Stretchable Conductors Research Articles

Double-layer stretchable composite conductive graphene-hydrogel with wide-range linear sensing and thermal-humidity management for health monitoring

Laser-induced graphene (LIG) is widely used in stretchable electrical conductors due to its excellent properties and low manufacturing cost. However, there is usually a mechanical mismatch or poor interface effect between LIG and the flexible substrate, and LIG will inevitably crack when stretched, which leads to LIG-based stretchable conductors usually have poor operating range, linearity and sensitivity stability. In this study, a double-layer stretchable composite conductive (DSCCGH) was prepared based on laser-induced MXene-decorated graphene (LIMG), with LIMG transferred to the hydrogel using a repeated reversible freeze-thaw method and a biomimetic structural design incorporated. The strong mechanical interlocking between LIMG and the hydrogel, along with the graphene-coated MXene derivative TiO2 providing additional electrical pathways when LIG’s conductive networks are disconnected, allowed this DSCCGH to achieve wide-range sensing linearity (0–366%, R2 = 0.9948) and stable sensitivity. This setup enabled stable and reliable muscle movement and pulse monitoring. Furthermore, biomimetic structural design imparts the DSCCGH with perspiration and cooling functions, enhancing comfort and wearability for outdoor exercise. This innovative design provides significant potential for future developments in wearable electronics and human-computer interaction fields.

Electrofluids with Tailored Rheoelectrical Properties: Liquid Composites with Tunable Network Structures as Stretchable Conductors.

Flexible and stretchable electronics require both sensing elements and stretching-insensitive electrical connections. Conductive polymer composites and liquid metals are highly deformable but change their conductivity upon elongation and/or contain rare metals. Solid conductive composites are limited in mechanoelectrical properties and are often combined with macroscopic Kirigami structures, but their use is limited by geometrical restraints. Here, we introduce "Electrofluids", concentrated conductive particle suspensions with transient particle contacts that flow under shear that bridge the gap between classic solid composites and liquid metals. We show how Carbon Black (CB) forms large agglomerates when using incompatible solvents that reduce the electrical percolation threshold by 1 order of magnitude compared to more compatible solvents, where CB is well-dispersed. We analyze the correlation between stiffness and electrical conductivity to create a figure of merit of first electrofluids. Sealed elastomeric tubes containing different types of electrofluids were characterized under uniaxial tensile strain, and their electrical resistance was monitored. We found a dependency of the piezoresistivity with the solvent compatibility. Electrofluids enable the rational design of sustainable soft electronics components by simple solvent choice and can be used both as sensor and electrode materials, as we demonstrate.

Multifunctional PDMS/Schiff base/SiO2 gel assisted fabrication of printed, stretchable and straight copper conductors

Printable, stretchable, and highly conductive metal conductors have emerged as potentially promising interconnects in the field of soft electronics. However, current techniques relying on geometric engineering have been criticized for complicated fabrication process and defective geometric structure. Here, we report a printed, straight, and omnidirectionally stretchable copper conductor, which is facilely fabricated on a flat substrate without requiring prestretching, assisted by a multifunctional gel medium layer. The PSS gel, composited by polydimethylsiloxane, Schiff base and fumed SiO2, offers a unique combination of stretchability, printability, and moldability. Additionally, this gel enables the reduction of silver ions directly on its surface by partially bonded crosslinker molecules. The fully exposed catalyst exhibits excellent activity in catalyzing electroless copper deposition reaction, on which high-quality copper coating is facilely achieved. The produced copper patterns exhibit high conductivity (4.93 × 107 S m−1) and excellent stretchability with fracture strains of ∼ 191 % and ∼ 135 % for the uni- and omni-directionally stretchable patterns respectively, which are superior to the majority of stretchable metals in published literature. The stretchable wrinkles derive form the gel medium layer molded by templates, realizing straight metal lines on flat substrate. This process combines the advantages of traditional in-plane and out-of-plane strategies, demonstrating great potential in practical application.

Liquid-free, tough and transparent ionic conductive elastomers based on nanocellulose for multi-functional sensors and triboelectric nanogenerators

Stretchable and transparent ionic conductors have attracted great interest due to their promising applications in flexible wearable electronics. The obvious drawbacks of gel-based ionic conductors such as hydrogels (e.g., evaporation or freezing of water) have driven the demand for liquid-free ionic conductors. This paper reports a new strategy for fabricating transparent, liquid-free ionic conductive elastomers based on renewable nanocellulose. A three-dimensional cellulose skeleton was constructed through ionic cross-linking, and the physically and chemically cross-linked dual network structure was prepared by in situ polymerization of the polymerizable deep eutectic solvent (PDES) therein. The homogeneous three-dimensional cross-linked network provides a site for energy dissipation and ionic migration. Results show that the elastomers retain good transparency and achieve significantly improved mechanical strength, toughness and ionic conductivity. Therefore, they can be applied as multi-functional sensors and triboelectric nanogenerators (TENG). For the optimized TENG, output voltage, current and charge reach 115 V, 6 μA, and 40 nC, respectively. A maximum output power density of 0.35 W/m2 is achieved, and the collected mechanical energy can light up LEDs and power an electronic clock. In addition, the elastomers maintain reliable performance even at low/high temperatures, enabling use in harsh environments. In conclusion, this study developed a promising strategy for the construction of sustainable liquid-free ionic conductors utilizing natural polysaccharides.

Exploring the Elastomer Influence on the Electromechanical Performance of Stretchable Conductors.

Stretchable electronics has received major attention in recent years due to the prospects of integrating electronics onto and into the human body. While many studies investigate how different conductive fillers perform in stretchable composites, the effect of different elastomers on composite performance, and the related fundamental understanding of what is causing the performance differences, is poorly understood. Here, we perform a systematic investigation of the elastomer influence on the electromechanical performance of gold nanowire-based stretchable conductors based on five chemically different elastomers of similar Young's modulus. The choice of elastomer has a huge impact on the electromechanical performance of the conductors under cyclic strain, as some composites perform well, while others fail rapidly at 100% strain cycling. The lack of macroscopic crack formation in the failing composites indicates that the key aspect for good electromechanical performance is not homogeneous films on the macroscale but rather beneficial interactions on the nanoscale. Based on the comprehensive characterization, we propose a failure mechanism related to the mechanical properties of the elastomers. By improving our understanding of elastomer influence on the mechanisms of electrical failure, we can move toward rational material design, which could greatly benefit the field of stretchable electronics.

Surfactant Self-Assembly Enhances Tribopositivity of Stretchable Ionic Conductors for Wearable Energy Harvesting and Motion Sensing.

Boosting stretchability and electric output is critical for high-performance wearable triboelectric nanogenerators (TENG). Herein, for the first time, a new approach for tuning the composition of surface functional groups through surfactant self-assembly to improve the tribopositivity, where the assembly increases the transferred charge density and the relative permittivity of water polyurethane (WPU). Incorporating bis(trifluoromethanesulfonyl)imide (TFSI-) and alkali metal ions into a mixture of WPU and the surfactant forms a stretchable film that simultaneously functions as positive tribolayer and electrode, preventing the conventional detachment of tribolayer and electrode in long term usage. Further, the conductivity of the crosslinked film reaches 3.3 × 10-3 mS cm-1 while the elongation at break reaches 362%. Moreover, the surfactant self-assembly impedes the adverse impact of the fluorine-containing groups on tribopositivity. Consequently, the charge density reaches 155 µC m-2, being the highest recorded for WPU and stretchable ionic conductor based TENG. This work introduces a novel approach for boosting the output charge density while avoiding the adverse effect of ionic salts in solid conductors through a universal surfactant self-assembly strategy, which can be extended to other materials. Further, the device is used to monitor and harvest the kinetic energy of human body motion.

Recent development on the design, preparation, and application of stretchable conductors for flexible energy harvest and storage devices

AbstractThe intensifying energy crisis has made it urgent to develop robust and reliable next‐generation energy systems. Except for conventional large‐scale energy sources, the imperceptible and randomly distributed energy embedded in daily life awaits comprehensive exploration and utilization. Harnessing the latent energy has the potential to facilitate the further evolution of soft energy systems. Compared with rigid energy devices, flexible energy devices are more convenient and suitable for harvesting and storing energy from dynamic and complex structures such as human skin. Stretchable conductors that are capable of withstanding strain (≫1%) while sustaining stable conductive pathways are prerequisites for realizing flexible electronic energy devices. Therefore, understanding the characteristics of these conductors and evaluating the feasibility of their fabrication strategies are particularly critical. In this review, various preparation methods for stretchable conductors are carefully classified and analyzed. Furthermore, recent progress in the application of energy harvesting and storage based on these conductors is discussed in detail. Finally, the challenges and promising opportunities in the development of stretchable conductors and integrated flexible energy devices are highlighted, seeking to inspire their future research directions.

Sensitivity‐Adjustable Flexible Conductors Based on Liquid Metals via Magnetic Printing

AbstractThe ability to shape and pattern conductors has been indispensable for manufacturing functional devices. Herein, a series of patterned liquid metals–based flexible electronics are fabricated via magnetic printing. The combination of highly conductive liquid metals and flexible photosensitive resins endows the as‐prepared devices with excellent electrical and mechanical properties, such as high electric conductivity (8 × 104 S m−1), low detection hysteresis, good durability, and environmental stability. What's more, strain‐insensitive flexible electronics can be achieved by designing the structure of liquid metal‐based conductors, and the resistance is only increased 0.13 Ω at 100% strain. Therefore, the as‐prepared liquid metals‐based flexible electronics have promising applications in stretchable conductors.

Effects of network structure on the viscoelastic creep and delayed fracture of polyelectrolyte elastomers

The past decade has witnessed a significant surge in the field of stretchable ionotronics, wherein stretchable ionic conductors play an essential role. Polyelectrolyte elastomers have been featured as the most promising stretchable ionic conductors for engineering practice owing to their leakage-free nature. However, the inherent viscosity of the ionized polymer networks confounds the responses of the materials thus hampering the applicability of the devices. Therefore, it is of significant importance to study the viscoelastic behaviors and understand the overlayed molecular mechanism. In this work, we study the effects of network structure on the viscoelastic creep and delayed fracture of polyelectrolyte elastomers. We design and synthesize poly(1-[2acryloyloxyethyl]-3-butylimidazolium bis(trifluoromethane) sulfonimide-co-methyl acrylate) elastomer and tune the network structure by changing the covalent crosslink density and the molar ratio between the ionic segment and the neutral segment, which are the two most crucial structural parameters. We perform tensile and creep tests for polyelectrolyte elastomers of different compositions and interpret the creep results, with emphasis placed on the effects of applied stress and strain hardening. In general, the creep deformation is mitigated with increasing covalent crosslink density and the molar ratio between the ionic segment and the neutral segment, suggesting that creep is mainly due to the breakage of ionic bonds and the slippage and movement of polymer chains. This work promotes the understanding of the viscoelastic behaviors of polyelectrolyte elastomers and provides insights for tailoring the mechanical properties through the rational design of polymer networks, which is expected to facilitate the development of next-generation stretchable ionotronic devices.

Strain‐Tolerant Nanocomposite Conductors with Engineered Bicontinuous Phase System by Additive Liquid/Solid Assembly

AbstractIntrinsically stretchable conductors are vital components in next‐generation flexible electronics. However, solid conductive networks are generally vulnerable to external deformations due to their incompatible mechanical properties with the highly elastic substrates or matrixes. It is still challenging to achieve precisely controlled conductive structure with stable electromechanical performances. Herein, a nanocomposite conductor with 3D engineered bicontinuous phase system is developed, which is constructed by alternately arranged robust solid‐phase and conductive liquid‐phase via additive liquid/solid assembly. The proper rheological properties and extraordinary structural compatibility of the liquid‐phase prompt the formation of bridged structure within the 3D periodic solid‐phase main skeleton, giving rise to stable and even strain‐enhanced electrical and electromagnetic interference (EMI) shielding properties under external deformation. A negative resistance change of −37% along with unabated EMI shielding effectiveness and mechanical properties are obtained at 100% strain, showing negligible performance degradation even after 10,000 cycles of rigorous stretching and releasing. A strain‐tolerant pressure‐sensing device is further demonstrated using the liquid/solid nanocomposite structure, delivering a unique function of precisely recognizing the local pressure at large tensile loading interference. This work provides a new paradigm for the manufacturing of nanocomposites with desired functionalities using the adequate nanofiller reserve toward practical strain‐tolerant applications.

A gelatin-based ionic conductor with a dense/loose alternating network enabled by surface active cellulose

Although conductive and stretchable ionic conductors are increasingly prepared for flexible sensing applications, such as health monitoring, tissue engineering, and electronic skin, achieving a balance between remarkable mechanical properties and high sensing sensitivity remains a significant challenge. In this work, a dense/loose alternating network was meticulously constructed through a pre-assembly process in sodium citrate solution and a subsequent re-assembly in FeCl3 solution. This approach ensured the robustness of the network structure and facilitated ion migration. Within the network, a gelatin network cross-linked via covalently and hydrogen bonds was significantly enhanced by a synergy of micro-enhancement and dynamic bidirectional crosslinking containing hydrogen bonds, imine bonds, and Fe coordinate bonds enabled by surface active cellulose. Benefiting from the dense network, the obtained composite hydrogels delivered a strong tensile strength (496.3 kPa), outstanding elastic modulus (917.6 kPa), and an excellent energy dissipation rate (55.6 %) at 80 % of strain. The loose polymer network can also endow Fe3+/Cl− ions with sensitive mobility, leading to an ultra-high sensing sensitivity (maximum GF: 20.06). These exceptional properties enable the ionic conductor to accurately detect a wide range of motions and pressures without being affected by environmental noise. With attributes such as reusability, biodegradability, strength, and sensitivity, this ionic conductor emerges as a reliable soft material with promising applications in the field of healthcare monitoring, wearable strain sensor sensors, and writing anti-counterfeiting materials.

Enhancing Electrical Conductivity of Stretchable Liquid Metal-Silver Composites through Direct Ink Writing.

Structure-property-process relationships are a controlling factor in the performance of materials. This offers opportunities in emerging areas, such as stretchable conductors, to control process conditions during printing to enhance performance. Herein, by systematically tuning direct ink write (DIW) process parameters, the electrical conductivity of multiphase liquid metal (LM)-silver stretchable conductors is increased by a maximum of 400% to over 1.06 × 106 S·m-1. This is achieved by modulating the DIW print velocity, which enables the in situ elongation, coalescence, and percolation of these multiphase inclusions during printing. These DIW printed filaments are conductive as fabricated and are soft (modulus as low as 1.1 MPa), stretchable (strain limit >800%), and show strain invariant conductivity up to 80% strain. These capabilities are demonstrated through a set of electromagnetic induction coils that can transfer power wirelessly through air and water, even under deformation. This work provides a methodology to program properties in stretchable conductors, where the combination of material composition and process parameters leads to greatly enhanced performance. This approach can find use in applications such as soft robots, soft electronics, and printed materials for deformable, yet highly functional devices.

Zwitterionic liquid crystal elastomer with unusual dependence of ionic conductivity on strain and temperature for smart wearable fabric

Stretchable ionic conductors are appealing for soft electronics, yet frequently had their conductivities limited below the level desired by many applications, and also lack the ability of simultaneously actuating and sensing their own motions, resembling living organisms’ neuromuscular behaviors. In this study, by introducing a zwitterionic chain extender into liquid crystal elastomers (LCEs) backbones, a unique type of zwitterionic LCEs (iLCEs) fiber with ionic conductivity of 2.3 S m−1 and self-sensing properties was designed. By controlling the anisotropy solvent evaporation, the iLCEs fiber with certain degree of mesogenic alignment was easily obtained. The mesogenic alignment not only enabled a reversible actuation behavior, but also boosted the formation of ionic channels for an abnormal dependence of ionic conductivity on temperature and elongation. The resultant iLCEs fibers showed an increasing resistance at high temperatures, and a decreasing resistance at large tensile strain, in contrast to the liquid-like mechanism of ion transport for ionogel systems. This combination of actuation and ionic conductance promises great potential in applications of self-sensing actuation and self-perception of soft robots.

A Solid-Liquid Bicontinuous Fiber with Strain-Insensitive Ionic Conduction.

Stretchable ionic conductors are crucial for enabling advanced iontronic devices to operate under diverse deformation conditions. However, when employed as interconnects, existing ionic conductors struggle to maintain stable ionic conduction under strain, hindering high-fidelity signal transmission. Here, it is shown that strain-insensitive ionic conduction can be achieved by creating a solid-liquid bicontinuous microstructure. A bicontinuous fiber from polymerization-induced phase separation, which contains a solid elastomer phase interpenetrated by a liquid ion-conducting phase, is fabricated. The spontaneous partitioning of dissolved salts leads to the formation of a robust self-wrinkled interface, fostering the development of highly tortuous ionic channels. Upon stretch, these meandering ionic channels are straightened, effectively enhancing ionic conductivity to counteract the strain effect. Remarkably, the fiber retains highly stable ionic conduction till fracture, with only 7% resistance increase at 200% strain. This approach presents a promising avenue for designing durable ionic cables capable of signal transmission with minimal strain-induced distortion.

3D Woven Liquid Metals for Radio‐Frequency Stretchable Circuits

AbstractMechanically flexible and stretchable inductive coils are critical for enabling communication, sensing, and wireless power transfer capabilities in wearable electronics that conform to the body for healthcare and Internet of Things (IoT) applications. Leading stretchable conductors such as liquid metals (LMs) offer conformability but sacrifice electromagnetic performance compared with Cu wires, leading to lossy radio‐frequency (RF) characteristics. Here, a strategy leveraging multistranded 3D woven ‘litz’ transmission lines is presented to amplify the resonant RF performance of LM inductors. Through comprehensive simulations and experiments, it is discovered that interwoven LM litz wires boost the Quality Factor (Q) by 80% compared to standard liquid metal wires. A fabrication methodology is also demonstrated for stretchable coils that retain high Q (>30), outperforming the previously reported LM coils and maintaining 98% of their wireless transmission efficiency under up to 30% biaxial strain. Moreover, the versatility of this approach is showcased by 3D printing four‐terminal ‘choke’ inductors optimized for RF filtering and inductance tunability, overcoming the fabrication limitations of traditional planar printed electronics. These results offer valuable insights into the design and implementation of 3D‐printed inductors for a diverse suite of electromagnetic device applications.

Self-Assembly Enabled Printable Asymmetric Self-Insulated Stretchable Conductor for Human Interface.

Soft and stretchable conductors with high electrical conductivity and tissue-like mechanical properties are crucial for both on-skin and implantable electronic devices. Liquid metal-based conductors hold great promise due to their metallic conductivity and minimal stiffness. However, the surface oxidation of liquid metal particles in polymeric matrices poses a challenge in forming a continuous pathway for highly conductive elastic composites. Here, it is reported a printable composite material based on liquid metal and conducting polymer that undergoes a self-assembly process, achieving high conductivity (2089 S cm-1) in the bottom surface while maintaining an insulated top surface, high stretchability (>800%), and a modulus akin to human skin tissue. This material is further applied to fabricate skin-interfaced strain sensors and electromyogram sensors through 3D printing.

Continuous production of ultratough semiconducting polymer fibers with high electronic performance.

Conjugated polymers have demonstrated promising optoelectronic properties, but their brittleness and poor mechanical characteristics have hindered their fabrication into durable fibers and textiles. Here, we report a universal approach to continuously producing highly strong, ultratough conjugated polymer fibers using a flow-enhanced crystallization (FLEX) method. These fibers exhibit one order of magnitude higher tensile strength (>200 megapascals) and toughness (>80 megajoules per cubic meter) than traditional semiconducting polymer fibers and films, outperforming many synthetic fibers, ready for scalable production. These fibers also exhibit unique strain-enhanced electronic properties and exceptional performance when used as stretchable conductors, thermoelectrics, transistors, and sensors. This work not only highlights the influence of fluid mechanical effects on the crystallization and mechanical properties of conjugated polymers but also opens up exciting possibilities for integrating these functional fibers into wearable electronics.

Highly conductive, super-stretchable and stable stretchable conductor with CNT and liquid metal alternating layered structure

Liquid metal (LM) composites, known for their excellent conductivity and fluidity, hold great promise in the fields of printed electronics and wearable technology. However, their high surface tension and fluidity can lead to circuit instability, such as leakage during deformation, resulting in short circuits. Therefore, additional methods are necessary to enhance the stability of LM within composite material systems. In this paper, Carbon nanotube (CNT) was introduced as a transition layer to enhance the adhesion of gallium-indium-copper-alloy ink (GCLA) on different elastic substrates and to obtain interface fusion conductive layers. The modified GCLA conductive fibers achieved high conductivity (conductivity up to 3.13 × 105 S/m), ultra-stretchability (1580 % of ultra-large deformation, the resistance changes less than 3 Ω), and ultra-stability (within 200 % of large deformation, the change of resistance less than 1 Ω). In addition, we also demonstrated the operability of GCLA in forming patterns on hydrophilic and hydrophobic substrate surfaces and obtained conductive patterns through printing, transfer printing, and hand drawing, making it a promising candidate for flexible electronics.

Stretchable conductors based on nanoporous Ag for flexible sensors

Conductive filler is essential to fabricate stretchable conductors for flexible electronics. In this paper, a novel kind of conductive filler, regularly nanoporous Ag (NP–Ag) is synthesized using a method called “complexation coprecipitation-rapid reduction”, which offers rapid, facile, and environment-friendly advantages. Owing to the nanoporous feature, the as-prepared conductor containing low contents of NP-Ag can exhibit excellent conductivity. Typically, the fluorine rubber-based conductor containing 45 wt% of NP-Ag can maintain a conductivity as high as 1578 S cm−1, while the composite loaded with the same content of fashionable filler (commercial Ag micro-flakes) remains non-conductive. Utilizing this conductor, a strain sensor with excellent sensitivity and repeatability can be fabricated conveniently. The synthetic advantages and regular porosity would make those microstructures a filler candidate for preparing large-area, low-cost, and high-through stretchable conductors.

Surface Embedded Metal Nanowire-Liquid Metal-Elastomer Hybrid Composites for Stretchable Electronics.

Both liquid metal (LM) and metallic filler-based conductive composites are promising stretchable conductors. LM alloys exhibit intrinsically high deformability but present challenges for patterning on polymeric substrates due to high surface tension. On the other hand, conductive composites comprising metallic fillers undergo considerable decrease in electrical conductivity under mechanical deformation. To address the challenges, we present silver nanowire (AgNW)-LM-elastomer hybrid composite films, where AgNWs and LM are embedded below the surface of an elastomeric matrix, using two fabrication approaches, sequential and mixed. We investigate and understand the process-structure-property relationship of the AgNW-LM-elastomer hybrid composites fabricated using two approaches. Different weight ratios of AgNWs and LM particles provide tunable electrical conductivity. The hybrid composites show more stable electromechanical performance than the composites with AgNWs alone. In particular, 1:2.4 (AgNW:LMP w/w) sequential hybrid composite shows electromechanical stability similar to that of the LM-elastomer composite, with a resistance increase of 2.04% at 90% strain. The sequential approach is found to form AgIn2 intermetallic compounds which along with Ga-In bonds, imparts large deformability to the sequential hybrid composite as well as mechanical robustness against scratching, cutting, peeling, and wiping. To demonstrate the application of the hybrid composite for stretchable electronics, a laser patterned stretchable heater on textile and a stretchable circuit including a light-emitting diode are fabricated.