COMMUNICATION procedure as illustrated in Figure 1 a. Figure 3 a shows the parallel AgNW/PDMS elastic conductors with a linewidth of 800 μ m; the performance was found to be the same as that of the wileyonlinelibrary.com COMMUNICATION cycles of stretching/releasing in a large range of tensile strain (0-50%). This stable electrical response is due to buckling of the AgNW/PDMS layer. The physics/mechanics origin of the buckling is due to irreversible sliding of the AgNWs in the PDMS matrix. Following a parallel fabrication approach, line and cross patterns of AgNWs were fabricated. Furthermore, a stretchable LED circuit and a capacitive strain sensor were demonstrated using the AgNW/PDMS elastic conductors as interconnects or electrodes. With their superior conductivity, stretchability and compatibility with existing fabrication/patterning technology, the reported AgNW/PDMS elastic conductors may fi nd broad applications in stretchable electronics, skin sensors, wearable communication devices, photovoltaics and energy storage. Experimental SectionSample Preparation : AgNWs were provided by Blue Nano, Inc. PDMS were prepared using Sylgard 184 (Dow Corning) by mixing the "base" and the "curing agent" with a ratio of 10:1. After air bubbles disappeared, the liquid mixture was then thermally cured at 65 ° C for 12 hours to form cross-linked and solid PDMS.Fabrication and Measurement of Capacitive Strain Sensors : Liquid PDMS layer was fi rst casted onto a Si substrate patterned with rectangular AgNW fi lms. Before curing, we placed an already patterned and cured AgNW/PDMS fi lm (with the AgNW surface facing up) on the Si substrate as well as the wet PDMS and oriented it to make sure that both patterns are perfectly aligned. Following that, the whole piece was thermally cured and peeled off the Si substrate. This way, the top and bottom surfaces of the PDMS were symmetrically covered by the patterned AgNW stretchable electrodes. Cutting a strip off the PDMS substrate produced a capacitive strain sensor, as schematically shown in Figure 4 a. The strain sensor were repeatedly stretched and released by a tensile testing stage (Ernest F. Fullam), while the capacitance was measured at the same time by a LCR (inductance, capacitance, resistance) meter (Stanford Research Systems, SR715).

Considerable efforts have been made to achieve highly sensitive and wearable sensors that can simultaneously detect multiple stimuli such as stretch, pressure, temperature or touch. Here we develop highly stretchable multifunctional sensors that can detect strain (up to 50%), pressure (up to ∼1.2 MPa) and finger touch with high sensitivity, fast response time (∼40 ms) and good pressure mapping function. The reported sensors utilize the capacitive sensing mechanism, where silver nanowires are used as electrodes (conductors) and Ecoflex is used as a dielectric. The silver nanowire electrodes are screen printed. Our sensors have been demonstrated for several wearable applications including monitoring thumb movement, sensing the strain of the knee joint in patellar reflex (knee-jerk) and other human motions such as walking, running and jumping from squatting, illustrating the potential utilities of such sensors in robotic systems, prosthetics, healthcare and flexible touch panels.

Since the first successful synthesis of graphene just over a decade ago, a variety of twodimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene.

Stretchable electronics are attracting intensive attention due to their promising applications in many areas where electronic devices undergo large deformation and/or form intimate contact with curvilinear surfaces. On the other hand, a plethora of nanomaterials with outstanding properties have emerged over the past decades. The understanding of nanoscale phenomena, materials, and devices has progressed to a point where substantial strides in nanomaterial-enabled applications become realistic. This review summarizes recent advances in one such application, nanomaterial-enabled stretchable conductors (one of the most important components for stretchable electronics) and related stretchable devices (e.g., capacitive sensors, supercapacitors and electroactive polymer actuators), over the past five years. Focusing on bottom-up synthesized carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal nanomaterials (e.g., metal nanowires and nanoparticles), this review provides fundamental insights into the strategies for developing nanomaterial-enabled highly conductive and stretchable conductors. Finally, some of the challenges and important directions in the area of nanomaterial-enabled stretchable conductors and devices are discussed.

The transfer of synthesized 2D MoS2 films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS2 due to different growth mechanisms and surface properties. Here we demonstrate a surface-energy-assisted process that can perfectly transfer centimeter-scale monolayer and few-layer MoS2 films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging the penetration of water between hydrophobic MoS2 films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS2 films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consists of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of poly(methyl methacrylate) (PMMA) that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS2 films than PMMA and is important for maintaining the integrity of the film during the transfer process. This surface-energy-assisted approach can be generally applied to the transfer of other 2D materials, such as WS2.