Cholesteric Liquid Crystal Polymeric Coatings for Colorful Artificial Muscles and Motile Humidity Sensor Skin Integrated with Magnetic Composites

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.

Self-flickering bioinspired actuator with autonomous motion and structural color switching

Color-tunable actuators with motion and color-changing functions have attracted considerable attention in recent years, yet it remains a challenge to achieve the autonomous regulation of motion and color. Inspired by Apatura ilia butterfly with dynamic structural color and Pelargonium carnosum plant with moisture responsive bilayer structure, an automatic color-tunable actuator is developed by integrating photonic crystals layer and hygroscopic layer. Taking advantage of the asymmetric hygroscopicity between two layers and the angle-dependent structural color of photonic crystals, this actuator can continuously self-flicker in humid environment by visual switching in structural color due to automated cyclic motion. The actuator is assembled into the self-flapping biomimetic butterfly with switchable color and the self-reporting information array with dynamic visual display, demonstrating its autoregulatory motion and color. This work provides a new strategy for developing automatic color-tunable actuator and suggests its potential in the intelligent robot and optical display.

Embedded Physical Intelligence in Liquid Crystalline Polymer Actuators and Robots

Responsive materials possess the inherent capacity to autonomously sense and respond to various external stimuli, demonstrating physical intelligence. Among the diverse array of responsive materials, liquid crystalline polymers (LCPs) stand out for their remarkable reversible stimuli-responsive shape-morphing properties and their potential for creating soft robots. While numerous reviews have extensively detailed the progress in developing LCP-based actuators and robots, there exists a need for comprehensive summaries that elucidate the underlying principles governing actuation and how physical intelligence is embedded within these systems. This review provides a comprehensive overview of recent advancements in developing actuators and robots endowed with physical intelligence using LCPs. This review is structured around the stimulus conditions and categorizes the studies involving responsive LCPs based on the fundamental control and stimulation logic and approach. Specifically, three main categories are examined: systems that respond to changing stimuli, those operating under constant stimuli, and those equip with learning and logic control capabilities. Furthermore, the persisting challenges that need to be addressed are outlined and discuss the future avenues of research in this dynamic field.

Multistimuli-Responsive Soft Actuators with Controllable Bionic Motions

Soft actuators with biomimetic self-regulatory intelligence have garnered significant scientific interest due to their potential applications in robotics and advanced functional devices. We present a multistimuli-responsive actuator made from a carbon nitride/carbon nanotube (CN/CNTs) composite film. This film features a molecular switch based on reversible hydrogen bonds, whose asymmetric distribution endows the film with the ability to absorb water unevenly and convert molecular motion into macroscopic movement. By incorporating carboxylated CNTs, the film demonstrates improved mechanical properties and actuation performance. Under ambient humidity stimuli, the actuator can autonomously generate walking and tumbling motions. The CN/CNTs composite film's actuating behaviors are programmable, enabling diverse deformation modes and complex biomimetic movements. Additionally, the film exhibits excellent photothermal conversion efficiency (74.10 °C/s), allowing for temperature and light-responsive actuation, which can be remotely controlled in real time. These features have enabled the creation of soft robots capable of complex biomimetic actions such as jumping, directional movement, and transporting objects. This research broadens the potential applications of CN-based actuators and paves the way for the development of intelligent soft robots.

A Proprioceptive Janus Fiber with Controllable Multistage Segments for Bionic Soft Robots

Smart fibers capable of integrating the multifunctionality of actuation and self-sensation into a single proprioceptive device have significant applications in soft robots and biomedicine. Especially, the achievement of self-sensing the movement patterns of different actuating segments in one fiber is still a great challenge. Herein, in this study, a fiber with the controllable Janus architecture is successfully proposed via an artful centrifugation-driven hierarchical gradient self-assembly strategy, which couples two functional components of piezoresistive carbon nanotubes and magnetic NdFeB nanoparticles into the upper and lower layers of this flexible fibrous framework with the porous sponge structure partially, respectively. As predicted, the final product exhibits the as-anticipated bionic proprioceptive behaviors of programmable actuating deformation and highly selective response to bending, stretching, and pressure with high washable stability and mechanical performances. More importantly, assisted by the different three-dimensional printing molds, the superlong Janus fibers with various controllable lengths of the reversed but sequential multistage segments can be fabricated, offering the hybrid magnetic actuation and proprioceptive sensation existing at arbitrary nodes. Therefore, several kinds of soft organism-inspired Janus fiber-derived soft robots with the arbitrarily controlled segmental characters were assembled as the proof-of-concept, which can not only realize a snake or inchworm-inspired successive contracting-stretching deformation and a sperm-inspired self-rotating crawling motion but also self-sense the signals of each segment themselves in real time and then be used to navigate an object to target position in a liquid-filled confined tube. It is believed that this work promotes the further development of proprioceptive soft robots.

Multiresponse Liquid Metal Bionic Flexible Actuator

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

Reprogrammable Magnetic Soft Actuators with Microfluidic Functional Modules via Pixel-Assembly

Magnetic soft actuators and robots have attracted considerable attention in biomedical applications due to their speedy response, programmability, and biocompatibility. Despite recent advancements, the fabrication process of magnetic actuators and the reprogramming approach of their magnetization profiles continue to pose challenges. Here, a facile fabrication strategy is reported based on arrangements and distributions of reusable magnetic pixels on silicone substrates, allowing for various magnetic actuators with customizable architectures, arbitrary magnetization profiles, and integration of microfluidic technology. This approach enables intricate configurations with decent deformability and programmability, as well as biomimetic movements involving grasping, swimming, and wriggling in response to magnetic actuation. Moreover, microfluidic functional modules are integrated for various purposes, such as on/off valve control, curvature adjustment, fluid mixing, dynamic microfluidic architecture, and liquid delivery robot. The proposed method fulfills the requirements of low-cost, rapid, and simplified preparation of magnetic actuators, since it eliminates the need to sustain pre-defined deformations during the magnetization process or to employ laser heating or other stimulation for reprogramming the magnetization profile. Consequently, it is envisioned that magnetic actuators fabricated via pixel-assembly will have broad prospects in microfluidics and biomedical applications.

Beyond Color Boundaries: Pioneering Developments in Cholesteric Liquid Crystal Photonic Actuators

Creatures in nature make extensive use of structural color adaptive camouflage to survive. Cholesteric liquid crystals, with nanostructures similar to those of natural organisms, can be combined with actuators to produce bright structural colors in response to a wide range of stimuli. Structural colors modulated by nano-helical structures can continuously and selectively reflect specific wavelengths of light, breaking the limit of colors recognizable by the human eye. In this review, the current state of research on cholesteric liquid crystal photonic actuators and their technological applications is presented. First, the basic concepts of cholesteric liquid crystals and their nanostructural modulation are outlined. Then, the cholesteric liquid crystal photonic actuators responding to different stimuli (mechanical, thermal, electrical, light, humidity, magnetic, pneumatic) are presented. This review describes the practical applications of cholesteric liquid crystal photonic actuators and summarizes the prospects for the development of these advanced structures as well as the challenges and their promising applications.

A Solvent-Templated Porous Liquid Crystal Elastomer with Tactile Sensation beyond Reversible Actuation toward Versatile Artificial Muscles

Liquid crystal elastomers (LCEs), as a classical two-way shape-memory material, are good candidates for developing artificial muscles that mimic the contraction, expansion, or rotational behavior of natural muscles. However, biomimicry is currently focused more on the actuation functions of natural muscles dominated by muscle fibers, whereas the tactile sensing functions that are dominated by neuronal receptors and synapses have not been well captured. Very few studies have reported the sensing concept for LCEs, but the signals were still donated by macroscopic actuation, that is, variations in angle or length. Herein, we develop a conductive porous LCE (CPLCE) using a solvent (dimethyl sulfoxide (DMSO))-templated photo-cross-linking strategy, followed by carbon nanotube (CNT) incorporation. The CPLCE has excellent reversible contraction/elongation behavior in a manner similar to the actuation functions of skeletal muscles. Moreover, the CPLCE shows excellent pressure-sensing performance by providing real-time electrical signals and is capable of microtouch sensing, which is very similar to natural tactile sensing. Furthermore, macroscopic actuation and tactile sensation can be integrated into a single system. Proof-of-concept studies reveal that the CPLCE-based artificial muscle is sensitive to external touch while maintaining its excellent actuation performance. The CPLCE with tactile sensation beyond reversible actuation is expected to benefit the development of versatile artificial muscles and intelligent robots.

Photonic Interpenetrating Polymer Network Fibers Comprising Intertwined Solid-State Cholesteric Liquid Crystal and Polyelectrolyte Networks for Sensor Applications

Uniform-sized photonic interpenetrating polymer network (IPN) fibers comprising intertwined solid-state cholesteric liquid crystal (CLCsolid) and anionic poly(acrylic acid) (PAA) or cationic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) networks (photonic IPNPAA or IPNPDMAEMA fibers) were developed for sensor applications. IPNPAA or IPNPDMAEMA fibers with a perfect photonic structure were fabricated inside Teflon tube templates without any treatments for realizing a planar orientation in those fibers. The dominant wavelength of the photonic color from a photograph taken with a cellular phone was used to measure the photonic color change. Photonic IPNPAA fibers treated with KOH (IPNKOH fibers) were used for sensing humidity and divalent metal ions. The linear ranges for relative humidity and Ca2+ detection were 21-92% and 0.5-3.5 mM, and their limits of detection (LODs) were 7.86% and 0.07 mM, respectively. The photonic IPNPAA (or IPNPDMAEMA) fiber immobilized with urease (IPNPAA-urease) (or glucose oxidase (IPNPDMAEMA-GOx)) was used for urea (or glucose) biosensor application. The photonic IPNPAA-urease (or IPNPDMAEMA-GOx) fiber was red-shifted in response to urea (or glucose) in the linear range of 10-60 mM (or 2-16 mM) with an LOD of 2.54 mM (or 0.76 mM). These photonic IPN fibers are promising because of their easy fabrication and miniaturization, battery-free device, cost-effectiveness, and visual detection without using sophisticated analytical instruments. The developed photonic IPN fibers provide new possibilities for the widespread use of photonic sensors in cutting-edge wearable technology and beyond.

Knittable Electrochemical Yarn Muscle for Morphing Textiles

Morphing textiles, crafted using electrochemical artificial muscle yarns, boast features such as adaptive structural flexibility, programmable control, low operating voltage, and minimal thermal effect. However, the progression of these textiles is still impeded by the challenges in the continuous production of these yarn muscles and the necessity for proper structure designs that bypass operation in extensive electrolyte environments. Herein, a meters-long sheath-core structured carbon nanotube (CNT)/nylon composite yarn muscle is continuously prepared. The nylon core not only reduces the consumption of CNTs but also amplifies the surface area for interaction between the CNT yarn and the electrolyte, leading to an enhanced effective actuation volume. When driven electrochemically, the CNT@nylon yarn muscle demonstrates a maximum contractile stroke of 26.4%, a maximum contractile rate of 15.8% s-1, and a maximum power density of 0.37 W g-1, surpassing pure CNT yarn muscles by 1.59, 1.82, and 5.5 times, respectively. By knitting the electrochemical CNT@nylon artificial muscle yarns into a soft fabric that serves as both a soft scaffold and an electrolyte container, we achieved a morphing textile is achieved. This textile can perform programmable multiple motion modes in air such as contraction and sectional bending.

Multimode opto-magnetic dual-responsive actuating fibers and fabrics programmed via direct ink writing

Current methods lack one-step actuation programming for weave structures that can achieve multimodal motions in fiber and fabric actuators. Fiber and fabric actuators with dual-response to magnetic fields and near-infrared (NIR) light were fabricated via direct ink writing (DIW) in this work, and have 105.3 J kg-1 energy density, enabling multimodal motions including rolling, grasping, and transportation.