Flexible Conductive Materials Research Articles

CHARACTERIZATION OF 3D PRINTED CONDUCTIVE FLEXIBLE MATERIALS FOR SOFT ROBOTIC APPLICATIONS

Soft robots composed of compliant and flexible materials can safely interact with humans and adapt to unstructured environments. However, integrating sensors, actuators, and control circuits into soft structures remains challenging. Additive manufacturing shows promise for fabricating soft robots with embedded electronics using conductive flexible composites. Nevertheless, there is still a limited understanding of the electromechanical behavior of 3D-printed conductive structures when subjected to the types of strains relevant to soft robotics applications. Optimized design requires characterizing the interplay between a soft component's changing shape and electrical properties during deformation. This study investigates the application of 3D printing technology to fabricate various geometries using a conductive, flexible material for soft robotic applications. The primary objective is to understand and characterize the behavior of differently shaped 3D-printed conductive materials under various mechanical stresses. Two distinct test setups are designed for conducting bending and tensile tests on the produced materials. Diverse geometries are printed using the conductive flexible material with desirable mechanical and electrical properties to employ tensile and bending tests. The experiments reveal a direct correlation between shape change and electrical resistance of the 3D printed materials, providing valuable insights into their adaptability for soft robotics. According to numerical results, honeycomb profiles are found to be the most linear and stable profile type. This research not only contributes to the field of flexible conductive materials but also lays the foundation for integrating these materials into future engineering designs, potentially enabling the development of highly responsive and adaptable devices for various industries.

A one step method to prepare polymer‐based flexible wires by continuous spatial‐confining network assembly technique

AbstractIn recent years, flexible electronics technology has been developing rapidly, and flexible electronic devices have been widely used in wearable gadgets and robotics. Flexible wires are the basic components of these devices, which usually consist of metallic materials with flexibility and ductility. However, the introduction of flexible conductive materials has broadened the application scope of flexible wires. In this paper, PDMS/SCF conductive composites exhibiting different conductivity properties on both sides are fabricated by adding meshes based on the continuous spatial‐confining network assembly method. The conductivity of PDMS/SCF (6 wt%) composites can be exhibited as 173.45 S/m in the conductive layer and 6.25 × 10−11 S/m in the insulating layer, which can be used as core and surface layers of conductors, respectively. A mathematical model is also developed to predict the electrical conductivity of the PDMS/SCF conductive composites, and the experimental data are in good agreement with the predicted results. This work provides a method for the continuous preparation of multifunctional composites that can be molded in one step with conductive layer and insulating layer, which can be used as the core and insulating layers of wires, thus simplifying the preparation process of composite‐based wires and expanding the range of conductive composites applications.

Cover Image, Volume 62, Issue 6

The eutectogels find promising applications as flexible conductive materials. The study from Jianhua Zhang of Tianjin University reports a facile strategy to prepare poly(N‐acryloyl glycinamide) eutectogels by one‐step in situ photopolymerization of N‐acryloyl glycinamide without any crosslinkers in a deep eutectic solvent composed of choline chloride and ethylene glycol. The obtained poly(N‐acryloyl glycinamide) eutectogels exhibit concentration‐dependent and adjustable mechanical properties, high electrical conductivity, and self‐healing abilities as well as good anti‐drying and antifreezing properties. DOI: 10.1002/pol.20230603 image

Smart materials for flexible electronics and devices: hydrogel.

In recent years, flexible conductive materials have attracted considerable attention for their potential use in flexible energy storage devices, touch panels, sensors, memristors, and other applications. The outstanding flexibility, electricity, and tunable mechanical properties of hydrogels make them ideal conductive materials for flexible electronic devices. Various synthetic strategies have been developed to produce conductive and environmentally friendly hydrogels for high-performance flexible electronics. In this review, we discuss the state-of-the-art applications of hydrogels in flexible electronics, such as energy storage, touch panels, memristor devices, and sensors like temperature, gas, humidity, chemical, strain, and textile sensors, and the latest synthesis methods of hydrogels. Describe the process of fabricating sensors as well. Finally, we discussed the challenges and future research avenues for flexible and portable electronic devices based on hydrogels.

Highly ion-conducting, robust and environmentally stable poly(vinyl alcohol) eutectic gels designed by natural polyelectrolytes for flexible wearable sensors and supercapacitors

Flexible conductive materials have received a lot of attention due to their versatility in wearable devices, etc. Eutectic gels are expected to replace the traditional hydrogel as a new generation of soft conductors. Here, sodium alginate (SA), a polysaccharide polyelectrolyte with surface charge, was ingeniously used to construct the energy dissipation and ion transport network. Thus, a double-network polyvinyl alcohol composite eutectic gel with various physical effects was prepared by salting out and solvent replacement process. The obtained gels showed excellent mechanical properties, frost resistance and stability; the maximum fracture strain and toughness can reach 3.87 MPa and 9.87 MJ/m3, respectively. Surprisingly, the –COO− contained in SA plays an active role in the ion transport, and the conductivity (0.30 S/m) was improved by 5 times. Based on this composite gel, a supercapacitor with a wide operating window of 1.7 V and good cyclic charging and discharging stability (capacitance retention at 82.1 % after 1500 cycles) was prepared. And the prepared sensor can sensitively monitor the movement of human joints and perform handwriting recognition. This strategy realizes the balance between mechanical properties and conductivity of eutectic gel and opens up the application of natural polyelectrolytes in the construction of high-performance ion-transport networks.

Tough soldering for stretchable electronics by small-molecule modulated interfacial assemblies

The rapid-developing soft robots and wearable devices require flexible conductive materials to maintain electric functions over a large range of deformations. Considerable efforts are made to develop stretchable conductive materials; little attention is paid to the frequent failures of integrated circuits caused by the interface mismatch of soft substrates and rigid silicon-based microelectronics. Here, we present a stretchable solder with good weldability that can strongly bond with electronic components, benefiting from the hierarchical assemblies of liquid metal particles, small-molecule modulators, and non-covalently crosslinked polymer matrix. Our self-solder shows high conductivity (>2×105 S m−1), extreme stretchability (~1000%, and >600% with chip-integrated), and high toughness (~20 MJ m−3). Additionally, the dynamic interactions within our solder’s surface and interior enable a range of unique features, including ease of integration, component substitution, and circuit recyclability. With all these features, we demonstrated an application as thermoforming technology for three-dimensional (3D) conformable electronics, showing potential in reducing the complexity of microchip interfacing, as well as scalable fabrication of chip-integrated stretchable circuits and 3D electronics.

Fabrication and Evaluation of Carrageenan Based Bioplastic with Graphite and Ag-Nanoparticles Addition as Flexible Electrode for EMG Signal Measurement

Electromyography (EMG) is a method for measuring muscle biopotential signals for monitoring muscle activity. Electrodes are placed on the skin to capture EMG signals from muscles underneath. The most common electrodes used in clinical EMG measurement are Ag/AgCl electrodes in the form of metal plates coated with electrode gel. Electrode gel enhances the contact between the electrode’s metal plate and the skin since it is essential for a good measurement signal quality. Meanwhile, flexible electrodes are made from flexible conductive materials that can be adjusted to the contour of the skin surface; therefore, they can improve the measured biopotential signal quality. This study developed a carrageenan-based bioplastic with the addition of graphite and silver nanoparticles (AgNP) hybrid as a flexible electrode for EMG signal measurement. Fabrication of graphite and AgNP hybrid starts with the functionalization of the graphite powder in a mixture of HNO3 and H2SO4. Next, AgNPs were added using the electrochemical method by utilizing SnCl2 and functionalized graphite powder to form an Ag-Sn/Graphite (Graphite-AgNPs) hybrid conductive material. In order to incorporate conductive materials into bioplastic, the Graphite-AgNPs hybrid conductive material is then mixed into the carrageenan-based bioplastic mixture. It is found that 25% w/w addition of these conductive materials already gives good electrical conductivity. The best electrical conductivity value was determined by varying several conductive material types and concentrations. Finally, the EMG signal was measured with the bioplastic flexible electrodes, and the performance was compared with the commercial Ag/AgCl electrodes.

Metal-grade laminated nanofiber films with outstanding EMI shielding performances and high-temperature resistance

With the growing demands in electronics, communications, and energy, conductive composites face a daunting challenge as criteria for conductivity and mechanical properties rise. Extensive search on nanocomposites and amorphous metals has been motivated by the quest for a conductive material that can enhance strength and deformability. However, finding conductive materials that can overcome the drawbacks of their conventional counterparts and exhibit great strength, toughness and quick charge transmission while accomplishing large-scale production remains a significant challenge. This paper describes an approach to high-temperature flexible conductive materials by utilizing composites composed of aramid nanofiber (ANF) and silver nanoparticles (Ag NPs). The ANF is derived from macro-aramid fibers and retains its excellent mechanical properties and heat resistance. Within the composite, Ag NPs effectively enter the special nanofiber matrix to build a tightly connected conductive pathway on the ANF, thus facilitating efficient charge transport. The resulting Ag-ANF composite films exhibit an impressive electrical conductivity of 36,989.68 S m−1 and a tensile strength of 30.3 MPa. Additionally, they provide outstanding EMI shielding with a value of 107.9 dB. This significant achievement opens up the possibility of metallizing the surface of insulated polymer fibers, thereby enabling effective EMI shielding applications even in challenging and extreme conditions.

A Highly Stretchable Force Sensitive and Temperature Sensitive Sensor Material with the Sandwich Structure of PDMS + PDMS/GaInSn + PDMS.

Flexible conductive sensor materials have received great attention for their sensitive electrical response to external conditions and their promising applications in flexible wearable and robotic applications. In this work, a highly stretchable force sensitive and temperature sensitive sensor material with a sandwich structure was prepared from the polydimethylsiloxane (PDMS) and the liquid metal (LM) gallium-indium-tin alloy (GaInSn). The sandwich structure (PDMS + PDMS/GaInSn + PDMS) was proven to prevent the "leakage" of LM. The preparation method of the sensing material was simple and time-saving (less than 1.5 h) and can be used for industrial production. The electrical performance analysis results confirmed that the resistance (R) of the material was sensitive to the external force, such as repeated stretching, compressing, bending, and impacting. The ΔR/R changed periodically and stably with the repeated stretching, when the GaInSn/Part A ≥ 0.4, the cyclic tensile strain ≤ 50%, and the cyclic tensile rate ≤ 2.5 mm/min. The R of the sensor materials was also responsive to the temperature, such as hot air and liquid nitrogen. In conclusion, this work provides a method for preparing sensing materials with the sandwich structure, which was confirmed to be sensitive to force and temperature without leaking LM, and it produced different types of R signals under different deformations and different temperatures.

Compressible, anti-freezing, and ionic conductive cellulose/polyacrylic acid composite hydrogel prepared via AlCl3/ZnCl2 aqueous system as solvent and catalyst

From the perspective of environmental sustainability, introducing cellulose into ionic conductive hydrogel is an inevitable trend for the development of flexible conductive materials. We report a double-network cellulose/polyacrylic acid (Cel/PAA) composite hydrogel based on the dissolving of cellulose by AlCl3/ZnCl2 aqueous system. The Cel/PAA composite hydrogel consists of rigid cellulose chains and flexible polyacrylic acid, which synergistically realize the improvement of the mechanical properties. The AlCl3/ZnCl2 aqueous system not only serves as the green solvent for cellulose, but also the Al3+ and Zn2+ metal ions can be served as a catalyst to activate the initiator for polymerization of acrylic acid. Compared with pure cellulose hydrogel, the compression strain of the Cel/PAA composite hydrogel was significantly improved to 80 %, and its conductivity increased by 28.1 %. In addition, its compression stress was enhanced over 2 times than pure PAA hydrogel. The Cel/PAA composite hydrogel exhibits excellent anti-freezing (−45 °C), weight retention (90 %), and conductivity (2.70 S/m) properties, still maintaining transparency and storage stability in the extreme environment. This work presents a facile strategy to develop an ionic conductive cellulose-based composite hydrogel with good conductivity and mechanical properties, which shows potential for the application fields of flexible sensors and 3D-printing functional materials.

Highly Conductive Polysiloxane Elastomers with Excellent Transparency, Resilience, and Stretchability.

Flexible transparent conductive materials show great potential in wearable electronics, flexible sensors, and so on. But the most used flexible conductive materials like hydrogels and ionogels suffer from evaporation and solvent leakage. For the application in these fields, integrated performances of preeminent resilience, transparency, stability, and conductivity that do not change with deformation are prerequisites. It is still challenging to handle the trade-off among these performances. Herein, a facile approach is established to balance these properties into one elastomer. Through the thiol-ene click reaction, mercaptopropyl-modified polydimethylsiloxane (mPDMS) is cross-linked and grafted by PEG-based macromonomers to prepare conductive elastomers. By anchoring with mPDMS through carbon-sulfur bonds, PEG can be evenly dispersed, resulting in ultratransparency (97%) and stable conductivity of as high as 1.68 × 10-2 S m-1, comparable to pure PEG/lithium salt conductivity. It also has a wide electrochemical stability window with a high voltage of 4.8 V. Moreover, the multibond network strategy is employed through grafting ligand 1-vinylimidazole to mPDMS to construct dynamic cross-links between Zn(II) and 1-vinylimidazol, bestowing excellent properties to the elastomers. Overall, elastomers with a well-balanced performance of high resilience, good conductivity, and ultratransparency are obtained, providing promising applications for soft electronics, lithium battery electrolytes, and flexible devices.

A Review on the Progress in Core-Spun Yarns (CSYs) Based Textile TENGs for Real-Time Energy Generation, Capture and Sensing.

This review is a critical analysis of the current state-of-the-art in core spun yarn textile triboelectric nanogenerators (CSY-T-TENGs) for self-powered smart sensing applications. The rapid expansion of wireless communication, flexible conductive materials, and wearable electronics over the last ten years is now demanding autonomous energy, which has created a new research space in the field of wearable T-TENGs. Current research is exploring T-TENGs made from CSYs as stable and reliable energy harvesters and sensing devices for modern wearable IoT platforms. CSY-TENGs are emerging as an important technology due to its simple structure, low cost, and excellent performance in converting mechanical energy into electrical energy and due to its sensing ability. This paper provides a critical review on current progress, it analyzes the unique advantages of CSYs T-TENGs over conventional T-TENGs, it describes fabrication techniques and discusses the materials used along with their properties and electrical performance characteristics, and it highlights the recent advancements in their integration with self-excitation circuits, charge storage devices and IoT-enabled smart sensing applications, such as environmental and health monitoring. In the conclusion, it discusses the challenges and future directions of CSYs T-TENGs and it provides a future road map for optimization, upscaling, and commercialization of the technology.

Low-Temperature Adaptive Dual-Network MXene Nanocomposite Hydrogel as Flexible Wearable Strain Sensors.

Flexible electronic devices and conductive materials can be used as wearable sensors to detect human motions. However, the existing hydrogels generally have problems of weak tensile capacity, insufficient durability, and being easy to freeze at low temperatures, which greatly affect their application in the field of wearable devices. In this paper, glycerol was partially replaced by water as the solvent, agar was thermally dissolved to initiate acrylamide polymerization, and MXene was used as a conductive filler and initiator promoter to form the double network MXene-PAM/Agar organic hydrogel. The presence of MXene makes the hydrogel produce more conductive paths and enforces the hydrogel's higher conductivity (1.02 S·m-1). The mechanical properties of hydrogels were enhanced by the double network structure, and the hydrogel had high stretchability (1300%). In addition, the hydrogel-based wearable strain sensor exhibited good sensitivity over a wide strain range (GF = 2.99, 0-200% strain). The strain sensor based on MXene-PAM/Agar hydrogel was capable of real-time monitoring of human movement signals such as fingers, wrists, arms, etc. and could maintain good working conditions even in cold environments (-26 °C). Hence, we are of the opinion that delving into this hydrogel holds the potential to broaden the scope of utilizing conductive hydrogels as flexible and wearable strain sensors, especially in chilly environments.

Rational design of MXene/Boron Carbon Nitride nanotube composite electrode for supercapacitors with improved electrochemical properties

3D structures obtained using 1D and 2D materials are promising materials in energy storage. Optimizing the interlayer spacing of 2D MXene through flexible conductive materials is an important step in synthesizing 3D composite electrodes. In this study, flexible and conductive MXene/BCN nanotube composite film electrodes were prepared by vacuum filtration using BCN nanotubes in different weight ratios (10 %, 20 %, and 30 %) in mixed and sandwiched structures. The mixed MXene/BCN10 (m-MX/BCN10) composite film electrode structure attained a specific capacitance of 678 F g−1, the highest of all tested electrodes, and exhibited remarkable cycle stability by retaining 95 % of its capacitance even after 10,000 charge/discharge cycles. In the symmetric supercapacitor (SSC), the cell exhibited a specific capacitance of 97 % after 10,000 charge/discharge cycles. Moreover, SSC showed an energy density of 32 Wh kg−1 and a power density of 11.5 kW kg−1. The composite electrodes made of 2D Ti3C2Tx MXene and 1D BCN nanotube materials open up a new avenue for synthesizing promising materials for energy storage in transition metal carbides and 1D nanotubes or nanowires.

Preparation and performance of self-cleaning photothermal-induced self-healing flexible sensors

Flexible conductive materials for wearable applications have seen remarkable advancements in recent years. However, a significant challenge that must be addressed is their susceptibility to accidental mechanical damage. In this study, a self-healing superhydrophobic flexible fabric sensor by incorporating fluorinated polyurethane with fluorinated side chains and graphene modified with Fe3O4 nanoparticles. The fabric composite coating exhibited excellent hydrophobicity and self-cleaning ability, allowing easy removal of dirt and dust by rainwater or other liquids. Due to its excellent conductivity and stretch recovery properties, the composite coating can be applied as a wearable fabric sensor. The sensor demonstrates remarkable sensitivity to strain and can be used to monitor the movements of fingers, wrists, elbows, and other joints. Most importantly, the damaged fabric coating could restore surface integrity, repair conductivity, and maintain superhydrophobicity, which is attributed to the dynamic reversible nature of disulfide bonds and the induced dipole polarization effect of fluorinated side chains. Moreover, the fabric sensor exhibits stable photothermal conversion performance based on the joint effect of graphene and Fe3O4. This property provides a foundation for light-induced self-healing. The combination of a fluorine-modified self-healing polyurethane, Fe3O4 nanoparticles-modified graphene (FFNs-iaG), and an elastic fabric creates a self-healing superhydrophobic conductive fabric coating. This coating has excellent properties for various applications, enhancing the longevity and reliability of the fabric sensor. The combination of these materials greatly improves the performance of the fabric composite coating, resulting in a versatile fabric sensor with unique characteristics suitable for a wide range of applications.

Liquid Metal Fibers with a Knitted Structure for Wearable Electronics.

Flexible conductive fibers have shown tremendous potential in diverse fields, including health monitoring, intelligent robotics, and human-machine interaction. Nevertheless, most conventional flexible conductive materials face challenges in meeting the high conductivity and stretchability requirements. In this study, we introduce a knitted structure of liquid metal conductive fibers. The knitted structure of liquid metal fiber significantly reduces the resistance variation under tension and exhibits favorable durability, as evidenced by the results of cyclic tensile testing, which indicate that their resistance only undergoes a slight increase (<3%) after 1300 cycles. Furthermore, we demonstrate the integration of these liquid metal fibers with various rigid electronic components, thereby facilitating the production of pliable LED arrays and intelligent garments for electrocardiogram (ECG) monitoring. The LED array underwent a 30 min machine wash, during which it consistently retained its normal functionality. These findings evince the devices' robust stable circuit functionality and water resistance that remain unaffected by daily human activities. The liquid metal knitted fibers offer great promise for advancing the field of flexible conductive fibers. Their exceptional electrical and mechanical properties, combined with compatibility with existing electronic components, open new possibilities for applications in the physiological signal detection of carriers, human-machine interaction, and large-area electronic skin.

Surface-Localized Chemically Modified Reduced Graphene Oxide Nanocomposites as Flexible Conductive Surfaces for Space Applications.

Thermoplastic polymers are a compelling class of materials for emerging space exploration applications due to their wide range of mechanical properties and compatibility with a variety of processing methods, including additive manufacturing. However, despite these benefits, the use of thermoplastic polymers in a set of critical space applications is limited by their low electrical conductivity, which makes them susceptible to static charging and limits their ability to be used as active and passive components in electronic devices, including materials for static charge dissipation, resistive heaters, and electrodynamic dust shielding devices. Herein, we explore the microstructural evolution of electrically conductive, surface-localized nanocomposites (SLNCs) of chemically modified reduced graphene oxide and a set of thermoplastic polymers as a function of critical thermal properties of the substrate (melting temperature for semi-crystalline materials or glass transition temperature for amorphous materials). Selected offsets from critical substrate temperatures were used to produce SLNCs with conductivities between 0.6-3 S/cm and surface structures, which ranged from particle-rich, porous surfaces to polymer-rich, non-porous surfaces. We then demonstrate the physical durability of these electrically conductive SLNCs to expected stress conditions for flexible conductive materials in lunar applications including tension, flexion, and abrasion with lunar simulant. Small changes in resistance (R/R0 < 2) were measured under uniaxial tension up to 20% strain in high density polyethylene and up to 500 abrasion cycles in polysulfone, demonstrating the applicability of these materials as active and passive flexible conductors in exterior lunar applications. The tough, electrically conductive SLNCs developed here could greatly expand the use of polymeric materials in space applications, including lunar exploration, micro- and nano-satellites, and other orbital structures.

Liquid metal flexible electronics: Past, present, and future

Flexible electronics is one of the most overwhelming and promising technologies available today, receiving widespread attention from both academia and industry. As it continues to evolve, demands on flexible conductive materials are becoming increasingly prominent. Liquid metals (LMs), which combine the compliance of fluids with the electrical conductivity of metals, are excellent candidates among various material options for fabricating flexible electronics. Following more than a decade of development, LM flexible electronics has become a rather promising research field. This review aims to provide a comprehensive summary and interpretation of the typical progress in LM flexible electronics so as to promote further advancement in the area. First, modification strategies of LMs, such as oxidation and composite approaches (doped particles or mixed polymers), are systematically digested to improve their performances and increase the formability involved. Furthermore, we divide the LM flexible electronics into three major architectures (LM flexible electronics with channels, LM printed electronics, and LM/polymer flexible electronics) and introduce the core patterning methods for each type. In addition, we provide an overview of the representative applications of LM flexible electronics in the categories of soft sensors, biomedicine, flexible energy, electronic fabrics, etc. Finally, the current challenges and potential directions of LM flexible electronics are discussed. Overall, the past progress, current situation, and future outlook as outlined in full view in the present article is expected to inspire tremendous new opportunities for further fundamental research or practical explorations of LM flexible electronics in the coming time.

A review of wearable supercapacitors fabricated from highly flexible conductive fiber materials

A review of wearable supercapacitors fabricated from highly flexible conductive fiber materials

Stretchable Broadband Absorbers Based on the Hauberk Structure

One of the major challenges in the design of flexible microwave absorbers is to achieve flexibility, particularly the stretchability of the overall structure while ensuring broadband absorption. In general, the comprehensive properties of flexible dielectric materials are good enough, whereas those of flexible conductive materials are relatively poor. Materials with both good conductivity and stretchability are rarely reported. In this letter, a stretchable absorber was designed with the conductive material made into the hauberk structure, utilizing the relative movement of the rings of the hauberk to realize the stretchability. At the same time, this structure can achieve extra-broad-band absorption at large incident angles. Approximately 80% absorption under vertically incident TE and TM waves was measured in (10–40) GHz, showing good agreement with the simulation results. When the overall structure was stretched uniaxially by 10%, experiment results showed that the absorption decreased slightly but was still broadband, which agreed well with the simulation results. What is more, the structure maintained broadband absorption under 0°, 45°, and 75° incident angles.