A facile solvent-exchange strategy is devised to fabricate anti-drying, self-healing and transparent organohydrogels for stretchable humidity sensing applications.

Ionic hydrogels, a class of intrinsically stretchable and conductive materials, are widely used in soft electronics. However, the easy freezing and drying of water-based hydrogels significantly limit their long-term stability. Here, a facile solvent-replacement strategy is developed to fabricate ethylene glycol (Eg)/glycerol (Gl)-water binary antifreezing and antidrying organohydrogels for ultrastretchable and sensitive strain sensing within a wide temperature range. Because of the ready formation of strong hydrogen bonds between Eg/Gl and water molecules, the organohydrogels gain exceptional freezing and drying tolerance with retained deformability, conductivity, and self-healing ability even stay at extreme temperature for a long time. Thus, the fabricated strain sensor displays a gauge factor of 6, which is much higher than previously reported values for hydrogel-based strain sensors. Furthermore, the strain sensor exhibits a relatively wide strain range (0.5−950%) even at −18 °C. Various human motions with different strain levels are monitored by the strain sensor with good stability and repeatability from −18 to 25 °C. The organohydrogels maintained the strain sensing capability when exposed to ambient air for nine months. This work provides new insight into the fabrication of stable, ultrastretchable, and ultrasensitive strain sensors using chemically modified organohydrogel for emerging wearable electronics.

Conductive hydrogels have drawn significant attention in the field of stretchable/wearable sensors due to their intrinsic stretchability, tunable conductivity, biocompatibility, multistimuli sensitivity, and self-healing ability. Recent advancements in hydrogel- and organohydrogel-based sensors, including a novel sensing mechanism, outstanding performance, and broad application scenarios, suggest the great potential of hydrogels for stretchable electronics. However, a systematic summary of hydrogel- and organohydrogel-based sensors in terms of their working principles, unique properties, and promising applications is still lacking. In this spotlight, we present recent advances in hydrogel- and organohydrogel-based stretchable sensors with four main sections: improved stability of hydrogels, fabrication and characterization of organohydrogel, working principles, and performance of different types of sensors. We particularly highlight our recent work on ultrastretchable and high-performance strain, temperature, humidity, and gas sensors based on polyacrylamide/carrageenan double network hydrogel and ethylene glycol/glycerol modified organohydrogels obtained via a facile solvent displacement strategy. The organohydrogels display higher stability (drying and freezing tolerances) and sensing performances than corresponding hydrogels. The sensing mechanisms, key factors influencing the performance, and application prospects of these sensors are revealed. Especially, we find that the hindering effect of polymer networks on the ionic transport is one of the key mechanisms applicable for all four of these kinds of sensors.

wileyonlinelibrary.comhydrogels exhibit slow macroscale response with a magnitude of minutes to hours. [10][11][12][13] Meanwhile, they are typically mechanically weak or brittle, [ 14,15 ] resulting in unstable performance after cycles of stimulation. In addition, most of these hydrogels possess isotropic porous structures, showing size changes evenly, making desirable locomotion (e.g., bending, twisting, and folding) diffi cult to achieve. [ 16,17 ] Modulation of pores size and their distributions is essential for manipulating hydrogels properties. Several strategies including gas foaming, [ 18 ] fi ber bonding, [ 19 ] and porogen leaching [ 20 ] were developed to fabricate homo geneous macropore sized hydrogels for rapid responses. However, the orientation responses of such materials were limited, causing diffi culties in anisotropic locomotion. Moreover, mechanical strength of these hydrogels was relatively weak due to the fragile macrosized pore structures. On the other hand, electrophoresis-assisted porogen leaching and hydrogel layering methods [21][22][23][24] have been developed to produce responsive hydrogels with stepwisedistributed pore structures to enable their anisotropic responsive capabilities. However, these hydrogels have some adverse properties including less pore interconnectivity, decelerated mass transport, and being prone to delamination, causing their slow response to stimuli and poor mechanical properties. To date, synthesis of hydrogels with simultaneously rapid thermal response kinetics, robust mechanical strength, and desirable anisotropic locomotion remains an unsolved challenge.In this study, we presented a heterobifunctional crosslinker enabled hydrothermal process, forming hydrogels with gradient porous structure to address these issues. The hydrothermal synthesis is performed in closed systems of relatively high temperatures and pressures, in which only water is used as the reactive medium. At elevated temperatures, hydrothermal process can prompt a variety of chemical reactions such as vinyl polymerizations and intermolecular dehydration. [25][26][27] N -isopropylacrylamide (NIPAM), a well-known thermo-responsive material bearing two highly reactive double bonds, [ 28,29 ] was used as monomer. 4-hydroxybutyl acrylate (4HBA), an acrylic ester possessing a reactive double bond and a less reactive hydroxyl group at either end of the molecule, was innovatively applied Programmable locomotion of responsive hydrogels has gained increasing attention for potential applications in soft robotics, microfl uidic components, actuators, and artifi cial muscle. Modulation of hydrogel pore structures is essential for tailoring their mechanical strength, response speeds, and motion behaviors. Conventional methods forming hydrogels with homogeneous or stepwise-distributed pore structures are limited by the required compromise to simultaneously optimize these aspects. Here, a heterobifunctional crosslinker enabled hydrothermal process is introduced to synthesize responsive hydrogels with well-defi...

limits the diffusion of chemical species and their interactions with active sites in MOFs. [ 10 ] One of successful strategy from zeolites, silica, and carbon is the fabrication of mesopore structure which has expanded a large variety of potential and existing commercial applications. [1][2][3] Hence, it is worthwhile to develop methods to fabricate MOFs with mesopores so as to enhance the molecular diffusion while preserving their molecular sieve properties.To date, two major synthetic strategies have been explored to synthesize mesoporous-MOFs (meso-MOFs). [ 11 ] One is through ligand extension, either to increase the length of organic ligands [ 12 ] or to use bulky organic scaffolds [ 13 ] to form mesopores inside MOFs. In this case, the largest pore size reported is 9.8 nm in MOF-74 by increasing the length of organic linker to 5 nm. [ 14 ] Besides the diffi culties in complex ligands synthesis, interpenetration, disintegration, and instability of frameworks almost inevitably occur in MOFs with extended organic linkers, which prevent this functionalization method from being generally adopted in the formation of meso-MOFs. Another approach, the surfactanttemplate method, [ 4d , 15 ] has been introduced to increase the pore size in MOFs. For example, the Zhou group has successfully used cetyltrimethylammonium bromide (CTAB) as soft template to build meso-MOFs. [ 16 ] In this system, surfactant mole cules fi rst self-assembled into micelles serving as a soft template for MOFs growth and were subsequently removed to generate mesopores. The pore diameter of the resulting MOFs could be tuned from 3.8 to 31.0 nm. Nevertheless, as is well known, small molecular micelles are usually unstable under the synthesis conditions of most MOFs, so that only a few series of MOFs (such as carboxylic acid ligands) can be obtained by the surfactant-template method. Recently, some new methods have been successively developed to prepare the meso-MOFs, such as the gelation process, [ 17 ] and switchable solvent. [ 18 ] Moreover, the above methods are suitable for preparation of intrinsic meso-MOFs, but lack of control over the shape, position, and space distribution of mesopores in MOFs makes it hard to meet the demand for the growing applications of MOFs. It is well known that the potential applications of MOFs can be further developed and extended by encapsulating various nanoparticles (NPs) within the frameworks matrix so that the functionalized MOFs can exhibit the novel chemical and physical properties endowed by NPs. [ 7b , 19 ] Thus, to the best of our knowledge, general and versatile strategies of synthesizing functionalized MOFs with size-, shape-, and space-distribution-controlled mesopores have been rarely reported, in spite of the need and the signifi cance in application of functionalized meso-MOFs.Herein, we report a facile strategy of crafting mesopores inside MOFs through encapsulation of NPs followed by etching. Especially, the mesopore morphology, hierarchical structure, and space Porous materials, such as sili...

Controllable integration of nanoparticles (NPs) and metal−organic frameworks (MOFs) is crucial for expanding the applications of MOF-based materials. In this study, we demonstrate the facile encapsulation of presynthesized NPs into carboxylic acid based MOFs using NPs@metal oxide core−shell nanostructures as the self-template. The shell dissolved gradually in the mildly acidic growth solution created by dissociation of the ligands and thus directing the growth of the MOF crystals by providing metal ions. With protection of the metal oxide shell, various NPs (Au NPs, Au nanorods, Pd nanocubes, and Pt-on-Au dendritic NPs) could be encapsulated easily without being aggregated or dissolved in the reaction mixture. Importantly, instead of forming the exact replicate of the self-template, the obtained NP@MOF heterostructures exhibited a yolk−shell morphology with a central cavity and a certain degree of mesoporosity. The formation of the well-defined yolk−shell structure was demonstrated to be dependent on both the choice of the solvent and the dissolution behavior of the metal oxide shell. Finally, the obtained heterostructures were employed for heterogeneous catalysis, in which the size selectivity of the MOF shell was perfectly retained.

An ultrastretchable thermistor that combines intrinsic stretchability, thermal sensitivity, transparency, and self-healing capability is fabricated. It is found the polyacrylamide/carrageenan double network (DN) hydrogel is highly sensitive to temperature and therefore can be exploited as a novel channel material for a thermistor. This thermistor can be stretched from 0 to 330% strain with the sensitivity as high as 2.6%/°C at extreme 200% strain. Noticeably, the mechanical, electrical, and thermal sensing properties of the DN hydrogel can be self-healed, analogous to the self-healing capability of human skin. The large mechanical deformations, such as flexion and twist with large angles, do not affect the thermal sensitivity. Good flexibility enables the thermistor to be attached on nonplanar curvilinear surfaces for practical temperature detection. Remarkably, the thermal sensitivity can be improved by introducing mechanical strain, making the sensitivity programmable. This thermistor with tunable sensitivity is advantageous over traditional rigid thermistors that lack flexibility in adjusting their sensitivity. In addition to superior sensitivity and stretchability compared with traditional thermistors, this DN hydrogel-based thermistor provides additional advantages of good transparency and self-healing ability, enabling it to be potentially integrated in soft robots to grasp real world information for guiding their actions.

Low-cost, one-step, and hydrothermal synthesized 3D reduced graphene oxide hydrogel (RGOH) is exploited to fabricate a high performance NO2 and NH3 sensor with an integrated microheater. The sensor can experimentally detect NO2 and NH3 at low concentrations of 200 ppb and 20 ppm, respectively, at room temperature. In addition to accelerating the signal recovery rate by elevating the local silicon substrate temperature, the microheater is exploited for the first time to improve the selectivity of NO2 sensing. Specifically, the sensor response from NH3 can be effectively suppressed by a locally increased temperature, while the sensitivity of detecting NO2 is not significantly affected. This leads to good discrimination between NO2 and NH3. This strategy paves a new avenue to improve the selectivity of gas sensing by using the microheater to raise substrate temperature.