Smart conducting polymer composites having zero temperature coefficient of resistance

A Review of Conductive Hydrogel-Based Wearable Temperature Sensors

Conductive hydrogel has garnered significant attention as an emergent candidate for diverse wearable sensors, owing to its remarkable and tailorable properties such as flexibility, biocompatibility, and strong electrical conductivity. These attributes make it highly suitable for various wearable sensor applications (e.g., biophysical, bioelectrical, and biochemical sensors) that can monitor human health conditions and provide timely interventions. Among these applications, conductive hydrogel-based wearable temperature sensors are especially important for healthcare and disease surveillance. This review aims to provide a comprehensive overview of conductive hydrogel-based wearable temperature sensors. First, this work summarizes different types of conductive fillers-based hydrogel, highlighting their recent developments and advantages as wearable temperature sensors. Next, this work discusses the sensing characteristics of conductive hydrogel-based wearable temperature sensors, focusing on sensitivity, dynamic stability, stretchability, and signal output. Then, state-of-the-art applications are introduced, ranging from body temperature detection and wound temperature detection to disease monitoring. Finally, this work identifies the remaining challenges and prospects facing this field. By addressing these challenges with potential solutions, this review hopes to shed some light on future research and innovations in this promising field.

Toward flexible piezoresistive strain sensors based on polymer nanocomposites: a review on fundamentals, performance, and applications

The fundamentals, performance, and applications of piezoresistive strain sensors based on polymer nanocomposites are summarized herein. The addition of conductive nanoparticles to a flexible polymer matrix has emerged as a possible alternative to conventional strain gauges, which have limitations in detecting small strain levels and adapting to different surfaces. The evaluation of the properties or performance parameters of strain sensors such as the elongation at break, sensitivity, linearity, hysteresis, transient response, stability, and durability are explained in this review. Moreover, these nanocomposites can be exposed to different environmental conditions throughout their lifetime, including different temperature, humidity or acidity/alkalinity levels, that can affect performance parameters. The development of flexible piezoresistive sensors based on nanocomposites has emerged in recent years for applications related to the biomedical field, smart robotics, and structural health monitoring. However, there are still challenges to overcome in designing high-performance flexible sensors for practical implementation. Overall, this paper provides a comprehensive overview of the current state of research on flexible piezoresistive strain sensors based on polymer nanocomposites, which can be a viable option to address some of the major technological challenges that the future holds.

Recent Research on Preparation and Application of Smart Joule Heating Fabrics

Multifunctional wearable heaters have attracted much attention for their effective applications in personal thermal management and medical therapy. Compared to passive heating, Joule heating offers significant advantages in terms of reusability, reliable temperature control, and versatile coupling. Joule-heated fabrics make wearable electronics smarter. This review critically discusses recent advances in Joule-heated smart fabrics, focusing on various fabrication strategies based on material-structure synergy. Specifically, various applicable conductive materials with Joule heating effect are first summarized. Subsequently, different preparation methods for Joule heating fabrics are compared, and then their various applications in smart clothing, healthcare, and visual indication are discussed. Finally, the challenges faced in developing these smart Joule heating fabrics and their possible solutions are discussed.

New Carbon Materials for Multifunctional Soft Electronics

Soft electronics are garnering significant attention due to their wide-ranging applications in artificial skin, health monitoring, human-machine interaction, artificial intelligence, and the Internet of Things. Various soft physical sensors such as mechanical sensors, temperature sensors, and humidity sensors are the fundamental building blocks for soft electronics. While the fast growth and widespread utilization of electronic devices have elevated life quality, the consequential electromagnetic interference (EMI) and radiation pose potential threats to device precision and human health. Another substantial concern pertains to overheating issues that occur during prolonged operation. Therefore, the design of multifunctional soft electronics exhibiting excellent capabilities in sensing, EMI shielding, and thermal management is of paramount importance. Because of the prominent advantages in chemical stability, electrical and thermal conductivity, and easy functionalization, new carbon materials including carbon nanotubes, graphene and its derivatives, graphdiyne, and sustainable natural-biomass-derived carbon are particularly promising candidates for multifunctional soft electronics. This review summarizes the latest advancements in multifunctional soft electronics based on new carbon materials across a range of performance aspects, mainly focusing on the structure or composite design, and fabrication method on the physical signals monitoring, EMI shielding, and thermal management. Furthermore, the device integration strategies and corresponding intriguing applications are highlighted. Finally, this review presents prospects aimed at overcoming current barriers and advancing the development of state-of-the-art multifunctional soft electronics.

Multifunctional MoSe2 @MXene Heterostructure-Decorated Cellulose Fabric for Wearable Thermal Therapy

A booming demand for wearable electronic devices urges the development of multifunctional smart fabrics. However, it is still facing a challenge to fabricate multifunctional smart fabrics with satisfactory mechanical property, excellent Joule heating performance, highly efficient photothermal conversion, outstanding electromagnetic shielding effectiveness, and superior anti-bacterial capability. Here, a MoSe₂ @MXene heterostructure-based multifunctional cellulose fabric is fabricated by depositing MXene nanosheets onto cellulose fabric followed by a facile hydrothermal method to grow MoSe₂ nanoflakes on MXene layers. A low-voltage Joule heating therapy platform with rapid Joule heating response (up to 230 °C in 25 s at a supplied voltage of 4 V) and stable performance under repeated bending cycles (up to 1000 cycles) is realized. Besides, the multifunctional fabric also exhibits excellent photothermal performance (up to 130 °C upon irradiation for 25 s with a light intensity of 400 mW cm^-2 ), outstanding electromagnetic interference shielding effectiveness (37 dB), and excellent antibacterial performances (>90% anti-bacterial rate toward Escherichia coli, Bacillus subtilis, and Staphylococcus aureus). This work offers an efficient avenue to fabricate multifunctional wearable thermal therapy devices for mobile healthcare and personal thermal management.

Independent Heating Performances in the Sub-Zero Environment of MWCNT/PDMS Composite with Low Electron-Tunneling Energy

The structural stability of various structures (railroads, bridges, buildings, etc.) is lowered due to freezing because of the decreasing outside temperature in winter. To prevent damage from freezing, a technology for de-icing has been developed using an electric-heating composite. For this purpose, a highly electrically conductive composite film with multi-wall carbon nanotubes (MWCNTs) uniformly dispersed in a polydimethylsiloxane (PDMS) matrix through a three-roll process was fabricated by shearing the MWCNT/PDMS paste, through a two-roll process. The electrical conductivity and the activation energy of the composite were 326.5 S/m and 8.0 meV at 5.82 Vol% of MWCNTs, respectively. The dependence of the electric-heating performance (heating rate and temperature change) on the applied voltage and environmental temperature (from -20 °C to 20 °C) was evaluated. The heating rate and effective-heat-transfer characteristics were observed to decrease as the applied voltage increased, while they showed the opposite tendency when the environmental temperature was at sub-zero temperatures. Nevertheless, the overall heating performance (heating rate and temperature change) was maintained with little significant difference in the considered external-temperature range. The unique heating behaviors can result from the low activation energy and the negative-temperature (T) coefficient of resistance (R) (NTCR, dR/dT < 0) of the MWCNT/PDMS composite.

Electrical and Thermal Properties of Carbon Nanotube Polymer Composites with Various Aspect Ratios

In response to the rising need for flexible and lightweight materials capable of efficient heat transport, many studies have been conducted to improve the thermal properties of polymers via nanofillers. Among the various nanofillers, carbon nanotubes (CNTs) are considered as the most promising, owing to their excellent thermal and electrical properties. Accordingly, CNT/polymer composites can be used as flexible and lightweight heat transfer materials, owing to their low density. In this study, we fabricated multi-walled CNT (MWCNT)/polymer composites with different aspect ratios to investigate their effects on electrical and thermal properties. Through a three-roll milling process, CNTs were uniformly dispersed in the polymer matrix to form a conductive network. Enhanced electrical and thermal properties were observed in MWCNT composite with a high aspect ratio as compared to those with a low aspect ratio. The thermal conductivity of composites obtained as a function of the filler content was also compared with the results of a theoretical prediction model.

Temperature-dependent electrical resistivity of macroscopic graphene nanoplatelet strips

This paper studies the temperature dependence of the electrical resistivity of low-cost commercial graphene-based strips, made from a mixture of epoxy and graphene nanoplatelets. An equivalent homogenous resistivity model is derived from the joint use of experimental data and simulation results obtained by means of a full three-dimensional (3D) numerical electrothermal model. Three different types of macroscopic strips (with surface dimensions of cm²) are analyzed, differing in their percentage of graphene nanoplatelets. The experimental results show a linear trend of resistivity in a wide temperature range (-60°C to +60°C), and a negative temperature coefficient . The derived analytical model of temperature-dependent resistivity follows the simple law commonly adopted for conventional conducting materials, such us copper. The model is then validated by using the graphene strips as heating elements by exploiting the Joule effect. These results suggest that such materials can be used as thermistors in sensing or heating applications.

A Switching-Type Positive Temperature Coefficient Behavior Exhibited by PPy/(PhSe)2 Nanocomposite Prepared by Chemical Oxidative Polymerization

In this study, we prepared a polypyrrole-diphenyl diselenide [PPy/(PhSe)₂] nanocomposite by oxidative chemical polymerization for the purpose of temperature sensing applications. Fourier transform infrared spectroscopy, X-ray diffraction and scanning transmission electron microscopy confirmed the synthesis of the above material. Thermogravimetry (TG) revealed enhanced thermal stability as compared to pristine polypyrrole (PPy). Dielectric study showed the material to have a dielectric constant of colossal value. The material has been found to exhibit correlated barrier hopping conduction (CBH) wherein hopping of charge carriers takes place over the insulating (PhSe)₂ barrier. The maximum barrier height was found to be 0.224 eV. The nanocomposite material was found to exhibit a switching-type positive temperature coefficient (PTC) behavior with a Curie temperature of 400 K. This has been explained by a CBH model wherein PPy chains expand upon heating, thereby reducing the barrier height to facilitate current flow. However, above 400 K, disruption of PPy chains allows to reflect a PTC behavior. This has been in agreement with TG data.

Uniform conductivity in stretchable silicones via multiphase inclusions

Many soft robotic components require highly stretchable, electrically conductive materials for proper operation. Often these conductive materials are used as sensors or as heaters for thermally responsive materials. However, there is a scarcity of stretchable materials that can withstand the high strains typically experienced by soft robots, while maintaining the electrical properties necessary for Joule heating (e.g., uniform conductivity). In this work, we present a silicone composite containing both liquid and solid inclusions that can maintain a uniform conductivity while experiencing 200% linear strains. This composite can be cast in thin sheets enabling it to be wrapped around thermally responsive soft materials that increase their volume or stretchability when heated. We show how this material opens up possibilities for electrically controllable shape changing soft robotic actuators, as well as all-silicone actuation systems powered only by electrical stimulus. Additionally, we show that this stretchable composite can be used as an electrode material in other applications, including a strain sensor with a linear response up to 200% strain and near-zero signal noise.

Single-layer metamaterial bolometer for sensitive detection of low-power terahertz waves at room temperature

This study demonstrates a metamaterial bolometer that can detect terahertz (THz) waves by measuring variations in electrical resistance. A metamaterial pattern for enhanced THz waves absorption and a composite material with a high temperature coefficient of resistance (TCR) are incorporated into a single layer of the bolometer chip to realize a compact and highly sensitive device. To detect the temperature change caused by the absorption of the THz waves, a polydimethylsiloxane mixed with carbon black microparticles is used. The thermosensitive composite has TCR ranging from 1.88%/K to 3.11%/K at room temperature (22.2-23.8^°C). In addition, a microscale metamaterial without a backside reflector is designed to enable the measurement of the resistance and to enhance the sensitivity of the bolometer. The proposed configuration effectively improves thermal response of the chip as well as the absorption of the THz waves. It was confirmed that the irradiated THz waves can be detected via the increment in the electrical resistance. The resistance change caused by the absorption of the THz waves is detectable in spite of the changes in resistance originating from the background thermal noise. The proposed metamaterial bolometer could be applied to detect chemical or biological molecules that have fingerprints in the THz band by measuring the variation of the resistance without using the complex and bulky THz time-domain spectroscopy system.

Flexible PEBAX/graphene electromagnetic shielding composite films with a negative pressure effect of resistance for pressure sensors applications

In the current work, we fabricated flexible poly(ether-block-amide) (PEBAX)/graphene composite films by a combination of facile melt blending and compression molding technique. The graphene content significantly affects the mechanical properties, electrical conductivity and electromagnetic interference (EMI) shielding performance. An electrically conductive percolation threshold of 1.75 vol% graphene was obtained in the PEBAX/graphene composites. With the introduction of 4.45 vol%, and 8.91 vol% graphene content, the average EMI SE of composite films could reach 16.6 and 30.7 dB, respectively. More interestingly, the PEBAX/graphene composite exhibited a nearly-linear negative pressure coefficient (NPC) effect of resistance with increasing outer pressure stimulation, which was attributed to the formation of more conductive pathways caused by the decreased distance between adjacent graphene. In addition, these composites demonstrated good sensing stability, recoverability and reproducibility after stabilization by cyclic pressure loading. The current study provides guidelines for the large-scale preparation of elastomer NPC sensors and smart EMI shielding devices.

Graphene/PEBAX composite films present high-efficiency EMI shielding properties and good sensitivity as well as sensing stability.

Effect of Dispersion by Three-Roll Milling on Electrical Properties and Filler Length of Carbon Nanotube Composites

For practical use of carbon nanotube (CNT) composites, especially in electronic applications, uniform dispersion of a high concentration of CNTs in a polymer matrix is a critical challenge. Three-roll milling is one of most reliable dispersion techniques. We investigate the effect of three-roll milling time on CNT length and the electrical properties of a CNT/polydimethylsiloxane composite film with 10 wt% CNTs. During the milling process, the CNT length is decreased from 10 to 1-4 μm by mechanical shear forces. The electrical conductivity increases after 1.5 min of milling owing to dispersion of the CNTs but decreases with increasing milling time owing to the decrease in the CNT length. Considering the changes in the electrical conductivity of the CNT composite and CNT length, we determined how to optimize the three-roll milling time to obtain a suitable dispersion state.

Silver-Coated Poly(dimethylsiloxane) Beads for Soft, Stretchable, and Thermally Stable Conductive Elastomer Composites

We introduce an elastomer composite filled with silver (Ag) flakes and Ag-coated poly(dimethylsiloxane) (PDMS) beads that exhibits electrical conductivity that is 2 orders of magnitude greater than that of elastomers in which the same concentration of Ag filler is uniformly dispersed. In addition to the dramatic enhancement in conductivity, these composites exhibit high mechanical compliance (strain limit, >100%) and robust thermal stability (conductivity change, <10% at 150 °C). The incorporation of Ag-coated PDMS beads introduces an effective phase segregation in which Ag flakes are confined to the "grain boundaries" between the embedded beads. This morphological control aids in the percolation of the Ag flakes and the formation of conductive bridges between neighboring Ag shells. The confinement of Ag flakes also suppresses thermal expansion and changes in electrical conductivity of the percolating networks when the composite is heated. We demonstrate potential applications of thermally stable elastic conductors in wearable devices and soft robotics by fabricating a highly stretchable antenna for a "smart" furnace glove and a strain sensor for soft gripper operation in hot water.

Modeling the electrical resistivity of polymer composites with segregated structures

Hybrid carbon nanotube composites with two different types of fillers have attracted considerable attention for various advantages. The incorporation of micro-scale secondary fillers creates an excluded volume that leads to the increase in the electrical conductivity. By contrast, nano-scale secondary fillers shows a conflicting behavior of the decreased electrical conductivity with micro-scale secondary fillers. Although several attempts have been made in theoretical modeling of secondary-filler composites, the knowledge about how the electrical conductivity depends on the dimension of secondary fillers was not fully understood. This work aims at comprehensive understanding of the size effect of secondary particulate fillers on the electrical conductivity, via the combination of Voronoi geometry induced from Swiss cheese models and the underlying percolation theory. This indicates a transition in the impact of the excluded volume, i.e., the adjustment of the electrical conductivity was measured in cooperation with loading of second fillers with different sizes.

Improved Electromagnetic Interference Shielding Properties Through the Use of Segregate Carbon Nanotube Networks

We report the enhanced electromagnetic interference (EMI) shielding properties of hybrid carbon nanotube (CNT) composites consisting of more than two kinds of fillers through the use of segregate conducting networks. An excluded volume was created by micro-sized silica particles that concentrate the CNT network, resulting in improved electrical conductivity and microwave properties. To achieve the optimal dispersion of CNTs and silica particles, high shear force was applied to the pre-cured composite mixture via three-roll milling. Depending on the micro-silica content ratio, we observed improved electrical conductivity and EMI shielding properties. For a quantitative comparison to observe the excluded-volume effects, a CNT composite without micro-silica was measured in parallel with the other sample.

Laser patterned, high-power graphene paper resistor with dual temperature coefficient of resistance

Printing of electronic devices on a paper substrate using 2D graphene-based ink is an opening gate to innovative applications, where devices would be biodegradable, eco-friendly and can be disposed of with negligible impact on the environment.

Accelerated Curing and Enhanced Material Properties of Conductive Polymer Nanocomposites by Joule Heating

Joule heating is useful for fast and reliable manufacturing of conductive composite materials. In this study, we investigated the influence of Joule heating on curing conditions and material properties of polymer-based conductive composite materials consisting of carbon nanotubes (CNTs) and polydimethylsiloxane (PDMS). We applied different voltages to the CNT nanocomposites to investigate their electrical stabilization, curing temperature, and curing time. The result showed that highly conductive CNT/PDMS composites were successfully cured by Joule heating with uniform and fast heat distribution. For a 7.0 wt % CNT/PDMS composite, a high curing temperature of around 100 °C was achieved at 20 V with rapid temperature increase. The conductive nanocomposite cured by Joule heating also revealed an enhancement in mechanical properties without changing the electrical conductivities. Therefore, CNT/PDMS composites cured by Joule heating are useful for expediting the manufacturing process for particulate conductive composites in the field of flexible and large-area sensors and electronics, where fast and uniform curing is critical to their performance.

Electron tunnelling and hopping effects on the temperature coefficient of resistance of carbon nanotube/polymer nanocomposites

Effectively tailoring the temperature coefficient of resistance (TCR) is critical for multifunctional carbon nanotube (CNT) polymer composites with sensing capability. By developing a new multiscale percolation network model, this work reveals theoretically that the zero-TCR could be achieved by adjusting competing contributions from thermally assisted tunnelling transport at CNT junctions and thermal expansion of matrices. On the other hand, the negative temperature coefficient of nanocomposites above glass transition temperature could be greatly enhanced because the transport mechanism at the CNT junctions experienced a transition from tunnelling to hopping. Both tube-tube and/or tube-matrix interactions at conjunction and the structural distortion of nanotubes are considered in the newly proposed model. To validate the model, CNT/polymer nanocomposites with nearly constant resistance values (zero-TCR) below the glass transition temperature and a high TCR (98% resistance change ratio) resulting from the glass transition of the polymer matrix are successfully developed. The study also suggests that the desired parameters to achieve the zero-TCR property and the potential resistance change ratio could be improved by the glass transition in nanocomposites. This could be beneficial for the development of high quality sensing materials.

Wrinkle-stabilized metal-graphene hybrid fibers with zero temperature coefficient of resistance

The interfacial adhesion between graphene and metals is poor, as metals tend to generate superlubricity on smooth graphene surface. This problem renders the free assembly of graphene and metals to be a big challenge, and therefore, some desired conducting properties (e.g., stable metal-like conductivities in air, lightweight yet flexible conductors, and ultralow temperature coefficient of resistance, TCR) likely being realized by integrating the merits of graphene and metals remains at a theoretical level. This work proposes a wrinkle-stabilized approach to address the poor adhesion between graphene surface and metals. Cyclic voltammetry (CV) tests and theoretical analysis by Scharifker-Hills models demonstrate that multiscale wrinkles effectively induce nucleation of metal particles, locking in metal nuclei and guiding the continuous growth of metal islands in an instantaneous model on rough graphene surface. The universality and practicability of the wrinkle-stabilized approach is verified by our investigation through the electrodeposition of nine kinds of metals on graphene fibers (GF). The strong interface bonding permits metal-graphene hybrid fibers to show metal-level conductivities (up to 2.2 × 10⁷ S m^-1, a record high value for GF in air), reliable weatherability and favorable flexibility. Due to the negative TCR of graphene and positive TCR of metals, the TCR of Cu- and Au-coated GFs reaches zero at a wide temperature range (15 K-300 K). For this layered model, the quantitative analysis by classical theories demonstrates the suitable thickness ratio of graphene layer and metal layer to achieve zero TCR to be 0.2, agreeing well with our experimental results. This wrinkle-stabilized approach and our theoretical analysis of zero-TCR behavior of the graphene-metal system are conducive to the design of high-performance conducting materials based on graphene and metals.