Strain-release assembly of nanowires on stretchable substrates

Current On-Skin Flexible Sensors, Materials, Manufacturing Approaches, and Study Trends for Health Monitoring: A Review

Due to an ever-increasing amount of the population focusing more on their personal health, thanks to rising living standards, there is a pressing need to improve personal healthcare devices. These devices presently require laborious, time-consuming, and convoluted procedures that heavily rely on cumbersome equipment, causing discomfort and pain for the patients during invasive methods such as sample-gathering, blood sampling, and other traditional benchtop techniques. The solution lies in the development of new flexible sensors with temperature, humidity, strain, pressure, and sweat detection and monitoring capabilities, mimicking some of the sensory capabilities of the skin. In this review, a comprehensive presentation of the themes regarding flexible sensors, chosen materials, manufacturing processes, and trends was made. It was concluded that carbon-based composite materials, along with graphene and its derivates, have garnered significant interest due to their electromechanical stability, extraordinary electrical conductivity, high specific surface area, variety, and relatively low cost.

Magnetic Manipulation of Nanowires for Engineered Stretchable Electronics

Nanowires are often key ingredients of high-tech composite materials. The properties and performance of devices created using these, depend heavily on the structure and density of the embedded nanowires. Despite significant efforts, a process that can be adapted to different materials, compatible with current nanowire deposition methods, and that is able to control both variables simultaneously has not been achieved yet. In this work, we show that we can use low magnetic fields (80 mT) to manipulate nanowires by electrostatically coating them with superparamagnetic iron oxide nanoparticles in an aqueous solution. Monolayers, multilayers, and hierarchical structures of oriented nanowires were achieved in a highly ordered manner using vacuum filtration for two types of nanowires: silver and gold-coated titanium dioxide nanowires. The produced films were embedded in an elastomer, and the strain-dependent electrical properties of the resulting composites were investigated. The orientation of the assembly with respect to the tensile strain heavily impacts the performance of the composites. Composites containing nanowires perpendicular to the strain direction exhibit an extremely low gauge factor. On the other hand, when nanowires are arranged parallel to the strain direction, the composites have a high gauge factor. The possibility to orient nanowires during the processing steps is not only interesting for the shown strain sensing application but also expected to be useful in many other areas of material science.

Recent progress in post treatment of silver nanowire electrodes for optoelectronic device applications

Owing to the economical and practical solution synthesis and coating strategies, silver nanowires (AgNWs) have been considered as one of the most suitable alternative materials to replace commercial indium tin oxide (ITO) transparent electrodes. The primitive AgNW electrode cannot meet the requirements for preparing high performance optoelectronic devices due to its high contact resistance, large surface roughness and poor stability. Thus, various post-treatments for AgNW film optimization are needed before its actual applications, such as welding treatment to decrease contact resistance and passivation to increase film stability. This review investigates recent progress on the preparation and optimization of AgNWs. Moreover, some unique fabrication strategies to produce highly oriented AgNW films with unique anisotropic properties have also been carried out with detailed analysis. The representative devices based on the AgNW electrode have been summarized and discussed at the end of this review.

Metal-free plasmonic refractory core-shell nanowires for tunable all-dielectric broadband perfect absorbers

In this work, we numerically demonstrate a new facile strategy for all-dielectric broadband optical perfect absorbers. A monolayer refractory titanium oxide and nitride (TiN/TiO₂) core-shell nanowires array is used to form the grating on the opaque TiN substrate. Multiple resonant absorption bands are observed in the adjacent wavelength range, which therefore leads to the formation of an ultra-broadband absorption window from the visible to the infrared regime. The maximal absorption reaches 95.6% and the average absorption efficiency in the whole range (0.5-1.8 µm) is up to 85.4%. Moreover, the absorption bandwidth can be feasibly adjusted while the absorption efficiency can be still maintained in a high level via tuning the polarization state. Furthermore, the absorption window is observed to be highly adjustable in the wavelength range, showing a nearly linear relationship to the shell's index. These features not only confirm the achievement of the broadband perfect absorption but also introduce feasible ways to artificially manipulate the absorption properties, which will hold wide applications in metal-free plasmonic optoelectronic devices such as the solar harvesting, photo-detection, and thermal generation and its related bio-medical techniques.

Tunable Piezoelectricity of Multifunctional Boron Nitride Nanotube/Poly(dimethylsiloxane) Stretchable Composites

Boron nitride nanotubes (BNNT) uniformly dispersed in stretchable materials, such as poly(dimethylsiloxane) (PDMS), could create the next generation of composites with augmented mechanical, thermal, and piezoelectric characteristics. This work reports tunable piezoelectricity of multifunctional BNNT/PDMS stretchable composites prepared via co-solvent blending with tetrahydrofuran (THF) to disperse BNNTs in PDMS while avoiding sonication or functionalization. The resultant stretchable BNNT/PDMS composites demonstrate augmented Young's modulus (200% increase at 9 wt% BNNT) and thermal conductivity (120% increase at 9 wt% BNNT) without losing stretchability. Furthermore, BNNT/PDMS composites demonstrate piezoelectric responses that are linearly proportional to BNNT wt%, achieving a piezoelectric constant (|d₃₃ |) of 18 pmV^-1 at 9 wt% BNNT without poling, which is competitive with commercial piezoelectric polymers. Uniquely, BNNT/PDMS accommodates tensile strains up to 60% without plastic deformation by aligning BNNTs, which enhances the composites' piezoelectric response approximately five times. Finally, the combined stretchable and piezoelectric nature of the composite was exploited to produce a vibration sensor sensitive to low-frequency (≈1 kHz) excitation. This is the first demonstration of multifunctional, stretchable BNNT/PDMS composites with enhanced mechanical strength and thermal conductivity and furthermore tunable piezoelectric response by varying BNNT wt% and applied strain, permitting applications in soft actuators and vibration sensors.

Transparent and Flexible Electronics Assembled with Metallic Nanowire-Layered Nondrying Glycerogel

There has been increasing demand for transparent and mechanically durable electrical conductors for their uses in wearable electronic devices. It is common to layer metallic nanowires on transparent but stiff poly(dimethylsiloxane) (PDMS) or stretchable but opaque Ecoflex-based substrates. Here, we hypothesized that layering metallic nanowires on a stretchable and hygroscopic gel would allow us to assemble a transparent, stretchable, and durable conductor. The hygroscopic property of the gel was attained by partially replacing water in the preformed polyacrylamide hydrogel with glycerol. The resulting gel, denoted as a glycerogel, could remain hydrated for over 6 months in air by taking up water molecules from the air. The glycerogel was tailored to be stretchable up to 8 times its original length by tuning the amount of the cross-linker and acrylamide. The resulting glycerogel allowed for deposition of wavy silver nanowires using the prestrain method up to 400% prestrain, without causing kinks and interfacial cracks often found with nanowires layered onto PDMS. With a prestrain of 100%, the resulting nanowire-gel conductor exhibited optical transparency (85%) and electrical conductivity (17.1 ohm/sq) even after 5000 cycles of deformation. The results of this study would broadly be useful to improve the performance of the next generation of flexible electronic devices.

In-plane aligned assemblies of 1D-nanoobjects: recent approaches and applications

One-dimensional (1D) nanoobjects have strongly anisotropic physical properties which are averaged out and cannot be exploited in disordered systems. We reviewed the in plane alignment approaches and potential applications with perspectives shared.

Stretchable Transparent Conductors: from Micro/Macromechanics to Applications

Stretchable transparent conductors (STCs), generally consisting of conducting networks and stretchable transparent elastomers, can maintain stable conductivity and transparency even at large tensile strain, beyond the reach of rigid/flexible transparent conductors. They are essential components in stretchable/wearable electronics for using on irregular 3D conformable surfaces and have attracted tremendous attention in recent years. This review aims to provide systematical correlation of the conducting element-substrate interaction with the structural stability of conducting networks, as well as the properties and device applications of STCs. It starts with the micromechanics for stretching of conducting elements on substrates, including the mechanical mismatch, distribution/level of interfacial shear stress, and the deformation behavior of conducting elements on substrates. The macromechanics for stretching of conducting networks on substrates are then further illustrated from a more statistical point of view, namely sliding/preferred orientation of percolation networks, unfolding of buckled structures, and unit cell distortion/distributed rupture of nanomeshes. The structure-dependent properties as well as the state-of-the-art applications of STCs are summarized before ending with the conclusions and outlooks for STCs.

Nanowire Assemblies for Flexible Electronic Devices: Recent Advances and Perspectives

The fabrication of nanowire (NW)-based flexible electronics including wearable energy storage devices, flexible displays, electrical sensors, and health monitors has received great attention both in fundamental research and market requirements in our daily lives. Other than a disordered state after synthesis, NWs with designed and hierarchical structures would not only optimize the intrinsic performance, but also create new physical and chemical properties, and integration of individual NWs into well-defined structures over large areas is one of the most promising strategies to optimize the performance of NW-based flexible electronics. Here, the recent developments and achievements made in the field of flexible electronics composed of integrated NW structures are presented. The different assembly strategies for the construction of 1D, 2D, and 3D NW assemblies, especially the NW coassembly process for 2D NW assemblies, are comprehensively discussed. The improvements of different NW assemblies on flexible electronics structure and performance are described in detail to elucidate the advantages of well-defined NW assemblies. Finally, a short summary and outlook for future challenges and perspectives in this field are presented.

Large-Area, Ultrathin Metal-Oxide Semiconductor Nanoribbon Arrays Fabricated by Chemical Lift-Off Lithography

Nanoribbon- and nanowire-based field-effect transistors (FETs) have attracted significant attention due to their high surface-to-volume ratios, which make them effective as chemical and biological sensors. However, the conventional nanofabrication of these devices is challenging and costly, posing a major barrier to widespread use. We report a high-throughput approach for producing arrays of ultrathin (∼3 nm) In2O3 nanoribbon FETs at the wafer scale. Uniform films of semiconducting In2O3 were prepared on Si/SiO2 surfaces via a sol-gel process prior to depositing Au/Ti metal layers. Commercially available high-definition digital versatile discs were employed as low-cost, large-area templates to prepare polymeric stamps for chemical lift-off lithography, which selectively removed molecules from self-assembled monolayers functionalizing the outermost Au surfaces. Nanoscale chemical patterns, consisting of one-dimensional lines (200 nm wide and 400 nm pitch) extending over centimeter length scales, were etched into the metal layers using the remaining monolayer regions as resists. Subsequent etch processes transferred the patterns into the underlying In2O3 films before the removal of the protective organic and metal coatings, revealing large-area nanoribbon arrays. We employed nanoribbons in semiconducting FET channels, achieving current on-to-off ratios over 107 and carrier mobilities up to 13.7 cm2 V-1 s-1. Nanofabricated structures, such as In2O3 nanoribbons and others, will be useful in nanoelectronics and biosensors. The technique demonstrated here will enable these applications and expand low-cost, large-area patterning strategies to enable a variety of materials and design geometries in nanoelectronics.

Biomimetic twisted plywood structural materials

Biomimetic designs based on micro/nanoscale manipulation and scalable fabrication are expected to develop new-style strong, tough structural materials. Although the mimicking of nacre-like ‘brick-and-mortar’ structure is well studied, many highly ordered natural architectures comprising 1D micro/nanoscale building blocks still elude imitation owing to the scarcity of efficient manipulation techniques for micro/nanostructural control in practical bulk counterparts. Herein, inspired by natural twisted plywood structures with fascinating damage tolerance, biomimetic bulk materials that closely resemble natural hierarchical structures and toughening mechanisms are successfully fabricated through a programmed and scalable bottom-up assembly strategy. By accurately engineering the arrangement of 1D mineral micro/nanofibers in biopolymer matrix on the multiscale, the resultant composites display optimal mechanical performance, superior to many natural, biomimetic and engineering materials. The design strategy allows for precise micro/nanostructural control at the macroscopic 3D level and can be easily extended to other materials systems, opening up an avenue for many more micro/nanofiber-based biomimetic designs.

Highly Conformable, Transparent Electrodes for Epidermal Electronics

We present a highly conformable, stretchable, and transparent electrode for application in epidermal electronics based on polydimethylsiloxane (PDMS) and Ag nanowire (AgNW) networks. With the addition of a small amount of a commercially available nonionic surfactant, Triton X, PDMS became highly adhesive and mechanically compliant, key factors for the development of conformable and stretchable substrates. The polar functional groups present in Triton X interacted with the Pt catalyst present in the PDMS curing agent, thereby hindering the cross-linking reaction of PDMS and modulating the mechanical properties of the polymer. Due to the strong interactions that occur between the polar functional groups of Triton X and AgNWs, AgNWs were effectively embedded in the adhesive PDMS (a-PDMS) matrix, and the highly enhanced conformability, mechanical stretchability, and transparency of the a-PDMS matrix were maintained in the resulting AgNW-embedded a-PDMS matrix. Finally, wearable strain and electrocardiogram (ECG) sensors were fabricated from the AgNW-embedded a-PDMS. The a-PDMS-based strain and ECG sensors exhibited significantly improved sensing performances compared with those of the bare PDMS-based sensors because of the better stretchability and conformability to the skin of the former sensors.

Patterned electromagnetic alignment of magnetic nanowires

A combination of electromagnetic alignment and topological pattern assisted alignment to position magnetic nanowires, which is referred to as the Patterned Electromagnetic Alignment (PEA), is developed and examined. Electrodeposited, FeNiCo nanowires with different lengths were used as the test nanomaterial, and the microscale grooved surface was formed by UV nanoimprint lithography. The accuracy of the PEA with FeNiCo nanowires was evaluated by measuring the deviation angle from the direction of the magnetic field line for different magnetic field strengths and nanowire lengths, and a statistical alignment distribution was reported for different nanowire length groups. The results were compared with those of the electromagnetic alignment on flat surfaces and in grooved-patterned substrates without electromagnetic alignment. Overall, the deviation angle for the PEA was lower than that for the electromagnetic alignment when all other experimental conditions were identical, indicating that the alignment accuracy along the direction of the magnetic field lines was enhanced in the presence of surface micro grooves. This can be attributed to the fact that, upon attachment of nanowires to the substrate surface, the surface micro grooves in the PEA add additional deterministic characteristics to the otherwise stochastic nature of the nanowire deposition and solvent evaporation processes compared to the sole electromagnetic alignment.

Exploiting both optical and electrical anisotropy in nanowire electrodes for higher transparency

Transparent electrodes such as indium tin oxide and random meshes of silver nanowires (AgNWs) have isotropic in-plane properties. However, we show that imparting some alignment to AgNWs can create anisotropic transparency and electrical conductivity characteristics that may benefit many applications. For example, liquid crystal displays and the touch sensors on top of them often only need to be transparent to one type of polarized light as well as predominantly conductive in only one direction. Herein, AgNWs are slightly preferentially aligned during their deposition by rod coating. Compared to randomly oriented AgNW films, the alignment boosts the transparency to perpendicularly polarized light, as well as achieves a higher transparency for a given sheet resistance in one direction compared to randomly oriented AgNWs films. These factors together increase the transparency of a 16 Ω/sq electrode by 7.3 percentage points. The alignment technique is cheap and scalable, compatible with roll-to-roll processes, and most importantly does not require extra processing steps, as rod coating is already a standard process for AgNW electrode fabrication.

Highly Transparent, Conductive, and Bendable Ag Nanowire Electrodes with Enhanced Mechanical Stability Based on Polyelectrolyte Adhesive Layer

In this paper, a highly transparent, conductive, and bendable Ag nanowire (AgNW)-based electrode with excellent mechanical stability was prepared through the introduction of an adhesive polyelectrolyte multilayer between AgNW networks and a polyethylene terephthalate (PET) substrate. The introduction of the adhesive layer was performed based on a peel-assembly-transfer procedure, and the adhesive polyelectrolyte greatly improved the mechanical stability of the AgNW transparent conductive films (TCFs) without obviously attenuating the morphology and optoelectrical properties of the AgNW networks. The as-prepared AgNW TCFs simultaneously possess high optical transparency, good conductivity, excellent flexibility, and remarkable mechanical stability. It is believed that the proposed strategy would pave a new way for preparing flexible transparent electrodes with a long-term stability, which is significant in the development and practical applications of flexible transparent electronic devices operated in severe environments.

Stretchable Substrates for the Assembly of Polymeric Microstructures

The directed assembly of micro-/nanoscale objects relies on physical or chemical processes to generate structures that are not possible via self-assembly alone. A relatively unexplored strategy in directed assembly is the "active" manipulation of building blocks through deformations of elastomeric substrates. This manuscript reports a method which uses macroscopic mechanical deformations of chemically modified silicone films to realize the rational assembly of microscopic polymer structures. Specifically, polystyrene microparticles are deposited onto polydimethylsiloxane substrates using microcontact-printing where, through a process that involved stretching/relaxing the substrates and bonding of the particles, they are elaborated into microstructures of various sizes, shapes, symmetries, periodicities, and functionalities. The resulting polymeric microstructures can be released and redeposited onto planar/nonplanar surfaces. When building blocks with different properties (e.g., those with fluorescent and catalytic properties) are used, it is possible to fabricate structures with heterogeneous functionality. This method can be extended to the assembly of numerous micro-/nanoscale building blocks (e.g., colloidal organic/inorganic materials) with rational control over the size, shape, and functionality of the product. As a strategy, the use of substrate deformations to enable the micromanipulation and fabrication of a potentially diverse set of assemblies represents a powerful tool useful to, for example, nanotechnology and micromanufacturing.

Recent Advances in Silicon Nanowire Biosensors: Synthesis Methods, Properties, and Applications

The application of silicon nanowire (SiNW) biosensor as a subtle, label-free, and electrical tool has been extensively demonstrated by several researchers over the past few decades. Human ability to delicately fabricate and control its chemical configuration, morphology, and arrangement either separately or in combination with other materials as lead to the development of a nanomaterial with specific and efficient electronic and catalytic properties useful in the fields of biological sciences and renewable energy. This review illuminates on the various synthetic methods of SiNW, with its optical and electrical properties that make them one of the most applicable nanomaterials in the field of biomolecule sensing, photoelectrochemical conversion, and diseases diagnostics.

Magnetite Core-Shell Nanoparticles in Nondestructive Flaw Detection of Polymeric Materials

Nondestructive flaw detection in polymeric materials is important but difficult to achieve. In this research, the application of magnetite nanoparticles (MNPs) in nondestructive flaw detection is studied and realized, to the best of our knowledge, for the first time. Superparamagnetic and highly magnetic (up to 63 emu/g) magnetite core-shell nanoparticles are prepared by grafting bromo-end-group-functionalized poly(glycidyl methacrylate) (Br-PGMA) onto surface-modified Fe₃O₄ NPs. These Fe₃O₄-PGMA NPs are blended into bisphenol A diglycidylether (BADGE)-based epoxy to form homogeneously distributed magnetic epoxy nanocomposites (MENCs) after curing. The core Fe₃O₄ of the Fe₃O₄-PGMA NPs endows the MENCs with magnetic property, which is crucial for nondestructive flaw detection of the materials, while the shell PGMA promotes colloidal stability and prevents NP aggregation during curing. The eddy current testing (ET) technique is first applied to detect flaws in the MENCs. Through the brightness contrast of the ET image, surficial and subsurficial flaws in MENCs can be detected, even for MENCs with low content of Fe₃O₄-PGMA NPs (1 wt %). The incorporation of Fe₃O₄-PGMA NPs can be easily extended to other polymer and polymer-based composite systems and opens a new and very promising pathway toward MNP-based nondestructive flaw detection in polymeric materials.

Moving beyond flexible to stretchable conductive electrodes using metal nanowires and graphenes

Stretchable and/or flexible electrodes and their associated electronic devices have attracted great interest because of their possible applications in high-end technologies such as lightweight, large area, wearable, and biointegrated devices. In particular, metal nanowires and graphene derivatives are chosen for electrodes because they show low resistance and high mechanical stability. Here, we review stretchable and flexible soft electrodes by discussing in depth the intrinsic properties of metal NWs and graphenes that are driven by their dimensionality. We investigate these properties with respect to electronics, optics, and mechanics from a chemistry perspective and discuss currently unsolved issues, such as how to maintain high conductivity and simultaneous high mechanical stability. Possible applications of stretchable and/or flexible electrodes using these nanodimensional materials are summarized at the end of this review.

Aligned silver nanowire-based transparent electrodes for engineering polarisation-selective optoelectronics

We herein report on a remarkably simple, fast, and economic way of fabricating homogeneous and well oriented silver nanowires (AgNWs) that exhibit strong in-plane electrical and optical anisotropies. Using a small quantity of AgNW suspension, the horizontal-dip (H-dip) coating method was applied, in which highly oriented AgNWs were deposited unidirectionally along the direction of coating over centimetre-scale lengths very rapidly. In applying the H-dip-coating method, we adjusted the shear strain rate of the capillary flow in the Landau-Levich meniscus of the AgNW suspension, which induced a high degree of uniaxial orientational ordering (0.37-0.43) of the AgNWs, comparable with the ordering seen in archetypal nematic liquid crystal (LC) materials. These AgNWs could be used to fabricate not only transparent electrodes, but also LC-alignment electrodes for LC devices and/or polarising electrodes for organic photovoltaic devices, having the potential to revolutionise the architectures of a number of polarisation-selective opto-electronic devices for use in printed/organic electronics.

Nano-Bioelectronics

Nano-bioelectronics represents a rapidly expanding interdisciplinary field that combines nanomaterials with biology and electronics and, in so doing, offers the potential to overcome existing challenges in bioelectronics. In particular, shrinking electronic transducer dimensions to the nanoscale and making their properties appear more biological can yield significant improvements in the sensitivity and biocompatibility and thereby open up opportunities in fundamental biology and healthcare. This review emphasizes recent advances in nano-bioelectronics enabled with semiconductor nanostructures, including silicon nanowires, carbon nanotubes, and graphene. First, the synthesis and electrical properties of these nanomaterials are discussed in the context of bioelectronics. Second, affinity-based nano-bioelectronic sensors for highly sensitive analysis of biomolecules are reviewed. In these studies, semiconductor nanostructures as transistor-based biosensors are discussed from fundamental device behavior through sensing applications and future challenges. Third, the complex interface between nanoelectronics and living biological systems, from single cells to live animals, is reviewed. This discussion focuses on representative advances in electrophysiology enabled using semiconductor nanostructures and their nanoelectronic devices for cellular measurements through emerging work where arrays of nanoelectronic devices are incorporated within three-dimensional cell networks that define synthetic and natural tissues. Last, some challenges and exciting future opportunities are discussed.

Highly Transparent and Stretchable Conductors Based on a Directional Arrangement of Silver Nanowires by a Microliter-Scale Solution Process

We report an effective method for fabricating highly transparent and stretchable large-area conducting films based on a directional arrangement of silver nanowires (AgNWs) driven by a shear force in a microliter-scale solution process. The thin conducting films with parallel AgNWs or cross-junctions of AgNWs are deposited on the coating substrate by dragging a microliter drop of the coating solution trapped between two plates. The optical and electrical properties of the AgNW thin films are finely tuned by varying the simple systematic parameters in the coating process. The transparent thin films with AgNW cross-junctions exhibit the superior electrical conductivity with a sheet resistance of 10 Ω sq(-1) at a transmittance of 85% (λ = 550 nm), which is well described by the high ratio of DC to optical conductivity of 276 and percolation theory in a two-dimensional matrix model. This simple coating method enables the deposition of AgNW thin films with high optical transparency, flexibility, and stretchability directly on plastic substrates.

Simple assembly of long nanowires through substrate stretching

Although nanowire (NW) alignment has been previously investigated, minimizing limitations such as process complexity and NW breakage, as well as quantifying the quality of alignment, have not been sufficiently addressed. A simple, low cost, large-area, and versatile alignment method is reported that is applicable for NWs either grown on a substrate or synthesized in solution. Metal and semiconductor NWs with average lengths of up to 16 μm are aligned through the stretching of polyvinyl alcohol (PVA) films, which compared to other stretching methods results in superior alignment because of the large stretching ratio of PVA. Poly[oxy(methyl-1,2-ethanediyl)] is employed as lubricant to prevent NW breakage. To quantify NW alignment, a simple and effective image processing method is presented. The alignment process results in an order parameter (S) of NW alignment as high as 0.97.

Thickness-Gradient Films for High Gauge Factor Stretchable Strain Sensors

High-gauge-factor stretchable strain sensors are developed by utilizing a new strategy of thickness-gradient films with high durability, and high uniaxial/isotropic stretchability based on the self-pinning effect of SWCNTs. The monitoring of detailed damping vibration modes driven by weak sound based on such sensors is demonstrated, making a solid step toward real applications.

Floating compression of Ag nanowire networks for effective strain release of stretchable transparent electrodes

Manipulation of the configuration of Ag nanowire (NW) networks has been pursued to enhance the performance of stretchable transparent electrodes. However, it has remained challenging due to the high Young's modulus and low yield strain of Ag NWs, which lead to their mechanical failure when subjected to structural deformation. We demonstrate that floating a Ag NW network on water and subsequent in-plane compression allows convenient development of a wavy configuration in the Ag NW network, which can release the applied strain. A greatly enhanced electromechanical stability of Ag NW networks can be achieved due to their wavy configuration, while the NW networks maintain the desirable optical and electrical properties. Moreover, the produced NW networks can be transferred to a variety of substrates, offering flexibility for device fabrication. The Ag NW networks with wavy configurations are used as compliant electrodes for dielectric elastomer actuators. The study demonstrates their promising potential to provide improved performance for soft electronic devices.

Nanomaterial-enabled stretchable conductors: strategies, materials and devices

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.

Anisotropic optical properties of large-scale aligned silver nanowire films via controlled coffee ring effects

Preparation of thin films with one-dimensional nanostructures and unique physical properties for high-performance electronic, optoelectronic, and electromechanical systems.

Highly conductive and stretchable Ag nanowire/carbon nanotube hybrid conductors

Fabricating stretchable conductors through simple, cost-effective and scalable methods is a challenge. Here, we report on an approach used to develop nanowelded Ag nanowire/single-walled carbon nanotube (AgNW/SWCNT) hybrid films to be used as high-performance stretchable conductors. Plasmonic welding, which was done at the junctions of AgNWs in order to form hybrid AgNW/SWCNT conductors on an Ecoflex substrate, enabled excellent electrical and mechanical stability under large tensile strains of over 480% without the need to pre-strain. Furthermore, we demonstrate highly stretchable circuits that are used to power LED arrays. The LED arrays are formed using the plasmonic-welded AgNW/SWCNT/Ecoflex hybrid material, which demonstrates suitability for interconnector applications in flexible electronics.

Assembly and densification of nanowire arrays via shrinkage

Chemically synthesized semiconductor nanowires (NWs) have demonstrated substantial promise for nanoelectronics, nanoenergy, and nanobiotechnology, but the lack of an effective and controllable assembly process has limited the wide adoption of NWs in these areas. Here we demonstrate a facile, robust, and controllable approach to assembling and densifying a parallel array of NWs using shrinkable shape memory polymers. Using thermal-induced shrinkage of polystyrene, we were able to successfully assemble and densify NW arrays up to close-packing and, furthermore, achieve tunable density (up to ∼300% amplification of density) by controlling the shrinkage process. We also demonstrate scalable assembly and densification of NWs on a 2.5 × 6 inch scale to explore the manufacturability of the shrink-induced assembly process. Finally, we demonstrate the successful transfer of the shrink-assembled NW arrays onto various 2-dimensional and 3-dimensional substrates without compromising the integrity of NW assembly and density.

Stretchable and reversibly deformable radio frequency antennas based on silver nanowires

We demonstrate a class of microstrip patch antennas that are stretchable, mechanically tunable, and reversibly deformable. The radiating element of the antenna consists of highly conductive and stretchable material with screen-printed silver nanowires embedded in the surface layer of an elastomeric substrate. A 3-GHz microstrip patch antenna and a 6-GHz 2-element patch array are fabricated. Radiating properties of the antennas are characterized under tensile strain and agree well with the simulation results. The antenna is reconfigurable because the resonant frequency is a function of the applied tensile strain. The antenna is thus well suited for applications like wireless strain sensing. The material and fabrication technique reported here could be extended to achieve other types of stretchable antennas with more complex patterns and multilayer structures.

Anisotropic surface strain in single crystalline cobalt nanowires and its impact on the diameter-dependent Young's modulus

Understanding and measuring the size-dependent surface strain of nanowires are essential to their applications in various emerging devices. Here, we report on the diameter-dependent surface strain and Young's modulus of single-crystalline Co nanowires investigated by in situ X-ray diffraction measurements. Diameter-dependent initial longitudinal elongation of the nanowires is observed and ascribed to the anisotropic surface stress due to the Poisson effect, which serves as the basis for mechanical measurements. As the nanowire diameter decreases, a transition from the "smaller is softer" regime to the "smaller is tougher" regime is observed in the Young's modulus of the nanowires, which is attributed to the competition between the elongation softening and the surface stiffening effects. Our work demonstrates a new nondestructive method capable of measuring the initial surface strain and estimating the Young's modulus of single crystalline nanowires, and provides new insights on the size effect.

Flexible nanodielectric materials with high permittivity for power energy storage

Study of flexible nanodielectric materials (FNDMs) with high permittivity is one of the most active academic research areas in advanced functional materials. FNDMs with excellent dielectric properties are demonstrated to show great promise as energy-storage dielectric layers in high-performance capacitors. These materials, in common, consist of nanoscale particles dispersed into a flexible polymer matrix so that both the physical/chemical characteristics of the nanoparticles and the interaction between the nanoparticles and the polymers have crucial effects on the microstructures and final properties. This review first outlines the crucial issues in the nanodielectric field and then focuses on recent remarkable research developments in the fabrication of FNDMs with special constitutents, molecular structures, and microstructures. Possible reasons for several persistent issues are analyzed and the general strategies to realize FNDMs with excellent integral properties are summarized. The review further highlights some exciting examples of these FNDMs for power-energy-storage applications.

Silver nanowire/optical adhesive coatings as transparent electrodes for flexible electronics

We present new flexible, transparent, and conductive coatings composed of an annealed silver nanowire network embedded in a polyurethane optical adhesive. These coatings can be applied to rigid glass substrates as well as to flexible polyethylene terephthalate (PET) plastic and elastomeric polydimethylsiloxane (PDMS) substrates to produce highly flexible transparent conductive electrodes. The coatings are as conductive and transparent as indium tin oxide (ITO) films on glass, but they remain conductive at high bending strains and are more durable to marring and scratching than ITO. Coatings on PDMS withstand up to 76% tensile strain and 250 bending cycles of 15% strain with a negligible increase in electrical resistance. Since the silver nanowire network is embedded at the surface of the optical adhesive, these coatings also provide a smooth surface (root mean squared surface roughness<10 nm), making them suitable as transparent conducting electrodes in flexible light-emitting electrochemical cells. These devices continue to emit light even while being bent to radii as low as 1.5 mm and perform as well as unstrained devices after 20 bending cycles of 25% tensile strain.

Highly bendable, conductive, and transparent film by an enhanced adhesion of silver nanowires

Recently, silver nanowires (AgNWs) have attracted considerable interest for their potential application in flexible transparent conductive films (TCFs). One challenge for the commercialization of AgNW-based TCFs is the low conductivity and stability caused by the weak adhesion forces between the AgNWs and the substrate. Here, we report a highly bendable, conductive, and transparent AgNW film, which consists of an underlying poly(diallyldimethyl-ammonium chloride) (PDDA) and AgNW composite bottom layer and a top layer-by-layer (LbL) assembled graphene oxide (GO) and PDDA overcoating layer (OCL). We demonstrated that PDDA could increase the adhesion between the AgNW and the substrate to form a uniform AgNW network and could also serve to improve the stability of the GO OCL. Hence, a highly bendable, conductive, and transparent AgNW-PDDA-GO composite TCF on a poly(ethylene terephthalate) (PET) substrate with Rs ≈ 10 Ω/sq and T ≈ 91% could be made by an all-solution processable method at room temperature. In addition, our AgNW-PDDA-GO composite TCF is stable without degradation after exposure to H2S gas or sonication.

Fabrication of functional nanowire devices on unconventional substrates using strain-release assembly

We report three representative nanowire (NW) devices for applications in stretchable electronics, strain sensing, and optical sensing. Fabrication of such devices is based on a recently developed strain-release assembly method. NWs are first aligned transversely on an elastomeric substrate using the strain-release assembly. Constant resistance is achieved in silicon (Si) NW devices stretched up to ~40% of axial strain, highlighting a new concept of transverse buckling. Combining the NW assembly with transfer printing extends suitable device substrates beyond elastomers to other unconventional materials (e.g., flexible and transparent materials). Following this combined process, flexible SiNW strain sensors are fabricated on plastics capable of sensing up to 1.6% bending strain and gauge factors >1000; flexible zinc oxide NW ultraviolet sensors are demonstrated with quick recovery (~2 s) and excellent repeatability on plastics. Our results show promise for the strain-release assembly as a simple and cost-effective process to fabricate NW devices on unconventional substrates.

Stretchable photonic crystal cavity with wide frequency tunability

We report a new approach for realizing a flexible photonic crystal (PC) cavity that enables wide-range tuning of its resonance frequency. Our PC cavity consists of a regular array of silicon nanowires embedded in a polydimethylsiloxane (PDMS) matrix and exhibits a cavity resonance in the telecommunication band that can be reversibly tuned over 60 nm via mechanical stretching-a record for two-dimensional (2D) PC structures. These mechanically reconfigurable devices could find potential applications in integrated photonics, sensing in biological systems, and smart materials.

Large-scale integration of semiconductor nanowires for high-performance flexible electronics

High-performance flexible electronics has attracted much attention in recent years due to potential applications in flexible displays, artificial skin, radio frequency identification, sensor tapes, etc. Various materials such as organic and inorganic semiconductor nanowires, carbon nanotubes, graphene, etc. have been explored as the active semiconductor components for flexible devices. Among them, inorganic semiconductor nanowires are considered as highly promising materials due to their relatively high carrier mobility, reliable control on geometry and electronic properties, and cost-effective synthesis processes. In this review, recent progress on the assembly of high-performance inorganic semiconductor nanowires and their applications for large-scale flexible electronics will be summarized. In particular, nanowire-based integrated circuitry and high-frequency electronics will be highlighted.

Buckling of aligned carbon nanotubes as stretchable conductors: a new manufacturing strategy

A new manufacturing strategy for buckling of aligned carbon nanotubes is developed, which does not involve prestretching the substrate but relies on the interface interaction between the nanotubes and the substrate. More specifically, upon stretching the substrate the nanotubes slide on the substrate, but upon releasing the nanotubes buckle. Following this manufacturing strategy, stretchable conductors based on aligned carbon nanotubes are demonstrated.

Static friction between silicon nanowires and elastomeric substrates

This paper reports the first direct measurements of static friction force and interfacial shear strength between silicon (Si) nanowires (NWs) and poly(dimethylsiloxane) (PDMS). A micromanipulator is used to manipulate and deform the NWs under a high-magnification optical microscope in real time. The static friction force is measured based on "the most-bent state" of the NWs. The static friction and interface shear strength are found to depend on the ultraviolet/ozone (UVO) treatment of PDMS. The shear strength starts at 0.30 MPa without UVO treatment, increases rapidly up to 10.57 MPa at 60 min of treatment and decreases for longer treatment. Water contact angle measurements suggest that the UVO-induced hydrophobic-to-hydrophilic conversion of PDMS surface is responsible for the increase in the static friction, while the hydrophobic recovery effect contributes to the decrease. The static friction between NWs and PDMS is of critical relevance to many device applications of NWs including NW-based flexible/stretchable electronics, NW assembly and nanocomposites (e.g., supercapacitors). Our results will enable quantitative interface design and control for such applications.

Flexible and semitransparent strain sensors based on micromolded Pd nanoparticle-carbon μ-stripes

Flexible resistive strain sensors have been fabricated by micromolding Pd alkanethiolate on polyimide substrates and subjecting to thermolysis in air. Thus produced stripes were ∼1 μm wide with spacing of ∼0.5 μm and contained Pd nanoparticles in carbon matrix. The nanoparticle size and the nature of carbon are much dependent on the thermolysis temperature as is also the resistance of the microstripes. Generally, lower thermolysis temperatures (<230 °C) produced stripes containing small Pd nanoparticles with significant fraction of carbon from the precursor decomposition. The stripes were poorly conducting yet interestingly, exhibited change of resistance under tensile and compressive strain. Particularly noteworthy are the stripes produced from 195 °C thermolysis, which showed a high gauge factor of ∼390 with strain sensitivity, 0.09%. With molding at 230 °C, the stripes obtained were highly conducting, and amazingly did not change the resistance with strain even after several bending cycles. The latter are ideal as flexible conduits and interconnects. Thus, the article reports a method of producing flexible sensitive strain sensors on one hand and on the other, flexible conduits with unchanging resistance, merely by fine-tuning the precursor decomposition under the molding conditions.

Controlled 3D buckling of silicon nanowires for stretchable electronics

Silicon (Si) nanowire (NW) coils were fabricated on elastomeric substrates by a controlled buckling process. Si NWs were first transferred onto prestrained and ultraviolet/ozone (UVO)-treated poly(dimethylsiloxane) (PDMS) substrates and buckled upon release of the prestrain. Two buckling modes (the in-plane wavy mode and the three-dimensional coiled mode) were found; a transition between them was achieved by controlling the UVO treatment of PDMS. Structural characterization revealed that the NW coils were oval-shaped. The oval-shaped NW coils exhibited very large stretchability up to the failure strain of PDMS (∼104% in our study). Such a large stretchability relies on the effectiveness of the coil shape in mitigating the maximum local strain, with a mechanics that is similar to the motion of a coil spring. Single NW devices based on coiled NWs were demonstrated with a nearly constant electrical response in a large strain range. In addition to the wavy shape, the coil shape represents an effective architecture in accommodating large tension, compression, bending, and twist, which may find important applications for stretchable electronics and other stretchable technologies.