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water splitting (2H 2 O → 2H 2 + O 2 ) comprises two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Theoretically, the decomposition voltage of water splitting is 1.23 V. [4] Nevertheless, the actual voltage is greater than the theoretical voltage of water dissociation due to the large activation energy. [5] To accelerate the sluggish HER and OER kinetics, efficient electrocatalysts are required to reduce kinetic energy barriers. Currently, the most efficient electrocatalysts toward water splitting are precious metal-based catalysts such as Pt/C for HER and RuO 2 or IrO 2 for OER. [6] Unfortunately, the large-scale application is hindered by the high price and scarcity of noble metals. Therefore, it is urgent to seek highly active and inexpensive electrocatalysts.Metal-organic frameworks (MOFs) are an emerging class of porous crystalline materials constructed by metal ions or clusters and organic ligands. [7][8][9] Due to their unique merits of large porosities, diversified structures, and designable compositions, MOFs have gained extensive attention in various applications, such as chemical sensors, [10] gas absorption, [11] and energy conversion/storage. [12][13][14][15][16] From an electrochemical perspective, MOFs can be perceived as a spatial architecture, where discrete functional units can be designed to lie in close proximity to facilitate bond breaking/forming reactions. [17] Generally, active sites and conductivity are considered as the key factors affecting the electrochemical performance of HER and OER catalysts. Despite abundant intrinsic molecular metal sites, large size and poor conductivity (10 −10 S cm −1 ) of bulk MOFs restrict the full use of their distinct advantages and lead to poor electrochemical performance. [18] To improve electrocatalytic activity of MOFs, they can be used as sacrificial precursors or templates to fabricate various conductive carbides, [19] oxides/hydroxides, [20] phosphides, [21] chalcogenides, [22] single-atom catalysts [23] or pure carbon materials with rich morphologies and sizes. [24,25] However, the high temperatures calcination inevitably leads to the loss of intrinsic active sites and long-range orders in MOFs. Therefore, improving the catalytic properties of pristine MOFs for highly efficient electrocatalysts is quite appealing.Compared with bulk MOFs, 2D MOFs emerge as a category of promising electrocatalysts toward HER or OER due to their unique characteristics, including enlarged surface areas, rapid mass transport, and enhanced conductivities (Figure 1). [26,27] Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemi...

Ideal conductive hydrogels for flexible, wearable strain sensors should be tough, highly resilient, adhesive, and anti-freezing. However, such hydrogels are difficult to design. Herein, a multifunctional macromolecular crosslinker (MC) based on poly(hydroxyethyl-L-glutamine) was designed and used to synthesize the hydrogels. Cross-linking with the MC leads to a reduced inhomogeneity of the gel network. Therefore, the mechanical properties of the gels are significantly improved compared with the ordinary hydrogels cross-linked with the conventional cross-linker N,N-methylenebisacrylamide (BIS). The MCcross-linked gels also exhibit high resilience. At the same time, replacing BIS with MC significantly improves the adhesive properties of the gel, which is attributed to the introduction of a large amount of adhesive groups with the MC. The gels can stick to various substrates including skin. The good tissue adhesiveness of the gel allows it to stick to skin by itself without using any straps or adhesive tapes when used as a flexible wearable strain sensor. Both large and subtle human movements were successfully monitored using the sensor. The signals are highly stable and reliable, thanks to the high resilience of the gel. The introduction of the polar groups also improved dramatically the anti-freezing properties of the gels. Even at −20 °C, the gels still remained highly flexible and stretchable, therefore allowing the gel-based sensor to work at sub-zero temperatures. The excellent toughness, resilience, tissueadhesiveness, and anti-freezing properties of the gel make it a good choice for a flexible wearable sensor.

Three-dimensional (3D) multicellular spheroids are a new generation in vitro cell model, however, their applications are severely limited by difficulties in their generation. Here patterned poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel films were synthesized for their generation. Instead of polymerization of HEMA monomers in the presence of a cross-linker, here the PHEMA films were synthesized by cross-linking furan-functionalized linear PHEMA, PHEMA-furan, and maleimide-functionalized linear PHEMA, PHEMA-mal, via Diels−Alder (DA) reaction between furan and maleimide groups. A thermal treatment temperature of 75 °C was chosen for the cross-linking reaction. The occurrence of DA reaction was confirmed by IR spectra. Using this method, cross-linked PHEMA films with smooth surface were successfully synthesized in situ in the well of cell culture plates. The films were then patterned by simply adding water to swell them. Highly ordered, honeycomb-like wrinkling patterns were successfully obtained by adjusting the furan and maleimide contents in the precursor linear polymers. The patterned hydrogel films were used to generate multicellular spheroids. Guided by the patterns, 3D spheroids with narrow size distribution, tunable size, and high cell viability were successfully obtained. The patterned PHEMA films reported here exhibited a lot of advantages. The patterning method was quite simple and required no template or special equipment. They were synthesized in situ in commercial cell culture plates. Particularly, thanks to the clean nature of the DA reaction, no low molecular weight monomer, cross-linker, initiator, or catalyst, which were potentially cytotoxic, was involved in the film synthesis, and no byproduct was produced and left in the film. The resulting films presented a high biocompatibility, allowing the avoidance of the tedious washing step. The films synthesized here were expected to have high potential for massive production of well-defined multicellular spheroids.

Elastomeric dielectrics are crucial for reliably governing the carrier densities in semiconducting channels during deformation in soft/stretchable field‐effect transistors (FETs). Uncontrolled stacking of polymeric chains renders elastomeric dielectrics poorly insulated at nanoscale thicknesses, thereby thick films are usually required, leading to high voltage or power consumption for on/off operations of FETs. Here, layer‐by‐layer assembly is exploited to build 15‐nm‐thick elastomeric nanodielectrics through alternative adsorption of oppositely charged polyurethanes (PUs) for soft and hysteresis‐free FETs. After mild thermal annealing to heal pinholes, such PU multilayers offer high areal capacitances of 237 nF cm−2 and low leakage current densities of 3.2 × 10−8 A cm−2 at 2 V. Owing to the intrinsic ductility of the elastomeric PUs, the nanofilms possess excellent dielectric properties at a strain of 5% or a bending radius of 1.5 mm, while the wrinkled counterparts show mechanical stability with negligible changes of leakage currents after repeated stretching to a strain of 50%. Besides, these nanodielectrics are immune to high humidity and conserve their properties when immersed into water, despite their assembly occurs aqueously. Furthermore, the PU dielectrics are implemented in carbon nanotube FETs, demonstrating low‐voltage operations (< 1.5 V) and negligible hysteresis without any encapsulations.