Near‐Infrared Light‐Driven Shape‐Morphing of Programmable Anisotropic Hydrogels Enabled by MXene Nanosheets

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.

Magneto-Orientation of Magnetic Double Stacks for Patterned Anisotropic Hydrogels with Multiple Responses and Modulable Motions

Reported here is a multi-response anisotropic poly(N-isopropylacrylamide) hydrogel developed by using a rotating magnetic field to align magnetic double stacks (MDSs) that are fixed by polymerization. The magneto-orientation of MDSs originates from the unique structure with γ-Fe₂ O₃ nanoparticles sandwiched by two silicate nanosheets. The resultant gels not only exhibit anisotropic optical and mechanical properties but also show anisotropic responses to temperature and light. Gels with complex ordered structures of MDSs are further devised by multi-step magnetic orientation and photolithographic polymerization. These gels show varied birefringence patterns with potentials as information materials, and can deform into specific configurations upon stimulations. Multi-gait motions are further realized in the patterned gel through dynamic deformation under spatiotemporal light and friction regulation by imposed magnetic force. The magneto-orientation assisted fabrication of hydrogels with anisotropic structures and additional functions should bring opportunities for gel materials in biomedical devices, soft actuators/robots, etc.

Dual-Driven Mechanically and Tribologically Adaptive Hydrogels Solely Constituted of Graphene Oxide and Water

Although mythologies and fictions have recorded living creatures fully composed of inorganics, it is however hard to turn inorganic constituents into lifelike materials in reality as they usually do not possess characteristics required for constructing a living organism. Here, we report to our knowledge the first biomimetic hydrogel in response to both pH and temperature variations that solely comprises graphene oxide and water. The hydrogel is capable of abruptly and reversibly switching its mechanical and tribological properties by more than 10-fold and 5-fold magnitudes, respectively, as a result of pH- and/or thermal-induced topological reconfiguration of its internal microstructure and ordering. Such behavior closely mimics some natural living organisms such as muscles and sea cucumbers. The hydrogel also shows a low coefficient of friction at pH 2 and room temperature, indicating it a potent smart lubricant free of any flammable and toxic organic base oils and additives.

Modulating the Activity of Enzyme in Metal-Organic Frameworks Using the Photothermal Effect of Ti3C2 Nanosheets

Enzymes are versatile catalysts with high potential in various applications, and much attention has been paid to the stability improvement of native enzymes and activity modulation. Encapsulation in metal-organic frameworks (MOFs) as an efficient strategy for protecting fragile native enzymes while modulating the activity of enzymes remotely, which is practically demanded, has rarely been explored in MOF-encapsulated enzymes. Herein, Ti₃C₂ nanosheets exhibiting photothermal effect and biocompatibility were encapsulated in Cyt c-embedded ZIF-8 to tailor the enzymatic activity remotely by near-infrared (NIR) irradiation for the first time. By exposure to NIR light, the temperature of an aqueous solution containing Ti₃C₂/Cyt c@ZIF-8 increases obviously (up to 15 °C), while that of Cyt c@ZIF-8 shows no change. The enzymatic activity in the composites with a certain amount of nanosheets increases, which is attributed to the created defect and transformed microenvironment caused by the introduction of nanosheets. Importantly, the enzymatic activity in ZIF-8 can be further enhanced up to 150% under NIR light irradiation, and this enhancement can be modulated flexibly by varying laser power density. Our investigations indicate that Ti₃C₂ nanosheets are promising candidates for modulating the activity of encapsulated enzymes remotely.

MXene chemistry, electrochemistry and energy storage applications

The diverse and tunable surface and bulk chemistry of MXenes affords valuable and distinctive properties, which can be useful across many components of energy storage devices. MXenes offer diverse functions in batteries and supercapacitors, including double-layer and redox-type ion storage, ion transfer regulation, steric hindrance, ion redistribution, electrocatalysts, electrodeposition substrates and so on. They have been utilized to enhance the stability and performance of electrodes, electrolytes and separators. In this Review, we present a discussion on the roles of MXene bulk and surface chemistries across various energy storage devices and clarify the correlations between their chemical properties and the required functions. We also provide guidelines for the utilization of MXene surface terminations to control the properties and improve the performance of batteries and supercapacitors. Finally, we conclude with a perspective on the challenges and opportunities of MXene-based energy storage components towards future practical applications.

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.

Fast-Response Bioinspired Near-Infrared Light-Driven Soft Robot Based on Two-Stage Deformation

Herein, we report a remotely controlled soft robot employing a photoresponsive nanocomposite synthesized from liquid crystal elastomers (LCEs), high elastic form-stable phase change polymer (HEPCP), and multiwalled carbon nanotubes (MWCNTs). Possessing a two-stage deformation upon exposure to near-infrared (NIR) light, the LCE/HEPCP/MWCNT (LHM) nanocomposite allows the soft robot to exhibit an obvious, fast, and reversible shape change with low detection limitations. In addition to the deformation and bending of the LCE molecular chains itself, the HEPCP in the composite material can also be triggered by a reversible solid-liquid transition due to the temperature rise caused by MWCNTs, which further promotes the change of the LCE. In particular, the proposed photodriven LHM soft robot can bend up to 180° in 2 s upon NIR stimulation (320 mW, distance of 5 cm) and generate recoverable, dramatic, and sensitive deformation to execute various tasks including walking, twisting, and bending. With the capacity of imitating biological behaviors through remote control, the disruptive innovation developed here offers a promising path toward miniaturized untethered robotic systems.

Programming Soft Shape-Morphing Systems by Harnessing Strain Mismatch and Snap-Through Bistability: A Review

Multi-modal and controllable shape-morphing constitutes the cornerstone of the functionalization of soft actuators/robots. Involving heterogeneity through material layout is a widely used strategy to generate internal mismatches in active morphing structures. Once triggered by external stimuli, the entire structure undergoes cooperative deformation by minimizing the potential energy. However, the intrinsic limitation of soft materials emerges when it comes to applications such as soft actuators or load-bearing structures that require fast response and large output force. Many researchers have explored the use of the structural principle of snap-through bistability as the morphing mechanisms. Bistable or multi-stable mechanical systems possess more than one local energy minimum and are capable of resting in any of these equilibrium states without external forces. The snap-through motion could overcome energy barriers to switch among these stable or metastable states with dramatically distinct geometries. Attributed to the energy storage and release mechanism, such snap-through transition is quite highly efficient, accompanied by fast response speed, large displacement magnitude, high manipulation strength, and moderate driving force. For example, the shape-morphing timescale of conventional hydrogel systems is usually tens of minutes, while the activation time of hydrogel actuators using the elastic snapping instability strategy can be reduced to below 1 s. By rationally embedding stimuli-responsive inclusions to offer the required trigger energy, various controllable snap-through actuations could be achieved. This review summarizes the current shape-morphing programming strategies based on mismatch strain induced by material heterogeneity, with emphasis on how to leverage snap-through bistability to broaden the applications of the shape-morphing structures in soft robotics and mechanical metamaterials.

A facile approach for anisotropic hydrogel with light-regulated stiffness and its application to achieve mechanical toughening

Many load-bearing tissues in nature obtain high toughness by fabricating anisotropic structures with spatially regulated composition and modulus at macroscale. This reality inspires a toughening strategy for hydrogel based on the controlling of modulus heterogeneity. Herein, a facile approach to realize light-regulated spatial modulus heterogeneity with large contrast in hydrogel is proposed. Ferric citric acid complex is used as a light-responsive ionic crosslinker, which can first stiffen an alginate/polyacrylamide hydrogel by coordinating with the alginate to form another network, then realize light-triggered softening through photoreduction of ferric ions. Based on this, a stripe-patterned hydrogel with alternating stiff and soft segments can be fabricated through photopatterning. The modulus contrast between the stiff and soft phases can be adjusted by control of several influence factors and the maximum modulus contrast reach up to 87 times. As a result, the toughness of the stripe-patterned hydrogel is enhanced by 3.5 times comparing to that hydrogel without pattern. This approach shows great potential in synthesis of smart hydrogel with light-programmable mechanical performances, and may be widely applicable for the hydrogels with functional groups that can coordinate with metal ions. This article is protected by copyright. All rights reserved.

pH Oscillator-Driven Jellyfish-like Hydrogel Actuator with Dissipative Synergy between Deformation and Fluorescence Color Change

Development of soft actuators with complex practical functions is significant for imitating the behaviors of living organisms. However, it is still a challenge to fabricate artificial soft actuators with jellyfish-like synergistic deformation and fluorescence color change (SDFC) and autonomous dynamic behavior, but such a system could obviously endow the classic soft actuators with more functions. Herein, we proposed to utilize tetra(4-pyridylphenyl)ethylene (TPE-4N) luminogen with pH-responsive aggregation-induced emission (AIE) to fabricate the AIE active hydrogel, which could be further employed to obtain an anisotropic bilayer soft actuator based on strong interfacial adhesion with acrylic acid (AA) gels. Furthermore, artificial flower-shape actuators showing SDFC behaviors were demonstrated. On the basis of these findings, jellyfish-inspired autonomous gel actuators driven by a pH oscillator have been fabricated, in which periodical SDFC behaviors completely regulated by the system itself without repetitive on/off switches of external stimuli were well synchronized with the pH oscillator. The described combination of nonlinear chemistry and responsive hydrogels actuator opens pathways toward out-of-equilibrium SDFC devices with autonomous behavior useful for biomimetic fields.

Near-Infrared Light-Driven Shape-Programmable Hydrogel Actuators Loaded with Metal-Organic Frameworks

Shape-programmable hydrogel-based soft actuators that can adaptively respond to external stimuli are of paramount significance for the development of bioinspired aquatic smart soft robots. Herein, we report the design and synthesis of near-infrared (NIR) light-driven hydrogel actuators through in situ photopolymerization of poly(N-isopropylacrylamide) (PNIPAM) hydrogels loaded with metal-organic frameworks (MOFs) onto the surface of the poly(dimethylsiloxane) (PDMS) thin film. The MOFs can not only function as an excellent photothermal nanotransducer but also accelerate the adsorption/desorption of water due to their porous nanostructure, which speeds up the response rate of the actuators. Shape-programmable hydrogel actuators are fabricated by tailoring the patterning of PDMS thin film, and thus different shape-morphing modes such as directional bending and chiral twisting are observed under the NIR light irradiations. As the proof-of-concept demonstrations, an artificial hand, biomimetic mimosa, and flower are conceptualized with light-driven MOF-containing hydrogel actuators. Interestingly, we are able to achieve an octopus-inspired light-driven soft swimmer upon cyclic NIR illumination due to the fast photoresponsiveness of as-prepared hydrogel actuators. This work can offer insights for fabricating programmable and reconfigurable smart aquatic soft actuators, thus shining a light into their potential applications in emerging fields including soft robots, biomedical devices, and beyond.

A Shift from Efficiency to Adaptability: Recent Progress in Biomimetic Interactive Soft Robotics in Wet Environments

Research field of soft robotics develops exponentially since it opens up many imaginations, such as human-interactive robot, wearable robots, and transformable robots in unpredictable environments. Wet environments such as sea and in vivo represent dynamic and unstructured environments that adaptive soft robots can reach their potentials. Recent progresses in soft hybridized robotics performing tasks underwater herald a diversity of interactive soft robotics in wet environments. Here, the development of soft robots in wet environments is reviewed. The authors recapitulate biomimetic inspirations, recent advances in soft matter materials, representative fabrication techniques, system integration, and exemplary functions for underwater soft robots. The authors consider the key challenges the field faces in engineering material, software, and hardware that can bring highly intelligent soft robots into real world.

Hydrogels with both mechanical strength and luminescence anisotropy

How to produce luminescent materials with anisotropic emission properties in a universal manner remains an open issue. We here report a range of hydrogels with both anisotropic mechanical strength and...

Precisely Defining Local Gradients of Stimuli-Responsive Hydrogels for Complex 2D-to-4D Shape Evolutions

The intellectualization and complication of existing self-shaping materials are limited by the inseparable monotonic relationship between their deformation rate and deformation degree (i.e., a higher deformation rate is accompanied by a high deformation degree). This causes that they can only deform from 2D to 3D states. Here, a simple yet versatile strategy to decouple the monotonic correlation between the deformation rate and deformation degree of self-shaping hydrogels is presented for achieving complex deformations from 2D to temporary 3D to 3D (2D-to-4D). It is demonstrated that when the gradient hydrogels prepared by photopolymerization possess dense polymer networks, the local regions with a high deformation rate can exhibit a low deformation degree. The resulting hydrogels can thus deform in a novel 2D-to-4D mode under external stimuli. During the deformation, they first transform into the temporary shapes induced by the local deformation rate difference, and then transform into the final shapes determined by the local deformation degree difference. Through controlling the ultraviolet irradiation direction and time to precisely program the local gradients of self-shaping hydrogels, they can be designed to produce various unprecedented yet controllable 2D-to-4D shape evolutions on demand, such as transformable origami, sequential gesture actions in finger-guessing games, mobile octopuses, time switch, etc.

Bioinspired ionic hydrogel materials with excellent antifouling properties and high conductivity in dry and cold environments

A bioinspired ionic hydrogel-based antifouling material with excellent adaptability has been constructed, featured with ultralow adhesion to various solid/viscous liquid deposition, high ionic conductivity, and excellent mechanical properties.

Recent advances in precursor-derived ceramics integrated with two-dimensional materials

This review focused on the recent advances in precursor-derived ceramics integrated with two-dimensional materials. Their fabrication methods, structures and applications were discussed in detail and the perspectives in this field were presented.

Mechanochromic, Shape-Programmable and Self-Healable Cholesteric Liquid Crystal Elastomers Enabled by Dynamic Covalent Boronic Ester Bonds

Endowing a cholesteric liquid crystal elastomer (CLCE) exhibiting helicoidal nanostructure with dynamically tailorable functionalities is of paramount significance for its emerging applications in diverse fields such as adaptive optics and soft robotics. Here, a mechanochromic, shape-programmable and self-healable CLCE is judiciously designed and synthesized through integrating dynamic covalent boronic ester bonds into the main-chain CLCE polymer network. The circularly polarized reflection of CLCEs can be reversibly and dynamically tuned across the entire visible spectrum by mechanical stretching. Thanks to the introduction of dynamic boronic ester bonds, the CLCEs were found to show robust reprogrammable and self-healing capabilities. The research disclosed herein can provide new insights into the development of 4D (color and 3D shape) programmable photonic actuators towards bioinspired camouflage, adaptive optical systems, and next-generation intelligent machines.

Specific recognition and photothermal release of circulating tumor cells using near-infrared light-responsive 2D MXene nanosheets@hydrogel membranes

2D materials with attractive optical properties are promising for individualized cancer immunotherapy. Isolation, capture, and release of circulating tumor cells (CTCs) are of great significance for promoting the process of early diagnosis of cancers. MXene nanosheets incorporated gelatin hydrogel offers the possibility of achieving near-infrared (NIR) light response to initiate the photothermal effect. Herein, the design and preparation of Ti₃C₂T_x MXene nanosheets-embedded thermoresponsive gelatin hydrogel membrane with NIR light-responsive for the specific capture and release of CTCs were reported. The membrane was fabricated by casting Ti₃C₂T_x-embedded gelatin onto a substrate and then modified with epithelial-cell adhesion-molecule antibody (anti-EpCAM) for the specific recognition and separation of CTCs from whole blood. The captured cells can be released without damage with dual-mode containing temperature-responsive release (gelatin deconstructed at 37 °C) and photothermal site-release (Ti₃C₂T_x induced by NIR light). Furthermore, we were able to achieve an average efficient release rate of 89 % of captured cells with stable cell viability of 87 % via the NIR light irradiation. This work may provide the promising potential for retrieval of single cells in clinical diagnosis.

Patterned Electrode Assisted One-Step Fabrication of Biomimetic Morphing Hydrogels with Sophisticated Anisotropic Structures

Anisotropic structures are ubiquitous in nature, affording fascinating morphing behaviors. Biomimetic morphing materials can be developed by spatially controlling the orientations of molecules or nanofillers that produce anisotropic responses and internal stresses under external stimuli. However, it remains a serious challenge to fabricate materials with sophisticated anisotropic architectures. Here, a facile strategy to fabricate morphing hydrogels with elaborately ordered structures of nanosheets, which are oriented under distributed electric field and immobilized by polymerization to form a poly(N-isopropylacrylamide) matrix, is proposed. Diverse sophisticated anisotropic structures are obtained by engineering the electric field through the patterns and relative locations of the electrodes. Upon heating, the monolithic hydrogels with through-thickness and/or in-plane gradients in orientation of the nanosheets deform into various three-dimensional configurations. After incorporating gold nanoparticles, the hydrogels become photoresponsive and capable of programmable motions, for example, dynamic twisting and flipping under spatiotemporal stimuli. Such a strategy of using patterned electrodes to generate distributed electric field should be applicable to systems of liquid crystals or charged particles/molecules to direct orientation or electrophoresis and form functional structures. The biomimetically architectured hydrogels would be ideal materials to develop artificial muscles, soft actuators/robots, and biomedical devices with versatile applications.

A conductive polyacrylamide hydrogel enabled by dispersion-enhanced MXene@chitosan assembly for highly stretchable and sensitive wearable skin

MXene is recognized as an ideal material for sensitive wearable strain sensors because of its unique advantages of conductivity, hydrophilicity and mechanical properties. However, conventional hydrogel sensors utilizing MXene as a conductive material inevitably encounter the excessive accumulation of MXene nanosheets during the process of synthesis, which limits the electron transmission, reduces the conductivity, and concurrently weakens the mechanical capability and sensitivity of sensors. Herein, we construct a dispersion-enhanced MXene hydrogel (DEMH) through a chitosan-induced self-assembly strategy for the first time. Charge transfer is carried out through the flow of a material or a collection of material microstructures, and thus the highly interconnected 3D MXene@Chitosan network provides fast transport channels for electrons, and the DEMH exhibits excellent conductivity and sensibility simultaneously. Besides, the electrostatic self-assembly between MXene and chitosan, and the supramolecular interactions between MXene, chitosan and polyacrylamide chain segment result in excellent mechanical strength (of up to 1900%) and flexibility of DEMH. Furthermore, the introduction of chitosan which possesses a high density of positively charged groups and MXene with semiconducting properties also endows sensor versatility, such as self-adhesion properties and antibacterial activity. This work develops a simple and cut-price strategy for combining MXene unaggregated into a hydrogel as a sensor with high conductivity, sensibility and flexibility. A simple and inexpensive strategy for avoiding self-stacking of two-dimensional conductive materials is proposed, which paves the way for a broad range of applications in electronic skin, human motion detection and intelligent devices.

Bifunctional Smart Hydrogel Dressing with Strain Sensitivity and NIR-Responsive Performance

Smart response hydrogel has a broad application prospect in human health real-time monitoring due to its responses to a variety of stimuli. In this study, we developed a novel smart hydrogel dressing based on conductive MXene nanosheets and a temperature-sensitive PNIPAm polymer. γ-Methacryloxypropyltrimethoxysilane (KH570) was selected to functionalize the surface of MXene further to improve the interface compatibility between MXene and PNIPAm. Our prepared K-M/PNIPAm hydrogel was found to have a strain-sensitive property, as well as a respond to NIR phase change and volume change. When applied as a strain flexible sensor, this K-M/PNIPAm hydrogel exhibited a high strain sensitivity with a gauge factor (GF) of 4.491, a broad working strain range of ≈250%, a fast response of ∼160 ms, and good cycle stability (i.e., 3000 s at 20% strain). Besides, this K-M/PNIPAm hydrogel can be used as an efficient NIR light-controlled drug release carrier to achieve on-demand drug release. This work paved the way for the application of smart response hydrogel in human health real-time monitoring and NIR-controlled drug release functions.

One-Step, Continuous Three-Dimensional Printing of Multi-Stimuli-Responsive Bilayer Microactuators via a Double-Barreled Theta Pipette

Although there has been extensive development and exploration of small-scale robots, the technological challenges associated with their complicated and high-cost fabrication processes remain unresolved. Here, we report a one-step, bi-material, high-resolution three-dimensional (3D) printing method for the fabrication of multi-stimuli-responsive microactuators. This method exploits a two-phase femtoliter ink meniscus formed on a double-barreled theta micropipette to continuously print a freestanding bilayer microstructure, which undergoes an asymmetric volume change upon the adsorption or desorption of water. We show that the 3D-printed bilayer microstructures exhibit reversible, reproducible actuation in ambient humidity or under illumination with infrared light. Our 3D printing approach can assemble bilayer segments for programming microscale actuation, as demonstrated by proof-of-concept experiments. We expect that this method will serve as the basis for flexible, programmable, one-step routes for the assembly of small-scale intelligent actuators.

Rapid Photothermal Responsive Conductive MXene Nanocomposite Hydrogels for Soft Manipulators and Sensitive Strain Sensors

Stimulus-responsive hydrogels are of great significance in soft robotics, wearable electronic devices, and sensors. Near-infrared (NIR) light is considered an ideal stimulus as it can trigger the response behavior remotely and precisely. In this work, a smart flexible stimuli-responsive hydrogel with excellent photothermal property and decent conductivity are prepared by incorporating MXene nanosheets into the physically cross-linked poly(N-isopropyl acrylamide) hydrogel matrix. Because of outstanding photothermal effect and dispersion of MXene, the composite hydrogel exhibits rapid photothermal responsiveness and excellent photothermal stability under the NIR irradiation. Furthermore, the anisotropic bilayer hydrogel actuator shows fast and controllable light-driven bending behavior, which can be used as a light-controlled soft manipulator. Meanwhile, the hydrogel sensor exhibits cycling stability and good durability in detecting various deformation and real-time human activities. Therefore, the present study involving the fabrication of MXene nanocomposite hydrogels for potential applications in remotely controlled actuator and wearable electronic device provides a new method for the development of photothermal responsive conductive hydrogels.

Stimulus-driven liquid metal and liquid crystal network actuators for programmable soft robotics

Sophisticated soft matter engineering has been endorsed as an emerging paradigm for developing untethered soft robots with built-in electronic functions and biomimetic adaptation capacities. However, the integration of flexible electronic components into soft robotic actuators is challenging due to strain mismatch and material incompatibilities. Herein, we report a general strategy to integrate electrically conductive liquid metals (LMs) and shape-morphing liquid crystal networks (LCNs) towards multifunctional and programmable soft robotics. A unique colloidal LM ink with superior adhesion and photothermal conversion efficiency was judiciously designed and fabricated by ultrasonicating LMs and miniature carboxylated gold nanorods (MiniGNR-COOH) in an aqueous suspension of biological bacterial cellulose. The designed nanocellulose-based colloidal LM ink is used for shape-deformable and electrically conductive LM-LCN soft robots that can be electro- and photo-thermally actuated. As proof-of-concept demonstrations, we present a light-fueled soft oscillator, an inchworm-inspired soft crawler and programmable robotic Shadow Play exhibiting multifunctional controllability. The strategy disclosed here could open up a new technological arena for advanced multifunctional soft materials with potential utility in bioinspired soft machines, integrated soft electronics, human-computer interaction and beyond.

Flexible Photodriven Actuator Based on Gradient-Paraffin-Wax-Filled Ti3C2Tx MXene Film for Bionic Robots

Due to their high flexibility and adaptability, bionic robots have great potential in applications such as healthcare, rescue, and surveillance. The flexible actuator is an essential component of the bionic robot and determines its performance. Even though much progress has been achieved in bionic robot research, there still exists a great challenge in preparing a flexible actuator with a large stroke, high sensitivity, fast response, low triggering power, and long lifetime. This study presents a flexible actuator based on a paraffin wax and Ti₃C₂T_x MXene (PW-MX) film composite. Such a flexible actuator delivers an excellent actuation performance, including a large curvature change (2.2 × 10² m^-1), high thermal sensitivity (4.6 m^-1/°C), low triggering power of light (76 mW/cm²), wavelength selectivity, fast response (0.38 s), and long lifetime (>20000 cycles). Due to the high thermal sensitivity and the strong infrared absorption of the PW-MX film, crawling motion of an inchworm robot based on PW-MX film can be triggered by infrared irradiation from the human finger. To mimic living organisms with bioluminescence, we prepared a PW-MX actuator with green fluorescence by doping PW-MX film with CdSe/ZnS quantum dots. The integration of luminescent function enables the PW-MX actuator to deliver information under light stimulation and to camouflage under a background of green foliage actively. With its merits of ease of fabrication and high actuation performance, the flexible PW-MX actuator is expected to lend itself to more applications in the future.

Light-Driven Self-Oscillating Actuators with Phototactic Locomotion Based on Black Phosphorus Heterostructure

Developing self-oscillating soft actuators that enable autonomous, continuous, and directional locomotion is significant in biomimetic soft robotics fields, but remains great challenging. Here, an untethered soft photoactuators based on covalently-bridged black phosphorus-carbon nanotubes heterostructure with self-oscillation and phototactic locomotion under constant light irradiation is designed. Owing to the good photothermal effect of black phosphorus heterostructure and thermal deformation of the actuator components, the new actuator assembled by heterostructured black phosphorus, polymer and paper produces light-driven reversible deformation with fast and large response. By using this actuator as mechanical power and designing a robot configuration with self-feedback loop to generate self-oscillation, an inchworm-like actuator that can crawl autonomously towards the light source is constructed. Moreover, due to the anisotropy and tailorability of the actuator, an artificial crab robot that can simulate the sideways locomotion of crabs and simultaneously change color under light irradiation is also realized.

Healable and Rearrangeable Networks of Liquid Crystal Elastomers Enabled by Diselenide Bonds

Based on liquid crystal elastomer (LCE) materials, hierarchically structured soft actuators can meet some requirements for "human-friendly" working mode and execute complex tasks with intelligent adaptation to environmental changes. However, few researchers have paid much attention to the preparation methods of multicomponent/hierarchical LCE actuators. In this communication, we demonstrate the successful integration of an exchangeable diselenide chain extender for the preparation of dynamic LCEs, which could be reprogrammed on heating or under visible light illumination. Moreover, the rearrangeable polydiselenide networks could be applied to develop the self-welding technology toward fabricating hierarchically structured LCE actuators with sophisticated deformability without using any auxiliary reagent (adhesive, tape, catalysts or initiator) during the assembling process.

MXene-Based Humidity-Responsive Actuators: Preparation and Properties

Water is a significant and abundant resource as well as a pure natural energy source. Many researchers have been reported on humidity-responsive actuators that mimick the humidity responsive behavior that widely exists in nature. Benefiting from advantages such as hydrophilicity, high electrical conductivity, and good dispersibility, MXenes (Ti₃ C₂ T_x ) show promising performance when applied to humidity-responsive actuators. This Minireview describes the preparation methods and structural characteristics of MXenes, and the mechanism of humidity-responsive actuators. Recent important advances of MXene materials in actuators are objectively reviewed and evaluated, and existing issues are discussed. In addition, the development of these systems is outlined from the aspects of MXene preparation, structure control, design and assembly, and applications, and provides new ideas and guidance for the development of the next generation of high-performance MXene-based humidity-responsive actuators.