High-Resolution X-ray Spectromicroscopy Reveals Process-Structure Correlations in sub-5-µm Diameter Carbon Nanotube-Polymer Composite Dry-Spun Yarns.

A persistent lack of detailed and quantitative structural analysis of these hierarchical carbon nanotube (CNT) ensembles precludes establishing processing-structure-property relationships that are essential to enhance macroscale performance (e.g., in mechanical, electrical, thermal applications). Here, we use scanning transmission X-ray microscopy (STXM) to analyze the hierarchical, twisted morphology of dry-spun CNT yarns and their composites, quantifying key structural characteristics such as density, porosity, alignment, and polymer loading. As the yarn twist density increases (15,000 to 150,000 turns per meter), the yarn diameter decreased (4.4-1.4 µm) and density increased (0.55-1.26 g·cm-3), as intuitively expected. Yarn density, ρ, ubiquitously scaled with diameter d according to ρ ∼ d-2 for all parameters studied here. Spectromicroscopy probes with 30 nm resolution and elemental specificity were employed to analyze the radial and longitudinal distribution of the oxygen-containing polymer content (∼30% weight fraction), demonstrating nearly perfect filling of the voids between CNTs with a vapor-phase polymer coating and cross-linking process. These quantitative correlations highlight the intimate connections between processing conditions and yarn structure with important implications for translating the nanoscale properties of CNTs to the macroscale.

A simple, highly efficient route to electroless gold plating on complex 3D printed polyacrylate plastics.

Compared to tedious, multi-step treatments for electroless gold plating of traditional thermoplastics, this communication describes a simpler three-step procedure for 3D printed crosslinked polyacrylate substrates. This allows for the synthesis of ultralight gold foam microlattice materials with great potential for architecture-sensitive applications in future energy, catalysis, and sensing.

Magnetic torsional actuation of carbon nanotube yarn artificial muscle.

Magnetically driven torsional actuation of a multiwalled carbon nanotube (MWNT) yarn was realized by first biscrolling NdFeB magnetic particles into helical yarn corridors to make a magnetic MWNT yarn. The actuating device comprised a pristineMWNT yarn that was connected to the magnetic MWNT yarn, with a paddle attached between these yarns. The application of a magnetic field reversibly drove torsional actuation of up to 80° within ∼0.67 seconds. This magnetic actuator was remotely powered, and its actuation stroke was the same when the muscle array was at 20 °C and at -100 °C.

Harvesting electrical energy from carbon nanotube yarn twist.

Mechanical energy harvesters are needed for diverse applications, including self-powered wireless sensors, structural and human health monitoring systems, and the extraction of energy from ocean waves. We report carbon nanotube yarn harvesters that electrochemically convert tensile or torsional mechanical energy into electrical energy without requiring an external bias voltage. Stretching coiled yarns generated 250 watts per kilogram of peak electrical power when cycled up to 30 hertz, as well as up to 41.2 joules per kilogram of electrical energy per mechanical cycle, when normalized to harvester yarn weight. These energy harvesters were used in the ocean to harvest wave energy, combined with thermally driven artificial muscles to convert temperature fluctuations to electrical energy, sewn into textiles for use as self-powered respiration sensors, and used to power a light-emitting diode and to charge a storage capacitor.

Ultrastretchable Analog/Digital Signal Transmission Line with Carbon Nanotube Sheets.

Stretchable conductors can be used in various applications depending on their own characteristics. Here, we demonstrate simple and robust elastomeric conductors that are optimized for stretchable electrical signal transmission line. They can withstand strains up to 600% without any substantial change in their resistance (≤10% as is and ≤1% with passivation), and exhibit suppressed charge fluctuations in the medium. The inherent elasticity of a polymeric rubber and the high conductivity of flexible, highly oriented carbon nanotube sheets were combined synergistically, without losing both properties. The nanoscopic strong adhesion between aligned carbon nanotube arrays and strained elastomeric polymers induces conductive wavy folds with microscopic bending of radii on the scale of a few micrometers. Such features enable practical applications such as in elastomeric length-changeable electrical digital and analog signal transmission lines at above MHz frequencies. In addition to reporting basic direct current, alternating current, and noise characterizations of the elastomeric conductors, various examples as a stretchable signal transmission line up to 600% strains are presented by confirming the capability of transmitting audio and video signals, as well as low-frequency medical signals without information distortion.

Electrochemically Powered, Energy-Conserving Carbon Nanotube Artificial Muscles.

While artificial muscle yarns and fibers are potentially important for many applications, the combination of large strokes, high gravimetric work capacities, short cycle times, and high efficiencies are not realized for these fibers. This paper demonstrates here electrochemically powered carbon nanotube yarn muscles that provide tensile contraction as high as 16.5%, which is 12.7 times higher than previously obtained. These electrochemical muscles can deliver a contractile energy conversion efficiency of 5.4%, which is 4.1 times higher than reported for any organic-material-based artificial muscle. All-solid-state parallel muscles and braided muscles, which do not require a liquid electrolyte, provide tensile contractions of 11.6% and 5%, respectively. These artificial muscles might eventually be deployed for a host of applications, from robotics to perhaps even implantable medical devices.

Ultralow Stress, Thermally Stable Cross-Linked Polymer Films of Polydivinylbenzene (PDVB).

Although closely related to polystyrene, poly(divinylbenzene) (PDVB) has found limited utility due to the difficulties associated with its synthesis. As a highly cross-linked polymer, PDVB is infusible and insoluble and thus nearly impossible to shape into films by either melt or solvent-based processes. Here, we report the initiated chemical vapor deposition (iCVD) of nearly stress-free, highly transparent, free-standing films of PDVB up to 25 µm thick. Films initially grow under tensile intrinsic stress but become more compressive with thickness and eventually converge to zero-stress values once they reach ≥10 µm in thickness. Upon initial heating, the evaporative loss of unreacted monomer left in the polymer matrix induces between 35 and 45 MPa of tensile stress in the films. Afterward, subsequent heating cycles induce reversible stress and film expansion behaviors. We estimate the degree of cross-linking to be 44%, resulting in high thermal stability (up to 300 °C) and mechanical stiffness (Young's modulus of 5.2 GPa). The low stress combined with high cross-linking makes iCVD PDVB an excellent candidate for protective coatings in harsh environments.

Improvement of system capacitance via weavable superelastic biscrolled yarn supercapacitors.

Yarn-based supercapacitors having improved performance are needed for existing and emerging wearable applications. Here, we report weavable carbon nanotube yarn supercapacitors having high performance because of high loadings of rapidly accessible charge storage particles (above 90 wt% MnO2). The yarn electrodes are made by a biscrolling process that traps host MnO2 nanoparticles within the galleries of helically scrolled carbon nanotube sheets, which provide strength and electrical conductivity. Despite the high loading of brittle metal oxide particles, the biscrolled solid-state yarn supercapacitors are flexible and can be made elastically stretchable (up to 30% strain) by over-twisting to produce yarn coiling. The maximum areal capacitance of the yarn electrodes were up to 100 times higher than for previously reported fibres or yarn supercapacitors. Similarly, the energy density of complete, solid-state supercapacitors made from biscrolled yarn electrodes with gel electrolyte coating were significantly higher than for previously reported fibre or yarn supercapacitors.

Stretchable Triboelectric Fiber for Self-powered Kinematic Sensing Textile.

Stretchable fiber and yarn triboelectric nanogenerator are sought for such applications as wearable sensing system such as cloth communication devices, electronic textiles, and robotic sensory skin. Unfortunately, previously reported triboelectric fiber and yarn are difficult to have stretchable property. We introduce here a new type of stretchable and weavable triboelectric fibers with microdiameter dimensions. The stretchable triboelectric fibers can be reversibly stretched up to 50% in tensile direction while generating voltage output proportional to the applied tensile strain. The reversible distance change induced by the Poisson's ratio difference between the core fiber (silver-coated nylon/polyurethane) and the shell (wrinkled polyvinylidene fluoride-co-trifluoroethylene/carbon nanotube layer) during tensile deformation is the key working principle for electrical generation. Owing to exceptional structural stability, the stretchable triboelectric fibers show high performance retention after 10,000 times repeated stretching/releasing cycle. Furthermore, the stretchable triboelectric fibers are mechanically strong to be woven into a commercial textile for textile based sensors, which can detect magnitude as well as direction of the motion.

Conducting Fibers: Downsized Sheath-Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors (Adv. Mater. 25/2016).

Using intelligent textiles for clothing represents one possibility for weavable superelastic conducting fibers that can store energy, sense body motions, and detect biochemicals. On page 4998, S. Yin, R. H. Baughman, and co-workers demonstrate that these hair-like-diameter fibers, comprising buckled carbon nanotube sheaths on a rubber core, can be used as glucose sensors, supercapacitors, ultrafast strain sensors, and electrical interconnectors. The performance of these structures is maintained also under giant strain.

Downsized Sheath-Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors.

Hair-like-diameter superelastic conducting fibers, comprising a buckled carbon nanotube sheath on a rubber core, are fabricated, characterized, and deployed as weavable wires, biosensors, supercapacitors, and strain sensors. These downsized sheath-core fibers provide the demonstrated basis for glucose sensors, supercapacitors, and electrical interconnects whose performance is undegraded by giant strain, as well as ultrafast strain sensors that exploit strain-dependent capacitance changes.

Strong, Twist-Stable Carbon Nanotube Yarns and Muscles by Tension Annealing at Extreme Temperatures.

A high-speed incandescent tension annealing process (ITAP) is used to increase the modulus and strength of twist-spun carbon nanotube yarns by up to 12-fold and 2.6-fold, respectively, provide remarkable resistance to oxidation and powerful protonating acids, and freeze yarn untwist. This twist stability enables torsional artificial-muscle motors having improved performance and minimizes problematic untwist during weaving nanotube yarns.

Woven-Yarn Thermoelectric Textiles.

The fabrication and characterization of highly flexible textiles are reported. These textiles can harvest thermal energy from temperature gradients in the desirable through-thickness direction. The tiger yarns containing n- and p-type segments are woven to provide textiles containing n-p junctions. A high power output of up to 8.6 W m(-2) is obtained for a temperature difference of 200 °C.

Bio-inspired, Moisture-Powered Hybrid Carbon Nanotube Yarn Muscles.

Hygromorph artificial muscles are attractive as self-powered actuators driven by moisture from the ambient environment. Previously reported hygromorph muscles have been largely limited to bending or torsional motions or as tensile actuators with low work and energy densities. Herein, we developed a hybrid yarn artificial muscle with a unique coiled and wrinkled structure, which can be actuated by either changing relative humidity or contact with water. The muscle provides a large tensile stroke (up to 78%) and a high maximum gravimetric work capacity during contraction (2.17 kJ kg(-1)), which is over 50 times that of the same weight human muscle and 5.5 times higher than for the same weight spider silk, which is the previous record holder for a moisture driven muscle. We demonstrate an automatic ventilation system that is operated by the tensile actuation of the hybrid muscles caused by dew condensing on the hybrid yarn. This self-powered humidity-controlled ventilation system could be adapted to automatically control the desired relative humidity of an enclosed space.

Stretchable, weavable coiled carbon nanotube/MnO2/polymer fiber solid-state supercapacitors.

Fiber and yarn supercapacitors that are elastomerically deformable without performance loss are sought for such applications as power sources for wearable electronics, micro-devices, and implantable medical devices. Previously reported yarn and fiber supercapacitors are expensive to fabricate, difficult to upscale, or non-stretchable, which limits possible use. The elastomeric electrodes of the present solid-state supercapacitors are made by using giant inserted twist to coil a nylon sewing thread that is helically wrapped with a carbon nanotube sheet, and then electrochemically depositing pseudocapacitive MnO2 nanofibers. These solid-state supercapacitors decrease capacitance by less than 15% when reversibly stretched by 150% in the fiber direction, and largely retain capacitance while being cyclically stretched during charge and discharge. The maximum linear and areal capacitances (based on active materials) and areal energy storage and power densities (based on overall supercapacitor dimensions) are high (5.4 mF/cm, 40.9 mF/cm(2), 2.6 µWh/cm(2) and 66.9 µW/cm(2), respectively), despite the engineered superelasticity of the fiber supercapacitor. Retention of supercapacitor performance during large strain (50%) elastic deformation is demonstrated for supercapacitors incorporated into the wristband of a glove.

Three-dimensionally bonded spongy graphene material with super compressive elasticity and near-zero Poisson's ratio.

It is a challenge to fabricate graphene bulk materials with properties arising from the nature of individual graphene sheets, and which assemble into monolithic three-dimensional structures. Here we report the scalable self-assembly of randomly oriented graphene sheets into additive-free, essentially homogenous graphene sponge materials that provide a combination of both cork-like and rubber-like properties. These graphene sponges, with densities similar to air, display Poisson's ratios in all directions that are near-zero and largely strain-independent during reversible compression to giant strains. And at the same time, they function as enthalpic rubbers, which can recover up to 98% compression in air and 90% in liquids, and operate between -196 and 900 °C. Furthermore, these sponges provide reversible liquid absorption for hundreds of cycles and then discharge it within seconds, while still providing an effective near-zero Poisson's ratio.

Primary liver cells cultured on carbon nanotube substrates for liver tissue engineering and drug discovery applications.

Here, we explore the use of two- and three-dimensional scaffolds of multiwalled-carbon nanotubes (MWNTs) for hepatocyte cell culture. Our objective is to study the use of these scaffolds in liver tissue engineering and drug discovery. In our experiments, primary rat hepatocytes, the parenchymal (main functional) cell type in the liver, were cultured on aligned nanogrooved MWNT sheets, MWNT yarns, or standard 2-dimensional culture conditions as a control. We find comparable cell viability between all three culture conditions but enhanced production of the hepatocyte-specific marker albumin for cells cultured on MWNTs. The basal activity of two clinically relevant cytochrome P450 enzymes, CYP1A2 and CYP3A4, are similar on all substrates, but we find enhanced induction of CYP1A2 for cells on the MWNT sheets. Our data thus supports the use of these substrates for applications including tissue engineering and enhancing liver-specific functions, as well as in in vitro model systems with enhanced predictive capability in drug discovery and development.

All-solid-state carbon nanotube torsional and tensile artificial muscles.

We report electrochemically powered, all-solid-state torsional and tensile artificial yarn muscles using a spinnable carbon nanotube (CNT) sheet that provides attractive performance. Large torsional muscle stroke (53°/mm) with minor hysteresis loop was obtained for a low applied voltage (5 V) without the use of a relatively complex three-electrode electromechanical setup, liquid electrolyte, or packaging. Useful tensile muscle strokes were obtained (1.3% at 2.5 V and 0.52% at 1 V) when lifting loads that are ∼25 times heavier than can be lifted by the same diameter human skeletal muscle. Also, the tensile actuator maintained its contraction following charging and subsequent disconnection from the power supply because of its own supercapacitor property at the same time. Possible eventual applications for the individual tensile and torsional muscles are in micromechanical devices, such as for controlling valves and stirring liquids in microfluidic circuits, and in medical catheters.

Superior rechargeability and efficiency of lithium-oxygen batteries: hierarchical air electrode architecture combined with a soluble catalyst.

The lithium-oxygen battery has the potential to deliver extremely high energy densities; however, the practical use of Li-O2 batteries has been restricted because of their poor cyclability and low energy efficiency. In this work, we report a novel Li-O2 battery with high reversibility and good energy efficiency using a soluble catalyst combined with a hierarchical nanoporous air electrode. Through the porous three-dimensional network of the air electrode, not only lithium ions and oxygen but also soluble catalysts can be rapidly transported, enabling ultra-efficient electrode reactions and significantly enhanced catalytic activity. The novel Li-O2 battery, combining an ideal air electrode and a soluble catalyst, can deliver a high reversible capacity (1000 mAh g(-1) ) up to 900 cycles with reduced polarization (about 0.25 V).

Artificial muscles from fishing line and sewing thread.

The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications. We demonstrated that inexpensive high-strength polymer fibers used for fishing line and sewing thread can be easily transformed by twist insertion to provide fast, scalable, nonhysteretic, long-life tensile and torsional muscles. Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.