Smart Polymeric Biomaterials for Clinical Use.

Stimuli-sensitive polymers can respond to certain external environmental changes ("Stimulus") by altering properties such as shape, permeability, or color. The ability of a material to recognize a stimulus and respond to it is derived from stimuli- sensitive processes on the molecular and/or supramolecular level, as well as on the level of phase morphology, which are translated and amplified to the macroscopic level. The response to a stimulus often involves several processes, which take place at different hierarchical levels (e.g. photoisomerization reaction on the molecular level and phase transition on the morphology level in liquid crystalline elastomers). They can also occur on different time scales. In pH-responsive hydrogels the protonation/deprotonation of functional groups of the polymer network chains is a fast process compared to the diffusion of water molecules in the polymer network structure or the phase separation leading to the shrinkage of the gel. In some cases the material needs to be taught by physical processes to gain stimuli-responsiveness (e.g. shape-memory effect, (SME)). The field of stimuli-sensitive polymers is presently progressing rapidly. Current research topics involve extending the repertoire of suitable stimuli, realization of multifunctionality in one material, exploring concepts for materials, which can continuously adapt their (structural) properties to the requirements of their environment by implementing self- regulating processes, and investigating the influence of specimen dimensions on the shape changing behavior, e.g. on nano- and microparticles. On the other hand applications are being realized based on stimuli-sensitive polymers for various areas including aerospace, packaging, textiles, microfluidics, sensors and actuators as well as bioengineering. In this special issue three important classes of stimuli-sensitive polymers are comprehensively described in reviews and progress reports: shape-memory polymers (SMPs), stimuli-responsive gels, and liquid crystalline elastomers (LCE). In addition, advances in oscillating gels, and molecular modeling approaches as predictive tools for stimuli-responsive polymers are covered. Finally, exciting recent results in the field of stimuli- sensitive polymers are presented in selected communication articles such as multiphase polymer networks capable of a reversible triple shape effect (Zotzmann et al., DOI: 10.1002/adma.200904202) as well as LCE-based nanocomposites with partially aligned carbon nanotubes (CNT) having anisotropic and frequency dependent optical properties (Terentjev and co-workers, DOI: 10.1002/adma.200904103). The design principles for stimuli-sensitive polymers are elucidated exemplarily for photosensitive polymers. Rhodopsin, a sensory molecule for the visual perception, is an example of a photosensitive polymer from nature. It consists of the protein opsin and the photochromic molecule retinal, which can undergo a photoisomerization from 11-cis to all-trans retinal. This isomerization causes a conformational change in opsin, whereby the associated G-protein is activated. Photochromic molecules such as azobenzene or diarylethene (Figure 1a–b) are the starting point for Russew and Hecht (DOI: 10.1002/adma.200904102) to explain how responsive materials with switchable macroscopic properties can be created based on such stimuli-sensitive molecular processes. The photosensitive groups enabled photo-controllable self-assembly and self-organization of block copolymers as well as of low molecular weight gelator molecules in solution, switch assemblies at surfaces and photo-induced swelling and shrinkage of gels, e.g. functionalized poly(N-isopropylacrylamide). The incorporation of diazobenzene groups into LCE leads to photo-induced shape-changing materials, whereby the isomerization induces a liquid crystalline phase transition. It is notable that this field so far has been limited to only a few selected photochromic molecules most of which are already known for a long time. Therefore the design and synthesis of novel photosensitive molecules is a challenging area for future research. As the existing molecules require UV or visible light, the development of molecules sensitive to the NIR range would be desirable especially for biomedical applications, where a deep penetration of light without harming tissue is required. Schematic illustrations of reversible photoisomerization (a–b) and photochemical reactions (c–d). a) E/Z-photoisomerization of azobenzene groups. b) Ring-closure/ring-opening reaction of dithienylethene. c) Photo-induced ionic dissociation of triphenylmethane leuco derivatives. d) Photodimerization of cinnamic acid group. (a,c,d) reproduced with permission from.3 (b) adapted from Russew and Hecht, DOI: 10.1002/adma.200904102. Besides photochromic molecules, molecules containing thermolabile bonds are investigated as molecular switches in stimuli-sensitive polymers. Polyphthalaldehyde is an example of a self-amplified depolymerization molecule, in which the breaking of a single bond induces the spontaneous depolymerization of the entire polymer. This relatively fast chemical reaction can be performed in an impressively high spatial accuracy by application of a hot silicon tip, whereby 700 °C was determined to be a sufficient heater temperature corresponding to a polymer temperature of 300–400 °C. Arbitrary three-dimensional patterns with 40 nm lateral and 1 nm vertical resolution could be created and characterized. It is expected that this technology will be applied in printing optics on chips or the creation of nanoscale three- dimensional templates for shape matching self-assembly of nanorods or -cubes (Knoll et al., DOI: 10.1002/adma.200904386). The most prominent molecular architecture for stimuli-sensitive polymers is entropy-elastic polymer networks, consisting of chemical or physical crosslinks and highly flexible network chains (Figure 2). These networks serve as a molecular skeleton for the covalent attachment of functional groups (e.g. photochromic molecules) or as a matrix for particles (e.g. magnetic nanoparticles or CNT) or molecules (e.g. molecules forming liquid crystallines) acting as gates for translating the stimulating signal. The network segments may also act by themselves as a functional unit, whereby the chemical structure of their repeating units as well as the sequence structure in case of copolymer- based segments have to be tailored. Different molecular architectures for shape-memory polymers. a) Covalent network; b) multiblock copolymer; multimaterial systems: c) blend, and d) IPN. Gray and black lines: amorphous polymer chain segments, blue and red lines: crystalline polymer chain segments. Image adapted from Behl et al., DOI: 10.1002/adma.200904447. LCEs are polymer networks, which are loaded with a liquid crystalline material or contain covalently bound mesogenic groups (Ohm et al., DOI: 10.1002/adma.200904059). The mesogen can be part of the polymer backbone or be attached as a side chain via a flexible spacer ("end-on" or "side on"). Several synthetic pathways have been developed to crosslink the LCE in a way such that the materials can be oriented into liquid crystalline mono-domains. If the anisotropy of the liquid crystalline phase gets lost, e.g. because it is heated into the isotropic phase, a shape change of the LCE occurs, which is reversed on cooling when the anisotropic phase is regained. Photosensitive LCEs were obtained by incorporation of photoswitchable mesogens. Shape changes in electric fields were achieved by resistive heating. According to Ohm et al., a future potential for LCE might be in microscopically structured devices as they can be processed by soft molding, microfluidics or ink-jet printing. In general the long-term functionality/stability of LCEs needs to be improved. SMPs contain reversible crosslinks in addition to the netpoints determining the original permanent shape of the polymer network. Reversible crosslinks can be based on stimuli-depending physical interactions of functional groups or switching segments. In thermo-sensitive SMPs switching domains formed by such switching segments are able to temporarily fix a mechanical deformation by solidification caused by crystallization or vitrification. The fixation of the temporary shape as well as the recovery of the permanent shape is related to the thermal transition temperature associated with the switching domains. Reversible crosslinks can also be achieved by reversible chemical reactions, such as the photodimerization of cinnamate-groups (Figure 1d). In this way a light-induced shape-memory effect could be obtained. Different polymer architectures for polymers capable of undergoing a thermally-induced SME are displayed in Figure 2. Computational models for the thermally activated shape-memory effect are being developed as predictive tools facilitating the optimization of shape-memory properties (Nguyen and co-workers, DOI: 10.1002/adma.200904119). A prerequisite for the shape-memory effect is the creation of the temporary shape, which requires external manipulation. Once the original, permanent shape is recovered, a new temporary shape needs to be created to enable an additional SME. In this context the shape-memory effect is a one-way effect. A reversible shape change could be achieved for SMP containing a crystallizable switching segment kept under constant stress. During cooling under constant stress conditions a crystallization-induced elongation (CIE) occurred. This increased elongation is reversed by a melting-induced contraction (MIC) driven by entropy when the sample was reheated. Triple-shape polymers are able to perform two subsequent shape changes occurring at two different switching temperatures when heated. The triple-shape effect is in analogy to the classic shape-memory effect (dual-shape effect) an irreversible one-way effect. Zotzmann et al. (DOI: 10.1002/adma.200904202) report on a reversible triple-shape effect of polymer networks containing polypentadecalactone- and poly(ε-caprolactone)-segments, which was observed when a constant stress was applied. Suitable values for the constant stress level and the cooling rate were found to ensure two substantial CIEs and two MICs with similar contribution from both segments. Shape-memory polymers and gels as well as LCEs are able to self-sufficiently change their shape on demand and thus all belong to the class of actively moving polymers.1, 2 However the mechanisms for the shape changes are different, e.g. LCE change their shape as long as they are exposed to a suitable stimulus while the temporary shape of an SMP stays unchanged until exposed to the stimulus. Inspired by the complex and diverse requirements of potential applications, e.g. in biomedicine or in aerospace vehicles, multifunctional SMP are explored intensively. Here the shape-memory effect is combined with other functions (e.g. degradability, electrical conductivity, magnetic sensitivity, or radio-opacity), whereby the different functions shall be independent from each other. There are two different approaches to achieve multifunctionality (Behl et al. DOI: 10.1002/adma.200904447): multi-material systems, in which each material component contributes a certain function, and one-component materials, where several functions (e.g. SME and hydrolytic degradability) are integrated. An example of a multi-material approach are (nano)composites, in which a SMP matrix is combined with particulate fillers. Sellinger et al. (DOI: 10.1002/adma.200904107) report a fundamental study on the electromechanical effect of a nanocomposite from polyimide filled with CNT. The application of electrical current results in an increase in the temperature due to resistive heating. Thermal expansion of the polymer leads to a reversible shape change of the nanocomposite. Mechanical softening associated with the reversible phase transition at Tg can also be utilized to achieve substantial strain increases over a small ΔT. An impressive example of a multi-material system, composed of a main-chain thermotropic liquid crystalline copolymer (LCP) and CNT is described in the communication from Ji et. al. (DOI: 10.1002/adma.200904103). The LCP exhibits an enhanced compatibility with different pristine CNT, which was achieved by end group functionalization of the copolymer with pyrene- moieties. A particularly interesting result of this study is that a monodomain CNT-LCE composite could be prepared, in which LCP wrapped CNT were at least partially aligned. An anisotropy and frequency-dependency of its refractive index makes this material system useful for optical applications in the GHz-THz region. Stimuli-sensitive polymeric gels are able to swell and shrink in response to certain changes in their environmental conditions such as solvent composition, temperature, light, ionic strength, or pH. For example photosensitive gels were obtained by incorporation of triphenylmethane leuco derivatives, which dissociate into ion pairs upon exposure to UV light irradiation (Figure 1c). The back reaction recombining the ion pair occurs thermally in the dark. The reversible variation of electrostatic repulsion between photo-generated charges in gels results in expansion or shrinkage.4 For a general overview about this wide field looking back over more than 30 years of research, we refer the readership to references5 and.6. Although SME has been demonstrated in hydrogels as early as the mid 1990s,7 only few reports have been published about shape-memory gels since then. In this special issue four current research topics from the area of stimuli-responsive gels are addressed: microgels, porous hydrogels at interfaces, self-oscillating gels and finally enzymatically degradable hydrogels as temporary substitute of the extracellular matrix. Romeo et al. (DOI: 10.1002/adma.200904189) investigated suspensions of microgels from crosslinked poly(N-isopropylacry­lamide) having a lower critical solution temperature (LCST) at 33 °C. At temperatures higher than LCST, the gel particles shrink while at the same time the physical interaction between the particles increases, which can lead to the formation of a gel. For this reason concentrated suspensions of poly(N-isopropylacrylamide)-microgels behaved like a colloidal glass below LCST, like a liquid in the range of LCST, and exhibited properties like a colloidal gel above LCST. Structuring of stimuli-responsive gels lead to novel functions e.g. the capability to control the mass transport through intelligent membranes. Tokarev and Minko (DOI: 10.1002/adma.201000165) report on porous hydrogel films, where controlled closing and opening of pores can be achieved by external stimuli such as pH, ionic strength, or temperature. Such systems can be applied as plain films or capsules e.g. for filtration, separation, controlled release of drugs, sensors, or actuators. Self-oscillating gels perform swelling-shrinking cycles without requiring an external stimulus to control this process (Yoshida, DOI: 10.1002/adma.200904075). This effect is driven by an oscillating chemical reaction (Belousov-Zhabotinsky reaction). The catalyst of the reaction is covalently bound to a poly(N-isopropylacrylamide)-network, while the reactants are added to the solvent. Potential applications of these gels are actuators as well as active mass transport systems. Stimuli-responsive hydrogels have a high application potential in biomedical applications especially in regenerative therapies where they can act as a temporary substitute of the extracellular matrix. Two types of stimuli-sensitive functions are explored for hydrogels in this application area: volume changes and degradation. Hydrogels exhibiting volume changes on demand can be applied as coating in cell culture devices to detach cell layers without application of enzymes, e.g. by slightly increasing the temperature.8 Enzymatically degradable hydrogels (Kloxin et al., DOI: 10.1002/adma.200904179) can be used as implantable scaffolds for induced autoregeneration as well as miniscaffolds or injectable in-situ forming hydrogel systems for cell therapies. Cells experience these gels solely in their local microenvironment, e.g. via focal adhesion. Therefore the characterization of the local mechanical properties and the degradation behavior of the gels are of high importance. Atomic force microscopy under physiological conditions and tracer particle microrheology are modern methods to determine gel modulus and viscoelastic properties in spatial resolution. The insights obtained by such studies might contribute to a knowledge-based design of scaffolds, which will be capable of guiding e.g. cell differentiation and tissue formation. We wish to thank all authors for their contributions to this special issue and hope that the research on stimuli- sensitive polymers presented in this issue will inspire and stimulate the readership to further develop this fascinating field of materials science. Andreas Lendlein is Director of the Institute of Polymer Research at GKSS Research Center in Teltow, Germany and Member of the Board of Directors of the Berlin-Brandenburg Center for Regenerative Therapies. He is Professor at University of Potsdam and Honorary Professor at Freie Universität Berlin as well as Member of the Medical Faculty of Charité University Medicine Berlin. He completed his Habilitation in Macromolecular Chemistry in 2002 at RWTH Aachen University and received his doctoral degree in Materials Science from the Swiss Federal Institute of Technology (ETH) in Zürich in 1996. His current research interests include (multi)functional polymer-based materials and their interaction with physiological environments. V. Prasad Shastri is the Bioss Professor of Cell Signaling Environments and Professor of Biofunctional Macromolecular Chemistry, and director of the Institute for Macromolecular Chemistry. Shastri received his PhD degree from the Rensselaer Polytechnic Institute in 1995 and received his postdoctoral training at the Massachusetts Institute of Technology under Professor Robert Langer. Shastri has made seminal contributions in development of new degradable polymers, in vivo engineering of organs and tissue, and drug delivery. His research interests include biofunctional polymers, nanoscale engineering of surfaces, systems for controlling cell functions and imaging, in vivo engineering of tissues, and transdermal delivery.

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Liquid crystalline (LC) elastomers (LCEs) enable large-scale reversible shape changes in polymeric materials; however, they require intensive, irreversible programming approaches in order to facilitate controllable actuation. We have implemented photoinduced dynamic covalent chemistry (DCC) that chemically anneals the LCE toward an applied equilibrium only when and where the light-activated DCC is on. By using light as the stimulus that enables programming, the dynamic bond exchange is orthogonal to LC phase behavior, enabling the LCE to be annealed in any LC phase or in the isotropic phase with various manifestations of this capability explored here. In a photopolymerizable LCE network, we report the synthesis, characterization, and exploitation of readily shape-programmable DCC-functional LCEs to create predictable, complex, and fully reversible shape changes, thus enabling the literal square peg to fit into a round hole.

Multifunctional composites can accomplish multiple tasks such as shape morphing, sensing, and load bearing using a single structure. Smart materials including liquid crystal elastomers (LCE) and shape memory polymers (SMP) have long been used as the primary components of multifunctional composites because of their shape and property changes in response to external stimuli. However, LCEs can generate rapid and reversible shape changes but are soft and require a constant temperature to retain their deformed shape; SMPs have favorable mechanical properties but few can achieve reversible actuations. Moreover, both LCEs and SMPs have limited capability for tunable shape morphing. Multi‐material 3D printing of smart materials, also known as 4D printing, has seen significant advances enabling the fabrication of composites with novel functionality. In this work, 4D printing is leveraged to create an LCE‐SMP composite that can achieve not only rapid and reversible shape changes, but also cooling‐rate regulated tunable shape morphing. The latter is achieved by harnessing the distinct time‐dependent thermomechanical properties of LCEs and SMPs. Furthermore, the composite has a high stiffness at low temperature to support heavy loads. The LCE‐SMP composite hence offers a novel approach to achieve tunable shape morphing for future engineering applications.

Liquid crystal elastomers (LCEs) are well‐known for their significant, reversible, and anisotropic shape changes when exposed to different external stimuli due to their crosslinked polymer networks that merge the elastic properties of rubbers with the anisotropic characteristics of liquid crystals. LCEs with stimuli‐responsive behaviors are preferable for applications in robotics, bio‐medics, electronics, optics, and energy. In this review, the state‐of‐the‐art advances in stimuli‐responsive LCEs are reviewed, including thermo‐responsive LCEs, photo‐responsive LCEs, electro‐responsive LCEs, and other responsive LCEs, such as chemical‐responsive LCEs, magnetic‐responsive LCEs, humidity‐responsive LCEs, radio‐frequency‐responsive LCEs, ultrasonic‐responsive LCEs, and photothermal‐responsive LCEs, all of which have contributed to the resurgence in LCE research. The LCEs exhibit remarkable performance, spanning several orders of magnitude. They typically achieve actuation strains of 5–500%, stresses of 0.01–20 MPa, and fast response speeds, making them promising systems for actuators and artificial muscles. Furthermore, the applications of these responsive LCEs in information encryption, force sensors, and material transportation are demonstrated, which have significant potential for further development of the next‐generation advanced functional materials. Finally, this review concludes with a summary and perspective on the current challenges and emerging research opportunities for high‐performance LCEs endowed with remarkable properties.

Programmable soft materials have shown applications in artificial muscles, soft robotics, flexible electronics, and biomedicines due to their adaptive structural transformations. As an ordered soft material, directional shape changes of liquid crystal elastomer (LCE) can be easily achieved via external stimuli thanks to its anisotropic elasticity. However, harnessing the interplay between molecular ordering, geometry, and shape morphing in this anisotropic material to create programmable and complex shape changes remains a challenge. Here, by integrating the concepts of kirigami or Chinese paper cutting “JianZhi” in the light‐actuated LCE encoded with controlled molecular orientations, various complex 3D shape morphing behaviors are demonstrated. Versatile combinations of fundamental shape changes such as bending, folding, twisting, and rolling are enabled by fine‐tuning the molecular orientations and geometries in the monolithic LCE kirigami. Furthermore, various functions such as fluttering of the Chinese crane bird “QianZhiHe,” arbitrary directional locomotion in the annulus and linear locomotion in the complex Chinese character are also realized. These complex, fast‐response, untethered, remote, reversible, and programmable shape morphologies actuated in a monolith of LCE kirigami will open opportunities in soft robotics and smart materials.

This chapter focuses on the integration of oriented Liquid-Crystalline Elastomers (LCEs) in MEMS/MOEMS for the development and construction of devices for mechanical actuation. These anisotropic soft materials combine the entropic elasticity of the polymer backbone with the degree of order from the mesogens. When aligned, such materials undergo a change in shape which can be used to generate stresses and mechanical work. Thus, these smart materials are good candidates to be used as artificial muscles. We will first describe the chemistry and the orientational processes for the development of LCEs, as well as the key parameters for the obtaining of a successful smart soft material and the external stimuli which can be applied to trigger the corresponding actuation.

Liquid crystal elastomers (LCE) are highly attractive for 4D printing due to their ability to undergo large, rapid, and reversible shape changes in response to external stimuli. While direct ink writing (DIW) enables mesogen alignment during extrusion, it remains limited in resolution and geometric complexity. In contrast, digital light processing (DLP) offers fast, high-resolution fabrication of complex architectures but lacks an intrinsic mechanism for aligning mesogens, which prevents reversible actuation. Here, we present a scalable and versatile strategy for DLP 4D printing of LCEs, based on partially cured printed structures subjected to mechanical programming followed by photo-crosslinking to fix mesogen alignment. This two-stage photo-crosslinking approach enables the fabrication of monodomain nematic LCEs with tunable thermo-mechanical properties and programmable, multimodal, and large actuation strains up to 45%. The strategy is demonstrated through complex LCE architectures, including an octopus model that undergoes consistent, reversible actuation over 100 thermal cycles. Additionally, sports-themed stickman models based on these LCEs show how a single printed object can be programmed with different actuation modes, such as bending, twisting, or contraction, and their combinations, by selecting the ink best suited to the targeted actuation. These results highlight the design and programming flexibility of the method, establishing DLP as a compelling alternative to DIW for fabricating functional soft actuators.

Monodomain liquid crystal elastomers (LCEs) are new materials uniquely suitable for artificial muscles, as they undergo large reversible uniaxial shape changes, with strains of 20-500% and stresses of 10-100 kPa, falling exactly into the dynamic range of a muscle. LCEs exhibit little to no fatigue over thousands of actuation cycles. Their practical use has been limited, however, owing to the difficulty of synthesizing components, achieving consistent alignment during cross-linking across the whole material and often a high nematic-isotropic phase transition temperature. The most widely studied method for LC alignment involves mechanical stretching of the material during one of two cross-linking steps, which makes fabrication difficult to control and lends itself mainly to samples that can be easily grasped (with sizes of the order of mm). In this article, we describe a method of adapting the LCE synthesis to microscale objects, achieving monodomain alignment with a single cross-linking step, and lowering the cycling temperature. LCE precursor droplets are embedded in and then stretched in a polymer matrix at high temperature. Confinement of the uniaxially stretched droplets maintains the alignment achieved during stretching and allows us to eliminate one of the cross-linking steps and the variability associated with it. Adding a comonomer during the polymerization leads to lowering of the nematic-to-isotropic transition temperature (58 °C), significantly expanding the range of potential applications for these micromuscles. We demonstrate reversible thermal switching of the micromuscles in line with the largest strain changes observed for side-chain LCEs and a differential scanning calorimetry characterization of the material phase transitions. The method demonstrates the parallel fabrication of many microscale actuators and is amenable to further scale-up and manufacturing.

Liquid crystal elastomers (LCEs) are smart materials that integrate the anisotropic properties of liquid crystals and the elasticity of polymers, enabling large, reversible shape changes in response to various external stimuli. These distinctive properties make LCEs a promising candidate for applications in actuators, soft robotics, sensors, and optics. The morphing behaviors of LCEs are fundamentally governed by the alignment of mesogenic molecules, which transition from ordered to disordered states upon stimulation, resulting in controllable shape transformations. Various alignment techniques exploiting the manipulation of mesogenic molecules are continuously explored as a way to effectively actuate morphing behaviors. This review provides an overview of key alignment techniques, including surface anchoring, field effect, and mechanical alignment, and explores how these methods support the design of tailored morphing properties for specific applications. The relationship between alignment and morphing behaviors in LCEs is discussed, offering a comprehensive overview of alignment-based morphing design strategies. Furthermore, the review highlights the significant potential of LCEs in advanced applications such as artificial muscles, actuators, and reconfigurable optical devices. By providing a foundational understanding of LCEs' alignment and morphing, this review aims to inspire more scientific innovations and technical advances in their design and application.

Liquid crystal elastomers (LCE) exhibit a combination of elasticity and mesogenic ordering, yielding large thermally stimulated changes in shape. These LCE systems although well characterised, still yield open questions in the nature of how the crosslinking affects the LCE phase transition. Therefore calorimetry and deuteron-nuclear magnetic resonance were used to study the isotropic-nematic phase transition of uniformly ordered LCE. We observed that the density of crosslinkers strongly affects the nematic-isotropic phase transition. The observed spread critical transitions are explained with a dispersion of local mechanical fields that yields a weakly disordered orientational state composed of regions that exhibit temperature profiles of the nematic order parameter ranging from first order to supercritical. On increasing crosslinking density, the predominantly first order thermodynamic response transforms into a predominantly supercritical one.Additionally, to illustrate the response of these actuating systems, it was demonstrated that a LCE can be electrically heated. The insulating LCE network was reprocessed using conducting nanoparticles dispersed in a solvent with high LCE swelling capability. This results in a low electrical resistivity surface layer of LCE network with a high concentration of conducting nanoparticles. The reprocessing allows the effective resistivity of a LCE film to be reduced from highly insulating values to values useable for electrical actuation. This layer in addition withstands large changes in geometrical shape both in contraction and expansion. Utilizing a resistive “Joule” heating effect, the reprocessed system exhibits an indirect electromechanical effect characterised by a 150% length change that can be cycled for more than 10, 000 times.

Liquid crystal elastomers are currently of great interest due to their large thermally stimulated changes in shape. Here the authors show that by using an existing network and conducting carbon nanoparticles dispersed in a solvent with high swelling capability, a surface integrated layer can be created. This layer allows the effective resistivity to be reduced from highly insulating to usable values for electrical actuation and withstands large changes in geometrical shape both in contraction and expansion. Utilizing a resistive “Joule” heating effect, the reprocessed system shows a 150% length change and can be cycled beyond 10kcycles.

Photomechanics of blanket and patterned liquid crystal elastomer films

Liquid crystal elastomers (LCEs) are unique among shape-responsive materials in that they exhibit large and reversible shape changes and can respond to a variety of stimuli. However, only a handful of studies have explored LCEs for biomedical applications. Here, we demonstrate that LCE nanocomposites (LCE-NCs) exhibit a fast and reversible electromechanical response and can be employed as dynamic substrates for cell culture. A two-step method for preparing conductive LCE-NCs is described, which produces materials that exhibit rapid (response times as fast at 0.6 s), large-amplitude (contraction by up to 30%), and fully reversible shape changes (stable to over 5000 cycles) under externally applied voltages (5-40 V). The electromechanical response of the LCE-NCs is tunable through variation of the electrical potential and LCE-NC composition. We utilize conductive LCE-NCs as responsive substrates to culture neonatal rat ventricular myocytes (NRVM) and find that NRVM remain viable on both stimulated and static LCE-NC substrates. These materials provide a reliable and simple route to materials that exhibit a fast, reversible, and large-amplitude electromechanical response.

Recent work has used self-folding origami inspired composites to produce complex, scalable, affordable, and lightweight morphing structures [1]. These characteristics are of interest for engineering applications, in fields including aerospace [2] and medical devices [3]. Due to these advantages, research on self-folding smart composites has grown, with a particular focus on the use of laminate manufacturing techniques that stack layers of heterogeneous materials to generate functional composites. Previous work used this approach to manufacture self-folding origami inspired robots [1]. A simple shape memory composite design consists of a smart material (e.g. a one-way shape memory polymer, or SMP) sandwiched between patterned rigid layers. These SMPs change their shape in response to an external stimulus (e.g. temperature). Upon heating above the phase transition temperature of the polymer (Tt), the SMP contracts, causing the laminate to fold. The SMPs used in self-folding laminate composites are unidirectional and thus the laminate is unable to recover its original state without application of external force. In this work, we study the use of thermal responsive liquid crystal elastomers (LCE) for reversible self-folding and actuation of origami inspired composites using laminate manufacturing. LCEs are smart materials that exhibit reversible deformation, good strain recoverability, and tailorable properties (i.e. phase transition temperature, strain, and orientation of deformation) [4–6]. We explore two composite hinge designs using laminate manufacturing process [1, 7] with a Joule heating layer to enable self-folding: one where the LCE acts as a tensile actuator connected only on the edges of the rigid layer, which we call a tensional hinge, and a second where the LCE is attached along the patterned rigid layer hinge, which we call a flexural hinge. The angular displacements of these two hinge designs are estimated using geometric models that account for the contraction of the LCE upon heating, and compared against experimental measurements. The maximum blocked torque of the composite hinges is also measured experimentally. To demonstrate the use of LCE as an active layer for origami inspired composites, we also present a laminate crawler robot. The crawling locomotion is controlled with an electrical heating layer laminated on the LCE. These results demonstrate the possibility of using LCE to achieve rapid, reversible folding and to generate similar torques, as compared to previous work in origami inspired self-folding composite.