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

Interfacial functionalization and capillary force welding of enhanced silver nanowire-cellulose nanofiber composite electrodes for electroluminescent devices

The development of flexible, intelligent, and lightweight optoelectronic devices based on flexible transparent conductive electrodes (FTCEs) utilizing silver nanowires (AgNWs) has garnered increasing attention. However, achieving low surface resistance, strong adhesion to the flexible substrate, low surface roughness, and green degradability remains a challenge. Here, a composite electrode combining natural polymer cellulose nanofibers (TCNFs) with AgNWs was prepared. This process includes non-covalent interface embedding between TCNFs and AgNWs as well as a strong capillary force between the hydrophilic TCNF substrate and AgNWs during water mist capillary force cold welding. By adding ethanolamine and employing a rapid water mist wetting-drying process (within 10 s), the performance of TCNF/AgNW FTCEs significantly improves with reduced sheet resistance (8.3 Ω sq.-1), high light transmission (84.9 %), and low surface roughness (9.9 nm). Additionally, the composite electrode exhibits excellent stability and durability under various conditions such as bending, adhesion, and tensile stress. The prepared flexible electroluminescent device achieves high luminous intensity (43.6 cd m-2) and excellent operational stability, thanks to the outstanding performance of the composite electrode. This study presents a simple strategy for fabricating FTCEs using nanocellulose combined with AgNWs offering a potential material option for key components in green flexible optoelectronic devices.

MXene-MWCNT Conductive Network for Long-Lasting Wearable Strain Sensors with Gesture Recognition Capabilities

In this work, a conductive composite film composed of multi-walled carbon nanotubes (MWCNTs) and multi-layer Ti3C2Tx MXene nanosheets is used to construct a strain sensor on sandpaper Ecoflex substrate. The composite material forms a sophisticated conductive network with exceptional electrical conductivity, resulting in sensors with broad detection ranges and high sensitivities. The findings indicate that the strain sensing range of the Ecoflex/Ti3C2Tx/MWCNT strain sensor, when the mass ratio is set to 5:2, extends to 240%, with a gauge factor (GF) of 933 within the strain interval from 180% to 240%. The strain sensor has demonstrated its robustness by enduring more than 33,000 prolonged stretch-and-release cycles at 20% cyclic tensile strain. Moreover, a fast response time of 200 ms and detection limit of 0.05% are achieved. During application, the sensor effectively enables the detection of diverse physiological signals in the human body. More importantly, its application in a data glove that is coupled with machine learning and uses the Support Vector Machine (SVM) model trained on the collected gesture data results in an impressive recognition accuracy of 93.6%.

A Highly Sensitive Strain Sensor with Self-Assembled MXene/Multi-Walled Carbon Nanotube Sliding Networks for Gesture Recognition

Flexible electronics is pursuing a new generation of electronic skin and human-computer interaction. However, effectively detecting large dynamic ranges and highly sensitive human movements remains a challenge. In this study, flexible strain sensors with a self-assembled PDMS/MXene/MWCNT structure are fabricated, in which MXene particles are wrapped and bridged by dense MWCNTs, forming complex sliding conductive networks. Therefore, the strain sensor possesses an impressive sensitivity (gauge factor = 646) and 40% response range. Moreover, a fast response time of 280 ms and detection limit of 0.05% are achieved. The high performance enables good prospects in human detection, like human movement and pulse signals for healthcare. It is also applied to wearable smart data gloves, in which the CNN algorithm is utilized to identify 15 gestures, and the final recognition rate is up to 95%. This comprehensive performance strain sensor is designed for a wide array of human body detection applications and wearable intelligent systems.

Bioinspired High-Linearity, Wide-Sensing-Range Flexible Stretchable Bioelectronics Based on MWCNTs/GR/Nd2Fe14B/PDMS Nanocomposites for Human-Computer Interaction and Biomechanics Detection

In recent years, flexible and stretchable strain sensors have emerged as a prominent area of research, primarily due to their remarkable stretchability and extremely low strain detection threshold. Nevertheless, the advancement of sensors is currently constrained by issues such as complexity, high costs, and limited durability. To tackle the aforementioned issues, this study introduces a lepidophyte-inspired flexible, stretchable strain sensor (LIFSSS). The stretchable bioelectronics composites were composed of multiwalled carbon nanotubes, graphene, neodymium iron boron, and polydimethylsiloxane. Unique biolepidophyted microstructures and magnetic conductive nanocomposites interact with each other through synergistic interactions, resulting in the effective detection of tensile strain and magnetic excitation. The LIFSSS exhibits a 170% tensile range, a linearity of 0.99 in 50-170% strain (0.96 for full-scale range), and a fine durability of 7000 cycles at 110% tensile range. The sensor accurately detects variations in linear tensile force, human movement, and microexpressions. Moreover, LIFSSS demonstrates enhanced efficacy in sign language recognition for individuals with hearing impairments and magnetic grasping for robotic manipulators. Hence, the LIFSSS proposed in this study shows potential applications in various fields, including bioelectronics, electronic skin, and physiological activity monitoring.

Fabrication of Textured Ni-Coated Carbon Tubes for a Flexible Strain Sensor: Effect of the Device Elastic Modulus on Sensor Performance

The exploration of flexible resistive sensors with excellent performance remains a challenge. In this paper, a nickel-coated carbon tube with a textured structure was prepared as a conductive sensitive material and inserted into the poly(dimethylsiloxane) (PDMS) polymer; interestingly, the sensor performance was controlled by the elastic modulus of the matrix resin. The results show that Pd²⁺ may be adsorbed by the active groups on the surface of a plant fiber as a catalytic center for the reduction of Ni²⁺. After 300 °C annealing, the inner plant fiber would be carbonized and attached to the outside of the nickel tube; to be precise, the textured Ni-encapsulated C tube was fabricated successfully. It is worth noting that the C tube serves as a layer of support for the external Ni coating, providing sufficient mechanical strength. In addition, resistance sensors with different properties were prepared by controlling the elasticity modulus of the PDMS polymer by introducing different contents of curing agents. The limit uniaxial tensile strain was enhanced from 42 to 49% and sensitivity reduced from 0.2 to 2.0% with the elasticity modulus of the matrix resin increasing from 0.32 to 2.2 MPa. As expected, the sensor is obviously appropriate for the detection of elbow joints, human speaking, and human joints with the reduction of the elasticity modulus of the matrix resin. To be precise, the optimal elastic modulus of the sensor matrix resin would facilitate the improvement of its sensitivity to monitor different human behaviors.

Research status of polysiloxane-based piezoresistive flexible human electronic sensors

Flexible human body electronic sensor is a multifunctional electronic device with flexibility, extensibility, and responsiveness. Piezoresistive flexible human body electronic sensor has attracted the extensive attention of researchers because of its simple preparation process, high detection sensitivity, wide detection range, and low power consumption. However, the wearability and affinity to the human body of traditional flexible human electronic sensors are poor, while polysiloxane materials can be mixed with other electronic materials and have good affinity toward the human body. Therefore, polysiloxane materials have become the first choice of flexible matrixes. In this study, the research progress and preparation methods of piezoresistive flexible human electronic sensors based on polysiloxane materials in recent years are summarized, the challenges faced in the development of piezoresistive flexible human electronic sensors are analyzed, and the future research directions are prospected.

Highly Stretchable Printed Poly(vinylidene fluoride) Sensors through the Formation of a Continuous Elastomer Phase

Stretchable piezoelectric stress/strain sensing materials have attracted substantial research interest in the fields of wearable health monitoring, motion capturing, and soft robotics. These sensors require operation under dynamic loading conditions with high strain range, changing strain/loading rates, and varying pre-stretch states, which are challenging conditions for existing sensors to produce reliable measurements. To overcome these challenges, an intrinsically stretchable poly(vinylidene fluoride) (PVDF) sensor is developed through the polymer blending of PVDF and acrylonitrile butadiene rubber (NBR). Through precipitation printing and vulcanization, the resulting PVDF/NBR blends exhibit strong β phase PVDF and a blend morphology with submicron-level phase separation, but also strains up to 544%. Both the blend morphology and the mechanical properties indicate that this PVDF/NBR blend can be considered as a continuous elastomer phase above micron scale. After electric poling and adding electrodes, the PVDF/NBR blends have excellent piezoelectric properties to be used as both stretching mode strain sensors and compression mode stress/force sensors. The stretching mode sensors can measure strain up to 70% without strain rate and pre-stretch dependence, while the compression mode sensors have a loading-rate-independent linear voltage-stress relationship up to 4.8 MPa stress and a negligible pre-stretch dependence. Therefore, the PVDF/NBR sensors can provide accurate and reliable stress/strain measurements when attached to soft structures, which paves the way for sensing and calibration of soft robots under dynamic loading conditions.

Fabrication and characterization of highly sensitive flexible strain sensor based on biodegradable gelatin nanocomposites and double strain layered structures with crack for gesture recognition

Flexible sensors have attracted extensive attention in the field of human-computer interaction. However, it is still a challenging task to realize accuracy gesture recognition with flexible sensor, which requires sensor not only have high sensitivity, but also have appropriate strain detection range. Here, a high gauge factor flexible sensor (gauge factor ∼ 1296 under 12-20 % strain) based on crack structure is reported. The sensor is made of a biodegradable and stretchable gelatin composite combined with fabric bases, with good repeatability (6000 cycles) and a fast response (60 ms). Because of the double-layer structure, it has a suitable detection range (20 % strain). The sensor is manufactured by a screen-printing process, and it has been used to make data gloves and has realized 9 gestures recognition with machine learning algorithm (99.6 % accuracy). In general, this study offers a wearable gestures recognition scheme through the proposed sensor.

Ag NW-Embedded Coaxial Nanofiber-Coated Yarns with High Stretchability and Sensitivity for Wearable Multi-Sensing Textiles

The emerging intelligent piezoresistive yarn/textile-based sensors are of paramount importance for skin-interface electronics, owing to their unparalleled features including softness, breathability, and easy integration with functional devices. However, employing a facile way to fabricate 1D sensing yarns with mechanical robustness, multi-functional integration, and comfortability is still demanded for satisfying the practical applications. Herein, a facile one-step synchronous conjugated electrospinning and electrospraying technique is innovatively employed to continuously construct an Ag NW-embedded polyurethane (PU) nanofiber sensing yarn (AENSY) with hierarchical architecture. This 1D AENSY with weavability and stretchability can be woven into AENSY textile-based sensors integrated with functions of strain and pressure sensing. In this embedded multi-scale architecture, Ag NWs are evenly embedded and locked in the oriented and twisted PU nanofiber (PUNF) scaffold, forming the hierarchical mechanical sensing layer on the surface of the AENSY with favorable stability. Meanwhile, the presence of the elastic PUNFs enhances porosity, elasticity, and considerable deformation space, which in turn endow the AENSY textile-based sensor with a gauge factor (GF) up to 1010, a pressure sensitivity up to 16.7 N^-1, high stretchability up to 160%, and high stability under long-term cycles. In addition, the AENSY textile-based sensor exhibits light weight and the unique advantage of skin-friendliness with the human body, which can be directly and conformally attached to the curved human skin to monitor the various human movements. Furthermore, the weavable AENSYs can be integrated into smart textiles with sensing arrays, which are capable for spatial pressure and strain mapping. Thus, the continuous one-step developing process and the stable embedded-twisted fiber structure provide a promising strategy to develop innovative smart yarns and textiles for personalized healthcare and human-machine interfaces.

Hierarchically Structured Carbon Nanofiber-Enabled Skin-Like Strain Sensors with Full-Range Human Motion Monitoring and Autonomous Self-Healing Capability

Flexible strain sensors that mimic the properties of human skin have recently attracted tremendous attention. However, integrating multiple functions of skin into one strain sensor, e.g., stretchability, full-range motion response, and self-healing capability, is still an enormous challenge. Herein, a skin-like strain sensor was presented by the construction of hierarchically structured carbon nanofibers (CNFs), followed by encapsulation of elastic self-healing polyurethane (PU). The hierarchical sensing structure was composed of diversified CNFs with orientations from highly aligned to randomly oriented, and their different fracture mechanisms enabled the resultant strain sensor to successfully integrate key sensing properties including high sensitivity (gauge factor of 90), wide sensing range (∼80% strain), and fast response (52 ms). These properties, combined with high stretchability (870%) and excellent stability (>2000 cycles), allowed the sensor to precisely detect full-range human motions from large joint motions to subtle physiological signals. Moreover, the strain sensor had spontaneous self-healing capability at room temperature with high healing efficiencies of 97.7%, while the healing process could substantially be accelerated by the natural sunlight (24 h → 0.5 h). The healed sensor possessed comparable stretchability, sensing performance, and accurate monitoring ability of subtle body signals with the original sensor. The biomimetic self-healing functionality along with skin-like sensing properties makes it attractive for next-generation wearable electronics.

Miniaturized retractable thin-film sensor for wearable multifunctional respiratory monitoring

As extremely important physiological indicators, respiratory signals can often reflect or predict the depth and urgency of various diseases. However, designing a wearable respiratory monitoring system with convenience, excellent durability, and high precision is still an urgent challenge. Here, we designed an easy-fabricate, lightweight, and badge reel-like retractable self-powered sensor (RSPS) with high precision, sensitivity, and durability for continuous detection of important indicators such as respiratory rate, apnea, and respiratory ventilation. By using three groups of interdigital electrode structures with phase differences, combined with flexible printed circuit boards (FPCBs) processing technology, a miniature rotating thin-film triboelectric nanogenerator (RTF-TENG) was developed. Based on discrete sensing technology, the RSPS has a sensing resolution of 0.13 mm, sensitivity of 7 P·mm-1, and durability more than 1 million stretching cycles, with low hysteresis and excellent anti-environmental interference ability. Additionally, to demonstrate its wearability, real-time, and convenience of respiratory monitoring, a multifunctional wearable respiratory monitoring system (MWRMS) was designed. The MWRMS demonstrated in this study is expected to provide a new and practical strategy and technology for daily human respiratory monitoring and clinical diagnosis.

Electronic Supplementary Material

Supplementary material (additional figures and movies, including the production process of respiratory monitoring straps, the mechanical analysis of RSPS, RTF-TENG versus vector TENG sensors, the simulation studies of TE-TENG and FT-TENG, the additional characterization of RTF-TENG, the tensile and robustness tests of RSPS, the characterizations of the MWRMS during different sleeping positions, detailed circuit schematic of the MWRMS, the displacements and phase relations of RSPS, MWRMS for multifunctional respiratory monitoring) is available in the online version of this article at 10.1007/s12274-023-5420-1.

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.

Reconfigurable, Stretchable Strain Sensor with the Localized Controlling of Substrate Modulus by Two-Phase Liquid Metal Cells

Strain modulation based on the heterogeneous design of soft substrates is an effective method to improve the sensitivity of stretchable resistive strain sensors. In this study, a novel design for reconfigurable strain modulation in the soft substrate with two-phase liquid cells is proposed. The modulatory strain distribution induced by the reversible phase transition of the liquid metal provides reconfigurable strain sensing capabilities with multiple combinations of operating range and sensitivity. The effectiveness of our strategy is validated by theoretical simulations and experiments on a hybrid carbonous film-based resistive strain sensor. The strain sensor can be gradually switched between a highly sensitive one and a wide-range one by selectively controlling the phases of liquid metal in the cell array with a external heating source. The relative change of sensitivity and operating range reaches a maximum of 59% and 44%, respectively. This reversible heterogeneous design shows great potential to facilitate the fabrication of strain sensors and might play a promising role in the future applications of stretchable strain sensors.

Synthesis of a grape-like conductive carbon black/Ag hybrid as the conductive filler for soft silicone rubber

Conductive silicone rubber (CSR) is an outstanding stretchable conductive composite due to its excellent mechanical properties and stable conductivity. In this paper, silver nanoparticles were deposited on carbon black (CB) through a reduction reaction. The uniform dispersion of silver particles on the surface of CB as well as the grape-like branch structure of hybrid particles was formed by the condensation reaction of the hydroxyl groups of CB with (3-mercaptopropyl) trimethoxysilane (KH-590), along with the interattraction between sulfhydryl groups of KH-590 and silver ions. This sulfhydryl modified conductive carbon black/Ag hybrid filler (SMCB@Ag) avoided the high processing viscosity of CSR caused by the hydroxyl groups of CB. The percolation threshold of CSR made from SMCB@Ag was 5.5 wt% according to the percolation equation. With the addition amount of SMCB@Ag increasing to 10 wt%, the conductivity of CSR increased from 10^-5 to about 10¹. Moreover, the conductivity of this CSR showed excellent stability with extension of storage time and increase of stretching-recovery cycles.

Smart Textile Based on 3D Stretchable Silver Nanowires/MXene Conductive Networks for Personal Healthcare and Thermal Management

Wearable electronics have enriched daily lives by providing smart functions as well as monitoring body health conditions. However, the realization of wearable electronics with personal healthcare and thermal comfort management of the human body is still a great challenge. Furthermore, manufacturing such on-skin wearable electronics on traditional thin-film substrates results in limited gas permeability and inflammation. Herein, we proposed a personal healthcare and thermal management smart textile with a three-dimensional (3D) interconnected conductive network, formed by silver nanowires (AgNWs) bridging lamellar structured transition-metal carbide/carbonitride (MXene) nanosheets deposited on nonwoven fabrics. Benefiting from the interconnected conductive network synergistic effect of one-dimensional (1D) AgNWs bridging two-dimensional (2D) MXene, the strain sensor exhibits excellent durability (>1500 stretching cycles) and high sensitivity (gauge factor (GF) = 1085) with a wide strain range limit (∼100%), and the details of human body activities can be accurately recognized and monitored. Moreover, thanks to the excellent Joule heating and photothermal effect endowed by AgNWs and MXene, the multifunctional smart textile with direct temperature visualization and solar-powered temperature regulation functions was successfully developed, after further combination of thermochromic and phase-change functional layers, respectively. The smart textiles with a stretchable AgNW-MXene 3D conductive network hold great promise for next-generation personal healthcare and thermal management wearable systems.

Low-Temperature Wearable Strain Sensor Based on a Silver Nanowires/Graphene Composite with a Near-Zero Temperature Coefficient of Resistance

Currently, the exploration of wearable strain sensors that can work under subzero temperatures while simultaneously possessing anti-interference capability toward temperature is still a grand challenge. Herein, we present a low-temperature wearable strain sensor that is constructed via the incorporation of a Ag nanowires/graphene (Ag NWs/G) composite into the polydimethylsiloxane (PDMS) polymer. The Ag NWs/G/PDMS strain sensor exhibits promising flexibility at a very low temperature (-40 °C), outstanding fatigue resistance with low hysteresis energy, and near-zero temperature coefficient of resistance (TCR). The Ag NWs/G/PDMS strain sensor shows excellent sensing performance under subzero temperatures with a very high gauge factor of 9156 under a strain of >36%, accompanied by a noninterference characteristic to temperature (-40 to 20 °C). The Ag NWs/G/PDMS strain sensor also demonstrates the feasibility of monitoring various human movements such as finger bending, arm waving, wrist rotation, and knee bending under both room temperature and low-temperature conditions. This work initiates a new promising strategy to construct next-generation wearable strain sensors that can work stably and effectively under very low temperatures.