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Hydrogel-based flexible strain sensors have shown great potential in body movement tracking, early disease diagnosis, noninvasive treatment, electronic skins, and soft robotics. The good self-healing, biocompatible, sensitive and stretchable properties are the focus of hydrogel-based flexible strain sensors. Dual network (DN) hydrogels are hopeful to fabricate self-healing hydrogels with the above properties. Here, multifunctional DN hydrogels are prepared via a combination of host-guest interaction of β-cyclodextrin and ferrocene with dynamic borate ester bonds of poly(vinyl alcohol) and borax. Carbon nanotubes are used to endow the DN hydrogels with good conductivity. The obtained DN composite hydrogels possess good biocompatibility, stretchability (436%), fracture strength (41.0 KPa), self-healing property (healing efficiency of 95%), and high tensile strain sensitivity (gauge factor of 5.9). The DN composite hydrogels are used as flexible strain sensors to detect different human motions. After cutting, the healed hydrogels also can monitor human motions and have good stability. In addition, the hydrogel sensors may track the respiratory movement of a pig lung in vitro . This work exhibits new ideas and approaches to develop multifunctional self-healing hydrogels for constructing flexible strain sensors.

Multifunctional hydrogels with good stretchability and self‐healing properties have attracted considerable attention in the field of flexible electronic devices. However, hydrogels constructed by traditional chemical crosslinking usually have poor mechanical properties and lack self‐healing, adhesion, and biocompatibility, which cannot meet the requirements of flexible wearable devices. This work introduced multiple dynamic crosslinking, including borate ester bond, Schiff‐base, and host–guest interaction, into polymer networks to fabricate triple‐network gelatin/polyvinyl alcohol/carbon nanotube composite hydrogels with good self‐healing ability (healing efficiency of 95.0%), mechanical strength (54.6 kPa), stretchability (778%), biocompatibility, conductivity, adhesion, and remodeling properties. Carbon nanotubes can serve as the filler to improve the electrical conductivity of the triple‐network hydrogels. The resulting hydrogel can be made into a strain sensor with high strain sensitivity (gauge factor up to 16.0). More importantly, the strain sensor can be used to monitor various human and organ movements. Owing to its good self‐healing ability, the self‐healed hydrogel sensor also can detect human movements with almost the same electrical signal strength as the original one. This study gives novel insights for the design of multifunctional self‐healing hydrogels that have great potential in various application fields such as electronic skins, soft robots, and wearable devices. [ABSTRACT FROM AUTHOR]

With the growing demand for eco-friendly materials in wearable smart electronic devices, renewable, biocompatible, and low-cost hydrogels based on natural polymers have attracted much attention. Cellulose, as one of the renewable and degradable natural polymers, shows great potential in wearable smart electronic devices. Multifunctional conductive cellulose-based hydrogels are designed for flexible electronic devices by adding sodium carboxymethyl cellulose and MXene into polyacrylic acid networks. The multifunctional hydrogels possess excellent mechanical property (stress: 310 kPa; strain: 1127 %), toughness (206.67 KJ m−3), conductivity (1.09 ± 0.12 S m−1) and adhesion (82.19 ± 3.65 kPa). The multifunctional conductive hydrogels serve as strain sensors (Gauge Factor (GF) = 5.79, 0–700 % strain; GF = 14.0, 700–900 % strain; GF = 40.36, 900–1000 % strain; response time: 300 ms; recovery time: 200 ms) and temperature sensors (Temperature coefficient of resistance (TCR) = 2.5755 °C−1 at 35 °C- 60 °C). The sensor detects human activities with clear and steady signals. A distributed array of flexible sensors is created to measure the magnitude and distribution of pressure and a hydrogel-based flexible touch keyboard is also fabricated to recognize writing trajectories, pressures and speeds. Furthermore, a flexible hydrogel-based supercapacitor powers the LED and exhibits good cyclic stability over 15,000 charge-discharge cycles. [Display omitted] [ABSTRACT FROM AUTHOR]

Photocatalysts immobilized on hydrogels is a win-win mode, which not only improves photocatalysis but also successfully prevents catalyst loss, making it easy to separate and reuse during catalytic process. Here, ZnO-Ti 3 C 2 T X photocatalysts are loaded into the chitosan/polyacrylic acid hydrogel networks, realizing the efficiently photocatalytic degradation of norfloxacin. The chitosan-based composite hydrogel features rich functional groups and a dense pore structure, which is beneficial to antibiotic enrichment and photocatalytic degradation. The effects of different catalyst ratios, dosage, initial concentrations and pH on the degradation efficiency are investigated. The norfloxacin degradation rate constant is 0.012 min−1 and its degradation efficiency reaches up to 90 % after 240 min. Importantly, the photocatalytic composite hydrogel still retains 85 % degradation efficiency after 6 cycles. Moreover, e− plays a significant role in the degradation process. This work converts the traditional powder photocatalysts into bulk photocatalysts (photocatalytic hydrogels) to accomplish efficient degradation and rapid recycling for contaminant removal. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Preparation of hydrogel materials. • Strategies for enhancing water evaporation efficiency. • Photothermal hydrogel composites for water evaporation. • Utilization of solar energy. • Challenges and opportunities of hydrogel-based solar evaporators. Water scarcity has forced people to develop advanced water purification techniques. Solar-driven water treatment has the potential to produce clean water through utilizing solar energy to alleviate the global water crisis. The development of advanced materials with new properties contribute to enhancing the photothermal conversion efficiency. Hydrogels have emerged as encouraging material platforms for solar-driven water purification because of good water absorption, water retention and water transport. The rational design of hydrogel structures can lower the heat loss during heat transfer process to improve the photothermal conversion efficiency. This current review summarizes the recent advance on hydrogel-based solar evaporators. Hydrogel materials are generally prepared by chemical or physical cross-linking. The strategies to enhance water evaporation efficiency is also highlighted, which mainly involve the reduction of evaporation enthalpy, photothermal management of interfaces, highly interconnected hole structures and salt resistance. Hydrogels are combined with photothermal materials (including carbon-based materials, polymer materials, semiconductor materials, plasmonic materials and hybrid materials) to prepare hydrogel-based solar evaporators. Finally, challenges of hydrogel-based solar evaporators for water purification are discussed. This review inspires future efforts to design hydrogel solar evaporators for mitigating water shortage. [ABSTRACT FROM AUTHOR]

Biopolymer-derived hydrogels with low-cost and sustainable features have been considered as fascinating supported materials for metal nanoparticles. Cellulose, as the most abundant biopolymer, is a renewable raw material to prepare biopolymer-derived hydrogels for catalysis. Here, a cellulose-based hydrogel is designed to load bimetallic (AuAg, AuPd and AgPd) nanoparticles. 4-Nitrophenol reduction and Suzuki–Miyaura coupling reactions are selected to evaluate and compare the catalytic performance of the resulting bimetallic nanoparticle-loaded cellulose-based composite hydrogels. The bimetallic nanocomposite hydrogels are easy to be recycled over 10 times during the catalytic experiments and possess good applicability and generality for various substrates. The catalytic activity of bimetallic nanocomposite hydrogels was compared with recent literatures. In addition, the possible catalytic mechanism is also proposed. This work is expected to give a new insight for designing and preparing bimetallic nanoparticle-based cellulose hydrogels and proves its applicability and prospect in the catalytic field. [Display omitted] [ABSTRACT FROM AUTHOR]

Metal nanoparticle composite hydrogels (MNCHs) have attracted considerable attention in the catalytic field because hydrogels can provide appropriate microenvironment and enable catalysts to be conveniently separated and recycled. Here, a novel and well-defined gallol-functionalized hydrogel Gel-G was synthesized by ring-opening metathesis polymerization (ROMP). The resulting hydrogel Gel-G can be used as template to in situ prepare catalytically active gold nanoparticles hydrogel (AuNPs-Gel-G) and sliver nanoparticles hydrogel (AgNPs-Gel-G) without use of any external reductant based on the strong reducibility and coating property of the gallol moiety. The two MNCHs were well characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). Both AuNPs-Gel-G and AgNPs-Gel-G exhibited high catalytic activity for the 4-nitrophenol (4-NP) reduction to 4-aminophenol (4-AP), and can be easily separated and reused at least five times. [Display omitted] • Gallol-containing polymer hydrogels are synthesized by ROMP. • Gallol-containing polymer hydrogels are used for in situ fabrication of AuNPs and AgNPs composites. • AuNPs and AgNPs hydrogel composites exhibit good catalytic activity and reusability for 4-nitrophenol reduction. [ABSTRACT FROM AUTHOR]

Hydrogels with stimuli-responsive behavior have been of great interest to researchers from many fields, such as biotechnology, drug delivery, and sensors, owing to their "smart" change in properties under external stimuli such as temperature, pH, pressure, redox, magnetic and electric fields. In this work, using the third generation Grubbs' catalyst as the initiator, both temperature and β-cyclodextrin (β-CD) stimuli-responsive ferrocenyl hydrogels were prepared with different proportions of hydrophilic norbornene monomer NTEG , ferrocene-containing monomer NFc and crosslinker 1 by using one-pot ROMP (ring-opening metathesis polymerization) reaction. Based on the mechanical and rheological tests, it was found that NFc not only played a crucial role in maintaining the structure of ferrocenyl hydrogels, but also enhanced their physical strength. Comparatively, the increase of the NTEG content in the hydrogel system can improve the toughness of the resulting hydrogels. Furthermore, Results of the swelling ratio test demonstrated that these hydrogels possessed temperature-responsiveness, in which the NTEG block, which contained dendritic oligoethylene glycol structures in the side chain, played a decisive role. Simultaneously, it is worth noting that these ferrocenyl hydrogels also exhibited responsive behavior to β-CD solution due to the host-guest interaction of ferrocene unit with β-CD macromolecule. Therefore, we believe that this study provides a new idea for the design of covalently crosslinked ferrocenyl stimuli-responsive smart hydrogels. Thermo/β-cyclodextrin-responsive ferrocenyl hydrogels were constructed by the ROMP reaction of a hydrophilic norbornene monomer containing dendritic oligoethylene glycol structures in the side chain and a hydrophobic side-chain ferrocene-containing monomer, and their mechanical and swelling properties could regulated by the hydrophilic-hydrophobic ratio, environmental temperature and the addition of β-cyclodextrin. [Display omitted] • Hydrogels with pendent ferrocene and dendritic oligoethylene glycol were prepared. • Ferrocenyl hydrogels were covalently cross-linked through the ROMP reaction. • Ferrocenyl hydrogels had excellent toughness and compression resilience. • Mechanical/swelling behavior of hydrogels can be regulated by changing feed ratio. • Ferrocenyl hydrogels possessed dual-responsiveness to temperature and β-CD. [ABSTRACT FROM AUTHOR]

This review focuses on various classes of ferrocene-containing hydrogels via covalently cross-linking and supramolecular strategies and analyzes their gelling mechanisms, characteristic structures, properties, and functional applications. • Ferrocene hydrogels fabricated by covalent cross-linking. • Ferrocene hydrogels fabricated by supramolecular assemblies. • Ferrocene hydrogels as biomedical, shape-memory and self-healing materials. • Ferrocene hydrogels present fascinating properties change upon redox switch. • Ferrocene microgels composed of cross-linked three-dimensional polymer networks. Stimuli-responsive hydrogels have lately attracted a lot of attention in the chemistry and material fields because of the "smart" change of their properties under outside stimuli including light, temperature, electric or magnetic field, pH, chemicals, shear stress, and redox reagents. Ferrocenyl (Fc) is often employed as a redox-responsive building unit due to its properties of chemical and electrochemical redox reversibility. This property involves reversible change between hydrophobicity and hydrophilicity, which endows hydrogels with unexpected features. Also, Fc derivatives are used as guest molecules featuring host–guest interactions with macrocyclic host molecules, mainly including cyclodextrins and pillararenes, commonly leading to the formation of supramolecular hydrogels with shape-memory, self-healing and sol–gel transition performances. This review focuses on the fabrication of various kinds of Fc-containing hydrogels and describes their gelling mechanisms, characteristic structures and properties, as well as functional applications. The review is divided into covalently cross-linked hydrogels and supramolecular cross-linked hydrogels. Furthermore, Fc-containing microgels constructed by chemically cross-linked three-dimensional polymer networks that are related to traditional hydrogels are also discussed. Fc-containing hydrogels and microgels are becoming more and more important as advanced functional materials, especially biomedical, shape-memory and self-healing materials. [ABSTRACT FROM AUTHOR]