wileyonlinelibrary.comSkin-mountable and wearable sensors can be attached onto the clothing or even directly mounted on the human skin for the real-time monitoring of human activities. [ 10 ] Besides their high effi ciency, they must fulfi ll several minimum requirements including high stretchability, fl exibility, durability, low power consumption, biocompatibility, and lightweight. These demands become even more severe for epidermal electronic devices where mechanical compliance like human skin and high stretchability ( ε > 100% where ε is the strain) are required. [ 11,12 ] Recently, several types of skin-mountable and wearable sensors have been proposed by using nanomaterials coupled with fl exible and stretchable polymers. Indeed, nanomaterials are utilized as functional sensing elements owing to their outstanding electrical, mechanical, optical, and chemical properties while polymers are employed as fl exible support materials thanks to their fl exibility, stretchability, humanfriendliness, and durability. [ 13 ] Examples of those innovative sensors include strain sensors, [ 10,[12][13][14] pressure sensors, [ 5,[15][16][17] electronic skins (e-skins), [ 3,[16][17][18][19][20] and temperature sensors. [ 2,21,22 ] Particularly, various skin-mountable and wearable strain sensors have been developed because of their broad applications in personalized heath-monitoring, human motion detection, human-machine interfaces, and soft robotics. [ 10,12,[23][24][25][26][27][28][29][30][31] This paper aims to survey fabrication processes, working mechanisms, strain sensing performances, and applications of stretchable strain sensors. The article is organized as follows: fi rst, common operation mechanisms of stretchable strain sensors are described. Here, we summarize novel functional nanomaterials and techniques for the fabrication of stretchable strain sensors in details. Second, mechanisms involved in the strain-responsive behavior of resistive-type and capacitive-type sensors are explained. We show that the infl uence of traditional mechanisms like geometrical changes and piezoresistivity of materials on the strain sensing performance of fl exible strain sensors are very small whereas mechanisms such as disconnection between sensing elements, crack propagation in thin fi lms, and tunneling effect can potentially be employed for highly stretchable and sensitive strain sensing. Third, we emphasize the performance parameters of stretchable strain sensors in terms of stretchability, sensitivity, linearity, hysteresis behavior, response time, overshooting, and durability. We demonstrate

This paper reviews state-of-the-art dielectric elastomer actuators (DEAs) and their future perspectives as soft actuators which have recently been considered as a key power generation component for soft robots. This paper begins with the introduction of the working principle of the dielectric elastomer actuators. Because the operation of DEA includes the physics of both mechanical viscoelastic properties and dielectric characteristics, we describe theoretical modeling methods for the DEA before introducing applications. In addition, the design of artificial muscles based on DEA is also introduced. This paper reviews four popular subjects for the application of DEA: soft robot hand, locomotion robots, wearable devices, and tunable optical components. Other potential applications and challenging issues are described in the conclusion.

This paper reports soft actuator based tactile stimulation interfaces applicable to wearable devices. The soft actuator is prepared by multi-layered accumulation of thin electro-active polymer (EAP) films. The multi-layered actuator is designed to produce electrically-induced convex protrusive deformation, which can be dynamically programmable for wide range of tactile stimuli. The maximum vertical protrusion is and the output force is up to 255 mN. The soft actuators are embedded into the fingertip part of a glove and front part of a forearm band, respectively. We have conducted two kinds of experiments with 15 subjects. Perceived magnitudes of actuator's protrusion and vibrotactile intensity were measured with frequency of 1 Hz and 191 Hz, respectively. Analysis of the user tests shows participants perceive variation of protrusion height at the finger pad and modulation of vibration intensity through the proposed soft actuator based tactile interface.

Transition metal dichalcogenides (TMDs) layers of molecular thickness, in particular molybdenum disulfide (MoS2), become increasingly important as active elements for mechanically flexible/stretchable electronics owing to their relatively high carrier mobility, wide bandgap, and mechanical flexibility. Although the superior electronic properties of TMD transistors are usually integrated into rigid silicon wafers or glass substrates, the achievement of similar device performance on flexible substrates remains quite a challenge. The present work successfully addresses this challenge by a novel process architecture consisting of a solution‐based polyimide (PI) flexible substrate in which laser‐welded silver nanowires are embedded, a hybrid organic/inorganic gate insulator, and multilayers of MoS2. Transistors fabricated according to this process scheme have decent properties: a field‐effect‐mobility as high as 141 cm2 V−1 s−1 and an Ion/Ioff ratio as high as 5 × 105. Furthermore, no apparent degradation in the device properties is observed under systematic cyclic bending tests with bending radii of 10 and 5 mm. Overall electrical and mechanical results provide potentially important applications in the fabrication of versatile areas of flexible integrated circuitry.

A polymer-waveguide-based transparent and flexible force sensor array is proposed, which satisfies the principal requirements for a tactile sensor working on curvilinear surfaces, such as thinfilm architecture (thickness < 150 μm), localized force sensing (ca. 0-3 N), multiple-point re cognition (27 points), bending robustness (10.8% degradation at R = 1.5 mm), and fast response (bandwidth > 16 Hz).

In this paper, we propose a shape memory alloy (SMA)-based wearable robot that assists the wrist motion for patients who have difficulties in manipulating the lower arm. Since SMA shows high contraction strain when it is designed as a form of coil spring shape, the proposed muscle-like actuator was designed after optimizing the spring parameters. The fabricated actuator shows a maximum force of 10 N and a maximum contraction ratio of 40%. The SMA-based wearable robot, named soft wrist assist (SWA), assists 2 degrees of freedom (DOF) wrist motions. In addition, the robot is totally flexible and weighs 151g for the wearable parts. A maximum torque of 1.32 Nm was measured for wrist flexion, and a torque of larger than 0.5 Nm was measured for the other motions. The robot showed the average range of motion (ROM) with 33.8, 30.4, 15.4, and 21.4 degrees for flexion, extension, ulnar, and radial deviation, respectively. Thanks to the soft feature of the SWA, time cost for wearing the device is shorter than 2 min as was also the case for patients when putting it on by themselves. From the experimental results, the SWA is expected to support wrist motion for diverse activities of daily living (ADL) routinely for patients.

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Developing tunable lenses, an expansion-based mechanism for dynamic focus adjustment can provide a larger focal length tuning range than a contraction-based mechanism. Here, we develop an expansion-tunable soft lens module using a disk-type dielectric elastomer actuator (DEA) that creates axially symmetric pulling forces on a soft lens. Adopted from a biological accommodation mechanism in human eyes, a soft lens at the annular center of a disk-type DEA pair is efficiently stretched to change the focal length in a highly reliable manner. A soft lens with a diameter of 3 mm shows a 65.7% change in the focal length (14.3-23.7 mm) under a dynamic driving voltage signal control. We confirm a quadratic relation between lens expansion and focal length that leads to large focal length tunability obtainable in the proposed approach. The fabricated tunable lens module can be used for soft, lightweight, and compact vision components in robots, drones, vehicles, and so on.