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.

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.

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.

Tough Self-Healing Elastomers Based on the Host-Guest Interaction of Polycyclodextrin.

Inspired by animal muscles, we developed a kind of tough elastomers combining high strength and high stretchability with autonomous self-healing capability. A key structural feature is the construction of a double network (DN) connected by the hydrogen bond and host-guest interactions. The first network is the classic elastomer polyacrylate matrix cross-linked by strong hydrogen bonding. The second network is formed through the host-guest interactions between polycyclodextrin and the adamantane (Ad) groups on the side of the polyacrylate chain. Supramolecular interactions between two networks make them miscible and interpenetrate in the molecular level and then can share the load as the sample was stretched. The host-guest interactions act not only as sacrificial bonds for energy dissipation but also as self-healing driving forces. The tensile strength of the DN elastomer reaches about 6.7 MPa and the strain is as high as about 950%. The DN elastomer can be easy to repair by touching the damaged surface together at ambient conditions when broken or cut. The recovered tensile strength can reach over 4.5 MPa, which is better than the most pristine strength of existing spontaneous self-healing elastomers.

Facile stabilization of a cyclodextrin metal-organic framework under humid environment via hydrogen sulfide treatment.

Increasing resistance to humid environments is a major challenge for the application of γ-CD-K-MOF (a green MOF) in real-world utilisation. γ-CD-K-MOF-H2S with enhanced moisture tolerance was obtained by simply treating MOF with H2S gas. XPS, Raman and TGA characterizations indicated that the H2S molecules coordinated with the metal centers in the framework. H2S acting as a newly available water adsorption potential well near the potassium centers protects the metal-ligand coordination bond from attack by water molecules and thus improves the moisture stability of MOF. After 7 days exposure in 60% relative humidity, γ-CD-K-MOF-H2S retained its crystal structure and morphology, while γ-CD-K-MOF had nearly collapsed. In addition, the formaldehyde uptake tests indicated that γ-CD-K-MOF retain their permanent porosity after interaction with H2S. This simple and facile one-step strategy would open a new avenue for preparation of moisture stable MOFs for practical applications.

Leeches-Inspired Hydrogel-Elastomer Integration Materials.

Inspired by the functions of leeches, for the first time homogeneous materials integrating hydrogels and elastomers were achieved by free radical polymerization. 2-Methoxyethyl acrylate (MEA) was used as elastomer monomer and Pluronics functionalized with vinyl groups acted as cross-linkers to impart the hydrogel property to the materials. The resulting Pluronic/PMEA gels possess a swelling ratio of about 210% and good water-retaining ability. Compression tests of Pluronic/PMEA gels at swelling equilibrium state show a stress up to 1.6 MPa under 85% strain. The gels act as elastomer after dehydration. Uniaxial tensile fracture stress and the elongation reached 1200 kPa and 500%, respectively, and compression stress was above 22 MPa. Furthermore, the Pluronic/PMEA gels also show self-healing properties. Owing to the excellent mechanical performance in both wet and dry conditions, this hydrogel-elastomer integrated material may have potential applications in tissue engineering, soft robotics, and biomedical devices.