A Flexible and Multipurpose Piezoresistive Strain Sensor Based on Carbonized Phenol Formaldehyde Foam for Human Motion Monitoring

High‐performance flexible strain sensors are extensively studied for various applications including healthcare, robots, and human–computer interaction. In most of the reported research, the fabrication of these sensors involves conductive polymer composites containing expensive metallic or carbon nanomaterials. In this study, commercial phenol formaldehyde foam (PFF) is carbonized by a simple high‐temperature pyrolysis treatment and encapsulated by polydimethylsiloxane (PDMS) to fabricate a flexible and multipurpose piezoresistive strain sensor. The as‐fabricated PDMS‐cPFF strain sensor is capable of detecting various strain modes, including tension, compression, and three‐point bending. Furthermore, the sensor exhibits a high sensitivity with a gauge factor (GF) of ?20.5 under tension and stable signal responses in a frequency range of 0.01–0.5 Hz. The sensor is also capable of accurately monitoring a subtle bending strain of 0.05%. In addition, the sensor shows excellent durability in cyclic loading/unloading tests up to 1000 cycles. The applications of this strain sensor in both large‐ (finger bending and neck movement) and small‐scale human motion monitoring (facial micro‐expression and phonation) are demonstrated, showing its potential for applications in wearable electronics. This work also offers an alternative route to reuse waste thermosetting resins which would otherwise be difficult to recycle.     

Double Network Hydrogel Sensors with High Sensitivity in Large Strain Range

Owing to their preferable flexibility and facilitation to integrate with various apparel products, flexible sensors with high sensitivity are highly favored in the fields of environmental monitoring, health diagnosis, and wearable electronics. However, great challenges still remain in integrating high sensitivity with wide sensing range in one single flexible strain sensor. Herein, a new stretchable conductive gel-based sensor exhibiting remarkable properties regarding stretchability and sensitivity is developed via improving the ionic conductivity of the PVA/P(AM-AANa) double network hydrogel. Specifically, the strain sensor developed exhibits an excellent elongation of 549%, good fatigue resistance, and recovery performance. Simultaneously, the hydrogel strain sensor shows a high conductivity of 25 mS cm⁻¹, fast response time of 360 ms, and a linear response (gauge factor = 4.75) to external strain (≈400%), which endow the sensor with accurate and reliable capacities to detect various human movements. Integrating the merits of flexibility, environment friendliness, and high sensitivity, the conductive gel-based sensor has promising application prospects in human–machine interfaces, touchpads, biosensors, electronic skin, wearable electronic devices, and so on.     

Highly Stretchable,Transparent, and Bio‐Friendly Strain Sensor Based on Self‐Recovery Ionic‐Covalent Hydrogels for Human Motion Monitoring

Stretchable, flexible, and strain‐sensitive hydrogels have gained tremendous attention due to their potential application in health monitoring devices and artificial intelligence. Nevertheless, it is still a huge challenge to develop an integrated strain sensor with excellent mechanical properties, broad sensing range, high transparency, biocompatibility, and self‐recovery. Herein, a simple paradigm of stretchable strain sensor based on multifunctional hydrogels is prepared by constructing synergistic effects among polyacrylamide (PAM), biocompatible macromolecule sodium alginate (SA), and Ca ion in covalently and ionically crosslinked networks. Under large deformation, the dynamic SA‐Ca²⁺ bonds effectively dissipate energy, serving as sacrificial bonds, while the PAM chains bridge the crack and stabilize the network, endowing hydrogels with outstanding mechanical performances, for instance, high stretchability and compressibility, as well as excellent self‐recovery performance. The hydrogel is assembled to be a transparent and wearable strain sensor, which has good sensitivity and very wide sensing range (0–1700%), and can precisely detect dynamic strains, including both low and high strains (20–800% strain). It also exhibits fast response time (800 ms) and long‐time stability (200 cycles). The sensor can monitor and distinguish complicated human motions, opening up a new route for broad potential applications of eco‐friendly flexible strain‐sensing devices.     

Preparation of multifunctional self-healing MXene/PVA double network hydrogel wearable strain sensor for monitoring human body and organ movement

In this work, a kind of conductive, self-healing hydrogel was prepared. Then it is assembled into a flexible wearable sensor for human motion detection and human-computer interaction. MXene/PVA-CBA hydrogel has super mechanical properties and excellent self-healing ability (1.8 s). It is assembled into a flexible sensor with high sensitivity, which can accurately detect various movements of the human body (ranging from frowning, speaking, and coughing on the face to bending of fingers and wrists, and body movements). Furthermore, it can be used for handwriting recognition. When it is installed on the artificial limb, it can realize the function of touching the capacitive screen. It solves the problem of using silicone prostheses to control the screen and has broad research potential in the field of intelligent robots. Therefore, the flexible wearable sensor composed of MXene/PVA-CBA hydrogel has great potential in human motion detection, bionic intelligent robot, and intelligent detection.     

二维导电材料/柔性聚合物复合材料基可穿戴压阻式应变传感器的研究进展

可穿戴应变传感器在人体运动检测、健康监测、可穿戴电子设备和柔性电子皮肤等新兴领域具有极大的应用前景。近年来,由二维（2D）导电材料和柔性聚合物基体组成的可穿戴压阻式应变传感器具有较高的灵敏度、良好的拉伸性和柔韧性、优异的耐久性、可调的应变传感性和易加工等特点,受到广泛关注。基于此,本文对基于2D导电材料/柔性聚合物复合材料（2D-CPC）的可穿戴压阻式应变传感器的类型、传感机理、性能指标、影响因素及应用等进行了综述,并对其未来发展趋势进行了展望。     

A Skin-Inspired Triboelectric Nanogenerator with an Interpenetrating Structure for Motion Sensing and Energy Harvesting

Rapid advancements in wearable electronics impose the challenge on power supply devices. Herein, a flexible single-electrode triboelectric nanogenerator (SE-TENG) that enables both human motion sensing and biomechanical energy harvesting is reported. The SE-TENG is fabricated by interpenetrating Ag-coated polyethylene terephthalate (PET) nanofibers within a polydimethylsiloxane (PDMS) elastomer. The Ag coating and PDMS are performed as the electrode and dielectric material for the SE-TENG, respectively. The Ag-coated PET nanofibers enlarge the electrode surface area, which is beneficial to increase sensing sensitivity. The flexible SE-TENG sensor shows the capability of outputting alternating electrical signals with an open-circuit voltage up to 50 V and a short-circuit current up to 200 nA in response to externally applied pressure. It is used to sense various types of human motions and harvest electric energy from body motion. The harvested energy can successfully power wearable electronics, such as an electronic watch and light-emitting diode. Therefore, the as-prepared SE-TENG sensor with a pressure response and self-powered capability provides potential applications in wearable sensors or flexible electronics for personal healthcare and human–machine interfaces.     

Highly flexible fabric strain sensor based on graphene nanoplatelet–polyaniline nanocomposites for human gesture recognition

Flexible and wearable smart fabrics are becoming increasingly popular in healthcare and motion monitoring because of their potential applications in flexible and stretchable electronics. The integration of ordinary fabric with conductive fillers provides the fabric with new and intriguing functions, such as sensation. In this study, a low‐cost and efficient manner was used to fabricate a highly reliable conductive composite on fabric as an effective sensing material for gesture recognition. A strain sensor was fabricated by the incorporation of the highly conductive polyaniline (PANI) polymer, graphene nanoplatelets (GNPs), and a handful of silicon rubber (SR) onto elastic Lycra fabric via a spin‐coating method. We demonstrated that the fabric strain sensor was able to detect and monitor the bending angle of a human finger. By means of the covered structure of the PANI and GNPs, the composite fabric could bear a 40% maximum strain and possess the pleasant characteristic of stretching and bending. The gauge factor of the fabric strain sensor reached 67.3; this was an improvement of approximately four times compared to sensors without PANI microparticles. Finally, the superior performance of our strain sensor through the integration of five strain sensors on a glove for the motion detection of fingers was demonstrated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45340.     

Self-Healing Polyurethane Elastomer Based on Crosslinked-Linear Interpenetrating Structure: Preparation,Properties, and Applications

Despite tremendous advances in the preparation of self-healing flexible strain sensor devices, it remains a great challenge to large-area synthesis, relatively high mechanical strength, and sensitive sensing properties into one self-healing materials system, which have received increasing interests because of their many potential applications. Herein, the design and synthesis of a crosslinked-linear interpenetrating structure polymer containing disulfide and hydrogen bonds is reported to address this conundrum. The resulting self-healable polymer is highly stretchable (up to 551.7%) with a high tensile strength (4.14 MPa) and excellent healing efficiency of similar to 90% at mild temperature without using any external reagents. Furthermore, a novel method for fabricating flexible sensor is also proposed to endow the resulted sensor with large-area (20 cm × 12 cm), low cost, and outstanding sensitivity to strain, which makes it very suitable for human motion monitoring applications. This work will provide afflatus on future design, fabrication, and application of self-healing flexible strain sensor devices.     

Highly Stretchable,Tough, and Conductive Ag@Cu Nanocomposite Hydrogels for Flexible Wearable Sensors and Bionic Electronic Skins

Flexible conductive materials and flexible electronic devices are driving the development of the next generation of cutting-edge wearable electronics. However, the existing hydrogel-based flexible conductive materials have limited tensile capacity, low toughness, and poor anti-fatigue performance, resulting in narrow sensing area and insufficient durability. In this paper, a conductive nanocomposite hydrogel with high ductility, toughness, and fatigue resistance is prepared by combining silver coated copper (Ag@Cu) nanoparticles with gelatin followed by one-step immersion in sodium sulfate (Na₂SO₄) solution. The salting-out of gelatin in Na₂SO₄ solution greatly improve the mechanical properties of this gelatin-based hydrogel. The uniform distribution of Ag@Cu nanoparticles inside the whole hydrogel endow the composite hydrogel with excellent electrical conductivity (1.35 S m⁻¹). In addition, it displayed high and stable tensile strain sensitivity over a wide strain range (gauge factor = 2.08). Therefore, the Ag@Cu-Gel hydrogel is sensitive and stable enough to be successfully utilized as flexible wearable sensor for detecting human motion signals in real time, such as bending of human joints, swallowing, and throat vocalization. Furthermore, this hydrogel is also suitable for application as electronic skin for bionic robots. The above results demonstrate the promising application of Ag@Cu-Gel hydrogel for wearable electronics.     

Durable and highly sensitive flexible sensors for wearable electronic devices with PDMS-MXene/TPU composite films

MXenes, as two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides, have very excellent electrical properties and surface activity and are increasingly used in supercapacitors, batteries, electromagnetic interference shielding, and composite materials. Still, the poor stability of MXene when exposed to aqueous oxygen and the poor ability to interact with the polymer matrix have become important factors limiting its’ practical applications. To enhance stability, highly conductive and stretchable Ti₃C₂MXene/TPU sensing elements were prepared by a simple spraying process using thermoplastic polyurethane (TPU) as a substrate, and the sensing elements were encapsulated by polydimethylsiloxane (PDMS) to obtain MXene-TPU/PDMS constructed flexible strain sensors with excellent performance. This strain sensor features low detection limits (less than 0.005%, 0.5 μm), a wide sensing range (0–90%), a short response time (120.1 ms), and excellent durability (>3000 cycles). This strain sensor can be applied to a range of applications such as health detection, motion signals, detection of robot movements, and wearable electronic devices.     

A supersensitive wearable sensor constructed with PDMS porous foam and multi-integrated conductive pathways structure

In recent years, wearable multifunctional strain sensors have attracted attention for their promising applications in wearable electronics and portable devices. To achieve a high-performance wearable strain sensor with a wide sensing range and high gauge factor (GF), wisely choosing appropriate conductive materials and a rational structural design is essential. Herein, we develop a supersensitive sensor that contains one-dimensional conductive material CNT and two-dimensional material MXene built on a PDMS porous foam that is made based on a sugar template. The one-dimensional carbon nanotube (CNT) functionalizes as a conductive scale layer through solvent swelling and evaporation on the surface of the PDMS skeleton. The two-dimensional MXene is applied on top of the CNT layer to form final conductive pathways. The PDMS/CNT@MXene (PCM) sensor has a wide sensing range (150%), high sensitivity (GF = 26438), rapid response speed (response/recovery time of 60/71 ms), and exceptional durability (>1000 cycles) owing to its unique porous structure with scale layers and graded fracture of conductive pathways. Moreover, the PCM sensor is capable of monitoring subtle and significant human activities and is used for wireless sensing and medical diagnostics, even for solvent identification. The superior performance of the PCM sensor provides vast application potential in human movement, health monitoring, and warning devices.     

双重动态共价键交联纳米纤维素导电水凝胶及其柔性传感器

自愈合导电水凝胶因其良好的自愈合性能与导电性能,在柔性可穿戴设备中具有巨大的应用前景。以4-甲酰基苯硼酸(Bn)交联聚乙烯醇(PVA)和聚乙烯亚胺(PEI)构建基于硼酸酯键和亚胺键的双重动态交联水凝胶网络,引入聚吡咯修饰的纤维素纳米纤维(PPy@CNF)构建了具有良好自愈合和导电性的PBP-PPy@CNF纳米复合水凝胶。结果表明,当PPy@CNF的质量分数为0.8%时,水凝胶的力学性能最佳,其最大应力可达6.65kPa,断裂拉伸应变可达2080%,电导率为2174μS/m。基于该水凝胶的电阻式传感器具有良好的稳定性和重复性,在应变检测范围0～800%内,灵敏因子GF可分为三个线性响应区域,分别是0～200%(GF₁=2.82)、200%～600%(GF₂=7.15)和600%～800%(GF₃=12.85),该传感器能有效检测人体不同部位的运动,可应用于可穿戴传感设备。     

A biodegradable wearable flexible sensor based on natural wheat flour polymer for human motion and sweat monitoring

Fabrication of economical and eco-friendly flexible sensors is highly demanded in developing wearable biomedical devices. In this study, an eco-friendly strain and sweat sensor has been successfully assembled based on innovative composites consisting of carbon black (CB) and natural flour polymer, which are entirely green and biodegradable. A film-based sensing device has been developed to measure electrical resistance and strain simultaneously and continuously, owing to the highly sensitive resistance responsibility of the as-developed hybrid of CB and wheat flour (CB-dough). Furthermore, the influence of simulated sweat on the electric response of such sensor was also investigated. Repeatable relationships in resistance versus strain were revealed and obtained for varying samples with different content of simulated sweat, in which the amplitude of resistance was directly proportional to the concentration of NaCl. Accordingly, the as-developed CB-dough sensor was efficaciously used for durable surveillance of human activity and real-time feedback of sweat secretion during body motion. This work offers a simple and cost-effective way to manufacture flexible strain sensors, with a broad application prospect in wearable electronics, artificial intelligence systems, and other bio-sensors.     

不同模式多功能柔性传感器研究进展

柔性传感器能够实现压力、应变、温度、湿度及气体等与人体健康相关信号多功能识别及监测,在可穿戴人工智能设备的开发中展现出巨大的应用前景。本文综述了具有多种模式监测功能的柔性电化学式传感器领域最新研究成果,包括双模式传感器、三模式传感器和多模式传感器;重点介绍了传感器实现多功能监测的途径和传感机理。研究表明,多模式传感性的实现方法主要包括结构设计和多功能材料制备两种。而基于先进功能材料(包括纳米金属、纳米碳及导电聚合物)和柔性基体材料(如水凝胶、气凝胶及弹性聚合物)所制造的柔性多功能复合材料可有效降低多模式传感器的复杂性。最后,对比并指出了不同类型的功能材料在制造多功能柔性传感器中的特点与优势,为多功能柔性传感器的研究提供借鉴意义。     

Multifunctional,Wash Durable and Re-usable Conductive Textiles for Wearable Electro/Physiological Monitoring

Intelligent functionalities coupled textiles have received massive attention in the wearable health monitoring devices owing to their lightweight, ease of wearability, intrinsic flexibility, and comfortable texture. This article describes multifunctional, highly conductive, wash durable, and re-usable reduced graphene oxide (RGO) coated polyester knitted textiles by a simple, cost effective, and scalable dip-dry-reduce approach and a novel approach to integrate as fabricated RGO coated polyester knitted textile (RGT) with the wearable elastic band for electro/physiological signal monitoring. The RGT exhibits high conductivity of 0.23 KΩ sq⁻¹ and excellent wash durability. The RGT textile as a strain sensor shows a high sensitivity of 4.1 and 8 at a strain of 5% along the course and wale direction, respectively, stability and durability for more than 1500 cycles and also shows its excellent performance in human motion detection. Obtained electrophysiological signals from the RGT electrodes integrated wearable elastic band are comparable to those of the conventional Ag/AgCl electrodes. Owing to its comfortable texture, ease of wearability, stability, and wash durability, the RGT has potential applications in the future wearable electronic devices.     

Microstructure Engineering of Stretchable Resistive Strain Sensors with Discrimination Capabilities in Transverse and Longitudinal Directions

Featuring simple device structure, high sensitivity, and excellent reliability, stretchable resistive sensors have developed rapidly due to the high demand for flexible and wearable electronics. Nevertheless, it remains critically challenging to evaluate external stimuli using one simple device for diverse application scenarios. Here, a microstructure is engineered for a stretchable sensor by a facile replication/transferring and a prestretching/releasing process, enabling the device to have discrimination capabilities in the transverse direction (X-axis) and longitudinal direction (Y-axis). Consisting of silver nanowires (Ag NWs)/transition metal carbides (MXene)/poly(3,4-ethylenedioxythiophene):poly (styrene-sulfonate) (PEDOT:PSS) conducting layer and polydimethylsiloxane (PDMS)/Ecoflex elastomer, the microstructured sensor has a broad strain range of 120% along the X-axis and a large gauge factor (GF) of 37.44 along the Y-axis, and shows good stability during 1000 stretching/releasing cycles along two directions, indicating the excellent interfacial connection between the sensing layer and elastomer. As a result, taking advantages of distinct performance along two directions, the proposed stretchable sensor is demonstrated to monitor a variety of human movements and physical stimuli as a wearable and flexible device, revealing its promising potential in diverse application scenarios.     

MXene Reinforced PAA/PEDOT:PSS/MXene Conductive Hydrogel for Highly Sensitive Strain Sensors

Conductive hydrogel has a vital application prospect in flexible electronic fields such as electronic skin and force sensors. Developing conductive hydrogel with significant toughness and high sensitivity is urgently needed for application research. In this work, a strong and sensitive strain sensor based on conductive hydrogel is demonstrated by introducing MXene (Ti3C2Tx) into the micelle crosslinked polyacrylic acid (PAA)/poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) hydrogel network. The functional polymer micelle crosslinkers can dissipate external stress by deformation, endowing the hydrogel with high strength. The combination of MXene both improves the polymer network structure and the conductive pathways, further enhancing the mechanical properties and sensing performance. Resultantly, the flexible strain sensor base on PAA/PEDOT:PSS/MXene conductive hydrogel exhibits excellent sensing performance with a high gauge factor of 20.86, a large strain detection range of 1000%, as well as good adhesion on different interfaces. Thus, it can be used to monitor various movements of the human body and identify all kinds of handwriting, showing great potential into wearable electronics.     

Flexible strain sensors with high sensitivity and large working range prepared from biaxially stretched carbon nanotubes/polyolefin elastomer nanocomposites

Flexible strain sensors are a new generation of flexible and stretchable electronic devices that attracted increasing attention due to their practical applications in many fields. However, maintaining a wide detectable strain range while improving the sensitivity of flexible strain sensors remains challenging. In this study, flexible strain sensors with a large working range based on biaxially stretched carbon nanotubes (CNTs)/polyolefin elastomer (POE) nanocomposites were fabricated. Biaxial stretching was demonstrated to enhance the uniform dispersion and orientation of CNTs, thereby improving the performance of sensors. The optimal stretching ratios (SRs) of nanocomposites were investigated and the data revealed an increment in the sensitivity of sensors with SRs, while the working range first increased after biaxial stretching and decreased at higher SRs. Compared to the 9 wt% CNT/POE-1.0 sensor with a gauge factor (GF) value of 2.37 and a detectable range of 0.5%–230%, the CNT/POE-2.0 sensor exhibited an enhanced sensitivity (GF = 3935.12) coupled with a wider detectable range (0.5%–710%) and better stability. Besides, CNT/POE-2.0 sensor also achieved the monitoring of head movements, mouth opening, facial expression, and physiological signals, showing a potential for use in wearable electronic products.     

Flexible Sandwich Structural Strain Sensor Based on Silver Nanowires Decorated with Self‐Healing Substrate

Flexible and stretchable conducting composites that can sense stress or strain are needed for several emerging fields including human motion detection and personalized health monitoring. Silver nanowires (AgNWs) have already been used as conductive networks. However, once a traditional polymer is broken, the conductive network is subsequently destroyed. Integrating high pressure sensitivity and repeatable self‐healing capability into flexible strain sensors represents new advances for high performance strain sensing. Herein, superflexible 3D architectures are fabricated by sandwiching a layer of AgNWs decorated self‐healing polymer between two layers of polydimethylsiloxane, which exhibit good stability, self‐healability, and stretchability. For better mechanical properties, the self‐healing polymer is reinforced with carbon fibers (CFs). The sensors based on self‐healing polymer and AgNWs conductive network show high conductivity and excellent ability to repair both mechanical and electrical damage. They can detect different human motions accurately such as bending and recovering of the forearm and shank, the changes of palm, fist, and fingers. The fracture tensile stress of the reinforced self‐healing polymer (9 wt% CFs) is increased to 10.3 MPa with the elongation at break of 8%. The stretch/release responses under static and dynamic loads of the sensor have a high sensitivity, large sensing range, excellent reliability, and remarkable stability.     

Highly sensitive flexible strain sensor based on carbon nanotube/styrene butadiene styrene@ thermoplastic polyurethane fiber with a double percolated structure

The combination of a high sensitivity and a wide strain detection range in conductive polymer composites-based flexible strain sensors is still challenging to achieve. Herein, a double-percolation structural fiber strain sensor based on carbon nanotubes (CNT)/styrene butadiene styrene (SBS)@thermoplastic polyurethane (TPU) composite was fabricated by a simple melt mixing and fused filament fabrication strategy, in which the CNT/SBS and TPU were the conductive and insulating phases, respectively. Compared with the sensor without the double percolated structure, the CNT/SBS@TPU sensor achieved a lower percolation threshold (from 2.0 to 0.5 wt%, a reduction of 75%), and better electrical and sensing performance. It is shown that the strain detection range of the CNT/SBS@TPU sensor increases with increasing CNT loading. An opposite trend was observed for the sensitivity. The 1%-CNT/SBS@TPU sensor exhibited a high conductivity (1.08 × 10⁻³ S/m), high sensitivity (gauge factor of 2.65 × 10⁶ at 92% strain), wide strain detection range (0.2%–92% strain), high degree of linearity (R² = 0.954 at 0–10% strain), broad monitoring frequencies (0.05–0.5 Hz), and excellent stability (2000 cycles). Moreover, the CNT/SBS@TPU sensor was shown to successfully monitor a range of human physiological activities and to be capable of tactile perception and weight distribution sensing.