Extreme Temperature-Tolerant Conductive Gel with Antibacterial Activity for Flexible Dual-Response Sensors

Stimuli-responsive mesoporous silica nanoplatforms for smart antibacterial therapies: From single to combination strategies

The demand for new antibacterial therapies is urgent and crucial in the clinical setting because of the growing degree of antibiotic resistance and the limits of conventional antibacterial therapies. Stimuli- responsive nanoplatforms, are sensitive to endogenous or exogenous stimulus (pH, temperature, light, and magnetic fields, etc.) which activate cargo release locally and on-demand, hold great potential in developing next generation personalized precision medicine. For instance, pH-sensitive nanoplatforms can selectively release antibacterial agents in the acidic environment of infection sites. To achieve the stimuli-responsive delivery, mesoporous silica nanoplatforms (MSNs) have demonstrated as prospective candidates for efficient cargo loading and controlled release through strategies such as tunable pore engineering, versatile surface modification/coating, and tailored framework composition. Furthermore, aiming for more precise delivery of MSNs, current research interests are increasingly shifting from single-stimuli antibacterial strategy to integrated strategy that combine multiple-stimulus. In this review, we briefly discuss the microenvironment of bacterial infections and provide a comprehensive summary of current stimuli-responsive strategies, and associated materials design principles of stimuli-responsive mesoporous silica-based smart nanoplatforms (SRMSNs). Additionally, integrative antibacterial strategies with synergistic effects, combining chemodynamic, photodynamic, photothermal, sonodynamic and gas therapies, have also been elaborated. Present research advances and limitations of SRMSNs-based antibacterial therapies, such as limited biodegradability and potential cytotoxicity, have been overviewed with future outlooks presented. This review aims to inspire and guide future research in developing novel antibacterial strategies with integrative solutions.

Ultra-stretchable, adhesive, fatigue resistance, and anti-freezing conductive hydrogel based on gelatin/guar gum and liquid metal for dual-sensory flexible sensor and all-in-one supercapacitors

Benefiting from the tissue-like mechanical properties, conductive hydrogels have emerged as a promising candidate for manufacturing wearable electronics. However, the high water content within hydrogels will inevitably freeze at subzero temperature, causing a degradation or loss of functionality, which severely prevent their practical application in wearable electronics. Herein, an anti-freezing hydrogel integrating high conductivity, superior stretchability, and robust adhesion was fabricated by dissolving choline chloride and gallium in gelatin/guar gum network using borax as the cross-linker. Based on the synergistic effect of dynamic borate ester bonds and hydrogen bonds, the hydrogel exhibited rapid self-healing property and excellent fatigue resistance. Profiting from these fascinating characteristics, the hydrogel was assembled as strain sensor to precisely detect various human activities with high strain sensitivity and fast response time. Meanwhile, the hydrogel was demonstrated high sensitivity and rapid response to temperature, which can be used as thermal sensor to monitor temperature. Moreover, the conductive hydrogel was encapsulated into supercapacitors with high areal capacitance and favorable cycle stability. Importantly, the flexible sensor and supercapacitors still maintain stable sensing performance and good electrochemical performance even at subzero temperature. Therefore, our work broaden hydrogels application in intelligent wearable devices and energy storage in extreme environments.

Triple-Responsive, Multimodal, Visual Electronic Skin toward All-in-One Health Management for Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) is one of the most common metabolic disorders during pregnancy, leading to serious complications for pregnant women and a threat to life safety of infants. Therefore, it is particularly important to establish a multipurpose monitoring pathway to important physiological indicators of pregnant women. In this work, three kinds of double network hydrogels are prepared with poly(vinyl alcohol) (PVA), borax, and cellulose ethers with varying substituents of methyl (methyl cellulose, MC), hydroxypropyl (hydroxypropyl cellulose, HPC), or both (hydroxypropyl methyl cellulose, HPMC), respectively. The corresponding toughness (143.9, 102.3, and 135.9 kJ cm-3) and conductivity (0.69, 0.45, and 0.51 S m-1) of the hydrogels demonstrate that PB-MC was endowed with the prominent performance. Molecular dynamics simulations further revealed the essence that hydrogen bond interactions between PVA and cellulose ethers play a critical role in regulating the structure and properties of hydrogels. Thermochromic capsule powders (TCPs) were subsequently doped in to achieve a composite hydrogel (TCPs@PB-MC) to indicate the change in human body temperature. Furthermore, the process of the TCPs@PB-MC response to glucose, pH, and temperature was tracked in-depth through the electrochemical window. This work provides a novel strategy for all-in-one health management of GDM.

Nacre-Mimetic Structure Multifunctional Ion-Conductive Hydrogel Strain Sensors with Ultrastretchability, High Sensitivity, and Excellent Adhesive Properties

Recently, conductive hydrogels have emerged as promising materials for smart, wearable devices. However, limited mechanical properties and low sensitivity greatly restrict their lifespan. Based on the design of biomimetic-layered structure, the conductive hydrogels with nacre-mimetic structure were prepared by using layered acrylic bentonite (AABT) and phytic acid (PA) as multifunctional "brick" and "mortar" units. Among them, the unique rigid cyclic multihydroxyl structure of the "organic mortar" PA preserves both ultrastretchability (4050.02%) and high stress (563.20 kPa) of the hydrogel, which far exceeds most of the reported articles. Because of the synergistic effect of AABT and PA, the hydrogel exhibits an excellent adhesive strength (87.74 kPa). The role of AABT in the adhesive properties of hydrogels is proposed for the first time, and a general strategy for improving the adhesive properties of hydrogels by using AABT is demonstrated. Furthermore, AABT provides ion channels and PA ionizes abundant H+, conferring a high gauge factor (GF = 14.95) and excellent antimicrobial properties to the hydrogel. Also, inspired by fruit batteries, simple self-powered flexible sensors were developed. Consequently, this study provides knowledge for functional bentonite filler modified hydrogel, and the prepared multifunctional ionic conductive hydrogel shows great application potential in the field of intelligent wearable devices.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Multifunctional Conductive Double-Network Hydrogel Sensors for Multiscale Motion Detection and Temperature Monitoring

As typical soft materials, hydrogels have demonstrated great potential for the fabrication of flexible sensors due to their highly compatible elastic modulus with human skin, prominent flexibility, and biocompatible three-dimensional network structure. However, the practical application of wearable hydrogel sensors is significantly constrained because of weak adhesion, limited stretchability, and poor self-healing properties of traditional hydrogels. Herein, a multifunctional sodium hyaluronate (SH)/borax (B)/gelatin (G) double-cross-linked conductive hydrogel (SBG) was designed and constructed through a simple one-pot blending strategy with SH and gelatin as the gel matrix and borax as the dynamic cross-linker. The obtained SBG hydrogels exhibited a moderate tensile strength of 25.3 kPa at a large elongation of 760%, high interfacial toughness (106.5 kJ m-3), strong adhesion (28 kPa to paper), and satisfactory conductivity (224.5 mS/m). In particular, the dynamic cross-linking between SH, gelatin, and borax via borate ester bonds and hydrogen bonds between SH and gelatin chain endowed the SBG hydrogels with good fatigue resistance (>300 cycles), rapid self-healing performance (HE (healing efficiency) ∼97.03%), and excellent repeatable adhesion. The flexible wearable sensor assembled with SBG hydrogels demonstrated desirable strain sensing performance with a competitive gauge factor and exceptional stability, which enabled it to detect and distinguish various multiscale human motions and physiological signals. Furthermore, the flexible sensor is capable of precisely perceiving temperature variation with a high thermal sensitivity (1.685% °C-1). As a result, the wearable sensor displayed dual sensory performance for temperature and strain deformation. It is envisioned that the integration of strain sensors and thermal sensors provide a novel and convenient strategy for the next generation of multisensory wearable electronics and lay a solid foundation for their application in electronic skin and soft actuators.

Recent advances in wearable iontronic sensors for healthcare applications

Iontronic sensors have garnered significant attention as wearable sensors due to their exceptional mechanical performance and the ability to maintain electrical performance under various mechanical stimuli. Iontronic sensors can respond to stimuli like mechanical stimuli, humidity, and temperature, which has led to exploration of their potential as versatile sensors. Here, a comprehensive review of the recent researches and developments on several types of iontronic sensors (e.g., pressure, strain, humidity, temperature, and multi-modal sensors), in terms of their sensing principles, constituent materials, and their healthcare-related applications is provided. The strategies for improving the sensing performance and environmental stability of iontronic sensors through various innovative ionic materials and structural designs are reviewed. This review also provides the healthcare applications of iontronic sensors that have gained increased feasibility and broader applicability due to the improved sensing performance. Lastly, outlook section discusses the current challenges and the future direction in terms of the applicability of the iontronic sensors to the healthcare.

Intrinsically Stretchable Conductive Self-Healable Organogels for Strain, Pressure, Temperature, and Humidity Sensing

Intrinsically stretchable conductive self-healable organogels containing poly(lipoic acid), Al3+ ion, tannic acid, and reduced graphene oxide are produced in this report. These noncovalent networks interlocked through physical (hydrogen and coordination) bonds offered high stretchabilities and mechanical strengths as well as fast self-healing behaviors. The optimum organogel-based sensor showed outstanding pressure sensitivities (0.94 kPa-1 up to 10 and 1.07 kPa-1 for 10-50 kPa) and high strain responses (corresponding gauge factors of 1.1 and 0.4 for 0-50 and 50-100% stretching ratios). This organogel also revealed high stabilities at ambient atmosphere due to the presence of binary solvents of dimethyl sulfoxide and glycerol. Additionally, this stretchable thermistor displayed remarkable two-stage sensitivities of -2.6 and -0.4%/°C ranging over 0-30 and 30-80 °C, respectively. Besides, the signal variations of water droplet addition and removal with different temperatures were recorded by the organogel sensor to elucidate the practical applicabilities as a temperature sensor. Moreover, the organogel was utilized to demonstrate humidity sensing, where individual sensitivities of 0.89 and 0.55 were obtained in the respective relative humidity ranges of 10-30 and 40-90%. In the meanwhile, the sensor device illustrated distinct humidity signals during respiration monitoring of nose and mouth breathing detection. Accordingly, these quad-functional sensor applications in strain, pressure, temperature, and humidity detection enable this gel to act as a promising material for future multifunctional flexible electronics.

Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement

In recent years, conductive hydrogels have received increasing attention as wearable electronics due to the electrochemical properties of conductive polymers combined with the softness of hydrogels. However, conventional hydrogels are complicated to prepare, require high temperature or UV radiation to trigger monomer polymerization, and are frozen at low temperatures, which seriously hinder the application of flexible wearable devices. In this paper, a conductive sensor integrating mechanical properties, adhesion, UV shielding, anti-dehydration, and anti-freeze was prepared based on Ca²⁺-initiated radical polymerization at room temperature using the synergy of sodium lignosulfonate, acrylamide (AM), and calcium chloride (CaCl₂). Metal ions can activate ammonium persulfate to generate free radicals that allow rapid gelation of AM monomers at room temperature without external stimuli. Due to ionic cross-linking and non-covalent interaction, the hydrogels have good tensile properties (1153% elongation and 168 kPa tensile strength), high toughness (758 KJ·m^-3), excellent adhesive properties (48.5 kPa), high ionic conductivity (7.2 mS·cm^-1), and UV resistance (94.4%). CaCl₂ can inhibit ice nucleation, so that the hydrogels have anti-dehydration and frost resistance properties and even at -80 °C can maintain flexibility, high conductivity, and adhesion. Assembled into a flexible sensor, it can sense various large and small movements such as compression, bending, and talking, which is a flexible sensing material with wide application prospects.

Rigid, eco-friendly and superhydrophobic SiO2-Polyvinyl alcohol composite sponge for durable oil remediation

Development of durable and eco-friendly adsorbents for oil remediation is in great demands. However, most of adsorbents were designed to pursue large capabilities while ignored their strength after adsorbing oil, which might cause secondary oil spilling during complex salvage process. Herein, an eco-friendly and superhydrophobic SiO₂-modified polyvinyl alcohol composite (H-SiO₂-G-PVA) sponge with extraordinary rigid structure after oil adsorption is designed for durable oil remediation. Through a two-step hydrolysis-condensation process including deposition of silica microparticles and introduction of hexadecyltrimethoxysilane (HDTMS), a superhydrophobic H-SiO₂-G-PVA sponge has been successfully constructed. The sponge presents stable superhydrophobicity in various complex environments，therefore it efficiently adsorbs oil from water (up to 6 g g^-1) and separate surfactant-stabilized water/oil emulsion with high efficiency (>99%). Noticeably, the H-SiO₂-G-PVA sponge maintains tough strength (3.5 MPa) after oil adsorption, which ideally overcomes secondary oil spilling problem and endows the sponge with excellent recycling performances (>20 cycles). Meanwhile, the excellent biocompatibility of the sponge (high cell viability of 91.85%) ensures the potential for practical applications. This rigid, eco-friendly oil-adsorbing sponge that achieves stable superhydrophobicity and recyclability, fulfills the application needs for durable oil remediation.

Flexible porous Gelatin/Polypyrrole/Reduction graphene oxide organohydrogel for wearable electronics

Conductive hydrogel-based flexible electronics have attracted immense interest in wearable sensor, soft robot and human-machine interface. However, the application of hydrogels in flexible electronics is limited by the deterioration of mechanical and electrical properties due to freezing at low temperature and desiccation after long-term use. Meanwhile, flexible electronics based on hydrogel are usually not breathable, which has a great impact on wearing comfort and signal stability in long-term sensing. In this work, an adjustable porous gelatin/polypyrrole/reduction graphene oxide (Gel/PPy/rGO) organohydrogel with high breathability (14 g∙cm^-2∙h^-1), conductivity (5.25 S/m), mechanical flexibility, anti-freezing and long-term stability is prepared via the combination method of biological fermentation and salt-out toughening crosslinking. The sensor fabricated from the prepared porous organohydrogel exhibits excellent sensing sensitivity, fast response ability, and good endurance, which monitors both weak and intense human activities effectively like finger bending, elbow bending, walking and running, and tiny pulse beating. A pressure sensor array prepared from the porous organohydrogel detects pressure variation in 2D sensitively. Furthermore, the porous organohydrogel is utilized as flexible electrodes for the accurate collection and recognition of human physiological signals (EMG, ECG) and as an interface between human and machine.

Temperature-Responsive Ionic Conductive Hydrogel for Strain and Temperature Sensors

Flexible wearable devices have achieved remarkable applications in health monitoring because of the advantages of multisignal collecting and real-time wireless transmission of information. However, the integration of bulky sensing elements and rigid metal circuit components in traditional wearable devices may lead to a mechanical and signal-conducting mismatch between wearable devices and biological tissues, thus restricting their wide applications in the human body. The excellent mechanical properties, conductivity, and high tissue resemblance of conductive hydrogel contribute to its application in flexible electronic sensors to monitor human health. In this work, a dual-network, temperature-responsive ionic conductive hydrogel with excellent stretchability, fast temperature responsiveness, and good conductivity was developed by introducing a polyvinylpyrrolidone (PVP)/ tannic acid (TA)/ Fe3+ cross-linked network into the N,N-methylene diacrylamide (MBAA) cross-linked poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAAm-co-AM)) network. Furthermore, the introduction of the PVP/TA/Fe3+ cross-linked network endowed the hydrogel with excellent stretchability and conductivity. By adjusting the molar ratio of TA and Fe3+ to 3:5, a hydrogel with a maximal stretching ratio of 720% and sensitive strain response (GF = 3.61) was achieved, showing a promising application in wearable strain sensors to monitor both large and fine human motions. Moreover, by introducing PNIPAAm with a lower critical solution temperature (LCST), the hydrogel may be used to monitor the environmental temperature through the temperature-conductivity responsiveness, which can be applied as a wearable temperature sensor to detect fever or tissue hyperthermia in the human body.

Efficient uranium adsorbent with antimicrobial function constructed by grafting amidoxime groups on ZIF-90 via malononitrile intermediate

Herein, a dual-function Zeolitic Imidazole Frameworks (ZIFs) ZIF-90 grafted with malononitrile by Knoevenagel reaction and following with an amidoximation reaction to form an efficient U (VI) adsorbent (ZIF-90-AO). The strong chelation power of amidoxime groups (AO) with uranium and ZIF-90's mesoporous structure afforded ZIF-90-AO high maximum uranium adsorption capacity of 468.3 mg/g (pH = 5). In addition, the factors affecting uranium adsorption process were investigated by a batch of adsorption tests under different adsorption conditions. ZIF-90-AO displayed good selectivity to UO₂²⁺ in the solution containing multiple co-existing ions and good regeneration property. More importantly, ZIF-90-AO showed excellent antimicrobial property against both E. coli and S. aureus. Therefore, ZIF-90-AO is a U-adsorbent with great application value for removing U (VI) from wastewater due to the high U (VI) adsorption capacity in weak acid condition and good anti-biofouling properties.

Flexible Zinc-Air Battery with High Energy Efficiency and Freezing Tolerance Enabled by DMSO-Based Organohydrogel Electrolyte

With the emergence of various flexible electronics, the flexible zinc-air battery (ZAB) is considered a promising energy source with low cost, high energy density, and safety. However, gel electrolytes that improve the freezing tolerance and energy efficiency of ZABs are rarely explored. Herein, an organohydrogel electrolyte (OHE) is fabricated by soaking poly(2-acrylamido-2-methylpropanesulfonic acid)/polyacrylamide (PAMPS/PAAm) double-network hydrogel in aqueous KOH electrolyte with dimethyl sulfoxide (DMSO) additive. The prepared OHE exhibits high mechanical strength and excellent ionic conductivity. In addition, the introduction of DMSO effectively improves freezing tolerance and electrochemical performance especially in energy efficiency of ZABs due to that DMSO can break the hydrogen bonds between water molecules and alter the path of the conventional oxygen evolution reaction in ZAB simultaneously. Compared with the control hydrogel electrolyte, the optimized OHE enables flexible ZABs to not only exhibit an exceptionally low charge voltage of 1.63 V, high energy efficiency of 74.2%, and long cycling life of 177 cycles, but also to operate with an excellent specific capacity of 562 mAh g^-1 and energy density of 523.4 Wh kg^-1 at -40 °C. Moreover, the obtained flexible ZABs keep a stable output under deformations and extreme low temperature, manifesting a great potential for functional wearable devices.

Multifunctional conductive hydrogels and their applications as smart wearable devices

Recently, hydrogel-based conductive materials and their applications as smart wearable devices have been paid tremendous attention due to their high stretchability, flexibility, and excellent biocompatibility. Compared with single functional conductive hydrogels, multifunctional conductive hydrogels are more advantageous to match various demands for practical applications. This review focuses on multifunctional conductive hydrogels applied for smart wearable devices. Representative strategies for conduction of hydrogels are discussed firstly: (1) electronic conduction based on the conductive fillers and (2) ionic conduction based on charged ions. Then, the common and intensive research on multiple functionalities of conductive hydrogels, such as mechanical properties, conductive and sensory properties, anti-freezing and moisturizing properties, and adhesion and self-healing properties is presented. The applications of multifunctional conductive hydrogels such as in human motion sensors, sensory skins, and personal healthcare diagnosis are provided in the third part. Finally, we offer our perspective on open challenges and future areas of interest for multifunctional conductive hydrogels used as smart wearable devices.