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Hydrogel actuators showing shape morphing in response to external stimuli are of significant interest for their applications in soft robots, artificial muscles, etc. However, there is still a lack of hydrogel actuators with adjustable stimulus responsiveness for on-demand driving. In this study, an organohydrogel actuator was prepared by a two-step interpenetrating method, resulting in the coexistence of poly(N-isopropylacrylamide-co-4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine) (p(NIPAM-SVBP)) hydrophilic networks and poly(lauryl methacrylate) (pLMA) hydrophobic networks with gradient distribution. In the initial state, the organohydrogel actuator can be driven globally under thermal stimulation. Owing to the unique alkali-chromic performance of SVBP, the organohydrogel actuator can be endowed with photothermal properties and actuate locally under the stimulus of NIR light. More importantly, the organohydrogel will return to the original colorless state after being treated with acid solution. Our work provides a new insight into designing and fabricating novel actuators with adjustable stimulus responsiveness for on-demand morphing.

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Based on the demand for flexible human-machine interaction devices, it is urgent to develop high-performance stretchable ionic conductive materials. However, most gel-based ionic conductive materials are composed of crosslinked polymer networks that contain liquids, and suffer from limitations of solvent volatilization and leakage, and the cross-linking restricts the movement and diffusion of polymer chains, making it difficult for them to achieve adhesion. Here, we introduce flexible and adhesive liquid-free ionic conductive elastomers (ICE) with salt using a non-crosslinked polymer strategy. The ICE show a transparency of 89.5%, Tg of -51.2 °C, negligible weight loss at 200 °C, a tensile fracture strain of 289.5%, and an initial modulus of 45.7 kPa, and is adhesive to various solid surfaces with an interfacial toughness of 11.4 to 41.4 J m-2. Moreover, the ICE exhibit stable electrical conductivity under ambient conditions. Triboelectric nanogenerators (TENGs) were assembled on an electrical shell surface with the adhesive ICE as an electrostatic induction layer and were displayed for use as human-machine interactive keyboards. This approach opens a route to making adhesive and stable polymer ionic conductors for human-machine interaction.

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Mixed Gaussian and Random-valued impulse noise (RVIN) removal is still a big challenge in the field of image denoising. Existing denoising algorithms have defects in denoising performance and computational complexity. Based on the improved "detecting then filtering" strategy and the idea of inpainting, this paper proposes an efficient method to remove mixed Gaussian and RVIN. The proposed algorithm contains two phases: noise classification and noise removal. The noise classifier is based on Adaptive center-weighted median filter (ACWMF), three-sigma rule and extreme value processing. Different from the traditional "detecting then filtering" strategy, a preliminary RVIN removal step is added to the noise removal phase, which leads to three steps in this phase: preliminary RVIN removal, Gaussian noise removal and final RVIN removal. Firstly, RVIN is processed to obtain a noisy image approximately corrupted by Gaussian noise only. Subsequently, Gaussian noise is re-estimated and then denoised by Block Matching and 3D filtering method (BM3D). At last, the idea of inpainting is introduced to further remove RVIN. Extensive experimental results demonstrate that the proposed method outperforms quantitatively and visually to the state-of-the-art mixed Gaussian and RVIN removal methods. In addition, it greatly shortens the computation time.

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Bionic skin sensors based on conductive polymer gels have garnered interest for their potential applications in human-computer interaction, soft robotics, biomedical systems, sports, and healthcare, because of their intrinsic flexibility and stretchability embedded at the material level, and other such as self-healing, adhesion, high, and low temperature tolerance properties that can be tuned through macromolecular design. Here, important advances in polymer gel-based flexible sensors over recent years are summarized, from material design, sensor fabrication to system-level applications. This review focuses on the representative strategies of design and preparing of conductive polymer gels, and adjusting their conductivity, mechanics, and other properties such as self-healing and adhesiveness by controlling the macromolecular network structures. The state-of-art of present flexible pressure and strain sensors, temperature sensors, position sensors, and multifunctional sensors based on capacitance, voltage, and resistance sensing technologies, are also systematically reviewed. Finally, perspectives on issues regarding further advances and challenges are provided.

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Acute wounds subject to frequent deformations are difficult to be treated because the healing process was easily interfered by external mechanical forces. Traditional wound dressings have limited efficacy because of their poor mechanical properties and skin adhesiveness and difficulty in the delivery of therapeutic drugs effectively. As such, tough and skin-adhesive wound dressings with sustainable and stimuli-responsive drug release properties for treatment of those wounds are highly desirable. For this purpose, we have developed a mechano-responsive poly(sulfobetaine methacrylate) hydrogel which aims to control the delivery of antibiotic drug upon application of mechanical forces. Diacrylated Pluronic F127 micelles were used as a macro-cross-linker of the hydrogel and loaded with hydrophobic antimicrobial drugs. The micelle-cross-linked hydrogel has excellent mechanical properties, with the ultimate tensile strength and tensile strain of up to 112 kPa and 1420%, respectively, and compressive stress of up to 1.41 MPa. Adhesiveness of the hydrogel to the skin tissue was ∼6 kPa, and it did not decrease significantly after repetitive adhesion cycles. Protein adsorption on the hydrogel was significantly inhibited compared to that on commercial wound dressings. Because of the mechano-responsive deformation of micelles, the release of drug from the hydrogel could be precisely controlled by the extent and cycles of mechanical loading and unloading, endowing the hydrogel with superior antibacterial property against both Gram-positive and Gram-negative bacteria. In addition, drug penetration into the skin tissue was enhanced by mechanical stress applied to the hydrogel. The micelle-cross-linked zwitterionic hydrogel also showed good cell biocompatibility, negligible skin irritation, and healing capacity to acute skin wounds in mice. Such a tough mechano-responsive hydrogel holds great promise as wound dressings for acute wounds subjected to frequent movements.

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There is an increasing interest to develop a next generation of touch pads that require stretchability and biocompatibility to allow their integration with a human body, and even to mimic the self-healing behavior with fast functionality recovery upon damage. However, most touch pads are developed based on stiff and brittle electrodes with the lack of the important nature of self-healing. Polyzwitterion-clay nanocomposite hydrogels as a soft, stretchable, and transparent ionic conductor with transmittance of 98.8% and fracture strain beyond 1500% are developed, which can be used as a self-healing human-machine interactive touch pad with pressure-sensitive adhesiveness on target substrates. A surface-capacitive touch system is adopted to sense a touched position. Finger positions are perceived during both point-by-point touch and continuous moving. Hydrogel touch pads are adhered to curved or flat insulators, with the high-resolution and self-healable input functions demonstrated by drawing, writing, and playing electronic games.

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Inspired by biosystem, ionic hydrogels have been extensively studied as promising materials for wearable or implantable devices. Herein, we report novel ionic hydrogels that comprise dynamically crosslinked polyzwitterion and physically crosslinked polyvinyl alcohol, which demonstrate excellent mechanical properties, repeatable self-adhesion, and high and linear strain sensitivity. The obtained hydrogels can be directly attached to human skin as sensors to detect or monitor physiological signals.

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Intelligent hydrogels responsive to external stimuli have been widely studied due to their great potentials for applications in artificial muscles, soft robotics, sensors and actuators. However, the weak mechanical properties, narrow response range, and slow response speed of many responsive hydrogels have hindered practical applications. In this paper, tough multi-responsive hydrogels were synthesized by using vinyl-functionalized triblock copolymer micelles as macro-crosslinkers and N-isopropyl acrylamide (NIPAM) and acrylamide (AAm) or 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 2-acrylamido-2-methyl-1-propane-sulfonic acid (AMPS) as monomers. The P(NIPAM-co-AAm) hydrogels presented tensile strength of up to 1.6 MPa and compressive strength of up to 127 MPa and were tunable by changing their formulations. Moreover, the lower critical solution temperature (LCST) of the thermosensitive hydrogels was manipulated in a wide range by changing the molar ratio of NIPAM to AAm. Responsive hydrogel bilayers were fabricated through a two-step synthesis. A second layer of P(DMAEMA-co-AMPS) was synthesized on the first P(NIPAM-co-AAm) layer to obtain a bilayer hydrogel, which was responsive to temperature, pH and ionic strength changes to undergo fast and reversible shape transformation in a few minutes. This kind of strong and tough multi-responsive hydrogel device has broad prospects in soft actuators.

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It is desired to create skin strain sensors composed of multifunctional conductive hydrogels with excellent toughness and adhesion properties to sustain cyclic loadings during use and facilitate the electrical signal transmission. Herein, we prepared transparent, compliant, and adhesive zwitterionic nanocomposite hydrogels with excellent mechanical properties. The incorporated zwitterionic polymers can form interchain dipole-dipole associations to offer additional physical cross-linking of the network. The hydrogels show a high fracture elongation up to 2000%, a fracture strength up to 0.27 MPa, and a fracture toughness up to 2.45 MJ/m3. Moreover, the reversible physical interaction imparts the hydrogels with rapid self-healing ability without any stimuli. The hydrogels are adhesive to many surfaces including polyelectrolyte hydrogels, skin, glasses, silicone rubbers, and nitrile rubbers. The presence of abundant zwitterionic groups facilitates ionic conductivity in the hydrogels. The combination of these properties enables the hydrogels to act as strain sensors with high sensitivity (gauge factor = 1.8). The strategy to design the tough, adhesive, self-healable, and conductive hydrogels as skin strain sensors by the zwitterionic nanocomposite hydrogels is promising for practical applications.

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Responsive hydrogel actuators have promising applications in diverse fields. Most hydrogel actuators are limited by slow actuation or shape transformations. This work reports on snap-buckling motivated jumping of thermoresponsive hydrogel bilayers. The bilayers are composed of poly(NIPAM- co-DMAPMA)/clay hydrogel with different lower critical solution temperatures in each layer, and thus undergo slow reversible curling/uncurling at temperature changes. The gels are adhesive to numerous materials including aluminum. The adhesion between the gels and an aluminum ratchet is utilized to constrain the thermoresponsive deformation of the bilayers to store elastic energy. When the accumulated elastic energy overwhelms the gel-aluminum adhesion, snap-buckling takes place to abruptly release the accumulated energy, which motivates the bilayer to jump. The jumping direction, start time, height, and distance are controlled by the geometry of the bilayers or the ratchet. This work paves a novel way for the rapid actuation of responsive hydrogels in a controlled manner and may stimulate the development of novel hydrogel devices.

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Stimulus-responsive hydrogels are assembled into soft devices that transform their shape upon external stimuli. It is very important to understand the macroscopic assembly mechanisms and to modulate the interface stability of assemblies. In this study, polyelectrolyte hydrogels with outstanding mechanical performances and opposite charges were assembled into soft devices via electrostatic association. The interface strength could be tuned by varying the net charge density, which depends on the concentration of charged/chargeable monomers, the pH of the buffer solution, and the ionic strength of the solution. We propose that charge screening at the interface by free counterions causes a reduction of interface strength, whereas charge redistribution is helpful to strengthen the interface of the assemblies. The understanding of macroscopic assembly mechanisms provides significant guidelines for designing novel soft transducers and drivers and engineering their interface strength.

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Graphene composite hydrogels have wide potential applications as biomaterials. Herein, l-cysteine was used to reduce graphene oxide (GO) into hydrogels. Systematic investigations by FTIR, Raman spectroscopy, TEM, and other methods reveal that the reduced graphene oxide (rGO) nanosheets are likely bridged by l-cysteine molecules, forming porous hydrogels with the rGO nanosheets stacked into layered structures on the walls. Such layered structures, as well as the reduction, are critical for the improvement of conductivity by four to five orders of magnitude in comparison to graphene oxide. In vitro cell culture experiments demonstrate excellent cell adhesion and growth on these reduced graphene oxide hydrogels. These conductive rGO hydrogels may find applications in electrical stimulation to facilitate cell adhesion and growth.

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Multiresponsive polyelectrolyte hydrogels with extraordinary toughness have great potential in soft device applications. Previously we have demonstrated a series of tough and multiresponsive hydrogels by using multifunctional triblock copolymer (Pluronic F127 diacrylate, F127DA) micelles to cross-link cationic polyelectrolyte chains into 3D network. Herein, we further synthesize negatively charged hydrogels comprising 2-acrylamido-2-methyl propylsulfonic acid (AMPS) monomers by using F127DA micelles as cross-linkers. Similar to the positive nanomicelle (NM) hydrogels, the negative NM hydrogels exhibited a compressive strength up to 59 MPa with a fracture strain up to 98%, and tensile fracture strain higher than 2000%. These charged hydrogels were actuated by electric field when immersed in salt solutions. The effects of electrolyte concentration, electric field strength, and ionic monomer content on the electric actuation behavior of these electroactive hydrogels (EAHs) have been systematically investigated. It is concluded that the electroactive hydrogels show a fast actuation rate with a bending angle up to 87° at 120 s and the bending angle was cyclically reversed upon changing bias direction without a large decrease. This study demonstrates that such tough and multiresponsive electroactive hydrogels may have great potential in sensors, actuators, switches, and artificial muscles.

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Multi-responsive hydrogels with high strength have great significance for potential applications in smart soft devices. However, it remains a challenge to incorporate multiple responsive moieties with energy dissipation mechanisms. Herein, multi-responsive nanocomposite hydrogels with high compressive strength and toughness were synthesized via in situ copolymerization of N-isopropylacrylamide (NIPAM) and acryloyloxyethyltrimethyl ammonium chloride (DAC) in an aqueous dispersion of exfoliated LAPONITE® RDS with a minute amount of N,N'-methylenebisacrylamide (MBAA) as a crosslinker. The combined use of clay and MBAA is demonstrated to be favorable for the high strength and toughness, and helped in avoiding precipitation of clay nanosheets, which otherwise occurred upon addition of cationic DAC. The effect of the NIPAM/DAC molar ratio, MBAA and clay contents on the properties of the hydrogels has been systematically investigated. Compression tests showed a compressive strength up to 6.2 MPa, with fracture strain higher than 90%. The presence of ionic DAC moieties in the hydrogels rendered a very high swelling ratio up to 40 (g g-1). These hydrogels were responsive to temperature changes due to the presence of NIPAM units, with the transition temperature (Ttrans) dependent on the molar ratio of NIPAM and DAC monomers. The internal electrostatic repulsion of the NIPAM/DAC copolymer network changed upon exposure to solutions with different pH and/or ion strength. Cyclic swelling-shrinking was demonstrated by shuttling the gels between pure water and 0.1 mol L-1 NaCl solution.

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Responsive nanocomposite hydrogels are widely recognized as strong and tough soft materials for smart devices, but there are still challenges in their synthesis and in the fabrication of devices. We report here the versatile synthesis of nanocomposite polyelectrolyte hydrogels with high strength and toughness using either cationic or anionic monomers and demonstrate the simple and versatile fabrication of bilayer actuators by assembling the oppositely charged hydrogels through electrostatic attraction. Exfoliated sodium montmorillonite nanosheets were used as cross-linkers through the adsorption of monomers and initiators before in situ free radical polymerization. Nanocomposite hydrogels with negative charges were obtained by the copolymerization of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid, whereas positively charged nanocomposite gels were obtained using acrylamide and dimethylaminoethyl methacrylate methylchloride. The nanocomposite polyelectrolyte hydrogels were responsive to pH and the ionic strength of buffer solutions. They also showed outstanding fatigue resistance against cyclic compression loading and high tensile strength and toughness. The gels were able to recover at room temperature after tensile testing. The oppositely charged hydrogels were assembled with a robust interface based on the electrostatic attraction between the opposite charges. These hydrogels were actuated under an electric field. The oppositely charged hydrogels were assembled into bilayers that were reversibly actuated as a result of the contrasting responsiveness of each gel to the ionic strength.

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Correction for 'Multi-responsive nanocomposite hydrogels with high strength and toughness' by Jingli Yang et al., J. Mater. Chem. B, 2016, 4, 1733-1739.

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A versatile ion release mediated by GDL is demonstrated to achieve a controlled homogeneous crosslinking of alginate chains, which is critical for the synthesis of highly stretchable and notch-insensitive hybrid hydrogels with controlled properties.

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Nanocomposite hydrogels with unprecedented stretchability, toughness, and self-healing have been developed by in situ polymerization of acrylamide with the presence of exfoliated montmorillonite (MMT) layers as noncovalent cross-linkers. The exfoliated MMT clay nanoplatelets with high aspect ratios, as confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) results, are well dispersed in the polyacrylamide matrix. Strong polymer/MMT interaction was confirmed by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The effective cross-link densities of these hydrogels are estimated in the range of 2.2-5.7 mol m(-3). Uniaxial tensile tests showed a very high fracture elongation up to 11 800% and a fracture toughness up to 10.1 MJ m(-3). Cyclic loading-unloading tests showed remarkable hysteresis, which indicates energy dissipation upon deformation. Residual strain after cyclic loadings could be recovered under mild conditions, with the recovery extent depending on clay content. A mechanism based on reversible desorption/adsorption of polymer chains on clay platelets surface is discussed. Finally, these nanocomposite hydrogels are demonstrated to fully heal by dry-reswell treatments.

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Multi-stimuli responsive hydrogels are synthesized using self-assembled nanomicelles of Pluronic F127 diacrylate triblock copolymer as non-covalent macro-crosslinkers to in situ copolymerize with acrylamide and methyl chloride quaternized N,N-dimethylamino ethylacrylate monomers, generating positively charged hydrogels. These hydrogels showed high strength, toughness, and outstanding fatigue resistance, and are reversibly responsive to changes in pH and ionic strength.

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An interpenetrating network (IPN) strategy has been widely facilitated to construct strong and tough hydrogels, but most of the efforts have been focused on organic/organic networks. Herein, aqueous dispersible 2,2'-(ethylenedioxy)-diethanethiol (EDDET) cross-linked graphene oxide (E-cGO) skeleton was in situ incorporated into a PVA matrix, resulting in novel inorganic/organic IPN hydrogels with super mechanical and chondrocyte cell-adhesion properties. The unique interpenetrating structure and hydrogen bonding were demonstrated to play critical roles in enhancing the compressive property of the IPN hydrogels, in comparison to the GO and thermally reduced graphene oxide (T-rGO) filled hydrogels. It is critical that the E-cGO/PVA hydrogels have been demonstrated as being biocompatible, which make the E-cGO/PVA hydrogels promising candidate biomaterials for load-bearing biotissue substitution.

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