Two-Dimensional MoO2 Nanosheet Composite Hydrogels with High Transmittance and Excellent Photothermal Property for Near-Infrared Responsive Actuators and Microvalves

Ultrasensitive and light erasable surface-enhanced Raman scattering substrates based on Au-MoO2 heterostructures

Metal nanostructures are commonly utilized as surface-enhanced Raman scattering (SERS) substrates due to their strong localized surface plasmon resonance (LSPR) effects. However, these substrates are typically single-use because the analytes tend to remain on them. Molybdenum dioxide (MoO2), a metalloid oxide with excellent photothermal properties, allows for removal of residues through photothermolysis. Nevertheless, its inherent SERS performance is relatively limited, with a detection threshold around 10-7 M, approximately two orders of magnitude less sensitive than metal-based substrates. Here, we harnessed the synergistic interaction between Au and MoO2 to develop reusable and highly sensitive SERS substrates. Au nanoparticles (NPs) were selectively deposited onto MoO2 through a light-induced reduction method. The resulting Au-MoO2 heterostructures exhibit an excellent electric field at interstitial sites due to LSPR coupling and present remarkable SERS sensitivity towards rhodamine 6G (R6G), methyl blue (MB), rhodamine B (RhB), and crystal violet (CV). The limit of detection can be improved down to 10-9 M, achieving a maximum enhancement factor of 4.9 × 106. Additionally, when exposed to an 808 nm laser beam, the surface temperature of the Au-MoO2 nanocomposites can reach up to 529 °C, exceeding the decomposition temperatures of most organic compounds. Therefore, the residues can be entirely eliminated from the substrate under irradiation within 12 min, without sacrificing the SERS reproducibility and sensitivity. We expect our work to provide beneficial insights into the construction of ultrasensitive and recyclable SERS substrates.

Fast fabrication of stimuli-responsive MXene-based hydrogels for high-performance actuators with simultaneous actuation and self-sensing capability

Poly(N-isopropylacrylamide) (PNIPAM) composite hydrogels have recently emerged as promising candidates for soft hydrogel actuators. However, developing a facile and fast method to obtain multifunctional PNIPAM hydrogel actuators with simulating biological versatility remains a major challenge. Herein, we developed a fast-redox initiation system to prepare PNIPAM/sodium carboxymethyl cellulose (CMC)/T3C2Tx MXene nanocomposite hydrogel with multidirectional actuating behaviors and improved mechanical properties. The rapid thermoresponsive behavior of the PNIPAM/CMC/MXene layer bestows its corresponding bilayer actuator with an extraordinary actuation speed of 9.36°/s in hot water. Owing to the high photothermal conversion of MXenes, this PNIPAM/CMC/MXene hydrogel displays a range of remote-controlled actuations upon NIR light irradiation, including bending, rolling, displacement, and simulations of the sea eel's hunting behaviors in a water environment. More importantly, based on the excellent electrical properties of MXene, the PNIPAM/CMC/MXene-based hydrogel actuators have accomplished a self-sensing function by integrating the surface temperature-bending angle-the relative resistance changes during the NIR light-driven actuation process. The photothermal actuator's integrated actuation and sensing capabilities have facilitated the feedback of the contact and movement dynamics of the bioinspired artificial tongue. The straightforward preparation and multifunctional design of MXene-based hydrogel may facilitate the development of soft smart actuators.

A Photothermal-Responsive Soft Actuator Based on Biomass Carbon Nanosheets of Synergistic Bilateral Polymers

Currently, polymer actuators capable of photothermal response are being developed to be more sensitive and repeatable. In this work, a three-layered structured soft film actuator (NA/PET/NI-3) was designed by combining poly(N-isopropylacrylamide) (PNIPAM), poly(N-(2-aminoethyl)-acrylamide) (PANGA) and poly(ethylene glycol-co-terephthalate) (PET) film. Coconut water and PEI were used to synthesize a new kind of carbon nanosheet (PEI-CCS), which, when triggered by near-infrared light, will enable photothermal bending behavior in the micrometer-scale NA/PET/NI-n film, while PET served as the supporting and heat conducting layer. This three-layered actuator utilized the synergistic effects of two kind of polymers, PNIPAM and PNAGA, on either side of PET, with the upper critical solution temperatures and lower critical solution temperatures when subjected to temperature changes. This bilateral polymer design exhibited a rapid response under near-infrared light stimulation, bending to 180° within 4 s and recovering to its original shape within 30 s. When the bending process was set to 90° as in the standard experiment, NA/PET/NI-3 responded within 2 s and recovered within 8 s. NA/PET/NI-3 also demonstrated good reversibility and repeatability, capable of undergoing reversible driving over 120 times. The design and preparation of this actuator provided new ideas for the development of polymer soft actuators.

Aromatic Biobased Polymeric Materials Using Plant Polyphenols as Sustainable Alternative Raw Materials: A Review

In the foreseeable future, the development of petroleum-based polymeric materials may be limited, owing to the gradual consumption of disposable resources and the increasing emphasis on environmental protection policies. Therefore, it is necessary to focus on introducing environmentally friendly renewable biobased materials as a substitute for petroleum-based feed stocks in the preparation of different types of industrially important polymers. Plant polyphenols, a kind of natural aromatic biomolecule, exist widely in some plant species. Benefiting from their special macromolecular structure, high reactivity, and broad abundance, plant polyphenols are potent candidates to replace the dwindling aromatic monomers derived from petroleum-based resources in synthesizing high-quality polymeric materials. In this review, the most related and innovative methods for elaborating novel polymeric materials from plant polyphenols are addressed. After a brief historical overview, the classification, structural characteristics, and reactivity of plant polyphenols are summarized in detail. In addition, some interesting and innovative works concerning the chemical modifications and polymerization techniques of plant polyphenols are also discussed. Importantly, the main chemical pathways to create plant polyphenol-based organic/organic-inorganic polymeric materials as well as their properties and possible applications are systematically described. We believe that this review could offer helpful references for designing multifunctional polyphenolic materials.

An Intrinsic Photothermal Supramolecular Hydrogel with Robust Mechanical Strength and NIR-Responsive Shape Memory

Near-infrared (NIR)-triggered shape memory hydrogels with promising mechanical strength hold immense potential in the field of biomedical applications and soft actuators. However, the optical and mechanical properties of currently reported hydrogels usually suffer from limited solubility and dispersion of commonly used photothermal additives in hydrogels, thus restricting their practical implementations. Here,, a set of NIR-responsive shape memory hydrogels synthesized by polyaddition of diisocyanate-terminated poly(ethylene glycol), imidazolidinyl urea (IU), and p-benzoquinone dioxime (BQDO) is reported. The introduction of IU, a hydrogen bond reinforcing factor, significantly enhances the mechanical properties of the hydrogels, allowing for their tunable ranges of the ultimate tensile strength (0.4-2.5 MPa), elongation at break (210-450%), and Young's modulus (190-850 kPa). The unique hydrogels exhibit an intrinsic photothermal effect because of the covalently incorporated photothermal moiety (BQDO), and the photothermal supramolecular hydrogel shows controllable shape memory capabilities characterized by rapid recovery speed and high recovery ratio (>90%). This design provides new possibilities for applying shape memory hydrogels in the field of soft actuators.

Novel Acid-Driven Bioinspired Self-Resettable Bilayer Hydrogel Actuator Mimicking Natural Muscles

Soft robots have great potential applications in manufacturing, disaster rescue, medical treatment, etc. Artificial muscle is one of the most important components of a soft robot. In previous years, hydrogel actuators that can be controllably deformed by the stimuli of external signals have been developed as good candidates for muscle-like materials. In this article, we successfully prepared a chemical fuel-driven self-resettable bilayer hydrogel actuator mimicking natural muscles with the aid of a new negative feedback reaction network. The actuator can temporarily deform upon the addition of H+ (chemical fuel). Subsequently, H+ accelerated the reaction between BrO3- and Fe(CN)64-, which consume H+. It resulted in the spontaneous recovery of the pH as well as the shape of the actuator. Such an actuator exhibits a great similarity with natural muscles in actuation mechanisms and automaticity in the manipulation compared to the widely reported stimuli-responsive hydrogel actuators. This illustrates that fuel-driven self-resettable hydrogel is a promising dynamic material for mimicking the functions of living creatures.

Organohydrogel Actuators with Adjustable Stimulus Responsiveness for On-Demand Morphing

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.

Passive Automatic Switch Relying on Laplace Pressure for Efficient Underwater Low-Gas-Flux Bubble Energy Harvesting

The buoyancy potential energy contained in bubbles released by subsea geological and biological activities represents a possible in situ energy source for underwater sensing and detection equipment. However, the low gas flux of the bubble seepages that exist widely on the seabed introduces severe challenges. Herein, a passive automatic switch relying on Laplace pressure is proposed for efficient energy harvesting from low-gas-flux bubbles. This switch has no moving mechanical parts; it uses the Laplace-pressure difference across a curved gas-liquid interface in a biconical channel as an invisible "microvalve". If there is mechanical equilibrium between the Laplace-pressure difference and the liquid-pressure difference, the microvalve will remain closed and prevent the release of bubbles as they continue to accumulate. After the accumulated gas reaches a threshold value, the microvalve will open automatically, and the gas will be released rapidly, relying on the positive feedback of interface mechanics. Using this device, the gas buoyancy potential energy entering the energy harvesting system per unit time can be increased by a factor of more than 30. Compared with a traditional bubble energy harvesting system without a switch, this system achieves a 19.55-fold increase in output power and a 5.16-fold enhancement in electrical energy production. The potential energy of ultralow flow rate bubbles (as low as 3.97 mL/min) is effectively collected. This work provides a new design philosophy for passive automatic-switching control of gas-liquid two-phase fluids, presenting an effective approach for harvesting of buoyancy potential energy from low-gas-flux bubble seepages. This opens a promising avenue for in situ energy supply for subsea scientific observation networks.

Nanosheet-hydrogel composites: from preparation and fundamental properties to their promising applications

Hydrogels are an important class of soft materials with elastic and intelligent properties. Nevertheless, these traditional hydrogels usually possess poor mechanical properties and limited functions, which greatly restrict their further applications. With the rapid development of nanotechnology, there have been significant advances in the design and fabrication of functional nanocomposite hydrogels with unique properties and functions. Among various materials, nanosheets with planar topography, large specific surface areas, and versatile physicochemical properties have attracted intense research interest. Herein, this review summarises the synthesis mechanisms, fundamental properties, and promising applications of nanosheet-incorporated hydrogels. In particular, how the nanosheet structure is applied to improve the overall performance of the hydrogel in each application is emphasized. Additionally, the current challenges and prospects are briefly discussed in this area. We expect that the combination of nanosheets and hydrogels can attract more researchers' interest and bring new opportunities in the future.

Remotely Controlled Light/Electric/Magnetic Multiresponsive Hydrogel for Fast Actuations

As a kind of soft smart material, hydrogel actuators have extensive development prospects, but it is still difficult for these actuators to integrate multiresponsiveness, multiple remote actuation, high strength, fast responsiveness, and programmable complex deformation. Herein, we have explored an anisotropic bilayer hydrogel actuator with an Fe3O4/co-poly(isopropylacrylamide-4-benzoylphenyl acrylate) [Fe3O4/P(NIPAM-ABP)] active layer and an isotropic conductive adhesive (ICAs) passive layer based on the layer-by-layer method. Benefiting from the fibrosis and porosity of the Fe3O4/P(NIPAM-ABP) hydrogel, the ICAs-Fe3O4/P(NIPAM-ABP) hydrogel actuator has excellent mechanical strength (tensile strength of 3.1 ± 0.3 MPa) and response speed (temperature (45 °C): bending speed of 2400.3°/s; near-infrared (NIR) light: bending speed of 356.4°/s; electricity (2 V): bending speed of 180°/s; water (10 °C): recovery speed of 30.0°/s). In addition, the good photothermal properties and magnetic conductivity of Fe3O4 nanoparticles provide precise remotely controllable light- and magnetic-actuated properties for the hydrogel actuator. The Ag microsheets with excellent conductivity (1.4 × 104 S/cm) provide remotely controllable electrical-actuated property for the hydrogel actuator. Combined with the responsiveness of P(NIPAM-ABP), the actuator can achieve short-range actuation including temperature-, ethanol-, and salt-responses. More importantly, it can achieve remote actuation including light, electrical, and magnetic responses. Finally, the Fe3O4/P(NIPAM-ABP) fibers can provide excellent anisotropic structures for the actuator to achieve precise deformational programmability. Inspired by some phenomena in nature, several actuating devices with the above characteristics have been successfully developed. This study can provide a general method for multifunctional anisotropic hydrogel actuators and will provide a new strategy for exploring smart materials suitable for complex bioinspired systems.

Biomimetic ultra-strong, ultra-tough, degradable cellulose-based composites for multi-stimuli responsive shape memory

Fabrication of ultra-strong, ultra-tough, sustainable, and degradable bio-based composites is urgently needed but remains challenging. Here, a biomimetic sustainable, degradable, and multi-stimuli responsive cellulose/PCL/Fe₃O₄ composite with ultra-strong mechanical strength and ultra-high toughness was developed. To prepare the proposed composites, the soft poly(ε-caprolactone) (PCL) side chain was grafted onto the rigid cellulose backbone, then the cellulose graft copolymer (EC-g-PCL) reacted with rigid hexamethylenediamine modified Fe₃O₄ nanoparticle (Fe₃O₄-NH₂) to construct the crosslinking network using MDI-50 as a crosslinker. Given by the construction of crosslinking network and the "hard" and "soft" interactive structure, the composites showed ultra-strong mechanical strength (25.7 MPa) and ultra-high toughness (107.0 MJ/m³), and the composite specimen could lift a weight of approximately 21,200 times its mass. The composites also exhibited rapid degradation ability with high degradation efficiency. In addition, the composites showed excellent thermal responsive shape memory property with a shape recovery ratio above 96 %. Most importantly, the Fe₃O₄ nanoparticles endowed the composites with photothermal conversion property, the composites exhibited superior NIR light-triggered shape memory capability. The EC-g-PCL/Fe₃O₄ composites with ultra-strong mechanical strength and ultra-high toughness have promising applications in heavy-lift, object transportation, and self-tightening knots.

Poly(N-isopropylacrylamide) Based Electrically Conductive Hydrogels and Their Applications

Poly(N-isopropylacrylamide) (PNIPAM) based electrically conductive hydrogels (PNIPAM-ECHs) have been extensively studied in recent decades due to their thermal-responsive (leading to the volume change of hydrogels) and electrically conductive performance. The incorporation of conductive components into the PNIPAM hydrogel network makes it become conductive hydrogel, and as a result, the PNIPAM hydrogel could become sensitive to an electrical signal, greatly expanding its application. In addition, conductive components usually bring new stimuli-responsive properties of PNIPAM-based hydrogels, such as near-infrared light and stress/strain responsive properties. PNIPAM-ECHs display a wide range of applications in human motion detection, actuators, controlled drug release, wound dressings, etc. To summarize recent research advances and achievements related to PNIPAM-ECHs, this manuscript first reviews the design and structure of representative PNIPAM-ECHs according to their conductive components. Then, the applications of PNIPAM-ECHs have been classified and discussed. Finally, the remaining problems related to PNIPAM-ECHs have been summarized and a future research direction is proposed which is to fabricate PNIPAM-ECHs with integrated multifunctionality.

Silicene/poly(N-isopropylacrylamide) smart hydrogels as remote light-controlled switches

Smart hydrogels with good flexibility and biocompatibility have been widely used. The common near-infrared (NIR) photothermal agents are facing a trade-off between good photothermal-conversion efficiency and high biocompatibility. Therefore, developing new metal-free photothermal agents with low cost, high biocompatibility and excellent phase stability is still in urgent need. In this study, we successfully combined poly(N-isopropylacrylamide) (PNIPAM) with the two-dimensional (2D) silicene nanosheets via the in situ polymerization method. Attributed to the thermal-responsive nature of PNIPAM and the excellent photothermal properties of 2D silicene, the obtained silicene/PNIPAM composite hydrogels exhibited dual thermal and NIR responsive properties. This smart hydrogel showed rapid, reversible and repeatable NIR light-responsive behaviors. The volume of this smart hydrogels can shrink significantly under NIR irradiation and recover to its original size without the NIR irradiation. Remote near-infrared light-controlled microfluidic pipelines and electronic switches based on obtained silicene/PNIPAM composite hydrogels were also demonstrated. This work significantly broadens the application prospects of silicene-based hydrogels in remote light-controlled devices.

Green and sustainable cellulose-based shape memory composites with excellent conductivity for temperature warning

Green and sustainable cellulose-based composites containing poly(ε-caprolactone) (PCL) with temperature-induced shape memory properties and conductivity performance are presented. The composites are fabricated by in situ polymerization of ε-caprolactone (ε-CL) monomer in three-dimensional porous cellulose gels, and then silver-porous cellulose gel/poly(ε-caprolactone) (Ag-Cell/PCL) composites are fabricated by depositing Ag onto the surface of porous cellulose gel/poly(ε-caprolactone) (Cell/PCL) composites. The addition of PCL not only improves the mechanical properties of the Cell/PCL composites but also endows them with excellent shape memory properties. The Cell/PCL composites exhibit a high shape-fixing rate (98.9%) and can recover to their original shape within 8 s without external force. In addition, the Ag-Cell/PCL composites show superior and stable conductivity under different bending angles. Finally, a temperature warning sensor with fast performance is successfully designed using Ag-Cell/PCL composites. This work provides a means to develop temperature warning systems based on shape memory polymers.

A programmable bilayer hydrogel actuator based on the asymmetric distribution of crystalline regions

A novel strategy to fabricate bilayer hydrogel actuators based on the asymmetric distribution of crystalline regions across the bilayer structures was proposed. By employing PVA polymer chains into an alkali solvent-derived chitosan hydrogel matrix, chitosan/PVA hybrid bilayer hydrogels with both excellent responsive bending and mechanical properties were obtained as pH-controlled manipulators. In the design, the chitosan/PVA hydrogels upon treatment with freeze-thawing cycles were taken as the first monolayer, where excessive crystalline regions appeared. The original chitosan/PVA hydrogel as the second monolayer was then integrated into one bilayer device through the chemical-crosslinking of epichlorohydrin at the interface. The results showed that the resultant chitosan/PVA bilayer hydrogel actuator with a weight ratio of 3 : 1 displayed better sensitivity upon exposure to stimuli. The actuation behaviors are strongly dependent on experimental parameters such as the pH, PVA content and the chemical-crosslinking density. It is proposed that the driving force originates from the asymmetric distribution of crystalline regions, thus resulting in differential swelling ratios between the monolayers. In addition, programmable 3D shape transformations were achieved by using the bilayer hydrogel with designed 2D geometric patterns, and the tailored gripper-like hydrogel actuator can successfully capture and transport the cargo. Moreover, this actuation behavior can be erased and re-written on demand under certain conditions. Taking advantage of this universal strategy, more attractive actuators derived from synthetic or natural polymers in combination with PVA are highly expected, which can be used as smart soft robots in various fields such as manipulators, grippers, and cantilever sensors.

Stimulus-responsive nanomaterials under physical regulation for biomedical applications

Cancer is a growing threat to human beings. Traditional treatments for malignant tumors usually involve invasive means to healthy human tissues, such as surgical treatment and chemotherapy. In recent years the use of specific stimulus-responsive materials in combination with some non-contact, non-invasive stimuli can lead to better efficacy and has become an important area of research. It promises to develop personalized treatment systems for four types of physical stimuli: light, ultrasound, magnetic field, and temperature. Nanomaterials that are responsive to these stimuli can be used to enhance drug delivery, cancer treatment, and tissue engineering. This paper reviews the principles of the stimuli mentioned above, their effects on materials, and how they work with nanomaterials. For this aim, we focus on specific applications in controlled drug release, cancer therapy, tissue engineering, and virus detection, with particular reference to recent photothermal, photodynamic, sonodynamic, magnetothermal, radiation, and other types of therapies. It is instructive for the future development of stimulus-responsive nanomaterials for these aspects.