Flexible and transparent strain sensors based on super-aligned carbon nanotube films

Biodegradable poly(lactic acid) blocked polyurethane/carbon nanotubes coated cotton fabric prepared by ultrasonic-assisted inkjet printing for high performance strain sensors

In order to fulfill the demands for degradability, a broad working range, and heightened sensitivity in flexible sensors, biodegradable polyurethane (BTPU) was synthesized and combined with CNTs to produce BTPU/CNTs coated cotton fabric using an ultrasonic-assisted inkjet printing process. The synthesized BTPU displayed a capacity for degradation in a phosphate buffered saline solution, resulting in a weight loss of 25 % after 12 weeks of degradation. The BTPU/CNTs coated cotton fabric sensor achieved an extensive strain sensing range of 0-137.5 %, characterized by high linearity and a notable sensitivity (gauge factor (GF) of 126.8). Notably, it demonstrated a low strain detection limit (1 %), rapid response (within 280 ms), and robust durability, enabling precise monitoring of both large and subtle human body movements such as finger, wrist, neck, and knee bending, as well as swallowing. Moreover, the BTPU/CNTs coated cotton fabric exhibited favorable biocompatibility with human epidermis, enabling potential applications as wearable skin-contact sensors. This work provides insight into the development of degradable and high sensing performance sensors suitable for applications in electronic skins and health monitoring devices.

A Highly Sensitive Strain Sensor with Wide Linear Sensing Range Prepared on a Hybrid-Structured CNT/Ecoflex Film via Local Regulation of Strain Distribution

With the development of information technology, high-performance wearable strain sensors with high sensitivity and stretchability have played a significant role in motion detection. However, many high-sensitivity and outstanding-stretchability strain sensors possess a limited linear sensing range, which limits the enhancement of the flexible strain sensors' performance. Herein, we develop a hybrid-structured carbon nanotube (CNT)/Ecoflex strain sensor with laser-engraved grooves along with punched circular holes in a composite CNT/Ecoflex film by vacuum filtration and permeation. By optimizing the distribution of grooves and circular holes, the strain in the sensing layer can be locally regulated, which alters the morphology of cracks under strain and allows the hybrid-structured CNT/Ecoflex strain sensor to simultaneously exhibit high sensitivity (GF = 43.8) as well as a wide linear sensing range (200%). On the basis of excellent performance, the hybrid-structured CNT/Ecoflex strain sensor is capable of detecting movements in various parts of the human body, including movements of larynx and joint bending.

Nanocarbon-based sensors for the structural health monitoring of smart biocomposites

Structural health monitoring (SHM) is a critical aspect of ensuring the safety and durability of smart biocomposite materials used as multifunctional materials. Smart biocomposites are composed of renewable or biodegradable materials and have emerged as eco-friendly alternatives of traditional non-biodegradable glass fiber-based composite materials. Although biocomposites exhibit fascinating properties and many desirable traits, real-time and early stage SHM is the most challenging issue to enable their long-term use. Smart biocomposites are integrated with sensors for in situ identification of the progress of damage and composite failure. The sensitivity of such smart biocomposites is a key functionality, which can be tuned by the introduction of an appropriate filler. In particular, nanocarbons hold promising potential to be incorporated in SHM applications of biocomposites. This review focused on the potential applications of nanocarbons in SHM of biocomposites. The aspects related to fabrication techniques and working mechanism of sensors are comprehensively discussed. Furthermore, their unique mechanical and electrical properties and sustainable nature ensure seamless integration into biocomposites, allowing for real-time monitoring without compromising the material's properties. These sensors offer multi-parameter sensing capabilities, such as strain, pressure, humidity, temperature, and chemical exposure, allowing a comprehensive assessment of biocomposite health. Additionally, their durability and longevity in harsh conditions, along with wireless connectivity options, provide cost-effective and sustainable SHM solutions. As research in this field advances, ongoing efforts seek to enhance the sensitivity and selectivity of these sensors, optimizing their performance for real-world applications. This review highlights the significant advances, ongoing efforts to enhance the sensitivity and selectivity, and performance optimization of nanocarbon-based sensors along with their working mechanism in the field of SHM for smart biocomposites. The key challenges and future research perspectives facing the conversion of nanocarbons to smart biocomposites are also displayed.

Highly Sensitive and Wide Detection Range Thermoplastic Polyurethane/Graphene Nanoplatelets Multifunctional Strain Sensor with a Porous and Crimped Network Structure

High-performance flexible strain sensors have tremendous potential applications in wearable devices and health monitoring. However, developing a flexible strain sensor with high sensitivity over a wide strain range remains a significant challenge. In this study, a fibrous membrane with a porous and crimped structure was designed as the substrate material for TPU/GNPs flexible strain sensors. This structural design effectively balances sensitivity with the strain range. The TPU-PEO fibrous membrane prepared using electrospinning with water washing, resulted in a porous fibrous membrane with a TPU framework. Subsequently, the fibrous membrane was subjected to anhydrous ethanol stimulation to obtain a porous and crimped network structure. GNPs were modified on the TPU fibrous membrane through ultrasonic treatment. The produced flexible strain sensor exhibited high sensitivity (GF = 4047.5) within a large strain range (350%) and demonstrated excellent sensing performance, stability, and durability (>10,000 cycles). It not only captured basic movements but also efficiently recognized and measured bending angles, enabling a more sophisticated human-machine interaction experience. This advancement opens up possibilities for future intelligent wearable technology and human-machine interaction, contributing to the evolution of these fields.

Facile Fabrication of "Tacky", Stretchable, and Aligned Carbon Nanotube Sheet-Based Electronics for On-Skin Health Monitoring

Point-of-care monitoring of physiological signals such as electrocardiogram, electromyogram, and electroencephalogram is essential for prompt disease diagnosis and quick treatment, which can be realized through advanced skin-worn electronics. However, it is still challenging to design an intimate and nonrestrictive skin-contact device for physiological measurements with high fidelity and artifact tolerance. This research presents a facile method using a "tacky" surface to produce a tight interface between the ACNT skin-like electronic and the skin. The method provides the skin-worn electronic with a stretchability of up to 70% strain, greater than that of most common epidermal electrodes. Low-density ACNT bundles facilitate the infiltration of adhesive and improve the conformal contact between the ACNT sheet and the skin, while dense ACNT bundles lessen this effect. The stretchability and conformal contact allow the ACNT sheet-based electronics to create a tight interface with the skin, which enables the high-fidelity measurement of physiological signals (the Pearson's coefficient of 0.98) and tolerance for motion artifacts. In addition, our method allows the use of degradable substrates to enable reusability and degradability of the electronics based on ACNT sheets, integrating "green" properties into on-skin electronics.

Electric Resistance of Elastic Strain Sensors-Fundamental Mechanisms and Experimental Validation

Elastic strain sensor nanocomposites are emerging materials of high scientific and commercial interest. This study analyzes the major factors influencing the electrical behavior of elastic strain sensor nanocomposites. The sensor mechanisms were described for nanocomposites with conductive nanofillers, either dispersed inside the polymer matrix or coated onto the polymer surface. The purely geometrical contributions to the change in resistance were also assessed. The theoretical predictions indicated that maximum Gauge values are achieved for mixture composites with filler fractions slightly above the electrical percolation threshold, especially for nanocomposites with a very rapid conductivity increase around the threshold. PDMS/CB and PDMS/CNT mixture nanocomposites with 0-5.5 vol.% fillers were therefore manufactured and analyzed with resistivity measurements. In agreement with the predictions, the PDMS/CB with 2.0 vol.% CB gave very high Gauge values of around 20,000. The findings in this study will thus facilitate the development of highly optimized conductive polymer composites for strain sensor applications.

Highly Stretchable, Self-Adhesive, Antidrying Ionic Conductive Organohydrogels for Strain Sensors

As flexible wearable devices, hydrogel sensors have attracted extensive attention in the field of soft electronics. However, the application or long-term stability of conventional hydrogels at extreme temperatures remains a challenge due to the presence of water. Antifreezing and antidrying ionic conductive organohydrogels were prepared using cellulose nanocrystals and gelatin as raw materials, and the hydrogels were prepared in a water/glycerol binary solvent by a one-pot method. The prepared hydrogels were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The mechanical properties, electrical conductivity, and sensing properties of the hydrogels were studied by means of a universal material testing machine and LCR digital bridge. The results show that the ionic conductive hydrogel exhibits high stretchability (elongation at break, 584.35%) and firmness (up to 0.16 MPa). As the binary solvent easily forms strong hydrogen bonds with water molecules, experiments show that the organohydrogels exhibit excellent freezing and drying (7 days). The organohydrogels maintain conductivity and stable sensitivity at a temperature range (-50 °C-50 °C) and after long-term storage (7 days). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 6.47 (strain, 0-400%) could detect human motions. Therefore, multifunctional organohydrogel wearable sensors with antifreezing and antidrying properties have promising potential for human body monitoring under a broad range of environmental conditions.

Stretchable and Highly Sensitive Strain Sensor Based on a 2D MXene and 1D Whisker Carbon Nanotube Binary Composite Film

We fabricate a 2D MXene and 1D whisker carbon nanotube (WCNT) binary composite, where the MXene layer was sandwiched between two WCNT films, and assemble a flexible resistive-type strain sensor using this composite film. The deformations of the conductive networks trigged by the external mechanical stimuli cause the variations of the number of effective conductive paths, which result in the changes of the electric resistance of composite films. The resistances of the MXene/WCNT composite films that carry the strain information about the external mechanical stimuli are monitored. In addition, we demonstrate the role of the conductive MXene networks and the WCNT networks in responding to the external mechanical stimuli. The MXene networks dominate the variations of the resistance of the strain sensors in the low strain range. In the middle strain range, the deformations of both the MXene networks and the WCNT networks are responsible for the variations of the resistance of the strain sensors. In the high strain range, an "island bridge" like conductive network forms, where MXenes act as islands and WCNTs connect the adjacent MXene islands like bridges. The multiple types of conductive networks lead to the high sensitivity of the MXene/WCNT-based strain sensors over a wide strain range and a wide response window. This stretchable strain sensor exhibits good performances in detecting human muscle motions with a wide strain range and has the potentials of being applicable to wearable electronics.

Carbon Nanotube Coated Fibrous Tubes for Highly Stretchable Strain Sensors Having High Linearity

Strain sensors are currently limited by an inability to operate over large deformations or to exhibit linear responses to strain. Producing strain sensors meeting these criteria remains a particularly difficult challenge. In this work, the fabrication of a highly flexible strain sensor based on electrospun thermoplastic polyurethane (TPU) fibrous tubes comprising wavy and oriented fibers coated with carboxylated multiwall carbon nanotubes (CNTs) is described. By combining spraying and ultrasonic-assisted deposition, the number of CNTs deposited on the electrospun TPU fibrous tube could reach 12 wt%, which can potentially lead to the formation of an excellent conductive network with high conductivity of 0.01 S/cm. The as-prepared strain sensors exhibited a wide strain sensing range of 0-760% and importantly high linearity over the whole sensing range while maintaining high sensitivity with a GF of 57. Moreover, the strain sensors were capable of detecting a low strain (2%) and achieved a fast response time whilst retaining a high level of durability. The TPU/CNTs fibrous tube-based strain sensors were found capable of accurately monitoring both large and small human body motions. Additionally, the strain sensors exhibited rapid response time, (e.g., 45 ms) combined with reliable long-term stability and durability when subjected to 60 min of water washing. The strain sensors developed in this research had the ability to detect large and subtle human motions, (e.g., bending of the finger, wrist, and knee, and swallowing). Consequently, this work provides an effective method for designing and manufacturing high-performance fiber-based wearable strain sensors, which offer wide strain sensing ranges and high linearity over broad working strain ranges.

Preparation of stretchable and self-healable dual ionically cross-linked hydrogel based on chitosan/polyacrylic acid with anti-freezing property for multi-model flexible sensing and detection

As a kind of promising material for flexible wearable electronics, conductive hydrogels have attracted extensive interests of researchers for their inherent merits such as superior mechanical properties, biocompatibility, and permeability. Herein, we constructed a new type of highly stretchable, anti-freezing, self-healable, and conductive hydrogel based on chitosan/polyacrylic acid. The large amount of ions inside the network had five functions for the proposed hydrogel, including excellent mechanical behaviors, high conductivity, self-recovery, self-healing and anti-freezing capability. Consequently, the proposed hydrogel possessed tunable stretchability (1190-1550%), tensile strength (0.96-2.56 MPa), toughness (5.7-14.7 MJ/m³), superior self-healing property (self-healing efficiency up to 83.7%), high conductivity (4.58-5.76 S/m), and excellent anti-freezing capability. To our knowledge, the self-healable hydrogel with balanced tensile strength, toughness, conductivity, and low-temperature tolerance can hardly be achieved till now. Furthermore, the conductive hydrogels exhibited high sensitivity (gauge factor up to 10.8) in a broad strain window (0-1000%) and could detect the conventional motion signals of human body such as bending of a knuckle, swallowing, and pressure signal at both room temperature and -20 °C. Moreover, the hydrogels could also be fabricated as flexible detectors to identify different temperatures, different kinds of solutions, and different concentrations of the solution.

Ready-to-wear strain sensing gloves for human motion sensing

Integrating soft sensors with wearable platforms is critical for sensor-based human augmentation, yet the fabrication of wearable sensors integrated into ready-to-wear platforms remains underdeveloped. Disposable gloves are an ideal substrate for wearable sensors that map hand-specific gestures. Here, we use solution-based metallization to prepare resistive sensing arrays directly on off-the-shelf nitrile butadiene rubber (NBR) gloves. The NBR glove acts as the wearable platform while its surface roughness enhances the sensitivity of the overlying sensing array. The NBR sensors have a sheet resistance of 3.1 ± 0.6 Ω/sq and a large linear working range (two linear regions ≤70%). When stretched, the rough NBR substrate facilitates microcrack formation in the overlying metal, enabling high gauge factors (62 up to 40% strain, 246 from 45 - 70% strain) that are unprecedented for metal film sensors. We apply the sensing array to dynamically monitor gestures for gesture differentiation and robotic control.

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Sensing arrays are prepared on ready-to-wear nitrile butadiene rubber (NBR) gloves

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Solution-based deposition is used to pattern gold sensing arrays on the NBR surface

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The roughness-enhanced sensitivity offers high gauge factors (62-246) to 70% strain

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Motion sensing is demonstrated for gesture differentiation and robotic control

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.

Anisotropic, Wrinkled, and Crack-Bridging Structure for Ultrasensitive, Highly Selective Multidirectional Strain Sensors

Flexible multidirectional strain sensors are crucial to accurately determining the complex strain states involved in emerging sensing applications. Although considerable efforts have been made to construct anisotropic structures for improved selective sensing capabilities, existing anisotropic sensors suffer from a trade-off between high sensitivity and high stretchability with acceptable linearity. Here, an ultrasensitive, highly selective multidirectional sensor is developed by rational design of functionally different anisotropic layers. The bilayer sensor consists of an aligned carbon nanotube (CNT) array assembled on top of a periodically wrinkled and cracked CNT-graphene oxide film. The transversely aligned CNT layer bridge the underlying longitudinal microcracks to effectively discourage their propagation even when highly stretched, leading to superior sensitivity with a gauge factor of 287.6 across a broad linear working range of up to 100% strain. The wrinkles generated through a pre-straining/releasing routine in the direction transverse to CNT alignment is responsible for exceptional selectivity of 6.3, to the benefit of accurate detection of loading directions by the multidirectional sensor. This work proposes a unique approach to leveraging the inherent merits of two cross-influential anisotropic structures to resolve the trade-off among sensitivity, selectivity, and stretchability, demonstrating promising applications in full-range, multi-axis human motion detection for wearable electronics and smart robotics.

A multi-model, large range and anti-freezing sensor based on a multi-crosslinked poly(vinyl alcohol) hydrogel for human-motion monitoring

Conductive hydrogels are capturing intensive attention for versatile applications in flexible wearable devices on account of their unique combination of softness, stretchability, conductivity and biocompatibility. However, most of the hydrogel sensors can only serve as single-type sensors to detect strain or pressure, accompanied by a limited detection range. Moreover, the poor anti-freezing performance is also a serious problem to be addressed for their practical applications. Herein, a multi-model, large range and anti-freezing hydrogel sensor was constructed from a high-mechanical and ionic conductive multi-crosslinked poly(vinyl alcohol) (M-PVA) hydrogel, which was prepared via incorporating chain entanglement interaction and complexation between Fe3+ ions and hydroxyl groups into the microcrystalline network through immersion treatment in Fe2(SO4)3 solution. The three reversible and reconstructable crosslinks within the M-PVA hydrogel worked in tandem to achieve ultra-stretchability (1120%), supercompressibility (98%), high toughness, fast self-recoverability and excellent fatigue resistance. Meanwhile, the introduction of Fe3+ and SO42- ions endowed the M-PVA hydrogel with good ionic conductivity and remarkable anti-freezing properties (-50 °C), which benefited the M-PVA hydrogel to act as a freezing-tolerant dressing. The assembled multi-model hydrogel sensor can sensitively and stably detect large range elongation (∼900%), compression (∼70%), bend and pressure (up to 4.60 MPa) concurrently, as well as various human activities including speaking, finger bending and treading behavior. Notably, the hydrogel sensor was capable of maintaining excellent mechanical flexibility and sensitive sensing capacity at low temperature. The M-PVA hydrogel is a promising flexible sensing material for versatile applications in ionic skin, motion recognition and intelligent wearable devices.

Multi-Walled Carbon Nanotubes-Based Sensors for Strain Sensing Applications

The use of multi-walled carbon nanotube (MWCNT)-based sensors for strain-strain applications is showcased in this paper. Extensive use of MWCNTs has been done for the fabrication and implementation of flexible sensors due to their enhanced electrical, mechanical, and thermal properties. These nanotubes have been deployed both in pure and composite forms for obtaining highly efficient sensors in terms of sensitivity, robustness, and longevity. Among the wide range of applications that MWCNTs have been exploited for, strain-sensing has been one of the most popular ones due to the high mechanical flexibility of these carbon allotropes. The MWCNT-based sensors have been able to deduce a broad spectrum of macro- and micro-scaled tensions through structural changes. This paper highlights some of the well-approved conjugations of MWCNTs with different kinds of polymers and other conductive nanomaterials to form the electrodes of the strain sensors. It also underlines some of the measures that can be taken in the future to improve the quality of these MWCNT-based sensors for strain-related applications.

Extending Porous Silicone Capacitive Pressure Sensor Applications into Athletic and Physiological Monitoring

Porous polymer dielectric materials have been developed to increase the sensitivity of capacitive pressure sensors, so that they might expand capacitive sensor use, and promote the realization of the advantages of this class of sensor in further fields. However, their use has not been demonstrated in physiological monitoring applications such as respiration monitoring and body position detection during sleep; an area in need of unmet medical attention for conditions such as sleep apnea. Here, we develop and characterize a sensor comprised of a poly dimethylsiloxane (PDMS) sponge dielectric layer, and PDMS/carbon black (CB) blend electrode layers, with suitable compliance and sensitivity for integration in mattresses, pillows, and athletic shoe insoles. With relatively high pressure sensitivity (~0.1 kPa^-1) and mechanical robustness, this sensor was able to fulfill a wide variety of roles, including athletic monitoring in an impact mechanics scenario, by recording heel pressure during running and walking, and physiological monitoring, by detecting head position and respiration of a subject lying on a pad and pillow. The sensor detected considerably greater relative signal changes than those reported in recent capacitive sensor studies for heel pressure, and for a comparably minimal, resistive sensor during respiration, in line with its enhanced sensitivity.

Highly Sensitive Ultrathin Flexible Thermoplastic Polyurethane/Carbon Black Fibrous Film Strain Sensor with Adjustable Scaffold Networks

In recently years, high-performance wearable strain sensors have attracted great attention in academic and industrial. Herein, a conductive polymer composite of electrospun thermoplastic polyurethane (TPU) fibrous film matrix-embedded carbon black (CB) particles with adjustable scaffold network was fabricated for high-sensitive strain sensor. This work indicated the influence of stereoscopic scaffold network structure built under various rotating speeds of collection device in electrospinning process on the electrical response of TPU/CB strain sensor. This structure makes the sensor exhibit combined characters of high sensitivity under stretching strain (gauge factor of 8962.7 at 155% strain), fast response time (60 ms), outstanding stability and durability (> 10,000 cycles) and a widely workable stretching range (0-160%). This high-performance, wearable, flexible strain sensor has a broad vision of application such as intelligent terminals, electrical skins, voice measurement and human motion monitoring. Moreover, a theoretical approach was used to analyze mechanical property and a model based on tunneling theory was modified to describe the relative change of resistance upon the applied strain. Meanwhile, two equations based from this model were first proposed and offered an effective but simple approach to analyze the change of number of conductive paths and distance of adjacent conductive particles.

Adhesive, Stretchable, and Transparent Organohydrogels for Antifreezing, Antidrying, and Sensitive Ionic Skins

As a flexible wearable device, hydrogel-based sensors have attracted widespread attention in soft electronics. However, the application of traditional hydrogels at extreme temperatures or for a long-term stability still remain a challenge because of the existence of water. Herein, we reported an antifreezing and antidrying organohydrogel with high transparency (over 85% transmittance), high stretchability (up to 1200%), and robust adhesiveness to various substrates, which consist of polyacrylic acid, gelatin, AlCl³⁺, and tannic acid in a water/glycerin binary solvent as the dispersion medium. As the binary solvent easily forms strong hydrogen bonds with water molecules, organohydrogels exhibited excellent tolerance for drying and freezing. The organohydrogels maintained conductivity, adhesion, and stable sensitivity after a long-term storage or at subzero temperature (-14 °C). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 2.5 (strain, 0-100%) could detect both large-scale movements and subtle motions. Therefore, the multifunctional organohydrogel-wearable sensors with antifreezing and antidrying properties have promising potential for human-machine interfaces and healthcare monitoring under a broad range of environmental conditions.

Highly aligned carbon nanotubes and their sensor applications

Flexible electronics comprising carbon nanotube (CNT) membranes and polymer composites are used in diverse applications, including health monitoring. Devices prepared using such electronics need to exhibit acceptable sensitivity at high strains, with the advantage of negligible hysteresis. Herein, we report a simple, physically robust method to fabricate a highly sensitive and stretchable sensor that enables the detection of pressure, strain, and human activity with facial expressions based on the highly aligned carbon nanotubes embedded in polydimethylsiloxane (PDMS). The aligned CNT network in PDMS modulates the electron conduction path in a unidirectional manner and provides multimodal mechanical sensing ability with a wide sensing range and high sensitivity. The highly aligned CNT sensor demonstrates high-pressure sensitivity (1.29 kPa-1), excellent stability and repeatability (over 10 000 cycles) with negligible hysteresis, and a good strain sensitivity over a wide range (up to 65%) with a good linear response. We confirmed the applicability of the sensor to detect small signals, such as heartbeat and pulse rate, expressions, and voice recognition, and that it could distinguish between various human motions with a very short recovery time of approximately 50 ms.

M13 Bacteriophage-Assisted Morphological Engineering of Crack-Based Sensors for Highly Sensitive and Wide Linear Range Strain Sensing

Despite their extraordinary mechanosensitivities, most channel-like crack-based strain sensors are limited by their poor levels of stretchability and linearity. This work presents a simple yet efficient way of modulating the cracking structure of thin metal films on elastomers to facilitate the development of high-performance wearable strain sensors. A net-shaped crack structure based on a thin platinum (Pt) film can be produced by coating an elastomer surface with M13 bacteriophages (phages) and consequently engineering the surface strain upon stretching. This process produces a Pt-on-phage (PoP) strain sensor that simultaneously exhibits high levels of stretchability (24%), sensitivity (maximum gauge factor ≈ 845.6 for 20-24%), and linearity (R² ≈ 0.988 up to 20%). In addition, the sensor performance can be further modulated by either changing the phage coating volume or adding a silver nanowire coating to the PoP sensor film. The balanced strain-sensing performance, combined with fast response times and high levels of mechanical flexibility and operational stability, enables the devices to detect a wide range of human motions in real time after being attached to various body parts. Furthermore, PoP-based strain sensors can be usefully extended to detect more complex multidimensional strains through further strain engineering on a cross-patterned PoP film.

Highly Sensitive and Stretchable Strain Sensor Based on a Synergistic Hybrid Conductive Network

Highly sensitive and stretchable strain sensors have attracted considerable attention due to their promising applications in human motion detection, soft robot, wearable electronics, etc. However, there is still a trade-off between high sensitivity and high stretchability. Here, we reported a stretchable strain sensor by sandwiching reduced graphene oxide (RGO)-coated polystyrene microspheres (PS@RGO) and silver nanowires (AgNWs) conductive hybrids in an elastic polydimethylsiloxane (PDMS) matrix. Due to the synergistic effect of PS@RGO and AgNWs, the PDMS/PS@RGO/AgNWs/PDMS sensor exhibits a high initial electrical conductivity of 8791 S m^-1, wide working range of 0-230%, large gauge factor of 11 at 0-60% of strain and 47 at 100%-230% of strain with a high linear coefficient of 0.9594 and 0.9947, respectively, low limit of detection (LOD) of 1% of strain, and excellent long-term stability over 1000 stretching/releasing cycles under 50% strain. Furthermore, the strain sensor has been demonstrated in detecting human body motion and fan rotation with high stretchability and stability, showing potential application in intelligent robot and Internet of things.

A CNT-PDMS wearable device for simultaneous measurement of wrist pulse pressure and cardiac electrical activity

Simultaneous measurement of multi-physiological signals can provide effective diagnosis and therapeutic assessment of diseases. This paper reports a carbon nanotube (CNT) - Polydimethylsiloxane (PDMS) - based wearable device with piezo-resistive and voltage-sensing capabilities for simultaneously capturing wrist pulse pressure and cardiac electrical signal. The layout design of sensing elements in the device was guided by analyzing strain distribution and electric field distribution for minimizing the interference between wrist pulse and cardiac electric activity during measurement. Each device was preconditioned under the strain of 20% until the resistance change of the device reached equilibrium. After preconditioning, the relationship between the resistance change and the pressure was calibrated, which determined the device sensitivity to be 0.01 Pa^-1 and the linear pressure range of the device to be 0.4 kPa to 14.0 kPa. Mechanisms of CNT-PDMS for sensing strain signal and electrical pulse signal were explored by scanning electron microscopy (SEM) imaging and equivalent circuit modeling. The device was applied to monitor the wrist pulse and ECG signals of volunteers during the recovering process after physical exercises.

Vibration monitoring based on flexible multi-walled carbon nanotube/polydimethylsiloxane film sensor and the application on motion signal acquisition

Flexible sensors at small scales have potential applications in many fields. Until now, the research on high-performance vibration sensors based on soft materials with high sensitivity and precision, fast response and high stability are still in its infancy. In this work, a flexible, wearable and high precision film sensor based on multi-walled carbon nanotube (MWCNT) was prepared via a vacuum filtration process and then encapsulated within polydimethylsiloxane (PDMS). The sensor exhibits an ultrahigh sensitivity with gauge factor of 214.3 at flexural strain of 0.4%. When used to monitor the vibration responses of a carbon-fiber beam induced by the base excitation and impact hammer, the time and frequency responses were comparable with the results obtained by the accelerometer, with difference less than 1\!%. In addition, when the MWCNT/PDMS thin film was employed as an electronic skin sensor attached on the human body to detect human activities, the high sensitivity and repeatability demonstrate a great potential application in monitoring human motion.

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.

Tough and Stretchable Dual Ionically Cross-Linked Hydrogel with High Conductivity and Fast Recovery Property for High-Performance Flexible Sensors

As a kind of typical soft and wet material, hydrogel has been increasingly investigated as another way to develop flexible electronics. However, the traditional hydrogel with poor strain and strength performance cannot meet the requirements for stretchable electronics; fabricating a stretchable hydrogel with balanced tensile strength, toughness, and conductivity is still a big challenge. Herein, a new type of physically cross-linked hydrogel with poly(acrylamide-co-acrylic acid)-Fe³⁺ and chitosan-SO₄^2- dual ionic networks via facile free radical polymerization and soaking processes is developed to fabricate excellent high-performance flexible sensors. The abundant Fe³⁺ and SO₄^2- ions in the hydrogel can not only construct tough and strong dual ionic networks but also give the hydrogel high conductivity. Consequently, the optimal hydrogel possesses high tensile strength (∼5.1 MPa), large strain capacity (∼1225%), elasticity (∼1.13 MPa), high toughness (∼32.1 MJ/m³), and high conductivity (3.04 S/m at f = 0.1M), as well as rapid self-recovery property. Furthermore, the hydrogel conductor has high stretching sensitivity with a gauge factor of 6.0 at strain of 700% and was able to detect conventional motions of the human body such as the motions of the knuckle, speaking, and swallowing, which indicates that our ionic conductive hydrogels can be used to fabricate excellent high-performance flexible sensors.

Aerosol-Jet-Printed Preferentially Aligned Carbon Nanotube Twin-Lines for Printed Electronics

The alignment of carbon nanotubes (CNTs) is of great importance for the fabrication of high-speed electronic devices such as a transistor as the electron mobilities can be greatly enhanced with aligned CNT architectures. Here, we report, for the first time, a methodology to obtain preferentially aligned CNT traces on a flexible polyimide substrate utilizing the high-resolution aerosol jet printing technique and evaporation-driven self-assembly process. A self-assembled twin-line of CNT ("coffee-ring" effect) is observed in the deposit patterns, and the field-emission scanning electron microscopy (FESEM) images reveal highly self-ordered CNT in the resulting CNT twin-line. Various aerosol jet parameters have been investigated to obtain printed tracks in the range of 30-80 μm and conductive tracks (single CNT twin-line width) in the range of 600-1500 nm. The smallest CNT twin-line obtained in this experiment is found to be approximately 16 μm using a suitable sheath-to-atomizer flow ratio. Image analysis of FESEM images confirms the formation of aligned CNT traces at the ink periphery. The effect of the line width on the degree of alignment of the CNT is studied and evaluated. The electrical resistance of the CNT trace is adjustable by controlling the number of print passes and print speed.

Highly Stretchable, Adhesive, and Mechanical Zwitterionic Nanocomposite Hydrogel Biomimetic Skin

The artificial skin-like stretchable ionic sensor device usually requires a synergistic effect of reliable adhesion between human machine interface, reasonable mechanical strength, and visually displayable transparency. A plant-inspired zwitterionic hydrogel was prepared through rapid UV initiation in the existence of cellulose nanocrystals as physically crosslinker and reinforcing agent. The resulting transparent zwitterionic nanocomposite hydrogel successfully brings the synergistic advantages of robust adhesive strength between diversified substrates such as skins, plastics, glass, and steels with remarkable mechanical properties of a superior stretchability over 1000% strain, a mechanical tensile strength up to 0.61 MPa, and compressive strength up to 7.5 MPa, manifesting in superior ionic transport performance, simultaneously. Furthermore, the zwitterionic nanocomposite hydrogel was fabricated as a wearable compliant stretchable pressure-strain sensor in the modality of the skin-adhesive patch to be sensitive to human motion such as finger touch and speech recognition for personal healthcare of patient sensory rebuilding and physiological data acquisition. It maintains compressive cycling sensibility at diverse pressure during 0.5, 1.0, and 1.5 Hz, respectively. The multifunctional zwitterionic nanocomposite hydrogel could also be assembled into flexible electrical devices such as luminescent display and information transfer between human and robot communication for mechanosensory electronics and artificial intelligence.

Practical and Durable Flexible Strain Sensors Based on Conductive Carbon Black and Silicone Blends for Large Scale Motion Monitoring Applications

Presented is a flexible capacitive strain sensor, based on the low cost materials silicone (PDMS) and carbon black (CB), that was fabricated by casting and curing of successive silicone layers—a central PDMS dielectric layer bounded by PDMS/CB blend electrodes and packaged by exterior PDMS films. It was effectively characterized for large flexion-angle motion wearable applications, with strain sensing properties assessed over large strains (50%) and variations in temperature and humidity. Additionally, suitability for monitoring large tissue deformation was established by integration with an in vitro digestive model. The capacitive gauge factor was approximately constant at 0.86 over these conditions for the linear strain range (3 to 47%). Durability was established from consistent relative capacitance changes over 10,000 strain cycles, with varying strain frequency and elongation up to 50%. Wearability and high flexion angle human motion detection were demonstrated by integration with an elbow band, with clear detection of motion ranges up 90°. The device’s simple structure and fabrication method, low-cost materials and robust performance, offer promise for expanding the availability of wearable sensor systems.

Highly Stretchable and Sensitive Strain Sensor with Porous Segregated Conductive Network

Flexible strain sensors based on elastomeric conductive polymer composites (ECPCs) play an important role in wearable sensing electronics. However, the achievement of good conjunction between broad detection range and high sensitivity is still challenging. Herein, a highly stretchable and sensitive strain sensor was developed with the formation of porous segregated conductive network in the carbon nanotube/thermoplastic polyurethane composite via a facile and nontoxic compression-molding plus salt-leaching method. The strain sensor with porous segregated conductive network exhibited perfect combination of ultrawide sensing range (800% strain), large sensitivity (gauge factor of 356.4), short response time (180 ms) and recovery time (180 ms), as well as superior stability and durability. The integrated porous structure intensifies the deformation of segregated conductive network when tension strain is applied, which benefits enhancement of the sensitivity. Our sensor could monitor not only subtle oscillation and physiological signals but also energetic human motions efficiently, revealing promising potential applications in wearable motion monitoring systems. This work provides a unique and effective strategy for realizing ECPCs based strain sensors with excellent comprehensive sensing performances.

MoS2-Decorated Laser-Induced Graphene for a Highly Sensitive, Hysteresis-free, and Reliable Piezoresistive Strain Sensor

Advancement of sensing systems, soft robotics, and point-of-care testing requires the development of highly efficient, scalable, and cost-effective physical sensors with competitive and attractive features such as high sensitivity, reliability, and preferably reversible sensing behaviors. This study reports a highly sensitive and reliable piezoresistive strain sensor fabricated by one-step carbonization of the MoS₂-coated polyimide film to obtain MoS₂-decorated laser-induced graphene. The resulting three-dimensional porous graphene nanoflakes decorated with MoS₂ exhibit stable electrical properties yielding a reliable output for longer strain/release cycles. The sensor demonstrates high sensitivity (i.e., gauge factor, GF ≈1242), is hysteresis-free (∼2.75%), and has a wide working range (up to 37.5%), ultralow detection limit (0.025%), fast relaxation time (∼0.17 s), and a highly stable and reproducible response over multiple test cycles (>12 000) with excellent switching response. Owing to the outstanding performances of the sensor, it is possible to successfully detect various subtle movements ranging from phonation, eye-blinking, and wrist pulse to large human-motion-induced deformations.

Construction of sandwich-like porous structure of graphene-coated foam composites for ultrasensitive and flexible pressure sensors

Ultrasensitive and flexible pressure sensors that can perceive and respond to environmental stimuli have attracted considerable attention due to their potential applications in wearable electronics and electronic skin devices. Here, we report a simple and low-cost strategy to fabricate high-performance pressure sensors via constructing a unique conductive/insulating/conductive sandwich-like porous structure (SPS). Interpenetration of the conductive graphene network throughout the porous insulating interlayer produces a highly efficient transition from the non-conductive to the conductive state. Consequently, the SPS sensors exhibit an extreme resistance-switching behavior (resistance change of >105 at 30 kPa), high sensitivity (∼0.67 kPa-1, <1.5 kPa), fast response/recovery time (∼10 and ∼16 ms) and outstanding mechanical stability. Such SPS pressure sensors are applicable for detecting various mechanical deformation modes (press, bend and torsion) and different stress/strain levels (from gait feature, finger/wrist/elbow movement to breathing monitoring and real-time pulse wave), providing a new concept of device design for wearable electronic applications.

Highly Stretchable and Transparent Double-Network Hydrogel Ionic Conductors as Flexible Thermal-Mechanical Dual Sensors and Electroluminescent Devices

The latest generation flexible devices feature materials that are conductive, highly stretchable, and transparent to meet the requirements of a reliable performance. However, the existing conductors are mostly electronic conductors, which cannot satisfy these high-performance challenges. A robust hydrogel ionic conductor was rapidly fabricated with a facile one-pot approach by employing bioinspired agar with a physically cross-linked network, polyacrylamide (PAM) with a photoinitiated cross-linked network under appropriate UV intensity, and Li⁺ as conductive ions. The resulting Li⁺/agar/PAM ionic double-network hydrogels could be fabricated into various shapes through injection. The unique ionic hydrogel showed a remarkable stretchability with over 1600% extension, high tension/compression strength (0.22 MPa/3.5 MPa), and toughness (2.2 MJ/m³). Furthermore, it was demonstrated to possess dual sensory capabilities through the combination of both temperature and mechanical deformation. This hydrogel ionic conductor exhibited high stretching sensitivity with a gauge factor of 1.8 at strain 1100% and bending sensitivity in a broad angle range of 30-150°, respectively. Such highly optical transparency and elasticity endow the hydrogel-phosphor composites with promising luminescent properties. The multifunctional ionic hydrogel can be used as soft conductors for application in flexible devices such as ionic skin for wearable sensors and luminescence display.

Skin-inspired flexible and high-sensitivity pressure sensors based on rGO films with continuous-gradient wrinkles

Flexible electronic devices have received more and more attention. In particular, for pressure sensors, traditional methods for improving sensing performances are mostly based on the construction of microstructured templates. However, it still remains significantly challenging to conveniently fabricate thin-film sensors which possess flexibility, high sensitivity and location detection ability. Inspired by the microstructure of the human skin surface, herein, a new pressure sensor with a hierarchical structure and gradient reduced graphene oxide (rGO) wrinkles is reported. Benefiting from the skin-like structure, the pressure sensor demonstrates outstanding sensitivity, reaching 178 kPa-1; it can also detect pressure as small as 42 Pa. Furthermore, the concept of designing and constructing a gradient structure has been applied to achieve position detection, which is expected to find practical applications in human motion detection.

Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes

Although there have been remarkable improvements in stretchable strain sensors, the development of strain sensors with scalable fabrication techniques and which both high sensitivity and stretchability simultaneously is still challenging. In this work, a stretchable strain sensor based on overlapped carbon nanotube (CNT) bundles coupled with a silicone elastomer is presented. The strain sensor with overlapped CNTs is prepared by synthesizing line-patterned vertically aligned CNT bundles and rolling and transferring them to the silicone elastomer. With the sliding and disconnection of the overlapped CNTs, the strain sensor performs excellently with a broad sensing range (≥145% strain), ultrahigh sensitivity (gauge factor of 42 300 at a strain of 125-145%), high repeatability, and durability. The performance of the sensor is also tunable by controlling the overlapped area of CNT bundles. Detailed mechanisms of the sensor and its applications in human motion detection are also further investigated. With the novel structure and mechanism, the sensor can detect a wide range of strains with high sensitivity, demonstrating the potential for numerous applications including wearable healthcare devices.

Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.

High performance strain sensor based on buckypaper for full-range detection of human motions

A high-performance strain sensor based on buckypaper has been fabricated and studied. The sensor with an ultrahigh gauge factor of 20 216 can detect a maximum and a minimum strain range of 75% and 0.1%, respectively. During stretching, the strain sensor achieves a high stability and reproducibility of 10 000 cycles, and a fast response time of less than 87 ms. On the other hand, the sensor shows an excellent sensing performance upon pressure. The pressure range, pressure sensitivity and loading-unloading cycles are 0-1.68 MPa, 89.7 kPa-1 and 3000 cycles, respectively. A concept of the optimal value is utilized to evaluate the strain and pressure performances of the sensor. The optimal values of the sensor upon tensile strain and pressure are calculated to be 3.07 × 108 and 1.35 × 107, respectively, which are much higher than those of most strain and pressure sensors reported in the literature. Precise detection of full-range human motions, acoustic vibrations and even pulse waves at a small scale has been successfully demonstrated by the buckypaper-based sensor. Owning to its advantages including ultrahigh sensitivity, wide detection range and good stability, the buckypaper-based sensor suggests a great potential for applications in wearable sensors, electronic skins, micro/nano electromechanical systems, vibration sensing devices and other strain sensing devices.

Transparent and stretchable strain sensors based on metal nanowire microgrids for human motion monitoring

Optical transparency is increasingly considered as one of the most important characteristics required in advanced stretchable strain sensors for application in body-attachable systems. In this paper, we present an entirely solution-processed fabrication route to highly transparent and stretchable resistive strain sensors based on silver nanowire microgrids (AgNW-MGs). The AgNW-MG strain sensors are readily prepared by patterning the AgNWs on a stretchable substrate into a MG geometry via a mesh-template-assisted contact-transfer printing. The MG has a unique architecture comprising the AgNWs and can be stretched to ε = 35%, with high gauge factors of ∼6.9 for ε = 0%-30% and ∼41.1 for ε = 30%-35%. The sensor also shows a high optical transmittance of 77.1% ± 1.5% (at 550 nm) and stably maintains the remarkable optical performance even at high strains. In addition, the sensor responses are found to be highly reversible with negligible hysteresis and are reliable even under repetitive stretching-releasing cycles (1000 cycles at ε = 10%). The practicality of the AgNW-MG strain sensor is confirmed by successfully monitoring a wide range of human motions in real time after firmly laminating the device onto various body parts.

Surface-agnostic highly stretchable and bendable conductive MXene multilayers

Stretchable, bendable, and foldable conductive coatings are crucial for wearable electronics and biometric sensors. These coatings should maintain functionality while simultaneously interfacing with different types of surfaces undergoing mechanical deformation. MXene sheets as conductive two-dimensional nanomaterials are promising for this purpose, but it is still extremely difficult to form surface-agnostic MXene coatings that can withstand extreme mechanical deformation. We report on conductive and conformal MXene multilayer coatings that can undergo large-scale mechanical deformation while maintaining a conductivity as high as 2000 S/m. MXene multilayers are successfully deposited onto flexible polymer sheets, stretchable poly(dimethylsiloxane), nylon fiber, glass, and silicon. The coating shows a recoverable resistance response to bending (up to 2.5-mm bending radius) and stretching (up to 40% tensile strain), which was leveraged for detecting human motion and topographical scanning. We anticipate that this discovery will allow for the implementation of MXene-based coatings onto mechanically deformable objects.

Hierarchical self-entangled carbon nanotube tube networks

Three-dimensional (3D) assemblies based on carbon nanomaterials still lag behind their individual one-dimensional building blocks in terms of mechanical and electrical properties. Here we demonstrate a simple strategy for the fabrication of an open porous 3D self-organized double-hierarchical carbon nanotube tube structure with properties advantageous to those existing so far. Even though no additional crosslinking exists between the individual nanotubes, a high reinforcement effect in compression and tensile characteristics is achieved by the formation of self-entangled carbon nanotube (CNT) networks in all three dimensions, employing the CNTs in their high tensile properties. Additionally, the tubular structure causes a self-enhancing effect in conductivity when employed in a 3D stretchable conductor, together with a high conductivity at low CNT concentrations. This strategy allows for an easy combination of different kinds of low-dimensional nanomaterials in a tube-shaped 3D structure, enabling the fabrication of multifunctional inorganic-carbon-polymer hybrid 3D materials.

Low-dimensional nanomaterials are crucial conducting components of stretchable electronics, but their mechanical reinforcement remains challenging. Here, the authors infiltrate carbon nanotubes into a porous ceramic network to produce a 3D nanofelted self-entangled assembly with high conductivity and mechanical stability.