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and reversibly change rigidity is also attractive for artificial muscle actuators, [5,6] which are becoming increasingly suitable for wearable devices. The goal of rigidity tuning has been addressed using methods like solvent interactions, [7] pneumatic jamming, [8,9] electrostatic adhesion, [10] antagonistic actuator architectures, [11][12][13] fluidic flexible matrix composites, [14] phase-change materials, [15][16][17][18][19][20][21][22][23][24][25][26][27] and magnetorheological fluids. [28] The diversity of these methods results in an equally diverse range of technical challenges, such as long activation times, high activation voltages, [10,15] limited scalability and structural versatility, [11][12][13]26,27] and a dependence on bulky auxiliary equipment. [8,9,14,17,28] At present, there remains to be an electrically powered method for reversible rigidity tuning that exhibits <5 s, <20 V, actuation in a size-scalable architecture that allows for integration into a wide range of systems.In this work, we introduce a rigiditytuning material architecture that changes stiffness in response to moderate electrical voltage (Figure 1a,b and Video S1 (Supporting Information)). Furthermore, we demonstrate its feasibility in both tensile and flexural applications, via an active tendon in an underactuated robotic finger model [29] and a moldable splint (Figure 1c,d). The tendon consists of a conductive thermoplastic elastomer (cTPE) coated with a ≈10-140 µm layer of spray-deposited eutectic gallium-indium liquid metal alloy (EGaIn) [30] and embedded in a silicone matrix ( Figure 1b). Applying voltage to the EGaIn electrodes causes electric current to travel through the cTPE element. This induces rapid Joule heating in the cTPE, bringing it to the melting temperature, above which it softens and can no longer support a tensile load. When current is removed, the element cools and solidifies, and its stiffness is restored. Previously, shape memory polymers (SMPs) [16,[18][19][20][23][24][25]31] and low-melting-point alloys [20][21][22]26,27,32] have also been incorporated into reversible, stiffness-based adhesives and rigidity-tuning elements. Although some of these methods may have very large stiffness change ratios, they typically require external heating equipment or long activation times ( Table 1). Shan et al. exploited Joule heating to directly electrically activate cTPE; however, this technique required activation voltages of above 100 V. [15] We build on previous work with cTPE by introducing a novel design, in which a pair of liquid metal electrodes is oriented on opposite sides of the cTPE. This configuration minimizes the An electrically responsive composite is introduced that exhibits muscle-like changes in elastic stiffness (≈1-10 MPa) when stimulated with moderate voltages (5-20 V). The stiffness-tuning element contains an embedded layer of conductive thermoplastic elastomer (cTPE), composed of a propylene-ethylene copolymer and a percolating network of carbon black. Two opposite surfaces of the cTPE layer ar...

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Carbon nanotube-reinforced polymer composites were fabricated by high shear mixing. The microstructure and the electrical properties of the carbon nanotube-polymer composites were investigated by scanning electron microscopy and electrical resistance measurement. We found that the carbon nanotube composites showed high electrical conductivity (1.5 S m À1) at 7.0 wt% of carbon nanotubes, and the increase in thickness enhanced the electrical conductivity of the composites. The multifunctional properties of the carbon nanotube composites were also investigated for use in sensing the freezing temperature and also in deicing by self-heating. The results showed that the carbon nanotube-polymer composites had high temperature sensitivity in the freezing temperature range from À5 to 5 C and an excellent heating performance due to the Joule heating effect. The carbon nanotube composites are promising to be used as smart coating materials for deicing by self-heating as well as by detection of the freezing temperature.

Exactly 50 years ago, the first article on electrochromism was published. Today electrochromic materials are highly popular in various devices. Interest in nanostructured electrochromic and nanocomposite organic/inorganic nanostructured electrochromic materials has increased in the last decade. These materials can enhance the electrochemical and electrochromic properties of devices related to them. This article describes electrochromic materials, proposes their classification and systematization for organic inorganic and nanostructured electrochromic materials, identifies their advantages and shortcomings, analyzes current tendencies in the development of nanomaterials used in electrochromic coatings (films) and their practical use in various optical devices for protection from light radiation, in particular, their use as light filters and light modulators for optoelectronic devices, as well as methods for their preparation. The modern technologies of “Smart Windows”, which are based on chromogenic materials and liquid crystals, are analyzed, and their advantages and disadvantages are also given. Various types of chromogenic materials are presented, examples of which include photochromic, thermochromic and gasochromic materials, as well as the main physical effects affecting changes in their optical properties. Additionally, this study describes electrochromic technologies based on WO3 films prepared by different methods, such as electrochemical deposition, magnetron sputtering, spray pyrolysis, sol–gel, etc. An example of an electrochromic “Smart Window” based on WO3 is shown in the article. A modern analysis of electrochromic devices based on nanostructured materials used in various applications is presented. The paper discusses the causes of internal and external size effects in the process of modifying WO3 electrochromic films using nanomaterials, in particular, GO/rGO nanomaterials.

Joule heating is useful for fast and reliable manufacturing of conductive composite materials. In this study, we investigated the influence of Joule heating on curing conditions and material properties of polymer-based conductive composite materials consisting of carbon nanotubes (CNTs) and polydimethylsiloxane (PDMS). We applied different voltages to the CNT nanocomposites to investigate their electrical stabilization, curing temperature, and curing time. The result showed that highly conductive CNT/PDMS composites were successfully cured by Joule heating with uniform and fast heat distribution. For a 7.0 wt % CNT/PDMS composite, a high curing temperature of around 100 °C was achieved at 20 V with rapid temperature increase. The conductive nanocomposite cured by Joule heating also revealed an enhancement in mechanical properties without changing the electrical conductivities. Therefore, CNT/PDMS composites cured by Joule heating are useful for expediting the manufacturing process for particulate conductive composites in the field of flexible and large-area sensors and electronics, where fast and uniform curing is critical to their performance.

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A strain sensor using chain-structured ferromagnetic particles (FPs) in a multi-walled carbon nanotube (MWCNT)/polydimethylsiloxane (PDMS) nanocomposite was fabricated under a magnetic field and its strain sensitivity was evaluated at different material proportions. When the proportion of MWCNTs that are well dispersed in PDMS is higher than the percolation threshold, the strain sensitivity reduces with the increase of MWCNTs, in general; whereas a higher volume fraction of FPs produces a higher strain sensitivity when the chain-structure of FPs sustains. The mechanisms causing this interesting phenomenon have been demonstrated through the microstructural evolution and micromechanics-based modeling. These findings indicate that an optimal design of the volume fraction of FPs and MWCNTs exists to achieve the best strain sensitivity of this type of sensors. It is demonstrated that the nanocomposites containing 20 vol. % of nickel particles and 0.35 wt. % MWCNTs exhibit a high strain sensitivity of ∼80.