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(a) Chemical structure of the liquid crystal polymer before photo-cross-linking. (b and c) Photographs showing 3D shape change starting from flat LCE films: side-exchange actuation (the two sides exchange their respective shape) (b) and reversal actuation (the actuator reversibly switches between two opposite configurations) (c). Scale bars: 3 mm. Adapted with permission from ref 100. Copyright 2017 John Wiley and Sons.
Source publication
This Review presents and discusses the current state of the art in "exchangeable liquid crystalline elastomers", that is, LCE materials utilizing dynamically cross-linked networks capable of reprocessing, reprogramming, and recycling. The focus here is on the chemistry and the specific reaction mechanisms that enable the dynamic bond exchange, of w...
Citations
... As a result, the LCE can be programmed into a monodomain state. The LCEs with DCBs exhibit reprocessing, reshaping, and self-healing abilities and can be assembled with modulable building blocks [49,50]. At present, numerous DCBs have been incorporated into LCEs, such as ester groups [51][52][53][54][55][56], disulfide bonds [57][58][59], boronic esters [60,61], Diels-Alder reactions [62], diselenide bonds [63], and so on. ...
Liquid crystal elastomer (LCE) is one kind of soft actuating material capable of producing large and reversible actuation strain, versatile and programmable actuation modes, and high work density, which can be widely exploited for nextgeneration soft robots. However, the slow response speed and low power density in LCE-based actuators remain a challenge, limiting their practical applications. Researchers have been considering how to improve these performances. In this review, we discuss the fundamentals of the LCEs and emphasize the fast actuation strategies developed in recent years. Firstly, we introduce conventional preparation strategies. Then, we describe typical actuation mechanisms of LCEs, discussing their features and limitations. Subsequently, we summarize several possible approaches as case studies to enhance the actuation performance of LCEs, including reducing physical sizes, introducing active heating-cooling mechanisms, utilizing mechanical instability, and developing dielectric LCEs. Finally, we discuss the future research opportunities and challenges for rapid actuation of LCEs.
... And this is desirable for complex displays and is needed for obtaining complex shape variations in LC-actuators. Thus, crosslinking processes, which can be reverted or rearranged, find a lot of interest in LC-elastomers today [41,[56][57][58]. The gelled LC-materials described above fall naturally into this category. ...
... Such reworkable LC-elastomers, in which the network structure can be rearranged, can be made by different concepts. Recently, mostly the use of reversible chemical bond formation has been under investigation [41,[56][57][58] in so-called-"exchangeable liquid crystalline elastomers". These LC-elastomer materials utilize dynamically cross-linked networks capable of reprocessing, reprogramming, and recycling. ...
The topic of this review is the physical gelling of liquid crystalline (LC) phases. It allows the combination of order and mobility of the LC-phase with macroscopic stability, which makes it a soft material. Thus, the gelled LCs acquire properties of LC-elastomers without the need for complicated chemistry to allow polymerization and crosslinking. But, instead, an LC-material (either a pure compound or a mixture) can be mixed with a few percent of a gel-forming agent, which self-assembles into long fibers that span the volume of the gel and make it a soft-solid. The use of azo-containing gel-forming agents thereby allows us to make gelation not only thermo-responsive, but also photo-responsive (trans-cis isomerization). This review discusses the micro-morphology of the gelled LCs and their influence on the mechanical properties and the switching in external electric fields. In addition, the potential of reversibility is discussed, which is not only interesting for recycling purposes, but also offers a route to inscribe a complex director pattern into the gelled liquid crystal.
... Despite variations in laser power and scan speed, equivalent D values yielded consistent transmittance outcomes dynamic covalent bonds has emerged as a compelling strategy to introduce reversibility into the programming of LCEs. By leveraging these dynamic bonds, chain exchange reactions involving reversible breakage and reformation can be achieved in response to external stimuli 23 . Various chain exchange mechanisms such as reversible additionfragmentation chain transfer (RAFT) 24 , transesterification 25,26 , siloxane exchange 27 , thiourea bond exchange 28 and disulfide exchange 29 have been investigated and embraced to reconfigure the liquid crystalline structure of programmed LCEs. ...
Liquid crystal elastomers hold promise in various fields due to their reversible transition of mechanical and optical properties across distinct phases. However, the lack of local phase patterning techniques and irreversible phase programming has hindered their broad implementation. Here we introduce laser-induced dynamic crosslinking, which leverages the precision and control offered by laser technology to achieve high-resolution multilevel patterning and transmittance modulation. Incorporation of allyl sulfide groups enables adaptive liquid crystal elastomers that can be reconfigured into desired phases or complex patterns. Laser-induced dynamic crosslinking is compatible with existing processing methods and allows the generation of thermo- and strain-responsive patterns that include isotropic, polydomain and monodomain phases within a single liquid crystal elastomer film. We show temporary information encryption at body temperature, expanding the functionality of liquid crystal elastomer devices in wearable applications.
... LCEs are moderately crosslinked polymer networks consisting of liquid crystalline (LC) mesogens, flexible spacers, and a small amount of crosslinker [51][52][53][54] . As a result, LCEs have soft elasticity (modulus: kPa −MPa) and their glass transition temperatures T g are usually below room temperature. ...
Self-sustainable autonomous locomotion is a non-equilibrium phenomenon and an advanced intelligence of soft-bodied organisms that exhibit the abilities of perception, feedback, decision-making, and self-sustainment. However, artificial self-sustaining architectures are often derived from algorithms and onboard modules of soft robots, resulting in complex fabrication, limited mobility, and low sensitivity. Self-sustainable autonomous soft actuators have emerged as naturally evolving systems that do not require human intervention. With shape-morphing materials integrating in their structural design, soft actuators can direct autonomous responses to complex environmental changes and achieve robust self-sustaining motions under sustained stimulation. This perspective article discusses the recent advances in self-sustainable autonomous soft actuators. Specifically, shape-morphing materials, motion characteristics, built-in negative feedback loops, and constant stimulus response patterns used in autonomous systems are summarized. Artificial self-sustaining autonomous concepts, modes, and deformation-induced functional applications of soft actuators are described. The current challenges and future opportunities for self-sustainable actuation systems are also discussed.
... A new mesogen configuration is mechanically instilled after the bonds are dissociated and subsequently imprinted into the material upon their recombination once the bond-exchanging stimulus is removed. For further insight, Saed et al. provide a comprehensive review on this topic 41 . Another way to achieve reprogrammability that was more recently applied to LCEs but is commonly used in shape-memory polymers 42 , takes advantage of the persistent glassy state at room temperature in the LCE network. ...
Liquid crystal elastomers (LCEs) are shape-morphing materials that demonstrate reversible actuation when exposed to external stimuli, such as light or heat. The actuation’s complexity depends heavily on the instilled liquid crystal alignment, programmed into the material using various shape-programming processes. As an unavoidable part of LCE synthesis, these also introduce geometrical and output restrictions that dictate the final applicability. Considering LCE’s future implementation in real-life applications, it is reasonable to explore these limiting factors. This review offers a brief overview of current shape-programming methods in relation to the challenges of employing LCEs as soft, shape-memory components in future devices.
... In the last decade, covalent adaptable network LCE thermosets with dynamic covalent crosslinks have been developed, leading to recyclable and reprogrammable actuators. When the dynamic bonds are activated upon exposure to light, heat, or other external stimuli, covalent bonds can undergo exchange reactions, leading to thermoplastic behaviour [9,10]. In the ideal case, bond dynamics and film actuation are activated by orthogonal stimuli. ...
... Methyl methacrylate polymers outstand others with comparatively low glass transition temperature (T g ), viscosity, and good mechanical, thermal stabilities, high silicon-oxygen bond angles mobility [3]. These properties rendered them useful in liquid crystal display (LCD), smart soft materials [4][5][6], information storage, non-linear optics and other applications as liquid crystal polymers with low phase transition temperature. ...
Three liquid crystalline monomers were systematically designed, synthesised and characterised. Methyl methacrylic unit was linked to the mesogenic compound, 4′-Undecycloxybiphenyl-4-yl 4-octyloxy-2-(pent-4-en-1-yloxy) benzoate, ( Me ), using disiloxane, ester and siloxane units respectively. In the first compound, 4′-(undecyloxy)-[1,1′-biphenyl]-4-yl 2-((5-(3-(4-(methacryloyloxy)butyl)-1,1,3,3-tetramethyldisiloxanyl)pentyl)oxy)-4-(octyloxy)benzoate, ( M1 ), the methyl methacrylic unit was linked to the mesogen by disiloxane group, (Si–O–Si), while ester group, (COOC), was used in the second compound, 4′-(undecyloxy)-[1,1′-biphenyl]-4-yl 2-((5-(methacryloyloxy)pentyl)oxy)-4-(octyloxy)benzoate, ( M2 ), and siloxane unit, (Si–O), in the third compound, 4′-(undecyloxy)-[1,1′-biphenyl]-4-yl 2-((5-((4-(methacryloyloxy)butyl)dimethylsilyl)pentyl)oxy)-4-(octyloxy)benzoate, ( M3 ). The introduction of the methyl methacrylic unit caused the melting point of the compounds to reduce with Me melting at 53.3 °C, M1 at −8.1 °C and M2 at −12.5 °C. The three compounds showed characteristic nematic textures when observed under POM and when compared to the mesogenic compound ( Me , 71.7 °C), there were remarkable reduction in the clearing points of the compounds, with M1 clearing at 18.6 °C, M2 at 68.8 °C and M3 at 10.3 °C. Finally, there was appreciable increment in the nematic phase range for the compounds when compared to the mesogen, Me , 18.4 °C. The range was 26.7 °C for M1 , 81.3 °C for M2 and since there was no observable crystallization point for M3, the range was not determined.
... 17,22 Among these anisotropic materials, liquid crystalline elastomers (LCEs, Figure 1a) are of particular interest, as their deformation performance resembles muscle fibers in actuation strain, frequency, and modulus, 23 and can be tuned to display good biocompatibility 24 and degradability. 25 LCEs 11−14 are crosslinked elastomeric networks that incorporate liquid crystal mesogen units either as part of, or appended to, the polymeric backbone ( Figure 1a) and can undergo programmable directional order-to-disorder transitions. For more details please refer to further reviews on LCE chemistry, synthesis, and applications. ...
Conspectus
Synthetic structures that undergo controlled movement are crucial building blocks for developing new technologies applicable to robotics, healthcare, and sustainable self-regulated materials. Yet, programming motion is nontrivial, and particularly at the microscale it remains a fundamental challenge. At the macroscale, movement can be controlled by conventional electric, pneumatic, or combustion-based machinery. At the nanoscale, chemistry has taken strides in enabling molecularly fueled movement. Yet in between, at the microscale, top-down fabrication becomes cumbersome and expensive, while bottom-up chemical self-assembly and amplified molecular motion does not reach the necessary sophistication. Hence, new approaches that converge top-down and bottom-up methods and enable motional complexity at the microscale are urgently needed.
Synthetic anisotropic materials (e.g., liquid crystalline elastomers, LCEs) with encoded molecular anisotropy that are shaped into arbitrary geometries by top-down fabrication promise new opportunities to implement controlled actuation at the microscale. In such materials, motional complexity is directly linked to the built-in molecular anisotropy that can be “activated” by external stimuli. So far, encoding the desired patterns of molecular directionality has relied mostly on either mechanical or surface alignment techniques, which do not allow the decoupling of molecular and geometric features, severely restricting achievable material shapes and thus limiting attainable actuation patterns, unless complex multimaterial constructs are fabricated. Electromagnetic fields have recently emerged as possible alternatives to provide 3D control over local anisotropy, independent of the geometry of a given 3D object.
The combination of magnetic alignment and soft lithography, in particular, provides a powerful platform for the rapid, practical, and facile production of microscale soft actuators with field-defined local anisotropy. Recent work has established the feasibility of this approach with low magnetic field strengths (in the lower mT range) and comparably simple setups used for the fabrication of the microactuators, in which magnetic fields can be engineered through arrangement of permanent magnets. This workflow gives access to microstructures with unusual spatial patterning of molecular alignment and has enabled a multitude of nontrivial deformation types that would not be possible to program by any other means at the micron scale. A range of “activating” stimuli can be used to put these structures in motion, and the type of the trigger plays a key role too: directional and dynamic stimuli (such as light) make it possible to activate the patterned anisotropic material locally and transiently, which enables one to achieve and further program motional complexity and communication in microactuators.
In this Account, we will discuss recent advances in magnetic alignment of molecular anisotropy and its use in soft lithography and related fabrication approaches to create LCE microactuators. We will examine how design choices—from the molecular to the fabrication and the operational levels—control and define the achievable LCE deformations. We then address the role of stimuli in realizing the motional complexity and how one can engineer feedback within and communication between microactuator arrays fabricated by soft lithography. Overall, we outline emerging strategies that make possible a completely new approach to designing for desired sets of motions of active, microscale objects.
... In terms of obtaining programmable shape changes, liquid crystal elastomers (LCEs) are particularly promising due to facile control over the alignment of the LC mesogens during polymerization and large, anisotropic, and reversible stimuli-induced strains within the loosely crosslinked network. [13][14][15][16] As a result, advanced functionalities such as biomimetic shape-morphing, cilia-like fluidic pumping, and butterfly wing-like oscillation, have been recently demonstrated. [17][18][19] Self-healing LCE formulations have also been reported. ...
... [17][18][19] Self-healing LCE formulations have also been reported. [16,[20][21][22][23][24] This characteristic requires the incorporation of dynamic bonds into LCE design, which can be obtained using, e.g., hydrogen bonds, disulfide bonds, ester bonds, and others. [20,21,23,[25][26][27][28][29] However, a common problem with the examples demonstrated is the relatively low exchange rate of the dynamic bonds used, which leads to high stability at ambient conditions and compromises the dynamic behavior. ...
Shape‐changing polymeric materials have gained significant attention in the field of bioinspired soft robotics. However, challenges remain in versatilizing the shape‐morphing process to suit different tasks and environments, and in designing systems that combine reversible actuation and self‐healing ability. Here, we report halogen‐bonded liquid crystal elastomers (LCEs) that can be arbitrarily shape‐programmed and that self‐heal under mild thermal or photothermal stimulation. We incorporate halogen‐bond‐donating diiodotetrafluorobenzene molecules as dynamic supramolecular crosslinks into the LCEs and show that these relatively weak crosslinks are pertinent for their mechanical programming and self‐healing. Utilizing the halogen‐bonded LCEs, we demonstrate proof‐of‐concept soft robotic motions such as crawling and rolling with programmed velocities. Our results showcase halogen bonding as a promising, yet unexplored tool for the preparation of smart supramolecular constructs for the development of advanced soft actuators.
... Then, the alignment of the mesogens is fixed by the formation or rearrangement of the polymer network. The preparation of the LCEs has been extensively documented in the previous reviews [24][25][26][27][28][29][30][31][32]. It involves two steps: the alignment of the mesogens and the formation of the polymer networks. ...
Liquid crystal elastomers are active materials that combine the anisotropic properties of liquid crystals with the elasticity of polymer networks. The LCEs exhibit remarkable reversible contraction and elongation capabilities in response to external stimuli, rendering them highly promising for diverse applications, such as soft robotics, haptic devices, shape morphing structures, etc. However, the predominant reliance on heating as the driving stimulus for LCEs has limited their practical applications. This drawback can be effectively addressed by incorporating fillers, which can generate heat under various stimuli. The recent progress in LCE composites has significantly expanded the application potential of LCEs. In this minireview, we present the design strategies for soft actuators with LCE composites, followed by a detailed exploration of photothermal and electrothermal LCE composites as prominent examples. Furthermore, we provide an outlook on the challenges and opportunities in the field of LCE composites.
