Bioinspired Soft Robots Based on the Moisture-Responsive Graphene Oxide

Hydrogel-Based Continuum Soft Robots

This paper comprehensively reviews the latest advances in hydrogel-based continuum soft robots. Hydrogels exhibit exceptional flexibility and adaptability compared to traditional robots reliant on rigid structures, making them ideal as biomimetic robotic skins and platforms for constructing highly accurate, real-time responsive sensory interfaces. The article systematically summarizes recent research developments across several key dimensions, including application domains, fabrication methods, actuator technologies, and sensing mechanisms. From an application perspective, developments span healthcare, manufacturing, and agriculture. Regarding fabrication techniques, the paper extensively explores crosslinking methods, additive manufacturing, microfluidics, and other related processes. Additionally, the article categorizes and thoroughly discusses various hydrogel-based actuators responsive to solute/solvent variations, pH, chemical reactions, temperature, light, magnetic fields, electric fields, hydraulic/electro-osmotic stimuli, and humidity. It also details the strategies for designing and implementing diverse sensors, including strain, pressure, humidity, conductive, magnetic, thermal, gas, optical, and multimodal sensors. Finally, the paper offers an in-depth discussion of the prospective applications of hydrogel-based continuum soft robots, particularly emphasizing their potential in medical and industrial fields. Concluding remarks include a forward-looking outlook highlighting future challenges and promising research directions.

Untethered Soft Robots Based on 1D and 2D Nanomaterials

Biological structures exhibit autonomous and intelligent behaviors, such as movement, perception, and responses to environmental changes, through dynamic interactions with their surroundings. Inspired by natural organisms, future soft robots are also advancing toward autonomy, sustainability, and interactivity. This review summarizes the latest achievements in untethered soft robots based on 1D and 2D nanomaterials. First, the performance of soft actuators designed with different structures is compared. Then, the development of basic locomotion forms, including crawling, jumping, swimming, rolling, gripping, and multimodal, mimicking biological motion mechanisms under dynamic stimuli, is discussed. Subsequently, various self-sustained movements based on imbalance mechanisms under static stimuli are introduced, including light tracking, self-oscillating, self-crawling, self-rolling, and flying. Following that, the progress in soft actuators integrated with additional functionalities such as sensing, energy harvesting, and storage is summarized. Finally, the challenges faced in this field and the prospects for future development are discussed.

Reversible Antireflection Materials Inspired by Cicada Wings for Anticounterfeit and Photovoltaic Cells

Antireflection (AR) surfaces are essential for the fields of flexible displays, photovoltaic industry, medical endoscope, intelligent windows, etc. Although natural creatures with well-organized micro/nanostructures have provided some coupling design principles for the rapid development of bioinspired AR materials, the mechanical vulnerability, poor flexibility, and nonadjustability have been pointed out as the drawback of these nanostructures. Here, a bioinspired reversible AR film with 4% reflectivity, 90% transmittance, and 9% haze in broadband (400-900 nm) was prepared. The flexible switching of AR performance enhancement and weakening throughout the visible wavelength band has been achieved by controlling the reversible change in the morphology of the interface structure. A variety of patterned film samples can be obtained by simply changing the template, which can be used in intelligent identification fields such as anticounterfeiting. The cycle test and photoelectric test show that the bionic reversible antireflection structure has certain stability and can effectively reduce the loss of photovoltaic cell conversion efficiency caused by mechanical deformation. It has broad application prospects in the fields of anticounterfeiting, intelligent window, flexible display, photoelectric element, and so on.

Recyclable Chitosan-Modified Cellulose Fiber Porous Structure for Sensitive and Robust Moisture-Driven Actuators and Automatic Cooling Textiles

Moisture-driven actuators featuring programmable stimuli-responsiveness and a rapid response have garnered substantial research attention. Cellulose-based actuators face challenges, including prolonged and unstable responsiveness, along with inadequate interfacial bonding. Herein, we developed a bilayer structured moisture actuator by integrating multiscale cellulose fibers with chitosan. The protonated chitosan forms strong electrostatic attractions with negatively charged cellulose nanofibrils (CNF), achieving a robust interfacial interaction. Leveraging the hierarchically porous structure and varying hygroscopicity of microfibrillated cellulose (MFC) and CNF, the film establishes an effective wettability gradient, enabling a stable and rapid moisture actuation performance. The bilayer film exhibits large deformation toward moisture with a bending angle of 60°, a short response time of 12 s, good stability over 50 wetting and drying cycles, and promising recyclability. Harnessing these advantageous properties, the bilayer film was demonstrated for its applications in automatic cooling textiles, contactless electrical switches, and artificial moisture-activated muscles, showing great potential for practical use.

Main-Chain Azobenzene Poly(ether ester) Multiblock Copolymers for Strong and Tough Light-Driven Actuators

The stimulus-responsive polymeric materials have attracted great research interest, especially those remotely manipulated materials with potential applications in actuators and soft robotics. Here we report a photoresponsive main-chain actuator based on azobenzene poly(ether ester) multiblock copolymer (mBCP) thermoplastic elastomers, (PTAD-b-PTMO-b-PTAD)n, which were synthesized by a cascade polycondensation-coupling ring-opening polymerization method using poly(tetramethylene oxide) (PTMO) and azobenzene-containing cyclic oligoesters (COTADs) as monomers. The thermal, mechanical, and microphase separation behaviors of mBCPs could be flexibly tuned by altering the ratios of soft-to-hard segments and block number (n). The oriented azobenzene mBCP fibers were prepared by melt spinning, showing reversible photoresponsive properties with remarkably high strength (∼1000 MPa) and high elongation at break comparable to spider silks. Fast photoinduced bending and contraction were successfully achieved in these fibers with high work and power densities and energy conversion efficiency, enabling it to lift up about 250 times of its own weight. Moreover, it can take out materials inside the tube by UV-light control. These fibers could be applied in light-driven actuators or telecontrolled robot arms.

Is Low Polydispersity Always Beneficial? Exploring the Impact of Size Polydispersity on the Microstructure and Rheological Properties of Graphene Oxide

Graphene oxide (GO) is a promising material widely utilized in advanced materials engineering, such as in the development of soft robotics, sensors, and flexible devices. Considering that GOs are often processed using solution-based methods, a comprehensive understanding of the fundamental characteristics of GO in dispersion states becomes crucial given their significant influence on the ultimate properties of the device. GOs inherently exhibit polydispersity in solution, which plays a critical role in determining the mechanical behavior and flowability. However, research in the domain of 2D colloids concerning the effects of GO's polydispersity on its rheological properties and microstructure is relatively scant. Consequently, gaining a comprehensive understanding of how GO's polydispersity affects these critical aspects remains a pressing concern. In this study, we aim to investigate the dispersions and structure of GOs and clarify the effect of polydispersity on the rheological properties and yielding behavior. Using a rheometer, polarized optical microscopy, and small-angle X-ray scattering, we found that higher polydispersity in the same average size leads to overall improved rheological properties and higher flowability during yielding. Thus, our study can be beneficial in the employment of polydispersity in the processing of GO such as 3D printing and fiber spinning.

Laser-Printed All-Carbon Responsive Material and Soft Robot

Responsive materials and actuators are the basis for the development of various leading-edge technologies but have so far mostly been designed based on polymers, incurring key limitations related to sensitivity and environmental tolerance. This work reports a new responsive material, laser-printed carbon film (LPCF), produced via direct laser transformation of a liquid organic precursor and consists of graphitic and amorphous carbons. The high activity of amorphous carbon combined with the dual-gradient structure enables the LPCF to have a actuation speed of 9400° s-1 in response to the stimulus of organic vapor. LPCF exhibits a conductivity of 950 S m-1 and excellent resistance to various extreme environmental conditions, which are unachievable for polymer-based materials. Additionally, an LPCF-based all-carbon soft robot that can mimic the complex continuous backward somersaulting motions without manual intervention is constructed. The locomotion velocity of the robot reaches a value of 1.19 BL s-1, which is almost one to two orders of magnitude faster than that of reported soft robots. This work not only offers a new paradigm for highly responsive materials but also provides a great design and engineering example for the next generation of biomimetic robots with life-like performance.

Multiresponse Liquid Metal Bionic Flexible Actuator

The flexible actuator has attracted significant interest for its ability to respond flexibly to external stimuli, especially for renewable natural energy sources. However, the flexible actuator faces issues such as inadequate sensitivity and inability to achieve synergistic responses. Therefore, we prepared a highly sensitive flexible actuator by mixing liquid metal (LM) with poly(vinylpyrrolidone) (PVP), graphene oxide (GO), and coating the resulting mixtures onto poly(ethylene terephthalate) (PET) substrate materials using the rod coating process. The flexible actuator responds quickly to near-infrared light and humidity and can be rapidly transformed from flat to curved with a maximum angular change of 540°. By demonstrating the flexible actuator in action, it can be used to create a crawling robot that mimics the movement of an inchworm on a leaf, as well as a gripper capable of lifting objects 5 times its weight, and a crawling robot that moves forward, turns left, and then right. Flexible actuators hold significant promise for applications in emerging fields such as advanced bionics and artificial intelligence.

Preparation of patterned hydrogels for anti-counterfeiting and directional actuation by shear-induced orientation of cellulose nanocrystals

Hydrogels with anisotropic structures are of great interest in the fields of bionic actuators, sensing and anti-counterfeiting due to their unique optical and stimulus response properties. Here we report an anisotropic cellulose nanocrystals/polyacrylamide (CNC/PAM) hydrogel with a patterned structure obtained by shear-induced orientation of CNC in precursor solution. Due to the difference in affinity between different slider surfaces and the precursor, patterned structures with different interference colors were realized by adhering the polypropylene (PP) film with a specific pattern to the bottom glass slider, which leads to differences in CNC orientation in different areas. These interfere color patterns can only be observed between crossed polarization, allowing the hydrogel to be used in applications of anti-counterfeiting and information encryption. Moreover, a complex and controllable 3D deformation is realized by introducing "zebra crossing" structure in the hydrogel. The opening and closing processes of flowers are vividly mimicked using the reversible swelling and shrinking properties of hydrogels in water and salt solutions, making the hydrogel promising for soft actuators.

Single-step precision programming of decoupled multiresponsive soft millirobots

Stimuli-responsive soft robots offer new capabilities for the fields of medical and rehabilitation robotics, artificial intelligence, and soft electronics. Precisely programming the shape morphing and decoupling the multiresponsiveness of such robots is crucial to enable them with ample degrees of freedom and multifunctionality, while ensuring high fabrication accuracy. However, current designs featuring coupled multiresponsiveness or intricate assembly processes face limitations in executing complex transformations and suffer from a lack of precision. Therefore, we propose a one-stepped strategy to program multistep shape-morphing soft millirobots (MSSMs) in response to decoupled environmental stimuli. Our approach involves employing a multilayered elastomer and laser scanning technology to selectively process the structure of MSSMs, achieving a minimum machining precision of 30 μm. The resulting MSSMs are capable of imitating the shape morphing of plants and hand gestures and resemble kirigami, pop-up, and bistable structures. The decoupled multistimuli responsiveness of the MSSMs allows them to conduct shape morphing during locomotion, perform logic circuit control, and remotely repair circuits in response to humidity, temperature, and magnetic field. This strategy presents a paradigm for the effective design and fabrication of untethered soft miniature robots with physical intelligence, advancing the decoupled multiresponsive materials through modular tailoring of robotic body structures and properties to suit specific applications.

Direct-Write Printed Slippery Surface for Assembling a High-Quality Graphene Structure and Its Application in Flexible Electric Actuators

Graphene is a two-dimensional honeycomb-like nanomaterial generated by carbon atoms in sp2 hybridized orbitals to form a hexagonal lattice structure with excellent electrical, optical, and mechanical properties. The solution process method has been widely used to realize large-area patterned graphene structures for high-performance devices. In the method, graphene usually needs to be dispersed in solution, and the π-π bonding gravitational interactions between graphene sheets would lead to uncontrollable structures in solution and difficulty in obtaining high performance. In this work, a patterned graphene oxide (GO) structure with controllable thickness and layer spacing was realized by a direct-write printed slippery surface, which was used as a slippery limited template. After reducing GO into reduced graphene oxide (rGO), a flexible electric pattern with a conductivity of up to 6.425 × 103 S/m was realized. Furthermore, the patterned rGO structure was transferred on polydimethylsiloxane (PDMS), which could generate less than a 5% change in resistance after 10,000 consecutive bends, and an anisotropic expansion based on rGO and PDMS materials under electro-thermal coupling. The patterned rGO structures could meet the performance requirements of highly sensitive and complex deformation applications as flexible electric actuators. This study provides great research significance and application value for patterning high-quality graphene structures and realizing high-performance flexible electronic devices.

Interfacial Assembly of 2D Graphene-Derived Ion Channels for Water-Based Green Energy Conversion

The utilization of sustained and green energy is believed to alleviate increasing menace of global environmental concerns and energy dilemma. Interfacial assembly of 2D graphene-derived ion channels (2D-GDICs) with tunable ion/fluid transport behavior enables efficient harvesting of renewable green energy from ubiquitous water, especially for osmotic energy harvesting. In this review, various interfacial assembly strategies for fabricating diverse 2D-GDICs are summarized and their ion transport properties are discussed. This review analyzes how particular structure and charge density/distribution of 2D-GDIC can be modulated to minimize internal resistance of ion/fluid transport and enhance energy conversion efficiency, and highlights stimuli-responsive functions and stability of 2D-GDIC and further examines the possibility of integrating 2D-GDIC with other energy conversion systems. Notably, the presented preparation and applications of 2D-GDIC also inspire and guide other 2D materials to fabricate sophisticated ion channels for targeted applications. Finally, potential challenges in this field is analyzed and a prospect to future developments toward high-performance or large-scale real-word applications is offered.

Bioinspired Stimuli-Responsive Materials for Soft Actuators

Biological species can walk, swim, fly, jump, and climb with fast response speeds and motion complexity. These remarkable functions are accomplished by means of soft actuation organisms, which are commonly composed of muscle tissue systems. To achieve the creation of their biomimetic artificial counterparts, various biomimetic stimuli-responsive materials have been synthesized and developed in recent decades. They can respond to various external stimuli in the form of structural or morphological transformations by actively or passively converting input energy into mechanical energy. They are the core element of soft actuators for typical smart devices like soft robots, artificial muscles, intelligent sensors and nanogenerators. Significant progress has been made in the development of bioinspired stimuli-responsive materials. However, these materials have not been comprehensively summarized with specific actuation mechanisms in the literature. In this review, we will discuss recent advances in biomimetic stimuli-responsive materials that are instrumental for soft actuators. Firstly, different stimuli-responsive principles for soft actuators are discussed, including fluidic, electrical, thermal, magnetic, light, and chemical stimuli. We further summarize the state-of-the-art stimuli-responsive materials for soft actuators and explore the advantages and disadvantages of using electroactive polymers, magnetic soft composites, photo-thermal responsive polymers, shape memory alloys and other responsive soft materials. Finally, we provide a critical outlook on the field of stimuli-responsive soft actuators and emphasize the challenges in the process of their implementation to various industries.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Soft Fiber/Textile Actuators: From Design Strategies to Diverse Applications

Fiber/textile-based actuators have garnered considerable attention due to their distinctive attributes, encompassing higher degrees of freedom, intriguing deformations, and enhanced adaptability to complex structures. Recent studies highlight the development of advanced fibers and textiles, expanding the application scope of fiber/textile-based actuators across diverse emerging fields. Unlike sheet-like soft actuators, fibers/textiles with intricate structures exhibit versatile movements, such as contraction, coiling, bending, and folding, achieved through adjustable strain and stroke. In this review article, we provide a timely and comprehensive overview of fiber/textile actuators, including structures, fabrication methods, actuation principles, and applications. After discussing the hierarchical structure and deformation of the fiber/textile actuator, we discuss various spinning strategies, detailing the merits and drawbacks of each. Next, we present the actuation principles of fiber/fabric actuators, along with common external stimuli. In addition, we provide a summary of the emerging applications of fiber/textile actuators. Concluding with an assessment of existing challenges and future opportunities, this review aims to provide a valuable perspective on the enticing realm of fiber/textile-based actuators.

Fabrication and Actuation of Magnetic Shape-Memory Materials

Soft actuators are deformable materials that change their dimensions or shape in response to external stimuli. Among the various stimuli, remote magnetic fields are one of the most attractive forms of actuation, due to their ease of use, fast response, and safety in biological systems. Composites of magnetic particles with polymer matrices are the most common materials for magnetic soft actuators. In this paper, we demonstrate the fabrication and actuation of magnetic shape-memory materials based on hydrogels containing field-structured magnetic particles. These actuators are formed by placing the pregel dispersion into a mold of the desired on-field shape and exposing it to a homogeneous magnetic field until the gel point is reached. At this point, the material may be removed from the mold and fully gelled in the desired off-field shape. The resultant magnetic shape-memory material then transitions between these two shapes when it is subjected to successive cycles of a homogeneous magnetic field, acting as a large deformation actuator. For actuators that are planar in the off-field state, this can result in significant bending to return to the on-field state. In addition, it is possible to make shape-memory materials that twist under the application of a magnetic field. For these torsional actuators, both experimental and theoretical results are given.

Selaginella lepidophylla-Inspired Multi-Stimulus Cooperative Control MXene-Based Flexible Actuator

Predictable bending deformation, high cycle stability, and multimode complex motion have always been the goals pursued in the field of flexible robots. In this study, inspired by the delicate structure and humidity response characteristics of Selaginella lepidophylla, a new multilevel assisted assembly strategy was developed to construct MXene-CoFe2O4 (MXCFO) flexible actuators with different concentration gradients, to achieve predictable bending deformation and multi-stimulus cooperative control of the actuators, revealing the intrinsic link between the gradient change and the bending deformation ability of the actuator. The thickness of the actuator shows uniformity compared with the common layer-by-layer assembly strategy. And, the bionic gradient structured actuator shows high cycle stability, and it maintains excellent interlayer bonding after bending 100 times. The flexible robots designed based on the predictable bending deformation and the multi-stimulus cooperative response characteristics of the actuator initially realize conceptual models of humidity monitoring, climbing, grasping, cargo transportation, and drug delivery. The designed bionic gradient structure and unbound multi-stimulus cooperative control strategy may show great potential in the design and development of robots in the future.

High Performance and Multifunction Moisture-Driven Yin-Yang-Interface Actuators Derived from Polyacrylamide Hydrogel

High actuation performance of a moisture actuator highly depends on the presence of a large property difference between the two layers, which may cause interfacial delamination. Improving interfacial adhesion strength while increasing the difference between the layers is a challenge. In this study, a moisture-driven tri-layer actuator with a Yin-Yang-interface (YYI) design is investigated in which a moisture-responsive polyacrylamide (PAM) hydrogel layer (Yang) is combined with a moisture-inert polyethylene terephthalate (PET) layer (Yin) using an interfacial poly(2-ethylhexyl acrylate) (PEA) adhesion layer. Fast and large reversible bending, oscillation, and programmable morphing motions in response to moisture are realized. The response time, bending curvature, and response speed normalized by thickness are among the best compared with those of previously reported moisture-driven actuators. The excellent actuation performance of the actuator has potential multifunctional applications in moisture-controlled switches, mechanical grippers, and crawling and jumping motions. The Yin-Yang-interface design proposed in this work provides a new design strategy for high-performance intelligent materials and devices.

Facile Photoresponsive Actuators Based on Ferrocene-Doped Poly(butyl methacrylate)

This paper presents facile photoresponsive actuators comprising ferrocene as a guest chromophore and poly(butyl methacrylate) (PBMA) as a host matrix. The ferrocene-doped PBMA film exhibits mechanical expansion and contraction when a 445 nm laser is turned on and off, respectively. The photoresponsive film is attached by a commercially available acetylcellulose adhesive tape, which exhibits a bending motion that is controlled by turning the laser on and off. Thereafter, the double-layer film is employed to fabricate a table-shaped lifting machine (0.7 mg) that lifts a 10.5 mg object up and down by turning the laser on and off, respectively, and the mechanical force offered by the double-layer film is recorded. Additionally, the film is coated with gold and applied to an electric circuit that serves as a reversible photoresponsive switch. This film preparation technique is applied to other chromophores (e.g., Coumarin 343, Rhodamine 6G, Sudan Blue II, and Solvent Green 3) to independently control the motions of the films with 445, 520, and 655 nm lasers. The ferrocene-containing films also exhibit photoinduced healing from mechanical damage. Finally, the photoirradiation-accompanied morphological changes in the film are observed via small-angle X-ray scattering.

Edible Origami Actuators Using Gelatin-Based Bioplastics

The potential of ingestible medical devices can be greatly enhanced through the use of smart structures made from stimuli-responsive materials. While hydration is a convenient stimulus for inducing shape changes in biomaterials, finding robust materials that can achieve rapid actuation, facile manufacturability, and biocompatibility suitable for ingestible medical devices poses practical challenges. Hydration is a convenient stimulus to induce shape changes in smart biomaterials; however, there are many practical challenges to identifying materials that can achieve rapid actuation and facile manufacturability while satisfying constraints associated with biocompatibility requirements and mechanical properties that are suitable for ingestible medical devices. Herein, we illustrate the formulation and processability of a moisture-responsive genipin-crosslinked gelatin bioplastic system, which can be processed into complex three-dimensional shapes. Mechanical characterization of bioplastic samples showed Young's Modulus values as high as 1845 MPa and toughness values up to 52 MJ/m3, using only food-safe ingredients. Custom molds and UV-laser processing enabled the fabrication of centimeter-scale structures with over 150 independent actuating joints. These self-actuating structures soften and unfold in response to surrounding moisture, eliminating the need for additional stimuli or actuating elements.

Photoresponsive hydrogel-based soft robot: A review

Soft robots have received a lot of attention because of their great human-robot interaction and environmental adaptability. Most soft robots are currently limited in their applications due to wired drives. Photoresponsive soft robotics is one of the most effective ways to promote wireless soft drives. Among the many soft robotics materials, photoresponsive hydrogels have received a lot of attention due to their good biocompatibility, ductility, and excellent photoresponse properties. This paper visualizes and analyzes the research hotspots in the field of hydrogels using the literature analysis tool Citespace, demonstrating that photoresponsive hydrogel technology is currently a key research direction. Therefore, this paper summarizes the current state of research on photoresponsive hydrogels in terms of photochemical and photothermal response mechanisms. The progress of the application of photoresponsive hydrogels in soft robots is highlighted based on bilayer, gradient, orientation, and patterned structures. Finally, the main factors influencing its application at this stage are discussed, including the development directions and insights. Advancement in photoresponsive hydrogel technology is crucial for its application in the field of soft robotics. The advantages and disadvantages of different preparation methods and structures should be considered in different application scenarios to select the best design scheme.

Microfluidic Approaches for Microactuators: From Fabrication, Actuation, to Functionalization

Microactuators can autonomously convert external energy into specific mechanical motions. With the feature sizes varying from the micrometer to millimeter scale, microactuators offer many operation and control possibilities for miniaturized devices. In recent years, advanced microfluidic techniques have revolutionized the fabrication, actuation, and functionalization of microactuators. Microfluidics can not only facilitate fabrication with continuously changing materials but also deliver various signals to stimulate the microactuators as desired, and consequently improve microfluidic chips with multiple functions. Herein, this cross-field that systematically correlates microactuator properties and microfluidic functions is comprehensively reviewed. The fabrication strategies are classified into two types according to the flow state of the microfluids: stop-flow and continuous-flow prototyping. The working mechanism of microactuators in microfluidic chips is discussed in detail. Finally, the applications of microactuator-enriched functional chips, which include tunable imaging devices, micromanipulation tools, micromotors, and microsensors, are summarized. The existing challenges and future perspectives are also discussed. It is believed that with the rapid progress of this cutting-edge field, intelligent microsystems may realize high-throughput manipulation, characterization, and analysis of tiny objects and find broad applications in various fields, such as tissue engineering, micro/nanorobotics, and analytical devices.

Aromatic Zipper Topology Dictates Water-Responsive Actuation in Phenylalanine-Based Crystals

Water-responsive (WR) materials that reversibly deform in response to relative humidity (RH) changes are gaining increasing interest for their potential in energy harvesting and soft robotics applications. Despite progress, there are significant gaps in the understanding of how supramolecular structure underpins the reconfiguration and performance of WR materials. Here, three crystals are compared based on the amino acid phenylalanine (F) that contain water channels and F packing domains that are either layered (F), continuously connected (phenylalanyl-phenylalanine, FF), or isolated (histidyl-tyrosyl-phenylalanine, HYF). Hydration-induced reconfiguration is analyzed through changes in hydrogen-bond interactions and aromatic zipper topology. F crystals show the greatest WR deformation (WR energy density of 19.8 MJ m^-3 ) followed by HYF (6.5 MJ m^-3 ), while FF exhibits no observable response. The difference in water-responsiveness strongly correlates to the deformability of aromatic regions, with FF crystals being too stiff to deform, whereas HYF is too soft to efficiently transfer water tension to external loads.  These findings reveal aromatic topology design rules for WR crystals and provide insight into general mechanisms of high-performance WR actuation. Moreover, the best-performing crystal, F emerges as an efficient WR material for applications at scale and low cost.

Fabrication and Functionality Integration Technologies for Small-Scale Soft Robots

Small-scale soft robots are attracting increasing interest for visible and potential applications owing to their safety and tolerance resulting from their intrinsic soft bodies or compliant structures. However, it is not sufficient that the soft bodies merely provide support or system protection. More importantly, to meet the increasing demands of controllable operation and real-time feedback in unstructured/complicated scenarios, these robots are required to perform simplex and multimodal functionalities for sensing, communicating, and interacting with external environments during large or dynamic deformation with the risk of mismatch or delamination. Challenges are encountered during fabrication and integration, including the selection and fabrication of composite/materials and structures, integration of active/passive functional modules with robust interfaces, particularly with highly deformable soft/stretchable bodies. Here, methods and strategies of fabricating structural soft bodies and integrating them with functional modules for developing small-scale soft robots are investigated. Utilizing templating, 3D printing, transfer printing, and swelling, small-scale soft robots can be endowed with several perceptual capabilities corresponding to diverse stimulus, such as light, heat, magnetism, and force. The integration of sensing and functionalities effectively enhances the agility, adaptability, and universality of soft robots when applied in various fields, including smart manufacturing, medical surgery, biomimetics, and other interdisciplinary sciences.

Multistimulus-Responsive Graphene Oxide/Fe3O4/Starch Soft Actuators

Soft actuators that respond to external stimuli like moisture, magnetism, light, and temperature have received tremendous attention owing to their promising potential in many frontier applications, including smart switches, soft robots, sensors, and artificial muscles. However, most of the conventional actuators can only be triggered by a solo stimulus and demand advanced manufacturing techniques that utilize expensive, hazardous, and synthetic raw materials. Herein, we design and fabricate a multiple stimuli-responsive actuator using graphene oxide, Fe₃O₄ nanoparticles, and tapioca starch via a water evaporation-induced self-assembly method. The resultant hybrid actuator exhibits a bending speed of ∼72° s^-1 upon moisture exposure. Moreover, it can perform clockwise and counterclockwise rotations, linear motion, and magnetic object capture by regulating a magnetic field. As representative examples, the actuator is used to fabricate various smart devices such as smart curtains, biomimetic structures, and a smart gripper that undergo complex and consecutive motion under the influence of multiple stimuli.

High-Power Hydro-Actuators Fabricated from Biomimetic Carbon Nanotube Coiled Yarns with Fast Electrothermal Recovery

Bioinspired yarn/fiber structured hydro-actuators have recently attracted significant attention. However, most water-driven mechanical actuators are unsatisfactory because of the slow recovery process and low full-time power density. A rapidly recoverable high-power hydro-actuator is reported by designing biomimetic carbon nanotube (CNT) yarns. The hydrophilic CNT (HCNT) coiled yarn was prepared by storing pre-twist into CNT sheets and subsequent electrochemical oxidation (ECO) treatment. The resulting yarn demonstrated structural stability even when one end was cut off without the possible loss of pre-stored twists. The HCNT coiled yarn actuators provided maximal contractile work of 863 J/kg at 11.8 MPa stress when driven by water. Moreover, the recovery time of electrically heated yarns at a direct current voltage of 5 V was 95% shorter than that of neat yarns without electric heating. Finally, the electrothermally recoverable hydro-actuators showed a high actuation frequency (0.17 Hz) and full-time power density (143.8 W/kg).

Laser Fabrication of Titanium Alloy-Based Photothermal Responsive Slippery Surface

In recent years, biomimetic materials inspired from natural organisms have attracted great attention due to their promising functionalities and cutting-edge applications, emerging as an important research topic. For example, how to reduce the reflectivity of the solid surface and increase the absorption of the substrate surface is essential for developing light response smart surface. Suitable solutions to this issue can be found in natural creatures; however, it is technologically challenging. In this work, inspired from butterfly wings, we proposed a laser processing technology to prepare micro nanostructured titanium alloy surfaces with anti-reflection properties. The reflectivity is significantly suppressed, and thus, the light absorption is improved. Consequently, the anti-reflection titanium alloy surface can be further employed as a photothermal substrate for developing light-responsive slippery surface. The sliding behavior of liquid droplets on the smart slippery surface can be well controlled via light irradiation. This method facilitates the preparation of low-reflection and high-absorption metallic surfaces towards bionic applications.

Micro-/Nano-Structures Fabricated by Laser Technologies for Optoelectronic Devices

Due to unique optical and electrical properties, micro-/nano-structures have become an essential part of optoelectronic devices. Here, we summarize the recent developments in micro-/nano-structures fabricated by laser technologies for optoelectronic devices. The fabrication of micro-/nano-structures by various laser technologies is reviewed. Micro-/nano-structures in optoelectronic devices for performance improvement are reviewed. In addition, typical optoelectronic devices with micro-nano structures are also summarized. Finally, the challenges and prospects are discussed.

Electro-responsive actuators based on graphene

Electro-responsive actuators (ERAs) hold great promise for cutting-edge applications in e-skins, soft robots, unmanned flight, and in vivo surgery devices due to the advantages of fast response, precise control, programmable deformation, and the ease of integration with control circuits. Recently, considering the excellent physical/chemical/mechanical properties (e.g., high carrier mobility, strong mechanical strength, outstanding thermal conductivity, high specific surface area, flexibility, and transparency), graphene and its derivatives have emerged as an appealing material in developing ERAs. In this review, we have summarized the recent advances in graphene-based ERAs. Typical the working mechanisms of graphene ERAs have been introduced. Design principles and working performance of three typical types of graphene ERAs (e.g., electrostatic actuators, electrothermal actuators, and ionic actuators) have been comprehensively summarized. Besides, emerging applications of graphene ERAs, including artificial muscles, bionic robots, human-soft actuators interaction, and other smart devices, have been reviewed. At last, the current challenges and future perspectives of graphene ERAs are discussed.

A Ti3C2Tx MXene-Based Energy-Harvesting Soft Actuator with Self-Powered Humidity Sensing and Real-Time Motion Tracking Capability

A smart soft actuator with multiple capabilities of humidity-driven actuating, humidity energy harvesting, self-powered humidity sensing, and real-time motion tracking is reported. It is designed on the basis of an MXene/cellulose/polystyrene sulfonic acid (PSSA) composite membrane. This actuator is driven by asymmetric expansion under a moisture gradient during capture of the chemical potential of humidity to mechanical power. Meanwhile, the gradient moisture chemistry also induces directional proton diffusion to generate electricity with high power density and open-circuit voltage. A good linear correlation between the humidity sensitivity, electrical signal, and bending state of this actuator allows real-time tracking of motion modes with humidity change without an external power supply. This multifunctional soft actuator can be used for engineering smart switches, artificial fingers, and soft robots with trackable and distinguishable motion patterns, as well as sensitive noncontacting humidity sensor and breathing monitors.

Materials and Schemes of Multimodal Reconfigurable Micro/Nanomachines and Robots: Review and Perspective

Mechanically programmable, reconfigurable micro/nanoscale materials that can dynamically change their mechanical properties or behaviors, or morph into distinct assemblies or swarms in response to stimuli have greatly piqued the interest of the science community due to their unprecedented potentials in both fundamental research and technological applications. To date, a variety of designs of hard and soft materials, as well as actuation schemes based on mechanisms including chemical reactions and magnetic, acoustic, optical, and electric stimuli, have been reported. Herein, state-of-the-art micro/nanostructures and operation schemes for multimodal reconfigurable micro/nanomachines and swarms, as well as potential new materials and working principles, challenges, and future perspectives are discussed.

Bioinspired Multi-Stimuli Responsive Actuators with Synergistic Color- and Morphing-Change Abilities

The combination of complex perception, defense, and camouflage mechanisms is a pivotal instinctive ability that equips organisms with survival advantages. The simulations of such fascinating multi-stimuli responsiveness, including thigmotropism, bioluminescence, color-changing ability, and so on, are of great significance for scientists to develop novel biomimetic smart materials. However, most biomimetic color-changing or luminescence materials can only realize a single stimulus-response, hence the design and fabrication of multi-stimuli responsive materials with synergistic color-changing are still on the way. Here, a bioinspired multi-stimuli responsive actuator with color- and morphing-change abilities is developed by taking advantage of the assembled cellulose nanocrystals-based cholesteric liquid crystal structure and its water/temperature response behaviors. The actuator exhibits superfast, reversible bi-directional humidity and near-infrared (NIR) light actuating ability (humidity: 9 s; NIR light: 16 s), accompanying with synergistic iridescent appearance which provides a visual cue for the movement of actuators. This work paves the way for biomimetic multi-stimuli responsive materials and will have a wide range of applications such as optical anti-counterfeiting devices, information storage materials, and smart soft robots.