Applications In Stretchable Electronics Research Articles

Screen printing of silver nanowires: balancing conductivity with transparency while maintaining flexibility and stretchability

Printing metal nanowires are particularly attractive as compared to conventional coating methods due to the ease of processing, direct patterning, and large-scale fabrication capability. However, it is still challenging to print metal nanowire patterns that simultaneously have high conductivity, high transparency, flexibility, and stretchability. Three steps have been taken in this work to balance the transparency and conductivity of the screen-printed flexible and stretchable silver nanowire films, (1) selection of the ink formulation, (2) optimization of the printing parameters, and (3) posttreatment with a laser. The as-obtained silver nanowire patterns are large-area and demonstrate an ultralow sheet resistance of 1.9 ohm/sq, high transmittance (73%) at the wavelength of 550 nm, and an ultrahigh figure of merit (~136) as compared to the printed silver nanowire electrodes in the literature. The screen-printed transparent patterns exhibit excellent electrical stability and mechanical repeatability when subjected to 1000 bending cycles with a bending radius of 28 mm and 1000 stretch-release cycles with 10% strain, which makes the transparent patterns suitable for the fabrication of flexible, transparent microwave absorbers. The absorption performance of the prepared frequency selective surface absorbers indicates no obvious degradation after various manipulating configurations and multiple bending and stretching cycles. The results are promising enough to make this ink and screen-printing process suitable for many applications of flexible, stretchable, and transparent electronics.

Localization of wrinkle patterns by crack-tip induced plasticity: Experiments and simulations

Regulating surface wrinkles have received considerable attention because of their potential applications in stretchable electronics, microfluidics, bionics, sensors, stamps, optical devices and smart materials. In this work, optical microscopy and atomic force microscopy revealed that tunable film thickness and substrate stiffness effectively regulate localized wrinkle patterns in metal films deposited on soft polymer substrates. Theoretical analysis and numerical simulation found that the high tensile stress developed during the film deposition leads to channel cracks in the film and creates a stripe-like plastic zone around each crack. The subsequently developed compressive stress in the plastic zone is anisotropic, resulting in the formation of localized straight wrinkles perpendicular to the crack. The width of the wrinkle is consistent with the size of the plastic zone, which decreases with increasing the film thickness or substrate stiffness, in good agreement with the experimental observations. The report in this work not only elucidates how the subtle correlation among crack, plasticity and buckle leads to such localized wrinkling pattern but also demonstrates an efficient way to produce crack-network-guided localized wrinkle patterns.

Meandering growth of in-plane silicon nanowire springs

Despite the fundamental difference in material systems and temporal evolution, self-oscillating growth of silicon nanowires (SiNWs), led by metal droplets, resembles very much natural river meanders in terms of their sinuosity, fractal dimensions, and scaling law. Both of them are driven by the release of higher potential energy stored in the disorder hydrogenated amorphous Si (a-Si:H) matrix or at highlands, tailored by a streamwise flow mechanism and subject to an erodible boundary constraint imposed by the a-Si:H thin film or the soil banks, respectively. Under specific conditions, the cross-droplet/stream velocity difference can be magnified, during the in-plane growth of SiNWs, to stimulate regular swaggering dynamics that produce continuous and smooth SiNW meanders. This interesting phenomenon indicates a rather simple and highly efficient strategy to shape complex elastic channels with only a few control parameters. A kinetic model has been established to explain the underlying mechanism of the self-oscillating meandering growth, which has unique potential to transform rigid SiNW channels into elastic forms for flexible or stretchable electronic applications.

Skin-Inspired Surface-Microstructured Tough Hydrogel Electrolytes for Stretchable Supercapacitors.

Double-network tough hydrogels have raised increasing interest in stretchable electronic applications as well as electronic skin (e-skin) owing to their excellent mechanical properties and functionalities. While hydrogels have been extensively explored as solid-state electrolytes, stretchable energy storage devices based on tough hydrogel electrolytes are still limited despite their high stretchability and strength. A key challenge remains in the robust electrode/electrolyte interface under large mechanical strains. Inspired by the skin structure that involves the microstructured interface for the tight connection between the dermis and epidermis, we demonstrated that a surface-microstructured tough hydrogel electrolyte composed of agar/polyacrylamide/LiCl (AG/PAAm/LiCl) could be exploited to allow stretchable supercapacitors with enhanced mechanical and electrochemical performance. The prestretched tough hydrogel electrolyte was treated to generate surface microstructures with a roughness of tens of micrometers simply via mechanical rubbing followed by the attachment of activated carbon electrodes on both sides to realize the fabrication of the stretchable supercapacitor. Through investigating the properties of the tough hydrogel electrolyte and the electrochemical performance of the as-fabricated supercapacitors under varied strains, the surface-microstructured hydrogel electrolyte was shown to enable robust adhesion to electrodes, improving electrochemical behavior and capacitance, as well as having better performance retention under repeated stretching cycles, which surpassed the pristine hydrogel with smooth surfaces. Our approach could provide an alternative and general strategy to improve the interfacial properties between the electrode and the hydrogel electrolyte, driving new directions for functional stretchable devices based on tough hydrogels.

Tailoring Carbosilane Side Chains toward Intrinsically Stretchable Semiconducting Polymers

Carbosilane side chain-equipped isoindigo–bithiophene semiconducting polymers (PII2T) have been designed and synthesized for stretchable electronics applications. Systematically tailoring the lengt...

Direct Wiring of Eutectic Gallium-Indium to a Metal Electrode for Soft Sensor Systems.

For wider applications of liquid metal-based stretchable electronics, electrical interface has remained a crucial issue due to its fragile electromechanical stability and complex fabrication steps. In this study, a direct writing-based technique is introduced to form the writing paths of conductive liquid metal (eutectic gallium-indium, eGaIn) and electrical connections to off-the-shelf metal electrodes in a single process. Specifically, by extending eGaIn wires written on a silicone substrate, the eGaIn wires were physically connected to five different metal electrodes, of which stability as an electrical connection was investigated. Among the five different surface materials, the metal electrode finished by electroless nickel immersion gold (ENIG) was reproducible and had low contact resistance without time-dependent variation. In our experiments, it was verified that the electrode part made by an ENIG-finished flexible flat cable (FFC) was mechanically (strain, ≤100%; pressure, ≤600 kPa) and thermally (temperature, ≤180 °C) durable. By modifying the trajectories of eGaIn wires, soft sensor systems composed of 10 sensing units were fabricated and tested to measure finger joint angles and ground reaction forces, respectively. The proposed method enables eGaIn-based soft sensors or circuits to be connected to typical electronic components through FFCs or weldable surfaces, using only off-the-shelf materials without additional mechanical or chemical treatments.

Nanomeshed Si nanomembranes

One of the main challenges in stretchable electronics is to achieve high-performance stretchable semiconductors. Here, we introduce an innovative concept of nanomeshed semiconductor nanomembrane which can be regarded almost as intrinsically stretchable to conventional microelectronic layouts. By making a silicon film into homogeneous nanomeshes with spring-like nano traces, we demonstrated a high electron mobility of 50 cm2/V·s, and moderate stretchability with a one-time strain of 25% and cyclic strain of 14% after stretching for 1000 cycles, further improvable with optimized nanomesh designs. A simple analytic model covering both fractional material and trace sidewall surfaces well predicted the transport properties of the normally on silicon nanomesh transistors, enabling future design and optimizations. Besides potential applications in stretchable electronics, this semiconductor nanomesh concept provides a new platform for materials engineering and is expected to yield a new family of stretchable inorganic materials having tunable electronic and optoelectronic properties with customized nanostructures.

Direct 3D Printing of Stretchable Circuits via Liquid Metal Co‐Extrusion Within Thermoplastic Filaments

Liquid‐metal alloys are now extensively used for stretchable electronic applications due to their superior electrical conductivity, non‐toxicity, and mechanical stability in micro‐channels. Needle‐injection and direct‐writing are the most popular techniques for patterning micro‐structured liquid metal alloys. However, embedded channels often require a very high pressure to inject liquid‐metal, and direct writing by dispensing is relatively complex due to the low viscosities and high surface tension of the metal which cause liquid to normally dispense in droplets rather than a stream. In this work, a technique to co‐axially extrude liquid‐metal alloy within an encapsulating cover fluid has been presented to obtain a continuous stable stream of liquid‐metal. Fused deposition modeling (FDM) 3D printing has been adapted to co‐extrude a liquid‐metal core with a shell made from a thermoplastic elastomer. A custom extruder system is used to directly produce conductive micro‐wires (diameter: ≈25 μm) of liquid‐metal having an insulating shell of styrene‐ethylene‐butylene‐styrene which can be stretched up to four times the original length without any noticeable mechanical and electrical loss. The system is capable of printing in‐plane conductive pathways as well as out‐of‐plane functional devices with direct‐stable encapsulation of liquid‐metal wires. This technology has been successfully used to print 2D‐pressure and 3D‐strain sensors.

Freestanding Functional Structures by Aerosol‐Jet Printing for Stretchable Electronics and Sensing Applications

AbstractModern electronic devices, particularly those intended for wearable or human health monitoring applications, require high levels of flexibility and stretchability. Hence devices, as well as interconnects, need to be capable of retaining functionality even when being mechanically deformed. Most approaches towards achieving this rely on printing or transferring structures onto elastomeric substrates that can withstand stretching. However, the processing involved can often be cumbersome, and the structures themselves tend to suffer from poor fatigue and/or are limited by the mechanical properties of the underlying substrate. Here, we have developed a novel aerosol jet printing technique capable of building fully freestanding functional structures layer by layer, which are robust and reliable upon thousands of stretching cycles. The process involves printing a combination of layers of different materials with the desired functionality, onto a substrate coated with a sacrificial film that is subsequently dissolved to release the printed structure. Using this method, we demonstrate freestanding conductive wires that can be used as stretchable interconnects/electrodes, and that also function as strain‐sensors. Additionally, we show that a freestanding capacitive structure functions as a robust, stretchable humidity sensor, paving the way for the development of other multilayer, multifunctional stretchable devices and sensors.

A rapid technique for the direct metallization of PDMS substrates for flexible and stretchable electronics applications

Metallization of a polydimethylsiloxane (PDMS)-based substrate is a challenge due to the difficulties in forming crack-free polymer and metal features using standard deposition techniques. Frequently, additional adhesion layers, rigid substrates, multiple processing steps (lift-off and etching) and expensive metal sputtering techniques are required, to achieve such metal patterns. This work presents a novel and rapid technique for the direct metallization of PDMS substrates using photolithography and electroless copper plating. The method has the advantage of not requiring expensive vacuum processing or multiple metallization steps. Electroless copper layer is demonstrated to have a strong adhesion to PDMS substrate with a high conductivity of (3.6 ± 0.7) × 107 S/m, which is close to the bulk copper (5.9 × 107 S/m). The copper-plated PDMS substrate displays mechanical and electrical stability whilst undergoing stretching deformations up to 10% due to applied strain. A functional electronic circuit was fabricated as a demonstration of the mechanical integrity of the copper-plated PDMS after bending.

Highly conductive 3D metal-rubber composites for stretchable electronic applications

Stretchable conductors are critical building blocks for enabling new forms of wearable and curvilinear electronics. In this paper, we introduce a new method using the interfacial design to enable stretchable conductors with ultra-high conductivity and robustness to strain using three-dimensional helical copper micro-interconnects embedded in an elastic rubber substrate (eHelix-Cu). We studied the interfacial mechanics of the metal-elastomer to achieve highly reversible conductivities with strains. The stretchable eHelix-Cu interconnect has an ultra-high conductivity (∼105 S cm−1) that remains almost invariant when stretched to 170%, which is significantly higher than in other approaches using nanomaterials. The stretchable conductors can withstand strains of 100% for thousands of cycles, demonstrating remarkable durability for exciting potential wearable electronic applications.

Mechanics of buckled serpentine structures formed via mechanics-guided, deterministic three-dimensional assembly

Many forms of stretchable electronic systems incorporate planar, filamentary serpentine structures to realize high levels of stretchability in ways that allow the use of high performance of inorganic functional materials. Recent advances in mechanics-guided, deterministic three-dimensional (3D) assembly provide routes to transform these traditional, two-dimensional (2D) serpentine layouts into 3D architectures, with significantly improved mechanics and potential for applications in energy harvesters, pressure sensors, soft robotics, biomedical devices and other classes of technologies. One challenge is that, by comparison to other geometries, the relatively low bending stiffnesses of the serpentine structures create difficulties in overcoming the interfacial adhesion energy to allow delamination from the underlying elastomeric substrate, as an essential aspect of the assembly process. An additional complication is that many of the functional materials widely used in stretchable electronics have a low strain threshold for failure such that damage can occur during buckling-induced assembly. Therefore, a clear understanding of the mechanics of buckled serpentine structures is essential to their design, fabrication and application. Through theoretical modeling and finite element analysis, we present models for the phase diagram of buckled states and the maximum strain in the globally-buckled serpentine structures. The analysis yield formulae in concise and explicit forms, clearly showing the effect of geometry/material parameters and the prestrain. The results can be used in designing buckled serpentine structures to ensure their compatibility with the mechanics-guided, deterministic 3D assembly and facilitate their applications in stretchable electronics.

Intrinsically stretchable conductors and interconnects for electronic applications

Intrinsically stretchable conductors and interconnects with excellent performance made from different types of materials find various applications in stretchable electronics.

High-performance stretchable conductive nanocomposites: materials, processes, and device applications.

Highly conductive and intrinsically stretchable electrodes are vital components of soft electronics such as stretchable transistors and circuits, sensors and actuators, light-emitting diode arrays, and energy harvesting devices. Many kinds of conducting nanomaterials with outstanding electrical and mechanical properties have been integrated with elastomers to produce stretchable conductive nanocomposites. Understanding the characteristics of these nanocomposites and assessing the feasibility of their fabrication are therefore critical for the development of high-performance stretchable conductors and electronic devices. We herein summarise the recent advances in stretchable conductors based on the percolation networks of nanoscale conductive fillers in elastomeric media. After discussing the material-, dimension-, and size-dependent properties of conductive fillers and their implications, we highlight various techniques that are used to reduce the contact resistance between the conductive filler materials. Furthermore, we categorize elastomer matrices with different stretchabilities and mechanical properties based on their polymeric chain structures. Then, we discuss the fabrication techniques of stretchable conductive nanocomposites toward their use in soft electronics. Finally, we provide representative examples of stretchable device applications and conclude the review with a brief outlook for future research.

Crack propagation design in transparent polymeric conductive films via carbon nanotube fiber-reinforcement and its application for highly sensitive and mechanically durable strain sensors

Conductive thin films are typically subject to crack formation and propagation under tensile strain, turning into insulating films due to complete breakage at large strain. However, if such crack propagation can be intentionally designed, repetitive resistance change can be obtained and used for implementation of high-performance strain sensors that are suitable for biocompatible and stretchable electronic applications. In this work, therefore, we introduce a fiber-reinforced region, which is formed by additionally inkjet-printing a single-walled carbon nanotube thin film, in a poly(3, 4-ethylenedioxythophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS) thin film. The fiber-reinforced region well suppresses the crack propagation in the film under the tensile strain. The engineered PEDOT:PSS films are used to fabricate a strain sensor with a high gauge factor of ∼9 (at 50% strain) and an excellent working range of 70% even after 1000 cycle test at 50% tensile strain. Such a high-performance is explained via different fracture mechanisms between the fracture-designed and the fiber-reinforced regions in the PEDOT:PSS films. Our strategy of designing crack propagation using the inkjet-printing process enables not only to fabricate high-performance strain sensors that can detect human motions but also to provide a new insight for highly contuctive, but relatively brittle, materials toward the stretchable electronics applications.

Study on the oxidation of copper nanowire network electrodes for skin mountable flexible, stretchable and wearable electronics applications

Copper nanowires (Cu NWs) are suitable material as an electrode for flexible, stretchable and wearable devices due to their excellent mechanical properties, high transparency, good conductivity, and low cost, but oxidation problem limits their practical use and application. In order to use Cu NWs as an electrode for advanced flexible, stretchable and wearable devices attached directly to the skin, the influence of the body temperature on the oxidation of Cu NWs needs to be investigated. In this paper, the oxidation behavior of Cu NWs at high temperature (more than 80 °C) as well as body temperature is studied which has been remained largely questionable to date, and an effective encapsulation method is proposed to prevent the oxidation of Cu NWs electrode in the range of body temperatures.

Engineering Surface Patterns with Shape Memory Polymers: Multiple Design Dimensions for Diverse and Hierarchical Structures.

Deterministic design of surface patterns has seen a surge of interests because of their wide applications in flexible and stretchable electronics, microfluidics, and optical devices. Recently, instability of bilayer systems has been extensively utilized by which micro-/nano-patterns of a film can be easily achieved through macroscopically deforming the underlying substrate. For a bilayer system with traditional thermostable substrates, the pattern morphology is only determined by initial strain mismatch of the two layers, and the realization of localized patterns appears to be particularly challenging because of the difficulties associated with manipulating inhomogeneous deformations. In this work, we exploit cross-linked polyethylene ( cPE), a shape memory polymer (SMP), as the flexible substrate for building micro-/nano-structures of sputtered gold films. We find that the shape memory effect can offer new dimensions for designing diverse and hierarchical surface structures by harnessing film thickness orheating time and by globally or locally controlling the thermal field. By combining those strategies, we further demonstrate versatile hierarchical, superimposed, and local surface patterns based on this cPE/gold (Au) system. Piezoresistive pressure sensors are assembled with the obtained patterned surface, which have high sensitivity, operational range, and cyclic stability. These results highlight the unique advantages of SMPs for building arbitrary surface patterns.

Honeycomb-Lantern-Inspired 3D Stretchable Supercapacitors with Enhanced Specific Areal Capacitance.

Traditional stretchable supercapacitors, possessing a thin electrode and a 2D shape, have limited areal specific areal capacitance and are incompatible with 3D wearables. To overcome the limitations of 2D stretchable supercapacitors, it is highly desirable to develop 3D stretchable supercapacitors with higher mass loading and customizable shapes. In this work, a new 3D stretchable supercapacitor inspired by a honeycomb lantern based on an expandable honeycomb composite electrode composed of polypyrrole/black-phosphorous oxide electrodeposited on carbon nanotube film is reported. The 3D stretchable supercapacitors possessing device-thickness-independent ion-transport path and stretchability can be crafted into customizable device thickness for enhancing the specific areal energy storage and integrability with wearables. Notably, a 1.0 cm thick rectangular-shaped supercapacitor shows enhanced specific areal capacitance of 7.34 F cm-2 , which is about 60 times higher than that of the original 2D supercapacitor (120 mF cm-2 ) at a similar discharge rate. The 3D supercapacitor can also maintain a capacitance ratio of 95% even under the reversible strain of 2000% after 10 000 stretch-and-release cycles, superior to state-of-the-art stretchable supercapacitors. The enhanced specific areal energy storage and the customizablility in shapes of the 3D stretchable supercapacitors show immense promise in a wide range of applications in stretchable and wearable electronics.

Highly stretchable, rapid-response strain sensor based on SWCNTs/CB nanocomposites coated on rubber/latex polymer for human motion tracking

PurposeThe purpose of this study is to present a highly stretchable and flexible strain sensor with simple and low cost of fabrication process and excellent dynamic characteristics, which make it suitable for human motion monitoring under large strain and high frequency.Design/methodology/approachThe strain sensor was fabricated using the rubber/latex polymer as elastic carrier and single-walled carbon nanotubes (SWCNTs)/carbon black (CB) as a synergistic conductive network. The rubber/latex polymer was pre-treated in naphtha and then soaked in SWCNTs/CB/silicon rubber composite solution. The strain sensing and other performance of the sensor were measured and human motion tracking applications were tried.FindingsThese strain sensors based on aforementioned materials display high stretchability (500 per cent), excellent flexibility, fast response (approximately 45 ms), low creep (3.1 per cent at 100 per cent strain), temperature and humidity independence, superior stability and reproducibility during approximately 5,000 stretch/release cycles. Furthermore, the authors used these composites as human motion sensors, effectively monitoring joint motion, indicating that the stretchable strain sensor based on the rubber/latex polymer and the synergetic effects of mixed SWCNTs and CB could have promising applications in flexible and wearable devices for human motion tracking.Originality/valueThis paper presents a low-cost and a new type of strain sensor with excellent performance that can open up new fields of applications in flexible, stretchable and wearable electronics, especially in human motion tracking applications where very large strain should be accommodated by the strain sensor.

Fabrication of Highly Stretchable, Washable, Wearable, Water-Repellent Strain Sensors with Multi-Stimuli Sensing Ability.

Stretchable and wearable sensors with active response to various environmental stimuli possess numerous potential applications in stretchable electronics, motion sensors, environmental monitoring, and so on. Herein, we report a new method to realize control on the local conductive networks of strain sensors, thus, their sensing behavior. These multifunctional crack-based sensors were prepared via spray coating a mixture of carbon nanotube (CNT) and 3-aminopropyltriethoxysilane (KH550) with various ratios onto polydimethylsiloxane (PDMS). The conductive CNT/KH550 layer exhibits brittle mechanical behavior which triggers the formation of cracks upon stretching. This is thought to be responsible for the observed electromechanical behavior. These sensors exhibit adjustable gauge factors of 5-1000, stretchability (ε) of 2-250%, linearity (nonlinearity-linearity) and high durability over 1000 stretching-releasing cycles for mechanical deformation. Washable, wearable, and water-repellent sensors were prepared through such a method to successfully detect human physiological activities. Moreover, the variation in temperature or the presence of solvent can also be detected due to the thermal expansion and swelling of the PDMS layer. It is expected that such a concept could be used to prepare sensors for multiple applications, thanks to its multifunctionality, adjustable and robust performance, simple and low-cost fabrication strategy.