Linearly Responsive, Reliable, and Stretchable Strain Sensors Based on Polyaniline Composite Hydrogels

Xanthan gum toughen ionically conductive hydrogels for flexible and artificial epidermis sensors with multifunctionality and self-healability

This study investigates the influence of lay-up and load direction on embedded optical fiber Bragg gratings (FBGs) used as strain sensors. FBGs have shown great promise for application to structural health monitoring with advantages of small size and cylindrical geometry readily allowing for embedment within fiber reinforced composites. Characterization of the embedded FBGs is necessary to develop a rugged and reliable strain sensor. This paper specifically explores the effects of loading direction on the FBG strain outputs. A well behaved baseline case is established with results for gratings loaded parallel to the optical fiber direction while embedded parallel to the adjacent structural fibers in a quasi-isotropic composite. Results and analysis are also presented for a case involving a composite fabricated with the optical and structural fibers parallel to each other but perpendicular to the loading direction. Extremely good results are obtained relating FBG strain measurements with that of surface mounted resistance strain gauges.

Flexible wearable sensors are emerging as next-generation tools to collect information from the human body and surroundings in a smart, friendly, and real-time manner. A new class of such sensors with various functionality and amenability for the human body is essential for this goal. Unfortunately, the majority of the wearable sensors reported so far in the literature were of a single function (mostly strain sensors) and just a prototype without thinking of continuous mass production. In this paper, we report a series of multifunctional conductive hydrogel/ thermochromic elastomer hybrid fibers with core-shell segmental configuration and their application as flexible wearable strain and temperature sensors to monitor human motion and body/surrounding temperatures. Specifically, a conductive reduced-graphene-oxide-doped poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide (rGO-poly(AMPS-co-AAm)) hydrogel and a thermochromic elastomer containing silicon rubber and thermochromic microcapsules are chosen as strain-sensitive and thermosensitive materials, respectively. A core-shell segmental structure is realized by programming the extrusion of either conductive hydrogel precursor solution or a thermochromic elastomer prepolymer as a core layer via dual-core coaxial wet spinning. Depending on the assembly order and length of the conductive hydrogel and the thermochromic elastomer, the as-prepared hybrid fibers can be used for different purposes, i.e., human-motion monitoring, body or room temperature detection, and color decoration. The strategy described above, i.e., fabrication of core-shell segmental fibers via the wet-spinning method, is especially suitable for mass production in industry and can be further extended to fabricate flexible wearable devices with more components and more functions such as transistors, sensors, displays, and batteries.

In this paper, a novel conductive hydrogel was made by a one-pot method at 60 °C for 12 hours in one step without any cross-linking agent. The hydrogel was composited of Ti3C2 MXene nano slices, poly(vinyl alcohol) (PVA) and poly-N-(2-hydroxyethyl acrylamide) (PHEAA), named as MXene/PVA/PHEAA hydrogel. Compared with previous methods to make MXene hydrogel, this one-pot method was facile and fast. The as-prepared hydrogel was highly stretchable with the breaking elongation of 1300% and tensile strength of 1.1 MPa. MXene nano sheets dispersed in the hydrogel formed a three-dimensional conductive network, which increased the strength and conductivity of the gels. The conductivity changed with the gel’s deformation and could be applied as strain sensors. The strain sensors made of the hydrogels were highly stretchable, bendable and flexible with the response range of 1%–300% and a large gauge factor of 4.23. The sensors can be applied to detect human movements such as finger bending, leg bending, water drinking. The MXene/PVA/PHEAA hydrogel made by this facile one-pot method is promising to make strain sensors for health detection.

Conductive, water-retaining and knittable hydrogel fiber from xanthan gum and aniline tetramer modified-polysaccharide for strain and pressure sensors

Advances in conductive supramolecular hydrogels for applications in wearable electronics.

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

A ionic liquid enhanced conductive hydrogel for strain sensing applications

Synthesis and transport properties of poly(vinyl alcohol) acrylonitrile/2-hydroxyethyl methacrylate membranes—I. Synthesis of poly(vinyl alcohol) acrylonitrile 2-hydroxyethyl methacrylate graft copolymer

Soft humidity‐responsive materials are highly desirable for applications such as actuators, sensors, generators, and soft robots. However, it remains a huge challenge to develop a durable, cost‐effective, fast responsive version of such a smart material powered by water evaporation at ambient conditions. Herein, this challenge is addressed to demonstrate sustained response to humidity gradient from ambient water evaporation by using common poly(vinyl alcohol) (PVA) film as an actuator. The resultant PVA film displays strong mechanical properties in both dry and wet conditions, which cause rapid adsorption and desorption of water vapor to drive the film undergoing swift locomotion with flipping frequency of up to 65 r min‐1. Based on these features, a mimosa inspired humidity‐responsive actuator is developed which is far superior in response speed and durability than real mimosa. Furthermore, it is demonstrated that the film actuator can convert water evaporation energy into electricity when attached to a piezoelectric element.

Conductive hydrogels are revolutionizing the fields of wearable sensors, implantable bioelectronics, and soft robotics. However, achieving both mechanical robustness and high conductivity within a single system remains challenging. Here, inspired by the cooperative vascular-neural networks in biological tissues, we develop a nanofiber-reinforced conductive hydrogel composed of poly(vinyl alcohol) (PVA), aramid nanofibers (ANFs), and in situ polymerized PEDOT:PSS. Through solvent- and thermally induced structural reorganization, the hydrogel evolves into a bi-continuous architecture in which the mechanical and conductive networks are intimately coupled. The tough, ANF-reinforced porous PVA mimics the vascular system, providing mechanical support and maintaining toughness, while the poly(3,4-ethylenedioxythiophene) (PEDOT) network resembles neural pathways, enabling efficient electron transport. This structural evolution enables a rare synergy of high tensile strength (10.72MPa) and ultrahigh conductivity (452.75 S m-1) with excellent biocompatibility. The hydrogel maintains stable conduction under impact and complex deformation, supporting multimodal sensing from large-amplitude joint motion to low-amplitude electrophysiological signals: electrocardiographic and electromyographic. When integrated with a convolutional neural network, it achieves 99.54% accuracy in recognizing five complex hand gestures. This bioinspired strategy paves the way for developing robust and conductive hydrogels toward next-generation intelligent wearable electronics.

A semi-interpenetrating network ionic composite hydrogel with low modulus, fast self-recoverability and high conductivity as flexible sensor

Stretchable conductive materials have attracted great attention due to their potential applications as strain sensors, wearable electronics, soft robotics, and medical devices. The fabrication of these materials with customized object geometries is desirable, but the methods to achieve them are still highly limited. Additive manufacturing via vat photopolymerization can generate sophisticated object geometries, but there is still a significant need to print with materials that afford improved conductivity, mechanical properties, elastic recovery, and durability. Herein, stretchable strain sensors with a range of 3D printed designs are reported using vat photopolymerization. Ionic liquid resins are optimized for their printability using Sudan‐I as a photoabsorber and used to fabricate 3D objects that are subjected to compression, stretching, and bending loads that are detected as real‐time changes in current. Additionally, the self‐adhesive nature of these materials enables mechanically damaged structures to be mended together to regain its function as a strain sensor. These ionic liquid resins are compatible with commercial 3D printers, which enhances their applicability for on‐demand production of customized devices.

An adhesive, highly stretchable and low-hysteresis alginate-based conductive hydrogel strain sensing system for motion capture

Dual-network conductive hydrogel with rapid self-healing ability and great fatigue resistance as strain sensor for human motion monitoring