Stretchable Electronics Research Articles

Mechanically Durable Memristor Arrays Based on a Discrete Structure Design.

Memristors constitute a promising functional component for information storage and in-memory computing in flexible and stretchable electronics including wearable devices, prosthetics, and soft robotics. Despite tremendous efforts made to adapt conventional rigid memristors to flexible and stretchable scenarios, stretchable and mechanical-damage-endurable memristors, which are critical for maintaining reliable functions under unexpected mechanical attack, have never been achieved. Here, the development of stretchable memristors with mechanical damage endurance based on a discrete structure design is reported. The memristors possess large stretchability (40%) and excellent deformability (half-fold), and retain stable performances under dynamic stretching and releasing. It is shown that the memristors maintain reliable functions and preserve information after extreme mechanical damage, including puncture (up to 100 times) and serious tearing situations (fully diagonally cut). The structural strategy offers new opportunities for next-generation stretchable memristors with mechanical damage endurance, which is vital to achieve reliable functions for flexible and stretchable electronics even in extreme and highly dynamic environments.

A Catalytic and Interfacing PEDOT:PSS/CuPc Polymerized on Cloth Fiber to Electro‐Metalize Stretchable Copper Conductive Pattern

AbstractCopper conductive pattern should provide dependent flexibility on coated cloth‐fiber to support the signal transmission or current carrier for stretchable and reliable electronics. The design of copper phthalocyanine (CuPc) introduced in poly(3,4‐ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) polymer network promotes the realization of copper electro‐metallization from its inner active dots, leading to a structure of PEDOT:PSS/CuPc‐rooted copper coating. The bridge‐layer of elastic PEDOT:PSS/CuPc interfacing and catalyzing the copper layer with excellent ductility and antioxidation, is developed to sustain the required stretchability for electronics on regular cloth polyurethane fiber. Experimental results demonstrate that this design achieves a perfect 80% conductivity of bulk copper, an ignored resistance changes after bending test, tensile test, 10 days’ natural oxidation, and 10 times’ machine‐washing, respectively. Accordingly, a breakthrough is made to supply the compatible integrity of copper line on cloth fiber for wearable electronics.

Designable Integration of Silicide Nanowire Springs as Ultra-Compact and Stretchable Electronic Interconnections.

Stretchable electronics are finding widespread applications in bio-sensing, skin-mimetic electronics, and flexible displays, where high-density integration of elastic and durable interconnections is a key capability. Instead of forming a randomly crossed nanowire (NW) network, here, a large-scale and precise integration of highly conductive nickel silicide nanospring (SiNix -NS) arrays are demonstrated, which are fabricated out of an in-plane solid-liquid-solid guided growth of planar Si nanowires (SiNWs), and subsequent alloy-forming process that boosts the channel conductivity over 4 orders of magnitude (to 2 × 104 S cm-1 ). Thanks to the narrow diameter of the serpentine SiNix -NS channels, the elastic geometry engineering can be accomplished within a very short interconnection distance (down to ≈3µm), which is crucial for integrating high-density displays or logic units in a rigid-island and elastic-interconnection configuration. Deployed over soft polydimethylsiloxane thin film substrate, the SiNix -NS array demonstrates an excellent stretchability that can sustain up to 50% stretching and for 10000 cycles (at 15%). This approach paves the way to integrate high-density inorganic electronics and interconnections for high-performance health monitoring, displays, and on-skin electronic applications, based on the mature and rather reliable Si thin film technology.

Size effect on stretchability of Cu films on wrinkled polydimethylsiloxane substrates

Stretchable electronics demand that the metal thin film electrodes have the capacity to experience large level of strains without obvious changes in electrical performance. This work investigated the effect of film thickness and wrinkle dimension on the stretchability of Cu films on polydimethylsiloxane substrates. It is revealed that the wrinkle dimension highly depends on the ultraviolet/ozone treatment time. The fracture strain increases until reaching a maximum and then decreases with increasing film thickness for smaller wrinkles. By contrast, the fracture strain monotonically increases as the film thickness increases for the larger wrinkles within the thickness range studied. Generally, the larger wrinkle dimension leads to a higher fracture strain for a given film thickness. The evolution of fracture strain can be rationalized in terms of the competition between the intrinsic stretchability of Cu films related to the grain size and the extrinsic strain concentration related to film thickness and wrinkle dimension.

Rational Assembly of Liquid Metal/Elastomer Lattice Conductors for High‐Performance and Strain‐Invariant Stretchable Electronics

AbstractHighly stretchable and conductive composites have gained tremendous research interests due to the imperative demands in the fields of stretchable electronics and soft robotics. However, it is challenging to maintain the original performance of the composites under complex external deformations. Here, a one‐step dual‐material 3D printing technique is developed to rationally assemble liquid metal (LM) into an elastomer lattice. The 3D interconnected and deformable liquid conductive network is supported by a highly ordered and robust polydimethylsiloxane lattice skeleton, yielding the resultant composites high electrical conductivity (1.98 × 106 S m−1), stretchability (180%), and electromagnetic interference (EMI) shielding effectiveness (72 dB). Unlike those composites with dispersed fillers, the LM/elastomer lattice composites deliver negligible electromechanical coupling, showing a negative resistance change of only −2% at a large tensile strain of 100%. The composites also exhibit strain‐invariant EMI shielding performance in a strain range of 0–100%, and present exceptional stability over 1000 rigorous cycles of stretching and releasing. The applications of the composites in flexible display circuits, microwave shielding layer, and EMI shields in wireless power transmission systems are demonstrated. The current findings suggest an effective strategy for fabricating LM‐based composites with precisely controlled and unprecedented multi‐functionality.

Multifunctional Ternary Hybrid Hydrogel Sensor Prepared via the Synergistic Stabilization Effect.

Since highly stretchable hydrogels have demonstrated their promising applications in flexible tactile sensors and wearable devices, the current challenge has been imposed on stretchable and multifunctional electronics. Here, we report a multifunctional sensor composed of a liquid metal (LM) nanodroplet-adhered self-assembled polymeric network, anionic carboxymethylcellulose (CMC), and cationic polyacrylamide (PAAm). The synergistic effect, zeta potential reduction, by CMC and macromolecules enveloped by LM contributes to the stabilization of the ternary system during preparation and, thus, the homogenization of the products. By engineering and optimizing the ternary hybrid hydrogels, excellent extensibility (tensile strain near 300%), readily reversible hysteresis loops, and accessible deformability (low modulus of 104 Pa) are afforded. The fabricated sensor exhibits a high tensile strain gauge factor of around 0.7 and a high compressive stress sensitivity of up to 0.12 kPa-1, a fast response time below 125 ms, and a high stability and precision in usage. In a series of practical scenarios, the assembled sensor displays distinguished abilities to monitor bodily motions, record electrocardiograms, authenticate handwriting, discern temperature, and infer materials, making them highly promising for multifunctional intelligent soft sensing.

Structural Analysis and Material Characterization of Silver Conductive Ink for Stretchable Electronics

Stretchable electronic systems have become more popular in various applications such as medical, fabric, flexible sensors for personalized health care, etc. There are two major parts of flexible and stretchable circuit boards that are substrate (a plastic material) and conductive ink (formulated polymer with conductive metal). According to electrical measurements, conductive ink plays a very important role in stretchable electronic equipment. The main objective of this paper is to develop a silver (Ag) based conductive ink and characterize its mechanical and electrical properties. Conductive ink is prepared by mixing an epoxy resin, cross –linking agent, additives (adhesion promoter), organic solvent, catalyst and silver flakes all together. ASTM D412 Type C dog bone shaped cutter is used to make three samples of conductive ink. The stress-strain analysis of conductive ink is carried out using universal testing machine (UTM). The conductivity is measured using two-point probe digital multi-meter. Also, the microstructural analysis, morphology and characterization are done by scanning electron microscopy (SEM). The images are taken after curing and tensile testing. The formulated ink possesses high conductivity and stretchability up to 137% strain. The achieved conductivity of the ink is 4.167×104S/m. The maximum stress before failure, yield stress, Young’s and tangent moduli are calculated as 1.195 MPa, 0.86 MPa, 5.72 MPa and 2.08 MPa, respectively. The SEM analysis indicates that the distribution of silver particles is uniform and in a good density throughout the sample.

Synergistic contribution of flexoelectricity and piezoelectricity towards a stretchable robust nanogenerator for wearable electronics

The fabrication of a high-performance piezoelectric nanogenerator (PENG) with high stretchability and durability is desirable for the next-generation of stretchable and wearable electronics. Herein, an eco-friendly and stretchable flexoelectricity-enhanced piezoelectric nanogenerator (F-PENG) based on zinc-aluminum layered double hydroxide nanosheets (ZnAl:LDH Ns)-ZnO heterostructure is demonstrated on stretchable polydimethylsiloxane (PDMS) substrates. The vertically-oriented eco-friendly ZnAl:LDH Ns are facilely synthesized by dipping the 10 wt% aluminum-doped zinc oxide (AZO) thin films in deionized (DI) water at room temperature. The enhanced output performance of the F-PENG is demonstrated under tapping, bending, and stretching modes, and is attributed to the synergistic flexoelectric and piezoelectric effects. The achieved maximum output power density of F-PENG under tapping is ~2.7 µW/cm2. The pressure-sensing capability of the F-PENG is demonstrated by the generated outputs under the three applied modes. In addition, the biomechanical energy harvesting capability of the F-PENG is demonstrated by subjecting it to various biomechanical motions. The F-PENG exhibits an excellent mechanical durability in all three modes of operation. The present study not only paves the way towards the facile fabrication of stretchable and high-performance F-PENGs with combined flexoelectric and piezoelectric effects, but also validates a wide range of applications in the next generation of stretchable and wearable electronics.

Organic Semiconductor/Insulator Blends for Elastic Field‐Effect Transistors and Sensors

AbstractOrganic semiconductors encounter limitations in their practical applicability in future electronics due to their low environmental stability and poor charge carrier mobilities. Blending with isolation of thermoplastic polymers and elastomers circumvents these restrictions and even induces new material properties, opening the door to novel flexible and stretchable electronics that hold great potential for improving people's life. This review discusses next generation applications of solution processable organic semiconductor/insulator blends in organic field‐effect transistors (OFETs). The fundamental basis is a comprehensive understanding of the phase separation mechanism that determines the morphology formation and electronic properties of the thin blend film. Continuous charge carrier pathways in blend OFETs are established by controlled phase separation through the chemical structure of components and the processing conditions. Recent advances in organic semiconductor/insulator blends with enhanced device properties including charge carrier mobility, life‐time, sensing ability, and especially mechanical behavior are reviewed with emphasis on implication in flexible and stretchable electronics. The concept of tuning existing properties and creating new ones of electronically active materials by blending with well‐selected insulators has great potential also for other types of electronic devices and classes of semiconductors.

3D printing-directed flexible strain sensors of accordion-like architecture to achieve ultrastretchability with the assist of ultrasonic cavitation treatment

A new class of accordion-like cellular architecture with sinusoidal struts is designed to enhance the planar stretchability of cellular solids, aiming to fabricate flexible strain sensors with ultrastretchability. The combination manufacturing process of fused deposition modeling (FDM) 3D printing technique and ultrasonic cavitation-enabled treatment was introduced into the fabrication of flexible strain sensors made of thermoplastic polyurethane (TPU) substrate and carbon nanotubes (CNTs). A negative Poisson’s ratio (NPR) architecture made of TPU was firstly 3D-printed by FDM. The ultrasonic cavitation treatment was then conducted on the soft auxetic structure immersing in CNTs liquid, aiming to embed the CNTs into the surface layer of the flexible TPU substrate with NPR configurations. Instead of 3D printing the TPU matrix composite after hybridization inside the matrix material, the hybrid manufacturing procedure can ensure that the intrinsic excellent mechanical properties of TPU are not embrittled. Besides, the sinusoidal struts in accordion-like cellular architectures offer a design route to extend the material property chart to achieve ultrahigh stretchability in lightweight 3D printable flexible polymers for the applications that require combined stretchability, lightweight, and energy absorption such as soft robotics, stretchable electronics, and wearable protection shields.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems, including flexible electronics, stretchable electronics, and micro light-emitting diode (LED) displays. Here we report a design of switchable dry adhesive based on shape memory polymer (SMP) with hemispherical indenters, which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP. Experimental and numerical studies reveal the fundamental aspects of design, fabrication, and operation of the switchable dry adhesive. Demonstrations of this adhesive concept in transfer printing of flat objects (e.g., silicon wafers), three-dimensional (3D) objects (e.g., stainless steel balls), and rough objects (e.g., frosted glasses) in two-dimensional (2D) or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Stretchable Carbon Nanotube Thin-Film Transistor Arrays Realized by a Universal Transferable-Band-Aid Method

Stretchable electronics offer new possibilities for humans and artificial intelligent robots, in which stretchable transistors are the core components. Either geometric engineering or intrinsic material integration has its own limitations for realizing high-performance stretchable transistors. In this work, by combining the advantages of both approaches, we propose a novel transferable-Band-Aid method to realize stretchable carbon nanotube (CNT) thin-film transistor (TFT) arrays with high electrical performance and stretchability simultaneously. This method uses polyimide (PI) tapes to transfer devices to elastomer substrates. The mobility and ON/OFF-current ratio of the stretchable CNT-TFTs reach 24 cm2/ $\text{V}\cdot \text{s}$ and $1.1\times10$ 5, respectively. After stretched by 50% and 100% for 2000 cycles, the device performance is maintained on both polydimethylsiloxane and Eco-flex substrates. Neither obvious degradation on electrical properties nor displacement between the PI and the elastomer is observed during the stretching. The mechanical performance of the stretchable devices has also been verified and analyzed by simulation using ANSYS. Young’s modulus ratio between the stiff island and the elastomer substrate influences stretchability and stress distribution of the devices. The simulation result provides a guideline for realizing highly robust stretchable devices. The Band-Aid structure could generally be stuck to most elastomer substrates, providing a new solution to convert flexible devices into stretchable devices universally. The demonstrations of the design exhibit its excellent transferability, conformal capability, and versatility, showing its application potentials in wearable devices, biological-human interfaces and Internet of Things.

Theory of soft solid electrolytes: Overall properties of composite electrolytes, effect of deformation and microstructural design for enhanced ionic conductivity

Re-chargeable batteries are an important element in the solution to the future of portable energy storage. In that context, electrolytes play a critical role in the adjudication of a battery’s capacity, efficiency and operation. In most current embodiments of rechargeable batteries, including the widely researched lithium-ion system, electrolytes are liquid. That said, the use of solid electrolytes in lieu of their liquid counterparts is being increasingly explored. Solid electrolytes offer increased stability and improved safety and (specifically polymers) are also ideal candidates in the context of stretchable and flexible electronics. Unfortunately, most solid polymer electrolytes possess ionic conductivity that is orders of magnitude lower than typical liquid electrolytes. In this work, we explore the possibility of microstructure design to enhance the effective ionic conductivity of composite polymer electrolytes. We develop a theoretical framework, grounded in thermodynamics and principles of continuum mechanics, to model the coupled deformation, electrostatics and diffusion in heterogeneous electrolytes. Guided by numerical thought experiments, we establish a homogenization procedure that significantly reduces the complexity of the original non-linear problem and derive general analytical expressions for the effective behavior of electrolytes and closed-form solutions for specific microstructures. Using insights from our model, in addition to proposing suggestions for improving the ionic conductivity of solid electrolytes, we are also able to explain several perplexing experimental observations and address questions such as: (i) Why do some experiments show a dramatic improvement in the ionic conductivity of nanoparticle-impregnated polymers while others yield a decrease? (ii) What is the effect of deformation on ionic conductivity?

Stretchable, Washable, and Rewritable Electrophoretic Displays with Tough Hydrogel–Elastomer Interface

AbstractTremendous advances in stretchable and biocompatible electronics have led to broad requirements of wearable display devices. However, most devices suffer from insufficient reliability owing to structural mismatch and bubbles under repeated deformation. To address this issue, this article presents a stretchable electrophoretic display (SEPD) with reinforced adhesion by in situ adhesion decoration between the hydrogel electrode and the elastomer display layer. The SEPD with a maximum strain of over 80% can still operate properly. Moreover, the SEPD device is endowed with stretchable, washable, rewritable, and erasable properties owing to the inherent characteristics of the elastomer‐based materials used. Arbitrary texts and patterns can be drawn and maintained for over 43 days of bistability without a predetermined patterned electrode or a thin‐film transistor as the addressing switches. In addition, the casting and blade coatings used in this work are suitable for mass production. The proposed hybrid structure with tough adhesion provides a new paradigm for other wearable devices.

Self-Patterned Stretchable Electrode Based on Silver Nanowire Bundle Mesh Developed by Liquid Bridge Evaporation.

A new strategy is required to realize a low-cost stretchable electrode while realizing high stretchability, conductivity, and manufacturability. In this study, we fabricated a self-patterned stretchable electrode using a simple and scalable process. The stretchable electrode is composed of a bridged square-shaped (BSS) AgNW bundle mesh developed by liquid bridge evaporation and a stretchable polymer matrix patterned with a microcavity array. Owing to the BSS structure and microcavity array, which effectively concentrate the applied strain on the deformable square region of the BSS structure under tensile stretching, the stretchable electrode exhibits high stretchability with a low ΔR/R0 of 10.3 at a strain of 40%. Furthermore, by exploiting the self-patterning ability—attributable to the difference in the ability to form liquid bridges according to the distance between microstructures—we successfully demonstrated a stretchable AgNW bundle mesh with complex patterns without using additional patterning processes. In particular, stretchable electrodes were fabricated by spray coating and bar coating, which are widely used in industry for low-cost mass production. We believe that this study significantly contributes to the commercialization of stretchable electronics while achieving high performance and complex patterns, such as stretchable displays and electronic skin.

Recent Advances in Stretchable Electronics: Materials, Mechanisms, and Applications

Recent Advances in Stretchable Electronics: Materials, Mechanisms, and Applications

Heat- and freeze-tolerant organohydrogel with enhanced ionic conductivity over a wide temperature range for highly mechanoresponsive smart paint

Binary solvent-based fabrication permits the conductive organohydrogel to function well at low-temperature environments. However, the deep cryogenic and high temperatures are still threatening the performance of conductive organohydrogels in the application of stretchable electronics, biosensors, and intelligent coatings. Here, a radically new method is developed to introduce propylene and carbonate cellulose nanofibrils into freeze tolerance polymer matrix, and fabricate an antifreezing/antiheating organohydrogel integrated a high mechanical strength (1.6 MPa) and high level of ionic conductivity (4.2 S cm−1) over a wide temperature range (−40 to 100 °C). In this designed system, the propylene carbonate with low freezing point and high boiling point was shown to enhance antifreezing (−40 °C) and antiheating (100 °C) performance of organohydrogel. Furthermore, negative charge-rich cellulose nanofibrils (CNFs) were served as an ion transport channel and nanoreinforcements to boost the conductive and mechanical properties of the organohydrogel. In particular, Molecular Dynamics (MD) simulations reveal that propylene carbonate with high dielectric constant is capable of generating ion migration-facilitated effects, enabling the high ionic conductivity of organohydrogel. Tapping into these attributes, potential applications in mechanoresponsive smart coating have been demonstrated utilizing the appealing organohydrogel as a paint, rendering unprecedented protection and monitoring performance.

(Invited) Multimodal Flexible Sensor System with Wireless Feedback Alarm Function

Flexible and stretchable electronics are now widely studied for a next class of electric devices replaced from the silicon-based conventional inflexible devices. In particular, multi-functional sensor sheets can contribute to the applications for Internet of Things concepts applied to infrastructure, vehicles, plants, and human owing to macroscale and conformable covering onto curved and soft surfaces. One of the interesting approaches to use the flexible sensor sheet is a healthcare and medical application to monitor vital signals from skin surface. Because of soft and flexible sensor system, it can be attached onto skin conformally and comfortably, resulting in that more vital signals can be detected unlike conventional wearable watch-based devices. In this application, chemical information such as pH, glucose, sodium ion, and potassium ion levels are very important to monitor as well as physical information including activity, temperature, heartbeat, and electrocardiogram (ECG) signals. Furthermore, simultaneous multiple condition monitoring of physical and chemical information is a next class of wearable devices for convenient and safe human life by integrating with wireless and signal processing circuit systems.To realize the multi-functional flexible sensor system platforms, in this talk, multimodal flexible physical and chemical sensor sheets will be discussed by developing low-cost and macroscale fabrication methods. Especially, ECG, respiration, perspiration, skin temperature, and human activity are monitored by using the developed sensor system. After understanding the fundamental properties and mechanism of the flexible sensors, integrated device applications with several sensors on a flexible film are demonstrated as proof-of-concepts for multi-functional e-skins. Finally, wireless and feedback alarm system integrated with the multimodal flexible sensor sheet are briefly discussed.

Superstretchable electrode based on hierarchical assembly of triblock copolymer fiber membrane

Stretchable electronics have attracted great interest due to their mechanical flexibility similar to tissues. In this paper, a superstretchable fiber membrane is designed based on step by step assembly of electrospun poly(styrene-ethylene/butylene-styrene) (SEBS), carbon nanotubes (CNTs) and electropolymerized polyaniline (PANi). The prepared soft membrane has a hierarchical micro-nano structure, forms a good interface with the triblock copolymer fiber, and shows good electromechanical performance. The electrical conductivity of the membrane is about 2.40 S·cm−1, the tensile strength and the maximum tensile strain can reach 8.2 MPa and 1200% respectively, while the Young’s modulus is only about 0.26 MPa. The conductivity of the prepared membrane remains stable after 1000 cycles, and the fractured composite membrane can be healedby hot pressing. In addition, this stretchable composite membrane has been successfully demonstrated to detect various human motions, and can even work underwater. Our results provide a simple and controllable method to achieve the stable soft stretchable electronics with large strain, and demonstrate promising potential in the expanded application of human motion monitoring.

Lithographically patterned stretchable metallic microwiring on electrospun nanofiber mats

Stretchable and flexible electronic applications require mechanically suitable electrical wiring. This article presents, for the first time, the microfabrication of photolithographically patterned microwiring on an electrospun nanofiber mat. The use of a photoresist-based technology allows for better than 10 μm pattern resolution on a good quality nanofiber mat, providing an order of magnitude smaller feature sizes than what has been published before. We demonstrate metallic wiring patterned from a 75 nm thick coating on top of the nanofiber mat. A silicone elastomer was incorporated to serve as a matrix material and form a composite substrate and an encapsulation layer on top of the microwiring. We demonstrate clean and anisotropic dry etching of the elastomer to open electrode sites that can be smaller than 10 μm in size. We speculate that these structures will be mechanically robust while being soft at the same time and provide the properties necessary for potential use in stretchable and flexible electronics.