Soft Actuators
Snapping is a widely adopted strategy used by natural species to speed up their movements. In article number 2108919, Arri Priimagi, Hao Zeng, and Hongshuang Guo report a fully ...optically controlled snapping‐like actuator based on light‐responsive elastomer muscles and a heat‐sensitive latch. The method enables light‐driven jumping and catapult motions, as well as the integration of multiple launchers to act in parallel or in series.
Harnessing snapping, an instability phenomenon observed in nature (e.g., Venus flytraps), for autonomy has attracted growing interest in autonomous soft robots. However, achieving self‐sustained ...snapping and snapping‐driven autonomous motions in soft robots remains largely unexplored. Here, harnessing bistable, ribbon ring‐like structures for realizing self‐sustained snapping in a library of soft liquid‐crystal elastomer wavy rings under constant thermal and photothermal actuation are reported. The self‐sustained snapping induces continuous ring flipping that drives autonomous dancing or crawling motions on the ground and underwater. The 3D, free‐standing wavy rings employ either a highly symmetric or symmetry‐broken twisted shape with tunable geometric asymmetries. It is found that the former favors periodic self‐dancing motion in place due to isotropic friction, while the latter shows a directional crawling motion along the predefined axis of symmetry during fabrication due to asymmetric friction. It shows that the crawling speed can be tuned by the geometric asymmetries with a peak speed achieved at the highest geometric asymmetry. Lastly, it is shown that the autonomous crawling ring can also adapt its body shape to pass through a confined space that is over 30% narrower than its body size.
Achieving self‐sustained snapping and motion remains challenging in autonomous systems. Harnessing wavy ribbon rings for realizing self‐sustained snapping in liquid‐crystal elastomer rings under constant temperature and light are reported. It drives continuous flipping for either periodic self‐dancing in place in symmetric rings or self‐crawling in asymmetric rings on the ground and underwater, as well as self‐escaping from confined spaces.
A liquid‐crystal elastomer (LCE) iris inspired by the human eye is demonstrated. With integrated polyimide‐based platinum heaters, the LCE material is thermally actuated. The radial contraction ...direction, similar to a mammalian iris, is imprinted to the LCE by a custom‐designed magnetic field. Actuation of the device is reproducible over multiple cycles and controllable at intermediate contraction states.
Self‐Dancing Rings
By exploring self‐sustained snapping instabilities, in article number 2207372, Yao Zhao, Jie Yin, and co‐workers report a self‐dancing and crawling wavy ring under heat and light. ...This ring crawls like a front‐wheel or rear‐wheel car and can even self‐navigate through a narrow space that is smaller than its ring size via snapping and adaptive interactions.
Three-dimensional structures capable of reversible changes in shape, i.e., four-dimensional-printed structures, may enable new generations of soft robotics, implantable medical devices, and consumer ...products. Here, thermally responsive liquid crystal elastomers (LCEs) are direct-write printed into 3D structures with a controlled molecular order. Molecular order is locally programmed by controlling the print path used to build the 3D object, and this order controls the stimulus response. Each aligned LCE filament undergoes 40% reversible contraction along the print direction on heating. By printing objects with controlled geometry and stimulus response, magnified shape transformations, for example, volumetric contractions or rapid, repetitive snap-through transitions, are realized.
Inspired by heliotropism in nature, artificial heliotropic devices that can follow the sun for increased light interception are realized. The mechanism of the artificial heliotropism is realized via ...direct actuation by the sunlight, eliminating the need for additional mechatronic components and resultant energy consumption. For this purpose, a novel reversible photo‐thermomechanical liquid crystalline elastomer (LCE) nanocomposite is developed that can be directly driven by natural sunlight and possesses strong actuation capability. Using the LCE nanocomposite actuators, the artificial heliotropic devices show full‐range heliotropism in both laboratory and in‐field tests. As a result, significant increase in the photocurrent output from the solar cells in the artificial heliotropic devices is observed.
A photo‐thermomechanical liquid‐crystalline elastomer (LCE) nanocomposite is developed and utilized to realize artificial heliotropism. The LCE nanocomposite actuators respond to light and, in turn, drive solar cells to track the light source. Full‐range heliotropism is shown in both laboratory and in‐field tests under natural sunlight. A significant increase in the photocurrent output from the solar cells in the artificial heliotropic devices is observed.
Muscle‐driven actuation of biomimetic microfibrillar structures is achieved using integrative soft‐lithography on a backing splayed liquid‐crystal elastomer (LCE). Variation in the backing LCE layer ...thickness yields different modes of thermal deformation from a pure bend to a twist‐bend. Muscular motion and dynamic self‐cleaning of gecko toe pads are mimicked via this mechanism.
Liquid crystal elastomers (LCEs) are smart elastomers capable of a reversible shape change triggered by a stimulus such as heat or light. This change in shape is programmed into the material by ...orienting the mesogen molecules, often through the post-manufacturing alignment of the polymer chains by mechanical stretching. This method limits the possible orientations and necessitates a second curing step to lock the orientation in the structure. Direct ink writing allows for the curing of LCEs directly in an oriented state and for a fine control of the local orientation of the mesogens. The process parameters dictate the actuation and mechanical properties of the 4D-printed object, where time is the fourth dimension. We present in this paper a study of the influence of the printing speed and raster angle on the actuation strain and mechanical properties of the resulting LCE. Complex shape changes can be programmed into specimens that combine variations in printing angles and speeds. We printed two dimensional specimens actuating into helicoids and hinges. Results show the expected influence of printing speed on the level of orientation of the mesogens and on the actuation strain; that is faster printing equals more orientation and actuation strain. The change in raster angle demonstrates the immediate impact of incompatible strains in the structure, as the flat specimen (i.e. a printed rectangle of a couple of layers thick) actuates in a controlled 3D configuration. Finally, the hinge and helicoid specimens show the versatility of 4D-printed LCE systems, where the folding angle of the hinge or the pitch of the helicoid can be programmed through basic geometry, angle, and speed control at the printing stage. This transformative step in the preparation of LCE actuators opens the door to precise customization exploiting the mobility of mesogens on the polymer chains during printing and the flexibility of the direct ink writing process.
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•Direct ink writing simultaneously orients and cures the liquid crystal elastomer.•We study the influence of printing speed and raster angle on actuation and mechanical properties.•Faster printing improves orientation and actuation strain.•Incompatible strains lead flat specimens to actuate in a controlled 3D configuration.•Hinge folding angle or helicoid pitch can be programmed through geometry, angle, and speed.
Liquid crystal elastomers (LCEs) have long held significant promise as materials for artificial muscles and smart actuators. Recent advancements in this field have introduced innovative LCE ...structures at various scales, resulting in novel properties and functionalities that further accentuate their actuation advantages, bolstering their potential as future soft actuation systems. The ongoing pursuit of enhanced performance and functionality in LCE actuators, essential for advancing them towards superior material-based machines and devices, is intricately linked to the understanding of the fundamental structure-property-function relationships. This review provides a perspective on these relationships across multiple structural levels, encompassing chemical structures, mesophase structures, and micro-to-macroscale programmed structures. It delves into the impact of various LCE structures on key actuation-related properties, actuation features, and functionalities. This review aspires to provide valuable insights into the design of high-performance LCE actuators, the development of exceptional actuation modes and behaviors, and the expansion of achievable functionality.
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