, and to control their nano‐ and microscale geometry to create biomimetic scaffolds are discussed. Finally, in contrast to pre‐fabricated scaffolds composed of synthetic polymers, injectable biomimetic scaffolds based on either genetically engineered‐ or chemically synthesized‐peptides of which sequences are derived from the natural ECM are discussed. The presence of defined peptide sequences can trigger in situ hydrogelation via molecular self‐assembly and chemical crosslinking. A basic understanding of the entire spectrum of biomimetic scaffolds provides insight into how they can potentially be used in diverse tissue engineering, regenerative medicine, and drug delivery applications.     

A Single Component Conducting Polymer Hydrogel as a Scaffold for Tissue Engineering

Conducting polymers (CPs) have exciting potential as scaffolds for tissue engineering, typically applied in regenerative medicine applications. In particular, the electrical properties of CPs has been shown to enhance nerve and muscle cell growth and regeneration. Hydrogels are particularly suitable candidates as scaffolds for tissue engineering because of their hydrated nature, their biocompatibility, and their tissue‐like mechanical properties. This study reports the development of the first single component CP hydrogel that is shown to combine both electro‐properties and hydrogel characteristics. Poly(3‐thiopheneacetic acid) hydrogels were fabricated by covalently crosslinking the polymer with 1,1′‐carbonyldiimidazole (CDI). Their swelling behavior was assessed and shown to display remarkable swelling capabilities (swelling ratios up to 850%). The mechanical properties of the networks were characterized as a function of the crosslinking density and were found to be comparable to those of muscle tissue. Hydrogels were found to be electroactive and conductive at physiological pH. Fibroblast and myoblast cells cultured on the hydrogel substrates were shown to adhere and proliferate. This is the first time that the potential of a single component CP hydrogel has been demonstrated for cell growth, opening the way for the development of new tissue engineering scaffolds.     

Recent Advances in Design of Functional Biocompatible Hydrogels for Bone Tissue Engineering

Bone related diseases have caused serious threats to human health owing to their complexity and specificity. Fortunately, owing to the unique 3D network structure with high aqueous content and functional properties, emerging hydrogels are regarded as one of the most promising candidates for bone tissue engineering, such as repairing cartilage injury, skull defect, and arthritis. Herein, various design strategies and synthesis methods (e.g., 3D-printing technology and nanoparticle composite strategy) are introduced to prepare implanted hydrogel scaffolds with tunable mechanical strength, favorable biocompatibility, and excellent bioactivity for applying in bone regeneration. Injectable hydrogels based on biocompatible materials (e.g., collagen, hyaluronic acid, chitosan, polyethylene glycol, etc.) possess many advantages in minimally invasive surgery, including adjustable physicochemical properties, filling irregular shapes of defect sites, and on-demand release drugs or growth factors in response to different stimuli (e.g., pH, temperature, redox, enzyme, light, magnetic, etc.). In addition, drug delivery systems based on micro/nanogels are discussed, and its numerous promising designs used in the application of bone diseases (e.g., rheumatoid arthritis, osteoarthritis, cartilage defect) are also briefed in this review. Particularly, several key factors of hydrogel scaffolds (e.g., mechanical property, pore size, and release behavior of active factors) that can induce bone tissue regeneration are also summarized in this review. It is anticipated that advanced approaches and innovative ideas of bioactive hydrogels will be exploited in the clinical field and increase the life quality of patients with the bone injury.