Stretchable Conductors Research Articles

Highly Stretchable and Transparent Double-Network Hydrogel Ionic Conductors as Flexible Thermal-Mechanical Dual Sensors and Electroluminescent Devices.

The latest generation flexible devices feature materials that are conductive, highly stretchable, and transparent to meet the requirements of a reliable performance. However, the existing conductors are mostly electronic conductors, which cannot satisfy these high-performance challenges. A robust hydrogel ionic conductor was rapidly fabricated with a facile one-pot approach by employing bioinspired agar with a physically cross-linked network, polyacrylamide (PAM) with a photoinitiated cross-linked network under appropriate UV intensity, and Li+ as conductive ions. The resulting Li+/agar/PAM ionic double-network hydrogels could be fabricated into various shapes through injection. The unique ionic hydrogel showed a remarkable stretchability with over 1600% extension, high tension/compression strength (0.22 MPa/3.5 MPa), and toughness (2.2 MJ/m3). Furthermore, it was demonstrated to possess dual sensory capabilities through the combination of both temperature and mechanical deformation. This hydrogel ionic conductor exhibited high stretching sensitivity with a gauge factor of 1.8 at strain 1100% and bending sensitivity in a broad angle range of 30-150°, respectively. Such highly optical transparency and elasticity endow the hydrogel-phosphor composites with promising luminescent properties. The multifunctional ionic hydrogel can be used as soft conductors for application in flexible devices such as ionic skin for wearable sensors and luminescence display.

Superelastic Hard Carbon Nanofiber Aerogels.

Superelastic carbon aerogels have been widely explored by graphitic carbons and soft carbons. These soft aerogels usually have delicate microstructures with good fatigue resistance but ultralow strength. Hard carbon aerogels show great advantages in mechanical strength and structural stability due to the sp3 -C-induced turbostratic "house-of-cards" structure. However, it is still a challenge to fabricate superelastic hard carbon-based aerogels. Through rational nanofibrous structural design, the traditional rigid phenolic resin can be converted into superelastic hard carbon aerogels. The hard carbon nanofibers and abundant welded junctions endow the hard carbon aerogels with robust and stable mechanical performance, including superelasticity, high strength, extremely fast recovery speed (860 mm s-1 ), low energy-loss coefficient (<0.16), long cycle lifespan, and heat/cold-endurance. These emerging hard carbon nanofiber aerogels hold a great promise in the application of piezoresistive stress sensors with high stability and wide detection range (50 kPa), as well as stretchable or bendable conductors.

Photocurable Stretchable Conductors with Low Dynamic Resistance Variation

A simple and facile method is developed to prepare photocurable conductive resin for stretchable conductors. To prevent substrate damages, intense pulsed light (IPL) technology was introduced to simultaneously cure photoresins and sinter the silver nanomaterials at room temperature. During the IPL illuminating, the intense pulsed light not only can heat the silver nanomaterials locally but also cure the polymeric resin at the same time. Compared to a regular curing–heating process, this process can form a more effective conductive pathway with a low percolation threshold. The high aspect ratio of AgNWs makes its own low percolation threshold of only 10 wt %. A lower percolation threshold can not only save the amount of expensive conductive materials but also retain the stretchability of elastomer. The combination (AgNWs/Ag flakes) as conductive fillers can maintain great electrical conductivity during the stretching process with only 25 wt % loading of silver. The resistance ratio became only 1.5 times th...

A Highly Stretchable Liquid Metal Polymer as Reversible Transitional Insulator and Conductor.

Materials with a temperature-controlled reversible electrical transition between insulator and conductor are attracting huge attention due to their promising applications in many fields. However, most of them are intrinsically rigid and require complicated fabrication processes. Here, a highly stretchable (680% strain) liquid metal polymer composite as a reversible transitional insulator and conductor (TIC), which is accompanied with huge resistivity changes (more than 4 × 109 times) reversibly through a tuning temperature in a few seconds is introduced. When frozen, the insulated TIC becomes conductive and recovers after warming. Both the phase change of the liquid metal droplets and the rigidity change of the polymer contribute directly to transition between insulator and conductor. A simplified model is established to predict the expansion and connection of liquid metal droplets. Along with high stretchability, straightforward fabrication methods, rapid triggering time, large switching ratio, good repeatability, the TIC offers tremendous possibilities for numerous applications, like stretchable switches, semiconductors, temperature sensors, and resistive random-access memory. Accordingly, a system that can display numbers and letters via converting alternative TIC temperature to a binary signal on a computer is conceived and demonstrated. The present discovery suggests a general strategy for fabricating and stimulating a stretchable transitional insulator and conductor based on liquid metal and allied polymers.

Highly Stretchable Organogel Ionic Conductors with Extreme-Temperature Tolerance

Stretchable ionic conductors with multiple functions provide new opportunities for flexible electronics, soft robots, and wearable devices. However, the practical applications of the existing stret...

Biomimetic, recyclable, highly stretchable and self-healing conductors enabled by dual reversible bonds

Utilization of self-healing chemistry and biomimetic structure design, to develop stretchable conductors that are self-healing and recyclable, is of great interest. Herein, we utilize small molecular (triethanolamine, TEA) instead of common polymer segments as a spacer and cross-linker, which could render the dynamic covalent/noncovalent thermosets with higher density of reversible bonds (H-bonds and disulfide bonds) and more cross-linked points through controlling mole fraction due to more hydroxyl group per molecular weight of TEA. The as-prepared thermosets exhibit excellent thermal stability, shape memory effect, recyclability and healing efficiency at relatively moderate temperature. Then, a series of mechanically and electrically self-healing conductive tendrils composed of self-healing/shape memory polymers and conductive percolation networks are fabricated, which possess enhanced conductivity retention capability under stretching states. Moreover, our designed helical-structured conductors can be not only rehealed but also recycled and reprocessed due to the simultaneous destruction and reestablishment of the reversible bonds. Strikingly, the fabrication process of self-healing conductive tendrils is compatible to the conventional technique of conductors including silk-screen printing and adhesive bonding technique. This biomimetic approach opens a promising pathway to fabricate wearable intelligent devices with versatile functions.

Liquid metal-filled magnetorheological elastomer with positive piezoconductivity

Conductive elastic composites have been used widely in soft electronics and soft robotics. These composites are typically a mixture of conductive fillers within elastomeric substrates. They can sense strain via changes in resistance resulting from separation of the fillers during elongation. Thus, most elastic composites exhibit a negative piezoconductive effect, i.e. the conductivity decreases under tensile strain. This property is undesirable for stretchable conductors since such composites may become less conductive during deformation. Here, we report a liquid metal-filled magnetorheological elastomer comprising a hybrid of fillers of liquid metal microdroplets and metallic magnetic microparticles. The composite’s resistivity reaches a maximum value in the relaxed state and drops drastically under any deformation, indicating that the composite exhibits an unconventional positive piezoconductive effect. We further investigate the magnetic field-responsive thermal properties of the composite and demonstrate several proof-of-concept applications. This composite has prospective applications in sensors, stretchable conductors, and responsive thermal interfaces.

Stretchable and electrically conductive polyurethane- silver/graphene composite fibers prepared by wet-spinning process

In recent years, much research has been conducted on wearable electronic devices based on human-friendly factors, and fibers having stretchability and electrical conductivity are required as a component technology of fiber-based wearable electronics. Here we demonstrate stretchable and electrically conductive polyurethane (PU)-silver/graphene composite fibers prepared by wet-spinning process, in which silver nanoparticles (AgNPs) and graphene nanoplatelets (GNPs) are added as conductive fillers. Although the aspect ratio of AgNPs is relatively small compared to nanowires and nanotubes, high conductivity and stretchability have been observed in PU-AgNPs composites, which depends on the volume fraction of AgNPs in the composites. In addition, the conductivity was improved by curing treatment and addition of a small amount of GNPs, and a high conductivity of 82,874 S m−1 was achieved with a ∼150% strain. Conventional yarn-like conductive threads consisting of six strands of PU-AgNPs/GNPs composite fibers showed a relatively low resistance change up to 30% strain and operated as stretchable conductor up to 100% stretch in a circuit. Thus, it is expected that the PU-AgNPs/GNPs composite fibers can be applicable to human costume, industrial robots, and wearable electronics such as adaptive sensor for body movement measurement or medical monitoring, and biological integration systems for rehabilitation or injury prevention.

A nonequilibrium thermodynamics approach to the transient properties of hydrogels

A polymer network can imbibe copious amounts of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as ionic cables, stretchable conductors, and fire resistance materials, the properties of hydrogels may change in response to the environment stimuli. However, existing researches about the transient properties of hydrogels usually treat different physical processes (mass diffusion, heat transportation, electric conduction, etc.) separately, ignoring the fact that there exist correlations between them. In the present work, from the point of view of nonequilibrium thermodynamics, the coupled non-linear governing equations for the transient properties of hydrogels are derived. Instead of writing the free energy of hydrogels as a combination of the stretching of polymer networks and mixing with solvent, we derive the free energy in the form of swelling of hydrogels plus stretching of the swollen hydrogels, which is a function of water content and stretch ratio of hydrogels under external forces or constraints. Chemical potential and total deformation of polymer networks are the fundamental parameters to characterize hydrogel states in many previous research. These are hard to determine, and we are less interested in these parameters. The new form of free energy is more suitable to be used in the laboratory or for the application of hydrogels in real situations, and its physical meaning is clearer. At last, a numerical simulation of the drying process of a hydrogel film attached on a substrate is carried out based on the governing equations and free energy. The transient water content distribution, temperature profile and stress field are investigated, which may provide a guide to the diverse applications of hydrogels.

Three-Dimensionally Printed Stretchable Conductors from Surfactant-Mediated Composite Pastes.

A stretchable conductor is a critical prerequisite to achieve various forms of stretchable electronics. In particular, directly printable stretchable conductors have gathered considerable attention with recent growing interest in a variety of large-area, deformable electronics. In this study, we have developed a chemical pathway of incorporating a surfactant with a moderate hydrophilic-lipophilic balance in formulating composite pastes for printed stretchable conductors, with a possibility of a vertically stackable, three-dimensional printing process. We demonstrate that the addition of a nonionic surfactant, sorbitane monooleate (commonly called SPAN 80) in Ag flake-based composite pastes, allows a critical reduction in resistance variation under an external strain. The four-layer stacked, surfactant-added composite conductors show a resistance variation of merely 1.6 at a strain of 0.6 and excellent cycling durability over 1000 cycles. The effectiveness of the methods suggested in this study is demonstrated with basic light-emitting diode circuits and the thermal heating characteristics of stretchable conductors.

A General Approach for Buckled Bulk Composites by Combined Biaxial Stretch and Layer‐by‐Layer Deposition and Their Electrical and Electromagnetic Applications

AbstractFabrication of high quality stretchable conductors that show bulk structure, high electrical conductivity, and stable conductance under large deformations is crucial for wearable electronics and soft robots. A key difficulty here is to introduce buckled structure into the conductive phase of the bulk conductors and overcome the lateral crack problem in the conductive phase during buckle formation. In this paper, a general approach is introduced for fabricating novel buckled bulk composite (BBC) stretchable conductors based on common conductive nanomaterial by using sequential biaxial stretch–release and layer‐by‐layer deposition to provide alternating buckled conductive layers and elastomer layers. This biaxial stretch is used to keep the width of elastomers unchanged before and after stretching, to avoid the lateral crack formation of the conductive layer. This method can be applied to common nanoparticles, such as silver nanoparticles, silver nanowires, single‐walled carbon nanotubes, and graphenes. The BBCs exhibit high conductance, uniform structure, and stable conductance during deformation and temperature change, which are demonstrated as applications in electrical interconnects, electrothermally driven artificial muscles, electromagnetic interference shielding materials, and reconfigurable antennas.

Stretchable Nanocomposite Conductors Enabled by 3D Segregated Dual‐Filler Network

AbstractFlexible and stretchable electrical conductors are needed in a wide range of applications to maintain electrical conductance under large deformation. Herein a new type of stretchable electrical conductor capable of large deformation is reported. Using a simple and cost‐effective dip‐coating technique, conductive nanomaterials including sliver nanowires (AgNWs) and carbon nanofibers (CNFs) are deposited onto the surface of polyurethane foam, forming a 3D segregated conductive network. The pores are subsequently infused with elastomeric polymethylsiloxane. Upon curing, the resultant nanocomposite is highly conductive even under large strain. Compared with nanocomposites made of a single type of conductive nanomaterial, the nanocomposite obtained by sequential dip‐coating with AgNWs and then CNFs give the best performance, due to the synergetic effect between the two conductive nanomaterials. This highly conductive nanocomposites can withstand large mechanical deformations, including stretching, twisting, and bending while maintain its high electrical conductivity. With a low‐cost and simple fabrication process, these newly developed conductive nanocomposites offer a new type of stretchable conductors for wearable electronics.

Ionic liquid enabled electrical-strain tuning capability of carbon black based conductive polymer composites for small-strain sensors and stretchable conductors

Conductive carbon black (CB) filled styrene-butadiene rubber composites with tuneable electrical-strain behaviour were prepared based on the use of ionic liquid (IL). The addition of IL is also capable of improving electrical conductivity and flexibility, thus, enabling use in applications such as small-strain sensors and stretchable conductors. For small-strain sensors, at small strains up to 9%, the composite with IL/CB ratio of 1.5 gives sensitivity up to about 700% higher than that without IL. Also, the sensing consistency is enhanced with increased IL/CB ratios. For stretchable conductors, the use of IL makes the alignment of the aggregate more effective, thus allowing an extension of the maximum conducting strain. This can be extended from less than 40% to approximately 180% using an IL/CB ratio of 1.5 in combination with an adjusted mixing time.

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.

Flexible ITO-free organic solar cells over 10% by employing drop-coated conductive PEDOT:PSS transparent anodes

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonic acid) (PEDOT:PSS) has been explored to fabricate flexible and stretchable conductors. Generally, PEDOT:PSS transparent anodes are prepared by spin-coating method. In this article, we adopt a method by dropping PEDOT:PSS aqueous solution on the PET plastic substrate to fabricate flexible electrodes. Compared with spin coating, drop-coating is simple and cost-effective with large-area fabrications. Through this method, we fabricated highly transparent conductive electrodes and systematically studied their electrical, optical, morphological and mechanical properties. With dimethyl sulfoxide/methanesulfonic acid (DMSO/MSA) treated PEDOT:PSS electrode, bendable devices based on non-fullerene system displayed an open-circuit voltage of 0.925 V, a fill factor of 70.74%, and a high power conversion efficiency (PCE) of 10.23% under 100 mW cm−2 illumination, which retained over 80% of the initial PCE value after 1000 bending cycles. Based on the findings, drop-coated PEDOT:PSS electrodes exhibited high suitability for the development of large-area and high-efficiency printed solar cell modules in the future.

Superior Stretchable Conductors by Electroless Plating of Copper on Knitted Fabrics

Stretchability is critical to wearable devices to afford a large amplitude strain. Herein, a uniformly nanoscale copper layer was successfully deposited onto the surface of a knitted fabric by a fa...

Interface Modified Flexible Printed Conductive Films via Ag2O Nanoparticle Decorated Ag Flake Inks.

A new approach to stable, low resistance inexpensive printed flexible conductive inks is proposed. Silver inks have been extensively studied and commercialized for applications in printed electronics due to the inherent high conductivity and stability of silver, even in particulate-based percolation networks processed at temperatures compatible with low-cost polymer films such as poly(ethylene terephthalate) (PET). Recent interest in flexible and even stretchable circuits, however, has presented new challenges for particle-based inks as mechanical strains can result in the opening of critical particle-to-particle contacts. Here we report a facile, low-cost method for the single-step synthesis of stable, printable nanoscale Ag2O decorated Ag flake inks which can be converted to highly conductive Ag films at 150 °C curing temperature without the use of limited shelf life organometallics or low metal loading nanoparticles to modify the interface between silver flakes. Analysis indicates that decoration of Ag flakes with Ag2O nanoparticles (NPs) during ink synthesis improves the conductivity and flexibility of printed silver films by forming bridging interconnections between Ag flakes after low temperature reduction of the Ag2O NPs. In this work, printed nanodecorated silver conductors with starting oxide to metal weight ratios of 5:95 exhibited lateral resistivities lower than 1.5 × 10-5 Ω·cm, which was 35% less than films derived from undecorated Ag flake inks of the same total Ag loading and binder system. This resistivity difference increased to 45% after cyclic bend testing showing increased resilience to repeated flexing for the nanodecorated inks. Through detailed compositional and morphological characterizations, we demonstrate that such improved conductivity and flexibility are due to a more effective bridging afforded by the in situ synthesized Ag NPs on the surface of Ag flakes. These properties, combined with the simplified syntheses method of the nanoink, make the material a viable, advantageous alternative to the limited number of stretchable conductors currently available.

Heterogeneous Surface Orientation of Solution-Deposited Gold Films Enables Retention of Conductivity with High Strain—A New Strategy for Stretchable Electronics

Stretchable electronic devices rely on stretchable conductors to form device interconnects and electrodes that maintain electrical performance during deformation. Although the high conductivity of ...

Harnessing the Rheological Properties of Liquid Metals To Shape Soft Electronic Conductors for Wearable Applications.

Emerging applications of the Internet of Things in healthcare, wellness, and gaming require continuous monitoring of the body and its environment, fueling the need for wearable devices able to maintain intimate, reliable, and unobtrusive contact with the human body. This translates in the necessity to develop soft and deformable electronics that match the body's mechanics and dynamics. In recent years, various strategies have been proposed to form stretchable circuits and more specifically elastic electrical conductors embedded in elastomeric substrate using either geometrical structuring of solid conductors or intrinsically stretchable materials. Gallium (Ga)-based liquid metals (LMs) are an emerging class of materials offering a particularly interesting set of properties for the design of intrinsically deformable conductors. They concomitantly offer the high electrical conductivity of metals with the ability of liquids to flow and reconfigure. The specific chemical and physical properties of Ga-based LMs differ fundamentally from those of solid conductors and need to be considered to successfully process and implement them into stretchable electronic devices. In this Account, we report on how the key physical and chemical properties of Ga-based LMs can be leveraged to enable repeatable manufacturing and precise patterning of stretchable LM conductors. A comprehensive understanding of the interplay between the LM, its receiving substrate chemistry and topography, and the environmental conditions is necessary to meet the reproducibility and reliability standards for large scale deployment in next-generation wearable systems. In oxidative environments, a solid oxide skin forms at the surface of the LM and provides enough stiffness to counterbalance surface tension, and prevent the LM from beading up to a spherical shape. We review techniques that advantageously harness the oxide skin to form metastable structures such as spraying, 3D printing, or channel injection. Next, we explore how controlling the environmental condition prevents the formation or removes the oxide skin, thereby allowing for selective wetting of Ga lyophilic surfaces. Representative examples include selective plating and physical vapor deposition. The wettability of LMs can be further tuned by engineering the surface chemistry and topology of the receiving substrate to form superlyophobic or superlyophilic surfaces. In particular, our group developed Ga-superlyophilic substrates by engineering the surface of silicone rubber with microstructures and a gold coating layer. Thermal evaporation of Ga on such engineered substrates allows for the formation of smooth LM films with micrometric thickness control and design freedom. The versatility of the available deposition techniques facilitates the implementation of LM conductors in a wide variety of wearable devices. We review various epidermal electronic systems using LM conductors as interconnects to carry power and information, transducers and sensors, antennas, and complex hybrid (soft-rigid) electronic circuits. In addition, we highlight the limitations and challenges inherent to the use of Ga LM conductors that include electromigration, corrosion, solidification, and biocompatibility.

Self-healing Interconnects with Nearly Plastic Stretching of Repairs

Flexible electronic systems such as roll up displays, wearable devices etc. promise exciting possibilities that could change the way humans interact with the environment. However, they suffer from poor reliability of interconnects and devices. Interconnects on flex are prone to open circuit failures due to mechanical stress, electrostatic discharge and environmental degradation. Passive approaches such as the use of stretchable conductors and novel geometries improve their response to mechanical stress but cannot salvage the interconnect if a fault were to occur. Active approaches using self healing techniques can repair a fault and have been demonstrated using methods that either use relatively rare materials, change conventional interconnect fabrication processes, only address faults due to mechanical stress or do not permit stretching. In this work we discuss a self healing technique that overcomes these limitations and demonstrate heals having metallic conductivity and nearly plastic stretchability. This is achieved using a dispersion of conductive particles in an insulating fluid encapsulated over the interconnect. Healing is automatically triggered by the electric field appearing in the open gap of a failed interconnect, irrespective of the cause of failure. The field polarizes the conductive particles causing them aggregate and chain up to bridge the gap and repair the fault. Using copper-silicone oil dispersions, we show self healing interconnects with the stretchable heal having conductivity of about $5 \times 10^{5}$ Sm and allowing strains from 12 to 60. Previously, stretchable interconnects used materials other than copper. Here we effectively show self healing, stretchable copper. This work promises high speed, self healing and stretchable interconnects on flex thereby improving system reliability.