Stretchable Conductive Materials Research Articles

From grape seed extracts to extremely stable strain sensors with freezing tolerance, drying resistance and anti-oxidation properties

Conductive hydrogels were fascinating stretchable conductive materials that have found widespread application in electronic skin, wearable sensors and soft robots. Nonetheless, similar to conductive materials, conductive hydrogels experience severe performance degradation in strong UV environments and cold, dry climates, which severely restricts their practical applications. Using grape seed extract polymer (GSP), sodium chloride (NaCl), and a DMSO/ water binary solvent system, a type of hydrogel with a variety of fascinating multifunctional properties was quickly fabricated. The presence of GSP and NaCl endows hydrogel materials with remarkable UV-blocking (~82%), DPHH radical scavenging ability (~76.33%) and conductivity, allow them to be utilized as sensors. In addition, a DMSO/water binary solvent provides the hydrogel with superior long-term drying resistance and freezing tolerance, as well as excellent stretchability (>300%) and good conductivity even at -23 °C. This work investigates the potential of dynamic plant catechol chemistry in the field of hydrogel materials and anticipates that it will enable flexible sensors capable of adjusting to a variety of complex environments. • A Conductive hydrogel was designed by PVA and grape seed extract polymer (GSP). • GSP was used as a green source of antioxidant. • The hydrogel exhibited good UV-blocking and free radical scavenging properties. • The hydrogel exhibited excellent anti-drying and anti-freezing properties.

Rational design of sustainable diblock copolymers toward strong adhesives and stretchable ionic conductive materials

Sustainable diblock copolymers of poly (isobornyl acrylate)-block-poly (methyl acrylate) (PIBOA-b-PMA) were rationally designed and synthesized with terpenoid-derived monomer IBOA as the rigid segment. These PIBOA-b-PMA diblock copolymers show excellent mechanical performance comparable to traditional triblock copolymer elastomers. They can be successfully utilized as strong sustainable adhesives with good adhesion performance and high adhesive strength. Moreover, stretchable liquid-free ionic conductive materials can be successfully fabricated by incorporating lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) into PIBOA-b-PMA diblock copolymers without leakage or evaporation risks. Intriguingly, these ionic conductive materials show decreased tensile strength compared to that of pristine PIBOA-b-PMA diblock copolymer due to the plasticizing effect while still maintaining excellent extensibility. The resultant stretchable ionic conductive materials show accurate and dramatic resistance change in response to intermittent stretching-retraction deformations, finger joint bending/straightening curvatures, and elbow joint bending/straightening movements, demonstrating exceptional performance. This convenient and facile strategy can offer new opportunities for the development of novel bio-based diblock copolymers toward strong adhesives and stretchable ionic conductive materials.

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.

The influence of liquid silicone rubber on the properties of polyurethane elastomer/liquid silicone rubber/graphene nanoplatelets stretchable conductive materials

This work reports the influence of liquid silicone rubber (LSR) as a secondary matrix in the polyurethane elastomer (PUE)/LSR/graphene nanoplatelets (GnPs) stretchable conductive materials. PUE was prepared by mixing with diphenylmethane-4,4-diisocyanate (MDI) and 1,4 butanediol (BDO). The content of LSR varied from 0 to 50 vol.% at fixed 1.0 vol.% of graphene nanoplatelets (GnPs) as a conductive filler. Liquid silicone rubber was used as the secondary immiscible phase to localize GnPs into a path in the primary phase in order to obtain higher electrical conductivity value. The tensile strength of the PUE/LSR/GnPs decreased with increasing LSR content, while the tear strength shows the optimum value at 10 vol.% of LSR. The incorporation of 20 vol.% of silicone rubber has proven to enhance the thermal stability of the blends.

Mechanical behavior of stretchable conductive materials based on elastomeric core: experimental and theoretical simulation

The mechanical behavior of braided carbon nanotube yarns (CNTYs) on an elastomeric core to produce stretchable conductive materials were theoretically modeled and experimentally studied under tension. The elastomeric core served as the stretchable spring and the CNTYs braiding, with shape changing capabilities, as the conductive shell. A variety of samples were produced having various braiding angles on an elastomeric core and subsequently loaded in tension, and their stress–strain behavior was characterized. The model predicts the stress–strain behavior of the composite as a function of the initial braiding angle and the number of pitches. The innovative aspect was included in the model related to the friction between the braid and the core. Results indicated good agreement between the theoretical simulations and the experimental results which was not discussed in previous studies. Since the rate of the diameter decrease of the CNTYs braid was higher than that of the elastomeric core diameter, squeezing out of the core through the braid inter yarn space occurred. This limited the maximum potential extension of the braid. Thus, a critical strain was defined where the braid came into contact with the core. The addition of the friction stresses made a significant contribution to the overall stresses and the accuracy of the theoretical simulation, and its agreement with the experimental results. An apparent friction coefficient was proposed to account for the effect of the elastomer core/braid interactive restriction and squeezing out of the elastomer through the braiding, as observed in experimental results. As the CNTYs are conductive, a stretchable conductive composite was obtained having a resistivity of 9.05 × 10–4 Ohm*cm, which remained constant throughout the tensile loading until failure and under cyclic loading.

A Highly Conducting Polymer for Self-Healable, Printable, and Stretchable Organic Electrochemical Transistor Arrays and Near Hysteresis-Free Soft Tactile Sensors.

A stretchable and self-healable conductive material with high conductivity is critical to high-performance wearable electronics and integrated devices for applications where large mechanical deformation is involved. While there has been great progress in developing stretchable and self-healable conducting materials, it remains challenging to concurrently maintain and recover such functionalities before and after healing. Here, a highly stretchable and autonomic self-healable conducting film consisting of a conducting polymer (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS) and a soft-polymer (poly(2-acrylamido-2-methyl-1-propanesulfonic acid), PAAMPSA) is reported. The optimal film exhibits outstanding stretchability as high as 630% and high electrical conductivity of 320 S cm-1 , while possessing the ability to repair both mechanical and electrical breakdowns when undergoing severe damage at ambient conditions. This polymer composite film is further utilized in a tactile sensor, which exhibits good pressure sensitivity of 164.5 kPa-1 , near hysteresis-free, an ultrafast response time of 19ms, and excellent endurance over 1500 consecutive presses. Additionally, an integrated 5 × 4 stretchable and self-healable organic electrochemical transistor (OECT) array with great device performance is successfully demonstrated. The developed stretchable and autonomic self-healable conducting film significantly increases the practicality and shelf life of wearable electronics, which in turn, reduces maintenance costs and build-up of electronic waste.

Electromechanical tensile test equipment for stretchable conductive materials

The demand for soft and conductive materials has intensified due to the increased interest in soft robotics. Consequently, researchers strive to realize easy, fast, and cost-effective fabrication methods. To evaluate the mechanical properties of materials requires tensile testing. However, the availability of an electromechanical tensile test to assess the quality of the electromechanical properties of stretchable conductive materials has yet to be widely commercialized. This situation has hindered the development of soft and stretchable conductive materials. Here, we develop a customized electromechanical tensile test for soft and stretchable materials. We integrate three standalone devices using Python software and provide a graphic user interface (GUI) for easy operation of the equipment. We expect that our customized electromechanical tensile test will contribute to advances in soft robotics, especially soft and stretchable sensors. Furthermore, our electromechanical setup can aid in the development of laboratory equipment and the understanding of the electromechanical properties of stretchable conductive materials.

Research progress of silver nanowire-based stretchable transparent conductive film: materials, devices and applications

With the continuous development of electronic products in the direction of wearable and portable, the flexible electronic devices on stretchable substrates have attracted great interest. As one of the important components of electronic devices, stretchable transparent conductive films have become an important research direction. The traditional indium tin oxide cannot meet the application in flexible devices due to its poor flexibility and other problems. As a new type of one-dimensional nanomaterial, nanowires possess the size effect of nanomaterials and high electrical conductivity and give the stretchable transparent conductive films excellent optical properties and flexibility performance, which makes them very promising in stretchable conductive materials. This article reviews the synthesis methods of silver nanowires, and the research progress of stretchable transparent conductive films based on silver nanowires, and gives an outlook on the future development direction, to provide a reference for the preparation of high-performance silver nanowires transparent conductive films.

Zwitterionic dual-network strategy for highly stretchable and transparent ionic conductor

The development of flexible electronic devices depends on the research of stretchable conductive materials. In this work, we report a green and transparent ionic conductor with high stretchability based on green deep eutectic solvents (DESs) and biocompatible zwitterionic polymer. The dual-network (DN) strategy was achieved by two-step UV-initiated polymerization, and the DN ion gels consisting of the physically cross-linked zwitterionic poly(3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate) (PDMAPS) and chemically cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) were prepared, using choline chloride/ethylene glycol (ChCl/EG) DESs as solvents. The prepared DN ion gels present outstanding tensile properties (breaking strain over 1000%), high transparency (>90%), good room temperature ionic conductivity (0.315 S m−1), and good fatigue resistance. In addition, due to the ultra-low volatility and low freezing point of the ChCl/EG DESs, the DN ion gel shows high stability and frost resistance (−60 °C). Finally, the performance of the DN ionic gel-based flexible strain sensor is investigated to evaluate its application potential in wearable flexible electronic devices. The DN ion gel strain sensor can sensitively and stably monitor human motions. The outstanding characteristics such as non-toxicity and biocompatibility of both the conductive filler and polymer matrix for preparing the DN ion gels in this work provide a new view for the development of green flexible conductive devices with high performance and multiple functions.

Dynamic electro‐mechanical analysis of highly conductive particle‐elastomer composites

AbstractThe availability of stretchable conductive materials is a key requirement for the development of soft and wearable electronics. Although there are many promising materials, the characterization of these materials under realistic conditions is complex and a standardized and reliable procedure has not been etablished yet. We therefore introduce a comprehensive protocol for the practice‐oriented dynamic electro‐mechanical analysis of elastomer‐particle composites. In addition to strain dependence (0–100% strain) and fatigue strength (10,000 cycles), this protocol aims in particular to clarify the influence of strain rate (0–100% s−1) on conductivity. Samples with the commonly used filler representatives carbon black and silver flakes with 20 vol% each were prepared and investigated. Silicone elastomers of different stiffness were used as matrix in order to determine its influence. We found that while the conductivity of the carbon black composites of about 1 × 102 S m−1 proved to be fatigue resistant and largely independent of the strain rate, the silver flake composites lost their initially higher conductivity of 1 × 104 S m−1 at high strain rates and increasing numbers of cycles. In addition, the use of a softer silicone matrix improved the performance of both particle composites, which was also demonstrated on an exemplary wearable electronic device.

Hierarchically Structured Stretchable Conductive Hydrogels for High-Performance Wearable Strain Sensors and Supercapacitors

Summary Stretchable conductive materials, a critical building block of soft electronics, typically require multiple components that synergistically contribute good mechanical, electrical, and interfacial properties. The overall performance is often hindered by phase instability and poor miscibility of functional fillers within polymer matrices, compromising the conductive percolative network. We addressed this challenge with an ice-templated, low-temperature polymerization (ITLP) strategy and created stretchable conducting hydrogels. Owning a hierarchical dendritic microstructure with mitigated nanoaggregation, the material exhibited 29-fold enhancement in toughness and 83-fold increase in conductivity. Strain sensors using such gels demonstrated a broad detection range, high sensitivity, and health-monitoring capability. ITLP gel electrodes exhibited 888 F/g specific capacitance and 2,097 mF/cm2 areal capacitance (368 F/g) when used in solid-state supercapacitors. Flexible and stretchable wearable supercapacitors have been successfully made and can power LEDs. The ITLP strategy is anticipated to create diverse high-performance soft-electronic materials for broad applications in energy, healthcare, and robotics.

Stretchable Electronics Based on Laser Structured, Vapor Phase Polymerized PEDOT/Tosylate.

The fabrication of stretchable conductive material through vapor phase polymerization of poly(3,4-ethylenedioxythiophene) (PEDOT) is presented alongside a method to easily pattern these materials with nanosecond laser structuring. The devices were constructed from sheets of vapor phase polymerized PEDOT doped with tosylate on pre-stretched elastomeric substrates followed by laser structuring to achieve the desired geometrical shape. Devices were characterized for electrical conductivity, morphology, and electrical integrity in response to externally applied strain. Fabricated PEDOT sheets displayed a conductivity of 53.1 ± 1.2 S cm−1; clear buckling in the PEDOT microstructure was observed as a result of pre-stretching the underlying elastomeric substrate; and the final stretchable electronic devices were able to remain electrically conductive with up to 100% of externally applied strain. The described polymerization and fabrication steps achieve highly processable and patternable functional conductive polymer films, which are suitable for stretchable electronics due to their ability to withstand externally applied strains of up to 100%.

Fabrication of BiInSn alloy powder via the combination of ultrasonic crushing with dispersants

BiInSn alloy, one of the low-melting-point alloys, has attracted more and more attention in various fields recently. However, research into applying BiInSn powder is limited, and most existing methods of preparing metal powders are not quite suitable for BiInSn alloy due to its special properties. Here, an effective method has been provided to fabricate BiInSn alloy powder based on ultrasonic crushing in combination with the dispersant solution. The effects of the dispersant and the cooling rate on the size distribution and shape of the powder are discussed. The experiment results show that the size of BiInSn powder prepared with ethylene glycol with higher viscosity is much smaller. Compared with bulk metal, the BiInSn powder exhibit more superior properties in the application. For example, the sintered BiInSn powder has better performance in the micro-hardness measurement. The micro-hardness of the products using ethylene glycol increases by 90.7% compared with that of the bulk metal. Besides, the BiInSn powder can be used to fabricate stretchable conductive material.

Carbon Nanotube-Based Stretchable Hybrid Material Film for Electronic Devices and Applications.

To meet the increasing demand, for stretchable conductive materials in a wide range of applications, innovative conductors based on single wall carbon nanotubes (SWCNT) self-grafted on different polymer films, are assembled. Aiming at a simple technology for flexible and stretchable electronic devices, and contrary to what commonly reported for carbon nanotubes (CNT), no chemical functionalization of SWCNT is necessary for stable grafting onto several polymeric surfaces. The novelty and functionality of our composite materials stand in the synergy among the intrinsic biocompatibility of CNT, a fully inert material, their electrical conductivity, and the stretchable-viscoelastic properties of the polymer-nanotube bundles composites. Electrical characterization of both unstretched and strongly stretched planar film conductors is provided, demonstrating the use of this new composite material for technological application. Also, an insight into the mechanisms of strong adhesion to the polymer is obtained by scanning electron microscopy (SEM) of the surface composite. As an example of technological application of such stretchable circuitry, the electrical functionality of a carbon nanotube-based six-sensor (electrode) grid is used to record subdural electrocorticograms in freely-moving laboratory rats over approximately three months.

A Super-Stretchable Liquid Metal Foamed Elastomer for Tunable Control of Electromagnetic Waves and Thermal Transport.

It is remarkably desirable and challenging to design a stretchable conductive material with tunable electromagnetic‐interference (EMI) shielding and heat transfer for applications in flexible electronics. However, the existing materials sustained a severe attenuation of performances when largely stretched. Here, a super‐stretchable (800% strain) liquid metal foamed elastomer composite (LMF‐EC) is reported, achieving super‐high electrical (≈104 S cm−1) and thermal (17.6 W mK−1) conductivities under a large strain of 400%, which also exhibits unexpected stretching‐enhanced EMI shielding effectiveness of 85 dB due to the conductive network elongation and reorientation. By varying the liquid and solid states of LMF, the stretching can enable a multifunctional reversible switch that simultaneously regulates the thermal, electrical, and electromagnetic wave transport. Novel flexible temperature control and a thermoelectric system based on LMF‐EC is furthermore developed. This work is a significant step toward the development of smart electromagnetic and thermal regulator for stretchable electronics.

Highly stretchable polymer/silver nanowires composite sensor for human health monitoring

Flexible strain sensors exhibit outstanding advantages in terms of sensitivity and stability by detecting changes in physical signals. It can be easily attached to human skin and clothed to achieve monitoring of human motion and health. However, general sensing materials shows low stretchability and cannot respond to signals under large deformation. In this work, a highly stretchable polymer composite was developed by adding small amount (0.17 wt.%) of silver nanowires (AgNWs) in stretchable conductive polymer materials. The conductivity of polymer/AgNWs composite is 1.3 S/m with the stretchability up to 500%. The stretchable strain sensor based on the polymer/AgNWs composite can respond to strain signals in real time, even for 1% strain response, and shows excellent stability over 1,000 loading/unloading cycles. Moreover, the strain sensor can be attached to human skin and clothed to monitor joints, throat and pulse of the human body. The human body electrocardiogram (ECG) signal was detected successfully with the polymer/AgNWs electrode, which is comparable to the signal obtained by the commercial electrode. Overall, the sensors enable monitoring of human movement and health. These advantages make it a potential application in wearable devices and electronic skin.

Advances in Rational Design and Materials of High-Performance Stretchable Electromechanical Sensors.

Stretchable and wearable sensor technology has attracted significant interests and created high technological impact on portable healthcare and smart human-machine interfaces. Wearable electromechanical systems are an important part of this technology that has recently witnessed tremendous progress toward high-performance devices for commercialization. Over the past few years, great attention has been paid to simultaneously enhance the sensitivity and stretchability of the electromechanical sensors toward high sensitivity, ultra-stretchability, low power consumption or self-power functionalities, miniaturisation as well as simplicity in design and fabrication. This work presents state-of-the-art advanced materials and rational designs of electromechanical sensors for wearable applications. Advances in various sensing concepts and structural designs for intrinsic stretchable conductive materials as well as advanced rational platforms are discussed. In addition, the practical applications and challenges in the development of stretchable electromechanical sensors are briefly mentioned and highlighted.

Characterisation of Morphic Sensors for Body Volume and Shape Applications.

Stretchable conductive materials are originally conceived as radio frequency (RF) and electromagnetic interference (EMI) shielding materials, and, under stretch, they generally function as distributed strain-gauges. These commercially available conductive elastomers have found their space in low power health monitoring systems, for example, to monitor respiratory and cardiac functions. Conductive elastomers do not behave linearly due to material constraints; hence, when used as a sensor, a full characterisation to identify ideal operating ranges are required. In this paper, we studied how the continuous stretch cycles affected the material electrical and physical properties in different embodiment impressed by bodily volume change. We simulated the stretch associated with breathing using a bespoke stress rig to ensure reproducibility of results. The stretch rig is capable of providing constant sinusoidal waves in the physiological ranges of extension and frequency. The material performances is evaluated assessing the total harmonic distortion (THD), signal-to-noise ratio (SNR), correlation coefficient, peak to peak (P-P) amplitude, accuracy, repeatability, hysteresis, delay, and washability. The results showed that, among the three controlled variables, stretch length, stretch frequency and fabric width, the most significant factor to the signal quality is the stretch length. The ideal working region is within 2% of the original length. The material cut in strips of >3 mm show more reliable to handle a variety of stretch parameter without losing its internal characteristics and electrical properties.

From Silver Nanoflakes to Silver Nanonets: An Effective Trade-Off between Conductivity and Stretchability of Flexible Electrodes.

Flexible and stretchable conductive materials have received significant attention due to their numerous potential applications in flexible printed electronics. In this paper, we describe a new type of conductive filler for flexible electrodes—silver nanonets prepared through the “dissolution–recrystallization” solvothermal route from porous silver nanoflakes. These new silver fillers show characteristics of both nanoflakes and nanoparticles with propensity to form interpenetrating polymer–silver networks. This effectively minimizes trade-off between composite electrode conductivity and stretchability and enables fabrication of the flexible electrodes simultaneously exhibiting high conductivity and mechanical durability. For example, an electrode with uniform, networked silver structure from the flakiest silver particles showed the lowest increase of resistivity upon extension (3500%), compared to that of the electrode filled with less flaky (3D) particles (>50,000%).

The effect of graphene loading on natural rubber latex/graphene stretchable conductive material

The demand for the soft and flexible stretchable electronic has been increased due to the flourish development of electronic field nowadays. This can be achieved by incorporating graphene nanoplatelet (GNP) into natural rubber latex (NRL). This paper was aimed to investigate the effect of the GNP content on the properties of the NRL/GNP composites. The incorporating of GNPs into NRL increased the density and modulus of the composite but reduced the tensile strength, elongation at break and crosslink density. On the other hand, the NRL/GNP has reached the percolation threshold at 7phr of GNP, at this content the composite became electrically conductive.