Electrically driven liquid crystal network actuators.

Soft actuators based on liquid crystal networks (LCNs) have aroused great scientific interest for use as stimuli-controlled shape-changing and moving components for robotic devices due to their fast, large, programmable and solvent-free actuation responses. Recently, various LCN actuators have been implemented in soft robotics using stimulus sources such as heat, light, humidity and chemical reactions. Among them, electrically driven LCN actuators allow easy modulation and programming of the input electrical signals (amplitude, phase, and frequency) as well as stimulation throughout the volume, rendering them promising actuators for practical applications. Herein, the progress of electrically driven LCN actuators regarding their construction, actuation mechanisms, actuation performance, actuation programmability and the design strategies for intelligent systems is elucidated. We also discuss new robotic functions and advanced actuation control. Finally, an outlook is provided, highlighting the research challenges faced with this type of actuator.

Dynamic AIE crosslinks in liquid crystal networks: visualizing for actuation-guiding, re-bonding for actuation-altering.

Covalent adaptable liquid crystal networks (CALCNs) are highly potential actuating materials due to their actuation properties and shape reprogrammability. Given the importance of network crosslinking state in a CALCN actuator, we sought an all-in-one strategy to probe and visualize its dynamic network while ensuring actuation and reprogramming. Here, tetraphenylethylene derivatives were incorporated into liquid crystal networks via the Diels-Alder (DA) reaction, acting simultaneously as reversible crosslinkers and aggregation-induced emission (AIE) fluorescent probes. The thermally tunable fluorescence of the resulting network can correlate to and thus visualize the actuator's crosslinking status, actuation capability and temperature in real-time and in situ, yielding an intriguing actuation limit-alerting function. Furthermore, we verified unprecedented reprogrammability of the AIE-type CALCNs through both associative and dissociative exchange mechanisms of DA chemistry.

"Self-Lockable" Liquid Crystalline Diels-Alder Dynamic Network Actuators with Room Temperature Programmability and Solution Reprocessability.

Novel main-chain liquid crystalline Diels-Alder dynamic networks (LCDANs) were prepared that exhibit unprecedented ease for actuator programming and reprocessing compared to existing liquid crystalline network (LCN) systems. Following cooling from 125 °C, LCDANs are deformed with aligned mesogens self-locked at room temperature by slowly formed Diels-Alder (DA) bonds, which allows for the formation of solid 3D actuators capable of reversible shape change, and strip walker and wheel-capable light-driven locomotion upon either thermally or optically induced order-disorder phase transition. Any actuator can readily be erased at 125 °C and reprogrammed into a new one under ambient conditions. Moreover, LCDANs can be processed directly from melt (for example, fiber drawing) and from solution (for example, casting tubular actuators), which cannot be achieved with LCNs using exchangeable covalent bonds. The combined attributes of LCDANs offer significant progress toward developing easily programmable/processable LCN actuators.

Selective Decrosslinking in Liquid Crystal Polymer Actuators for Optical Reconfiguration of Origami and Light-Fueled Locomotion.

The ability to optically reconfigure an existing actuator of a liquid crystal polymer network (LCN) so that it can display a new actuation behavior or function is highly desired in developing materials for soft robotics applications. Demonstrated here is a powerful approach relying on selective polymer chain decrosslinking in a LCN actuator with uniaxial LC alignment. Using an anthracene-containing LCN, spatially controlled optical decrosslinking can be realized through photocleavage of anthracene dimers under 254 nm UV light, which alters the distribution of actuation (crosslinked) and non-actuation (decrosslinked) domains and thus determines the actuation behavior upon order-disorder phase transitions. Based on this mechanism, a single actuator having a flat shape can be reconfigured in an on-demand manner to exhibit reversible shape transformation such as self-folding into origami three-dimensional structures. Moreover, using a dye-doped LCN actuator, a light-fueled microwalker can be optically reconfigured to adopt different locomotion behaviors, changing from moving in the laser scanning direction to moving in the opposite direction.

Semi-IPNs with Moisture-Triggered Shape Memory and Self-Healing Properties.

Moisture or water has the advantages of being green, inexpensive, and moderate. However, it is challenging to endow water-induced shape memory property and self-healing capability to one single polymer because of the conflicting structural requirement of the two types of materials. In this study, this problem is solved through introducing two kinds of supramolecular interactions into semi-interpenetrating polymer networks (semi-IPNs). The hydrogen bonds function as water-sensitive switches, making the materials show moisture-induced shape memory effect. The host-guest interactions (ß-cyclodextrin-adamantane) serve as both permanent phases and self-healing motifs, enabling further increased chain mobility at the cracks and self-healing function. In addition, these polyvinylpyrrolidone/poly(hydroxyethyl methacrylate-co-butyl acrylate) semi-IPNs also show thermosensitive triple-shape memory effect.

Multifunctional bimetallic MOF with oxygen vacancy synthesized by microplasma for rapid total antioxidant capacity assessment in agricultural products.

The assessment of total antioxidant capacity (TAC) is crucial for evaluating overall antioxidant potential, predicting the risk of chronic diseases, guiding dietary and nutritional interventions, and studying the effectiveness of antioxidants. However, achieving rapid TAC assessment with high sensitivity and stability remains a challenge. In this study, Ce/Fe-MOF with abundant oxygen vacancies was synthesized using microplasma for TAC determination. The microplasma synthesis method was rapid (30 min) and cost-effective. The presence of oxygen vacancies and the collaboration between iron and cerium in Ce/Fe-MOF not only enhanced the catalyst's efficiency but also conferred multiple enzyme-like properties: peroxidase-like, oxidase-like, and superoxide dismutase mimetic activities. Consequently, a simple colorimetric assay was established for TAC determination in vegetables and fruits, featuring a short analysis time of 15 min, a good linear range of 5-60 µM, a low detection limit of 1.3 µM and a good recovery of 91 %-107 %. This method holds promise for rapid TAC assessment in agricultural products.

Desynchronized liquid crystalline network actuators with deformation reversal capability.

Liquid crystalline network (LCN) actuator normally deforms upon thermally or optically induced order-disorder phase transition, switching once between two shapes (shape 1 in LC phase and shape 2 in isotropic state) for each stimulation on/off cycle. Herein, we report an LCN actuator that deforms from shape 1 to shape 2 and then reverses the deformation direction to form shape 3 on heating or under light only, thus completing the shape switch twice for one stimulation on/off cycle. The deformation reversal capability is obtained with a monolithic LCN actuator whose two sides are made to start deforming at different temperatures and exerting different reversible strains, by means of asymmetrical crosslinking and/or asymmetrical stretching. This desynchronized actuation strategy offers possibilities in developing light-fueled LCN soft robots. In particular, the multi-stage bidirectional shape change enables multimodal, light-driven locomotion from the same LCN actuator by simply varying the light on/off times.

Biomimetic Locomotion of Electrically Powered "Janus" Soft Robots Using a Liquid Crystal Polymer.

Oriented liquid crystal networks (LCNs) can undergo reversible shape change at the macroscopic scale upon an order-disorder phase transition of the mesogens. This property is explored for developing soft robots that can move under external stimuli, such as light in most studies. Herein, electrically driven soft robots capable of executing various types of biomimetic locomotion are reported. The soft robots are composed of a uniaxially oriented LCN strip, a laminated Kapton layer, and thin resistive wires embedded in between. Taking advantage of the combined attributes of the actuator, namely, easy processing, reprogrammability, and reversible shape shift between two 3D shapes at electric power on and off state, the concept of a "Janus" soft robot is demonstrated, which is built from a single piece of the material and has two parts undergoing opposite deformations simultaneously under a uniform stimulation. In addition to complex shape morphing such as the movement of oarfish and sophisticated devices like self-locking grippers, electrically powered "Janus" soft robots can accomplish versatile locomotion modes, including crawling on flat surfaces through body arching up and straightening down, crawling inside tubes through body stretching and contraction, walking like four-leg animals, and human-like two-leg walking while pushing a load forward.

Shape Memory Polymers Based on Supramolecular Interactions.

Shape memory polymers (SMPs), with the capability to change from one or more temporary shapes to predetermined shapes in response to an external stimulus, have attracted much interest from both academia and industries. When introducing supramolecular interactions that have been featured as dynamic and reversible into the design of novel SMPs, intriguing and unique functionalities have been engendered and thereby broaden the potential applications of the SMPs to new territories. In this review, we summarize recent progress made in SMPs based on supramolecular interactions, provide insight into the material design and shape memory mechanism, elucidate and evaluate their properties and performance, and point out opportunities and applications of SMPs.

Light-, pH- and thermal-responsive hydrogels with the triple-shape memory effect.

Light-, pH- and thermal-responsive hydrogels were prepared by introducing dansyl-aggregations and azo-cyclodextrin inclusion complexes as switches. The resulting material showed dual shape memory behavior in response to light, pH or temperature, respectively, and exhibits the triple-shape memory effect in response to light and pH sequentially.

pH- and Thermal-Responsive Multishape Memory Hydrogel.

A multistimuli sensitive shape memory hydrogel with dual and triple shape memory properties was prepared by grafting dansyl groups into the network of polyacrylamide (PAAM). The hydrophobic aggregation of dansyl groups acted as molecular switches, which showed reversible aggregation-disaggregation transition in aqueous solution in response to the pH or temperature change.