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Salivary pH is one of the crucial biomarkers used for non-invasive diagnosis of intraoral diseases, as well as general health conditions. However, standard pH sensors are usually too bulky, expensive, and impractical for routine use outside laboratory settings. Herein, a miniature hydrogel sensor, which enables quick and simple colorimetric detection of pH level, is shown. The sensor structure was manufactured from non-toxic hydrogel ink and patterned in the form of a matrix with 5 mm × 5 mm × 1 mm individual sensing pads using a 3D printing technique (bioplotting). The authors' ink composition, which contains sodium alginate, polyvinylpyrrolidone, and bromothymol blue indicator, enables repeatable and stable color response to different pH levels. The developed analysis software with an easy-to-use graphical user interface extracts the R(ed), G(reen), and B(lue) components of the color image of the hydrogel pads, and evaluates the pH value in a second. A calibration curve used for the analysis was obtained in a pH range of 3.5 to 9.0 using a laboratory pH meter as a reference. Validation of the sensor was performed on samples of artificial saliva for medical use and its mixtures with beverages of different pH values (lemon juice, coffee, black and green tea, bottled and tap water), and correct responses to acidic and alkaline solutions were observed. The matrix of square sensing pads used in this study provided multiple parallel responses for parametric tests, but the applied 3D printing method and ink composition enable easy adjustment of the shape of the sensing layer to other desired patterns and sizes. Additional mechanical tests of the hydrogel layers confirmed the relatively high quality and durability of the sensor structure. The solution presented here, comprising 3D printed hydrogel sensor pads, simple colorimetric detection, and graphical software for signal processing, opens the way to development of miniature and biocompatible diagnostic devices in the form of flexible, wearable, or intraoral sensors for prospective application in personalized medicine and point-of-care diagnosis. [ABSTRACT FROM AUTHOR]

Wearable sensors based on ionic hydrogels have attracted increasing attention, but practical application is often hampered by poor environmental tolerance. Here, an environment-stable and tough hydrogel was constructed by introducing glycerin, sodium chloride and xanthan gum into a polyvinyl alcohol hydrogel network. Hydrogen bonds were formed among glycerin, xanthan gum and polyvinyl alcohol chains to enhance the toughness of hydrogel. Furthermore, the salting-out effect of polyvinyl alcohol chains induced by sodium chloride also improved the strength of hydrogel. Besides, the hydrogel was highly sensitive to mechanical deformation of tensile and compressive strain. The obtained hydrogel with sensitivity was capable of serving as a wearable sensor for monitoring human motions including speaking, bending articulation and so on. Notably, the hydrogel still maintained flexibility, toughness and conductivity under subzero or ambient conditions for a long time. This simple method paves an effective way for designing environment-stable and tough hydrogels. [ABSTRACT FROM AUTHOR]

Hydrogels with excellent flexibility, conductivity, and controllable mechanical properties are the current research hotspots in the field of biomaterial sensors. However, it is difficult for hydrogel sensors to regain their original function after being damaged, which limits their practical applications. Herein, a composite hydrogel (named SPBC) of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/cellulose nanofibers (CNFs)/sodium borate tetrahydrate was synthesized, which has good self-healing, electrical conductivity, and excellent mechanical properties. The SPBC0.3 hydrogel demonstrates rapid self-healing (<30 s) and achieves mechanical properties of 33.92 kPa. Additionally, it exhibits high tensile strain performance (4000%). The abundant internal ions and functional groups of SPBC hydrogels provide support for the good electrical conductivity (0.62 S/cm) and electrical response properties. In addition, the SPBC hydrogel can be attached to surfaces such as fingers and wrists to monitor human movements in real time, and its good rheological property supports three-dimensional (3D) printing molding methods. In summary, this study successfully prepared a self-healing, conductive, printable, and mechanically superior SPBC hydrogel. Its suitability for 3D-printing personalized fabrication and outstanding sensor properties makes it a useful reference for hydrogels in wearable devices and human motion monitoring. [ABSTRACT FROM AUTHOR]

Highlights The bioinspired self-sensing actuated gradient hydrogel was developed by a wettability-based method via precipitation of MoO2 nanosheets. Self-sensing actuated gradient hydrogel combined ultrafast thermo-responsive actuation (21° s–1), exceptional photothermal efficiency (3.7 °C s–1) and high sensing properties (GF = 3.94). The first self-sensing remote interaction system based on gradient hydrogel actuators and robotic hands was constructed.

Modern materials with unique structures and functions are inspired by nature. However, nature's craftsmanship is beyond the reach of traditional handmade techniques. 3D printing technology creates the possibility of creating bionic structures. In this study, inspired by the ability of spiders to sense the capture of prey through the vibration of their webs, a spider‐web‐like, highly sensitive hydrogel sensor is designed and printed. The hydrogel is made of eco‐friendly green polyvinyl alcohol cross‐linked with microcrystalline cellulose. Surprisingly, the 3D‐printed hydrogel sensor can manipulate the sensitivity by changing the angle and strain range of the structure. Accordingly, the Gauge factor (GF) of the 3D‐printed hydrogel (θ = 60°) is up to 58.23, which is 23.9 times higher than that of the non‐3D‐printed hydrogel sensor (GF = 2.43). The GF of the 3D‐printed hydrogel at other angles still remained high at 24.74 (θ = 90°) and 12.31 (θ = 120°), respectively. It can be seen that the 3D‐printed hydrogel sensor has advanced performance. In addition, an intelligent gesture recognition system for human–computer interaction is constructed using 3D‐printed hydrogel sensors, which offers potential prospects for helping people communicate. [ABSTRACT FROM AUTHOR]

Highlights: The bioinspired self-sensing actuated gradient hydrogel was developed by a wettability-based method via precipitation of MoO2 nanosheets. Self-sensing actuated gradient hydrogel combined ultrafast thermo-responsive actuation (21° s–1), exceptional photothermal efficiency (3.7 °C s–1) and high sensing properties (GF = 3.94). The first self-sensing remote interaction system based on gradient hydrogel actuators and robotic hands was constructed. The development of bioinspired gradient hydrogels with self-sensing actuated capabilities for remote interaction with soft-hard robots remains a challenging endeavor. Here, we propose a novel multifunctional self-sensing actuated gradient hydrogel that combines ultrafast actuation and high sensitivity for remote interaction with robotic hand. The gradient network structure, achieved through a wettability difference method involving the rapid precipitation of MoO2 nanosheets, introduces hydrophilic disparities between two sides within hydrogel. This distinctive approach bestows the hydrogel with ultrafast thermo-responsive actuation (21° s−1) and enhanced photothermal efficiency (increase by 3.7 °C s−1 under 808 nm near-infrared). Moreover, the local cross-linking of sodium alginate with Ca2+ endows the hydrogel with programmable deformability and information display capabilities. Additionally, the hydrogel exhibits high sensitivity (gauge factor 3.94 within a wide strain range of 600%), fast response times (140 ms) and good cycling stability. Leveraging these exceptional properties, we incorporate the hydrogel into various soft actuators, including soft gripper, artificial iris, and bioinspired jellyfish, as well as wearable electronics capable of precise human motion and physiological signal detection. Furthermore, through the synergistic combination of remarkable actuation and sensitivity, we realize a self-sensing touch bioinspired tongue. Notably, by employing quantitative analysis of actuation-sensing, we realize remote interaction between soft-hard robot via the Internet of Things. The multifunctional self-sensing actuated gradient hydrogel presented in this study provides a new insight for advanced somatosensory materials, self-feedback intelligent soft robots and human–machine interactions. [ABSTRACT FROM AUTHOR]

New portable hydrogel sensors for Al3+ and Fe3+ detection were designed based on the aggregation-induced emission (AIE) and color change of N-doped carbon dots (N-CDs). N-CDs with yellow fluorescence were prepared by a one-pot hydrothermal method from 2,5-dihydroxyterephthalic acid and acrylamide. The fluorescence of N-CDs was enhanced by Al3+ about 20 times and quenched by Fe3+. It was interesting that although Fe3+ showed obvious quenching on the fluorescence of N-CDs it did not cause a noticeable change in the fluorescence of N-CDs + Al3+. The colorless solution of N-CDs appeared blue in the presence of Fe3+ without the influence of Al3+. Therefore, the turn-on fluorometry and colorimetry systems based on N-CDs were constructed for the simultaneous detection of Al3+ and Fe3+. Furthermore, the portable sensing of Al3+ and Fe3+ was realized with the assistance of hydrogel, filter paper, cellulose acetate, and cellulose nitrate film. The proposed approach was successfully applied to the detection of Al3+ and Fe3+ in food samples and cell imaging. [ABSTRACT FROM AUTHOR]

Novel and practical low-temperature 3D printing technology composed of a low-temperature 3D printing machine and optimized low-temperature 3D printing parameters was successfully developed. Under a low-temperature environment of 0–−20 °C, poly (vinyl alcohol) (PVA) matrix hydrogels including PVA-sodium lignosulphonate (PVA-LS) hydrogel and PVA-sodium carboxymethylcellulose (PVA-CMC) hydrogel exhibited specific low-temperature rheology properties, building theoretical low-temperature 3D printable bases. The self-made low-temperature 3D printing machine realized a machinery foundation for low-temperature 3D printing technology. Combined with ancillary path and strut members, simple and complicated structures were constructed with high precision. Based on self-compiling G-codes of path structures, layered variable-angle structures with high structure strength were also realized. After low-temperature 3D printing of path structures, excellent electrical sensing functions can be constructed on PVA matrix hydrogel surfaces via monoplasmatic silver particles which can be obtained from reduced reactions. Under the premise of maintaining original material function attributes, low-temperature 3D printing technology realized functionalization of path structures. Based on "3D printing first and then functionalization" logic, low-temperature 3D printing technology innovatively combined structure–strength design, 3D printable ability and electrical sensing functions of PVA matrix hydrogels. [ABSTRACT FROM AUTHOR]

Salivary pH is one of the crucial biomarkers used for non-invasive diagnosis of intraoral diseases, as well as general health conditions. However, standard pH sensors are usually too bulky, expensive, and impractical for routine use outside laboratory settings. Herein, a miniature hydrogel sensor, which enables quick and simple colorimetric detection of pH level, is shown. The sensor structure was manufactured from non-toxic hydrogel ink and patterned in the form of a matrix with 5 mm × 5 mm × 1 mm individual sensing pads using a 3D printing technique (bioplotting). The authors’ ink composition, which contains sodium alginate, polyvinylpyrrolidone, and bromothymol blue indicator, enables repeatable and stable color response to different pH levels. The developed analysis software with an easy-to-use graphical user interface extracts the R(ed), G(reen), and B(lue) components of the color image of the hydrogel pads, and evaluates the pH value in a second. A calibration curve used for the analysis was obtained in a pH range of 3.5 to 9.0 using a laboratory pH meter as a reference. Validation of the sensor was performed on samples of artificial saliva for medical use and its mixtures with beverages of different pH values (lemon juice, coffee, black and green tea, bottled and tap water), and correct responses to acidic and alkaline solutions were observed. The matrix of square sensing pads used in this study provided multiple parallel responses for parametric tests, but the applied 3D printing method and ink composition enable easy adjustment of the shape of the sensing layer to other desired patterns and sizes. Additional mechanical tests of the hydrogel layers confirmed the relatively high quality and durability of the sensor structure. The solution presented here, comprising 3D printed hydrogel sensor pads, simple colorimetric detection, and graphical software for signal processing, opens the way to development of miniature and biocompatible diagnostic devices in the form of flexible, wearable, or intraoral sensors for prospective application in personalized medicine and point-of-care diagnosis.

Hydrogels with excellent flexibility, conductivity, and controllable mechanical properties are the current research hotspots in the field of biomaterial sensors. However, it is difficult for hydrogel sensors to regain their original function after being damaged, which limits their practical applications. Herein, a composite hydrogel (named SPBC) of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/cellulose nanofibers (CNFs)/sodium borate tetrahydrate was synthesized, which has good self-healing, electrical conductivity, and excellent mechanical properties. The SPBC0.3 hydrogel demonstrates rapid self-healing (

A fluorescent and colorimetric poly (acrylamide)-based copolymer probe P(AAm-co-RBNCH) has been designed via free radical polymerization of a commercial acrylamide monomer with a rhodamine-functionalized monomer RBNCH. Metal ion selectivity of RBNCH was investigated by fluorescence and colorimetric spectrophotometry. Upon addition of Fe3+, a visual color change from colorless to red and a large fluorescence enhancement were observed for the ring-opening of the rhodamine spirolactam mechanism. The monomer gives a sensitive method for quantitatively detecting Fe3+ in the linear range of 100–200 μM, with a limit of detection as low as 27 nM and exhibiting high selectivity for Fe3+ over 12 other metal ions. The hydrogel sensor was characterized by FTIR, and the effects of RBNCH amount on gel content and swelling properties were explored. According to the recipe of 1.0 mol% RBNCH to the total monomers, the fabricated hydrogel sensor displayed a good swelling property and reversibility performance and has potential for application in the imaging of Fe3+ level in industrial wastewater. [ABSTRACT FROM AUTHOR]

Smart sensing fabrics are becoming increasingly attractive in the emerging wearable areas of medical and military so far. Here, for the first time, we present a smart humidity sensing fabric (SHSF) based on a moisture‐sensitive polyacrylamide hydrogel for respiratory monitoring and non‐contact sensing. Fabricated by in situ cross‐linking of the hydrogel precursors on the fibers of the fabric, the flexible SHSF shows excellent sensitivity, outstanding flame retardance, air permeability, water retention capacity, and stability after treatment with lithium bromide solution. Specifically, its conductance increases more than 311 times as humidity increased from 11% to 98%. Besides, the humidity sensor features good repeatability and the ability to work normally under folding due to its flexible nature. As a clothing material, hydrogel–fabric composite exhibits 4.3 times the burning time compared to cotton fabric, illustrating better flame retardance. The SHSF is used to monitor human breathing and non‐contact finger approaching in real time, demonstrating its flexibility in practical applications. This work provides strategies for preparing high‐performance, flame‐retardant SHSF for emerging wearable electronic devices. [ABSTRACT FROM AUTHOR]

To realize continuously and stably work in a "moist/hot environment", flexible electronics with excellent humid resistance, anti-swelling, and detection sensitivity are demanding. Herein, a solvent-resistant and temperature-ultrasensitive hydrogel sensor was prepared by combining MXene and quaternized chitosan (QCS) with the binary polymer chain. The strong electrostatic interaction between the QCS chain and the poly(acrylic acid) (PAA) network endows the hydrogel stability against solvent erosion, high temperature, and high humidity. The strong dynamic interaction between MXene and polymer matrix significantly improves the mechanical properties and sensing (strain and temperature) sensitivity of the hydrogel. The hydrogel strain sensor exhibits a high gauge factor (5.53), temperature/humidity tolerance (equilibrium swelling ratio of 2.5% at 80 °C), and excellent cycle stability, which could achieve a remote and accurate perception of complex human motion and environment fluctuation under aquatic conditions. Moreover, the hydrogel sensor exhibits impressive thermal response sensitivity (−3.183%/°C), ultrashort response time (< 2.53 s), and a low detection limit (< 0.5 °C) in a wide temperature range, which is applied as an indicator of the body surface and ambient temperature. In short, this study broadens the application scenarios of hydrogels in persistent extreme thermal and wet environments. [ABSTRACT FROM AUTHOR]

Abstract Smart sensing fabrics are becoming increasingly attractive in the emerging wearable areas of medical and military so far. Here, for the first time, we present a smart humidity sensing fabric (SHSF) based on a moisture‐sensitive polyacrylamide hydrogel for respiratory monitoring and non‐contact sensing. Fabricated by in situ cross‐linking of the hydrogel precursors on the fibers of the fabric, the flexible SHSF shows excellent sensitivity, outstanding flame retardance, air permeability, water retention capacity, and stability after treatment with lithium bromide solution. Specifically, its conductance increases more than 311 times as humidity increased from 11% to 98%. Besides, the humidity sensor features good repeatability and the ability to work normally under folding due to its flexible nature. As a clothing material, hydrogel–fabric composite exhibits 4.3 times the burning time compared to cotton fabric, illustrating better flame retardance. The SHSF is used to monitor human breathing and non‐contact finger approaching in real time, demonstrating its flexibility in practical applications. This work provides strategies for preparing high‐performance, flame‐retardant SHSF for emerging wearable electronic devices.

Flexible hydrogels have shown promise as strain sensors in medical monitoring, human motion detection and intelligent robotics. For a hydrogel strain sensor, certain challenges need to be urgently addressed for practical applications, such as the damage caused by external effects, leading to equipment failure, and the inability to perceive ambient temperature, resulting in single functionality. Herein, a stretchable, self-healing and dual temperature-strain sensitive hydrogel, with a physically-crosslinked network, is designed by constructing multiple dynamic reversible bonds. Graphene oxide (GO) and iron ions (Fe3+) act as dynamic bridges in the cross-linked network and are mediated by the covalent and hydrogen bonding, rendering excellent stretchability to the hydrogel. The reversible features of coordination interactions and hydrogen interactions endow excellent recoverability and self-healing properties. Moreover, the incorporated N-isopropyl acrylamide (NIPAM) provides excellent temperature responsiveness to the hydrogel, facilitating the detection of external temperature changes. Meanwhile, the hydrogels exhibited strain-sensitivity, with a wide working range of 1%–300%, fast response and electrical stability, which can be used as flexible sensors to monitor body motions, e.g., speaking and the bending of finger, wrist, elbow and knee. Overall, the hydrogel possesses dual sensory capabilities, combining external temperature and strain, for potential applications in wearable multifunctional sensing devices. [ABSTRACT FROM AUTHOR]

Recent advancement in wearable and robot-assisted healthcare technology gives rise to the demand for smart interfaces that allow more efficient human-machine interaction. In this paper, a hydrogel-based soft sensor for subtle touch detection is proposed. Adopting the working principle of a biomedical imaging technology known as electrical impedance tomography (EIT), the sensor produces images that display the electrical conductivity distribution of its sensitive region to enable touch detection. The sensor was made from a natural gelatin hydrogel whose electrical conductivity is considerably less than that of human skin. The low conductivity of the sensor enabled a touch-detection mechanism based on a novel short-circuiting approach, which resulted in the reconstructed images being predominantly affected by the electrical contact between the sensor and fingertips, rather than the conventionally used piezoresistive response of the sensing material. The experimental results indicated that the proposed sensor was promising for detecting subtle contacts without the necessity of exerting a noticeable force on the sensor. [ABSTRACT FROM AUTHOR]

[Display omitted] • An AIE and ESIPT based fluorescent probe was created for detecting biogenic amines. • Agarose-based hydrogel sensor containing the fluorescent probe was fabricated. • The hydrogel sensor showed ratiometric fluorescence during the meat spoilage trial. • A smartphone detection platform achieved real-time monitoring meat freshness. Meat quality is closely linked to food safety and human health, with freshness being a key indicator. Therefore, it is crucial to explore reliable and effective methods for detecting meat freshness. This study presented a ratiometric fluorescent hydrogel sensor based on aggregation-induced emission (AIE) and excited state intramolecular proton transfer (ESIPT) for this purpose. Tetraphenylene (TPE) and 2-(2′-hydroxyphenyl)-benzothiazole (HBT) fluorophore were intersected to generate TPE-HBT, and then a novel biogenic amines (BAs) detection ratiometric fluorescent probe, TPE-HBTAc, was successfully prepared by hydroxyl acetylation. TPE-HBTAc displayed a large Stokes shift and good photostability. It showed a significant response to the representative biogenic amine trimethylamine (TMA), with a ratiometric fluorescence change and low detection limit (LOD = 65 nM). Furthermore, BAs detection hydrogel sensor was created by mixing the fluorescent probe TPE-HBTAc in an agarose hydrogel with a 3D polymer network. The hydrogel sensor displayed a distinct color change from blue to yellow when exposed to TMA vapor under UV light and had a low detection limit (LOD = 5.88 ppm). The hydrogel sensor was utilized in conjunction with a smartphone RGB analysis system to determine the freshness of chicken breast samples. Consumers can easily observe the chicken breast freshness in real-time and non-destructively through UV flashlight illumination. The results of this work suggest that ratiometric fluorescent hydrogel sensor based on AIE and ESIPT can serve as a promising strategy to monitor food freshness. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multifunctional hydrogels that couple actuation and strain sensing were developed. • The hydrogel can be used for constructing smart bioelectronics and soft robotics. • Adhesive strain sensor detects motion and electrocardiogram and writing behavior. • Near-infrared controlled liquid valve and bimodal electronic switch were fabricated. Biomimetic intelligent conductive hydrogels integrating flexibility, signal sensing, and contactless actuation has become ideal candidates for the production of electronic skins, wearable devices, and soft robotics. However, the development of multifunctional smart hydrogels capable of both sensing and actuating simultaneously to enhance intelligent interaction interfaces remains a significant challenge. Herein, a multifunctional hydrogel composite with integrated sensing and actuating capabilities is proposed through the incorporation of Fe3+-tannic acid (TA) coordination compound-coated carbon nanotubes (Fe-TA@SWNT) and the in-situ formation of conductive polymer polypyrrole (PPy) inside the hydrogel networks composed of carboxymethylcellulose (CMC) and a copolymer of acrylic acid (AA) and [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA). Benefiting from multiple effective dynamic interactions within the internal matrix, the prepared hydrogel exhibits excellent stretchability (1283.42 % elongation) and toughness (1.11 MJ/m3). Additionally, the catechol groups carried by Fe-TA@SWNT, along with the zwitterionic monomer SBMA, provide the hydrogel with repeatable self-adhesion performance. Based on the outstanding photothermal effect of Fe-TA@SWNT and PPy, the hydrogel demonstrates reversible and stable photothermal responsiveness, exhibiting exceptional actuation performance under near-infrared (NIR) radiation. As a strain sensor, the hydrogel shows excellent sensitivity (GF = 3.32), rapid response time, and reliable stability, enabling precise monitoring of various physiological signals in the human body, including joint movements, facial expressions, and heartbeats. Therefore, this work significantly expands the application prospects of multifunctional smart hydrogels in fields of wearable electronics, soft robotics, and remote light-controlled devices. [ABSTRACT FROM AUTHOR]

Novel and practical low-temperature 3D printing technology composed of a low-temperature 3D printing machine and optimized low-temperature 3D printing parameters was successfully developed. Under a low-temperature environment of 0–−20 °C, poly (vinyl alcohol) (PVA) matrix hydrogels including PVA-sodium lignosulphonate (PVA-LS) hydrogel and PVA-sodium carboxymethylcellulose (PVA-CMC) hydrogel exhibited specific low-temperature rheology properties, building theoretical low-temperature 3D printable bases. The self-made low-temperature 3D printing machine realized a machinery foundation for low-temperature 3D printing technology. Combined with ancillary path and strut members, simple and complicated structures were constructed with high precision. Based on self-compiling G-codes of path structures, layered variable-angle structures with high structure strength were also realized. After low-temperature 3D printing of path structures, excellent electrical sensing functions can be constructed on PVA matrix hydrogel surfaces via monoplasmatic silver particles which can be obtained from reduced reactions. Under the premise of maintaining original material function attributes, low-temperature 3D printing technology realized functionalization of path structures. Based on “3D printing first and then functionalization” logic, low-temperature 3D printing technology innovatively combined structure–strength design, 3D printable ability and electrical sensing functions of PVA matrix hydrogels.

A fluorescent and colorimetric poly (acrylamide)-based copolymer probe P(AAm-co-RBNCH) has been designed via free radical polymerization of a commercial acrylamide monomer with a rhodamine-functionalized monomer RBNCH. Metal ion selectivity of RBNCH was investigated by fluorescence and colorimetric spectrophotometry. Upon addition of Fe3+, a visual color change from colorless to red and a large fluorescence enhancement were observed for the ring-opening of the rhodamine spirolactam mechanism. The monomer gives a sensitive method for quantitatively detecting Fe3+ in the linear range of 100–200 μM, with a limit of detection as low as 27 nM and exhibiting high selectivity for Fe3+ over 12 other metal ions. The hydrogel sensor was characterized by FTIR, and the effects of RBNCH amount on gel content and swelling properties were explored. According to the recipe of 1.0 mol% RBNCH to the total monomers, the fabricated hydrogel sensor displayed a good swelling property and reversibility performance and has potential for application in the imaging of Fe3+ level in industrial wastewater.

A novel fluorescein-based probe FLA-Boe was developed for detecting H2O2. Modified by 2-Bromomethylphenylboronic acid pinacol ester, FLA-Boe is a Fluorescein derivative with eminent photostability and remarkable H2O2 sensitivity and selectivity. FLA-Boe was utilized to synthesize hydrogel sensors in the manner of guest–host interaction by taking advantage of its aforementioned features. The hydrogel sensor can be used to detect H2O2 effectively in both flowing and static water environments with satisfactory performance. It is expected that this application may open a new page to develop a neoteric fluorescent property analysis method aiming at H2O2 detection. [ABSTRACT FROM AUTHOR]

Flexible wearable sensors are of interest for underwater applications such as aquatic robots and marine exploration. Non‐swelling hydrogels would be a preferable candidate for underwater sensing with stable sensing performance. Herein, hydrogels comprised of MXene and polyhydroxyethyl methacrylate (PHEMA) are designed and manufactured. Owing to the synergistic effect of phase separation zones and hydrophilic/hydrophobic interaction, MXene/PHEMA hydrogels exhibit non‐swelling performance in various liquid media (e.g., water, seawater) and effectively avoid the adhesion of proteins and bacteria. In addition, MXene/PHEMA hydrogels exhibit a large sensing range (more than 400%), high tensile strength (0.54 MPa), and skin‐compliance modulus (120 kPa). Furthermore, the resulted hydrogel‐based sensors possess superior sensibility (4.42 at 150–450% strain), fast responsiveness (200 ms), and durable sensing ability (over 1000 cycles). The hydrogel‐based sensors can accurately monitor human motions (both large and subtle motions) in air and in various liquid media. Benefiting from the non‐swelling and anti‐fouling ability, the hydrogels could maintain high mechanical strength and cyclic stability after being immersed in water for 30 days, and precisely detect the movement of human and shark models underwater. It is conceivable that non‐swelling and anti‐fouling MXene/PHEMA hydrogels provide a feasible material to construct electronic skins for various harsh liquid environments. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal carbides (Ti 3 C 2 T x MXene) have demonstrated substantial application potential across various fields, owing to their excellent metallic conductivity and solution processability. However, the rapid oxidation of Ti 3 C 2 T x in aqueous environments, leading to a loss of stability within mere days, poses a significant obstacle for its practical applications. Herein, we introduce an antioxidant strategy that combines free radical scavenging with surface passivation, culminating in the design and synthesis of imidazolium-based ionic liquids (ILs) incorporating siloxane groups. By deploying a straightforward hydrolysis-addition reaction, we successfully fabricated IL-modified Ti 3 C 2 T x materials (Ti 3 C 2 T x -IL). The Ti 3 C 2 T x -IL not only displayed exceptional conductivity exceeding 3.85 × 10 4 S/m and hydrophilic contact angles below 45° but also showcased its superior chemical stability and antioxidation mechanisms through various analyses, including visual color change experiments, spectroscopic and energy spectrum characterization, free radical scavenging tests, and density-functional-theory-based molecular simulations. Furthermore, when utilized as a conductive filler in the fabrication of a poly(vinyl alcohol)/nanocellulose fiber (PVA/CNF) composite hydrogel (PCMIL), the resultant sensors exhibited remarkable mechanical performance with up to 535% strain, 1.59 MPa strength, 4.35 MJ/m 3 toughness, and a conductivity of 3.40 mS/cm, as well as a high sensitivity gauge factor of 3.3. Importantly, even after 45 days of storage, the PCMIL retained most of its functionalities, demonstrating superior performance in human-machine interaction applications compared to hydrogels made from unmodified Ti 3 C 2 T x . This research establishes a robust antioxidant protection strategy for Ti 3 C 2 T x , offering substantial technical reinforcement for its prospective applications in the realm of flexible electronics and sensing technologies.

Conductive hydrogel-based sensors offer diverse applications in artificial intelligence, wearable electronic devices and character recognition management. However, it remains a significant challenge to maintain their satisfactory performances under extreme climatic conditions. Herein, a stretchable, self-adhesive, self-healing and environmentally stable conductive hydrogel was developed through free radical polymerization of hydroxyethyl acrylate (HEA) and poly(ethylene glycol) methacrylate (PEG) as the skeleton, followed by the incorporation of polyaniline-coated cellulose nanocrystal (CNC@PANI) as the conductive and reinforced nanofiller. Encouragingly, the as-prepared hydrogel (CHP) exhibited decent mechanical strength, satisfactory self-adhesion, prominent self-healing property (95.04 % after 60 s), excellent anti-freezing performance (below −60 °C) and outstanding moisture retention. The assembled sensor derived from CHP hydrogel possessed a low detection limit (0.5 % strain), high strain sensitivity (GF = 1.68) and fast response time (96 ms). Remarkably, even in harsh environmental temperatures from −60 °C to 80 °C, it reliably detected subtle and large-scale human motion for a long-term process (>10,000 cycles), manifesting its exceptional environmental tolerance. More interestingly, this hydrogel-based sensor could be assembled into a "writing board" for accurate handwritten numeral recognition. Therefore, the as-obtained multifunctional hydrogel could be a promising material applied in human motion detection and character recognition platforms even in harsh surroundings. [Display omitted] [ABSTRACT FROM AUTHOR]

Recent advancement in wearable and robot-assisted healthcare technology gives rise to the demand for smart interfaces that allow more efficient human-machine interaction. In this paper, a hydrogel-based soft sensor for subtle touch detection is proposed. Adopting the working principle of a biomedical imaging technology known as electrical impedance tomography (EIT), the sensor produces images that display the electrical conductivity distribution of its sensitive region to enable touch detection. The sensor was made from a natural gelatin hydrogel whose electrical conductivity is considerably less than that of human skin. The low conductivity of the sensor enabled a touch-detection mechanism based on a novel short-circuiting approach, which resulted in the reconstructed images being predominantly affected by the electrical contact between the sensor and fingertips, rather than the conventionally used piezoresistive response of the sensing material. The experimental results indicated that the proposed sensor was promising for detecting subtle contacts without the necessity of exerting a noticeable force on the sensor.

[Display omitted] Polydopamine (PDA)-based self-adhesive hydrogel sensors are extensively explored but it is still a challenge to construct PDA-based hydrogels by free radical polymerization. Herein, a new approach to construct self-adhesive hydrogels by conducting free radical polymerization in both aqueous phase and micelle phase is developed. The following two-phase polymerization processes account for the formation of the self-adhesive hydrogels. The first one is the polymerization of acrylamide (AM) and dopamine (DA) in aqueous phase to form adhesive component PAM-PDA (PAM, polyacrylamide; PDA, polydopamine). The second one is the polymerization of hydrophobic monomer 2-methoxyethyl acrylate (MEA) in micelles of an amphiphilic block copolymer Pluronic F127 diacrylate (F127DA). The poly(2-methoxyethyl acrylate) (PMEA) networks help to maintain the high robustness of the hydrogel. Because PMEA and PDA form in relatively separated phases, the inhibition effect of PDA on the free radical polymerization process of PMEA is weakened. Based on this mechanism, mechanically strong and adhesive hydrogels are achieved. The introduced ions during preparation process, such as Na+, OH– and K+, endow the resulting hydrogels ionic conductivity. Resistive strain sensor of the hydrogel achieves a high gauge factor (GF) of 5.26, a response time of 0.25 s and high sensing stability. Because of the adhesiveness, such hydrogel sensor can be applied as wearable sensors in monitoring various human motions. To further address the freezing and drying problems of the hydrogels, organohydrogels are constructed in glycerol-water mixed solvent. The organohydrogels exhibit outstanding anti-freezing property and moisture retention ability, and their adhesiveness is well maintained in subzero conditions. Capacitive pressure sensors of the organohydrogels possessing a GF of 2.05 kPa−1, high sensing stability and reversibility, are demonstrated and explored in monitoring diverse human motions. [ABSTRACT FROM AUTHOR]

The pH-sensitive smart hydrogel sensor was studied. The hydrogel was prepared from polymerization reaction between the 2-hydroxyethyl methacrylate and diallyldimethyammonium chloride using potassium persulfate as an initiator, N, N′-methylenebisacrylamide as a cross-linker and N, N, N′, N′-tetramethylethylenediamine as an accelerator. Bromothymol blue was added to the polymerization system to produce the pH-sensitive hydrogel. The results showed that the hydrogel exhibited properties as a sensor capable of detecting acid–base solutions. The fluid sensing of the hydrogel showed sign of a change in color of the bromothymol blue in the gel. Factor affecting the properties of the hydrogel sensors such as the color change, recovery from bending, and swelling property was the concentration of the diallyldimethyammonium chloride. [ABSTRACT FROM AUTHOR]

A novel fluorescein-based probe FLA-Boe was developed for detecting H2O2. Modified by 2-Bromomethylphenylboronic acid pinacol ester, FLA-Boe is a Fluorescein derivative with eminent photostability and remarkable H2O2 sensitivity and selectivity. FLA-Boe was utilized to synthesize hydrogel sensors in the manner of guest–host interaction by taking advantage of its aforementioned features. The hydrogel sensor can be used to detect H2O2 effectively in both flowing and static water environments with satisfactory performance. It is expected that this application may open a new page to develop a neoteric fluorescent property analysis method aiming at H2O2 detection.

[Display omitted] • Fast gelation hydrogel conductor was developed via dynamic self-catalytic strategy. • The interpenetrating network endowed the gel with superior self-healing properties. • The hydrogel was a promising candidate for constructing smart bioelectronics. • Novel and reliable eye mask was fabricated to detect sleeping behavior chronically. Multifunctional conductive hydrogels exhibit tremendous potential for applications in human–machine interfaces, flexible electronic devices and soft robotics. However, the time-consuming and energy-intensive preparation process, as well as the inefficient and unstable self-healing capabilities, limit their practicality and reusability. Herein, we utilized lignin sulfonate (LS)-Fe3+ dynamic redox system to induce the generation of ammonium persulfate (APS) radicals at room temperature. Additionally, the polymerization of acrylamide (AM), sodium acrylate (AAS), and 3-acrylamidophenylboronic acid (APBA) formed polymer networks through multiple dynamic crosslinking via the synergistic interactions of Fe3+ and cellulose nanocrystals (CNC). The dynamic and reversible boronic ester bonds, ion coordination bonds, and hydrogen bonds endowed the hydrogel with high stretchability (1170 %), low hysteresis and efficient self-healing (91.76 %, 2 h) capability. Interestingly, the intermediate catechol groups generated by the LS-Fe3+ dynamic catalytic system provided the hydrogel with repeatable and reliable adhesive performance. Thanks to the excellent ionically conductivity, the fabricated hydrogel-based sensors exhibited a wide sensing range (500 %), rapid response time (139 ms), and high sensitivity (GF = 8.98), that accomplished specific voice recognition and subtle body motion detection for real-time elderly health and sleep management. This multifunctional hydrogel is of great significance for efficiently developing prolonged lifespan wearable electronics and healthcare flexible devices. [ABSTRACT FROM AUTHOR]

The rapid developments of artificial intelligence have attracted attention in designing electronic skin (e-skin) to realize the mechanical and sensory properties of human skin. To better imitate the tactile sensing properties of human skin, a stretchable and transparent hydrogel is produced. Thus, an elastic and capacitive strain sensor was successfully produced through the as-prepared hydrogel. The sensor was elastic with a high conductive stability and could detect the strain changes in different states, which had very short response time that could be applied into the detection of large and small deformations and would shed light on its application in e-skin. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel PMs reinforced conductive and transparent PAAm hydrogels was prepared. • The hydrogel exhibited a recovery performance of 92% under 200% strain. • The hydrogel revealed excellent response time of only 150 ms at 100% strain. • The sensor showed remarkable stability and reproducibility over 1000 cycles. The development of medical sensor materials is progressing towards flexibility, wearability, and intelligence. Hydrogel, a new type of medical material, possesses excellent mechanical properties, high flexibility, and good biocompatibility. It is expected to become the foundation of a new generation of wearable devices. However, traditional hydrogels have limitations in recovery, conductivity, and stability, which restricts their practical applications as sensors. To address these limitations, this study presents a novel approach by preparing polyelectrolyte microgels (PMs) to reinforce conventional polyacrylamide (PAAm) hydrogels. The resulting double cross-linker hydrogel (DC-PMs/PAAm), containing PMs and N, N' -methylenebis (acrylamide) as cross-linkers, exhibits good transparency (90% at 550 nm) and mechanical properties (147 kPa, 880%). Notably, the DC-PMs/PAAm hydrogel demonstrates graded sensitivity factors in different strain ranges (GF at 0–150%, 150–350%, and 350–800% strain, were 2.47, 5.34, and 9.01, respectively). In addition, the hydrogel has a fast response time of 150 ms, which can detect large and subtle movements of the human body in real-time. Furthermore, DC-PMs/PAAm hydrogel maintains a stable resistance change rate during 1000 rapid cycles (100% strain) monitoring. Therefore, it is expected to be used as wearable device in the medical and health fields. [ABSTRACT FROM AUTHOR]

Concanavalin A (Con A)-based hydrogels have been widely studied in the field of glucose detection as Con A can selectively bond to glucose molecules. Many sensors have been proposed for detecting glucose using fluorescence or optical methods. However, these sensors suffer from limited sensitivity and fabrication issues. In this study, a label-free optical-fiber long-period grating-based glucose sensor is proposed. Con A-based UV-curable hydrogel was used as the sensing material, which can be rapidly prototyped using the micro-3D printing technique to realize microscale structures. Polymerized hydrogel grating structures patterned onto the fiber surface can periodically modulate the refractive index of the fiber modes affecting the resonant spectrum of the device. The structures swelled upon the addition of a glucose solution, which caused a wavelength shift in the transmission spectrum. The experiments showed that glucose-induced swelling/deswelling was reversible, and the wavelength versus glucose concentration was linear with a response sensitivity of 0.206 nm/mM over a wide range of 0–25 mM with high linearity (R2 = 0.998). The device works well at room temperature and has a short response time. It has significant potential for use in real time continuous glucose monitoring. • A hydrogel-based long period grating as a glucose sensor is proposed. • The hydrogel glucose sensor is directly printed by a micro DLP 3D printer. • The printed device is selectively sensitive to glucose. [ABSTRACT FROM AUTHOR]

To improve the sensitivity and shorten the testing time of hydrogel-based bio-toxin sensors, we introduce an optical fiber Fabry–Perot interferometer (FPI) to detect the changes in the optical properties induced by the reactions between the target agent and the hydrogel. The concept is demonstrated by a polyacrylamide hydrogel filled into a cavity of an optical fiber FPI with a length of 150 \mu \textm . Dithiothreitol (DTT) solutions with different concentrations were used as target agent. The degradation of the hydrogel by the DTT solution leads to a long and indistinct change in optical properties, which is difficult to be observed by conventional microscopy methods, but which can be detected by measuring the unique shifting process of the interfering spectrum caused by the hydrogel cleavage in the FPI cavity. Compared to our previous hydrogel-based sensor based on the microscopy observation, the sensitivity of the optic fiber FPI is improved by 2000 times, and the testing time is shortened from hundreds of hours to a few hours. Our approach opens up an avenue for highly sensitive, high-speed in-field detection of bio-toxins in live samples. [ABSTRACT FROM PUBLISHER]

The potential hazard to human health caused by antibiotic residues demands a rapid and sensitive detection method. However, the current rapid methods, such as paper-based sensors with reduced detection sensitivity caused by probe solidification due to printing, affect detection accuracy. Here, a hydrogel sensor has been prepared by an ion-induced enhanced optical probe using lutetium ions (Lu3+) coordinated with CdTe quantum dots (QDs). The hydrogel integrates the advantages of the liquid-phase in- situ detection environment and higher accuracy. With the addition of doxycycline (Dox), the β-diketone structure of Dox forms complexes with Lu3+, leading to a change in the Dox molecular structure, and absorbs electrons from QDs that provide a significant color change from red to green. The probe reacts within 2 s with a limit of detection (LOD) of 36.6 nM. Integrated with the advantages of the hydrogel and color analyzer app, the LOD achieved 53.7 nM, which is close to the detection sensitivity in water. On this basis, a smartphone sensing platform has been constructed for the rapid, visual and quantitative analysis of Dox. The hydrogel-based sensing method for rapid visual quantification detection of Dox provides a new strategy for food safety detection. [Display omitted] • A ratiometric fluorescent probe with Lu3+ as ligand inducing fluorescence enhanced was constructed. • The Lu/CdTe probe exhibited visual and sensitive detection of doxycycline. • The hydrogel sensor overcomes the problem of reduced detection sensitivity due to probe curing. • The platform utilizing smartphone producing dosage-sensitive color response to doxycycline. [ABSTRACT FROM AUTHOR]

Gel-sol transition-based biosensors are a promising and popular alternative for portable, cost-effective, and user-friendly point-of-care testing (POCT). However, the improvement of sensitivity and practicability is highly demanded. In this work, a Fe-NC single-atom catalyst (SAC) is successfully synthesized and used as a signal amplification element for highly sensitive gel-sol transition-based biosensing. The Fe-NC SAC owns excellent peroxidase-like activity of 188 U/mg due to its definite atomically active centers and maximum atomic utilization of active metal atoms. As a proof-of-concept, the Fe-NC SAC is uniformly encapsulated in gelatin hydrogel to obtain a hydrogel sensor that allows colorimetric detection of trypsin based on gel-sol transition. The gelatin hydrogel network collapses derived from the hydrolysis by trypsin, and thereby the released Fe-NC SAC leads to the colorimetric sensing process. The designed hydrogel sensor offers a low detection limit of 1 ng/mL with a range from 1 to 100 ng/mL toward trypsin detection, exhibiting excellent selectivity and sensitivity, and well-performed practical detection in human serum. This work offers a successful paradigm for designing a promising SACs-related detection strategy and paves a new way to develop high-performance gel-sol transition-based sensors and various POCT applications. [ABSTRACT FROM AUTHOR]

Developing a ready-to-use miniaturized thermosensor is a great challenge due to its individual use on a large scale for daily business such as food industry and healthcare. Herein, a polyethylene glycol (PEG)-modified graphene oxide (GO)-based hydrogel thermosensor was established with a fluorescent dye-labeled peptide nucleic acid (F-PNA). The size-tunable hydrogel with high water content and sufficient solidity allowed free movement of the oligonucleotides through the pores and improved usability for handling the sensor. In the PEG-GO hydrogel, the DNA/F-PNA duplex could be denatured by increasing the temperature, followed by selective PNA capture on the PEG-GO. Using this principle, the PEG-GO hydrogel exhibited a change in the fluorescence signal of F-PNA in a temperature-dependent manner, allowing real-time visualization of temperature on a large scale. The temperature detection range of this system can be adjusted by designing the PNA strands based on the melting temperature of the DNAzyme/PNA duplex. Its sensing specificity and detection range could be increased and broadened by observing multi-color detection using PNA probes labeled with different fluorescent dyes of different lengths in a single hydrogel. In addition, the hydrogel platform is easy to store for long time periods via dehydration and can be restored with the addition of water, allowing easy transport, storage, and use of the thermosensor in everyday life. A simple and storable hydrogel thermosensor for visualizing temperature was developed using PEG-GO and Dz/PNA. [Display omitted] • Here, we developed PEG-GO hydrogel-based fluorescent temperature sensor using PNA and DNA. • The hydrogel sensor is easy to handle and operate for measuring the target temperature on a large scale. • The hydrogel sensor is portable and retains its function after drying/swelling. • The sensor has potential applicability in food safety. [ABSTRACT FROM AUTHOR]

[Display omitted] • Robust, ultra stretchable and transparent nanocomposite hydrogel based on synergetic ionic interactions. • Quantitative characterization of intermolecular interactions using atomic force microscopy. • Ionic conductive skin-like materials with excellent strain/pressure sensing performances. • Machine learning module integrated for the recognition of human handwriting. Hydrogel-based ionic skins are ionic conductive artificial skin-like materials that are promising for a broad range of applications such as wearable sensory devices, soft robotics and machines, and bioelectronics. However, fabricating hydrogel skins with satisfying mechanical performance and intelligent sensing functions is still a significant challenge. Herein, we have developed an ionic conductive nanocomposite hydrogel with ultra stretchability and self-evolving sensing functions. By leveraging the dynamic feature of synergetic interfacial ionic interactions, a trace amount of carbon nanotubes endows the hydrogel networks with excellent mechanical performances (i.e., tensile strength, stretchability and toughness up to 1.09 MPa, 4075 % and 12.8 MJ/m3, respectively). Additionally, the hydrogel is soft, elastic, transparent and self-healing. The rational combination of the mechanical and electrical properties renders the as-prepared hydrogel with excellent sensing performances and cycling stability, and therefore enables it to perform as a sensory unit of a complete platform for the recognition of some complicated human behaviors, outperforming the previously reported hydrogels due to its intelligent sensing functions. Specifically, with the integration of machine learning module, the hydrogel-based platform exhibits great recognition accuracies to human handwriting motions from single letters to words, phrases, and short sentences after proper training. The combination of superior mechanical performances and self-evolving sensing functions within this hydrogel-based ionic skin unlocks its potential as the intelligent human-device interface, which promotes the application of artificial intelligence in customized electronic devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • A conductive hydrogel with good biocompatibility was prepared with SF,TA and PPy. • A multifunctional hydrogel based on dynamic non-covalent crosslinks was designed. • Conductive hydrogel has excellent underwater self-healing and wet adhesion. • Hydrogel sensor can detect human motion signals in underwater environment. • Hydrogel can realize underwater communication through Morse code expression. Wearable strain sensors based on conductive hydrogels have attracted much attention due to their great application potential in fields such as human motion and health monitoring. Under the guidance of application, the design of conductive hydrogels with multiple functions has been developed. However, due to the weakening or elimination of some properties of hydrogels by water molecules, the design of wearable sensors suitable for use in wet or underwater environments and with biosafety is still a challenge. In this paper, the conductive polymer polypyrrole (PPy) combined with silk fibroin (SF) and tannic acid (TA) were introduced into the same gel network by in situ polymerization, and the SF/TA@PPy conductive hydrogel was successfully constructed. The hydrogel has stretchability, skin compliance, antibacterial and biocompatibility properties. In air or underwater, SF/TA@PPy can perform self-healing of mechanical properties, electrical properties, and sensing properties without external stimulation. At the same time, the SF/TA@PPy showed excellent wet adhesion to various materials. It has a wide working range, high strain sensitivity and fast resistance response. It still has stable electrical performance after multiple stretching cycles. As a wearable strain sensor, the SF/TA@PPy can directly adhere to the surface of the human body without the aid of external objects. In addition, it can quickly perceive large strain body movements such as finger joints, wrist joints, elbow joints and knee joints. Changes in small-strain physiological signals such as smile, frown, cough and respiration can also be accurately captured. Body motion detection can be carried out not only in air but also in water. Based on stable electrical properties, SF/TA@PPy can achieve effective underwater information transmission by controlling body movement changes to express the Morse code. [ABSTRACT FROM AUTHOR]

Alzheimer's disease (AD), one of the leading senile disorders in the world, causes severe memory loss and cognitive impairment. To date, there is no clear cure for AD. However, early diagnosis and monitoring can help mitigate the effects of this disease. In this study, we reported a platform for diagnosing early-stage AD using microRNAs (miRNAs) in the blood as biomarkers. First, we selected an appropriate target miRNA (miR-574-5p) using AD model mice (4-month-old 5XFAD mice) and developed a hydrogel-based sensor that enabled high-sensitivity detection of the target miRNA. This hydrogel contained catalytic hairpin assembly (CHA) reaction-based probes, leading to fluorescence signal amplification without enzymes and temperature changes, at room temperature. This sensor exhibited high sensitivity and selectivity, as evidenced by its picomolar-level detection limit (limit of detection: 1.29 pM). Additionally, this sensor was evaluated using the plasma of AD patients and non-AD control to validate its clinical applicability. Finally, to use this sensor as a point-of-care-testing (POCT) diagnostic system, a portable fluorometer was developed and verified for feasibility of application. [ABSTRACT FROM AUTHOR]

In this article, the IPN hydrogels were prepared with acrylamide-modified agar (AMA), oxidized alginate (ADA) and polyacrylamide (PAM). The agar was firstly homogenously modified with acrylamide to facilitate the formation of hydrogen bonds and enhancement of the ionic conductivity of the hydrogel. The prepared IPN hydrogels showed high flexibility and conductivity. The composite hydrogel prepared with 20 wt% acrylamide, 0.25 wt% AMA and 5% ADA (hydrogel-5) possessed high toughness and can be stretched 20 times of its origin length. The hydrogel-5 also showed highest elastic modulus of 110 kPa compared with other hydrogels. The free potassium and sodium ions in acrylamide-modified agar and oxidized alginate chains endowed the hydrogel with conductive properties. The IPN hydrogels showed strain sensitive properties and can be used to detect various motion of human beings. The hydrogel still possessed high flexibility and conductivity after storing for six months. The IPN hydrogels with conductivity and high mechanical properties can be applied in various engineering fields. [Display omitted] • The hydrogel shows high toughness and conductive properties. • The hydrogel shows high stability and is sensitive to the strain. • The hydrogel can be used to monitor different motions. [ABSTRACT FROM AUTHOR]

Poly(2-hydroxyethylmethacrylate) (PHEMA) is one of the most well known synthetic hydrogels. In this study, PHEMA gel containing a charge transfer dye was synthesized using a copolymerization method. This polymer gel swelled appropriately in water with a swelling ratio ( Q) of approximately 1.0, and swelled significantly in aprotic polar solvents, such as DMF and DMSO, with Q values of >8.0, whereas it never swelled in nonpolar solvents. The fluorescence (FL) emission band of the hydrogel polymer showed a red shift exclusively in water, whereas it showed a blue shift in the other solvents tested. The emission band also shifted to shorter wavelengths with increasing proton activity (with decrease in pH) or iron cation concentration, and the FL intensity increased gradually. The FL intensity also decreased significantly with increasing absolute humidity. Graphical Abstract: A charge transfer dye-based PHEMA gel was synthesized using a copolymerization method. The FL emission band of the hydrogel polymer showed a red shift exclusively in water. The emission band shifted to shorter wavelengths with increasing in proton activity or iron cation concentration and the FL intensity gradually increased. The FL intensity deceased significantly with increasing absolute humidity.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

An elastomeric hydrogel membrane containing dissolved salt is demonstrated to be a linear strain sensor for large strains up to 25%. AC sensing circuitry allows the ionic conductor to function as a sensor without polarization effects. The homogeneous gel is shown to be lacking a relaxation or hysteresis response found in the soft composite piezoresistors that are commonly used for large strain sensing. This allows operation of the sensor with minimal post-processing and increased accuracy during real-time operation. The gauge factor of the gel sensor is shown to be near two, consistent with the observation that the strain sensing of the material is solely derived from the deformation of the bulk. The very low Young's modulus of the gel (~1 kPa) allows deformation of a fabric substrate to be followed without significantly constraining the fabric. A prototype sleeve that uses these sensors for proprioceptive sensing of joint angle motion on a robotic arm is demonstrated as a potential application. Water loss from the hydrogel due to evaporation from the surface is slow. We show that the resulting trend in the impedance of the sensor can be easily corrected by simple signal processing. [ABSTRACT FROM PUBLISHER]

[Display omitted] • The mechanical strength of SF hydrogel is reinforced by TA binary solvent. • Hydrogels show high compressibility, anti-bacterial and wide temperature tolerance. • Hydrogel sensor can precisely detect the various human motion. • The sensors exhibit superior mechanical-thermal dual sensitivity. Designing the high-performance hydrogel sensor with skin-compatible performances, such as mechanical/thermal sensitivity, excellent durability, extreme temperature-tolerance, and anti-bacterial property, is crucial for its application in skin-mimicking field. However, simultaneous realization of multifunctionalities in a single hydrogel sensor remains a great challenge. Herein, a multifunctional integrated hydrogel sensor was fabricated to imitate natural skins utilizing a robust silk-based hydrogel. This synthetic hydrogel exhibits wide-spectrum mechanical property, together with extreme temperature-tolerance, long-lasting moisture and excellent anti-bacterial property. Meanwhile, the synergistic performances of excellent mechanical performance, dynamic physical bonding interaction between networks and the thermoelectric effect of organic conductor within this hydrogel endow multiple sensory capabilities toward strain, stress and temperature with high resolution. This work provides a feasible method to construct robust hydrogel sensor with multi-functionalities, and significantly promotes the practical development of bioelectric skins to mimic natural skins with sophisticated intelligence. [ABSTRACT FROM AUTHOR]

The reversible and reproducible phase transition behavior of “stimuli-responsive” or “smart” hydrogels enables their use as materials for chemical and pH sensors in liquids. Using the quartz crystal microbalance (QCMB) technique, the principal behavior of hydrogel coated single sensors in dependence of changes in pH is presented. The general sensor properties of polyelectrolytic hydrogels, the detection possibilities and limits, and the response time of such sensors were investigated and discussed. [Copyright &y& Elsevier]

With an ageing population, hearing disorders are predicted to rise considerably in the following decades. Thus, developing a new class of artificial auditory system has been highlighted as one of the most exciting research topics for biomedical applications. Herein, a design of a biocompatible piezoresistive-based artificial hair cell sensor is presented consisting of a highly flexible and conductive polyvinyl alcohol (PVA) nanocomposite with vertical graphene nanosheets (VGNs). The bilayer hydrogel sensor demonstrates excellent performance to mimic biological hair cells, responding to acoustic stimuli in the audible range between 60 Hz to 20 kHz. The sensor output demonstrates stable mid-frequency regions (∼4-9 kHz), with the greatest sensitivity as high frequencies (∼13-20 kHz). This is somewhat akin to the mammalian auditory system, which has remarkable sensitivity and sharp tuning at high frequencies due to the "active process". This work validates the PVA/VGN sensor as a potential candidate to play a similar functional role to that of the cochlear hair cells, which also operate over a wide frequency domain in a viscous environment. Further characterizations of the sensor show that increasing the sound amplitude results in higher responses from the sensor while taking it to the depth drops the sensor outputs due to attenuation of sound in water. Meanwhile, the acoustic pressure distribution of sound waves is predicted through finite element analysis, whereby the numerical results are in perfect agreement with experimental data. This proof-of-concept work creates a platform for the future design of susceptible, flexible biomimetic sensors to closely mimic the biological cochlea.

Hydrogels flexible wearable sensors have gained widespread attention. However, there was a great challenge to prepare the flexible hydrogel sensor with rapid response, wide strain monitoring ranges, great sensitivity and self-adhesion. Therefore, the greatly sensitive and rapidly responsive strain sensor with wide monitoring range and self-adhesion was prepared by introducing sodium caseinate (SC) and reduced graphene oxide (rGO) into the polyacrylamide hydrogel due to ions and electrons provide two different ways of conducting electricity. Casein not only imparted excellent adhesion to the hydrogel, but also provided a good ionic conductive pathway. In addition, the introduction of rGO could offer an electronic pathway. Therefore, the obtained hydrogel exhibited excellent rapid response (190 ms), high sensitivity (Gauge factor, GF = 13.14) and wide strain monitoring range (0.1–1100%). As a result, SC/rGO/PAAm hydrogels could detect sensitive locomotor signals (for example speaking) and large-scale mankind movements for example knee bending). It was believed that the ion and electron conduction would provide a new mechanism to preparing greatly sensitive hydrogel strain sensor. Ionic-electronic synergistic conductive hydrogel with high sensitivity. Image 1 • Sodium caseinate endowed hydrogel with excellent adhesion for seamless contact. • The hydrogel with ultra-stretchability and fatigue resistance for strain sensors. • The synergistic effect of ion transfer and electron conduction can improve the sensor properties of the hydrogel. • Hydrogel with high sensitivity served as sensors for monitoring human motions. [ABSTRACT FROM AUTHOR]

The eye is an extension of the central nervous system (CNS) and contains aqueous humor (AH), which is a fluid rich in biomolecules secreted from intraocular tissues; thus, this organ allows for non-invasive visualization of early changes in CNS disorders. There is a growing interest in developing implantable devices, such as intraocular-lens (IOL), for specific medical uses, including intraocular monitoring. We describe a novel IOL-sensing system for detecting AH biomarkers via biocompatible enzyme-activatable fluorogenic hydrogel sensors. Matrix-metalloproteinase-9, a biomarker of degenerative CNS and eye disorders, was selected as a target. A peptide-probe-incorporated fluorogenic IOL (FIOL) was developed using diacrylamide-group-modified poly(ethyleneglycol) (PEGDAAm) biocompatible hydrogels, adjusting the hydrogel mesh size to allow selective penetration of the target while blocking non-targets, using label-free detection with semi-permanently implantable sensors, and demonstrating the clinical feasibility of FIOL through in vivo testing. This novel FIOL-based sensing system represents a promising approach for liquid biopsy of intraocular fluids. • The eye allows for early, non-invasive visualization of CNS disorders. • Implantable devices (intraocular-lens) are suitable for intraocular monitoring. • We detected AH biomarkers using enzyme-activatable fluorogenic hydrogel sensors. • FIOL was developed using PEGDAAm biocompatible hydrogels. • We proved the feasibility of FIOL-based sensing system for ocular liquid biopsy. [ABSTRACT FROM AUTHOR]