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Pump it up Carbon nanotube yarns can be used as electrochemical actuators because infiltration with ions causes a contraction in length and an expansion in diameter. Either positive or negative ions can cause this effect. Chu et al. constructed an all-solid-state muscle that eliminated the need for an electrolyte bath, which may expand the potential for its use in applications. By infiltrating the yarns with charged polymers, the fibers start partially swollen, so the length can increase through the loss of ions. It is thus possible to increase the overall stroke of the muscle. Further, these composite materials show a surprising increase in stroke with scan rate. Science , this issue p. 494

Semiconductor yarns with unique functional characteristics have great potential applications in next-generation electronic devices. However, scalable inorganic semiconductor yarns with excellent mechanical and electrical properties, and environmental stability have not been discovered. In this study, we explored a unique fluid-spinning strategy to obtain a series of scalable inorganic semiconductor yarns including neat and hybrid semiconductor yarns. Different from the conventional yarn spinning strategy through a mechanical motor, we utilized the fluid force from the triple-phase interface to assemble and twist inorganic nanofiber building blocks simultaneously, and eventually obtained highly oriented inorganic nanowire-based semiconductor yarns. The obtained semiconductor yarns showed an excellent flexibility (curvature exceeding 2 cm−1) and mechanical strength (tensile strength of 443 MPa) because of their highly oriented hierarchical nanostructures, which make them coiling able with highly twisted insertion. Additionally, coiled yarns were obtained by combining the host core material and functional guest sheath in a fluid-spinning process, which are flexible in deep cryogenic temperature owing to the pure inorganic building blocks (26.28% tensile strain in liquid nitrogen). In particular, inorganic yarn-based electrochromic actuators can obtain as high as 15.3% tensile stroke and 0.82 J g−1 work capacity by electrochemical charge injection-associated multicolor switching.

Stretching a coiled carbon nanotube (CNT) yarn can provide large, reversible electrochemical capacitance changes, which convert mechanical energy to electricity. Here, it is shown that the performance of these "twistron" harvesters can be increased by optimizing the alignment of precursor CNT forests, plastically stretching the precursor twisted yarn, applying much higher tensile loads during precoiling twist than for coiling, using electrothermal pulse annealing under tension, and incorporating reduced graphene oxide nanoplates. The peak output power for a 1 and a 30 Hz sinusoidal deformation are 0.73 and 3.19 kW kg

While previous guest-filled carbon nanotube yarn muscles provide remarkable performance, whose actuation is driven by the volume change of a guest. They also have problems. For instance (a) carbon nanotube yarns are expensive and (b) input energy is provided to the entire muscle, but the central region of the muscle contributes little to muscle contraction. Here, we eliminated this problem by driving the muscle using a sheath, which can be on an inexpensive coiled yarn or fiber core. This configuration change dramatically increases muscle power and enables cheap commercialized yarns to replace expensive carbon nanotube yarns. For different application purposes, we coated different types of polymer or other materials to optimized the actuation performance under different trigger conditions. For instance, the electrochemical SRAM which using the CNT as the sheath and Nylon 6 yarn as the core can generate 1.98 W/g of average contractile power - 40 times that for human muscle and 9.0 times that of the highest power alternative electrochemical muscle. The electrothermal PEO-SO3@CNT SRAMs (PEO-SO3 as the sheath and CNT as the core) operated in air and in room-temperature water to produce 2.6 W/g (at 9 Hz) and 9.0 W/g (at 12 Hz) of full-cycle contractile power, respectively, compared with the 0.05 W/g typical of human muscle. Additionally, coating vapors or moisture response material on the twisted core material, we can obtain a muscle which operates torsional and tensile actuation under different vapor concentration or humidity. Because sheath-run muscles can be made so cheaply without using carbon nanotubes, and are easily upscaled, we fabricated a comfort adjusting textiles that respond to temperature, humidity and the presence or absence of perspiration by opening and closing pores(1). Reference (1) Mu, Jiuke, et al. "Sheath-run artificial muscles." Science 365.6449 (2019): 150-155.

Two-dimensional (2D) materials have recently opened a new avenue to flexible thermoelectric materials with enhanced performance because of their unique electronic transport properties. Here, we report a feasible approach to improve the thermoelectric performance of transition-metal dichalcogenides by effectively decorating 2D MoS2 with Au nanoparticles using in situ growth. The present Au-decorated MoS2-assembled heterojunction system shows a certain decoupled phenomenon, that is, the Seebeck coefficient and conductivity increased simultaneously. This is due to the occurrence of p-type doping of the MoS2 2H phase and injection energy filtering of dopant-originated carriers around the local band bending at the interface. The composite flexible films can achieve a power factor value of 166.3 μW m–1 K–2 at room temperature, which have great potential for harvesting human body heat.

While the emerging stretchable electronic sensors have been demonstrated as promising wearable functional devices, challenges in achieving highly stretchable and self-powered fiber-like sensors still exist. Here, a stretchable sheath-core structural triboelectric fiber (SSCTEF) is developed to serve as a self-powered multifunctional kinematic sensor. Owing to the advanced built-in wavy structure design, the fiber-like sensor exhibits an ultrahigh working strain (100%) and demonstrates high sensitivity in response to not only stretching but also to bending and compressing. The working principle of the SSCTEF is verified by the coupling of numerical calculations and experimental measurements. A comprehensive study is carried out to investigate the factors that influence the output performance of the SSCTEF. By wearing, it is capable of detecting and discriminating the joint movements of human bodies. By further weaving and construction, it also shows potential for detecting the deformation in two-dimensional region. This work provides new opportunities for wearable and self-powered sensing fibers with full potential in human motion monitoring.

Flexible actuators are widely in demand for many real-life applications. Considering that existing actuators based on polymers, low-dimensional materials and pore-rich materials are mostly limited by slow response rate, high driving voltage and poor stability, we report here a novel metal based flexible actuator which is fabricated simply through partial oxidation and nano-function of copper foil with the assistance of reduced graphene oxide. The obtained asymmetric metallic actuator is (electric-)thermally driven and exhibits fast response rate (∼2 s) and large curvature (2.4 cm−1) under a low voltage (∼1 V) with a sustainable operation of up to ∼50 000 cycles. The actuator can also be triggered by infrared irradiation and direct-heating under various conditions including air, water, and vacuum.

Twisting is cool Rubber bands that are stretched and held in an extended shape for a while will extract heat from their surroundings as they are allowed to relax, owing to a reversal of stress-induced crystallization, which is an exothermic process. Wang et al. examine the potential for solid-state cooling of twisted fibers, along with configurations such as supercoiling, for materials including natural rubber, polyethylene, and nickel-titanium fibers. The cooling is related to the change in entropy of the material as it is mechanically deformed. Science , this issue p. 216

Although guest-filled carbon nanotube yarns provide record performance as torsional and tensile artificial muscles, they are expensive, and only part of the muscle effectively contributes to actuation. We describe a muscle type that provides higher performance, in which the guest that drives actuation is a sheath on a twisted or coiled core that can be an inexpensive yarn. This change from guest-filled to sheath-run artificial muscles increases the maximum work capacity by factors of 1.70 to 2.15 for tensile muscles driven electrothermally or by vapor absorption. A sheath-run electrochemical muscle generates 1.98 watts per gram of average contractile power-40 times that for human muscle and 9.0 times that of the highest power alternative electrochemical muscle. Theory predicts the observed performance advantages of sheath-run muscles.

Thermoelectric (TE) generators, being able to convert temperature gradients into electricity, are an appealing eco-friendly energy harvesting technology. In order to broaden the applications of TE materials, many researchers have focused their efforts towards flexible TE materials to substitute the commercial rigid TE devices. Most of these works used polymers as flexible TE materials, but their lower efficiency and melting point in comparison to inorganic TE materials are limiting factors for many applications. Here, we report the fabrication of a flexible and thermostable TE generator based on porous all-graphene films. The output power of the generator is up to 0.43 μW for a temperature gradient of 75 K. In addition, it runs at 550 K higher temperature than current flexible TE devices, which allows its application at much higher temperatures than conventional flexible TE.

Using a 3D stretching method, a highly elastic reduced graphene oxide (rGO)/polyacrylic ester hierarchically wrinkled elastic transparent conductor (HWETC) is fabricated. Periodic hierarchical N-rGO layer wrinkling allows the HWETC to show high conductivity (100-457 Ω ◻-1 ) and transmittance (67-85%) under substantial stretching (>400%) and bending deformation (≈180°), which enables electrothermal actuation and strain sensing.

Wearable electronic devices such as medical sensors, hearing aids and wristwatches can run on power of μW levels. Thermoelectric (TE) generators have the potential to provide power to wearable electronic devices, because they can effectively convert body heat into electrical energy. This study presents a flexible TE generator by applying single-walled carbon nanotube/polyaniline (SWNT/PANI) composites to polyesters, and it was found that ultrasonic induction can enhance the interfacial stability between SWNT/PANI composites and polyesters. The flexible TE generator can generate a TE voltage of 3.82 mV at a temperature difference of 115 K. Moreover, the TE generator can supply a maximum output power of 48.0 nW to the load at a temperature difference of 75 K. In addition to good flexibility and ventilation, the SWNT/PANI textile TE generator displays no change in performance on repeated bending for 150 cycles with a radius of 5 mm, which is important for wearable electronic devices.

Graphene nanosheet devices have attracted significant interest in transistor and photodetector applications owing to photothermoelectric effects of graphene. However, their solar energy conversion ability has not been fully explored due to disadvantages of their transparency and zero gap semiconductor in this special field. Here, we propose the potential of graphene-carbon nanotube papers for energy conversion and storage under sunlight and heat. We show that macroscopic 3-dimensional structure of graphene-carbon nanotube papers provides advantages over graphene nanomaterials in converting large area power sources, namely solar and thermal energy into considerable current and voltage. We also report a graphene-carbon nanotube paper stack with p-n interfaces that incorporates a solar/thermal energy cell and a p–n junction capacitor, which can be simultaneously used for energy conversion and storage with a charge-storage capacity of 70.5 μC cm−2 at one sun intensity.

Two-dimensional (2D) materials have recently opened a new avenue to flexible thermoelectric materials with enhanced performance because of their unique electronic transport properties. Here, we report a feasible approach to improve the thermoelectric performance of transition-metal dichalcogenides by effectively decorating 2D MoS

We here show that infiltrated bridging agents can convert inexpensively fabricated graphene platelet sheets into high-performance materials, thereby avoiding the need for a polymer matrix. Two types of bridging agents were investigated for interconnecting graphene sheets, which attach to sheets by either π–π bonding or covalent bonding. When applied alone, the π–π bonding agent is most effective. However, successive application of the optimized ratio of π–π bonding and covalent bonding agents provides graphene sheets with the highest strength, toughness, fatigue resistance, electrical conductivity, electromagnetic interference shielding efficiency, and resistance to ultrasonic dissolution. Raman spectroscopy measurements of stress transfer to graphene platelets allow us to decipher the mechanisms of property improvement. In addition, the degree of orientation of graphene platelets increases with increasing effectiveness of the bonding agents, and the interlayer spacing increases. Compared with other materials that are strong in all directions within a sheet, the realized tensile strength (945 MPa) of the resin-free graphene platelet sheets was higher than for carbon nanotube or graphene platelet composites, and comparable to that of commercially available carbon fiber composites. The toughness of these composites, containing the combination of π–π bonding and covalent bonding, was much higher than for these other materials having high strengths for all in-plane directions, thereby opening the path to materials design of layered nanocomposites using multiple types of quantitatively engineered chemical bonds between nanoscale building blocks.

The multilaminated ZnO/TiO2 heterojunction films were successfully deposited on conductive substrates including fluorine-doped tin oxide (FTO) glass and flexible indium tin oxide coated poly(ethylene terephthalate) via the layer-by-layer (LBL) self assembly method from the oxide colloids without using any polyelectrolytes. The positively charged ZnO nanoparticles and the negatively charged TiO2 nanoparticles were directly used as the components in the consecutive deposition process to prepare the heterojunction thin films by varying the thicknesses. Moreover, the crystal growth of both oxides could be efficiently inhibited by the good connection between ZnO and TiO2 nanoparticles even after calcination at 500 °C, especially for ZnO which was able to keep the crystallite size under 25 nm. The as-prepared films were used as the working electrodes in the three-electrode photoelectrochemical cells. Because the well-contacted nanoscale heterojunctions were formed during the LBL self-assembling process, the ZnO...

Origami-inspired self-folding graphene papers show remote control grasping, manipulation, and walking behaviors., Origami-inspired active graphene-based paper with programmed gradients in vertical and lateral directions is developed to address many of the limitations of polymer active materials including slow response and violent operation methods. Specifically, we used function-designed graphene oxide as nanoscale building blocks to fabricate an all-graphene self-folding paper that has a single-component gradient structure. A functional device composed of this graphene paper can (i) adopt predesigned shapes, (ii) walk, and (iii) turn a corner. These processes can be remote-controlled by gentle light or heating. We believe that this self-folding material holds potential for a wide range of applications such as sensing, artificial muscles, and robotics.

Mechanical actuators driven by water that respond to multiple stimuli, exhibit fast responses and large deformations and generate high stress have potential in artificial muscles, motors and generators. Meeting all these requirements in a single device remains a challenge. We report a graphene monolayer paper that undergoes reversible deformation. Its graphene oxide cells wrinkle and extend in response to water desorption and absorption, respectively. Its fast (~0.3 s), powerful (>100 MPa output stress, 7.5 × 105 N kg−1 unit mass force) and controllable actuation can be triggered by moisture, heat and light. The graphene monolayer paper has potential in artificial muscles, robotic hands and electromagnetic-free generators.

Freestanding, flexible/foldable, and wearable bifuctional ultrathin graphene paper for heating and cooling is fabricated as an active material in personal thermal management (PTM). The promising electrical conductivity grants the superior Joule heating for extra warmth of 42 °C using a low supply voltage around 3.2 V. Besides, based on its high out-of-plane thermal conductivity, the graphene paper provides passive cooling via thermal transmission from the human body to the environment within 7 s. The cooling effect of graphene paper is superior compared with that of the normal cotton fiber, and this advantage will become more prominent with the increased thickness difference. The present bifunctional graphene paper possesses high durability against bending cycles over 500 times and wash time over 1500 min, suggesting its great potential in wearable PTM.