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

Actuation performance of MXenes in response to moisture gradients: A systematic investigation

Humidity-responsive actuators (HRA) have garnered significant interest across various domains. Since 2020, MXene have been extensively studied for their potential in HRA, demonstrating remarkable performance. Thus far, more than 70 MXene materials have been found. However, no systematic research regarding the distinctions in their applications in the field of HRA has been carried out, limiting the exploration of their broader potential. Herein, a systematic investigation of the HRA performance within the MXene (Nb2CTx, V2CTx, Ti3C2Tx, and Nb4C3Tx) family has been conducted. The influences of atomic layer number, oxygen content, transition metal elements, film thickness, and moisture gradient on the actuation performance are thoroughly examined. It is demonstrated that the number of atomic layers plays an indispensably critical role in both the bending angle and the response/recovery time. Nb2CTx and V2CTx with fewer atomic layers show excellent performance. At 50 % relative humidity, their maximum bending angles reach 210° and 202° respectively. The HRA are exemplified by the creation of a bionic butterfly and an intelligent switch, showcasing its broad application potential.

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.

MXene-based solvent-responsive actuators with a polymer-intercalated gradient structure

Actuators based on electrically conductive and hydrophilic two-dimensional (2D) Ti3C2T X MXene are of interest for fast and specific responses in demanding environments, such as chemical production. Herein, Ti3C2T X -based solvent-responsive bilayer actuators were developed, featuring a gradient polymer-intercalation structure in the active layer. These actuators were assembled using negatively charged pristine Ti3C2T X nanosheets as the passive layer and positively charged polymer-tethered Ti3C2T X as the active layer. 2D wide-angle X-ray scattering and simulations related the gradient polymer intercalated microstructure in the polymer/MXene composite active layer to the counterintuitive actuation behavior. The bending of the bilayer films in solvent vapor is triggered by the gradient polymer-intercalation and the differing diffusion rate of solvent molecules through the MX and MX-polymer layers of the bilayer actuator. With their ease of fabrication, remote light-control capabilities, and excellent actuation performance, the Ti3C2T X -based bilayer actuators reported here may find applications in areas such as sensors for monitoring chemical production, infrared camouflage, smart switches, and excavators in toxic solvent environments.

Emerging innovations in electrically powered artificial muscle fibers

This review systematically explores the inherent structural advantages of fiber over conventional film or bulk forms for artificial muscles, emphasizing their enhanced mechanical properties and actuation, scalability, and design flexibility. Distinctive merits of electrically powered artificial muscle fiber actuation mechanisms, including electrothermal, electrochemical and dielectric actuation, are highlighted, particularly for their operational efficiency, precise control capabilities, miniaturizability and seamless integration with electronic components. A comprehensive overview of significant research driving performance enhancements in artificial muscle fibers through materials and structural innovations is provided, alongside a discussion of the diverse design methodologies that have emerged in this field. A detailed comparative assessment evaluates the performance metrics, advantages and manufacturing complexities of each actuation mechanism, underscoring their suitability for various applications. Concluding with a strategic outlook, the review identifies key challenges and proposes targeted research directions to advance and refine artificial muscle fiber technologies.

Cellulose nanofibers-based composite film with broadening MXene layer spacing and rapid moisture separation for humidity sensing and humidity actuators

Based on the basic idea of expanding the interlayer spacing of MXene, utilizing the effect of gallic acid-modified cellulose nanofibers for rapid moisture separation, the flexible sensing and driving composite film with a perfect balance among humidity signal response and mechanical properties was prepared. Inspired by the stacking of autumn fallen leaves, the cellulose nanofibers-based composite films were formed by self-assembly under vacuum filtration of blending gallic acid-modified cellulose nanofibers with MXene. The enhanced mechanical properties (tensile strength 131.1 MPa, puncture load 0.88 N, tearing strength 165.55 N/mm, and elongation at break 16.14 %), humidity sensing (the stable induced voltage 63.7 mV and response/recovery time 3.2/5.1 s), and humidity driving (154.7° bending angle) properties were observed. The synergistic effect of hydrogen bonds, the "pinning effect" arising from the side chains, and the hierarchical layered microstructure contributed to the enhanced performance. This work exemplifies the application of green natural product for preparing intelligent sensing, wearable devices, and biomimetic robots.

A tough, reversible and highly sensitive humidity actuator based on cellulose nanofiber films by intercalation modulated plasticization

Cellulose nanofiber was an ideal candidate for humidity actuators based on its wide availability, biocompatibility and excellent hydrophilicity. However, conventional cellulose nanofiber-based actuators faced challenges like poor water resistance, flexibility, and sensitivity. Herein, water-resistant, flexible, and highly sensitive cross-linked cellulose nanofibers (CCNF) single-layer humidity actuators with remarkable reversible humidity responsiveness were prepared by combining the green click chemistry modification and intercalation modulated plasticization (IMP). The incorporation of phenyl ring and the crosslinked network structure in CCNF films contributed to its improved water resistance and mechanical properties (with a stress increased from 85.9 ± 3.1 MPa to 141.2 ± 21.5 MPa). SEM analysis confirmed enhanced interlaminar sliding properties facilitated by IMP. This resulted in increased flexibility and toughness of CCNF films, with a strain of 11.5 % and toughness of 9.9 MJ/m3. These improvements efficiently enhanced humidity sensitivity for cellulose nanofiber, with a 4.8-fold increase in bending curvature and a response time of only 3.4 ± 0.1 s. Finally, the good humidity sensitivity of modified CNF can be easily imparted to carbon nanotubes (CNTs) via simple self-assembly method, thus leading to a high-performance humidity-responsive actuator. The click chemistry modification and IMP offer a new avenue to fabricate tough, reversible and highly sensitive humidity actuator based on cellulose nanofiber.

Research Progress on Ti3C2Tx-Based Composite Materials in Antibacterial Field

The integration of two-dimensional Ti3C2Tx nanosheets and other materials offers broader application options in the antibacterial field. Ti3C2Tx-based composites demonstrate synergistic physical, chemical, and photodynamic antibacterial activity. In this review, we aim to explore the potential of Ti3C2Tx-based composites in the fabrication of an antibiotic-free antibacterial agent with a focus on their systematic classification, manufacturing technology, and application potential. We investigate various components of Ti3C2Tx-based composites, such as metals, metal oxides, metal sulfides, organic frameworks, photosensitizers, etc. We also summarize the fabrication techniques used for preparing Ti3C2Tx-based composites, including solution mixing, chemical synthesis, layer-by-layer self-assembly, electrostatic assembly, and three-dimensional (3D) printing. The most recent developments in antibacterial application are also thoroughly discussed, with special attention to the medical, water treatment, food preservation, flexible textile, and industrial sectors. Ultimately, the future directions and opportunities are delineated, underscoring the focus of further research, such as elucidating microscopic mechanisms, achieving a balance between biocompatibility and antibacterial efficiency, and investigating effective, eco-friendly synthesis techniques combined with intelligent technology. A survey of the literature provides a comprehensive overview of the state-of-the-art developments in Ti3C2Tx-based composites and their potential applications in various fields. This comprehensive review covers the variety, preparation methods, and applications of Ti3C2Tx-based composites, drawing upon a total of 171 English-language references. Notably, 155 of these references are from the past five years, indicating significant recent progress and interest in this research area.

A review of cellulose-based catechol-containing functional materials for advanced applications

With the increment in global energy consumption and severe environmental pollution, it is urgently needed to explore green and sustainable materials. Inspired by nature, catechol groups in mussel adhesion proteins have been successively understood and utilized as novel biomimetic materials. In parallel, cellulose presents a wide class of functional materials rating from macro-scale to nano-scale components. The cross-over among both research fields alters the introduction of impressive materials with potential engineering properties, where catechol-containing materials supply a general stage for the functionalization of cellulose or cellulose derivatives. In this review, the role of catechol groups in the modification of cellulose and cellulose derivatives is discussed. A broad variety of advanced applications of cellulose-based catechol-containing materials, including adhesives, hydrogels, aerogels, membranes, textiles, pulp and papermaking, composites, are presented. Furthermore, some critical remaining challenges and opportunities are studied to mount the way toward the rational purpose and applications of cellulose-based catechol-containing materials.

Hydrophobic modification of cellulose nanofibers by gallic acid and the application in pressure sensing

Via rational molecular structure design and using gallic acid (GA) for hydrophobic modification of cellulose nanofibers (CNF), the "polymer dipole" CNF-GA with hydrophilic main chains and hydrophobic side chains was prepared, which improved the poor piezoelectric properties of CNF used for preparing pressure sensors. Due to the appearance of the side chains, the elongation at break of the CNF-GA-2, compared with CNF, was enhanced by 186 %, and the excellent tensile strength, puncture load, and tearing strength were displayed. Moreover, the significant glass transition temperature (Tg) near the human body temperature was exhibited for CNF-GA, making it possible to be applied in temperature sensing. Most importantly, the CNF-GA-2 showed the maximum hydrophobicity, with a contact angle of 76.77°. Finally, the CNF-GA-2/MXene nanocomposite film was prepared by the CNF-GA-2 with MXene through vacuum filtration. The results indicated that the film had excellent piezoelectric properties (d33 = 63.283), the generated stable induced voltage (125.6 mV), the preferable piezoresistive performance (ΔR/R0 = 2.15), the fast response/recovery time (48/61 ms), which could achieve dynamic and static responses. Moreover, this film could be used for real-time detection of limb movements (such as wrists).

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.

Recent strategies, progress, and prospects of two-dimensional metal carbides (MXenes) materials in wastewater purification: A review

With the rapid development of industrialization, water pollution directly leads to the serious shortage of fresh water. As reported by the World Water Council, nearly 3.8 billion people will face water scarcity by 2030. Therefore, developing advanced nanomaterials to realize wastewater purification is a major challenge. Two-dimensional (2D) transition metal carbides (MXenes), as the emerging 2D layered nanomaterials, have been investigated for the applications of water purification treatment since first reported in 2011. Over 40 different MXenes have been developed for environmental remediation, and dozens more structures and properties are theoretically predicted. Here, we review the advances from the aspects of synthesis strategies for MXenes, purification mechanism, and their applications in wastewater treatment processes. The major points are 1) the synthesis and modification approaches for MXenes such as multi-layered stacked MXenes and delaminated MXenes 2) a discussion of current water remediation over MXene-based materials, 3) a brief introduction for removal behaviors and deep interaction mechanisms, 4) optimization strategies and key points for boosting the remediation performance of MXenes.

Multivapor-Responsive Controlled Actuation of Starch-Based Soft Actuators

Multivapor-responsive biocompatible soft actuators have immense potential for applications in soft robotics and medical technology. We report fast, fully reversible, and multivapor-responsive controlled actuation of a pure cassava-starch-based film. Notably, this starch-based actuator sustains its actuated state for over 60 min with a continuous supply of water vapor. The durability of the film and repeatability of the actuation performance have been established upon subjecting the film to more than 1400 actuation cycles in the presence of water vapor. The starch-based actuators exhibit intriguing antagonistic actuation characteristics when exposed to different solvent vapors. In particular, they bend upward in response to water vapor and downward when exposed to ethanol vapor. This fascinating behavior opens up new possibilities for controlling the magnitude and direction of actuation by manipulating the ratio of water to ethanol in the binary solution. Additionally, the control of the bending axis of the starch-based actuator, when exposed to water vapor, is achieved by imprinting-orientated patterns on the surface of the starch film. The effect of microstructure, postsynthesis annealing, and pH of the starch solution on the actuation performance of the starch film is studied in detail. Our starch-based actuator can lift 10 times its own weight upon exposure to ethanol vapor. It can generate force ∼4.2 mN upon exposure to water vapor. To illustrate the vast potential of our cassava-starch-based actuators, we have showcased various proof-of-concept applications, ranging from biomimicry to crawling robots, locomotion near perspiring human skin, bidirectional electric switches, ventilation in the presence of toxic vapors, and smart lifting systems. These applications significantly broaden the practical uses of these starch-based actuators in the field of soft robotics.

Robust, ultrathin and flexible electromagnetic interference shielding paper designed with all-polysaccharide hydrogel and MXene

An effective strategy was demonstrated to design an electromagnetic interference (EMI) shielding paper via a facile surface treatment on paper. TEMPO-oxidized cellulose nanofibers (TOCN) were first integrated with Ti3C2Tx MXene, and subsequently cast onto a filter paper with cationic guar gum (CGG) in a sequential way. TOCN and CGG generated a self-assembling hydrogel and formed a MXene-containing hydrogel film on top of the filter paper. The hydrogel film enhanced the tensile strength (9.49 MPa) of composite paper, and resulted in a 17 % increase as compared to the control. The composite paper containing 80 mg MXene (namely, 2.07 mg·cm-2) showed a conductivity of 3843 S·m-1 and EMI shielding effectiveness (EMI SE) of 49.37 dB. Furthermore, the 2-layer assembled TC-M 80 hydrogel composite paper achieved an EMI SE of 73.99 dB. Importantly, this composite paper showed higher EMI SE and lower thickness than a lot of reported materials. The presence of TOCN and CGG also protected MXene against several solvents and the incorporation of polydimethylsiloxane (PDMS) further improved the durability of the composite paper. This work provides a novel and simple strategy to design robust, ultrathin and flexible EMI shielding materials, and it might also inspire other work in paper-based functional materials.

Multistimuli-Responsive Actuators Derived from Natural Materials for Entirely Biodegradable and Programmable Untethered Soft Robots

Untethered soft robots have attracted growing attention due to their safe interaction with living organisms, good flexibility, and accurate remote control. However, the materials involved are often nonbiodegradable or are derived from nonrenewable resources, leading to serious environmental problems. Here, we report a biomass-based multistimuli-responsive actuator based on cuttlefish ink nanoparticles (CINPs), wood-derived cellulose nanofiber (CNF), and bioderived polylactic acid (PLA). Taking advantage of the good photothermal conversion performance and exceptionally hygroscopic sensitivity of the CINPs/CNF composite (CICC) layer and the opposite thermally induced deformation behavior between the CICC layer and PLA layer, the soft actuator exhibits reversible deformation behaviors under near-infrared (NIR) light, humidity, and temperature stimuli, respectively. By introducing patterned or alignment structures and combining them with a macroscopic reassembly strategy, diverse programmable shape-morphing from 2D to 3D such as letter-shape, coiling, self-folding, and more sophisticated 3D deformations have been demonstrated. All of these deformations can be successfully predicted by finite element analysis (FEA) . Furthermore, this actuator has been further applied as an untethered grasping robot, weightlifting robot, and climbing robot capable of climbing a vertical pole. Such actuators consisting entirely of biodegradable materials will offer a sustainable future for untethered soft robots.

Addition of fibers derived from paper mill sludge in paper coatings: impact on microstructure, surface and optical properties

Traditionally, cellulose nanofiber (CNF) production has primarily relied on virgin cellulose sources. Yet, the shift to using paper mill sludge (PMS) as a source for CNF underscores the significance of reusing and recycling industrial byproducts. PMS contains significant amounts of cellulose that can be extracted as a raw material. The purpose of present study is to provide a sustainable approach to PMS utilization as a paper coating additive in the cellulose nanofibrils (CNFPMS) form via simply scalable wire-wound rod coating method. The effect of CNFPMS additive amounts at two coating layers on microstructure and surface properties of coatings such as porosity, air permeability surface roughness and optical properties such as brightness, gloss and CIE L*a*b* is studied, which they can also provide insight for the eventual print performance. Results indicated that the obtained CNFPMS in paper coating shows 52% decrease in porosity, presenting significant improvement in the coating microstructure. The marginal increase in permeability coefficient and surface roughness, 54% and 10%, respectively, suggests improving color reproduction and preventing color density losses. Optical analysis showed slight decrease in brightness and gloss, as was expected. Notably, the lightness was improved, which also indicates increasing color gamut volume in printing applications. As a result, the current work offers a sustainable approach to manage PMS for use in paper coatings as a high-value-added material.

Cellulose-Based Intelligent Responsive Materials: A Review

Due to the rapid development of intelligent technology and the pursuit of green environmental protection, responsive materials with single response and actuation can no longer meet the requirements of modern technology for intelligence, diversification, and environmental friendliness. Therefore, intelligent responsive materials have received much attention. In recent years, with the development of new materials and technologies, cellulose materials have become increasingly used as responsive materials due to their advantages of sustainability and renewability. This review summarizes the relevant research on cellulose-based intelligent responsive materials in recent years. According to the stimuli responses, they are divided into temperature-, light-, electrical-, magnetic-, and humidity-responsive types. The response mechanism, application status, and development trend of cellulose-based intelligent responsive materials are summarized. Finally, the future perspectives on the preparation and applications of cellulose-based intelligent responsive materials are presented for future research directions.

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.

Codesign of Biobased Cellulose-Filled Filaments and Mesostructures for 4D Printing Humidity Responsive Smart Structures

Hygromorphic smart structures are advantageous as passively actuated systems for generating movement, with applications ranging from weather-responsive architectural building skins to adaptive wearables and microrobotics. Four-dimensional (4D) printing is a valuable method for multiscale fabrication and physical programming of such structures. However, material limitations in terms of printability, responsiveness, and mechanical properties are major bottlenecks in achieving reliable and repeatable humidity-responsive actuation. We propose a codesign method for 4D printing hygromorphic structures through fused filament fabrication, incorporating parallel development of (1) biobased cellulose-filled filaments with varying stiffness and hygroresponsiveness, and (2) designed mesoscale structuring in printed elements. We first describe the design of a pallet of filaments produced by compounding cellulose powder in mass ratios of 0-30% within two matrix polymers with high and low stiffness. We then present the design, fabrication, and testing of a series of 4D-printed prototypes tuned to change shape, that is, open and close, in response to relative humidity (RH). The structures can fully transform in conditions of 35-90% RH, which corresponds to naturally occurring shifts in RH in daily and seasonal weather cycles. Furthermore, their motion is fast (within the range of minutes), fully reversible, and repeatable in numerous cycles. These results open new opportunities for the utilization of 4D printing and natural resources for the development of functional humidity-responsive smart structures.

Hemin-Functionalized Microfluidic Chip with Dual-Electric Signal Outputs for Accurate Determination of Uric Acid

Herein, we develop a hemin-functionalized microfluidic chip with dual-electric signal outputs for accurate determination of uric acid (UA). Hemin is designed as the catalyst, which could trigger a built-in reference signal. Carbon nanotube (CNT) and alkalinized titanium carbide (alk-Ti₃C₂T_x) are used as attachment substrates to strengthen the signal. Benefiting from the synergistic action of hemin, CNT, and alk-Ti₃C₂T_x, the hybrid functionalized sensor shows prominent electrochemical capacity, desirable catalytic activity, and unique built-in signal ability. Through density functional theory calculations, the structure-reactivity relationship and possible signal output mechanism are deeply investigated. The functionalized sensor is further integrated into a microfluidic chip to prepare a portable electrochemical sensing platform, in which multiple sample processing steps including primary filtration, target enrichment, and reliable analysis can be conducted step-by-step. Based on the abovementioned designs, the developed functionalized microfluidic platform presents desirable performance in UA determination with a detection limit of 0.41 μM. Furthermore, it is capable of accurately detecting UA in urine samples, providing a promising idea for biomolecule monitoring.

A Flexible Vertical-Section Wood/MXene Electrode with Excellent Performance Fabricated by Building a Highly Accessible Bonding Interface

Cross-section wood (CW) is generally used as a host for free-standing electrodes, as the abundant opened pores can provide large space for loading guest materials with high electrical conductivity and electrochemical activity. However, there is still a challenge for CW to be used in flexible supercapacitors (SCs) because of its low mechanical strength. Herein, as an alternative to CW, vertical-section wood (VW) with excellent mechanical strength and good flexibility is developed and used as a free-standing and flexible electrode by using Ti₃C₂T_x (MXene) with ultrahigh conductivity and good electrochemical activity as a guest material. In particular, the highly accessible bonding interface for Ti₃C₂T_x is first built by delignification on VW to generate abundant pores for continuously absorbing Ti₃C₂T_x and to expose cellulose with abundant oxygen-containing groups for stable combination with Ti₃C₂T_x. Then, cyclic pressing is used to form negative pressure to pump the Ti₃C₂T_x suspension into VW, combining with a preheating process to trigger layer-by-layer self-assembly of Ti₃C₂T_x nanosheets onto a wood cell wall by evaporating water in the suspension. As a result, the free-standing electrode has a large Ti₃C₂T_x loading mass proportion of 33 wt %, a high conductivity of 3.14 S cm^-1, and good flexibility with much higher mechanical strength of 15.1 MPa than 0.4 MPa of CW. The symmetric SC delivers a good specific capacitance of 805 mF cm^-2 at 0.5 mA cm^-2, a remarkably high rate capability of 84% to 10 mA cm^-2, and an energy density of 13.85 μW h cm^-2 at 87.5 μW cm^-2. Additionally, this SC shows a long lifespan of 90.5% after 10,000th charge and discharge cycles even at a constant bending angle of 90°, suggesting promising potential in flexible devices.

Development and challenges of smart actuators based on water-responsive materials

Water-responsive (WR) materials, due to their controllable mechanical response to humidity without energy actuation, have attracted lots of attention to the development of smart actuators. WR material-based smart actuators can transform natural humidity to a required mechanical motion and have been widely used in various fields, such as soft robots, micro-generators, smart building materials, and textiles. In this paper, the development of smart actuators based on different WR materials has been reviewed systematically. First, the properties of different biological WR materials and the corresponding actuators are summarized, including plant materials, animal materials, and microorganism materials. Additionally, various synthetic WR materials and their related applications in smart actuators have also been introduced in detail, including hydrophilic polymers, graphene oxide, carbon nanotubes, and other synthetic materials. Finally, the challenges of the WR actuator are analyzed from the three perspectives of actuator design, control methods, and compatibility, and the potential solutions are also discussed. This paper may be useful for the development of not only soft actuators that are based on WR materials, but also smart materials applied to renewable energy.

Smart Silk Origami as Eco-sensors for Environmental Pollution

Origami folding is an easy, cost-effective, and scalable fabrication method for changing a flat material into a complex 3D functional shape. Here, we created semicrystalline silk films doped with iron oxide particles by mold casting and annealing. The flat silk films could be loaded with natural dyes and folded into 3D geometries using origami principles following plasticization. They performed locomotion under a magnetic field, were reusable, and displayed colorimetric stability. The critical parameters for the design of the semi-autonomous silk film, including ease of folding, shape preservation, and locomotion in the presence of a magnetic field, were characterized, and pH detection was achieved by eye and by digital image colorimetry with a response time below 1 min. We demonstrate a practical application—a battery-free origami silk boat—as a colorimetric sensor for waterborne pollutants, which was reusable at least five times. This work introduces silk eco-sensors and merges responsive actuation and origami techniques.

Robust and Highly Sensitive Cellulose Nanofiber-Based Humidity Actuators

The design of humidity actuators with high response sensitivity (especially actuation time) while maintaining favorable mechanical properties is important for advanced intelligent manufacturing, like soft robotics and smart devices, but still remains a challenge. Here, we fabricate a robust and conductive composite film-based humidity actuator with synergetic benefits from one-dimensional cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) as well as two-dimensional graphene oxide (GO) via an efficient vacuum-assisted self-assembly method. Owing to the excellent moisture sensitivity of CNF and GO, the hydrophobic CNT favoring rapid desorption of water molecules, and the unique porous structure with numerous nanochannels for accelerating the water exchange rate, this CNF/GO/CNT composite film delivers excellent actuation including an ultrafast response/recovery (0.8/2 s), large deformation, and sufficient cycle stability (no detectable degradation after 1000 cycles) in response to ambient gradient humidity. Intriguingly, the actuator could also achieve a superior flexibility, a good mechanical strength (201 MPa), a desirable toughness (6.6 MJ/m³), and stable electrical conductivity. Taking advantage of these benefits, the actuator is conceptually fabricated into various smart devices including mechanical grippers, crawling robotics, and humidity control switches, which is expected to hold great promise toward practical applications.