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Soft robots have better flexibility than their rigid counterparts thus offering greater adaptability to changing environments. These robots made from flexible materials are generally with low stiffness and have limited capability to perform tasks that require the application of large forces. Soft artificial muscles (AMs) with variable stiffness are promising solutions for soft robots to handle larger payloads. However, the existing actuators need to overcome their small range of stiffness variation and slow response. Further, it is difficult to integrate self‐sensing into existing actuators. Herein, a novel electric AM with variable stiffness and self‐sensing referencing smooth muscle contraction in nature is proposed. The AM consists of two pieces of soft anode mesh, a flexible cathode and a polyvinyl chloride (PVC) gel dielectric layer, where the cathode is also a resistive sensor. The stiffness variation is induced by the friction change caused by electrostatic adsorption. Compared with the existing PVC gel actuation technology, the new design integrated the dielectric and the cathode layers and combined the sensing function. Also, the rigid anode and cathode are replaced by the soft ones, respectively, which makes the AM suitable for soft robotics or wearable devices. [ABSTRACT FROM AUTHOR]

Fingertip‐mounted tactile displays of softness are needed for various virtual‐ or augmented‐reality applications such as surgical simulation, tele‐operation, computer‐aided design, 3D model exploration, and tele‐presence. Displaying a virtual softness on a fingertip requires the generation of quasi‐static large displacements at moderate forces (as opposed to high‐frequency small vibrations at high forces), via a deformable surface, to control both the contact area and the indentation depth of the skin. State‐of‐the‐art actuation technologies are unable to combine simple structure, low weight, and low size, as well as energy efficiency and silent operation. Here, the progress on the development of a non‐vibratory display of softness made of electroactive polymers is reported. It consists of a hydrostatically coupled dielectric elastomer actuator, shaped as a bubble interfaced to the fingertip, having a weight of 6 g. Prototypes can generate displacements up to 3.5 mm and forces up to 1 N. By combining this technology with a compact hand tracking sensor, a simple and cost‐effective virtual‐reality system is demonstrated. A psychophysical study engaging 15 volunteers in poke and pinch tactile tasks shows that users can properly distinguish between different stimuli rendered by the display, with an accuracy correlated to the perceptual difficulty of the tactile comparative task. [ABSTRACT FROM AUTHOR]

Smart heating devices with reliable self-regulating performances and high efficiency, combined with additional properties like mechanical flexibility, are of particular interest in healthcare, soft robotics, and smart buildings. Unfortunately, the development of smart heaters necessitates managing normally conflicting requirements such as good self-regulating capabilities and efficient Joule heating performances. Here, a simple and universal materials design strategy based on a series connection of different conductive polymer composites (CPC) is shown to provide unique control over the pyroresistive properties. Hooke's and Kirchhoff's laws of electrical circuits can simply predict the overall pyroresistive behavior of devices connected in series and/or parallel configurations, hence providing design guidelines. An efficient and mechanically flexible Joule heating device is hence designed and created. The heater is characterized by a zero temperature coefficient of resistance below the self-regulating temperature, immediately followed by a large and sharp positive temperature coefficient (PTC) behavior with a PTC intensity of around 106. Flexibility and toughness is provided by the selected elastomeric thermoplastic polyurethane (TPU) matrix as well as the device design. The universality of the approach is demonstrated by using different polymer matrices and conductive fillers for which repeatable results are consistently obtained. [ABSTRACT FROM AUTHOR]

The strain and thermal dependence of the dynamic mechanical properties of carbon black filler networks underpin the performance of many rubber components. These effects are examined by varying the surface energetics of carbon black. This has a profound influence on the level of flocculation of the carbon black network in the final crosslinked compounds. Filler networks comprised of thermally deactivated carbon blacks are significantly more strain-sensitive - shifting the onset of the Payne effect to smaller dynamic strains. Using free vibration equipment to precisely probe the thermal sensitivity of the linear viscoelastic properties of the filled compounds, it is shown that carbon black deactivation results in carbon black networks which are more thermally sensitive than corresponding unmodified carbon black networks. Increased thermal dependence of the dynamic moduli results in the appearance of a secondary increase in tan δ as a function of increasing temperature well above the rubber Tg - which is not correlated with any thermal transitions in calorimetric experiments. Such effects are prevalent in the relevant literature for various rubber-filler combinations but their physical origins are often misinterpreted or unexplained. A rationalization of these effects based on the dynamics of the filler network is presented. [ABSTRACT FROM AUTHOR]

The process of particulate flocculation in natural rubber melts and subsequently crosslinked samples is investigated using carbon blacks of varying surface free energy, surface area and morphology. The surface free energies are varied via thermal treatment of the carbon blacks (graphitization). Reduction in surface free energy of the particulates accelerates the flocculation processes in the melt as measured by rheological experiments and reduces the percolation volume fraction threshold as determined by D/C conductivity measurements. The consequent effects on amplification of the small strain storage moduli of the crosslinked compounds are dramatic. Reduced polymer-filler interactions result in both an increased small strain modulus versus the unmodified carbon black-filled materials and an increased mechanical fragility of the fractal networks. Examination of the dynamic elastic moduli of crosslinked carbon black-filled samples loaded at volume fractions below the onset of network development reveal a significant temperature dependence isolated at the filler-rubber interface. [ABSTRACT FROM AUTHOR]

Small strain Young's moduli of natural rubber (NR)/organoclay nanocomposites were estimated using the Guth-Gold, Halpin-Tsai (HT), and Krieger-Dougherty (KD) models, and compared with experimental measurements of NR vulcanizates containing organo-montmorillonite (OM) or organo-sepiolite (OS). To account for the effect on modulus of the NR matrix of the vulcanization-active modifier in the organoclay, a matrix modulus correction (MMC) term was derived from the vulcanization parameters of the nanocomposites. The KD model gave a better empirical fit with the experimental data than the Guth-Gold model, with both giving good agreement with particle shape factors estimated from transmission electron microscope (TEM) images. The HT model gave the best fit with experiment for both types of nanocomposite, and use of the MMC term meant that the empirical shape factor was sufficiently close to that estimated from TEM images that the model could potentially be used to accurately predict the Young's moduli of NR/OM and NR/OS nanocomposites. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1621-1627, 2011 [ABSTRACT FROM AUTHOR]

The effect of silica nanofiller surface chemistry on compounded particle size and high strain particle dewetting in a semitransparent nanosilica-filled elastomer composite was determined using backscattered visible light and transmitted light, respectively. The integrated intensities of backscattered light from the samples were collected at various visible wavelengths for thin-film composites using ultraviolet-visible spectrometer with an integrating sphere. The data revealed strong Rayleigh-type scattering from compounded filler particles. Size information was extracted and found to broadly correlate with scanning electron microscopy image analysis of fracture surface. Incorporation of a siloxane surface treatment chemical during compounding resulted in a reduced average filler particle size in the cured composite. On extension of the samples, an optical transition was observed only in the filled composites. At high strains, the semi transparent samples displayed an abrupt drop in transparency becoming opaque. This was quantified using a simple light transmission-sample extension technique. Strain-induced crystallization was discounted as the cause for the transition by X-ray diffraction analysis. The onset yield stress for the optical transition was found to be filler surface-chemistry-dependent with the siloxane-treated filler exhibiting a greatly increased onset stress value. These observations were discussed and rationalized in terms of filler particle-matrix dewetting and cavitation at high strains. Matrix-filler dewetting was distinguished from matrix cavitation by comparison with Beer-Lambert behavior derived from unstrained samples. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011. [ABSTRACT FROM AUTHOR]

This article discusses a study on the use of microstructural finite element models investigating the modulus of filled elastomer over a wider range of strains than has been possible previously. This study offered insight into rigid filler reinforcement and the filler interaction mechanisms. The study considered medium thermal (MT)-type carbon-black filled rubber behavior due to the large amount of experimental data available in the literature.