Search Results (116)

Recently, flexible electronics have attracted widespread attention in personalized health monitoring, soft robotics, and smart human-machine interactions due to intrinsic high stretchability. Among them, constructing buckled structures in flexible devices is one of the most effective strategies to achieve flexibility and stretchability. Flexible electronic devices based on buckled structure (FEDB) have gained significant research progress, owing to their outstanding advantages such as simple fabrication processes, excellent structural stability, and broad applicability. Furthermore, its application areas are expanding to emerging scenarios, including the human body, underwater environments, the oceans, and space. However, there are few systematic reviews concerning their progresses, although researchers show increasing interest in the emerging applications of FEDB. This review summarizes recent research progress in FEDB. First, this review explains the buckled instability mechanism, listing the common conductive and substrate materials. The polymeric substrates discussed mainly include PDMS, TPU, SBS, PC, and hydrogel, which provide the flexibility and deformability required for FEDB. In addition, this review summarizes several methods for constructing buckled structures, including prestretch-release, solvent swelling, thermal, mold, and 3D printing as well as techniques for controlling morphology. Second, this review summarizes the applications of FEDB, such as flexible electrodes, strain and pressure sensors, and energy devices. Particularly, it provides a detailed introduction to the expansion of emerging scenarios, involving underwater monitoring, in vitro and in vivo physiological signal detection, human-machine interactions, and portable capsule devices. Finally, this review points out the current challenges of FEDB, including long-term service stability, adaptability to extreme environments, conformal attachment to complex curved surfaces, and large-scale manufacturing. Full article

Wearable biosensors have swiftly progressed from stiff laboratory prototypes to flexible, skin-like systems capable of ongoing physiological monitoring. Yet, the active and mechanically intense nature of athletic performance reveals the limits of static device designs made solely through traditional 3D printing. This review offers a thorough analysis of the shift from custom 3D-printed platforms to adaptive 4D-printed wearable biosensors that include time-sensitive, stimuli-responsive materials. We carefully investigate how nanomaterial-engineered transducers, including carbon nanomaterials, MXenes, and metallic nanostructures, improve electrochemical sensitivity, signal stability, and mechanical durability in sweat-based and electrophysiological sensing. Additionally, we examine the integration of thermoresponsive polymers, moisture-activated hydrogels, shape-memory materials, and self-healing networks that support autonomous control of skin–sensor contact, microfluidic sweat management, and structural stability under high mechanical strain. Case studies focused on sports monitoring demonstrate how these innovations enable multimodal measurement of mechanical, chemical, and molecular biomarkers in real time. Lastly, we address manufacturing scalability, regulatory issues, and translational challenges necessary to move from proof-of-concept to clinical deployment. By combining nanomaterial-driven electrochemical precision with 4D-printed mechanical adaptability, this review maps a path toward adaptive, self-regulating wearable platforms that sustain analytical accuracy even under extreme physiological demands. Full article

Hydrogel-based sensors have emerged as transformative soft-sensing platforms, featuring tissue-matched compliance, high water content, stimuli responsiveness, and chemical tunability, properties which are unachievable with conventional rigid sensors. Despite substantial advances, the existing reviews focus on individual polymer categories, discrete transduction mechanisms, or targeted standalone applications, failing to establish an integrated pipeline from material design to final sensing performance. This review fills these crucial gaps by systematically correlating polymer chemistry, crosslinking tactics, and fabrication protocols with the selection of transduction mechanisms and resultant sensing performance across biomedical and environmental fields. We conduct a critical assessment of natural and synthetic polymers together with chemical, physical, and hybrid composite crosslinking methodologies. Multiple sensing modalities, including piezoresistive, capacitive, thermogalvanic, electrochemical, colorimetric, ratiometric fluorescence, and piezoionic sensing are elaborated alongside representative quantitative performance parameters. Emerging platforms, including self-powered thermogalvanic sensors, SERS-integrated biosensors, and MXene/MOF composites, are highlighted as underexplored frontiers. In addition, persistent bottlenecks including dehydration-derived signal drift, inferior long-term operational stability, unsatisfactory target selectivity, and obstacles toward large-scale manufacturability are rigorously analyzed. Ultimately, this review constructs a holistic unified framework bridging polymer molecular design, fabrication engineering, signal transduction, and practical end-use applications, laying a clear developmental roadmap for next-generation flexible and smart hydrogel-based sensing systems. Full article

Recently, conductive hydrogels have gained extensive applications in flexible wearable electronics and have garnered considerable attention. However, their inherent swelling behaviour and limited mechanical strength have hindered their further development. In this study, a polyacrylamide/sodium alginate (PAM/SA, PS)-based hydrogel with high mechanical strength and anti-swelling properties was prepared by combining mechanical stretching–drying pretreatment with a bimetallic ion (Li⁺/multivalent metal ion) post-soaking strategy. Among multivalent metal ions (Ca²⁺, Al³⁺, and Zr⁴⁺), the Al³⁺-crosslinked hydrogel (PS-Al³⁺) demonstrated outstanding overall performance. It exhibited excellent mechanical properties, with tensile strength, elongation at break, and impact strength reaching 9.71 MPa, 993.53%, and 75 MJ/m³, respectively. Its dense network structure also gave it excellent anti-swelling properties (swelling ratio of 14%). As a strain sensor, the PS-Al³⁺ hydrogel displayed good conductivity (1.33 S/m), high sensitivity (GF = 2.25), fast response (response time of 403 ms), and negligible hysteresis (recovery time of 407 ms). Benefiting from its exceptional resistance to expansion, the material’s sensor response signals in underwater environments are highly consistent with those in air. Furthermore, this sensor has been successfully applied to swimming motion monitoring and data transmission in underwater environments. This study proposes a novel, low-cost, and simple approach for developing flexible sensors suitable for underwater environments. Full article

This study focused on a three-dimensional cross-linked hydrophobic association (PS) hydrogel framework. Phytic acid (PA) was selected as both a dopant and an antifreeze agent, and it was combined with an ethylene glycol/water binary solvent to construct a dual antifreeze system. The resulting composite conductive hydrogel, E/PS/PA-PPy, exhibited synergistically enhanced electrical conductivity, mechanical strength, and antifreeze properties. At a PA concentration of 0.1 M, a structurally uniform and ordered three-dimensional network was formed. The PS/PA-PPy hydrogel exhibited an elongation at break of 2595.7% and a high conductivity of 1.8 S/m, while maintaining excellent flexibility and adhesion. Owing to the synergistic antifreeze effect, the freezing point of the E/PS/PA-PPy hydrogel was reduced to −42.3 °C, and after 35 days of room-temperature storage, the weight loss was less than 7%, indicating outstanding water retention. The assembled flexible strain sensor exhibited a sensitivity of 2.09, with response and recovery times both less than 0.25 s. Notably, it exhibited good cyclic stability and accurately monitored human movements. Furthermore, the sensing performance remained stable without significant attenuation even at −20 °C. The results demonstrate the broad application prospects of the hydrogel in flexible electronics such as wearable health monitoring systems and human–machine interfaces in extreme environments. Full article

Polyvinyl alcohol (PVA)-based hydrogels suffer from insufficient mechanical strength, while aramid nanofibers (ANF) have intrinsic insulation that limits their sensing applications, and the synergistic effect of composite fillers remains underexplored. This study aims to develop a multifunctional PVA/ANF/NaCl composite organohydrogel for high-performance flexible sensors. The gel was fabricated via freeze–thaw crosslinking, solvent exchange and NaCl impregnation, with systematic investigations of its microstructure, mechanical, electrical and multifunctional sensing properties, and a corresponding triboelectric nanogenerator (TENG) and self-powered handwriting recognition system were constructed. Results show that 2% ANF significantly enhances the gel’s mechanical performance, 0.5 M NaCl achieves optimal mechanical-electrical balance, the gel-based sensor exhibits excellent distance, pressure and strain sensing with high cyclic stability, the TENG delivers stable electrical output, and the recognition system achieves 95% accuracy on the test set. This work provides a new material and design strategy for advanced flexible electronic devices. Full article

Conductive hydrogels have emerged as promising candidates for flexible strain sensors owing to their high water content, low elastic modulus, and intrinsic ionic conductivity. However, conventional hydrogel networks often suffer from an inherent trade-off among conductivity, mechanical robustness, and long-term stability, which limits their practical deployment in wearable sensing scenarios. The introduction of metal–ligand coordination bonds into hydrogel networks offers a versatile strategy to address these challenges: dynamic coordination cross-links can dissipate energy under deformation and reform upon unloading, thereby enhancing toughness, enabling self-healing, and contributing to ionic transport. This review focuses on metal-ion-coordinated conductive hydrogels designed for strain-sensing applications. Representative coordination systems based on Fe³⁺, Ca²⁺, Zn²⁺, Al³⁺, Cu²⁺, Ti⁴⁺, and Zr⁴⁺ are surveyed, with emphasis on their characteristic polymer matrices, ligand chemistries, and network-construction strategies. Key sensing-relevant properties—including ionic conductivity, mechanical stretchability, self-healing capability, interfacial adhesion, freezing resistance, and resistance to dehydration—are discussed in relation to coordination network design. Typical application demonstrations in large-deformation motion monitoring and subtle physiological signal detection are reviewed. Unlike existing reviews that survey conductive hydrogels broadly by conductive mechanism or sensor type, this review takes metal-ion coordination as the central organizing principle and systematically traces its influence across the full design chain—from ion–ligand coordination chemistry through network architecture to macroscopic sensing output. By comparatively analyzing seven representative metal-ion systems within a unified framework, this work aims to clarify how the choice of metal ion governs the interplay among conductivity, mechanical robustness, self-healing, and strain sensitivity—a perspective that has not yet been systematically addressed in prior reviews. Finally, current challenges—including the conductivity–mechanics coupling bottleneck, insufficient long-term stability, biosafety concerns for skin-contact deployment, the lack of standardized evaluation protocols, and device-integration barriers—are identified, and future directions for this field are outlined. Full article

A self-powered graphene-enhanced hydrogel sensor (SGHS) with high contact-pressure sensitivity and mechanical robustness was developed for precise dynamic biomechanical and material contact sensing. The device generates transient electrical signals via contact electrification and electrostatic induction during contact–separation events, eliminating the need for any external power supply. The optimized SGHS achieves a maximum peak power density of 0.23 mW·m⁻², with contact-pressure sensitivities of 0.6 kPa⁻¹ and 0.26 kPa⁻¹ in the pressure ranges of 0.25–5 kPa and 5–25 kPa, respectively, which is competitive with or exceeds that of other externally powered and self-powered flexible dynamic stress sensors in the low-pressure range. Comprehensive analyses reveal that the pressure response originates from the enhanced piezodielectric effect in the graphene hydrogel layer under compression. The SGHS exhibits excellent mechanical durability, maintaining stable output after 10,000 loading–unloading cycles. Moreover, the pulse intensity, width, and waveform of its self-generated output provide distinctive features for identifying the type and surface characteristics of contacting objects. These results highlight SGHS as a promising candidate for next-generation intelligent, self-powered, and flexible dynamic sensing systems. Full article

Flexible conductive hydrogels hold great promise in wearable electronics and biomonitoring applications, yet their practical use is constrained by issues such as poor low-temperature tolerance, susceptibility to dehydration, and limited multifunctional sensing capabilities. This study successfully synthesized a dual-conductive lithium-ion and calcium-ion hydrogel based on acrylamide/gelatin via a simplified low-temperature one-pot polymerization method. At 60 °C, mixing acrylamide, gelatin, lithium chloride, and calcium chloride within 40 min constructed a network structure featuring covalent bonds, ionic bonds, and hydrogen bonds. The resulting material exhibited exceptional extensibility with a break elongation of 1408.5% and tensile strength of 134.2 kPa while maintaining strong adhesion to nine different substrates. It retained flexibility at −20 °C and demonstrated minimal mass loss (3% of initial value) after 10 days of aging. As a sensor, this hydrogel reliably responds to pressure, temperature, large-amplitude body movements, and subtle physiological signals like pulse and vocal cord vibrations. In animal simulation monitoring, its electrical resistance signals increased linearly with model body weight and remained stable between −20 °C and 20 °C. These results demonstrate the hydrogel’s broad application potential in wearable sensing, ecological monitoring, and smart agriculture. Full article

Deep eutectic solvent (DES), when used as the continuous phase of eutectogels, can significantly improve their electrical and mechanical properties due to its excellent conductivity, freeze resistance and chemical stability. The development of eutectogels effectively solves the key limitations of traditional hydrogels and organogels, such as low-temperature freezing, high-temperature volatilization, and organic solvent leakage. It also realizes the collaborative optimization of environmental friendliness and comprehensive performance, which makes it show broad application prospects in the field of flexible sensing. This review summarizes the design principles, material selection, sensing mechanisms, and flexible sensing applications of eutectogels. By examining the design of eutectogels, the selection of DES, and the synthesis of the gel network, it provides a theoretical basis for the development of eutectogel-based sensor devices. A detailed description of the sensing mechanism is provided to elucidate the signal generation and transition in eutectogels toward the purpose of the practical applications. Finally, the application prospects of eutectogels for high-performance sensors and detection devices are discussed. Additionally, we provide a theoretical support for their structural design, performance optimization, and practical application. Full article

Smart healthcare is rapidly emerging as a transformative paradigm, enabling simultaneous health monitoring, therapeutic intervention, and early prediction of disease onset. In this context, electrochemical monitoring systems have attracted growing interest due to their cost-effectiveness, ease of operation, miniaturization and compatibility with wearable platforms. Accordingly, conductive hydrogel-based electrochemical (bio)sensors have gained significant attention for health monitoring owing to their soft mechanical properties, high water content, excellent biocompatibility, and ability to form intimate, conformal interfaces with biological tissues. Their three-dimensional polymeric networks facilitate efficient ion transport and mechanical flexibility, making them particularly suitable for wearable and noninvasive sensing and monitoring applications. However, the intrinsically limited conductivity and catalytic activity of pristine hydrogels often constrain their electrochemical performance. To overcome these limitations, functional nanomaterials such as metal–organic frameworks (MOFs) and MXene (MX) nanosheets have been increasingly integrated into hydrogel matrices to enhance conductivity and electrochemical activity. This review provides a comprehensive and critical comparison of recent advances in MOF- and MX-integrated conductive hydrogels for electrochemical health monitoring. In addition to material design strategies and sensing performance, emerging trends in data-driven sensing aimed at improving signal interpretation and multi-analyte discrimination are systematically discussed. Key challenges related to long-term stability, biocompatibility, scalability, and intelligent system integration are critically assessed, and the future potential of these platforms within closed-loop architectures is highlighted, paving the way for next-generation conductive hydrogel-based electrochemical sensors in smart healthcare applications. Full article

With the rapid development of artificial intelligence and wearable electronics, there is an increasing demand for skin-like, flexible, and self-powered sensors capable of continuously perceiving mechanical stimuli and human motions. Triboelectric nanogenerator (TENG)-based sensors incorporating stretchable conductive gels represent a promising approach to meet these requirements by combining soft mechanical compliance with efficient electromechanical signal transduction. However, conventional metallic or composite electrodes often suffer from mechanical mismatch with soft skin-like systems, motivating the exploration of intrinsically soft and stretchable conductive gels. In this review, we present a comprehensive and structured overview with comparative perspectives of stretchable skin-like conductive gel-based triboelectric devices. First, different classes of conductive gels, including hydrogels, organogels, ionogels, and other emerging gel systems, are systematically summarized and compared in terms of their composition, crosslinking strategies, conductivity, and mechanical characteristics. Next, the pivotal role of conductive gels in bridging skin-like sensing functions and triboelectric applications is elucidated, highlighting how their intrinsic softness, stretchability, self-healing capability, and interfacial conformability enable intimate skin contact and reliable electromechanical coupling. The key performance attributes of gel-based skin-like triboelectric sensors, including stretchability, self-healing behavior, optical and thermal tolerance, electrical durability, and environmental stability, are critically discussed with representative examples and comparative analysis. Typical device configurations, such as thin-film, fiber-shaped, and textile-based architectures, are further reviewed to illustrate structure–function relationships and application-oriented design strategies. Finally, current challenges, limitations, and future research directions for stretchable conductive gel-based triboelectric systems are outlined, aiming to provide practical guidelines and insights for the rational design of high-performance skin-like triboelectric sensors based on conductive gels. Full article

Conductive hydrogels that simultaneously exhibit high mechanical robustness, reliable electrical conductivity, and interfacial adhesion are highly desirable for flexible sensing applications; however, achieving these properties in a single system remains challenging due to intrinsic structure–property trade-offs. Herein, a multifunctional conductive hydrogel (ASP hydrogel) is developed based on a polyacrylamide (PAM)/sodium alginate (SA) double-network architecture using a gallic acid (GA)–Fe³⁺–pyrrole (Py) coupling strategy. In this design, GA provides metal-coordination sites for Fe³⁺, while Fe³⁺ simultaneously serves as an oxidant to trigger the in situ polymerization of pyrrole, enabling the homogeneous integration of polypyrrole (PPy) conductive networks within the hydrogel matrix. The resulting ASP hydrogel exhibits a markedly enhanced fracture strength of 2.95 MPa compared with PAM (0.26 MPa) and PAM–SA (0.22 MPa) hydrogels, together with stable electrical conductivity and reproducible strain-dependent electrical responses. Moreover, the introduction of dynamic metal–phenolic coordination and hydrogen-bonding interactions endows the hydrogel with intrinsic self-healing capability and strong adhesion to diverse substrates. Rather than relying on simple filler incorporation, this work demonstrates an integrated network design that balances mechanical strength, conductivity, and adhesion, providing a versatile material platform for flexible strain sensors and wearable electronics. Full article

In this work, a flexible, stretchable, tough, highly ionic conductive, and anti-freezing hydrogel based on acrylamide/two-dimensional transition metal carbide (MXene)/montmorillonite (MMT) was precisely designed. In the hydrogel, MXene and MMT acted as both cross-linking agents and conductive fillers, delivering high stretchability (1037%) with a strength of up to approximately 67 kPa and high conductivity. As a result, the usual trade-off between conductivity and mechanical properties of hydrogels could be alleviated to some extent. Therefore, the hydrogel could be used as an electrolyte for supercapacitors (SCs) and strain sensors to monitor physical signals. The hydrogel-based SC exhibited outstanding electrochemical performance over a wide temperature range. Moreover, it could easily withstand various deformations, such as bending, twisting, and compression. The hydrogel also exhibited excellent sensing properties, with a short tensile response time and a high-sensitivity factor (GF = 14.8) in the 0–400% range (0 denotes the original state, where both the strain and stretch are zero as there is no deformation at this point). Due to its high conductivity, the prepared hydrogel could be used as a flexible electrode to replace commercial electrodes and record electromyographic (EMG) signals. This work proposes a novel approach for balancing the conductivity and mechanical strength of hydrogels. Full article

Functionalized hydrogels represent an emerging class of smart materials being explored for advancing next-generation wearable technologies, owing to their flexibility, biocompatibility, stimuli-responsiveness, and tunable properties. This review provides an overview of recent developments in hydrogel-based wearables, highlighting their potential to enhance adaptive, multifunctional, and environmentally sustainable devices and textiles. It begins by examining progress in wearable sensors, energy storage and harvesting, biosignal monitoring, and smart textiles, as well as the associated challenges, including limited battery life, inadequate skin adhesion, user discomfort, and constrained functionality. The review further explores the synthesis, fabrication techniques, properties, and types of hydrogels tailored for wearable technologies, followed by a detailed discussion of their applications in smart batteries, supercapacitors, sensors, nanogenerators, fabrics and hybrid systems. It also highlights integrating artificial intelligence (AI) and the Internet of Things (IoT) to improve designs; enhance performance through real-time monitoring, data analytics, and user interaction; and expand functionality. Also, it analyzes key limitations of current hydrogels—particularly in energy density, dehydration resistance, fatigue behaviour, and large-scale reproducibility—and outlines strategies based on hierarchical material design, sustainable and biodegradable formulations, and standardized testing and regulatory alignment. The review concludes by affirming the role of hydrogel-based technologies in shaping the future of wearable innovations across healthcare, lifestyle, and beyond and outlines promising research directions. Full article