Mikhail E. Kozlov

Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles

Improved electrically powered artificial muscles are needed for generating force, moving objects, and accomplishing work. Carbon nanotube aerogel sheets are the sole component of new artificial muscles that provide giant elongations and elongation rates of 220% and (3.7 × 104)% per second, respectively, at operating temperatures from 80 to 1900 kelvin. These solid-state–fabricated sheets are enthalpic rubbers having gaslike density and specific strength in one direction higher than those of steel plate. Actuation decreases nanotube aerogel density and can be permanently frozen for such device applications as transparent electrodes. Poissons ratios reach 15, a factor of 30 higher than for conventional rubbers. These giant Poissons ratios explain the observed opposite sign of width and length actuation and result in rare properties: negative linear compressibility and stretch densification.

Ultrafast charge and discharge biscrolled yarn supercapacitors for textiles and microdevices

Flexible, wearable, implantable and easily reconfigurable supercapacitors delivering high energy and power densities are needed for electronic devices. Here we demonstrate weavable, sewable, knottable and braidable yarns that function as high performance electrodes of redox supercapacitors. A novel technology, gradient biscrolling, provides fast-ion-transport yarn in which hundreds of layers of conducting-polymer-infiltrated carbon nanotube sheet are scrolled into ~20 μm diameter yarn. Plying the biscrolled yarn with a metal wire current collector increases power generation capabilities. The volumetric capacitance is high (up to ~179 F cm(-3)) and the discharge current of the plied yarn supercapacitor linearly increases with voltage scan rate up to ~80 V s(-1) and ~20 V s(-1) for liquid and solid electrolytes, respectively. The exceptionally high energy and power densities for the complete supercapacitor, and high cycle life that little depends on winding or sewing (92%, 99% after 10,000 cycles, respectively) are important for the applications in electronic textiles.

Artificial Muscles from Fishing Line and Sewing Thread

Toward an Artificial Muscle In designing materials for artificial muscles, the goals are to find those that will combine high strokes, high efficiency, long cycle life, low hysteresis, and low cost. Now, Haines et al. (p. 868; see the Perspective by Yuan and Poulin) show that this is possible. Twisting high-strength, readily available polymer fibers, such as those used for fishing lines or sewing thread, to the point where they coil up, allowed construction of highly efficient actuators that could be triggered by a number of stimuli. Polymer fibers can be transformed into highly efficient artificial muscles through the application of extreme twist. [Also see Perspective by Yuan and Poulin] 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.

Torsional Carbon Nanotube Artificial Muscles

Carbon nanotube yarns are used to make fast, multirotational torsional actuators. Rotary motors of conventional design can be rather complex and are therefore difficult to miniaturize; previous carbon nanotube artificial muscles provide contraction and bending, but not rotation. We show that an electrolyte-filled twist-spun carbon nanotube yarn, much thinner than a human hair, functions as a torsional artificial muscle in a simple three-electrode electrochemical system, providing a reversible 15,000° rotation and 590 revolutions per minute. A hydrostatic actuation mechanism, as seen in muscular hydrostats in nature, explains the simultaneous occurrence of lengthwise contraction and torsional rotation during the yarn volume increase caused by electrochemical double-layer charge injection. The use of a torsional yarn muscle as a mixer for a fluidic chip is demonstrated.

Electrically, Chemically, and Photonically Powered Torsional and Tensile Actuation of Hybrid Carbon Nanotube Yarn Muscles

Nanotube Yarn Actuators Actuators are used to convert heat, light, or electricity into a twisting or tensile motion, and are often described as artificial muscles. Most materials that show actuation either provide larger forces with small-amplitude motions, such as the alloy NiTi, or provide larger motions with much less force, such as polymeric materials. Other problems with such actuators can include slow response times and short lifetimes. Lima et al. (p. 928, see the Perspective by Schulz) show that a range of guest-filled, twist-spun carbon nanotube yarns can be used for linear or torsional actuation, can solve the problems of speed and lifetime, and do not require electrolytes for operation. Thermally driven actuators use a guest material within carbon nanotube yarns to generate fast torsional and tensile motions. Artificial muscles are of practical interest, but few types have been commercially exploited. Typical problems include slow response, low strain and force generation, short cycle life, use of electrolytes, and low energy efficiency. We have designed guest-filled, twist-spun carbon nanotube yarns as electrolyte-free muscles that provide fast, high-force, large-stroke torsional and tensile actuation. More than a million torsional and tensile actuation cycles are demonstrated, wherein a muscle spins a rotor at an average 11,500 revolutions/minute or delivers 3% tensile contraction at 1200 cycles/minute. Electrical, chemical, or photonic excitation of hybrid yarns changes guest dimensions and generates torsional rotation and contraction of the yarn host. Demonstrations include torsional motors, contractile muscles, and sensors that capture the energy of the sensing process to mechanically actuate.

Synergistic toughening of composite fibres by self-alignment of reduced graphene oxide and carbon nanotubes

The extraordinary properties of graphene and carbon nanotubes motivate the development of methods for their use in producing continuous, strong, tough fibres. Previous work has shown that the toughness of the carbon nanotube-reinforced polymer fibres exceeds that of previously known materials. Here we show that further increased toughness results from combining carbon nanotubes and reduced graphene oxide flakes in solution-spun polymer fibres. The gravimetric toughness approaches 1,000 J g−1, far exceeding spider dragline silk (165 J g−1) and Kevlar (78 J g−1). This toughness enhancement is consistent with the observed formation of an interconnected network of partially aligned reduced graphene oxide flakes and carbon nanotubes during solution spinning, which act to deflect cracks and allow energy-consuming polymer deformation. Toughness is sensitive to the volume ratio of the reduced graphene oxide flakes to the carbon nanotubes in the spinning solution and the degree of graphene oxidation. The hybrid fibres were sewable and weavable, and could be shaped into high-modulus helical springs.

Elastomeric Conductive Composites Based on Carbon Nanotube Forests

Electrically conductive materials capable of substantial elastic stretch and bending are needed for such applications as smart clothing, flexible displays, stretchable circuits, strain gauges, implantable devices, high-stroke microelectromechanical systems, and dielectric elastomer actuators. A variety of approaches involving carbon nanotubes (CNTs) and elastic polymers have been suggested for the fabrication of conductive elastic composites. In particular, diverse active and passive electronic components have been embedded in rubber sheet by several research groups to obtain stretchable electronic devices. Sekitani et al. developed rubber-like conductive composites by mixing millimeter-long single-walled carbon nanotubes (SWNTs), an ionic liquid, and a fluorinated copolymer. The stretchability of the resulting composite was enhanced by creating perforated films with a net-shaped structure using a mechanical punching system. Cao et al. fabricated flexible electrodes by incorporating SWNTnetworks in plastics consisting of polyimide, polyurethane, and polyamic acid films. Although quite successful, these studies indicated that high loading of CNTs (or other conductive additive) was necessary to obtain a highly conducting composite. On the other hand, incorporation of high concentrations of CNTs into an elastic polymer increases the stiffness of the resulting composite and decreases its stretchability. In other words, the significant difference in the Young’s modulus of extremely rigid CNTs and the elastic polymer filler makes the creation of a highly stretchable conductive composites a challenging task. It is known that CNTs can be fabricated into macroscopic assemblies, such as mats (bucky paper), yarns, and fibers that possess useful electrical properties, and that these assemblies can be used for the fabrication of conductive polymer composites. While these assemblies are often more elastic than the individual CNTs, the achievable elastic strain range is still quite limited, normally less than 10%. We found that a combination of high stretchability and high electrical conductivity can be obtained for composites prepared from three-dimensional CNT structures, such as CNT forests (vertically aligned arrays of CNTs). Unlike previous methods involving casting CNT/ polymer dispersions as a film, our composites were prepared by the direct infiltration of multiwalled carbon nanotube (MWNT) forests with a polyurethane (PU) solution. Using this procedure, we obtained rubber-like forest/PU composites that combined high stretchability with high electrical conductivity. These composites provide highly reversible stress–strain behavior and little degradation of mechanical and electrical properties even when stretched over a wide strain range. The developed preparation procedure appears scalable for material fabrication on an industrial scale, though transition from present batchbased forest growth processes to continuous forest growth processes would be needed for applications that are price sensitive and depend on sheet weight, rather than the area of elastomeric sheet. The aligned arrays of MWNTs (MWNT forests) used in this study were grown on iron-catalyst-coated silicon wafers using a conventional chemical vapor deposition (CVD) method. Nanotubes in the forests typically had a diameter of about 10 nm; their length could be controlled across a wide range by changing the growth time and other fabrication conditions. The forest-covered area on the substrate used for the preparation of the composites typically had dimensions of about 50 100mm; the height of nanotubes in the forest was about 50mm as determined by the conventional optical microscopy. Since the nanotubes in the forests formed a three-dimensionally interconnected network, the forests were electrically conductive in all directions. The MWNT forests were infiltrated with a PU solution in N,N-dimethylformamide (DMF) using a simple drop-casting procedure, as shown in Figure 1a. The PU used was poly[4,40methylene-bis(phenyl isocyanate)-alt-1,4-butanediol/poly(butylene adipate)]. After evaporation of the solvent, we obtained about 250mm thick forest/PU composite sheets that could be peeled off the underlying Si wafer. Figure 1b shows a photograph of the MWNT/PU composite sheet taken at low magnification. One side of the prepared film facing the substrate (forest side) was black and conductive, and the other side (PU side) was white and insulating. The material was soft, flexible, and highly stretchable in the sheet plane. Figure 1c shows a SEM image of a cross-section of the composite sheet with the top ( 50mm in thickness) being the forest side and the bottom ( 200mm in thickness) being the PU side. A highmagnification image of the forest side is shown in the

Biscrolling nanotube sheets and functional guests into yarns.

Carbon nanotube sheets can support very large fractions of a second material, such as a superconductor or a catalyst. Multifunctional applications of textiles have been limited by the inability to spin important materials into yarns. Generically applicable methods are demonstrated for producing weavable yarns comprising up to 95 weight percent of otherwise unspinnable particulate or nanofiber powders that remain highly functional. Scrolled 50-nanometer-thick carbon nanotube sheets confine these powders in the galleries of irregular scroll sacks whose observed complex structures are related to twist-dependent extension of Archimedean spirals, Fermat spirals, or spiral pairs into scrolls. The strength and electronic connectivity of a small weight fraction of scrolled carbon nanotube sheet enables yarn weaving, sewing, knotting, braiding, and charge collection. This technology is used to make yarns of superconductors, lithium-ion battery materials, graphene ribbons, catalytic nanofibers for fuel cells, and titanium dioxide for photocatalysis.

Sign Change of Poisson's Ratio for Carbon Nanotube Sheets

Most materials shrink laterally like a rubber band when stretched, so their Poissons ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poissons ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poissons ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poissons ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poissons ratio is associated with a large positive Poissons ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poissons ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.

Fuel-Powered Artificial Muscles

Artificial muscles and electric motors found in autonomous robots and prosthetic limbs are typically battery-powered, which severely restricts the duration of their performance and can necessitate long inactivity during battery recharge. To help solve these problems, we demonstrated two types of artificial muscles that convert the chemical energy of high–energy-density fuels to mechanical energy. The first type stores electrical charge and uses changes in stored charge for mechanical actuation. In contrast with electrically powered electrochemical muscles, only half of the actuator cycle is electrochemical. The second type of fuel-powered muscle provides a demonstrated actuator stroke and power density comparable to those of natural skeletal muscle and generated stresses that are over a hundred times higher.