Understanding the effect of liquid crystal content on the phase behavior and mechanical properties of liquid crystal elastomers

Dynamic Liquid Crystal Elastomers for Body Heat- and Sunlight- Driven Self-Sustaining Motion via Material-Structure Synergy

Self-sustained actuators powered by natural, low-energy sources based on liquid crystal elastomers (LCEs) are attractive as they offer high safety, abundant energy availability, and practicality in applications. However, achieving stable self-sustaining motion with low-energy sources requires high actuation strain rates within a narrow temperature range near ambient conditions - a great challenge as LCEs with low nematic-to-isotropic transition temperatures (Tni) generally exhibit reduced actuation strain and strain rates. To address this, we synthesized a carbon nanotube-doped LCE with a low Tni and reversible Diels-Alder crosslinks, termed DALCE, and readily (re)fabricated it into specific structures (e.g., twisted-and-coiled or bimorph shapes). By leveraging material-structure synergy, we achieved both low Tni and high actuation strain rates, enabling self-rolling, self-breathing and autonomous twisting-untwisting movements powered by ambient/body temperature or natural sunlight. This low-energy, self-sustained actuator design opens new possibilities for LCE-based biomedical applications and naturally powered automatic devices.

Tailoring Diels-Alder Cross-Linked Liquid Crystal Elastomers for Spatially Programmable Monolithic Actuators

Liquid crystal elastomers with thermo-reversible Diels-Alder cross-links (DALCEs) offer exceptional reprocessability and mild-temperature reprogrammability, enabling repeated fabrication of diverse actuators. However, optimizing their molecular design and refabrication protocols remains crucial to further unlocking their potential. This work systematically investigates DALCEs synthesized via aza-Michael addition reactions between RM82, furfurylamine, and various chain extenders (phenylethylamine, ethylamine, butylamine, hexylamine, octylamine, and 6-amino-1-hexanol). The effects of cross-linking density and chain extender selection on phase behavior, thermomechanical properties, and actuation performance have been thoroughly examined. The results show that a PEA-based formulation with moderate cross-linking density achieves the most balanced performance. Based on this optimized formulation, a novel (re)fabrication strategy is introduced by harnessing DALCEs' intrinsic reprocessability, reprogrammability, and self-healing properties. This strategy employs multilevel fiber programming before monolithic actuator formation, enabling spatially controlled liquid crystal alignment and facilitating iterative actuator refinement through reconstruction. Consequently, complex morphing behaviors in disk films and stress-modulating functions in tubular actuators were demonstrated. This work establishes a versatile, easily synthesized material platform for spatially programmable, dynamic monolithic actuators, paving the way for advanced applications in soft robotics and adaptive devices.

Liquid Crystal Elastomers: 30 Years After

This is a Review that attempts to cast a look at the whole history of liquid crystal elastomers and the evolution of this field from its inception to the current state of the art. The exposition is limited by deliberately omitting several important elements of this field, such as densely cross-linked networks or smectic elastomers, focusing solely on the nematic phase of these elastomers. In this more narrow topic, we first discuss the current developments and perspectives in the materials chemistry. This is followed by three sections, each dedicated to one of the three main points of interest in the nematic liquid crystal elastomers: the reversible actuation, the soft elasticity, and the viscoelastic dynamics of nematic elastomers. In each of these directions, there have been significant developments over recent years but equally significant new avenues emerging for the research to follow.

Exploiting photopolymerization to modulate liquid crystalline network actuation

Liquid Crystalline Networks (LCNs) are widely investigated to develop actuators, from soft robots to artificial muscles. Indeed, they can produce forces and movements in response to a plethora of external stimuli, showing kinetics up to the millisecond time-scale. One of the most explored preparation technique involves the photopolymerization of an aligned layer of reactive mesogens. Following this approach, side-chain polymers are widely described, while a detailed comparison of light-responsive LCNs with different architectures is not properly addressed. In this paper, two synthetic approaches are exploited leading to photoresponsive LCNs with different architectures. Mixed main-chain/side-chain LCNs are obtained in one-pot through a thiol-acrylate chain-transfer reaction, while main-chain LCNs are achieved by a two-step approach involving an aza-Michael addition followed by acrylate crosslinking. Comparison among the two materials highlighted the superior performances in terms of tension developed upon light-activation of the former one, showing muscle-like force production comparable to standard side-chain LCNs combined with the greater ability to contract from common main-chain LCNs.

Cuboidal Deformation of Multimaterial Composites Prepared by 3-D Printing of Liquid Crystalline Elastomers

Multimaterial 3-D printing (3DP) of isotropic (IsoE) and liquid crystalline elastomers (LCE) yields spatially programmed elements that undergo a cuboidal shape transformation upon heating. The thermomechanical deformation of 3DP elements is determined by the geometry and extent of the isotropic and anisotropic regions. The synthesis and experimental characterization of the 3DP elements are complemented by finite element analysis (FEA). Calculations emphasize that the cuboidal deformation of the myriad 3DP elements is a manifestation of local stress gradients imparted by local control of the material composition and anisotropy. Varying the rectilinear spatial distribution of the multimaterial elastomer composites produces complex, multistable states that provide insights into how stress gradients drive multimaterial elastomer actuation. The thermomechanical stimuli response of the multimaterial elements is explored as a tactile element.

Synthesis of body temperature-triggerable dynamic liquid crystal elastomers using Diels-Alder crosslinkers

Novel liquid crystal elastomers (LCEs) with solely Diels-Alder dynamic covalent bonds (DADCBs) as crosslinks and body temperature sensitivity have been developed. The appealing attributes of the material, such as recyclability, reprogrammability and reconfigurability, have led to soft actuators capable of reversible deformation stimulated by shifting between ambient and body temperature, highlighting the potential for innovative applications in the biomedical field.

Phase patterning of liquid crystal elastomers by laser-induced dynamic crosslinking

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.

Light-driven peristaltic pumping by an actuating splay-bend strip

Despite spectacular progress in microfluidics, small-scale liquid manipulation, with few exceptions, is still driven by external pumps and controlled by large-scale valves, increasing cost and size and limiting complexity. By contrast, optofluidics uses light to power, control and monitor liquid manipulation, potentially allowing for small, self-contained microfluidic devices. Here we demonstrate a soft light-propelled actuator made of liquid crystal gel that pumps microlitre volumes of water. The strip of actuating material serves as both a pump and a channel leading to an extremely simple microfluidic architecture that is both powered and controlled by light. The performance of the pump is well explained by a simple theoretical model in which the light-induced bending of the actuator competes with the liquid's surface tension. The theory highlights that effective pumping requires a threshold light intensity and strip width. The proposed system explores the benefits of shifting the complexity of microfluidic systems from the fabricated device to spatio-temporal control over stimulating light patterns.

Degree of Orientation in Liquid Crystalline Elastomers Defines the Magnitude and Rate of Actuation

The anisotropy of liquid crystalline elastomers (LCEs) is derived from the interaction-facilitated orientation of the molecular constituents. Here, we correlate the thermomechanical response of a series of LCEs subjected to mechanical alignment to measurements of the Hermans orientation parameter. The LCEs were systematically prepared with varying concentrations of liquid crystalline mesogens, which affects the relative degree of achievable order. These compositions were subject to varying degrees of mechanical alignment to prepare LCEs with orientations that span a wide range of orientation parameters. The stimuli-response of the LCEs indicates that the liquid crystalline content defines the temperature of actuation, whereas the orientation parameter of the LCE is intricately correlated to both the total actuation strain of the LCE as well as the rate of thermomechanical response.

From Light-Powered Motors, to Micro-Grippers, to Crawling Caterpillars, Snails and Beyond-Light-Responsive Oriented Polymers in Action

"How would you build a robot, the size of a bacteria, powered by light, that would swim towards the light source, escape from it, or could be controlled by means of different light colors, intensities or polarizations?" This was the question that Professor Diederik Wiersma asked PW on a sunny spring day in 2012, when they first met at LENS-the European Laboratory of Nonlinear Spectroscopy-in Sesto Fiorentino, just outside Florence in northern Italy. It was not just a vague question, as Prof. Wiersma, then the LENS director and leader of one of its research groups, already had an idea (and an ERC grant) about how to actually make such micro-robots, using a class of light-responsive oriented polymers, liquid crystal elastomers (LCEs), combined with the most advanced fabrication technique-two-photon 3D laser photolithography. Indeed, over the next few years, the LCE technology, successfully married with the so-called direct laser writing at LENS, resulted in a 60 micrometer long walker developed in Prof. Wiersma's group (as, surprisingly, walking at that stage proved to be easier than swimming). After completing his post-doc at LENS, PW returned to his home Faculty of Physics at the University of Warsaw, and started experimenting with LCE, both in micrometer and millimeter scales, in his newly established Photonic Nanostructure Facility. This paper is a review of how the ideas of using light-powered soft actuators in micromechanics and micro-robotics have been evolving in Warsaw over the last decade and what the outcomes have been so far.