Stretchable Conductive Composites Research Articles

Design and Optimization of NR-Based Stretchable Conductive Composites Filled with MoSi2 Nanoparticles and MWCNTs: Perspectives from Experimental Characterization and Molecular Dynamics Simulations

Stretchable conductive composites play a pivotal role in the development of personalized electronic devices, electronic skins, and artificial implant devices. This article explores the fabrication and characterization of stretchable composites based on natural rubber (NR) filled with molybdenum disilicide (MoSi2) nanoparticles and multi-walled carbon nanotubes (MWCNTs). Experimental characterization and molecular dynamics (MD) simulations are employed to investigate the static and dynamic properties of the composites, including morphology, glass transition temperature (Tg), electrical conductivity, and mechanical behavior. Results show that the addition of MoSi2 nanoparticles enhances the dispersion of MWCNTs within the NR matrix, optimizing the formation of a conductive network. Dynamic mechanical analysis (DMA) confirms the Tg reduction with the addition of MWCNTs and the influence of MoSi2 content on Tg. Mechanical testing reveals that the tensile strength increases with MoSi2 content, with an optimal ratio of 4:1 MoSi2:MWCNTs. Electrical conductivity measurements demonstrate that the MoSi2/MWCNTs/NR composites exhibit enhanced conductivity, reaching optimal values at specific filler ratios. MD simulations further support experimental findings, highlighting the role of MoSi2 in improving dispersion and mechanical properties. Overall, the study elucidates the synergistic effects of nanoparticles and nanotubes in enhancing the properties of stretchable conductive composites.

Surface‐Engineered Liquid Metal Particles for Printing Stretchable Conductive Composites with Enhanced Stability Under Different Strain Rates

AbstractIntegrating liquid metal (LM) particles into compliant polymers presents an innovative approach for developing intelligent and adaptable systems in stretchable electronics, wearable devices, soft robotics, and other emerging technologies. However, the inherent electrically insulative nature of these solid‐liquid composites, compounded by the gallium oxide shell surrounding LM droplets, poses a significant challenge in establishing conductive pathways, especially for small droplet sizes and ultrasoft elastomers. Here, an interface modification approach that addresses this bottleneck and enables the synthesis of highly stretchable and printable composites with LM microparticles (<2 µm) is presented. Polyvinylpyrrolidone (PVP) is used to functionalize these small LM inclusions, weakening the particle‐matrix interface, and facilitating the formation of a conductive percolating network under tensile strain. Optimized synthesis parameters result in printed conductive traces with excellent electrical conductivity (0.2 Ω cm−1), ultra‐high elongation at break (>900% strain), and minimal resistance change (≈131%). Furthermore, this comprehensive study of the electromechanical response of these stretchable conductors under various strain rates reveals their exceptional stability under dynamic loading conditions, surpassing the performance of conductive traces composed of sprayed liquid metal. Finally, the potential application of these multifunctional materials in stretchable circuitry, addressing the demand for high stretchability and stability in wearable electronics, is demonstrated.

Ultra-stable, waterproof and self-healing serpentine stretchable conductors based on WPU sheath-wrapped conductive yarn for stretchable interconnects and wearable heaters

Stretchable conductors (SCs) have shown promising potential in the field of stretchable electronic devices for their lightweight, high flexibility, stable performance, and easy integration. However, the conductivity of most reported stretchable conductive composites is sensitive to deformation, and it is still unsatisfactory due to their poor recoverability and low reliability under large strain. In this paper, we have fabricated a serpentine stretchable conductor (SSC) with high conductivity, stretchability, and ultra-stability by spontaneously wrapping elastic waterborne polyurethane (WPU) sheath on the surface of the multi-walled carbon nanotube (MWCNT)/WPU nanocomposite conductive yarn, which was prepared by a simple wet-spinning method. The WPU elastic sheath endows SSC with long-term tensile durability and recoverability. Remarkably, benefiting from the sharing effect of the serpentine structure on the tensile strain energy, SSC shows distinctive strain-insensitive behavior (ΔR/R0 less than 1.6%) up to 100% strain and ultra-high quality factor (Q = 105) at 80% strain. Furthermore, SSC also has temperature resistance, waterproof ability, and self-healing capability, making it capable of being used in harsh circumstances. The combination of high conductivity, stretchability, and super reliability renders many potential applications for SSC, such as stretchable interconnects and wearable heaters. Such serpentine stretchable device with distinctive strain-insensitive behavior provides a novel design idea for stretchy electronics.

A reconfigurable and automatic platform for the on-demand production of stretchable conductive composites

Stretchable conductive composites (SCCs) have been widely used as interconnects and sensors in stretchable electronic devices due to their tunable electromechanical properties and intrinsically high stretchability compared to solid metals. SCCs can be readily made by mixing (or breaking bulk) conductive fillers within an elastomeric polymer, which are subsequently cured. Despite the simplicity of this, most fabrication methods follow customized protocols and lack precise automatic control. These methods also require bulky and costly equipment (e.g. stirrers, mixers, ovens, and vacuuming machines). Also, variations in the production process make it challenging to maintain the consistency of SCC’s electrical and mechanical properties produced in different batches. To solve this problem, this work develops an automatic SCC production platform (ASPP) that can be programmed to produce SCCs with high consistency in properties. The versatility of ASPP is demonstrated by fabricating SCCs with single and hybrid fillers, and porous structures. The consistency of SCCs’ electromechanical properties is examined using samples fabricated in different batches following the same protocol. We further utilize the fabricated SCCs to realize various intelligent tactile sensing and heating platforms. The capability demonstrated for the ASPP shows its potential in fabricating SCCs for applications in soft robotics and wearable devices.

Stretchable Conductive Tubular Composites Based on Braided Carbon Nanotube Yarns with an Elastomer Matrix.

We report an innovative approach to creating stretchable conductive materials composed of a tubular shell made from braided carbon nanotube yarns (CNTYs) embedded in an elastomeric matrix. For stretchable electronics, both mechanical properties and electrical conductivities are of interest. Consequently, both the mechanical behavior and electrical conductivities under large deformations were investigated. A new hyperelastic composite model was developed to predict the large deformation response to applied stress for a braid in a tubular elastomer composite. The composite demonstrated a hyperelastic response due to the architecture of the braid, and the behavior was affected by the braiding angle, braid modulus, and volume fraction of fibers. The elastomer matrix was considered a neo-Hookean material and represented by the Yeoh model. An interaction parameter was proposed to account for the effect of the elastomer/braid cooperative restriction as observed in experimental and calculated results. This novel approach enabled the determination of the constitutive behavior of the composite in large deformations (>150%), taking into account the elastomer and yarn properties and braid configurations. The model exhibited good agreement with the experimental results. As the CNTYs are conductive, a stretchable conductive composite was obtained having a resistivity of 5.01 × 10-4 and 5.67 × 10-5 Ω·cm for the 1-ply and 4-ply composites, respectively. The resistivity remained constant through cyclic loading under large deformations in tension until mechanical failure. The material has potential for use in stretchable electronics applications.

Laser-Induced Remote Healing of Stretchable Diselenide-Containing Conductive Composites.

Remotely controlled on-demand functional healing is vital to components that are difficult to access and repair in distance such as satellites and unmanned cruising aircrafts. Compared with other stimuli, a blue laser is a better choice to input energy to the damaged area in distance because of its high energy density and low dissipation through the air. Herein, diselenide-containing polyurethane (PUSe) is first employed to fabricate visible light-responsive stretchable conductive composites with multiwalled carbon nanotubes (MWCNTs). Then, laser-induced remote healing was realized based on the characteristics of long-distance propagation of lasers and the dynamic properties of diselenide bonds. Moreover, the PUSe/MWCNT composite film can be used to transfer an electrical signal in the circuit containing a signal generator. This laser-induced remote healing of conductivity paves the way for developing healing conductors which are difficult to access and repair.

Novel insights into the design of stretchable electrical systems.

Soft electronics have recently gathered considerable interest because of their biomechanical compatibility. An important feature of deformable conductors is their electrical response to strain. While development of stretchable materials with high gauge factors has attracted considerable attention, there is a growing need for stretchable conductors whose response to deformation can be accurately engineered to provide arbitrary resistance-strain relationships. Rare studies addressing this issue have focused on deterministic geometries of single rigid materials, limiting the scope of these strategies. We introduce the novel concept of periodic stretchable patterns combining multiple conductive materials to produce tailored responses. Using shortest path algorithms, we establish a computationally efficient selection method to obtain the required resistance-strain relationship. Using this algorithm, we identify and experimentally demonstrate constant resistance-strain responses up to 50% elongation using a single microtextured material. Last, we demonstrate counterintuitive sinusoidal responses by integrating three materials, with interesting applications in sensing and soft robotics.

Hybrid‐Filler Stretchable Conductive Composites: From Fabrication to Application

Stretchable conductive composites (SCCs) are generally elastomer matrices filled with conductive fillers. They combine the conductivity of metals and carbon materials with the flexibility of polymers, which are attractive properties for applications such as stretchable electronics, wearable devices, and flexible sensors. Most conventional conductive composites that are filled with only one type of conductive filler face issues in mechanical and electrical properties. Recently, some studies introduced secondary fillers to create hybrid‐filler SCCs to solve these problems. The secondary fillers produce a synergistic effect with the primary fillers to enhance the electrical conductivity of the composites. They also improve the thermal conductivity and mechanical properties or impart composites with special functions like catalysis and self‐healing. Herein, the fabrication methods, stretchability enhancement strategies, and piezoresistivity of SCCs are analyzed, and their latest applications in stretchable electronics are introduced. Finally, the challenges and prospects of their development are discussed.

Amalgamation‐Assisted Lithography: Amalgamation‐Assisted Control of Profile of Liquid Metal for the Fabrication of Microfluidic Mixer and Wearable Pressure Sensor (Adv. Mater. Interfaces 10/2021)

In article number 2100038, Sheng Yan and co-workers present a simple and straightforward amalgamation-assisted method for creating liquid metal droplets and microdomes with highly controllable sizes. The microdome can generate the vortices, enhancing the flow mixing inside the microfluidic channel. Besides, the microdomes are patterned on the surface of stretchable conductive composites to develop a flexible and sensitive pressure sensor.

Vapor-Mediated Stretchable and Reversible Conductors from Microporous Liquid Metal Polymers.

Flexible electronic devices have penetrated into a variety of industry sectors (i.e., consumer electronics, automotive, and medical) in human life, and this calls for better properties of stretchable conductive composites as the crucial elements. Traditionally, conductive inorganic fillers are incorporated in flexible polymers to prepare conductive composites, which falls short of the required properties in more demanding devices nowadays due to limited deformation, low conductivity, and poor processability. Herein, liquid metals were successfully incorporated in microporous polymer matrixes using a simple codissolving and film casting/solvent evaporation approach. The microporous liquid metal-embedded polymer (LMEP) was insulative as fabricated due to discontinuous liquid metals (LMs), while it became conductive upon stretching. Interestingly, the LMEP films showed a reversible insulator-conductor transition due to the regenerated pores in polymer matrix under organic vapor. Negligible changes in the resistance value were seen even after 50 solvent exposure-tensile strain cycles, demonstrating the excellent stability of the electrical properties of these films. Furthermore, most of the commercially available soluble polymers including rigid plastics and soft elastomers are suitable for the fabrication of LMEP. With the ideal characteristics, they have been successfully exploited in model alarm systems to prevent temperature overloads and solvent leakage, showcasing the great potential in next generation sensors used in industry settings.

Amalgamation‐Assisted Control of Profile of Liquid Metal for the Fabrication of Microfluidic Mixer and Wearable Pressure Sensor

AbstractThe presence of microdomes can significantly increase the surface roughness, contact area, and deformability of materials, which have been adopted in many fields including microfluidics, wearable devices, and microanalysis systems. However, the shape of liquid metal (LM) droplet is defined by the density and surface energy, which has very limited room to tune. In this work, a simple, low‐cost method to effectively control the profile of LM using the masked amalgamation is presented. The LM amalgamates the masked copper surface to create the complex microdomes with various aspect ratios, sizes, profiles, and structures. The concave dome replicated from the LM mold has been demonstrated to enhance the microfluidic mixing performance. With a pattern transfer technique, the microconvex domes can be patterned on the surface of stretchable conductive composites to develop a flexible and sensitive pressure sensor. This sensor exhibits a fast response time, a wide working range, and an enhanced sensitivity for detecting small strains. As such, the fabricated microdomes exhibit a great potential to enable the fabrication of high‐performance sensors, microfluidic platforms, and micro total analysis systems.

Tough, self-healable and conductive elastomers based on freezing-thawing strategy

Flexible conductive elastomers with excellent mechanical properties and body-temperature self-healing are highly desirable for wearable electronics. However, it is still a challenge to develop such elastomers because mechanical strength and healing efficiency are contradictory and are difficult to realize the concurrent optimization. Herein, we design stretchable conductive composites by combining tough self-healable elastomeric matrix and in-situ formed polypyrrole network. The elastomeric matrix, fabricated by copolymerization of N-isopropylacrylamide and 2-methoxyethylacrylate, displays superior mechanical strength (8.2 MPa) and high body-temperature healing efficiency (94%), enabled by combination of the dynamic hydrogen bonding and microphase separation structures from freezing-thawing treatment. The resultant conductive elastomers exhibit excellent strength of 7.4 MPa, high stretchability of 410%, good electrical conductivity of 12.5 S/m and over 70% body-temperature healing efficiency. Moreover, it is revealed that the conductive elastomers could be used as strain sensors due to high sensitivity of stretchability. The tough self-healing conductive elastomers may find potential applications in stretchable electronics and soft robotics

Stretchable conductor based on carbon nanotube/carbon black silicone rubber nanocomposites with highly mechanical, electrical properties and strain sensitivity

Stretchable conductive polymer composites are important in the field of flexible electronic devices, in which stretchable conductive silicone rubber composites has drawn great attention. However, the poor mechanical properties of the SR composites limit their practical application. How to compensate this drawback without compromising its electrical conductivity is an issue deserved to investigate. Herein, conductive carbon black grafted with a silicone coupling agent was prepared by free radical polymerization, and further reacted with carbon nanotube to obtain a new carbon black-carbon nanotube hybrid filler. This hybrid filler can not only improve the mechanical properties of silicone rubber composites, but also greatly enhance their electrical conductivity. Consequently, a silicone rubber composite with 5.76 vol% hybrid filler has high tensile strength (4.5 MPa) and elongation at break (211%). The minimum conductive percolation threshold and maximum conductivity of the composite are 0.24 vol% and 248.8 S/m, respectively. Besides, the composite has high strain sensitivity and superior deformation recovery performance, and can be applied to detect human motion behavior under a low strain. The preparation method of the hybrid filler provides a novel approach to the preparation and production of stretchable conductive composites with good mechanical properties and potential application for flexible electronic devices.

A highly flexible tactile sensor with an interlocked truncated sawtooth structure based on stretchable graphene/silver/silicone rubber composites

A highly flexible and sensitive tactile sensor with an interlocked truncated sawtooth structure and stretchable conductive composites is demonstrated.

Extraordinarily High Conductivity of Stretchable Fibers of Polyurethane and Silver Nanoflowers.

Stretchable conductive composites have received considerable attention recently, and they should have high conductivity and mechanical strength. Here we report highly conductive stretchable fibers synthesized by the scalable wet spinning process using flower-shaped silver nanoparticles with nanodisc-shaped petals (Ag nanoflowers) and polyurethane. An extraordinarily high conductivity (41,245 S cm(-1)) was obtained by Ag nanoflowers, which is 2 orders of magnitude greater than that of fibers synthesized using spherical Ag nanoparticles. This was due to the enhanced surface area and vigorous coalescence of nanodisc-shaped petals during the curing process. There was a trade-off relationship between conductivity and stretchability, and the maximum rupture strain was 776%. An analytical model revealed that the enhanced adhesion between Ag nanoflowers and polyurethane provided a high Young's modulus (731.5 MPa) and ultimate strength (39.6 MPa) of the fibers. The fibers exhibited an elastic property after prestretching, and the resistance change of weft-knitted fabric was negligible up to 200% strain. The fibers with extraordinarily high conductivity, stretchability, and mechanical strength may be useful for wearable electronics applications.

Stretchable Conductive Composites Based on Metal Wools for Use as Electrical Vias in Soft Devices

Soft devices can be bent, stretched, and compressed reversibly, but conventional wires are rigid. This work describes stretchable composites that are easily fabricated with simple tools and commodity materials, and that can provide a strategy for electrical wiring that meets certain needs of soft devices. These composites are made by combining metal wool and elastomeric polymers. Embedding fine (average fiber width ≈25 μm) steel wool (or other metal wools) in a silicone polymer creates an electrically conductive path through the nonconductive elastomer. This composite is flexible, stretchable, compressible, inexpensive, and simple to incorporate into the bodies of soft devices. It is also electrically anisotropic, and shows maximum conductivity along the majority axis of the fibers, but maximum extension perpendicular to this axis. The utility of this composite for creating an electrically conductive path through an elastomer was demonstrated in several devices, including: a soft, solderless breadboard, a soft touch sensor, and a soft strain gauge.