Humidity-Sensing and Moisture-Steering Liquid Crystal Elastomer Actuator

A liquid crystal elastomer (LCE) actuator capable of colorimetric humidity sensing is realized. The designed LCE features acid protonated amino azobenzene side groups in its structure, which endow the actuator with the hygroscopicity and act as the humidity reporter via color changes. Given that the protonated and deprotonated chromophore absorb visible light at different wavelengths, when the protonated LCE is under higher humidity, it absorbs more water that deprotonates azobenzene and leads to a change in color. This humidity-dependent color change is fast, because surface protonation of the actuator is enough. The initial color and the sensitivity to humidity variation are determined by the extent of acid protonation, and the reversible color changes are distinguishable by the naked eye over a wide humidity range. The humidity sensing of LCE actuator in motion is demonstrated using thermally driven rolling rod actuators. Moreover, through spatial-selective exposure of the rolling rod actuator to water mist, the moisture can act as a stimulus to change or reverse the rolling direction and reduce the rolling speed. The achieved nature-inspired colorimetric humidity sensing capability represents an intelligent function for LCE actuators and may widen their application scope.

Reprogramming Photoresponsive Liquid Crystalline Elastomer via Force-Directed Evaporation

Incorporating photothermal agents into thermoresponsive liquid crystalline elastomers (LCEs) offers remote and spatio-temporal control in actuation. Typically, both the light responsiveness and actuation behaviors are fixed since the agent doping and mesogen alignment are conducted before network formation. Here, we report an approach that enables programming photoresponsive LCEs after synthesis via force-directed evaporation. Different photothermal agents can be doped or removed by swelling the fully cross-linked LCEs in a specific solution, achieving the introduction and erasing of the photoresponsiveness. Moreover, the network swelling deletes the registered alignment, which allows for redefining the molecular order via re-evaporating the solvent with force imposed. This "one stone, two birds" strategy paves the way to simultaneously program/reprogram the actuation mode and responsiveness of LCEs, even in a spatio-selective manner to achieve complex actuations. Our approach is expandable to three-dimensional (3D) printed LCEs to access geometrically sophisticated shape-changing.

Photochemically and Photothermally Controllable Liquid Crystalline Network and Soft Walkers

Developing intelligent soft robots capable to perform various responses to different stimulations has been a hot topic in recent years. Liquid crystalline networks (LCNs) have been considered as one of the most promising candidates in the fabrication of soft actuators because of the combination of elasticity of the polymer and anisotropy of the liquid crystals. However, the design and fabrication of advanced LCN materials with outstanding performances and multiple responsivities is highly demanded but still a challenge. In this work, a NIR-UV dual light-responsive LCN actuator was prepared by selectively coating a polydopamine (PDA) layer on an azobenzene-doped LCN film. This actuator presents UV responsivity in the uncoated region because of the photochemical isomerization of azobenzene and NIR sensitivity in the PDA-coated region originated from the striking photothermal effect. Thanks to the reprogrammable PDA coating, this dual-responsive LCN actuator was totally reprogrammable by coating and washing the PDA layer repeatedly. Based on the novel soft actuator, an artificial car that can imitate the switch of the "forward gear" to "neutral gear" of a real car was prepared. In normal mode, the actuator can move forward under NIR irradiation. After UV light excitation, the actuator cannot move under the same NIR irradiation, just like the car with the level in neutral. This novel actuator may provide inspirations for the fabrication of light-driven functional devices and soft actuators.

"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.

Liquid-Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new-generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high-performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid-crystalline elastomer (LCE) actuators to resolve this problem. By post-synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on-demand 3D-shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black-ink-patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.

Bioinspired Actuators Based on Stimuli-Responsive Polymers

Organisms exhibit strong environmental adaptability by controllably adjusting their morphologies or fast locomotion; thus providing constant inspiration for scientists to develop artificial actuators that not only have diverse and sophisticated shape-morphing capabilities, but can also further transfer dynamic and reversible shape deformations into macroscopic motion under the following principles: asymmetric friction, the Marangoni effect, and counteracting forces of the surrounding conditions. Among numerous available materials for fabricating bioinspired artificial actuators, stimuli-responsive polymers are superior in their flexible features and the ability to change their physicochemical properties dynamically under external stimuli, such as temperature, pH, light, and ionic strength. Herein, different mechanisms, working principles, and applications of stimuli-responsive polymeric actuators are comprehensively introduced. Furthermore, perspectives on existing challenges and future directions of this field are provided.

All-Optical Control of Shape

Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large-scale, macroscopic deformations when exposed to light. Here, surface-aligned, azobenzene-functionalized LCEs are prepared via a radical-mediated, thiol-acrylate chain transfer reaction. A long-lived, macroscopic shape deformation is realized in an LCE composed with an o-fluorinated azobenzene (oF-azo) monomer. Under UV irradiation, the oF-azo LCE exhibits a persistent shape deformation for >72 h. By contrasting the photomechanical response of the oF-azo LCE to analogs prepared from classical and m-fluorinated azobenzene derivatives, the origin of the persistent deformation is clearly attributed to the underlying influence of positional functionalization on the kinetics of cis→trans isomerization. Informed by these studies and enabled by the salient features of light-induced deformations, oF-azo LCEs are demonstrated to undergo all-optical control of shape deformation and shape restoration.