Microstructural origin of resistance-strain hysteresis in carbon nanotube thin film conductors

Adaptive nanotube networks enabling omnidirectionally deformable electro-driven liquid crystal elastomers towards artificial muscles

Artificial muscles that can convert electrical energy into mechanical energy promise broad scientific and technological applications. However, existing electro-driven artificial muscles have been plagued with problems that hinder their practical applications: large electro-mechanical attenuation during deformation, high-driving voltages, small actuation strain, and low power density. Here, we design and create novel electro-thermal-driven artificial muscles rationally composited by hierarchically structured carbon nanotube (HS-CNT) networks and liquid crystal elastomers (LCEs), which possess adaptive sandwiched nanotube networks with angulated-scissor-like microstructures, thus effectively addressing above problems. These HS-CNT/LCE artificial muscles demonstrate not only large strain (>40%), but also remarkable conductive robustness (R/R0 < 1.03 under actuation), excellent Joule heating efficiency (≈ 233 °C at 4 V), and high load-bearing capacity (R/R0 < 1.15 at 4000 times its weight loaded). In addition, our artificial muscles exhibit real-muscle-like morphing intelligence that enables preventing mechanical damage in response to excessively heavyweight loading. These high-performance artificial muscles uniquely combining omnidirectional stretchability, robust electrothermal actuation, low driving voltage, and powerful mechanical output would exert significant technological impacts on engineering applications such as soft robotics and wearable flexible electronics.

Ultrafast piezocapacitive soft pressure sensors with over 10 kHz bandwidth via bonded microstructured interfaces

Flexible pressure sensors can convert mechanical stimuli to electrical signals to interact with the surroundings, mimicking the functionality of the human skins. Piezocapacitive pressure sensors, a class of most widely used devices for artificial skins, however, often suffer from slow response-relaxation speed (tens of milliseconds) and thus fail to detect dynamic stimuli or high-frequency vibrations. Here, we show that the contact-separation behavior of the electrode-dielectric interface is an energy dissipation process that substantially determines the response-relaxation time of the sensors. We thus reduce the response and relaxation time to ~0.04 ms using a bonded microstructured interface that effectively diminishes interfacial friction and energy dissipation. The high response-relaxation speed allows the sensor to detect vibrations over 10 kHz, which enables not only dynamic force detection, but also acoustic applications. This sensor also shows negligible hysteresis to precisely track dynamic stimuli. Our work opens a path that can substantially promote the response-relaxation speed of piezocapacitive pressure sensors into submillisecond range and extend their applications in acoustic range.

Ultraconformable Capacitive Strain Sensor Utilizing Network Structure of Single-Walled Carbon Nanotubes for Wireless Body Sensing

Capture and real-time recording of precise body movements using strain sensors provide personal information for healthcare monitoring and management. To acquire this information, a sensor that conforms to curved irregular surfaces, including biological tissue, is desired to record complex body movements while acting like a second skin to avoid interference with the movements. In this study, we developed a thin-film-type capacitive strain sensor that is flexible and stretchable on the surface of a living body. We fabricated conductive polymeric ultrathin films ("nanosheets") comprising polystyrene-block-polybutadiene (SB) elastomers and single-walled carbon nanotubes (SWCNTs) (i.e., SWCNT-SB nanosheets) via gravure coating; the SWCNT-SB-coated nanosheets were used as the flexible electrode in a capacitive strain sensor. The dielectric (DE) layer was then prepared using the silicone elastomer Ecoflex 00-30 because its Young's modulus is comparable to that of the epidermis. The normalized capacitance changes (ΔC/C0) in the sensor increased with increasing tensile strain over a range from 0-100%, indicating that the proposed sensor can measure the strain of biological movements, including those of skin and blood vessels. To improve sensor conformability further, the effect of sensor thickness on the gauge factor (GF) was investigated using thinner DE layers by focusing on their flexural rigidity. As a result, the GF increased from 0.64 to 1.13 as the DE layer thickness decreased from 260 to 40 μm. Finally, we evaluated the fabricated sensor's signal stability and mechanical durability, including during wireless sensing when applied to human skin and a vascular model. The ΔC/C0 values varied in response to the bending motion of a finger, dilation of a blood vessel, and the swallowing movement of the throat. These results indicate that our capacitive strain sensor is conformable and functional on biological tissue to enable monitoring of dynamic biological movements (e.g., pulse rate and arterial dilation) without wearer discomfort.

In silico optimization of aligned fiber electrodes for dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) exhibit fast actuation and high efficiencies, enabling applications in optics, wearable haptics, and insect-scale robotics. However, the non-uniformity and high sheet resistance of traditional soft electrodes based on nanomaterials limit the performance and operating frequency of the devices. In this work, we computationally investigate electrodes composed of arrays of stiff fiber electrodes. Aligning the fibers along one direction creates an electrode layer that exhibits zero stiffness in one direction and is predicted to possess high and uniform sheet resistance. A comprehensive parameter study of the fiber density and dielectric thickness reveals that the fiber density primary determines the electric field localization while the dielectric thickness primarily determines the unit cell stiffness. These trends identify an optimal condition for the actuation performance of the aligned electrode DEAs. This work demonstrates that deterministically designed electrodes composed of stiff materials could provide a new paradigm with the potential to surpass the performance of traditional soft planar electrodes.

Simultaneous Visualization of Microscopic Conductivity and Deformation in Conductive Elastomers

Conductive elastomers are promising for a wide range of applications in many fields due to their unique mechanical and electrical properties, and an understanding of the conductive mechanisms of such materials under deformation is crucial. However, revealing the microscopic conduction mechanism of conductive elastomers is a challenge. In this study, we developed a method that combines in situ deformation nanomechanical atomic force microscopy (AFM) and conductive AFM to successfully and simultaneously characterize the microscopic deformation and microscopic electrical conductivity of nanofiller composite conductive elastomers. With this approach, we visualized the conductive network structure of carbon black and carbon nanotube composite conductive elastomers at the nanoscale, tracked their microscopic response under different compressive strains, and revealed the correlation between microscopic and macroscopic electrical properties. This technique is important for understanding the conductive mechanism of conductive elastomers and improving the design of conductive elastomers.

High-Fidelity, Low-Hysteresis Bionic Flexible Strain Sensors for Soft Machines

Stretchable flexible strain sensors based on conductive elastomers are rapidly emerging as a highly promising candidate for popular wearable flexible electronic and soft-mechanical sensing devices. However, due to the intrinsic limitations of low fidelity and high hysteresis, existing flexible strain sensors are unable to exploit their full application potential. Herein, a design strategy for a successive three-dimensional crack conductive network is proposed to cope with the uncoordinated variation of the output resistance signal arising from the conductive elastomer. The electrical characteristics of the sensor are dominated by the successive crack conductive network through a greater resistance variation and a concise sensing mechanism. As a result, the developed elastomer bionic strain sensors exhibit excellent sensing performance in terms of a smaller overshoot response, a lower hysteresis (∼2.9%), and an ultralow detection limit (0.00179%). What's more, the proposed strategy is universal and applicable to many conductive elastomers with different conductive fillers (including 0-D, 1-D, and 2-D conductive fillers). This approach improves the sensing signal accuracy and reliability of conductive elastomer strain sensors and holds promising potential for various applications in the fields of e-skin and soft robotic systems.

Piezoresistive Effect of Conductive and Non-Conductive Fillers in Bi-Layer Hybrid CNT Composites under Extreme Strain

Polymers mixed with conductive fillers hold significant potential for use in stretchable and wearable sensor devices. Enhancing the piezoresistive effect and mechanical stability is critical for these devices. To explore the changes in the electrical resistance under high strains, typically unachievable in single-layer composites, bi-layer structures were fabricated from carbon nanotubes (CNTs) and EcoFlex composites to see unobservable strain regions. Spherical types of non-conductive fillers composed of polystyrene and conductive filler, coated with Ni and Au on non-conductive fillers, were used as secondary fillers to improve the piezoresistive sensitivity of composites, and their respective impact on the conductive network was compared. The electrical and mechanical properties were examined in the static state to understand the impact of these secondary fillers. The changes in the electrical resistance under 100% and 300% tensile strain, and their dependence on the inherent electrical properties of the secondary fillers, were also investigated. Single-layer CNT composites proved incapable of withstanding 300% strain, whereas the bi-layer structures proved resilient. By implementing cyclic stretching tests, contrary to non-conductive fillers, reduced piezoresistive influence of the conductive secondary filler under extreme strain conditions could be observed.

The Effect of Filler Dimensionality and Content on Resistive Viscoelasticity of Conductive Polymer Composites for Soft Strain Sensors

Soft strain sensors based on conductive polymer composites (CPCs) provide a simple and feasible detection tool in wearable electronics, soft machines, electronic skin, etc. However, the CPCs-based soft strain sensors exhibit resistive viscoelasticity (or time-dependent properties) that hinder the intuitive reflection of the accurate strain and a simple calibration process. In this paper, CPCs with different carbon nanotubes (CNTs) and carbon black (CB) contents were prepared, and electro-mechanical experiments were conducted to study the effect of filler dimensionality and content on the resistive viscoelasticity of CPCs, aimed at guiding the fabrication of CPCs with low resistive viscoelasticity. Furthermore, resistive viscoelasticity and mechanical viscoelasticity were compared to study the origin of the resistive viscoelasticity of CPCs. We found that, at the vicinity of their percolation threshold, the CPCs exhibit high resistive viscoelasticity despite their high sensitivity. In addition, the secondary peaks for CB/SR composite were negligible when the CB concentration was low. Generally, compared with one-dimensional CNT-filled CPCs, the zero-dimensional CB-filled CPCs show higher sensitivity, lower resistive hysteresis, lower resistance relaxation ratio, and better cyclic performance, so they are more suitable for sensor usage. By comparing the resistive viscoelasticity and mechanical viscoelasticity of CPCs, it is indicated that, when the concentration of nanoparticles (NPs) approaches the percolation thresholds, the resistive viscoelasticity is mainly derived from the change of conductive network, while when the concentration of NPs is higher, it is primarily due to the unrecoverable deformations inside the material.

Poisson-Nernst-Planck framework for modelling ionic strain and temperature sensors

Ionically conductive hydrogels are gaining traction as sensing and structural materials for use bioelectronic devices. Hydrogels that feature large mechanical compliances and tractable ionic conductivities are compelling materials that can sense physiological states and potentially modulate the stimulation of excitable tissue because of the congruence in electro-mechanical properties across the tissue-material interface. However, interfacing ionic hydrogels with conventional DC voltage-based circuits poses several technical challenges including electrode delamination, electrochemical reaction, and drifting contact impedance. Utilizing alternating voltages to probe ion-relaxation dynamics has been shown to be a viable alternative for strain and temperature sensing. In this work, we present a Poisson-Nernst-Planck theoretical framework to model ion transport under alternating fields within conductors subject to varying strains and temperatures. Using simulated impedance spectra, we develop key insights about the relationship between frequency of the applied voltage perturbation and sensitivity. Lastly, we perform preliminary experimental characterization to demonstrate the applicability of the proposed theory. We believe this work provides a useful perspective that is applicable to the design of a variety of ionic hydrogel-based sensors for biomedical and soft robotic applications.

Printed Directional Bending Sensor with High Sensitivity and Low Hysteresis for Human Motion Detection and Soft Robotic Perception

We present a high-performance flexible bending strain sensor for directional motion detection of human hands and soft robotic grippers. The sensor was fabricated using a printable porous conductive composite composed of polydimethylsiloxane (PDMS) and carbon black (CB). The utilization of a deep eutectic solvent (DES) in the ink formulation induced a phase segregation between the CB and PDMS and led to a porous structure inside the printed films after being vapored. This simple and spontaneously formed conductive architecture provided superior directional bend-sensing characteristics compared to conventional random composites. The resulting flexible bending sensors displayed high bidirectional sensitivity (gauge factor of 45.6 under compressive bending and 35.2 under tensile bending), negligible hysteresis, good linearity (>0.99), and excellent bending durability (over 10,000 cycles). The multifunctional applications of these sensors, including human motion detection, object-shape monitoring, and robotic perceptions, are demonstrated as a proof-of-concept.

Modeling the resistive viscoelasticity of conductive polymer composites for sensor usage

With the development of fully printed electronics, soft sensors are in demand in various fields, such as wearable electronics, soft machines, etc. Most soft resistive sensors are made of conductive elements dispersed in a viscoelastic polymer binder, exhibiting resistive viscoelastic behavior. The resistance of soft resistive sensors is time-dependent due to the viscoelastic response of polymer binder and structural rearrangement of the conductive pathway. In this paper, experiments and theoretical modeling are used to study the resistive viscoelastic behavior of printed silver wires. The printed silver wire belongs to conductive polymer composites (CPCs) consisting of conductive silver-nanoparticle pathways in an elastic polymer binder. Based on tunneling theory, a multi-branch model is developed to capture the resistance variation of the printed silver wire under mechanical loading. Our experiment-validated model uses only a single set of parameters to predict the resistive relaxation behaviors of CPCs under different strain and different loading rates. Moreover, we demonstrated this numerical model could describe the resistance response under complex loading conditions, such as cyclic loading, similar to the sensor's working condition. The multi-branch model can be extended to any other soft resistive sensor, such as a strain sensor, and provide a new avenue to calibrate these soft sensors.

Interface Engineering in Chip-Scale GaN Optical Devices for Near-Hysteresis-Free Hydraulic Pressure Sensing

In this work, a compact, near-hysteresis-free hydraulic pressure sensor is presented through interface engineering in a GaN chip-scale optical device. The sensor consists of a monolithic GaN-on-sapphire device responsible for light emission and detection and a multilevel microstructured polydimethylsiloxane (PDMS) film prepared through a low-cost molding process using sandpaper as a template. The micro-patterned PDMS film functions as a pressure-sensing medium to effectively modulate the reflectance properties at the sapphire interface during pressure loading and unloading. The interface engineering endows the GaN optical device with near-hysteresis-free performance over a wide pressure range of up to 0-800 kPa. Verified by a series of experimental measurements on its dynamic responses, the tiny hydraulic sensor exhibits superior performance in hysteresis, stability, repeatability, and response time, indicating its considerable potential for a broad range of practical applications.

Transparent Conducting Films Based on Carbon Nanotubes: Rational Design toward the Theoretical Limit

Electrically conductive thin-film materials possessing high transparency are essential components for many optoelectronic devices. The advancement in the transparent conductor applications requires a replacement of indium tin oxide (ITO), one of the key materials in electronics. ITO and other transparent conductive metal oxides have several drawbacks, including poor flexibility, high refractive index and haze, limited chemical stability, and depleted raw material supply. Single-walled carbon nanotubes (SWCNTs) are a promising alternative for transparent conducting films (TCFs) because of their unique and excellent chemical and physical properties. Here, the latest achievements in the optoelectronic performance of TCFs based on SWCNTs are analyzed. Various approaches to evaluate the performance of transparent electrodes are briefly reviewed. A roadmap for further research and development of the transparent conductors using "rational design," which breaks the deadlock for obtaining the TCFs with a performance close to the theoretical limit, is also described.

Variations of tunnelling resistance between CNTs with strain in composites: non-monotonicty and influencing factors

The electro-mechanical response of conductive carbon-nanotube(CNT)-polymer composites is vital when they are used as smart-sensing materials. Clarifying the variation trend of resistance with strain is the key to design and regulate the piezoresistive property of such material. Here, we present some finite element simulations to predict the electro-mechanical response using a geometrical model comprising two hollow cylindrical CNTs and a cuboid matrix. The electrical contact between CNTs is represented by some elements which account for quantum tunnelling effects and capture the sensitivity of conductivity to separation. Different from classical simulations using solid model or one-dimensional beam model, in which the tunnelling resistance between two CNTs changes monotonously with strain, the results in this work show that the trend is non-monotonic in some cases, i.e. it increases at first and then decreases with the uniaxial compressive strain when the elastic modulus of the matrix is high. In addition, factors affecting the different variation trends are discussed in details, which include geometric model, elastic modulus and Poisson's ratio of the matrix, and orientation angle.

A biomimetic elastomeric robot skin using electrical impedance and acoustic tomography for tactile sensing

Human skin perceives physical stimuli applied to the body and mitigates the risk of physical interaction through its soft and resilient mechanical properties. Social robots would benefit from whole-body robotic skin (or tactile sensors) resembling human skin in realizing a safe, intuitive, and contact-rich human-robot interaction. However, existing soft tactile sensors show several drawbacks (complex structure, poor scalability, and fragility), which limit their application in whole-body robotic skin. Here, we introduce biomimetic robotic skin based on hydrogel-elastomer hybrids and tomographic imaging. The developed skin consists of a tough hydrogel and a silicone elastomer forming a skin-inspired multilayer structure, achieving sufficient softness and resilience for protection. The sensor structure can also be easily repaired with adhesives even after severe damage (incision). For multimodal tactile sensation, electrodes and microphones are deployed in the sensor structure to measure local resistance changes and vibration due to touch. The ionic hydrogel layer is deformed owing to an external force, and the resulting local conductivity changes are measured via electrodes. The microphones also detect the vibration generated from touch to determine the location and type of dynamic tactile stimuli. The measurement data are then converted into multimodal tactile information through tomographic imaging and deep neural networks. We further implement a sensorized cosmetic prosthesis, demonstrating that our design could be used to implement deformable or complex-shaped robotic skin.

Investigation of stretchable strain sensor based on CNT/AgNW applied in smart wearable devices

Stretchable strain sensor, an important paradigm of wearable sensor which can be attached onto clothing or even human skin, is widely used in healthcare, human motion monitoring and human-machine interaction. Pattern-available and facile manufacturing process for strain sensor is pursued all the time. A carbon nanotube (CNT)/silver nanowire (AgNW)-based stretchable strain sensor fabricated by a facile process is reported here. The strain sensor exhibits a considerable Gauge factor of 6.7, long-term durability (>1000 stretching cycles), fast response and recovery (420 ms and 600 ms, respectively), hence the sensor can fulfill the measurement of finger movement. Accordingly, a smart glove comprising a sensor array and a flexible printed circuit board is assembled to detect the bending movement of five fingers simultaneously. Moreover, the glove is wireless and basically fully flexible, it can detect the finger bending of wearer and display the responses distinctly on an APP of a smart phone or a host computer. Our strain senor and smart glove will broaden the materials and applications of wearable sensors.

Highly Sensitive Capacitive Pressure Sensors over a Wide Pressure Range Enabled by the Hybrid Responses of a Highly Porous Nanocomposite

Past research aimed at increasing the sensitivity of capacitive pressure sensors has mostly focused on developing dielectric layers with surface/porous structures or higher dielectric constants. However, such strategies have only been effective in improving sensitivities at low pressure ranges (e.g., up to 3 kPa). To overcome this well-known obstacle, herein, a flexible hybrid-response pressure sensor (HRPS) composed of an electrically conductive porous nanocomposite (PNC) laminated with an ultrathin dielectric layer is devised. Using a nickel foam template, the PNC is fabricated with carbon nanotubes (CNTs)-doped Ecoflex to be 86% porous and electrically conductive. The PNC exhibits hybrid piezoresistive and piezocapacitive responses, resulting in significantly enhanced sensitivities (i.e., more than 400%) over wide pressure ranges, from 3.13 kPa^-1 within 0-1 kPa to 0.43 kPa^-1 within 30-50 kPa. The effect of the hybrid responses is differentiated from the effect of porosity or high dielectric constants by comparing the HRPS with its purely piezocapacitive counterparts. Fundamental understanding of the HRPS and the prediction of optimal CNT doping are achieved through simplified analytical models. The HRPS is able to measure pressures from as subtle as the temporal arterial pulse to as large as footsteps.

Self-adaptive cardiac optogenetics device based on negative stretching-resistive strain sensor

Precision medicine calls for high demand of continuous, closed-loop physiological monitoring and accurate control, especially for cardiovascular diseases. Cardiac optogenetics is promising for its superiority of cell selectivity and high time-space accuracy, but the efficacy of optogenetics relative to the input of light stimulus is detected and controlled separately by discrete instruments in vitro, which suffers from time retardation, energy consumption, and poor portability. Thus, a highly integrated system based on implantable sensors combining closed-loop self-monitoring with simultaneous treatment is highly desired. Here, we report a self-adaptive cardiac optogenetics system based on an original negative stretching-resistive strain sensor array for closed-loop heart rate recording and self-adaptive light intensity control. The strain sensor exhibits a dual and synchronous capability of precise monitor and physiological-electrical-optical regulation. In an in vivo ventricular tachycardia model, our system demonstrates the potential of a negative stretching-resistive device in controlling-in-sensor electronics for wearable/implantable autodiagnosis and telehealth applications.

Recent Progress in Flexible Tactile Sensors for Human-Interactive Systems: From Sensors to Advanced Applications

Flexible tactile sensors capable of measuring mechanical stimuli via physical contact have attracted significant attention in the field of human-interactive systems. The utilization of tactile information can complement vision and/or sound interaction and provide new functionalities. Recent advancements in micro/nanotechnology, material science, and information technology have resulted in the development of high-performance tactile sensors that reach and even surpass the tactile sensing ability of human skin. Here, important advances in flexible tactile sensors over recent years are summarized, from sensor designs to system-level applications. This review focuses on the representative strategies based on design and material configurations for improving key performance parameters including sensitivity, detection range/linearity, response time/hysteresis, spatial resolution/crosstalk, multidirectional force detection, and insensitivity to other stimuli. System-level integration for practical applications beyond conceptual prototypes and promising applications, such as artificial electronic skin for robotics and prosthetics, wearable controllers for electronics, and bidirectional communication tools, are also discussed. Finally, perspectives on issues regarding further advances are provided.

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.

Inkjet-Deposited Single-Wall Carbon Nanotube Micropatterns on Stretchable PDMS-Ag Substrate-Electrode Structures for Piezoresistive Strain Sensing

Printed piezoresistive strain sensors based on stretchable roll-to-roll screen-printed silver electrodes on polydimethylsiloxane substrates and inkjet-deposited single-wall carbon nanotube micropatterns are demonstrated in this work. With the optimization of surface wetting and inkjet printing parameters, well-defined microscopic line patterns of the nanotubes with a sheet resistance of <100 Ω/□ could be deposited between stretchable Ag electrodes on the plasma-treated substrate. The developed stretchable devices are highly sensitive to tensile strain with a gauge factor of up to 400 and a pressure sensitivity of ∼0.09 Pa^-1, respond to bending down to a radius of 1.5 mm, and are suitable for mounting on the skin to monitor and resolve various movements of the human body such as cardiac cycle, breathing, and finger flexing. This study indicates that inkjet deposition of nanomaterials can complement well other printing technologies to produce flexible and stretchable devices in a versatile manner.

Advanced Flexible Skin-Like Pressure and Strain Sensors for Human Health Monitoring

Recently, owing to their excellent flexibility and adaptability, skin-like pressure and strain sensors integrated with the human body have the potential for great prospects in healthcare. This review mainly focuses on the representative advances of the flexible pressure and strain sensors for health monitoring in recent years. The review consists of five sections. Firstly, we give a brief introduction of flexible skin-like sensors and their primary demands, and we comprehensively outline the two categories of design strategies for flexible sensors. Secondly, combining the typical sensor structures and their applications in human body monitoring, we summarize the recent development of flexible pressure sensors based on perceptual mechanism, the sensing component, elastic substrate, sensitivity and detection range. Thirdly, the main structure principles and performance characteristic parameters of noteworthy flexible strain sensors are summed up, namely the sensing mechanism, sensitive element, substrate, gauge factor, stretchability, and representative applications for human monitoring. Furthermore, the representations of flexible sensors with the favorable biocompatibility and self-driven properties are introduced. Finally, in conclusion, besides continuously researching how to enhance the flexibility and sensitivity of flexible sensors, their biocompatibility, versatility and durability should also be given sufficient attention, especially for implantable bioelectronics. In addition, the discussion emphasizes the challenges and opportunities of the above highlighted characteristics of novel flexible skin-like sensors.

A shape-deformable liquid-metal-filled magnetorheological plastomer sensor with a magnetic field "on-off" switch

Flexible viscoelastic sensors have gained significant attention in wearable devices owing to their exceptional strain-dependent electrical resistance. Most of the strain sensors are elastic composites, thus the internal stress is often preserved during the deformation when they are attached to the uneven target. Therefore, there is a pressing need for viscoelastic composites with highly self-adapted electromechanical properties sensitive to multiexternal circumstances. This work reports a liquid-metal-filled magnetorheological plastomer (LMMRP) that shows a high response behavior to the external stimulus such as magnetic field, temperature, and force. The shape-deformable LMMRP can transform from an insulator to a conductor under applying a magnetic field, thus the further viscoelastic sensor possesses a magnetic field “on-off” switch effect. The microstructure-dependent magnetic/thermal/mechanical-electrical coupling characteristics are investigated, and several proof-of-concept sensor applications, such as magnetic control, environment recognition, and motion monitoring, are demonstrated. These LMMRP composites show a broad potential in flexible sensors and soft electronics.

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A shape-deformable liquid-metal-filled magnetorheological plastomer was created.

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The plastomer is sensitive to magnetic field, temperature, and force.

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The evolution of the particle microstructure in the plastomer was simulated.

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The plastomer has great potential in flexible electronics with magnetic control.

Large-Area, Crosstalk-Free, Flexible Tactile Sensor Matrix Pixelated by Mesh Layers

Tactile sensor arrays have attracted considerable attention for their use in diverse applications, such as advanced robotics and interactive human-machine interfaces. However, conventional tactile sensor arrays suffer from electrical crosstalk caused by current leakages between the tactile cells. The approaches that have been proposed thus far to overcome this issue require complex rectifier circuits or a serial fabrication process. This article reports a flexible tactile sensor array fabricated through a batch process using a mesh. A carbon nanotube-polydimethylsiloxane composite is used to form an array of sensing cells in the mesh through a simple "dip-coating" process and is cured into a concave shape. The contact area between the electrode and the composite changes significantly under pressure, resulting in an excellent sensitivity (5.61 kPa^-1) over a wide range of pressure up to 600 kPa. The mesh separates the composite into the arranged sensing cells to prevent the electrical connection between adjacent cells and simultaneously connects each cell mechanically. Additionally, the sensor shows superior durability compared with previously reported tactile sensors because the mesh acts as a support beam. Furthermore, the tactile sensor array is successfully utilized as a Braille reader via information processing based on machine learning.

Mechanical design of brush coating technology for the alignment of one-dimension nanomaterials

Widespread approaches to fabricate surfaces with aligned nanostructured topographies have been stimulated by opportunities to enhance interface performance by combing physical and chemical effects, in which brush-coating technology (BCT) is a cost-effective and feasible method for aligned film and large-scale production. Here, we reported a BCT process to realize the alignment of various 1D nanostructures through mechanical design that provides a more precise and higher shear force. By regulating the viscosity of dispersion, shear force is proved to be 24 and 20.3 times larger (when the volume ratio of water and glycerol is 1:3) according to the theoretical calculation and ANSYS simulating calculation results respectively, which plays a vital role in brush coating process. The universality was demonstrated by the alignment of one-dimension nanomaterials with different diameters, including silver nanowires (~80 nm), molybdenum trioxide nanobelts (~150 nm), vanadium pentoxide nanobelts (~150 nm) and bismuth sulfide nanobelts (~200 nm), et al., which in consequence have different alignment ratios. Meanwhile, anisotropic and flexible electrical conductors (the resistance anisotropic ratio was 2) and thermoelectric films (Seebeck coefficient was calculated to be 56.7 µV/K) were demonstrated.

Soft Electronics Based on Stretchable and Conductive Nanocomposites for Biomedical Applications

Research on the field of implantable electronic devices that can be directly applied in the body with various functionalities is increasingly intensifying due to its great potential for various therapeutic applications. While conventional implantable electronics generally include rigid and hard conductive materials, their surrounding biological objects are soft and dynamic. The mechanical mismatch between implanted devices and biological environments induces damages in the body especially for long-term applications. Stretchable electronics with outstanding mechanical compliance with biological objects effectively improve such limitations of existing rigid implantable electronics. In this article, the recent progress of implantable soft electronics based on various conductive nanocomposites is systematically described. In particular, representative fabrication approaches of conductive and stretchable nanocomposites for implantable soft electronics and various in vivo applications of implantable soft electronics are focused on. To conclude, challenges and perspectives of current implantable soft electronics that should be considered for further advances are discussed.

Near-hysteresis-free soft tactile electronic skins for wearables and reliable machine learning

Electronic skins are essential for real-time health monitoring and tactile perception in robots. Although the use of soft elastomers and microstructures have improved the sensitivity and pressure-sensing range of tactile sensors, the intrinsic viscoelasticity of soft polymeric materials remains a long-standing challenge resulting in cyclic hysteresis. This causes sensor data variations between contact events that negatively impact the accuracy and reliability. Here, we introduce the Tactile Resistive Annularly Cracked E-Skin (TRACE) sensor to address the inherent trade-off between sensitivity and hysteresis in tactile sensors when using soft materials. We discovered that piezoresistive sensors made using an array of three-dimensional (3D) metallic annular cracks on polymeric microstructures possess high sensitivities (> 107 Ω ⋅ kPa-1), low hysteresis (2.99 ± 1.37%) over a wide pressure range (0-20 kPa), and fast response (400 Hz). We demonstrate that TRACE sensors can accurately detect and measure the pulse wave velocity (PWV) when skin mounted. Moreover, we show that these tactile sensors when arrayed enabled fast reliable one-touch surface texture classification with neuromorphic encoding and deep learning algorithms.

Bending Properties of Carbon Nanotube/Polymer Composites with Various Aspect Ratios and Filler Contents

The key characteristics of bending sensors are piezoresistive effect, hysteresis, and durability. In this study, to investigate the influence of the aspect ratio and contents of multi-walled nanotubes (MWNTs) on the properties of bending sensors, MWNT/polydimethylsiloxane (PDMS) composites were fabricated with various aspect ratios and filler contents. The MWNTs were uniformly dispersed in the composites using the three-roll milling method. By increasing the bending angle gradually, the sensitivity of each composite was analyzed. Furthermore, discontinuous cyclic bending tests were conducted to investigate the piezoresistive effect and hysteresis. In addition, stable repeatability of the composites was confirmed through continuous cyclic bending tests. As a result, optimal aspect ratios and filler contents have been presented for application in bending sensors of MWNT composites.

Effect of Filler Alignment on Piezo-Resistive and Mechanical Properties of Carbon Nanotube Composites

Highly aligned multi-walled carbon nanotube (MWCNT) polymer composites were fabricated via a roll-to-roll milling process; the alignment of the MWCNTs could be controlled by varying the speed of the rotating rolls. The effect of MWCNT alignment on the polymer matrix was morphologically observed and quantitatively characterized using polarized Raman spectroscopy. To provide a more detailed comparison, MWCNT composites with alignment in the transverse direction and random alignment were fabricated and tested. Enhanced mechanical and electrical properties were obtained for the aligned MWCNT composite, which can be attributed to the efficient electrical network and load transfer, respectively. In addition, a cyclic stretching test was conducted to evaluate the piezo-resistive characteristics of the aligned MWCNT composites. The composites with an aligned filler configuration showed an exceptionally high degree of strain sensitivity compared to the other composites.

Gas-Permeable, Ultrathin, Stretchable Epidermal Electronics with Porous Electrodes

We present gas-permeable, ultrathin, and stretchable electrodes enabled by self-assembled porous substrates and conductive nanostructures. An efficient and scalable breath figure method is employed to introduce the porous skeleton, and then silver nanowires (AgNWs) are dip-coated and heat-pressed to offer electric conductivity. The resulting film has a transmittance of 61%, sheet resistance of 7.3 Ω/sq, and water vapor permeability of 23 mg cm^-2 h^-1. With AgNWs embedded below the surface of the polymer, the electrode exhibits excellent stability in the presence of sweat and after long-term wear. We demonstrate the promising potential of the electrode for wearable electronics in two representative applications: skin-mountable biopotential sensing for healthcare and textile-integrated touch sensing for human-machine interfaces. The electrode can form conformal contact with human skin, leading to low skin-electrode impedance and high-quality biopotential signals. In addition, the textile electrode can be used in a self-capacitance wireless touch sensing system.

Strain‑Sensing Properties of Multi‑Walled Carbon Nanotube/Polydimethylsiloxane Composites with Different Aspect Ratio and Filler Contents

For filler composite systems used in strain sensor applications, piezoresistive effect, strain hysteresis, and repeatability are critical factors, which have to be clearly evaluated and understood. To investigate the effects of the aspect ratio and content of a multi-walled carbon nanotube (MWCNT) on the strain sensor properties of the composite, MWCNT/Polydimethylsiloxane (PDMS) composites with varying filler contents and aspect ratios were fabricated. In order to uniformly disperse MWCNTs on the polymer matrix, we used a three-roll milling method to generate high shear force for de-bundling MWCNTs. Mechanical and electrical properties of the MWCNT composites were evaluated for each case. In addition, through the cyclic stretching test, we optimized the strain-sensing properties of the MWCNT composites by considering their piezoresistive effects and strain hysteresis.

Directional Response of Randomly Dispersed Carbon Nanotube Strain Sensors

Tests on a double lap bonded joint, with transverse strips of randomly oriented carbon nanotubes (CNT) sprayed onto an epoxy adhesive film, showed a positive increment in electrical resistance under tensile load, even though the transverse strains were negative. Other experiments included in this work involved placing longitudinal and transversal CNT sensors in a tensile loaded aluminum plate, and, as reported by other authors, the results confirm that the resistance change is not only dependent on the strains oriented with the electrode line, while the other strain components also influence the response. This behavior is quite different to that of conventional strain gages which have a near zero sensitivity to strains not aligned to the sensor direction. The dependence of the electrical response on all the strain components makes it quite difficult, possibly unfeasible, to experimentally determine the individual strain components with this kind of sensors; however, the manufacturing of aligned CNT sensors could deal with this issue.

Fundamental Limits to the Electrochemical Impedance Stability of Dielectric Elastomers in Bioelectronics

Incorporation of elastomers into bioelectronics that reduces the mechanical mismatch between electronics and biological systems could potentially improve the long-term electronics-tissue interface. However, the chronic stability of elastomers in physiological conditions has not been systematically studied. Here, using electrochemical impedance spectrum we find that the electrochemical impedance of dielectric elastomers degrades over time in physiological environments. Both experimental and computational results reveal that this phenomenon is due to the diffusion of ions from the physiological solution into elastomers over time. Their conductivity increases by 6 orders of magnitude up to 10^-8 S/m. When the passivated conductors are also composed of intrinsically stretchable materials, higher leakage currents can be detected. Scaling analyses suggest fundamental limitations to the electrical performances of interconnects made of stretchable materials.

Dynamic Ag-N Bond Enhanced Stretchable Conductor for Transparent and Self-Healing Electronic Skin

Stretchable conductors have been achieved by stacking conductive nanomaterials onto the surfaces of elastomeric substrates. However, many of them show a dramatic decrease in conductivity under strain without an efficient way for the conductive layer to release strain. Here, we report a transparent, stretchable, and self-healing conductor with excellent mechanoelectrical stability by introducing dynamic bonding between conductive nanomaterials and an elastomeric substrate. We prepare the conductor by semiembedding Ag nanowires (AgNWs) into a self-healing polydimethylsiloxane (PDMS)-based elastomer, which is modified with bipyridine (Bpy) ligand and further cross-linked by adding Zn²⁺ as coordinator (Zn-Bpy-PDMS). The dynamic Ag-N bonds not only improve the wettability of the substrate and facilitate the spreading of AgNWs but also reversibly break and reform to accommodate the deformation of AgNWs. As a result, the resistance increase of Zn-Bpy-PDMS/AgNWs is much smaller than that without the dynamic bonding (PDMS/AgNWs). Besides, this conductor exhibits excellent conductivity (76.2 Ω/sq) and transparency (86.6% @ 550 nm), as well as extraordinary self-healing property with a low resistance increase (ΔR/R₀ ∼ 1.4) after healing at room temperature for 1 day. This work provides insights into the future design of integrated electronic skin with transparency, stretchability, conductivity, and self-healing capability for applications in wearable optoelectronic devices.

Sensitivity-Tunable Strain Sensors Based on Carbon Nanotube@Carbon Nanocoil Hybrid Networks

A novel vanelike nanostructure based on the hybridization of carbon nanotubes and carbon nanocoils has been fabricated by a two-step chemical vapor deposition method. A flexible and sensitive strain sensor is prepared by coupling this hybrid structure with polydimethylsiloxane. By regulating the density and length of carbon nanotubes, the gauge factor and strain range of the sensors are tuned from 4.5 to 70 and 9 to 260%, respectively. These sensors exhibit high reliability and stability in a more than 10 000-cycle test and have a prompt response time of less than 37 ms. Owing to the tunable properties, these sensors show great potential in monitoring both subtle and large-scale displacements, which can meet the diverse demands of human motion monitoring.

Ultracompliant Carbon Nanotube Direct Bladder Device

The bladder, stomach, intestines, heart, and lungs all move dynamically to achieve their purpose. A long-term implantable device that can attach onto an organ, sense its movement, and deliver current to modify the organ function would be useful in many therapeutic applications. The bladder, for example, can suffer from incomplete contractions that result in urinary retention with patients requiring catheterization. Those affected may benefit from a combination of a strain sensor and electrical stimulator to better control bladder emptying. The materials and design of such a device made from thin layer carbon nanotube (CNT) and Ecoflex 00-50 are described and demonstrate its function with in vivo feline bladders. During bench-top characterization, the resistive and capacitive sensors exhibit stability throughout 5000 stretching cycles under physiology conditions. In vivo measurements with piezoresistive devices show a high correlation between sensor resistance and volume. Stimulation driven from platinum-silicone composite electrodes successfully induce bladder contraction. A method for reliable connection and packaging of medical grade wire to the CNT device is also presented. This work is an important step toward the translation of low-durometer elastomers, stretchable CNT percolation, and platinum-silicone composite, which are ideal for large-strain bioelectric applications to sense or modulate dynamic organ states.

Piezoresistive Behaviour of Additively Manufactured Multi-Walled Carbon Nanotube/Thermoplastic Polyurethane Nanocomposites

To develop highly sensitive flexible pressure sensors, the mechanical and piezoresistive properties of conductive thermoplastic materials produced via additive manufacturing technology were investigated. Multi-walled carbon nanotubes (MWCNTs) dispersed in thermoplastic polyurethane (TPU), which is flexible and pliable, were used to form filaments. Specimens of the MWCNT/TPU composite with various MWCNT concentrations were printed using fused deposition modelling. Uniaxial tensile tests were conducted, while the mechanical and piezoresistive properties of the MWCNT/TPU composites were measured. To predict the piezoresistive behaviour of the composites, a microscale 3D resistance network model was developed. In addition, a continuum piezoresistive model was proposed for large-scale simulations.

Mechanically Tunable Single-Walled Carbon Nanotube Films as a Universal Material for Transparent and Stretchable Electronics

Soft, flexible, and stretchable electronic devices provide novel integration opportunities for wearable and implantable technologies. Despite the existing efforts to endow electronics with the capability of large deformation, the main technological challenge is still in the absence of suitable materials for the manufacturing of stretchable electronic circuits and devices with active (sensitive) and passive (stable) components. Here, we present a universal material, based on single-walled carbon nanotube (SWCNT) films deposited on a polydimethylsiloxane (PDMS) substrate, which can act as a material being both sensitive and insensitive to strain. The diverse performance of SWCNT/PDMS structures was achieved by two simple dry-transfer fabrication approaches: SWCNT film deposition onto the as-prepared PDMS and on the prestretched PDMS surface. The correlation between applied strain, microstructural evolution, and electro-optical properties is discussed on the basis of both experimental and computational results. The SWCNT/PDMS material with the mechanically tunable performance has a small relative resistance change from 0.05 to 0.07, while being stretched from 10 to 40% (stable electrode applications). A high sensitivity of 20.1 of the SWCNT/PDMS structures at a 100% strain was achieved (strain sensing applications). Our SWCNT/PDMS structures have superior transparency and conductivity compared to the ones reported previously, including the SWCNT/PDMS structures, obtained by wet processes.

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS₂ ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

Inkjet-printed stretchable and low voltage synaptic transistor array

Wearable and skin electronics benefit from mechanically soft and stretchable materials to conform to curved and dynamic surfaces, thereby enabling seamless integration with the human body. However, such materials are challenging to process using traditional microelectronics techniques. Here, stretchable transistor arrays are patterned exclusively from solution by inkjet printing of polymers and carbon nanotubes. The additive, non-contact and maskless nature of inkjet printing provides a simple, inexpensive and scalable route for stacking and patterning these chemically-sensitive materials over large areas. The transistors, which are stable at ambient conditions, display mobilities as high as 30 cm² V⁻¹ s⁻¹ and currents per channel width of 0.2 mA cm⁻¹ at operation voltages as low as 1 V, owing to the ionic character of their printed gate dielectric. Furthermore, these transistors with double-layer capacitive dielectric can mimic the synaptic behavior of neurons, making them interesting for conformal brain-machine interfaces and other wearable bioelectronics.

The development of novel low-cost fabrication schemes for realizing stretchable transistor arrays with applicability in wearable electronics remains a challenge. Here, the authors report skin-like electronics with stretchable active materials and devices processed exclusively from ink-jet printing.

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.

Aggregate-driven reconfigurations of carbon nanotubes in thin networks under strain: in-situ characterization

This work focuses on the in-situ characterization of multi-walled carbon nanotube (CNT) motions in thin random networks under strain. Many fine-grain models have been devised to account for CNT motions in carbon nanotube networks (CNN). However, the validation of these models relies on mesoscopic or macroscopic data with very little experimental validation of the physical mechanisms actually arising at the CNT scale. In the present paper, we use in-situ scanning electron microscopy imaging and high-resolution digital image correlation to uncover prominent mechanisms of CNT motions in CNNs under strain. Results show that thin and sparse CNNs feature stronger strain heterogeneities than thicker and denser ones. It is attributed to the complex motions of individual CNTs connected to aggregates within thin and sparse CNNs. While the aggregates exhibit a collective homogeneous deformation, individual CNTs connecting them are observed to fold, unwind or buckle, seemingly to accommodate the motion of these aggregates. In addition, looser aggregates feature internal reconfigurations via cell closing, similar to foam materials. Overall, this suggests that models describing thin and sparse CNN deformation should integrate multiphase behaviour (with various densities of aggregates in addition to individual CNTs), heterogeneity across surface, as well as imperfect substrate adhesion.

Materials and structural designs of stretchable conductors

Stretchable conductors are essential building blocks for stretchable electronic devices used in next-generation wearables and soft robotics. Over 10 years of research in stretchable electronics has produced stretchable sensors, circuits, displays, and energy harvesters, mostly enabled by unique stretchable conductors. This review covers recent advances in stretchable conductors, which have been achieved by engineering their structures, materials, or both. Advantages, mechanisms, and limitations of the different classes of stretchable conductors are discussed to provide insight into which class of stretchable conductor is suitable for fabrication of various stretchable electronic devices. The significantly improved electronic performance and wide variety of stretchable conductors are creating a new paradigm in stretchable electronics.

Nanomaterials in Skin-Inspired Electronics: Toward Soft and Robust Skin-like Electronic Nanosystems

Skin-inspired wearable electronic/biomedical systems based on functional nanomaterials with exceptional electrical and mechanical properties have revolutionized wearable applications, such as portable Internet of Things, personalized healthcare monitors, human-machine interfaces, and even always-connected precise medicine systems. Despite these advancements, including the ability to predict and to control nanolevel phenomena of functional nanomaterials precisely and strategies for integrating nanomaterials onto desired substrates without performance losses, skin-inspired electronic nanosystems are not yet feasible beyond proof-of-concept devices. In this Perspective, we provide an outlook on skin-like electronics through the review of several recent reports on various materials strategies and integration methodologies of stretchable conducting and semiconducting nanomaterials, which are used as electrodes and active layers in stretchable sensors, transistors, multiplexed arrays, and integrated circuits. To overcome the challenge of realizing robust electronic nanosystems, we discuss using nanomaterials in dynamically cross-linked polymer matrices, focusing on the latest innovations in stretchable self-healing electronics, which could change the paradigm of wearable electronics.

Computational characterization and control of electrical conductivity of nanowire composite network under mechanical deformation

Quantitative models to predict the electrical performance of 1-D nanowire (NW) composite networks under external deformation such as bending and patterning are developed by Monte-Carlo based computations, and appropriate solutions are addressed to enhance the tolerance of the sheet resistance (R_s) of the NW networks under the deformation. In addition, several strategies are employed to improve further the robustness of the sheet resistance against the network deformation. In the case of bending, outstanding bending durability of a hybrid NW network coated on a 2-D sheet is confirmed with a numerical model, and a network of NWs aligned unidirectionally toward bend axis is introduced to alleviate the sheet resistance degradation. In the case of a narrowly patterned channel, the conductivity enhancement of a network of NWs aligned in parallel to the channel with reduced channel is validated, and a network made with two types of NWs with different lengths is suggested to enhance the tolerance of the electrical conductivity. The results offer useful design guidelines to the use of the 1-D NW percolation network for flexible transparent conducting electrodes.

Acid-Interface Engineering of Carbon Nanotube/Elastomers with Enhanced Sensitivity for Stretchable Strain Sensors

Stretchable strain sensors with high sensitivity or gauge factor (GF), large stretchability, and long-term durability are highly demanded in human motion detection, artificial intelligence, and electronic skins. Nevertheless, to develop high-sensitive sensors without sacrificing the stretchability cannot be realized using simple device configurations. In this work, an acid-interface engineering (AIE) method was proposed to develop a stretchable strain sensor with high GF and large stretchability. The AIE generates a layer of SiO _x at the interface between the carbon nanotube (CNT) film and Ecoflex, playing a key role in enhancing the sensor's GF. Compared to devices without AIE (GF = 2.4), the ones with AIE are significantly improved. At an AIE time of 10 min, the GF up to 1665.9 is achieved without sacrificing the stretchability (>100%). The AIE-generated cracks are found to modulate the electrical behaviors and enhance the GFs of sensors with AIE through the crack-induced rapid reduction in the electrical conduction pathway, which is manipulated by the CNTs bridging over the cracks. The device with AIE proves its high mechanical durability through a cycling test (>10 000 cycles) at a high strain up to ∼80%, further paving its practical applications in various human motion detections.