An adhesive, stretchable, and freeze-resistant conductive hydrogel strain sensor for handwriting recognition and depth motion monitoring.

Wearable electronics based on conductive hydrogels (CHs) offer remarkable flexibility, conductivity, and versatility. However, the flexibility, adhesiveness, and conductivity of traditional CHs deteriorate when they freeze, thereby limiting their utility in challenging environments. In this work, we introduce a PHEA-NaSS/G hydrogel that can be conveniently fabricated into a freeze-resistant conductive hydrogel by weakening the hydrogen bonds between water molecules. This is achieved through the synergistic interaction between the charged polar end group (-SO3-) and the glycerol-water binary solvent system. The conductive hydrogel is simultaneously endowed with tunable mechanical properties and conductive pathways by the modulation caused by varying material compositions. Due to the uniform interconnectivity of the network structure resulting from strong intermolecular interactions and the enhancement effect of charged polar end-groups, the resulting hydrogel exhibits 174 kPa tensile strength, 2105 % tensile strain, and excellent sensing ability (GF = 2.86, response time: 121 ms), and the sensor is well suited for repeatable and stable monitoring of human motion. Additionally, using the Full Convolutional Network (FCN) algorithm, the sensor can be used to recognize English letter handwriting with an accuracy of 96.4 %. This hydrogel strain sensor provides a simple method for creating multi-functional electronic devices, with significant potential in the fields of multifunctional electronics such as soft robotics, health monitoring, and human-computer interaction.

Batch Fabrication of Flexible Strain Sensors with High Linearity and Low Hysteresis for Health Monitoring and Motion Detection.

In recent years, flexible strain sensors have gradually come into our lives due to their superiority in the field of biomonitoring. However, these sensors still suffer from poor durability, high hysteresis, and difficulty in calibration, resulting in great hindrance of practical application. Herein, starting with interfacial interaction regulation and structure-induced cracking, flexible strain sensors with high performance are successfully fabricated. In this strategy, dopamine treatment is used to enhance the bonding between flexible substrates and carbon nanotubes (CNT). The combination within the conductive networks is then controlled by substituting the CNT type. Braid-like fibers are employed to achieve controllable expansion of the conductive layer cracks. Finally, we obtain strain sensors that possess high linearity (R2 = 0.997) with low hysteresis (5%), high sensitivity (GF = 60) and wide sensing range (0-50%), short response time (62 ms), outstanding stability, and repeatability (>10,000 cycles). Flexible strain sensors with all performances good are rarely reported. Static and dynamic respiration and pulse signal monitoring by the fiber sensor are demonstrated. Moreover, a knee joint monitoring system is constructed for the monitoring of various walking stances, which is of great value to the diagnosis and rehabilitation of many diseases.

Dually-crosslinked ionic conductive hydrogels reinforced through biopolymer gellan gum for flexible sensors to monitor human activities.

Human-machine interactions, monitoring of health equipment, and gentle robots all depend considerably on flexible strain sensors. However, making strain sensors have better mechanical behavior and an extensive sensing range remains an urgent difficulty. In this study, poly acrylamide-co-butyl acrylate with gellan gum (poly(AAm-co-BA)@GG) hydrophobic association networks and intermolecular hydrogen bonding interactions are used to fabricate dual cross-linked hydrogels for wearable resistive-type strain sensors. This could be an acceptable way to minimize the limitations in hydrogels previously identified. The robust fracture strength (870 kPa) and exceptional stretchability (1297 %) of the hydrogel arise from the collaborative action of intermolecular hydrogen bonding and hydrophobic associations. It also demonstrates exceptional resilience to repeated cycles of uninterrupted stretching and relaxation, retaining its structural integrity. The response and restoration times are 110 and 120 ms respectively. Furthermore, a wide sensing range (0-900 %), notable sensitivity across various strain levels, and an impressive gauge factor (GF) of 31.51 with high durability were observed by the dual cross-linked (DC) hydrogel-based strain sensors. The measured conductivity of the hydrogel was 0.32 S/m which is due to the incorporation of NaCl. Therefore, the hydrogels can be tailored to function as wearable strain sensors that can detect subtle human gestures like speech patterns, distinguish between distinct words, and recognize vibrations of the larynx during drinking, as well as large joint motions like wrist, finger, and elbow. Furthermore, these hydrogels are capable of reliably distinguishing and reproducing various printed text. These findings imply that any electronic device that demands strain-sensing functionality might make use of these developed materials.

Real-Time Plasmonic Strain Sensors Based on Surface Relief Diffraction Gratings.

Large-scale diffraction gratings were fabricated in surface relief on azobenzene thin films and transferred to flexible PDMS substrates using soft lift-off lithography. The PDMS gratings were strained along the grating vector axis and the resulting surface topography was analyzed using diffraction angle measurements, AFM imagery and surface plasmon resonance (SPR) spectra. All measurement methods exhibited a linear response in strain indicating the useability of these sensors in real-world applications. For SPR-based strain sensing, an increasing pitch and a decreasing modulation depth were observed with increasing strain. The SPR peak shifted by ~1.0 nm wavelength and the SPR intensity decreased by ~0.3 a.u. per percentage of applied strain. The tested PDMS samples retained their integrity even after multiple cycles of stretching and relaxation, making them a suitable strain sensor.

Fiber Laser Writing of Highly Sensitive Nickel Nanoparticle-Incorporated Graphene Strain Sensors.

Unlocking new dimensions in wearable sensor technology, this research highlights ultrasensitive stretchable strain sensors fabricated with the customized laser-induced graphene (LIG) decorated with uniformly distributed nickel nanoparticles with a fiber laser writing process. The nickel nanoparticle-incorporated LIG (Ni-NPs@LIG) strain sensors fabricated by a simple all-laser-based method utilize a commercial fiber laser. The Ni-NPs@LIG sensors showcase an impressive gauge factor, reaching up to 248 for strain values below 5%, demonstrating a sensitivity increase of up to 430% compared to the pure LIG sensors. Moreover, these sensors offer adjustable strain sensitivity based on laser fluence. The key advancement of this study lies in the direct laser writing of highly porous nickel-graphene nanostructures with adjustable properties, making them applicable across a broad range of applications. As an application demonstration, the strain sensors were employed to assess the small deformation of a pouch battery or track the large deformation of a balloon surface.

Tough and Elastic Cellulose Composite Hydrogels/Films for Flexible Wearable Sensors.

Cellulose and its composites, despite being abundant and sustainable, are typically brittle with very low flexibility/stretchability. This study reports a solution processing method to prepare porous, amorphous, and elastic cellulose hydrogels and films. Native cellulose dissolved in a water-ZnCl2 mixture can form ionic gels through in situ polymerization of acrylic acid (AA) to poly(acrylic acid) (PAA). The addition of up to 30 vol % AA does not change the solubility of cellulose in the water-ZnCl2 mixture. After polymerization, the formation of interpenetrated networks, resulting from the chemical cross-linking of PAA and the ionic/coordination binding among cellulose/PAA and ZnCl2, gives rise to strong, transparent, and ionically conductive hydrogels. These hydrogels can be used for wearable sensors to detect mechanical deformation under stretching, compression, and bending. Upon removal of ZnCl2 and drying the gels, semitransparent amorphous cellulose composite films can be obtained with a Young's modulus of up to 4 GPa. The rehydration of these films leads to the formation of tough, highly elastic composites. With a water content of 3-10.5%, cellulose-containing films as strong as paper also show typical characteristics of elastomers with an elongation of up to 1300%. Such composite films provide an alternative solution to resolving the material sustainability of natural polymers without compromising their mechanical properties.

One-Step Synthesis of Graphene-Covered Silver Nanowires with Enhanced Stability for Heating and Strain Sensing.

Solution-processed silver nanowire (AgNW) networks have been considered as promising electrode candidates for next-generation electronic devices. However, they suffer from poor thermal and electrical stability and low mechanical properties, hindering their practical applications. In this work, graphene nanosheets are successfully introduced into AgNW via a facile one-step solvothermal process. Benefiting from increased conductive paths, the resultant AgNW/graphene films exhibit high electrical conductivity. More importantly, the interlocking NW morphology can be maintained under high temperature and applied voltage due to suppressed Ag migration, which is enabled by the introduction of graphene. This feature leads to enhanced thermal and electrical stability, making them suitable for use as transparent heaters. Furthermore, the composite films present excellent mechanical performance, and negligible resistance change is observed after 10 000 repeated bending cycles. To demonstrate their feasibility toward sensor applications, sandwiched strain sensors are designed, which can endure larger tensile strains and show higher sensitivity and repeatability compared with pure AgNW-based device. Furthermore, various hand gestures can be easily recognized by the resultant sensors based on unique combinations of sensing response. This work not only provides a low-cost method to realize large-scale synthesis of AgNW/graphene composites but also offers guidance to prepare high-performance electrodes for advanced electronics.

Wide-humidity, anti-freezing and stretchable multifunctional conductive carboxymethyl cellulose-based hydrogels for flexible wearable strain sensors and arrays.

Hydrogels play an important role in the design and fabrication of wearable sensors with outstanding flexibility, high sensitivity and versatility. Since hydrogels lose and absorb water during changes in humidity and temperature, it is critical and challenging to obtain hydrogels that function properly under different environmental conditions. Herein, a dual network hydrogel based on tannic acid (TA) reinforced polyacrylamide (PAM) and sodium carboxymethylcellulose (CMC) was constructed, while the introduction of the green solvents Solketal and LiCl endowed the hydrogel with greater possibilities for further modification to improve the water content and consistency of the mechanical properties over 30-90 % RH. This composite hydrogel (PTSL) has long-term stability, excellent mechanical strength, and freezing resistance. As strain sensors, they are linear over the entire strain range (R2 = 0.994) and have a high sensitivity (GF = 2.52 over 0-680 % strain range). Furthermore, the hydrogel's exceptional electrical conductivity and freezing resistance are a result of the synergistic effect of Solketal and LiCl, which intensifies the contact between the water molecules and the colloidal phase. This research could address the suitability of hydrogels over a wide range of humidity and temperature, suggesting great applications for smart flexible wearable electronics in harsh environmental conditions.

High-Performance Multidirectional Flexible Strain Sensor for Human Motion and Health Monitoring.

Multidirectional strain sensors are pivotal for wearable electronic devices and human-computer interaction. In this investigation, we translocate carbon/graphene (CB/Gr) conductive nanocomposites onto an Ecoflex flexible substrate via a facile technique encompassing reverse molding and spraying, culminating in the fabrication of a 45° strain rosette-shaped multidirectional flexible strain sensor. The sensor distinguishes itself with extraordinary performance characteristics, including high sensitivity (boasting a gauge factor of 35), an extensive strain range from 0 to 100%, exceptional linearity, a rapid response time of merely 200 ms, remarkable stability, and outstanding durability, effortlessly withstanding over 5000 stretch-release cycles. The sensor exhibits its exceptional capability to discern intricate movements, particularly in detecting human hand and neck motions. The sensor's remarkable comprehensive performance and strain direction recognition ability underscore its significant potential for diverse applications, notably in human-computer interaction, human motion monitoring, and health monitoring.

Bioinspired Low-Angle-Dependent Photonic Crystal Elastomer for Highly Sensitive Visual Strain Sensor.

Photonic crystals (PCs) possess unique photonic band gap properties that can be used in the field of sensors and smart displays if modulated on the micronano structure. Both nonclose-packed (NCP) structure and high refractive index (RI) contrast of PC play important roles in response sensitivity during stretching. Herein, we constructed an NCP-structured PC strain sensor with high RI by a novel coating-etching strategy. Stretch-induced changes in structural color correspond to the strength of the force, enabling the detection of the strength of the acting force by the naked eye. The flexible 3D cross-linked network constructed by poly(ethylene glycol) phenyl ether acrylate and pentaerythritol tetrakis(3-mercaptopropionate) endows the sensor with excellent elasticity and robustness. The designed PC strain sensor achieves high mechanochromic sensitivity (∼8.3 nm/%, 0.02 to 4.21 MPa) and a substantial reflection peak shift (Δλ = 249 nm). More importantly, the high RI contrast (Δn = 0.43) between CdS and polymers imparts isotropic optical properties, ensuring a broad viewing angle while avoiding misleading signals. The research provides a novel fabrication strategy to construct sensitive PC strain sensors, expanding the prospective applicability to human movement monitoring and secure message encryption.

Highly Stretchable, Transparent, Self-Healing Ion-Conducting Elastomers for Long-Term Reliable Human Motion Detection.

The flexible electronic sensor is a critical component of wearable devices, generally requiring high stretchability, excellent transmittance, conductivity, self-healing capability, and strong adhesion. However, designing ion-conducting elastomers meeting all these requirements simultaneously remains a challenge. In this study, a novel approach is presented to fabricate highly stretchable, transparent, and self-healing ion-conducting elastomers, which are synthesized via photo-polymerization of two polymerizable deep eutectic solvents (PDESs) monomers, i.e., methacrylic acid (MAA)/choline chloride (ChCl) and itaconic acid (IA)/ChCl. The as-prepared ion-conducting elastomers possess outstanding properties, including high transparency, conductivity, and the capability to adhere to various substrates. The elastomers also demonstrate ultra-stretchability (up to 3900%) owing to a combination of covalent cross-linking and noncovalent cross-linking. In addition, the elastomers can recover up to 3250% strain and over 94.5% of their original conductivity after self-healing at room temperature for 5 min, indicating remarkable mechanical and conductive self-healing abilities. When utilized as strain sensors to monitor real-time motion of human fingers, wrist, elbow, and knee joints, the elastomers exhibit stable and strong repetitive electrical signals, demonstrating excellent sensing performance for large-scale movements of the human body. It is anticipated that these ion-conducting elastomers will find promising applications in flexible and wearable electronics.

In-situ forming ultra-mechanically sensitive materials for high-sensitivity stretchable fiber strain sensors.

Fiber electronics with flexible and weavable features can be easily integrated into textiles for wearable applications. However, due to small sizes and curved surfaces of fiber materials, it remains challenging to load robust active layers, thus hindering production of high-sensitivity fiber strain sensors. Herein, functional sensing materials are firmly anchored on the fiber surface in-situ through a hydrolytic condensation process. The anchoring sensing layer with robust interfacial adhesion is ultra-mechanically sensitive, which significantly improves the sensitivity of strain sensors due to the easy generation of microcracks during stretching. The resulting stretchable fiber sensors simultaneously possess an ultra-low strain detection limit of 0.05%, a high stretchability of 100%, and a high gauge factor of 433.6, giving 254-folds enhancement in sensitivity. Additionally, these fiber sensors are soft and lightweight, enabling them to be attached onto skin or woven into clothes for recording physiological signals, e.g. pulse wave velocity has been effectively obtained by them. As a demonstration, a fiber sensor-based wearable smart healthcare system is designed to monitor and transmit health status for timely intervention. This work presents an effective strategy for developing high-performance fiber strain sensors as well as other stretchable electronic devices.

Silver Nanowire-Based Flexible Strain Sensor for Human Motion Detection.

Accurately capturing human movements is a crucial element of health status monitoring and a necessary precondition for realizing future virtual reality/augmented reality applications. Flexible motion sensors with exceptional sensitivity are capable of detecting physical activities by converting them into resistance fluctuations. Silver nanowires (AgNWs) have become a preferred choice for the development of various types of sensors due to their outstanding electrical conductivity, transparency, and flexibility within polymer composites. Herein, we present the design and fabrication of a flexible strain sensor based on silver nanowires. Suitable substrate materials were selected, and the sensor's sensitivity and fatigue properties were characterized and tested, with the sensor maintaining reliability after 5000 deformation cycles. Different sensors were prepared by controlling the concentration of silver nanowires to achieve the collection of motion signals from various parts of the human body. Additionally, we explored potential applications of these sensors in fields such as health monitoring and virtual reality. In summary, this work integrated the acquisition of different human motion signals, demonstrating great potential for future multifunctional wearable electronic devices.

Strain Virtual Sensing Applied to Industrial Presses for Fatigue Monitoring.

The techniques that allow one to estimate measurements at the unsensed points of a system are known as virtual sensing. These techniques are useful for the implementation of condition monitoring systems in industrial equipment subjected to high cyclic loads that can cause fatigue damage, such as industrial presses. In this article, three different virtual sensing algorithms for strain estimation are tested using real measurement data obtained from a scaled bed press prototype: two deterministic algorithms (Direct Strain Observer and Least-Squares Strain Estimation) and one stochastic algorithm (Static Strain Kalman Filter). The prototype is subjected to cyclic loads using a hydraulic fatigue testing machine and is sensorized with strain gauges. Results show that sufficiently accurate strain estimations can be obtained using virtual sensing algorithms and a reduced number of strain gauges as input sensors when the monitored structure is subjected to static and quasi-static loads. Results also show that is possible to estimate the initiation of fatigue cracks at critical points of a structural component using virtual strain sensors.

Visual Strain Sensors Based on Fabry-Perot Structures for Structural Integrity Monitoring.

Strain sensors that can rapidly and efficiently detect strain distribution and magnitude are crucial for structural health monitoring and human-computer interactions. However, traditional electrical and optical strain sensors make access to structural health information challenging because data conversion is required, and they have intricate, delicate designs. Drawing inspiration from the moisture-responsive coloration of beetle wing sheaths, we propose using Ecoflex as a flexible substrate. This substrate is coated with a Fabry-Perot (F-P) optical structure, comprising a "reflective layer/stretchable interference cavity/reflective layer", creating a dynamic color-changing visual strain sensor. Upon the application of external stress, the flexible interference chamber of the sensor stretches and contracts, prompting a blue-shift in the structural reflection curve and displaying varying colors that correlate with the applied strain. The innovative flexible sensor can be attached to complex-shaped components, enabling the visual detection of structural integrity. This biomimetic visual strain sensor holds significant promise for real-time structural health monitoring applications.

Highly Biocompatible Antibacterial Hydrogel for Wearable Sensing of Macro and Microscale Human Body Motions.

Flexible electronics, like electronic skin (e-skin), rely on stretchable conductive materials that integrate diverse components to enhance mechanical, electrical, and interfacial properties. However, poor biocompatibility, bacterial infections, and limited compatibility of functional additives within polymer matrices hinder healthcare sensors' performance. This study addresses these challenges by developing an antibacterial hydrogel using polyvinyl alcohol (PVA), konjac glucomannan (KGM), borax (B), and flower-shaped silver nanoparticles (F-AgNPs), referred as PKB/F-AgNPs hydrogel. The developed hydrogel forms a hierarchical network structure, with a tensile strength of 96 kPa, 83% self-healing efficiency within 60 minutes, and 128% cell viability in Cell Counting Kit-8 (CCK-8) assays, indicating excellent biocompatibility. It also shows strong antibacterial efficacy against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus). Blue light irradiation enhances its antibacterial activity by 1.3-fold for E. coli and 2.2-fold for S. aureus. The hydrogel's antibacterial effectiveness is assessed by monitoring changes in electrical conductivity, providing a cost-effective alternative to traditional microbial culture assays. The PKB/F-AgNPs hydrogel's flexibility and electrical conductivity enable it to function as strain sensors for detecting body movements and facial expressions. This antibacterial hydrogel underscores its potential for future human-machine interfaces and wearable electronics.

Stretchable MXene/Carbon Nanotube Bilayer Strain Sensors with Tunable Sensitivity and Working Ranges.

Stretchable strain sensors have gained increasing popularity as wearable devices to convert mechanical deformation of the human body into electrical signals. Two-dimensional transition metal carbides (Ti3C2Tx MXene) are promising candidates to achieve excellent sensitivity. However, MXene films have been limited in operating strain ranges due to rapid crack propagation during stretching. In this regard, this study reports MXene/carbon nanotube bilayer films with tunable sensitivity and working ranges. The device is fabricated using a scalable process involving spray deposition of well-dispersed nanomaterial inks. The bilayer sensor's high sensitivity is attributed to the cracks that form in the MXene film, while the compliant carbon nanotube layer extends the working range by maintaining conductive pathways. Moreover, the response of the sensor is easily controlled by tuning the MXene loading, achieving a gauge factor of 9039 within 15% strain at 1.92 mg/cm2 and a gauge factor of 1443 within 108% strain at 0.55 mg/cm2. These tailored properties can precisely match the operation requirements during the wearable application, providing accurate monitoring of various body movements and physiological activities. Additionally, a smart glove with multiple integrated strain sensors is demonstrated as a human-machine interface for the real-time recognition of hand gestures based on a machine-learning algorithm. The design strategy presented here provides a convenient avenue to modulate strain sensors for targeted applications.

Flexible Strain Sensor Based on Nickel Microparticles/Carbon Black Microspheres/Polydimethylsiloxane Conductive Composites for Human Motion Detection.

Herein, we report a dual-functional flexible sensor (DFFS) using a magnetic conductive polymer composed of nickel (Ni), carbon black (CB), and polydimethylsiloxane (PDMS). The material selection for the DFFS utilizes the excellent elasticity of the PDMS matrix and the synergistic interaction between Ni and CB. The DFFS has a wide strain range of 0-170%, a high sensitivity of 74.13 (140-170%), and a low detection limit of 0.3% strain. The DFFS based on superior performance can accurately detect microstrain/microvibration, oncoming/contacting objects, and bicycle riding speed. Additionally, the DFFS can be used for comprehensive monitoring of human movements. Therefore, the DFFS of this work shows significant value for implementation in intelligent wearable devices and noncontact intelligent control.

Strain-Modulated Flexible Bio-Organic/Graphene/PET Sensors Based on DNA-Curcumin Biopolymer.

In recent years, there has been growing interest in the development of metal-free, environmentally friendly, and cost-effective biopolymer-based piezoelectric strain sensors (bio-PSSs) for flexible applications. In this study, we have developed a bio-PSS based on pure deoxyribonucleic acid (DNA) and curcumin materials in a thin-film form and studied its strain-induced current-voltage characteristics based on piezoelectric phenomena. The bio-PSS exhibited flexibility under varying compressive and tensile loads. Notably, the sensor achieved a strain gauge factor of 407 at an applied compressive strain of -0.027%, which is 8.67 times greater than that of traditional metal strain gauges. Furthermore, the flexible bio-PSS demonstrated a rapid response under a compressive strain of -0.08%. Our findings suggest that the proposed flexible bio-PSS holds significant promise as a motion sensor, addressing the demand for environmentally safe, wearable, and flexible strain sensor applications.

Gallium-Carbon: A Universal Composite for Sustainable 3D Printing of Integrated Sensor-Heater-Battery Systems in Wearable and Recyclable Electronics.

This study presents a novel three-dimensional (3D) printable gallium-carbon black-styrene isoprene styrene block copolymer (Ga-CB-SIS), offering a versatile solution for the rapid fabrication of stretchable and integrated sensor-heater-battery systems in wearable and recyclable electronics. The composite exhibits sinter-free characteristics, allowing for printing on various substrates, including heat-sensitive materials. Unlike traditional conductive inks, the Ga-CB-SIS composite, composed of gallium, carbon black, and styrene isoprene block copolymers, combines electrical conductivity, stretchability, and digital printability. By introducing carbon black as a filler material, the composite achieves promising electromechanical behavior, making it suitable for low-resistance heaters, batteries, and electrical interconnects. The fabrication process involves a simultaneous mixing and ball-milling technique, resulting in a homogeneous composition with a CB/Ga ratio of 4.3%. The Ga-CB-SIS composite showcases remarkable adaptability for digital printing on various substrates. Its self-healing property and efficient recycling technique using a deep eutectic solvent contribute to an environmentally conscious approach to electronic waste, with a high gallium recovery efficiency of ∼98%. The study's innovation extends to applications, presenting a fully digitally printed stretchable Ga-CB-SIS battery integrated with strain sensors and heaters, representing a significant leap in LM-based composites. This multifunctional and sustainable Ga-CB-SIS composite emerges as a key player in the future of wearable electronics, offering integrated circuits with sensing, heating, and energy storage elements.