Multistimulus Responsive Actuator with GO and Carbon Nanotube/PDMS Bilayer Structure for Flexible and Smart Devices

Self-Healing and Reconfigurable Actuators Based on Synergistically Cross-Linked Supramolecular Elastomer

Stimulus-responsive soft actuators show great potential in intelligent robot systems for their various virtues, such as arbitrary shape morphing, outstanding adaptability to environment, and multidegrees of freedom. However, it is extremely challenging to achieve a combination of excellent actuating performance and robust mechanical strength as well as self-healing property. Herein we report a near-infrared light-responsive soft actuator based on the synergistic effects of a crystalline physical cross-linked network and a hydrogen bonding supramolecular network. The actuator exhibits outstanding comprehensive performance including fast and reliable light-responsive behavior (bending angle over 90° within 1.6 s), robust mechanical strength (12.52 MPa), superfast self-healing speed (2 s), and satisfactory self-healing efficiency in both mechanical (87.68%) and actuating (99.50%) performance. In addition, it is convenient to fabricate and reconfigure the actuators by a mild-temperature molding strategy to acquire various three-dimensional structures, thus achieving diverse actuating locomotion. This work provides a powerful and facile strategy to prepare soft actuators with intriguing performance, allowing significant progress in broadening their practical application.

High-Sensitivity Wearable and Flexible Humidity Sensor Based on Graphene Oxide/Non-Woven Fabric for Respiration Monitoring

The popularity of humidity sensing for respiratory analysis of patients is gradually increasing because of its portability and cost-effectiveness. However, current flexible humidity sensors are mainly made of polymer films, whose poor hygroscopicity and breathability reduce their sensitivity and comfort. In this study, a highly sensitive humidity sensor was developed using non-woven fabric (NWF) coated with graphene oxide (GO). Bovine serum albumin was used to improve the adsorption of GO onto the NWF, and its effect on sensitivity was investigated by adjusting its concentration. High-humidity sensitivity was experimentally validated by testing different relative humidity levels, and its fast response and excellent feasibility under diverse breathing conditions were verified by successful monitoring of fast and deep breathing, differentiating nose and mouth breathing, and even identifying simple spoken words. This study developed a breathable and skin-friendly humidity sensor based on GO/NWF, which is a promising device for human healthcare.

Deterministic Role of Carbon Nanotube-Substrate Coupling for Ultrahigh Actuation in Bilayer Electrothermal Actuators

Here, the actuation response of an architectured electrothermal actuator comprising a single layer of carbon nanotube (CNT) film and a relatively thicker film of silk, cellulose, or polydimethylsiloxane is studied. An electric current is passed through the CNT film, which generates heat responsible for electrothermal actuation, in all samples, affixed as per doubly clamped beam configuration. All samples, including pure CNT film, show remarkable actuation such that actuation monotonically increases with the applied voltage. Cyclic pulsed electrical loading shows a lag in the electric current stimulus and the actuation. Remarkably, an ultrahigh actuation of ∼2.8%, which was 72 times more than that shown by pure CNT film, is measured in the CNT-cellulose film, that is, the architectured actuator with the natural polymer having the functional property of hygroexpansion and the structural hierarchy of the CNT film, however, at a significantly larger length scale. Overall, the synergetic contribution of the individual layers in these bilayered actuators enabled achieving ultrahigh electrothermal actuation compared to the homogeneous, synthetic polymer-based devices. A detailed discussion, which also includes examination of the role of the hierarchical substructure and the functional properties of the substrate and numerical analysis using the finite element method, is presented to highlight the actuation mechanism in the fabricated actuators.

NIR-UV Responsive Actuator with Graphene Oxide/Microchannel-Induced Liquid Crystal Bilayer Structure for Biomimetic Devices

Soft bilayer actuators with a simple fabrication process, diverse molecular alignment, and multistimulus response are displayed in this work. The microchannel method proposed by us can exquisitely program the molecular arrangement. Based on the mismatch in coefficient of thermal expansion (CTE) between graphene oxide (GO) and the azobenzene doped liquid crystal network (ALCN), bilayer actuators can exhibit reversible, rapid, and complex deformations under the control of heat, UV and NIR light. Furthermore, in addition to microchannels, various deformation behaviors of bilayer actuators can also be programmed by directionally arranging GO layers. Smart bilayer membranes can be customized into a range of delicate biomimetic devices, such as bionic butterfly, bionic leaf, and foot robot, promising their numerous applications in biomimetic and intelligent soft robotics fields.

Smart Textile-Integrated Microelectronic Systems for Wearable Applications

The programmable nature of smart textiles makes them an indispensable part of an emerging new technology field. Smart textile-integrated microelectronic systems (STIMES), which combine microelectronics and technology such as artificial intelligence and augmented or virtual reality, have been intensively explored. A vast range of research activities have been reported. Many promising applications in healthcare, the internet of things (IoT), smart city management, robotics, etc., have been demonstrated around the world. A timely overview and comprehensive review of progress of this field in the last five years are provided. Several main aspects are covered: functional materials, major fabrication processes of smart textile components, functional devices, system architectures and heterogeneous integration, wearable applications in human and nonhuman-related areas, and the safety and security of STIMES. The major types of textile-integrated nonconventional functional devices are discussed in detail: sensors, actuators, displays, antennas, energy harvesters and their hybrids, batteries and supercapacitors, circuit boards, and memory devices.

Flexible and Super-Sensitive Moisture-Responsive Actuators by Dispersing Graphene Oxide into Three-Dimensional Structures of Nanofibers and Silver Nanowires

Smart actuators with excellent flexibility, sensitive responsiveness, large-scale bending-deformation, and rapid deformation-recovery performance have been sought after by researchers. Two-dimensional graphene oxide (GO) is considered as an ideal candidate for humidity-responsive actuators because of its excellent moisture sensitivity. Herein, a flexible membrane-based actuator was prepared by evenly dispersing GO sheets into a three-dimensional network formed by one-dimensional PVA-co-PE nanofibers (NFs) and silver nanowires (AgNWs). The three-dimensional interlaced pore structure of the AgNWs/NFs/GO composite membrane ensured its larger contact area (19.33 m²/g), faster moisture exchange rate, and large bending deformation under moisture stimulation. In addition, a new explanation for the spatial distribution of adsorbed water molecules and their actuating effect on the bending behaviors of composite membranes is proposed. The adsorbed water lies between the interlayer and surface layer of the composite membrane. The interlayer water molecules make the film volume expand, resulting in a large bending angle of the membrane. On the other hand, the water on the surface layers of the membrane only leads to the change in film weight, having little effect on the bending behavior. Moreover, to make the soft actuator more practical and multifunctional, a conductive AgNWs-NFs/GO bilayer membrane-based actuator was prepared by layered spraying of a AgNW on the NFs/GO membrane, which can be directly used in switching control circuits. The novel flexible membrane-based actuators are of great significance for the soft robot and intelligent control systems in the future.

Spirally deformable soft actuators and their designable helical actuations based on a highly oriented carbon nanotube film

Spiral configurations and helical curlings of plant tendrils and seed pods are very common in nature. Many researchers have tried to develop spirally deformable actuators to mimic these natural spirals through several approaches, such as preforming helical shapes, processing diagonal stripes and employing anisotropic organic layers. However, these methods are usually complex and time-consuming. Here, we used an efficient method to produce a highly oriented carbon nanotube (CNT) film and develop a series of spirally deformable soft actuators which perform various controllable helical actuations. The actuator consists of a CNT layer with strong anisotropy and a silicone layer. By simply adjusting the orientations of the aligned CNTs, the prepared actuators can accomplish left- or right-handed spiral deformations with different helical forms when driven by electricity. Finite element analyses and simulations were conducted to investigate the mechanism. It is confirmed that it is the anisotropic moduli of the CNT film that regulate the internal stress distributions of the actuators and lead to helical actuations. Moreover, complex actuator designs and functional applications were also carried out. A V-shaped actuator can simultaneously achieve left- and right-handed curling with large angles (630°), which vividly imitates the spiral winding of a tendril. A Y-shaped actuator performed three-dimensional movements, which can manipulate lightweight objects deftly. By virtue of easy preparation and flexible function design, the spirally deformable actuators based on the oriented CNT film will be very promising in artificial muscles and bionic soft robots.

Fast-response, agile and functional soft actuators based on highly-oriented carbon nanotube thin films

Highly-oriented carbon nanotube (CNT) film, which is made from super-aligned CNT array, is an even, tough and soft material. This CNT film has strong anisotropy in electrical and mechanical properties. The electrical conductivity and Young's modulus of the CNT film (2.8 × 10⁴S m^-1, 3000 MPa) along the CNT aligned direction are one magnitude larger than those (2.3 × 10³S m^-1, 200 MPa) along the vertical direction. In virtue of easy preparation and good processability, it is competent as high-performance flexible electrodes for soft actuators, advanced film capacitors and batteries. Here, we use this highly-oriented CNT film as a heating electrode to make fast-response soft actuators. The actuator has a thin bilayer composite structure and is driven by current heating. It takes a typical miniaturized actuator only 0.9 s to perform fast and large-angle deformations (270° bending, curvature 4.8 cm^-1), and its bending speed can reach 300° s^-1by low power driving (2.4 W). Based on this CNT film, graphical designs and fine processing were carried out to make patterned electrodes and functional actuators, such as cross-shaped and hand-shaped ones. Notably, a well-designed gripper-like actuator can even deftly grab and manipulate some tiny things, e.g. a grain of rice. Moreover, the anisotropic properties of the CNT film also effectively influence and regulate the deformation forms of the actuators. In virtue of good and unique performances in electrical, mechanical and thermal aspects, the high-oriented CNT film would have promising application prospects in various emerging soft devices.

One-Pot and One-Step Fabrication of Salt-Responsive Bilayer Hydrogels with 2D and 3D Shape Transformations

Bilayer hydrogels are one of the most promising materials for use as soft actuators, artificial muscles, and soft robotic elements. Therefore, the development of new and simple methods for the fabrication of such hydrogels is of particular importance for both academic research and industrial applications. Herein, a facile, one-pot, and one-step methodology was used to prepare bilayer hydrogels. Specifically, several common monomers, including N-isopropyl acrylamide, acrylamide, and N-(2-hydroxyethyl)acrylamide, as well as two salt-responsive zwitterionic monomers, 3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (VBIPS) and dimethyl-(4-vinylphenyl)ammonium propane sulfonate (DVBAPS), were chosen and employed with different combinations and ratios to understand the formation and structural tunability of the bilayer hydrogels. The results indicated that a salt-responsive zwitterionic-enriched copolymer, which could precipitate from water, plays a dominant role in the formation of the bilayer structure and that the ratio between the common monomer and the zwitterionic monomer had a significant effect on the structure. Due to the salt-responsive properties of polyVBIPS and polyDVBAPS, the resultant bilayer hydrogels exhibited excellent bidirectional bending properties in response to the salt solution. With the optimal monomer pair and ratio determined, the bend of the hydrogel could be reversed from ∼-360 to ∼266° in response to a switch between water and a 1.0 M NaCl solution. Additionally, this method was further used to fabricate small-scaled patterns with structural and compositional distinction in two-dimensional hydrogel sheets. These two-dimensional hydrogel sheets exhibited complex and reversible three-dimensional shape transformations due to the different bending behaviors of the patterned hydrogel stripes under the action of an external stimulus. This work provides greater insight into the mechanism of the one-step, one-pot method fabrication of bilayer hydrogels, demonstrates the ability of this method for the preparation of small-scale patterns in hydrogel sheets to endow the complex with a three-dimensional shape transformation capability, and hopefully opens up a new pathway for the design and fabrication of smart hydrogels.

Leaf-inspired multiresponsive MXene-based actuator for programmable smart devices

Natural leaves, with elaborate architectures and functional components, harvest and convert solar energy into chemical fuels that can be converted into energy based on photosynthesis. The energy produced leads to work done that inspired many autonomous systems such as light-triggered motion. On the basis of this nature-inspired phenomenon, we report an unprecedented bilayer-structured actuator based on MXene (Ti3C2T x )-cellulose composites (MXCC) and polycarbonate membrane, which mimic not only the sophisticated leaf structure but also the energy-harvesting and conversion capabilities. The bilayer actuator features multiresponsiveness, low-power actuation, fast actuation speed, large-shape deformation, programmable adaptability, robust stability, and low-cost facile fabrication, which are highly desirable for modern soft actuator systems. We believe that these adaptive soft systems are attractive in a wide range of revolutionary technologies such as soft robots, smart switch, information encryption, infrared dynamic display, camouflage, and temperature regulation, as well as human-machine interface such as haptics.

Photothermal and Moisture Actuator Made with Graphene Oxide and Sodium Alginate for Remotely Controllable and Programmable Intelligent Devices

Functional materials with energy storage and conversion properties have been useful for actuating devices. Here, a new kind of torsional fiber-based actuator including graphene oxide (GO) and natural sodium alginate was prepared by traditional wet spinning and twisting methods, during which the fiber structure was reconstructed, and the mechanical energy was prestored. When the twisted GO/SA (graphene oxide/sodium alginate) fiber was stimulated by infrared light or moisture, the torsional structure of the fiber was activated instantaneously to generate rapid and reversible rotational motion, thus realizing the automatic release and re-storage process of rotational kinetic energy. In addition, the full revolutions of the twisted GO/SA fiber have no attenuation after 100 reversible rotations when stimulated by moisture, which proves the excellent rotational stability. Due to its excellent flexibility and wettability, the twisted GO/SA fiber can be woven into a network or prepared into a series of programmable intelligent devices, which is of great significance for future flexible intelligent electronic devices.

Controllable Shape Changing and Tristability of Bilayer Composite

The programmable shape transition of a two-dimensional sheet to a three-dimensional (3D) structure in response to a variety of external stimuli has recently attracted increasing attention. Among the various shape changing materials, shape memory polymers (SMPs) can fix their temporary shape and/or their length and recover under proper thermal treatment. In this work, we create a bilayer composite by bonding one layer of elastomer with one layer of stretched SMPs, which can undergo a series of shape transitions via the storage and release of internal stresses. The programed shapes are achieved by adjusting the orientation and elongation of the SMPs. Meanwhile, the 3D structures exhibit tristability and can transit between hemihelical, left-handed helical, and right-handed helical shapes. Both theoretical analysis and finite element simulations were conducted to understand the mechanism of shape transformation and used to predict the deformed configuration by adjusting preprogramming parameters. Our work provides a new strategy and design space for fabricating smart reconfigurable structures and paves way for the design and development of bioinspired four-dimensional active matter for a broad range of applications in intelligent materials.