Multi-stimuli-responsive programmable biomimetic actuator

Multi-stimuli-responsive actuator based on bilayered thermoplastic film

Recently, soft actuators have attracted considerable interest owing to their biomimetic performance. Unfortunately, it remains a great challenge to fabricate multi-stimuli-responsive soft actuators by a facile but low-cost method. Herein, a thermoplastic film with bilayered architecture was designed and fabricated by a one-step method. This bilayered thermoplastic film can act as a soft actuator, demonstrating versatile shape-programmable performance in response to acetone vapor exposure and temperature change. Interestingly, diverse biomimetic devices including a worm-like self-walker, crawler-type robot and soft gripper can be realized, which highlights its promising applications in biomimetic robots, artificial muscles and automatic devices. Considering the one-step preparation process and the low-cost raw materials, this approach can be cost-effectively scaled up for practical production.

Engineered extracellular vesicles as intelligent nanosystems for next-generation nanomedicine

Extracellular vesicles (EVs), as natural carriers of bioactive cargo, have a unique micro/nanostructure, bioactive composition, and characteristic morphology, as well as fascinating physical, chemical and biochemical features, which have shown promising application in the treatment of a wide range of diseases. However, native EVs have limitations such as lack of or inefficient cell targeting, on-demand delivery, and therapeutic feedback. Recently, EVs have been engineered to contain an intelligent core, enabling them to (i) actively target sites of disease, (ii) respond to endogenous and/or exogenous signals, and (iii) provide treatment feedback for optimal function in the host. These advances pave the way for next-generation nanomedicine and offer promise for a revolution in drug delivery. Here, we summarise recent research on intelligent EVs and discuss the use of "intelligent core" based EV systems for the treatment of disease. We provide a critique about the construction and properties of intelligent EVs, and challenges in their commercialization. We compare the therapeutic potential of intelligent EVs to traditional nanomedicine and highlight key advantages for their clinical application. Collectively, this review aims to provide a new insight into the design of next-generation EV-based theranostic platforms for disease treatment.

Untethered small-scale magnetic soft robot with programmable magnetization and integrated multifunctional modules

Intelligent magnetic soft robots capable of programmable structural changes and multifunctionality modalities depend on material architectures and methods for controlling magnetization profiles. While some efforts have been made, there are still key challenges in achieving programmable magnetization profile and creating heterogeneous architectures. Here, we directly embed programmed magnetization patterns (magnetization modules) into the adhesive sticker layers to construct soft robots with programmable magnetization profiles and geometries and then integrate spatially distributed functional modules. Functional modules including temperature and ultraviolet light sensing particles, pH sensing sheets, oil sensing foams, positioning electronic component, circuit foils, and therapy patch films are integrated into soft robots. These test beds are used to explore multimodal robot locomotion and various applications related to environmental sensing and detection, circuit repairing, and gastric ulcer coating, respectively. This proposed approach to engineering modular soft material systems has the potential to expand the functionality, versatility, and adaptability of soft robots.

Bidirectional Actuation of Silk Fibroin Films: Role of Water and Alcohol Vapors

Three-dimensional (3D) shape morphism observed in nature inspires the development of stimuli-responsive soft actuators. Vapor-responsive actuators are promising among the different stimuli-responsive materials due to their capability to produce macroscale movements in response to a minuscule amount of specific chemical vapor. Here, we report unusual multiple vapor-responsive bidirectional macroscale actuation behaviors of single-layer regenerated silk fibroin films. The vapor-responsive silk fibroin actuator exhibits antagonistic actuation characteristics in a reversible manner to both water and ethanol vapors. For instance, it produces an upward bending in the presence of water vapor and downward bending in ethanol vapor, which demonstrates the chemical vapor-specific actuation. However, the actuation characteristics remain largely invariant upon changing the polarity of alcohol molecules. The silk fibroin actuators effectively utilize the vapor-induced minuscule expansion and contraction of the film surface to produce large-scale actuation, which is fully reversible. The intrinsic water content of the films and the vapor pressure of the stimulants are exploited to control the actuation performance. Further, we demonstrated the 3D shape morphing ability of the actuator by generating an undulating wavelike motion via preprogrammed water and ethanol vapor exposure conditions. The change in the actuation direction is instantaneous, which ensures the sensitivity and rapid response of the fabricated actuators.

High Energy and Power Density Peptidoglycan Muscles through Super-Viscous Nanoconfined Water

Water-responsive (WR) materials that reversibly deform in response to humidity changes show great potential for developing muscle-like actuators for miniature and biomimetic robotics. Here, it is presented that Bacillus (B.) subtilis' peptidoglycan (PG) exhibits WR actuation energy and power densities reaching 72.6 MJ m^-3 and 9.1 MW m^-3 , respectively, orders of magnitude higher than those of frequently used actuators, such as piezoelectric actuators and dielectric elastomers. PG can deform as much as 27.2% within 110 ms, and its actuation pressure reaches ≈354.6 MPa. Surprisingly, PG exhibits an energy conversion efficiency of ≈66.8%, which can be attributed to its super-viscous nanoconfined water that efficiently translates the movement of water molecules to PG's mechanical deformation. Using PG, WR composites that can be integrated into a range of engineering structures are developed, including a robotic gripper and linear actuators, which illustrate the possibilities of using PG as building blocks for high-efficiency WR actuators.

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.

Fundamentals and utilization of solid/ liquid phase boundary interactions on functional surfaces

The affinity of macroscopic solid surfaces or dispersed nano- and bioparticles towards liquids plays a key role in many areas from fluid transport to interactions of the cells with phase boundaries. Forces between solid interfaces in water become especially important when the surface texture or particles are in the colloidal size range. Although, solid-liquid interactions are still prioritized subjects of materials science and therefore are extensively studied, the related literature still lacks in conclusive approaches, which involve as much information on fundamental aspects as on recent experimental findings related to influencing the wetting and other wetting-related properties and applications of different surfaces. The aim of this review is to fill this gap by shedding light on the mechanism-of-action and design principles of different, state-of-the-art functional macroscopic surfaces, ranging from self-cleaning, photoreactive or antimicrobial coatings to emulsion separation membranes, as these surfaces are gaining distinguished attention during the ongoing global environmental and epidemic crises. As there are increasing numbers of examples for stimulus-responsive surfaces and their interactions with liquids in the literature, as well, this overview also covers different external stimulus-responsive systems, regarding their mechanistic principles and application possibilities.

Programmable Multistimuli-Responsive and Multimodal Polymer Actuator Based on a Designed Energy Transduction Network

A polymer actuator typically responds to only one or two types of stimuli, where sensing and actuation are simultaneously exerted by the same responsive polymer. In cells, sensing and actuation are exerted separately by different biomolecules, which are integrated into nanoscale assemblies to construct the signaling network, making cells a multistimuli responsive and multimodal system. Inspired by the structure-function relationship of the signaling network in cells, we have developed a strategy to select and assemble proper functional polymers into assemblies, where sensing and actuation are exerted by different polymers, and the assemblies can present novel functions beyond that of each polymer component. Three polymers [polyaniline, PANi; poly(N-isopropylacrylamide), PNIPAm; and polydimethylsiloxane, PDMS] are integrated as nodes into a simple energy transduction network, which can be regulated by three molecular factors (pH, kosmotropic anions, and polyethylene glycol). PANi converts the light or electric stimulus into heat, which triggers the actuation of PNIPAm and PDMS. Relying on this energy transduction network, the polymer assembly can respond to six types of stimuli (light, electricity, temperature, water, ions, and organic solvents) and perform different actuation modes, serving as a powerful actuator. Programmable complex deformation upon multiple simultaneous or sequential stimuli has also been achieved by this actuator. An adaptive gripper to catch thin objects and a self-regulating switch to maintain environmental humidity illustrate the wide potential of this actuator for next-generation smart materials and soft robots.

A unimorph nanocomposite dielectric elastomer for large out-of-plane actuation

Dielectric elastomer actuators (DEAs) feature large, reversible in-plane deformation, and stacked DEA layers are used to produce large strokes in the thickness dimension. We introduce an electrophoretic process to concentrate boron nitride nanosheet dispersion in a dielectric elastomer precursor solution onto a designated electrode surface. The resulting unimorph nanocomposite dielectric elastomer (UNDE) has a seamless bilayer structure with 13 times of modulus difference. The UNDE can be actuated to large bending curvatures, with enhanced breakdown field strength and durability as compared to conventional nanocomposite dielectric elastomer. Multiple UNDE units can be formed in a simple electrophoretic concentration process using patterned electrode areas. A disc-shaped actuator comprising six UNDE units outputs large bidirectional stroke up to 10 Hz. This actuator is used to demonstrate a high-speed lens motor capable of varying the focal length of a two-lens system by 40 times.

Photoprogrammable Moisture-Responsive Actuation of a Shape Memory Polymer Film

Humidity-responsive polymeric actuators have gained considerable interest due to their great potential in the fields including soft robotics, artificial muscles, smart sensors, and actuators. However, most of them can only exhibit invariable shape changes, which severely restricts their further exploration and practical use. Herein, we report that programmable humidity-responsive actuating behaviors can be realized by introducing photoprogrammable hygroscopic patterns into shape memory polymers. Poly(ethylene-co-acrylic acid) is selected as a model polymer and the solvent-processed thin films are soft and elastic, whose external shapes can be programmed by a modified shape memory process. On another aspect, an Fe³⁺-carboxylate coordinating network formed by surface treatments can be spatially dissociated under UV, resulting in transient hygroscopic gradients as active joints for moisture-driven actuation. Moreover, we show that the shape memory effect can be an effective means to adjust the direction as well as the amplitude of the moisture-driven actuating behavior. The proposed strategy is convenient and can be generally extended to other shape memory polymers to realize programmable moisture-responsive actuating behaviors.

Localizable, Identifiable, and Perceptive Untethered Light-Driven Soft Crawling Robot

Soft robots based on bionics have attracted extensive attention in recent years. However, most of previous works focused on the motion of robots that were incapable of communication and perception. In this work, an untethered crawling robot is proposed with integration of motion, communication, and location based entirely on a flexible material, which is capable of being utilized as a sensing platform. The hydrophilic graphene oxide film, capable of photothermal conversion, allows the robot to undergo a large deformation stimulated by near-infrared light. Conductive fabric with low resistivity and high mechanical strength, replacing the traditional rigid circuit, is utilized to complete the communication of the robot. The designed communication module allows an electrical signal to be inductively coupled to the soft robot instead of being generated by batteries or through transmission lines. The perception of the robot is demonstrated by covering sensitive materials. Furthermore, the positioning and identification of the robot are verified by an external coil array. The proposed soft crawling robot provides an innovative strategy for the integration of multifunctional robots and shows great potential in bionic devices, intelligent robots, and advanced sensors.

Pressure-Perceptive Actuators for Tactile Soft Robots and Visual Logic Devices

Soft actuators with sensing capabilities are important in intelligent robots and human-computer interactions. However, present perceptive actuating systems rely on the integration of multiple functional units with complex circuit design. Here, a new-type pressure-perceptive actuator is reported, which integrates functions of sensing, actuating, and decision making at material level without complex combination. The actuator is composed of an actuating unit and a pressure-sensing unit, both of which are fabricated by carbon nanotube (CNT), silk, and polymer composite. On the one hand, the actuating unit can be driven by low voltages (<13 V), owing to a Joule-heating effect. On the other hand, the current passing the pressure-sensing unit can be controlled by tactile pressure. In the integrated actuator, it is able to control the deformation amplitude of actuating unit by applying different pressures on the pressure-sensing unit. A portable tactile-activated gripper is fabricated to operate an object through pressure control, demonstrating its application in tactile soft robots. Finally, three visual logic gates (AND, OR, and NOT) are proposed, which convert "tactile" inputs into "visible" deformation outputs, using the CNT-silk-based material for sensing and actuating in the decision-making process. This study provides a new path for intelligent soft robots and new-generation logic devices.

Optical fibre taper-enabled waveguide photoactuators

Photoactuators have attracted significant interest for soft robot and gripper applications, yet most of them rely on free-space illumination, which requires a line-of-site low-loss optical path. While waveguide photoactuators can overcome this limitation, their actuating performances are fundamentally restricted by the nature of standard optical fibres. Herein, we demonstrated miniature photoactuators by embedding optical fibre taper in a polydimethylsiloxane/Au nanorod-graphene oxide photothermal film. The special geometric features of the taper endow the designed photoactuator with microscale active layer thickness, high energy density and optical coupling efficiency. Hence, our photoactuator show large bending angles (>270°), fast response (1.8 s for 180° bending), and low energy consumption (<0.55 mW/°), significantly exceeding the performance of state-of-the-art waveguide photoactuators. As a proof-of-concept study, one-arm and two-arm photoactuator-based soft grippers are demonstrated for capturing/moving small objects, which is challenging for free-space light-driven photoactuators.

Despite promising devices, waveguide photoactuators actuating performances have been fundamentally restricted by the nature of standard optical fibres. To overcome these challenges, authors propose an optical fibre taper-enabled waveguide photoactuator and show enhanced performance.

Bioprinting of Complex Multicellular Organs with Advanced Functionality-Recent Progress and Challenges Ahead

Bioprinting has emerged as one of the most promising strategies for fabrication of functional organs in the lab as an alternative to transplant organs. While progress in the field has mostly been restricted to a few miniaturized tissues with minimal biological functionality until a few years ago, recent progress has advanced the concept of building three-dimensional multicellular organ complexity remarkably. This review discusses a series of milestones that have paved the way for bioprinting of tissue constructs that have advanced levels of biological and architectural functionality. Critical materials, engineering and biological challenges that are key to addressing the desirable function of engineered organs are presented. These are discussed in light of the many difficulties to replicate the heterotypic organization of multicellular solid organs, the nanoscale precision of the extracellular microenvironment in hierarchical tissues, as well as the advantages and limitations of existing bioprinting methods to adequately overcome these barriers. In summary, the advances of the field toward realistic manufacturing of functional organs have never been so extensive, and this manuscript serves as a road map for some of the recent progress and the challenges ahead.

Recent Progress in Active Mechanical Metamaterials and Construction Principles

Active mechanical metamaterials (AMMs) (or smart mechanical metamaterials) that combine the configurations of mechanical metamaterials and the active control of stimuli-responsive materials have been widely investigated in recent decades. The elaborate artificial microstructures of mechanical metamaterials and the stimulus response characteristics of smart materials both contribute to AMMs, making them achieve excellent properties beyond the conventional metamaterials. The micro and macro structures of the AMMs are designed based on structural construction principles such as, phase transition, strain mismatch, and mechanical instability. Considering the controllability and efficiency of the stimuli-responsive materials, physical fields such as, the temperature, chemicals, light, electric current, magnetic field, and pressure have been adopted as the external stimuli in practice. In this paper, the frontier works and the latest progress in AMMs from the aspects of the mechanics and materials are reviewed. The functions and engineering applications of the AMMs are also discussed. Finally, existing issues and future perspectives in this field are briefly described. This review is expected to provide the basis and inspiration for the follow-up research on AMMs.

Materials for Smart Soft Actuator Systems

In contrast to conventional hard actuators, soft actuators offer many vivid advantages, such as improved flexibility, adaptability, and reconfigurability, which are intrinsic to living systems. These properties make them particularly promising for different applications, including soft electronics, surgery, drug delivery, artificial organs, or prosthesis. The additional degree of freedom for soft actuatoric devices can be provided through the use of intelligent materials, which are able to change their structure, macroscopic properties, and shape under the influence of external signals. The use of such intelligent materials allows a substantial reduction of a device's size, which enables a number of applications that cannot be realized by externally powered systems. This review aims to provide an overview of the properties of intelligent synthetic and living/natural materials used for the fabrication of soft robotic devices. We discuss basic physical/chemical properties of the main kinds of materials (elastomers, gels, shape memory polymers and gels, liquid crystalline elastomers, semicrystalline ferroelectric polymers, gels and hydrogels, other swelling polymers, materials with volume change during melting/crystallization, materials with tunable mechanical properties, and living and naturally derived materials), how they are related to actuation and soft robotic application, and effects of micro/macro structures on shape transformation, fabrication methods, and we highlight selected applications.

Giving Penetrable Remote-Control Ability to Thermoresponsive Fibrous Composite Actuator with Fast Response Induced by Alternative Magnetic Field

An alternative magnetic field (AMF)-induced electrospun fibrous thermoresponsive composite actuator showing penetrable remote-control ability with fast response is shown here for the first time. The built-in heater of magnetothermal Fe₃O₄ nanoparticles in the actuator and the porous structure of the fibrous layer contribute to a fast actuation with a curvature of 0.4 mm⁻¹ in 2 s. The higher loading amount of the Fe₃O₄ nanoparticles and higher magnetic field strength result in a faster actuation. Interestingly, the composite actuator showed a similar actuation even when it was covered by a piece of Polytetrafluoroethylene (PTFE) film, which shows a penetrable remote-control ability.

Electronically integrated microcatheters based on self-assembling polymer films

Existing electronically integrated catheters rely on the manual assembly of separate components to integrate sensing and actuation capabilities. This strongly impedes their miniaturization and further integration. Here, we report an electronically integrated self-assembled microcatheter. Electronic components for sensing and actuation are embedded into the catheter wall through the self-assembly of photolithographically processed polymer thin films. With a diameter of only about 0.1 mm, the catheter integrates actuated digits for manipulation and a magnetic sensor for navigation and is capable of targeted delivery of liquids. Fundamental functionalities are demonstrated and evaluated with artificial model environments and ex vivo tissue. Using the integrated magnetic sensor, we develop a strategy for the magnetic tracking of medical tools that facilitates basic navigation with a high resolution below 0.1 mm. These highly flexible and microsized integrated catheters might expand the boundary of minimally invasive surgery and lead to new biomedical applications.

Programmable Mechanically Active Hydrogel-Based Materials

Programmable mechanically active materials (MAMs) are defined as materials that can sense and transduce external stimuli into mechanical outputs or conversely that can detect mechanical stimuli and respond through an optical change or other change in the appearance of the material. Programmable MAMs are a subset of responsive materials and offer potential in next generation robotics and smart systems. This review specifically focuses on hydrogel-based MAMs because of their mechanical compliance, programmability, biocompatibility, and cost-efficiency. First, the composition of hydrogel MAMs along with the top-down and bottom-up approaches used for programming these materials are discussed. Next, the fundamental principles for engineering responsivity in MAMS, which includes optical, thermal, magnetic, electrical, chemical, and mechanical stimuli, are considered. Some advantages and disadvantages of different responsivities are compared. Then, to conclude, the emerging applications of hydrogel-based MAMs from recently published literature, as well as the future outlook of MAM studies, are summarized.

Robust Jumping Actuator with a Shrimp-Shell Architecture

It is highly desirable to develop compact- and robust-film jumping robots that can withstand severe conditions. Besides, the demands for strong actuation force, large bending curvature in a short response time, and good environmental tolerance are significant challenges to the material design. To address these challenges, this paper reports the fabrication of a thin-film jumping actuator, which exhibits a shrimp-shell architecture, from a conjugated ladder polymer (cLP) that is connected by carbon nanotube (CNT) sheets. The hierarchical porous structure ensures the fast absorption and desorption of organic vapor, thereby achieving a high response rate. The actuator does not exhibit shape distortion at temperatures of up to 225 °C and in concentrated sulfuric acid, as well as when immersed in many organic solvents. This work avails a new design strategy for high-performance actuators that function under harsh and complicated conditions.

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.

Digital printing of shape-morphing natural materials

We demonstrate how programmable shape evolution and deformation can be induced in plant-based natural materials through standard digital printing technologies. With nonallergenic pollen paper as the substrate material, we show how specific geometrical features and architectures can be custom designed through digital printing of patterns to modulate hygrophobicity, geometry, and complex shapes. These autonomously hygromorphing configurations can be "frozen" by postprocessing coatings to meet the needs of a wide spectrum of uses and applications. Through computational simulations involving the finite element method and accompanying experiments, we develop quantitative insights and a general framework for creating complex shapes in eco-friendly natural materials with potential sustainable applications for scalable manufacturing.

Wearable Sunlight-Triggered Bimorph Textile Actuators

Photothermal bimorph actuators have attracted considerable attention in intelligent devices because of their cordless control and lightweight and easy preparation. However, current photothermal bimorph actuators are mostly based on films or papers driven by near-infrared sources, which are deficient in flexibility and adaptability, restricting their potential in wearable applications. Herein, a bimorph textile actuator that can be scalably fabricated with a traditional textile route and autonomously triggered by sunlight is reported. The active layer and passive layer of the bimorph are constructed by polypropylene tape and a MXene-modified polyamide filament. Because of the opposite thermal expansion and MXene-enhanced photothermal efficiency (>260%) of the bimorph, the textile actuator presents effective deformation (1.38 cm^-1) under low sunlight power (100 mW/cm²). This work provides a new pathway for wearable sunlight-triggered actuators and finds attractive applications for smart textiles.

Bioinspired soft microrobots actuated by magnetic field

In contrast to traditional large-scale robots, which require complicated mechanical joints and material rigidity, microrobots made of soft materials have exhibited amazing features and great potential for extensive applications, such as minimally invasive surgery. However, microrobots are faced with energy supply and control issues due to the miniaturization. Magnetic field actuation emerges as an appropriate approach to tackle with these issues. This review summarizes the latest progress of biomimetic soft microrobots actuated by magnetic field. Starting with an overview of the soft material and magnetic material adopted in the magnetic field actuated soft microrobots, the various fabrication methods and design structures of soft microrobots are summarized. Subsequently, practical and potential applications, such as targeted therapy, surgical operation, and the transportation of microscopic objects, in the fields of biomedicine and environmental remediation are presented. In the end, some current challenges, and the future development trends of magnetic soft microrobots are briefly discussed. This review is expected to offer a helpful guidance for the new researchers of biomimetic soft microrobots actuated by magnetic field.

Multifunctional Ti3C2Tx MXene/Low-Density Polyethylene Soft Robots with Programmable Configuration for Amphibious Motions

To diversify the motion modes of multifunctional soft robots capable of shape programming, we fabricate a biomimetic and programmable Ti₃C₂T_x MXene/low-density polyethylene (LDPE) bilayer actuator by spraying an aqueous dispersion of MXenes onto a plasma-activated LDPE film, followed by optimal thermal regulations. Because of the eminent light absorption and photothermal/electrothermal features of MXenes and the extremely mismatched thermal expansion coefficients between the two layers, the MXene/LDPE actuator can be sensitively driven by many stimuli of near-infrared light, electricity, and heat. The initial configuration of the bilayer actuator can be programmed by tuning the thermal regulation temperature, thereby assembling multiple actuation units to achieve biomimetic functions, such as artificial iris, mechanical arms, and flying birds. More importantly, in virtue of free shape cutting and programmable configuration, the MXene/LDPE bilayer actuator can perform untethered locomotion including crawling, rolling, and sailing. The soft robots can not only move on the ground in different forms but also sail on water along any designated routes and complete the surface cargo transportation driven by a near-infrared laser via the photothermal Marangoni effect. The shape-programmable methodology for the three amphibious motion modes lays foundations for wide applications of the MXene-based soft robots.

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.

Engineering Nanorobots for Tumor-Targeting Drug Delivery: From Dynamic Control to Stimuli-Responsive Strategy

Nanotechnology has been widely applied to the fabrication of drug delivery systems in the past decades. Recently, with the progress made in microfabrication approaches, nanorobots are steadily becoming a promising means for tumor-targeting drug delivery. In general, nanorobots can be divided into two categories: nanomotors and stimuli-responsive nanorobots. Nanomotors are nanoscale systems with the ability to convert surrounding energies into mechanical motion, whereas stimuli-responsive nanorobots are featured with activatable capacity in response to various endogenous and exogenous stimulations. In this minireview, the dynamic control of nanomotors and the rational design of stimuli-responsive nanorobots are overviewed, with particular emphasis on their contribution to tumor-targeting therapy. Moreover, challenges and perspectives associated with the future development of nanorobots are presented.

Programmable Humidity-Responsive Actuation of Polymer Films Enabled by Combining Shape Memory Property and Surface-Tunable Hygroscopicity

Most humidity-responsive polymeric actuators can only exhibit shape transformations between a planar shape in the dry state and a bended three-dimensional (3D) shape when exposed to moisture, and it is challenging to design and prepare hygroscopic actuators with programmable actuating behaviors displayed from sophisticated 3D structures. Herein, we demonstrate that the integration of shape memory property and surface treatment enabled hygromorphic responsivity endows a single-component polymer film with programmable moisture-driven actuating behaviors. The solvent-processed polyethylene-co-acrylic acid (EAA) copolymer film is soft and stretchable at room temperature, and has a good thermal-responsive shape memory property. By surface treatment using base/acid solutions, the reversible gradient conversion between carboxyl groups and carboxylate salts along the thickness direction enables the film to exhibit designed hygroscopic actuations. The shape memory property and moisture-driven actuating behaviors can be combined to realize 3D-3D morphing by first programming the films into 3D shapes and then conducting the surface treatments. Both shape programming and surface treatment processes can be reprogrammed to make the actuation behavior readily tunable. We also show that the created surface patterns can act as moisture-sensitive conducting paths to detect human breathes, and the combination of shape memory, moisture-responsive morphing and conductivity change leads to some interesting applications such as smart switch in conducting circuit. This work provides a new and general strategy for the design of advanced humidity-responsive actuators.

Tough and Multifunctional Composite Film Actuators Based on Cellulose Nanofibers toward Smart Wearables

Although humidity-responsive actuators serve as a promising candidate in smart wearables, artificial muscles, and biomimetic devices, most of them derived from synthetic polymers could not simultaneously achieve multifunctional properties. In this work, a cellulose nanofiber (CNF)-based film actuator with high mechanical properties, excellent Joule heating, and antibacterial capability is successfully constructed by integrating with Ti₃C₂T_x (MXene) and tannic acid (TA) via a vacuum-assisted filtration approach. Owing to the unique nacrelike structure and strong hydrogen bonds, the tensile strength and toughness of the composite film could reach 275.4 MPa and 10.2 MJ·m^-3, respectively. Importantly, the hydrophilic nature of CNFs and alterable interlayer spacing of MXene nanosheets endow the composite film with sensitive humidity response and extraordinary stability (1000 cycles). With the assistance of MXene nanosheets and TA, the composite film could not only present outstanding Joule heating but also possess remarkable antibacterial properties against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Benefiting from the above merits, the proof-of-concept smart garment is assembled by the as-prepared film and is capable of regulating humidity and temperature.

Stimuli-Responsive AIEgens

The unique advantages and the exciting application prospects of AIEgens have triggered booming developments in this area in recent years. Among them, stimuli-responsive AIEgens have received particular attention and impressive progress, and they have been demonstrated to show tremendous potential in many fields from physical chemistry to materials science and to biology and medicine. Here, the recent achievements of stimuli-responsive AIEgens in terms of seven most representative types of stimuli including force, light, polarity, temperature, electricity, ion, and pH, are summarized. Based on typical examples, it is illustrated how each type of systems realize the desired stimuli-responsive performance for various applications. The key work principles behind them are ultimately deciphered and figured out to offer new insights and guidelines for the design and engineering of the next-generation stimuli-responsive luminescent materials for more broad applications.

Design of MXene Composites with Biomimetic Rapid and Self-Oscillating Actuation under Ambient Circumstances

Although responsive actuators have been intensively investigated, it remains challenging to enable rapid and self-oscillating actuation under ambient circumstances without human intervention analogous to living organisms. By hybridizing a unique type of two-dimensional nanomaterials (i.e., MXene) with a particular hydrophilic polymer, a smart and flexible conductive composite was produced with rapid actuation and spontaneous oscillation near a moist surface. Due to the presence of layered microstructures and the moisture-sensitivity improved by surface roughness and intercalated polymeric layers, the composites could reversibly bend up to 180° in 2 s or 210° in 10 s on demand when the circumstantial humidity was varied, being superior or comparable to many actuators in the literature. More importantly, the composite was capable not only of flipping upside down repeatedly on the moist surface but also of self-oscillating ceaselessly under ambient gradient humidity without human intervention, e.g., an oscillation between 30 and 100° with an oscillation frequency of 0.08 Hz. This self-oscillation resulted from the occurrence of rapid asymmetrical hydration and dehydration of the composite between the regions of high and low humidity, which could further be modulated both by different hydrophilic polymers and by photoradiation owing to the photothermal effect of MXene nanosheets. Because of the ubiquitous presence of humidity gradient near the moist surface, this type of smart composite may not only offer a strategy for designing artificial materials that are capable of spontaneous actuation under ambient circumstance without human intervention but also promise potential applications in artificial muscles, autonomous robotics, and energy harvesting from environments.

Wirelessly Actuated Thermo- and Magneto-Responsive Soft Bimorph Materials with Programmable Shape-Morphing

Soft materials that respond to wireless external stimuli are referred to as "smart" materials due to their promising potential in real-world actuation and sensing applications in robotics, microfluidics, and bioengineering. Recent years have witnessed a burst of these stimuli-responsive materials and their preliminary applications. However, their further advancement demands more versatility, configurability, and adaptability to deliver their promised benefits. Here, a dual-stimuli-responsive soft bimorph material with three configurations that enable complex programmable 3D shape-morphing is presented. The material consists of liquid crystal elastomers (LCEs) and magnetic-responsive elastomers (MREs) via facile fabrication that orthogonally integrates their respective stimuli-responsiveness without detrimentally altering their properties. The material offers an unprecedented wide design space and abundant degree-of-freedoms (DoFs) due to the LCE's programmable director field, the MRE's programmable magnetization profile, and diverse geometric configurations. It responds to wireless stimuli of the controlled magnetic field and environmental temperature. Its dual-responsiveness allows the independent control of different DoFs for complex shape-morphing behaviors with anisotropic material properties. A diverse set of in situ reconfigurable shape-morphing and an environment-aware untethered miniature 12-legged robot capable of locomotion and self-gripping are demonstrated. Such material can provide solutions for the development of future soft robotic and other functional devices.

Stimulus-Responsive Graphene with Periodical Wrinkles on Grooved Microfiber Arrays: Simulation, Programmable Shape-Shifting, and Catalytic Applications

This work demonstrates a facile fabrication of stimulus-responsive, periodically wrinkled graphene sheets on grooved microfiber arrays with fast and reversible shape change, multiresponsiveness, and programmable deformation, with the aid of finite element analysis (FEA). The cellulose acetate (CA) microfibers, endowing responsiveness to humidity and solvents, are designed to grooved shape and assembled into a well-aligned fibrous mat by electrospinning. Under the guidance of FEA simulation, the stiff reduced graphene oxide (RGO) sheets, serving as a photoresponsive component, could ably bind on grooved CA microfibers with favorable interlocked interfacial-structure. Through simple direct-writing and hot-pressing, the grooved CA arrays interlocked the conformal RGO sheets by water-induced self-clamping, and enabled the generation of periodic wrinkles within RGO sheets to maximize interfacial areas. By simply adjusting the orientation of written RGO patterns relative to uniaxial CA microfibers, programmed and omnidirectional shape-shifting were obtained to minimize strain energy, consisting with the dynamic deformation process simulated by FEA. Upon remote light or contactless humidity stimuli, the RGO/CA mat shows a rapid response (≤1 s), large amplitude (angle change ≥150°, 1.62 cm^-1), sophisticated 3D motions, and lifts objects that weigh 12.7-times its own weight up to over 1/3 of own height within 1 s. After loading catalytical nanoparticles, the RGO/CA mat could rapidly move to the targeted position by continuous crawling even on a slippery surface, and served as a microchannel reactor to trigger a reaction in built-in microchannels with suppressing catalyst leaching while accelerating reaction kinetics by both nanoconfinement and photothermal effect.

Bio-Inspired Soft Grippers Based on Impactive Gripping

Grasping and manipulation are fundamental ways for many creatures to interact with their environments. Different morphologies and grasping methods of "grippers" are highly evolved to adapt to harsh survival conditions. For example, human hands and bird feet are composed of rigid frames and soft joints. Compared with human hands, some plants like Drosera do not have rigid frames, so they can bend at arbitrary points of the body to capture their prey. Furthermore, many muscular hydrostat animals and plant tendrils can implement more complex twisting motions in 3D space. Recently, inspired by the flexible grasping methods present in nature, increasingly more bio-inspired soft grippers have been fabricated with compliant and soft materials. Based on this, the present review focuses on the recent research progress of bio-inspired soft grippers based on impactive gripping. According to their types of movement and a classification model inspired by biological "grippers", soft grippers are classified into three types, namely, non-continuum bending-type grippers, continuum bending-type grippers, and continuum twisting-type grippers. An exhaustive and updated analysis of each type of gripper is provided. Moreover, this review offers an overview of the different stiffness-controllable strategies developed in recent years.

Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

Bioinspired Soft Robots Based on the Moisture-Responsive Graphene Oxide

Graphene oxide (GO), which has many oxygen functional groups, is a promising candidate for use in moisture-responsive sensors and actuators due to the strong water-GO interaction and the ultrafast transport of water molecules within the stacked GO sheets. In the last 5 years, moisture-responsive actuators based on GO have shown distinct advantages over other stimuli-responsive materials and devices. Particularly, inspired by nature organisms, various moisture-enabled soft robots have been successfully developed via rational assembly of the GO-based actuators. Herein, the milestones in the development of moisture-responsive soft robots based on GO are summarized. In addition, the working mechanisms, design principles, current achievement, and prospects are also comprehensively reviewed. In particular, the GO-based soft robots are at the forefront of the advancement of automatable smart devices.

The Study of pH Effects on Phase Transition of Multi-Stimuli Responsive P(NiPAAm-co-AAc) Hydrogel Using 2D-COS

The temperature and mechanism of phase transition of poly(N-isopropylacrylamide-co-acrylic acid) [P(NiPAAm-co-AAc)], which is one of the multi-stimuli responsive polymers, were investigated at various pHs using infrared (IR) spectroscopy, two-dimensional (2D) gradient mapping, and two-dimensional correlation spectroscopy (2D-COS). The determined phase transition temperature of P(NiPAAm-co-AAc) at pH 4, 3, and 2 based on 2D gradient mapping and principal component analysis (PCA) showed that it decreases with decreasing pH, because COOH group in AAc changes with variation of pH. The results of 2D-COS analysis indicated that the phase transition mechanism of P(NiPAAm-co-AAc) hydrogel at pH4 is different from that at pH2 due to the effect of COOH group of AAc.

Voxelated three-dimensional miniature magnetic soft machines via multimaterial heterogeneous assembly

Small-scale soft-bodied machines that respond to externally applied magnetic field have attracted wide research interest because of their unique capabilities and promising potential in a variety of fields, especially for biomedical applications. When the size of such machines approach the sub-millimeter scale, their designs and functionalities are severely constrained by the available fabrication methods, which only work with limited materials, geometries, and magnetization profiles. To free such constraints, here, we propose a bottom-up assembly-based 3D microfabrication approach to create complex 3D miniature wireless magnetic soft machines at the milli- and sub-millimeter scale with arbitrary multimaterial compositions, arbitrary 3D geometries, and arbitrary programmable 3D magnetization profiles at high spatial resolution. This approach helps us concurrently realize diverse characteristics on the machines, including programmable shape morphing, negative Poisson's ratio, complex stiffness distribution, directional joint bending, and remagnetization for shape reconfiguration. It enlarges the design space and enables biomedical device-related functionalities that are previously difficult to achieve, including peristaltic pumping of biological fluids and transport of solid objects, active targeted cargo transport and delivery, liquid biopsy, and reversible surface anchoring in tortuous tubular environments withstanding fluid flows, all at the sub-millimeter scale. This work improves the achievable complexity of 3D magnetic soft machines and boosts their future capabilities for applications in robotics and biomedical engineering.

Self-powered locomotion of a hydrogel water strider

Hydrogels are an exciting class of materials for new and emerging robotics. For example, actuators based on hydrogels have impressive deformability and responsiveness. Studies into hydrogels with autonomous locomotive abilities, however, are limited. Existing hydrogels achieve locomotion through the application of cyclical stimuli or chemical modifications. Here, we report the fabrication of active hydrogels with an intrinsic ability to move on the surface of water without operated stimuli for up to 3.5 hours. The active hydrogels were composed of hydrophobic and hydrophilic groups and underwent a dynamic wetting process to achieve spatial and temporal control of surface tension asymmetry. Using surface tension, the homogeneous active hydrogels propelled themselves and showed controlled locomotion on water, similar to common water striders.

Metal-Organic Framework-Based Stimuli-Responsive Polymers

Metal-organic framework (MOF) based stimuli-responsive polymers (coordination polymers) exhibit reversible phase-transition behavior and demonstrate attractive properties that are capable of altering physical and/or chemical properties upon exposure to external stimuli, including pH, temperature, ions, etc., in a dynamic fashion. Thus, their conformational change can be imitated by the adsorption/desorption of target analytes (guest molecules), temperature or pressure changes, and electromagnetic field manipulation. MOF-based stimuli responsive polymers have received great attention due to their advanced optical properties and variety of applications. Herein, we summarized some recent progress on MOF-based stimuli-responsive polymers (SRPs) classified by physical and chemical responsiveness, including temperature, pressure, electricity, pH, metal ions, gases, alcohol and multi-targets.

Core-Shell Magnetic Micropillars for Reprogrammable Actuation

Stimuli-responsive micro/nanostructures that exhibit not only programmable but also reprogrammable actuation behaviors are highly desirable for various advanced engineering applications (e.g., anticounterfeiting, information encoding, dynamic imaging and display, microrobotics, etc.) but yet to be realized with state-of-the-art technologies. Here we report a concept and a corresponding experimental technique for core-shell magnetic micropillars enabling simultaneously programmable and reprogrammable actuations using a simple magnetic field. The micropillars are composed of elastomeric hollow shells for shaping encapsulated with liquid magnetic nanocomposite resin cores for actuating. The spatial distribution of the magnetic nanoparticles inside the resin channels can be dynamically modulated within individual micropillars, which consequently regulates the magnetomechanical responses of the pillars upon actuation (bending deformation varied near 1 order of magnitude under the same actuation field). We demonstrate that the micropillars with contrasting bending responses can be configured in an arbitrary spatial pattern by direct magnetic writing, and the written pattern can then be easily magnetically erased to facilitate next-round rewriting and reconfiguration. This reprogrammable actuation capability of the micropillars is further demonstrated by their potential applications for rewritable paper and recyclable displays, where various microscale characteristics can be controlled to dynamically appear and disappear at the same or different locations of one single micropillar array. The core-shell magnetic micropillars reported here provide a universal prototype for reprogrammable responsive micro/nanostructures through rational design and facile fabrication from conventional materials.

Recent advances in colour-tunable soft actuators

In nature, some creatures have the capability to change shapes to adapt to ever-changing environments, which greatly inspire researchers to develop soft actuators. To endow soft actuators with capabilities to interact with environment and integrate more feedbacks is of great significance. Colour-tunable soft actuators that provide colour change feedbacks have therefore attracted extensive attention. Based on either chemical-colour or structural-colour based materials, a variety of colour-tunable soft actuators enabling shape deformations (or locomotion) and colour changes have been prepared and hold promise for applications in soft robotics and biomedical devices. This review summarizes the recent advances of colour-tunable soft actuators, with emphasis on their colour-change mechanisms and highlighting their applications. Existing challenges and future perspectives on colour-tunable soft actuators are presented.

Polarization-Modulated Multidirectional Photothermal Actuators

Photothermal actuators have attracted increasing attention due to their ability to convert light energy into mechanical deformation and locomotion. This work reports a freestanding, multidirectional photothermal robot that can walk along a predesigned pathway by modulating laser polarization and on-off switching. Magnetic-plasmonic hybrid Fe₃ O₄ /Ag nanorods are synthesized using an unconventional templating approach. The coupled magnetic and plasmonic anisotropy allows control of the rod orientation, plasmonic excitation, and photothermal conversion by simply applying a magnetic field. Once the rods are fixed with desirable orientations in a bimorph actuator by magnetic-field-assisted lithography, the bending of the actuator can be controlled by switching the laser polarization. A bipedal robot is created by coupling the rod orientation with the alternating actuation of its two legs. Irradiating the robot by a laser with alternating or fixed polarization synergistically results in basic movement (backward and forward) and turning (including left-, right-, and U-turn), respectively. A complex walk along predesigned pathways can be potentially programmed by combining the movement and turning modes of the robots. This strategy provides an alternative driving mechanism for preparing functional soft robots, thus breaking through the limitations in the existing systems in terms of light sources and actuation manners.