Surface Modification of Super Arborized Silica for Flexible and Wearable Ultrafast-Response Strain Sensors with Low Hysteresis

Flexible Stretchable Strain Sensor Based on LIG/PDMS for Real-Time Health Monitoring of Test Pilots

In the rapidly advancing era of intelligent technology, flexible strain sensors are emerging as a key component in wearable electronics. Laser-induced graphene (LIG) stands out as a promising fabrication method due to its rapid processing, environmental sustainability, low cost, and superior physicochemical properties. However, the stretchability and conformability of LIG are often limited by the substrate material, hindering its application in scenarios requiring high deformation. To address this issue, we propose a high-performance flexible and stretchable strain sensor fabricated by generating graphene on a polyimide (PI) substrate using laser induction and subsequently transferred onto a polydimethylsiloxane (PDMS). The resultant sensor demonstrates an ultra-low detection limit (0.1%), a rapid response time (150 ms), a wide strain range (40%), and retains stable performance after 1000 stretching cycles. Notably, this sensor has been successfully applied to the real-time monitoring of civil aviation test pilots during flight for the first time, enabling the accurate detection of physiological signals such as pulse, hand movements, and blink frequency. This study introduces a unique and innovative solution for the real-time health monitoring of civil aviation test pilots, with significant implications for enhancing flight safety.

High-Conductivity, Self-Healing, and Adhesive Ionic Hydrogels for Health Monitoring and Human-Machine Interactions Under Extreme Cold Conditions

Ionic conductive hydrogels (ICHs) are emerging as key materials for advanced human-machine interactions and health monitoring systems due to their unique combination of flexibility, biocompatibility, and electrical conductivity. However, a major challenge remains in developing ICHs that simultaneously exhibit high ionic conductivity, self-healing, and strong adhesion, particularly under extreme low-temperature conditions. In this study, a novel ICH composed of sulfobetaine methacrylate, methacrylic acid, TEMPO-oxidized cellulose nanofibers, sodium alginate, and lithium chloride is presented. The hydrogel is designed with a hydrogen-bonded and chemically crosslinked network, achieving excellent conductivity (0.49 ± 0.05 S m-1), adhesion (36.73 ± 2.28 kPa), and self-healing capacity even at -80 °C. Furthermore, the ICHs maintain functionality for over 45 days, showcasing outstanding anti-freezing properties. This material demonstrates significant potential for non-invasive, continuous health monitoring, adhering conformally to the skin without signal crosstalk, and enabling real-time, high-fidelity signal transmission in human-machine interactions under cryogenic conditions. These ICHs offer transformative potential for the next generation of multimodal sensors, broadening application possibilities in harsh environments, including extreme weather and outer space.

Highly stretchable, conductive, and self-adhesive starch-based hydrogel for high-performance flexible electronic devices

To achieve the green and sustainable development of environment, biocompatible hydrogels with exceptional ionic conductivity and flexibility are highly desired for intelligent and wearable sensors. However, it remains a great challenge to obtain biopolymer hydrogel-based sensors with high transparency, excellent mechanical properties, and good adhesion ability simultaneously. Herein, starch/polyacrylamide double-network hydrogel is achieved to endow the multifunctionality of traditional hydrogel sensor. Specifically, the resultant hydrogel sensor exhibits wide strain detection range of 2580 %, fast response time of 120 ms, high conductivity of 31.9 mS·m-1, superior sensitivity, remarkable fatigue resistance of 1350 cycles. In addition, multiple hydrogen bonding endows starch/polyacrylamide hydrogel with high mechanical properties and high transparency. Owing to these merits, the hydrogel sensor is capable of discriminating different human motions. Notably, the ionic conducting hydrogels could be employed as single-electrode TENGs for energy harvesting. The multifunctionality and biocompatibility of starch-based hydrogel sensor may offer an inspiration for the future development of next-generation sustainable and wearable electronics.

Advances in conducting nanocomposite hydrogels for wearable biomonitoring

Recent advancements in wearable biosensors and bioelectronics have led to innovative designs for personalized health management devices, with biocompatible conducting nanocomposite hydrogels emerging as a promising building block for soft electronics engineering. In this review, we provide a comprehensive framework for advancing biosensors using these engineered nanocomposite hydrogels, highlighting their unique properties such as high electrical conductivity, flexibility, self-healing, biocompatibility, biodegradability, and tunable architecture, broadening their biomedical applications. We summarize key properties of nanocomposite hydrogels for thermal, biomechanical, electrophysiological, and biochemical sensing applications on the human body, recent progress in nanocomposite hydrogel design and synthesis, and the latest technologies in developing flexible and wearable devices. This review covers various sensor types, including strain, physiological, and electrochemical sensors, and explores their potential applications in personalized healthcare, from daily activity monitoring to versatile electronic skin applications. Furthermore, we highlight the blueprints of design, working procedures, performance, detection limits, and sensitivity of these soft devices. Finally, we address challenges, prospects, and future outlook for advanced nanocomposite hydrogels in wearable sensors, aiming to provide a comprehensive overview of their current state and future potential in healthcare applications.

Ultraflexible Sensor Development via 4D Printing: Enhanced Sensitivity to Strain, Temperature, and Magnetic Fields

This paper addresses the trade-off between sensitivity and sensing range in strain sensors, while introducing additional functionalities through an innovative 4D printing approach. The resulting ultraflexible sensor integrates carbon nanotubes/liquid metal hybrids and iron powders within an Ecoflex matrix. The optimization of this composition enables the creation of an uncured resin ideal for Direct Ink Writing (DIW) and a cured sensor with exceptional electromechanical, thermal, and magnetic performance. Notably, the sensor achieves a wide linear strain range of 350% and maintains a stable Gauge Factor of 19.8, offering an ultralow detection limit of 0.1% strain and a rapid 83-ms response time. Beyond superior strain sensing capabilities, the sensor exhibits outstanding thermal endurance for temperatures exceeding 300 °C, enhanced thermal conductivity, and a consistent resistance-temperature relationship, making it well-suited for high-temperature applications. Moreover, the inclusion of iron particles provides magnetic responsiveness, enabling synergistic applications in location and speed detection, particularly in home care. Leveraging DIW facilitates the creation of complex-shaped sensors with multiple functional materials, significantly broadening the sensor's capabilities. This convergence of additive manufacturing and multifunctional materials marks a transformative step in advancing the performance of next-generation sensors across diverse domains.

Hydrogel Strain Sensors for Integrating Into Dynamic Organ-on-a-Chip

Current hydrogel strain sensors have never been integrated into dynamic organ-on-a-chip (OOC) due to the lack of sensitivity in aqueous cell culture systems. To enhance sensing performance, a novel strain sensor is presented in which the MXene layer is coated on the bottom surface of a pre-stretched anti-swelling hydrogel substrate of di-acrylated Pluronic F127 (F127-DA) and chitosan (CS) for isolation from the cell culture on the top surface. The fabricated strain sensors display high sensitivity (gauge factor of 290.96), a wide sensing range (0-100%), and high repeatability. To demonstrate its application, alveolar epithelial cells are cultivated on the top surface of the hydrogel strain sensor forming alveolar barriers, and then integrated into dynamic lung-on-a-chip (LOC) systems. This system can sensitively monitor normal physiological breathing, pathological inflammation stimulated by lipopolysaccharide (LPS), and alleviated inflammation through drug intervention.

Enhancement and mechanism of mechanical properties and functionalities of polyacrylamide/polyacrylic acid hydrogels by 1D and 2D nanocarbon

Highly flexible hydrogels are widely used in fields such as agriculture, drug delivery, and tissue engineering. However, the simultaneous integration of excellent mechanical properties, swelling properties, and high electrical conductivity into a hydrogel is still a great challenge. This work introduces 1D tubular multi-walled carbon nanotubes (MWCNTs) and 2D layered graphene oxide (GO) into polyacrylamide/poly-acrylic acid (PAM/PAA) hydrogels. The high specific surface area and oxygen-containing groups of GO contribute to excellent mechanical properties and water absorption of the PAM/PAA hydrogels, but the conductivity is poorly affected due to the presence of defects on GO surface. However, MWCNTs with large aspect ratios benefit to form continuous conductive paths in PAM/PAA hydrogels which further improves conductivity of the hydrogels. MWCNTs are entangled with PAM/PAA molecular chains to form a dense three-dimensional (3D) network structure, and this special structure improves the water absorption of PAM/PAA hydrogels by 3.7 g g-1. What's more, the MWCNTs/PAM/PAA hydrogel not only provides excellent mechanical properties (compressive strength up to 2.7 MPa), but also has high conductivity (2.3 S m-1). In particular, a strain sensor based on MWCNTs/PAM/PAA hydrogel exhibits exceptional sensitivity (gauge factor = 3.9 at 230-300 % strain) with a rapid response (200 ms) over a wide strain range (50 ∼ 200 %) which enables the ability to precisely and reliably monitor human motion. Therefore, the work provides a new insight into the design of multifunctional hydrogels with application on anatomical water plugging, electronic skin, and biosensors.

Piezoresistivity and strain-sensing behaviour of poly(butylene adipate-co-terephthalate)/multiwalled carbon nanotube nanocomposites

Multiwalled carbon nanotubes (MWCNTs) at different concentrations, ranging from 0.5 to 10 wt%, as a conductive filler, were incorporated into poly(butylene adipate-co-terephthalate) (PBAT), a flexible biodegradable copolyester, via melt-mixing, followed by compression moulding. The electrical conductivity of the prepared nanocomposites was evaluated by considering their volume resistivity value. The volume resistivity values of the nanocomposites suggested a quite low percolation threshold between the MWCNT concentration of 0.5 and 1 wt%. The corresponding volume resistivity values of the nanocomposites in the range of (6.90 ± 3.16) × 105 Ω cm to (1.24 ± 0.41) × 101 Ω cm implied their suitability for strain-sensing applications. The change in the electrical resistance of the nanocomposites was measured simultaneously with tensile testing to evaluate their piezoresistivity. The deformation behaviour of the nanocomposites was correlated with relative resistance change (ΔR/R 0) via a cyclic-strain test to investigate the stability of their strain-sensing behaviour. A non-linear and exponential-like increment in the ΔR/R 0 values of the nanocomposites as a function of mechanical strain during tensile stretching confirmed their piezoresistivity. ΔR/R 0 values fitted well with an increase in applied mechanical strain from 2% to 8% during the cyclic-strain test, which revealed the low-strain sensing potential of the nanocomposites, provided by their stable and intact microstructure formed via continuous stretching and releasing during the test; this was further supported by the reproducibility of the ΔR/R 0 values in the cyclic-strain test with 7% applied strain in 15 cycles. Additionally, the uniformly extended net-like morphology of the nanocomposites with an entangled network of MWCNTs throughout the polymer matrix was revealed by their electron micrographs.

Unique framework effect induced by uniform silk fibroin dynamic nanospheres enables multiscale hydrogel with outstanding elastic resilience and strain sensing performance

It is a significant challenge to obtain hydrogels simultaneously with low tensile energy dissipation, high compressive resilience and long durability. Herein, the uniform dynamic nanospheres (Sil-4H0.75) derived from 4-Hydroxybutyl acrylate glycidyl ether grafted silk fibroin is designed to overcome this issue. Due to its uniform and dynamic characteristic, Sil-4H0.75 could endow hydrogel with homogeneous multiscale structure and produce unique framework effect. Thus, transparent Sil-4H0.75 crosslinked acrylamide hydrogel doped with Ag nanowires APS3.75%/AgNW0.1 exhibits a high stretchability (1260 %) and outstanding elastic resilience. The tensile energy dissipation ratio maintains a low value of 9 % across a wide 800 % strain range. A high compression resilience ratio of 92.2 % is kept after ten compression cycles under 90 % compressive strain. The orderly AgNWs motion guided by framework effect also make it be used as both tensile and compressive sensors and exhibits high gauge factor of 7.35, outstanding compression sensitivity of 30.379 kPa-1 and excellent durability (up to 2000 cycles). The detection or other applications based on both two sensing modes are also demonstrated. In a word, this work affords a general strategy to achieve high-performance hydrogel based on uniform dynamic nanospheres which exhibits great potential in the applications of flexible wearable strain sensors.

Robust Treble-Weaving Wearable Textiles for Pressure and Temperature Monitoring in Harsh Environments

Wearable sensing textiles with continuous temperature monitoring, tactile feedback, and motion perception are highly desirable for personal safeguarding in extreme environments, such as fire scenes and extreme sports. However, it remains challenging for current wearable sensors to maintain reliable performance and provide point-of-care monitoring in harsh environments, such as high- and low-temperature or high-humidity conditions. Herein, a robust temperature and pressure sensing textile (TPST) with a hierarchical triple-weaving structure is developed using industrial weaving technology. The well-engineered interlacing configuration of the polyimide binding yarns in the triple-weaving structure tightly solidifies the carbon-based sensing yarns between two weaving layers, forming an integrated textile sensing array. The TPST not only exhibits excellent sensing sensitivity, reliability, and rapid response to pressure and temperature stimuli but also shows robust mechanical properties, flame resistance, and wearing comfort. Moreover, we demonstrate the application of the TPST for continuous temperature monitoring, human motion mapping, and vital sign monitoring. This technology offers significant potential for enhancing autonomous rescue operations and defense wearables.

Triple-Mechanism Enhanced Flexible SiO2 Nanofiber Composite Hydrogel with High Stiffness and Toughness for Cartilaginous Ligaments

Hydrogels are widely used in tissue engineering, soft robotics and wearable electronics. However, it is difficult to achieve both the required toughness and stiffness, which severely hampers their application as load-bearing materials. This study presents a strategy to develop a hard and tough composite hydrogel. Herein, flexible SiO2 nanofibers (SNF) are dispersed homogeneously in a polyvinyl alcohol (PVA) matrix using the synergistic effect of freeze-drying and annealing through the phase separation, the modulation of macromolecular chain movement and the promotion of macromolecular crystallization. When the stress is applied, the strong molecular interaction between PVA and SNF effectively disperses the load damage to the substrate. Freeze-dried and annealed-flexible SiO2 nanofibers/polyvinyl alcohol (FDA-SNF/PVA) reaches a preferred balance between enhanced stiffness (13.71 ± 0.28 MPa) and toughness (9.9 ± 0.4 MJ m-3). Besides, FDA-SNF/PVA hydrogel has a high tensile strength of 7.84 ± 0.10 MPa, super elasticity (no plastic deformation under 100 cycles of stretching), fast deformation recovery ability and excellent mechanical properties that are superior to the other tough PVA hydrogels, providing an effective way to optimize the mechanical properties of hydrogels for potential applications in artificial tendons and ligaments.

Sandwich-Like Flexible Breathable Strain Sensor with Tunable Thermal Regulation Capability for Human Motion Monitoring

High-performance flexible strain sensors with synergistic and outstanding thermal regulation function are poised to make a significant impact on next-generation multifunctional sensors. However, it has long been intractable to optimize the sensing performance and high thermal conductivity simultaneously. Herein, a novel flexible sandwich-like strain sensor with advanced thermal regulation capability was prepared by assembling electrospun thermoplastic polyurethane (TPU) fibrous membrane, MXene layer, and TPU/boron nitride nanosheet (BNNS) composite films. The as-prepared sensor demonstrates a wide strain working range (∼100% strain), an ultrahigh gauge factor (2080.9), and a satisfactory reliability. Meanwhile, benefiting from the uniform dispersion and promising orientation of BNNSs in TPU composites, the sensor possesses a high thermal conductivity of 1.5 W·m-1·K-1, guaranteeing wearer comfort. Additionally, the unique structure endows the sensor with high stretchability, breathability, biocompatibility, and tunable electromagnetic interference shielding performances. Furthermore, an integrated wireless motion monitoring device based on this sensor is rationally designed. It exhibits a fast response time, a wide recognition range, and the ability to maintain skin temperature during prolonged physical activity. These encouraging findings provide a new and feasible approach to designing high-performance and versatile flexible strain sensors with broad applications in advanced wearable technology.

Flexible Sensors with a Multilayer Interlaced Tunnel Architecture for Distinguishing Different Strains

The diversity of body joints and the complexity of joint motions cause flexible strain sensors to undergo complex strains such as stretching, compression, bending, and extrusion, which results in sensors that do not recognize different strains, facing great challenges in detecting the true motion characteristics of joints. Here, the monitoring of body joints' real motion characteristics has been realized by the sensor that can output response signals with different resistance trends for different strains. The sensor prepared by the sacrificial template method is characterized by a multilayered interlaced tunnel architecture and carbon black embedded in the inner wall of the tunnel. Stretching, compressive, and bending strains result in increasing, decreasing, and increasing resistance, followed by a decrease in resistance of the sensor, respectively. The sensor can still output distinguishable response signals, even in the presence of complex strains induced by squeezing. Low strain detection limits (0.03%) and wide detection ranges (>600%) are achieved due to the localized strain enhancement caused by the unique structure. The sensor can detect the motion characteristics of different joints in flexion-extension, abduction-adduction, and internal-external rotation, which, in turn, can be used for real-time monitoring of complex joint motions involved in limb rehabilitation. In addition, the sensor recognizes the 26 letters of the alphabet represented by sign language gestures. The above studies demonstrate the potential application of our prepared sensors in flexible, wearable devices.