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The mechanical soft–hard transition of hydrogels is desired in conditions‐adaptive deformation and mechanics reconfiguration applications. However, highly efficient, stimuli‐responsive, and reversible transition strategies are hard to achieve. Inspired by the supercooling of salt‐aqueous solutions, solid and supersaturated hydrogels are prepared based on a hydrophilic polymer network and salt‐aqueous solution. The inner crystallization‐ or melting‐induced reversible phase‐change realizes the switch between the soft hydrogel (modulus: 0.1 MPa) and rigid composite (modulus: 24.0 MPa). The soft and supersaturated hydrogels easily deform to achieve diverse new 3D models and the unfamiliar soft–hard transition makes temporary shapes be efficiently fixed (hardening). Interestingly, the initial hydrogel's shapes can be regenerated relying on the resilience of the polyacrylamide network when the crystal is melted (softening). Shape memory, complex surface morphology replication, rapid mold application, and self‐supporting laminated glass are accomplished by this unique crystallization‐melting introduced soft–hard transition. This phase change soft‐hard switching strategy will broaden the functionalities of hydrogels. [ABSTRACT FROM AUTHOR]

Ostwald ripening (OR), a classic solution theory describing molecular transfer from metastable crystal to stable one, is applied to design time‐dependent crystal hydrogels that can automatically change their mechanical properties. Using a system made from crosslinked polyacrylamide (PAM) and sodium acetate (NaAc), we demonstrate that metastable fibrous crystal networks of NaAc preferably form in PAM hydrogels via a polymer‐involving mismatch nucleation. These fibrous crystals would undergo OR and evolve into isolated bulk crystals, leading to a significant reduction in material rigidity (179 folds) and interfacial adhesion (20 folds). This transformation can be applied to program time‐dependent self‐recovery in shape and self‐delamination. Since OR is a ubiquitous, robust feature of various crystals, the approach reported here represents a new direction for designing advanced transient soft materials. [ABSTRACT FROM AUTHOR]

Ostwald ripening (OR), a classic solution theory describing molecular transfer from metastable crystal to stable one, is applied to design time‐dependent crystal hydrogels that can automatically change their mechanical properties. Using a system made from crosslinked polyacrylamide (PAM) and sodium acetate (NaAc), we demonstrate that metastable fibrous crystal networks of NaAc preferably form in PAM hydrogels via a polymer‐involving mismatch nucleation. These fibrous crystals would undergo OR and evolve into isolated bulk crystals, leading to a significant reduction in material rigidity (179 folds) and interfacial adhesion (20 folds). This transformation can be applied to program time‐dependent self‐recovery in shape and self‐delamination. Since OR is a ubiquitous, robust feature of various crystals, the approach reported here represents a new direction for designing advanced transient soft materials. [ABSTRACT FROM AUTHOR]

The hysteresis of deformation and weak interaction between sensors and targets would deteriorate the authenticity and reliability of signals outputted from the strain sensors. Here, low‐hysteretic and adhesive organohydrogel‐based strain sensors (P‐H‐A‐A) are prepared according to the homogeneous growth of the conductive polyaniline (PANi) network and heterogeneous growth of the adhesive poly(acrylic acid) (PAA) layer from the poly(hydroxyethyl acrylate) (PHEA) seeds. Via regulating the concentration of aniline (ANi) in the ANi‐nutrients, the mechanical hysteresis at 50% strain of P‐H‐A‐A organohydrogels increased from 12 to 1896 kJ m−3. The relative resistance (ΔR/R0) signals achieved from the low mechanical hysteresis P‐H‐A‐A (0.1 m) strain sensors are timelier and more intact than the one from the obviously mechanical hysteretic P‐H‐A‐A (0.5 m). The PAA layer makes the P‐H‐A‐A strain sensors conformally in contact with the skin, ensuring the reliability and integrity of signals. This work would give some helpful suggestions for designing advanced epidermal electronics. [ABSTRACT FROM AUTHOR]

The adhesion strategies of the gecko's toe through surface adaptation of spatulas to increase contact area and the snail's epiphragm via dehydration‐induced solidification to lock interfaces are combined to design a class of adhesion‐switchable hydrogels. The hydrogels are made via incorporating CH3COONa·3H2O salt (SA) into polyacrylamide (PAM) aqueous networks to construct supersaturated and stimuli‐responsive phase change materials (PAM‐SA). The crystallization dramatically strengthens the mechanical properties, and tensile Young's moduli are 340.7 and 0.1 MPa for crystalline C‐PAM‐SA‐120% and soft PAM hydrogel. As a result, PAM‐SA‐120% shows excellent adhesive performance (adhesion strength, 348 kPa) compared with PAM hydrogel adhesive (adhesion strength, 7 kPa). The stimuli‐induced crystallization from H‐PAM‐SA‐120% phase change hydrogels releases thermal controllably, which can be utilized for thermochromic materials and thermotherapy. [ABSTRACT FROM AUTHOR]

Conductive hydrogels generally possess dual network structures consisting of hydrophilic cross‐linked polymer frameworks and conductive paths. The post‐modification of their structures and performance for fabricating devices with 3D conductive patterns is challenging because of their covalently cross‐linked complex matrices. Here, an orthogonal‐growth strategy is described for modulating all‐aspects of conductive hydrogels, including size, pattern, conductivity, and conductive path. In this strategy, the matrices controlling samples' size and the functional components modulating samples' conductivity can be orthogonally varied by feeding corresponding nutrients. Using poly(hydroxyethyl acrylate)‐polyaniline organohydrogels (PHEA‐PANI) as an example, it is demonstrated that the samples can incorporate the hydroxyethyl acrylate monomer to expand (21.9‐fold in weight is shown) or integrate aniline into the conductive networks to tune conductivity (from 0 to 42.6 mS cm‐1) without changing the sample size. Moreover, the growth of PANI is spatially selective. Exploiting the selective modulation of various parameters through this method, strain sensors with post‐modulated sensing performance and 3D hydrogel circuit boards are fabricated. [ABSTRACT FROM AUTHOR]

Vitrimers are reprocessing and recycling thermosetting plastics. They possess reconfigurable polymer networks that allow for unlimited transformation in shape in principle. However, current strategies to reshape the vitrimers typically involve heat or light, which often induces undesirable oxidation and decomposition. To address this issue, here a water‐assisted approach is proposed for programming epoxy vitrimers’ shapes. In this design, water molecules are utilized to reversibly dissociate the hydrogen bonds in epoxy vitrimers to enable the polymer segments to move flexibly. The hydrated epoxy vitrimers can then be reprogrammed and retained into different temporary shapes by removing the water. Such samples would be recovered to their original shapes by rehydration, exhibiting water‐induced shape memory property. More than temporary deformation, the permanent figures of the hydrated vitrimers can also be permanently changed at room temperature (rt) or elevated temperatures in the presence of transesterification catalysts. Combing the shape memory and high temperature plasticity or utilizing rt plasticity, sophisticated shapes such as spiral shapes are demonstrated. It is envisioned that this water‐assisted methodology can be useful in programming cross‐linked polymers into diverse 3D structures, which has wide practical applications in soft robots, deployable devices, aerospace materials, etc. [ABSTRACT FROM AUTHOR]

Wearable stretchable strain sensors can have important applications in many areas. However, the high noise is a big hurdle for their application to monitor body movement. The noise is mainly due to the motion artifacts related to the poor contact between the sensors and skin. Here, wearable stretchable dry and self‐adhesive strain sensors that can always form conformal contact to skin even during body movement are demonstrated. They are prepared via solution coating and consist of two layers, a dry adhesive layer made of biocompatible elastomeric waterborne polyurethane and a sensing layer made of a non‐adhesive composite of reduced graphene oxide and carbon nanotubes. The adhesive layer makes the sensors conformal to skin, while the sensing layer exhibits a resistance sensitive to strain. The sensors are used to accurately monitor both small‐ and large‐scale body movements, including various joint movements and muscle movements. They can always generate high‐quality signals even on curvilinear skin surface and during irregular skin deformation. The sensitivity is remarkably higher while the noise is saliently lower than the non‐adhesive strain sensors. They can also be used to monitor the movements along two perpendicular directions, which cannot be achieved by the non‐adhesive strain sensors. [ABSTRACT FROM AUTHOR]

Stretchable electronic materials and devices have important applications in flexible electronic systems including wearable electronics and bioelectronics. Convenient electricity generation such as thermoelectric conversion is required for the flexible electronic systems. Hence, it is development of high‐performance thermoelectric materials with high mechanical stretchability would be highly desirable. Here, stretchable and transparent ionogels with high thermoelectric properties are demonstrated. The ionogels made of elastomeric waterborne polyurethane and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) are prepared by solution processing. Their mechanical and electrical properties depend on the loading of EMIM:DCA. The ionogels with 40 wt% EMIM:DCA can have a high mechanical stretchability of up to 156%, low tensile strength of 0.6 MPa, and low Young's modulus of 0.6 MPa. They also exhibit a high ionic thermovoltage of 34.5 mV K−1, high ionic conductivity of 8.4 mS cm−1 and low thermal conductivity of 0.23 W m−1 K−1 at a relative humidity of 90%. As a result, it can have a high ionic figure of merit (ZTi) of 1.3 ± 0.2. Both the thermovoltage and the ZTi value are the highest for stretchable thermoelectric materials. They can be used in ionic thermoelectric capacitors to convert heat into electricity. [ABSTRACT FROM AUTHOR]

Abstract Previous studies have shown that the commonly used Diels-Alder (DA) bonds (furan and maleimide) cleaved at low temperatures (about 100 °C), which limits further application of these self-healing materials at higher temperatures. Hence, a series of thermo stable self-healing polyurethane elastomers (PUAn-DA-x) have been developed by using the reversible DA/retro-DA reactions initiated by thermal and mechanical stimuli between anthryl and maleimide functions. The synthesized linear polyurethane bearing pendant anthryl groups (PUAn-0) was effectively toward DA reactions with a bismaleimide crosslinker (BMI) to get covalently crosslinked PUAn-DA-x networks. Tensile test showed that the stress at break of PUAn-DA-1/1 was increased by more than 261% after the PUAn-0 was effectively networked by DA bonds. At the same time, with the aid of hydrogen bonds and chains' rearrangement, the healing efficiency of PUAn-DA-1/1 could reach as high as 91.28% via the recombination of mechanically decoupled anthracene and maleimide without causing deformation of bulk specimens. In addition, the thermal stabilities of the DA bonds and PUAn-DA-x were studied. Graphical abstract Image 1 Highlights • Fabricated polyurethane could repair the cracks and restore the comprehensive mechanical performance. • Synthesized system is thermally stable as the Diels-Alder bonds between anthracene and maleimide are more heat-resisting. • Mechanical exhibition of the polyurethane elastomer was distinctly reinforced since the Diels-Alder crosslinking reactions. [ABSTRACT FROM AUTHOR]

Flexible electronics have been extensively investigated in recent years because of their potential applications in artificial skin, soft robotics, energy storage, healthcare technology, etc. However, since the rigid and brittle nature of the conductive or semi-conductive materials, it's a huge challenge to project stretchable, self-healable and recyclable soft electronics to mimic skin. Herein, the interpenetrating and synergistic dual-network hydrogel of Diels-Alder chemistry-based dynamic polyurethane substrates and polyaniline conductive polymer (PU-DA-1/1-PANI) was prepared and employed as a strain sensor. The PU-DA-1/1-PANI hydrogel exhibited large elongation at break of 500%, soft Young's modulus of 0.3 MPa, and robust tensile strength of 1.1 MPa. Meanwhile, the strain sensors with optimized composition presented an ideal conductivity of 7.9 S m−1, which could monitor the physical motions precisely and repeatedly with a gauge factor (GF) of 2.9. In addition, the mechanical performance and conductivity could be self-healed, which was enabled by the hydrogen bonds, ionic interactions, reversible DA covalent bonds, and rearrangement of polymer chains. Built on the thermally stimuli-responsive nature of the DA covalent cross-linking points, the PU-DA-1/1-PANI composites could be recycled. [Display omitted] • Interpenetrating and synergistic PU-DA-1/1-PANI hydrogels monitor diverse movements with a reliable signal. • PU-DA-1/1-PANI composites show ideal self-healing performance depended on multiple dynamic interactions. • PU-DA-1/1-PANI composites can be recycled separately for recycling the polyurethane matrix and the conductive polyaniline. [ABSTRACT FROM AUTHOR]

Widely applied waterborne polyurethane (WPUs) are expected to be functionalized with excellent mechanical property, ideal self-healing performance and desired recyclability, but it is a great challenge to integrate these attractive properties into a one-pot WPU platform. Herein, a novel double ingredients waterborne polyurethane (2K-WPU-DA-x), found on the furan modified WPU-F and maleimide decorated WPU-M dispersions for Diels-Alder (DA) cross-linking reactions, was designed and projected which exhibited interesting enhanced-mechanical, self-healable and recyclable properties. The stability and appearance of these novel waterborne polyurethane emulsions were well studied. In the following, the reversible nature of direct DA/retroDA reactions within the 2K-WPU-DA-x was extensively analyzed via FT-IR, UV–vis, and DSC measurements. On account of the DA bonds resulted a cross-linked structure of 2K-WPU-DA-1/1, the stress and strain at the break increased by more than 20 MPa and 250% respectively compared with linear structural WPU-F. At the same time, relying on the thermal-responsive dissociation/reassociation of DA covalent bonds, 2K-WPU-DA-x films shown outstanding self-healing ability, with a self-healing efficiency beyond 95%. Moreover, owing to the reversibility of 2K-WPU-DA-x, the networks exhibited readily recyclable and reshaping properties, showing a desired thermoplasticity. Living and stable two-component waterborne polyurethane emulsions for the applications of self-healing and recyclable cross-linked films based on reversible Diels-Alder reactions. Image 1 • The mixed emulsion of maleimide- and furan-modified waterborne polyurethane latexes is highly stable. • Cross-linked films show robust mechanical property. • Exhibiting excellent self-healing performance regarding cracks and physical properties. • Showing ideal recyclability of the thermosets relying on the reversible DA chemistry. [ABSTRACT FROM AUTHOR]

Growing polymer materials emerge as a class of new intelligent systems, but they are all covalently cross‐linked. Herein, an alternating growth strategy for enabling tough supramolecular hydrogels is described to integrate externally provided compounds to change their sizes, geometric shapes, mechanical properties, etc. The hydrogels have dynamic acid–base complex (ABC) structures made from acidic and basic monomers, in which the acid–base pairs equilibrate between ionic and nonionic states to show either strong electrostatic or weak acid–base interactions. The growth process includes swelling of acidic/basic monomers, polymerization of the absorbed monomers, and homogenization of the original and newborn polymers, in which the balance between electrostatic and acid–base interactions is reversibly shifted to enable the process. As a result of such repeatable growth cycles, the hydrogels can continuously expand without compromising material mechanical properties. The growing sub‐processes and the monomer compositions can be used to post‐modulate the structure and properties of the grown products. It is demonstrated that the hydrogels may be designed as artificial tissues capable of growth under force‐bearing states. [ABSTRACT FROM AUTHOR]

Considerable research efforts have aimed to control indoor temperatures to improve daily life and industrial production. However, current strategies, which involve active regulation of energy input or passive regulation of sunlight transmission, cannot control indoor temperatures under all weather conditions. Herein, a self‐adaptive thermal management device (STMD) that uses solar energy (heating) and radiative cooling (cooling) by exploiting the temperature differences across the device is reported. The adaptive heating and cooling functions are achieved by modulating sunlight transmittance and reflectance using a porous SiO2 coating separated/merged with a refractive index‐matched liquid which serves as a shutter. As a result, the STMD in the opaque mode exhibits low transmittance and in the transparent mode exhibits sufficiently high solar transmittance. Taking advantage of this self‐regulated shuttering mechanism, the developed STMD enables i) an increase of ≈10 °C relative to the ambient temperature under a solar intensity of 400 W m−2 in cold weather and ii) a temperature reduction of ≈5 °C under a solar intensity exceeding 900 W m−2 in hot weather. The described design strategy offers an approach for constructing smart light‐controlling devices and thermal‐controlling building materials. [ABSTRACT FROM AUTHOR]

The oil and bacteria adhesion during membrane separation process brings great challenges to the operation costs and membrane service life. Meantime, the strong chemical corrosion in sewage seriously limits the durability of membrane as well. Herein, a facile strategy is developed for fabricating highly stable and efficient zwitterionic nanofibrous membrane (NFM) with self-cleaning feature via the combination of in - situ cross-linking of poly (sulfobetaine methacrylate) (PSBMA) and electrospun poly (ether sulfone) (PES) nanofibers. Owing to the introduction of zwitterionic functional groups, the PSBMA/PES NFM exhibits superior antifouling ability (over 3 cycles of crude oil fouling/self-cleaning and up to 7 days of bacteria adhesion/repelling tests). Moreover, the membrane also presents remarkable chemical stability in acidic, alkaline and salty environments; and exhibits excellent separation performance for both layered oil/water mixture and oil-in-water emulsion as well. Furthermore, the membrane is capable to remove bacteria during the continuous oil/water mixture separation. Overall, the proposed strategy provides a new perspective into developing long-term antifouling membrane materials for complicated oily wastewater remediation in various corrosive environments. Unlabelled Image • Self-cleaning zwitterionic nanofibrous membranes (NFMs) are prepared. • The NFMs can resist not only crude oil fouling but also bacteria adhesion. • The NFMs present highly-efficient oil/water separation performance. • The NFMs show great chemical stability in acid, alkaline and salty environments. [ABSTRACT FROM AUTHOR]