Nanoparticle-Liquid Crystalline Elastomer Composites

Photothermal Thin Films with Highly Efficient NIR Conversion for Miniaturized Liquid-Crystal Elastomer Actuators

This work presents the development of highly efficient photothermal thin films (PTFs) and the demonstration of their application on miniaturized polymer-based soft actuators. The proposed PTF, which comprises acrylic-based black paint and EGaIn liquid metal (LM) microdroplets, serves as an excellent absorber for efficiently converting near-infrared (NIR) irradiation into heat for actuating liquid-crystal elastomer (LCE) actuators. The introduction of LM microdroplets into the PTFs effectively increases the overall thermal efficiency of PTFs. Miniaturized soft crawlers monolithically integrated with the NIR-driven LCE actuators are also implemented for demonstrating the application of the proposed PTF. The crawler’s locomotion, which is inspired by the rectilinear movement of snakes, is generated with the proposed PTF for inducing the LC-to-isotropic phase transition of the LCEs. The experimental results show that introducing LM microdroplets into the PTF can effectively reduce the thermal time constants of LCE actuators by 70%. Under periodic on/off NIR illumination cycles, the locomotion of crawlers with different dimensions is also demonstrated. The measurement results indicate that the proposed PTF is not only essential for enabling photothermal LCE actuation but also quite efficient and durable for repeated operation.

Vat Photopolymerization of Reinforced Styrene-Butadiene Elastomers: A Degradable Scaffold Approach

Vat photopolymerization (VP) is a high-throughput additive manufacturing modality that also offers exceptional feature resolution and surface finish; however, the process is constrained by a limited selection of processable photocurable resins. Low resin viscosity (<10 Pa·s) is one of the most stringent process-induced constraints on resin processability, which in turn limits the mechanical performance of printed resin systems. Recently, the authors created a VP-processable photosensitive latex resin, where compartmentalization of the high molecular weight polymer chains into discrete particles resulted in the decoupling of viscosity from molecular weight. However, the monomers used to form the hydrogel green body resulted in decreased ultimate material properties due to the high cross-link density. Herein, we report a novel scaffold that allows for facile UV-based AM and simultaneously enhances the final part's material properties. This is achieved with a chemically labile acetal-containing cross-linker in conjunction with N-vinylpyrrolidone, which forms a glassy polymer after photocuring. Subsequent reactive extraction cleaves the cross-links and liberates the glassy polymer, which provides mechanical reinforcement of the geometrically complex VP-printed elastomer. With only a 0.1 wt % loading of photoinitiator, G'/G'' crossover times of less than 1 s and green body plateau moduli nearing 10⁵ Pa are obtained. In addition, removal of the hydrophilic and thermally labile scaffold results in decreased water uptake and increased thermal stability of the final printed part. Ultimate strain and stress values of over 650% and 8.5 MPa, respectively, are achieved, setting a new benchmark for styrene-butadiene VP elastomers.

Magnetic Soft Materials and Robots

In conventional classification, soft robots feature mechanical compliance as the main distinguishing factor from traditional robots made of rigid materials. Recent advances in functional soft materials have facilitated the emergence of a new class of soft robots capable of tether-free actuation in response to external stimuli such as heat, light, solvent, or electric or magnetic field. Among the various types of stimuli-responsive materials, magnetic soft materials have shown remarkable progress in their design and fabrication, leading to the development of magnetic soft robots with unique advantages and potential for many important applications. However, the field of magnetic soft robots is still in its infancy and requires further advancements in terms of design principles, fabrication methods, control mechanisms, and sensing modalities. Successful future development of magnetic soft robots would require a comprehensive understanding of the fundamental principle of magnetic actuation, as well as the physical properties and behavior of magnetic soft materials. In this review, we discuss recent progress in the design and fabrication, modeling and simulation, and actuation and control of magnetic soft materials and robots. We then give a set of design guidelines for optimal actuation performance of magnetic soft materials. Lastly, we summarize potential biomedical applications of magnetic soft robots and provide our perspectives on next-generation magnetic soft robots.

Actuation performance of a liquid crystalline elastomer composite reinforced by eiderdown fibers

Liquid crystalline elastomer (LCE) materials have been developed and investigated for several decades. One important obstacle, which impedes the practical industrial application of LCE materials, is their modest robustness as actuator materials. In this work, we developed a LCE composite which was fabricated by incorporating eiderdown fibers into a polysiloxane-based main-chain LCE matrix. The eiderdown fibers were used as the flexible reinforcement phase suitable for the shape-morphing performance of LCE materials upon being stimulated. Due to the long fiber property, specific structure and surface characteristics of the eiderdown fibers, they constructed a reinforcement network in the LCE matrix and formed tight interfacial adhesion with the matrix. The LCE composite demonstrated enhanced actuation mechanical properties and robust actuation performance. Its actuation blocking stress and modulus were increased due to the reinforcement effect of the eiderdown fibers. The tensile strength and the performance of anti-fatigue failure under repeated actuation cycles and high loadings were greatly improved due to the crack-resisting effect and bridging effect of the eiderdown fibers. While other properties, such as the liquid crystalline phase structure, the stimulus deformation ratio, phase transition temperature of the LCE matrix, etc., did not deteriorate or change due to the high flexibility, thermal stability and chemical stability of the eiderdown fibers.

Shape memory materials for electrically-powered soft machines

We review the recent progress of electrically-powered artificial muscles and soft machines using shape memory alloy and liquid crystal elastomer.

Photo actuation of liquid crystalline elastomer nanocomposites incorporated with gold nanoparticles based on surface plasmon resonance

In this work, according to the characteristic of surface plasmon resonance (SPR) of metallic nanoparticles, we investigated the photo actuation performance of a liquid crystalline elastomer (LCE) nanocomposite with incorporated gold nanoparticles (nano-gold/LCE nanocomposite). The nano-gold/LCE nanocomposites were fabricated by incorporating gold nanoparticles into a polysiloxane-based LCE matrix via a novel experimental protocol, and characterized by a well-developed SPR absorption band in the visible spectrum range. The nano-gold/LCE nanocomposites demonstrated strong actuation upon irradiation with a quasi-daylight source; the reason lay in that the SPR response of gold nanoparticles performed efficient energy conversion from light energy to thermal energy, and thus offered an activation pathway for the nematic-isotropic transition of the LCE matrix. The nano-gold/LCE nanocomposites underwent rapid maximum axial contraction up to about one third of the original length under light irradiation, and this photo-stimulated muscle-like actuation was fully reversible via the on-off switching of the light source. The photo actuation properties of nano-gold/LCE nanocomposites with varying irradiation intensities and gold nanoparticle content were also investigated. In addition, the nano-gold/LCE nanocomposites demonstrated superior optical nonlinear properties, and revealed potential for the application area of mode-locking for laser technology.

Nanostructured Composites Based on Liquid-Crystalline Elastomers

Liquid-crystalline elastomers (LCEs) are the object of many research investigations due to their reversible and controllable shape deformations, and their high potential for use in the field of soft robots and artificial muscles. This review focuses on recent studies about polymer composites based on LCEs and nanomaterials having different chemistry and morphology, with the aim of instilling new physical properties into LCEs. The synthesis, physico-chemical characterization, actuation properties, and applications of LCE-based composites reported in the literature are reviewed. Several cases are discussed: (1) the addition of various carbon nanomaterials to LCEs, from carbon black to carbon nanotubes, to the recent attempts to include graphene layers to enhance the thermo-mechanic properties of LCEs; (2) the use of various types of nanoparticles, such as ferroelectric ceramics, gold nanoparticles, conductive molybdenum-oxide nanowires, and magnetic iron-oxide nanoparticles, to induce electro-actuation, magnetic-actuation, or photo-actuation into the LCE-based composites; (3) the deposition on LCE surfaces of thin layers of conductive materials (i.e., conductive polymers and gold nanolayers) to produce bending actuation by applying on/off voltage cycles or surface-wrinkling phenomena in view of tunable optical applications. Some future perspectives of this field of soft materials conclude the review.

Liquid Crystal Elastomers-A Path to Biocompatible and Biodegradable 3D-LCE Scaffolds for Tissue Regeneration

The development of appropriate materials that can make breakthroughs in tissue engineering has long been pursued by the scientific community. Several types of material have been long tested and re-designed for this purpose. At the same time, liquid crystals (LCs) have captivated the scientific community since their discovery in 1888 and soon after were thought to be, in combination with polymers, artificial muscles. Within the past decade liquid crystal elastomers (LCE) have been attracting increasing interest for their use as smart advanced materials for biological applications. Here, we examine how LCEs can potentially be used as dynamic substrates for culturing cells, moving away from the classical two-dimensional cell-culture nature. We also briefly discuss the integration of a few technologies for the preparation of more sophisticated LCE-composite scaffolds for more dynamic biomaterials. The anisotropic properties of LCEs can be used not only to promote cell attachment and the proliferation of cells, but also to promote cell alignment under LCE-stimulated deformation. 3D LCEs are ideal materials for new insights to simulate and study the development of tissues and the complex interplay between cells.

Reducing the actuation threshold by incorporating a nonliquid crystal chain into a liquid crystal elastomer

The incorporation of nonliquid crystal chains made the actuation threshold of LCE being obviously decreased, and the LCE material can be effectively actuated by a lower energy intensity of the applied stimulus.

Smart biomimetic micro/nanostructures based on liquid crystal elastomers and networks

A plethora of living organisms are equipped with smart functionalities that are usually rooted in their surface micro/nanostructures or underlying muscle tissues. Inspired by nature, extensive research efforts have been devoted to the development of novel biomimetic functional micro/nanostructured systems. Despite all the accomplishments, the emulation of biological adaptation and stimuli responsive actuation has been a longstanding challenge. The use of liquid crystal elastomers (LCEs) and networks (LCNs) for the fabrication of smart biomimetic micro/nanostructures has recently drawn extensive scientific attention and has become a growing field of research with promising prospects for emerging technologies. In this study, we review the recent progress in this field and portray the current challenges as well as the outlook of this field of research.

Pixelated Polymers: Directed Self Assembly of Liquid Crystalline Polymer Networks

Polymeric materials are pervasive in modern society, in part attributable to the diverse range of properties that are accessible in these materials. Polymers can be stiff or soft, dissipative or elastic, adhesive or nonstick. Localizing the properties of polymeric materials can be achieved by a number of methods, including self-assembly, lithography, or 3-d printing. Here, we detail recent advances in the preparation of "pixelated" polymers prepared by the directed self-assembly of liquid crystalline monomers to yield cross-linked polymer networks (liquid crystalline polymer networks, LCN, or liquid crystalline elastomers, LCE). Through the local and arbitrary control of the orientation of the liquid crystalline units, monolithic elements can be realized with spatial variation in mechanical, thermal, electrical, optical, or acoustic properties. Stimuli-induced variation of these properties may enable paradigm-shifting end uses in a diverse set of applications.

Influence of a Crosslinker Containing an Azo Group on the Actuation Properties of a Photoactuating LCE System

Photoactuating liquid crystalline elastomers (LCE) are promising candidates for an application as artificial muscles in microdevices. In this work, we demonstrate that by optimizing (1) the illumination conditions and (2) the mixture of azo monomer and azo crosslinker, thick films of an all-azo LCE can be prepared, which show a strong length change without bending during photoactuation. This becomes possible by working with white light (about 440 nm), whose absorption is low, leading to a large penetration depth. By adding an azo crosslinker to a previously prepared system, several improvements of the actuation properties-like a stronger photoactuation at lower operational temperatures-could be achieved. In addition, films of different crosslinker concentrations and thicknesses were produced by photopolymerization at varying temperatures within a magnetic field, and their thermo- and photoresponsive behavior was investigated. An extraordinarily strong maximal thermal actuation of 46% and-by exposure to white light at 70 °C-a photoresponsive change in length of up to 40% in just about 13 s could be obtained. Even densely crosslinked samples were still able to photoactuate remarkably. Isothermal back-deformation could either be achieved by irradiation with red light (7 min) or by keeping the film in the dark (13 min).

Electroactive polymers for sensing

Electromechanical coupling in electroactive polymers (EAPs) has been widely applied for actuation and is also being increasingly investigated for sensing chemical and mechanical stimuli. EAPs are a unique class of materials, with low-moduli high-strain capabilities and the ability to conform to surfaces of different shapes. These features make them attractive for applications such as wearable sensors and interfacing with soft tissues. Here, we review the major types of EAPs and their sensing mechanisms. These are divided into two classes depending on the main type of charge carrier: ionic EAPs (such as conducting polymers and ionic polymer–metal composites) and electronic EAPs (such as dielectric elastomers, liquid-crystal polymers and piezoelectric polymers). This review is intended to serve as an introduction to the mechanisms of these materials and as a first step in material selection for both researchers and designers of flexible/bendable devices, biocompatible sensors or even robotic tactile sensing units.

Energy localization and shape transformations in semiflexible polymer rings

Shape transformations in driven and damped molecular chains are considered. Closed chains of weakly coupled molecular subunits under the action of spatially homogeneous time-periodic external field are studied. The coupling between the internal excitations and the bending degrees of freedom of the chain modifies the local bending rigidity of the chain. In the absence of driving the array takes a circular shape. When the energy pumped into the system exceeds some critical value the chain undergoes a nonequilibrium phase transition: The circular shape of the aggregate becomes unstable and the chain takes the shape of an ellipse or, in general, of a polygon. The excitation energy distribution becomes spatially nonuniform: It localizes in such places where the chain is more flat. The weak interaction of the chain with a flat surface restricts the dynamics to a flat manifold.

Synthesis and characterization of chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens

A novel series of siloxane-based chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens were fabricated through synthesis involving a one-step hydrosilication reaction via a liquid crystalline crosslinking agent containing smectic and nematic phases.

Liquid Crystal Elastomer Microspheres as Three-Dimensional Cell Scaffolds Supporting the Attachment and Proliferation of Myoblasts

We report that liquid crystal elastomers (LCEs), often portrayed as artificial muscles, serve as scaffolds for skeletal muscle cell. A simultaneous microemulsion photopolymerization and cross-linking results in nematic LCE microspheres 10-30 μm in diameter that when conjoined form a LCE construct that serves as the first proof-of-concept for responsive LCE muscle cell scaffolds. Confocal microscopy experiments clearly established that LCEs with a globular, porous morphology permit both attachment and proliferation of C2C12 myoblasts, while the nonporous elastomer morphology, prepared in the absence of a microemulsion, does not. In addition, cytotoxicity and proliferation assays confirm that the liquid crystal elastomer materials are biocompatible promoting cellular proliferation without any inherent cytotoxicity.

Optical mechanotransduction with carbazole-based luminescent liquid single-crystal elastomers

Carbazole-based liquid single-crystal elastomers (LSCEs) are valuable fluorescent flexible materials to perform optical mechanotransduction under ambient conditions. Indeed, the covalent incorporation of carbazole derivatives into nematic LSCEs allows to tune their luminescence on demand under mechanical control in a quick and reversible fashion. Specifically, the fluorescence intensity for these materials can be switched back and forth in less than a second. Moreover, such a process can be performed several times without detecting any sign of fatigue in the system. In addition, these materials show excellent resistance to aging; 2 years after their preparation they exhibit the very same mechanofluorescent behavior as when freshly prepared. In fact, the here reported fluorescent systems are highly sensitive; the application of a force of 70 mN decreases the fluorescence in the elastomeric material by 7%. Thus, mechanical forces are attractive external stimuli to modulate the fluorescence of nematic elastomers rapidly and reversibly enabling thereby mechanotransduction.

Chiral surfactants for dispersing carbon nanotubes

Chiral surfactants induce excellent dispersion of CNTs. The dispersed CNTs improve the thermal properties and chiral stability of the dispersion medium.

Photomobile polymer materials with crosslinked liquid-crystalline structures: molecular design, fabrication, and functions

Crosslinked liquid-crystalline polymer materials that macroscopically deform when irradiated with light have been extensively studied in the past decade because of their potential in various applications, such as microactuators and microfluidic devices. The basic motions of these materials are contraction-expansion and bending-unbending, which are observed mainly in polysiloxanes and polyacrylates that contain photochromic moieties. Other sophisticated motions such as twisting, oscillation, rotation, and translational motion have also been achieved. In recent years, efforts have been made to improve the photoresponsive and mechanical properties of this novel class of materials through the modification of molecular structures, development of new fabrication methods, and construction of composite structures. Herein, we review structures, functions, and working mechanisms of photomobile materials and recent advances in this field.

Liquid Crystalline Network Composites Reinforced by Silica Nanoparticles

Liquid crystalline networks (LCNs) are a class of polymers, which are able to produce mechanical actuation in response to external stimuli. Recent creation of LCNs with exchangeable links (xLCNs) makes LCNs easy moldable. As the xLCNs need to be shaped at a high temperature, it is important to enhance their thermal and mechanical properties. In this paper, a series of xLCNs/SiO₂ composites containing 1%-7% SiO₂ nanoparitcles (SNP) were prepared and their thermal and mechanical properties were examined. The results show that xLCNs/SNP composites have lower liquid crystalline-isotropic phase transition temperature and higher decomposition temperature than pure LCN. The tensile strength and the elongation at break of xLCNs at high temperatures were also enhanced due to the addition of SNPs.

Photoactuation and thermal isomerisation mechanism of cyanoazobenzene-based liquid crystal elastomers

4-Alkoxy-4'-cyanoazobenzenes are organic chromophores with great applicability within present nanotechnology. However, the use of such azo dyes for obtaining light-triggered artificial muscle-like actuators remains still unexplored. Achieving further knowledge about the thermal back reaction and isomerisation mechanism for these types of azoderivatives is essential to get photo-actuators with the desired abilities. Despite the push-pull nature of the 4-alkoxy-4'-cyanoazobenzene chromophore, it has been experimentally demonstrated that it isomerises via an inversion mechanism. The opto-mechanics of the prepared elastomeric material has also been investigated. For this system, a maximum opto-mechanical response of ca. 2.5 kPa has been registered, which is independent of the working temperature of the photoactuator.

Actuators based on liquid crystalline elastomer materials

Liquid crystalline elastomers (LCEs) exhibit a number of remarkable physical effects, including the unique, high-stroke reversible mechanical actuation when triggered by external stimuli. This article reviews some recent exciting developments in the field of LCE materials with an emphasis on their utilization in actuator applications. Such applications include artificial muscles, industrial manufacturing, health and microelectromechanical systems (MEMS). With suitable synthetic and preparation pathways and well-controlled actuation stimuli, such as heat, light, electric and magnetic fields, excellent physical properties of LCE materials can be realized. By comparing the actuating properties of different systems, general relationships between the structure and the properties of LCEs are discussed. How these materials can be turned into usable devices using interdisciplinary techniques is also described.