Reversible Shape Research Articles

Liquid Crystalline Hydrogel with Thermally Induced Reversible Shape Change and Water-Triggered Shape Memory.

Liquid crystalline hydrogel (LCH) is synthesized through simultaneous polymerization of hydrophobic and hydrophilic monomers in an oil-in-water emulsion, resulting in phase-separated liquid crystalline network (LCN) embedded in a hydrogel matrix. This material features some properties and functions of both LCN and hydrogel, displaying stable LC phase over repeated hydration and dehydration cycles of the hydrogel matrix. Using mechanically stretched and photocrosslinked LCH, the thermally induced LC-isotropic phase transition in LCN domains can be translated into reversible macroscopic deformation of the LCH. Moreover, the LCH exhibits water absorption-controlled shape memory effect.

Programmable multistability for 3D printed reinforced multifunctional composites with reversible shape change

4D printing empowers 3D printed structures made of hydrogels, liquid crystals or shape memory polymers, with reversible morphing capabilities in response to an external stimulus. To apply reversible shape-change to stiff lightweight materials such as microfiber reinforced polymers, we developed a composite ink that can be printed using direct-ink-writing (DIW), and that exhibits multistability around its glass transition temperature. After curing at room temperature, the flat print thermally morphs into a predefined shape upon heating at an actuation temperature and cooling down. The sample can then reversibly snap between multiple stable shapes when heated above its glass transition temperature thanks to pre-stress-induced multistability. The key that allows thermal morphing and pre-stress multistability is the microstructuring of the 3D printed composites by shear-induced alignment of reinforcing microfibers. This alignment leads to local anisotropy in thermomechanical properties and the build-up of pre-stresses. Furthermore, the ink composition can be tuned to generate shape-dependant reversible functional properties, such as electrical conductivity. Based on finite element modelling and experimental results, the method proposed here can be used for variety of compositions and designs, for applications where stiffness, reconfigurability and shape-dependent functionalities can be exploited.

Shape-programmable liquid crystal elastomer structures with arbitrary three-dimensional director fields and geometries

Liquid crystal elastomers exhibit large reversible strain and programmable shape transformations, enabling various applications in soft robotics, dynamic optics, and programmable origami and kirigami. The morphing modes of these materials depend on both their geometries and director fields. In two dimensions, a pixel-by-pixel design has been accomplished to attain more flexibility over the spatial resolution of the liquid crystal response. Here we generalize this idea in two steps. First, we create independent, cubic light-responsive voxels, each with a predefined director field orientation. Second, these voxels are in turn assembled to form lines, grids, or skeletal structures that would be rather difficult to obtain from an initially connected material sample. In this way, the orientation of the director fields can be made to vary at voxel resolution to allow for programmable optically- or thermally-triggered anisotropic or heterogeneous material responses and morphology changes in three dimensions that would be impossible or hard to implement otherwise.

Biomechanical behavior of the dental implant macrodesign in mandibular implant-supported overdentures

Aim The present in vitro study aimed to evaluate the effect of different dental implant macrodesigns on the stress occurred on the implants and bone for two-implant-supported overdentures (IOD) in the rehabilitation of edentulous mandibles. Methods Six different implant brands and macro design were used in this study. Two groups, Group V (V-thread shape) and Group R (reverse buttress thread shape), were formed based on implant thread shape. Vertical and oblique loads were applied to the implants in order to evaluate tension, compression, and Von Mises stresses by implementing the three-dimensional finite element analysis. Results According to the stresses after the applied forces, the macrodesign of dental implants affects stress distribution in different directions. Group R exerted more stresses on the cortical bone, while Group V produced more stresses on the implants. Tissue level implants caused high stresses on bones and low stress on itself. As proposed, microthread neck design decreased the stresses on the cortical bone. Conclusion In the light of these biomechanical findings, considering the anterior region of the mandible often consists of dense cortical bone, it may be advantageous to prefer implants with a V-thread design and a microthread neck surface, which creates less stress in the cortical bone.

Multidimensional‐Encryption in Emissive Liquid Crystal Elastomers through Synergistic Usage of Photorewritable Fluorescent Patterning and Reconfigurable 3D Shaping

AbstractAdvanced encryption/decryption strategies are of great significance for information protection and data security. It is highly desirable yet quite challenging to develop functional materials for encrypting/decrypting information more effectively. Herein, a novel emissive liquid crystal elastomer (LCE) for multidimensional and multistage encryption is proposed for the first time, through synergistic utilization of phototunable fluorescence and the photoprogrammable shape. The fluorescent LCE is fabricated by incorporating an aggregation‐induced‐emission α‐cyanodiarylethene‐based hydrogen‐bonded complex and azobenzene derivative into the LCE networks through covalent bonding. Because of the photoisomerization of both these two photosensitive derivatives under the same exciting light, the consequent LCE films exhibit photoinduced reversible fluorescence changes and shape deformations that are suitable for data storage and encryption. On this basis, the collaborative usage of photolithography‐based 2D fluorescent images and photoprogramming 3D shape configurations can lead to multidimensional and multistage encryption. Moreover, this encryption strategy is reprogrammable, allowing for repeatable encoding and decrypting. The results demonstrated here reveal that the reversible phototunable fluorescent LCE materials exhibit promising applications in data storage and encryption.

A Liquid Crystal Elastomer-Based Unprecedented Two-Way Shape-Memory Aerogel.

With the advantage of reversible shape‐morphing between two different permanent shapes under external stimuli, the two‐way shape‐memory aerogel is expected to become a preferred aerogel for developing practical applications in actuators, sensors, robotics, and more. Herein, the first two‐way shape‐memory liquid crystal elastomer (LCE)‐based aerogel is prepared by an orthogonal heat and light curing strategy coupled with an intermediate mechanical stretching step. The differential scanning calorimetry, temperature‐varied wide‐angle X‐ray scattering, and polarizing optical microscope results indicate that the aerogel possesses a liquid crystal phase and the insider mesogens are well‐oriented along the stretching direction. In addition to having superior compressibility and excellent shape stability, this LCE‐based aerogel can perform a reversible shape deformation during the heating/cooling cycles with a shrinkage ratio of 37%. The work, that is disclosed here, realizes a truly two‐way shape‐memory behavior rather than the one‐way shape deformation of traditional polymer aerogel materials, and may promote potential applications of this novel LCE‐based aerogel material in control devices, soft actuators, and beyond.

Arbitrarily Reconfigurable and Thermadapt Reversible Two-Way Shape Memory Poly(thiourethane) Accomplished by Multiple Dynamic Covalent Bonds.

The fabrication of a single polymer network that exhibits a good reversible two-way shape memory effect (2W-SME), can be formed into arbitrarily complex three-dimensional (3D) shapes, and is recyclable remains a challenge. Herein, we design and fabricate poly(thiourethane) (PTU) networks with an excellent thermadapt reversible 2W-SME, arbitrary reconfigurability, and good recyclability via the synergistic effects of multiple dynamic covalent bonds (i.e., ester, urethane, and thiourethane bonds). The PTU samples with good mechanical performance simultaneously demonstrate a maximum tensile stress of 29.7 ± 1.1 MPa and a high strain of 474.8 ± 7.5%. In addition, the fraction of reversible strain of the PTU with 20 wt % hard segment reaches 22.4% during the reversible 2W-SME, where the fraction of reversible strain is enhanced by self-nucleated crystallization of the PTU. A sample with arbitrarily complex permanent 3D shapes can be realized via the solid-state plasticity, and that sample also exhibits excellent reversible 2W-SME. A smart light-responsive actuator with a double control switch is fabricated using a reversible two-way shape memory PTU/MXene film. In addition, the PTU networks are de-cross-linked by alcohol solvolysis, enabling the recovery of monomers and the realization of recyclability. Therefore, the present study involving the design and fabrication of a PTU network for potential applications in intelligent actuators and multifunctional shape-shifting devices provides a new strategy for the development of thermadapt reversible two-way shape memory polymers.

Novel two-way multiple shape memory effects of olefin block copolymer (OBC)/polycaprolactone (PCL) blends

The shape memory properties of olefin block copolymer (OBC)/polycaprolactone (PCL) crosslinked blends at different blend compositions were investigated. This is the first OBC-based two-way shape memory blends processes without external load under thermal and vapor stimuli. The deformation temperature of 80 °C was selected far away between the respective melting temperatures of PCL (56.5 °C) and OBC (110.3 °C) for the thermo-triggered two-way multiple shape memory behavior. The internal stress developed between the unmelted OBC crystalline region and crystallized PCL during cooling under pre-strain conditions played an important role in further melting-induced contraction and cooling-induced elongation without external load, even though the deformation temperature caused the complete melting of the crystalline region of PCL and the crystallinity of unmelted OBC was very small. As for the solvent vapor-triggered cases, the THF and n-hexane solvents were used. The bent sample shrank in response to the vapor stimulation, followed by expanding in the air. The blends remarkably showed rare solvent vapor-triggered reversible shape memory behavior without any external stress and pre-soaking treatment. This interesting finding was not reported earlier until now, to the best of our knowledge. This current finding unveils numerous possibilities to prepare multi-triggered blending systems with reversible shape memory behavior.

Modeling the one-way and two-way shape memory effects of semi-crystalline polymers

Polymers of two-way shape memory effects (SMEs) are of great application potential ranging from intelligent actuators to artificial muscles due to their reversible shape response on the basis of an on-off stimulus such as a cooling-heating cycle, which motivates the development of theoretical tools to support their design. In the paper, a three-dimensional phase transition finite deformation constitutive model is proposed to phenomenologically describe the underlying mechanisms of the one-way and two-way SMEs in thermal-responsive semi-crystalline polymers. A unified transition equation is proposed to depict the volume changes of crystalline phase during both crystallization and melting transitions. The crystallization-induced elongation and melting-induced contraction behaviours in two-way SMEs are assumed to be realized by an elongation-contraction element and its temperature-dependent strain evolution rules. The overall constitutive model is then implemented into Mathematica, and the model predictions are compared to and agree well with the experimental results. The paper provides an efficient method on modeling the thermal-induced two-way SMEs under stress conditions in semi-crystalline polymers and detailed discussion about the influence of several parameters such as the crosslinking density, the applied external load and the cooling/heating rate.

Single Particle Color Switching by Laser-Induced Deformation of Liquid Metal-derived Microcapsules

Active controlling of optical properties of metallic particles holds great promise for nonlinear nanophotonics and compact optoelectronic devices. Except for the electronic and chemical tuning of their properties, active control through fast and reversible shape modulation remains a significant challenge. Here, we report on the concept for changing the color and brightness of single particles by reversible/irreversible tuning of their shapes. As a family of plasmonic materials with low melting points and high flexibility, we synthesized liquid metal microparticles with different interior (dense/hollow) and morphology from Ga and its alloys (GaNi, GaCu). Utilizing near-infrared femtosecond laser pulses, we achieve two regimes for reversible/irreversible optical tuning due to consequent weak/strong perturbation of the microcapsules (MC) shapes. The chemical composition and MCs morphology significantly affect the tuning of color and brightness, as well as the rigidity of the MCs to extreme laser conditions.

Programmable and Reversible Self-assembly of 3D Architectures Actuated by Flexible Metal–Organic Frameworks

Respond rapidly and reversibly to external stimuli are critical to mimic biological systems. To design a multi-stimuli responsive actuator, we built a novel type of monolayer composite film that integrates the swellable metal-organic frameworks (MOF-88C) into the polymer P(VDF-CTFE). When exposed to polar solvent vapors (toluene, acetone, dichloromethane, and methanol), the composite-strip produces a reversible shape transformation derived from the intrinsic flexible of MOF crystal. Benefited to the active swelling MOF, the instant reversible bending (angle change >360° within 10 seconds) of the composite strip was realized at minute concentrations of dichloromethane vapor. Furthermore, to realized specific and programmable actuation, we use two fabrication strategies, templates and femtosecond laser system to control over swellable MOF crystals spatial distribution within the polymeric film. Interestingly, the composite film successfully mimicked natural plant tendrils, flowers that blooms or closes and soft robots that wriggle and grab objects. The composite material based on flexible MOF is a promising candidate for smart material of autonomous soft mechanical driving device.

Anomalous Temperature-Controlled Concave-Convex Switching of Curved Oil-Water Menisci.

While the curvature of the classical liquid surfaces exhibits only a weak temperature dependence, we demonstrate here a reversible temperature-tunable concave-convex shape switching in capillary-contained, surfactant-decorated, oil-water interfaces. The observed switching gives rise to a concave-convex shape transition, which takes place as a function of the width of the containing capillary. This apparent violation of Young's equation results from a hitherto-unreported sharp reversible hydrophobic-hydrophilic transition of the glass capillary walls. The transition is driven by the interfacial freezing effect, which controls the balance between the competing surfactants' adsorption on, and consequent hydrophobization of, the capillary walls and their incorporation into the interfacially frozen monolayer. Since capillary wetting by surfactant solutions is fundamental for a wide range of technologies and natural phenomena, the present observations have important implications in many fields, from fluid engineering to biology, and beyond.

Covalent Adaptable Liquid Crystal Networks Enabled by Reversible Ring-Opening Cascades of Cyclic Disulfides.

The development of covalent adaptable liquid crystal networks (LCNs) enabled by introducing dynamic covalent bonds has endowed liquid crystal actuators with self-healing properties and reversible shape programmability, broadening their applications in diverse soft robotic devices. However, the finite molecular design strategy limits the recyclability and the architectural diversity of these materials. Here, a strategy is first reported for fabricating photoresponsive polydisulfide-based covalent adaptable LCNs by ring-opening polymerization of cyclic dithiolane groups. Based on the disulfide metathesis, the resulting materials are self-healable, reshapable, and reprogrammable. Importantly, the equilibrium between the polymer backbones and the dithiolane-functionalized monomers enables catalytic depolymerization to recycle monomers, which could significantly weaken the disadvantage of subtractive manufacturing of photomechanical devices. This work rooted in chemistry would provide an economical and environmentally friendly strategy for the fabrication of functional soft robotics with excellent programmability and renewability and beyond.

Digital light fabrication of reversible shape memory polymers

Reversible shape memory polymers (SMPs) have received extensive attention because of their unique deformation mechanisms and diverse shape-changing behaviors. However, the traditional methods of fabricating reversible SMPs usually result in the limitation of the accessible shapes and the corresponding shape-shifting behaviors. Herein, a reversible SMP with two crystalline phases is digital light-cured via thiol-acrylate chemistry with the assist of a digital light processing (DLP) technology. Thus, a set of special geometric patterns can be obtained by digitally defined exposure regions with a specified time, allowing the achievement of the 3D kirigami or origami reversible actuation. In addition, this strategy can be extended to the emerging advanced 3D or 4D printing technologies by introducing z-axis dimension or time gradient, facilitating the fabrication of sophisticated reprogrammable reversible shape memory devices, including 4D printed grabber and 3D printed movable cartoon character. Our approach based on DLP enables the rapid and precise manipulation of shape-changing materials, which can extend the possibilities for future development in aerospace and other potential applications.

Multifunctional Polysiloxane with coordinative ligand for ion recognition, reprocessable elastomer, and reconfigurable shape memory

Incorporating supramolecular motifs into polysiloxanes has been attracting interest to thrive advanced materials with unprecedented properties, such as enhanced stiffness, high stretchability, self-healing ability, and interfacial adhesion. Herein, we designed and synthesized a novel coordinative polysiloxane functionalized with the 2-hydroxy-1-naphthyl imine group, featuring in the synergistic coordination of N and O atoms towards various metal ions. By making full use of its coordination ability, this novel polysiloxane has been utilized for multipurpose applications. Firstly, remarkable ion recognition ability for Fe3+, Zn2+ and Al3+ ions was demonstrated. Secondly, supramolecular elastomers with thermoplasticity and self-healing ability were prepared by coordinating this polysiloxane with various metal ions. Especially the P1-10%-Fe3+ elastomer exhibits unprecedented mechanical strength (8.8 MPa) and fracture toughness (33.22 MJ/m3) among the reported thermoplastic polysiloxane-based elastomers. Thirdly, reprocessable shape-memory materials were developed by combination of glass transition and topological rearrangement induced by temperature, expanding their potential for complex reversible shape shifting. This new type of polymer could open the door for wider applications of supramolecular polysiloxane materials.

Microfluidic Obstacle Arrays Induce Large Reversible Shape Change in Red Blood Cells.

Red blood cell (RBC) shape change under static and dynamic shear stress has been a source of interest for at least 50 years. High-speed time-lapse microscopy was used to observe the rate of deformation and relaxation when RBCs are subjected to periodic shear stress and deformation forces as they pass through an obstacle. We show that red blood cells are reversibly deformed and take on characteristic shapes not previously seen in physiological buffers when the maximum shear stress was between 2.2 and 25 Pa (strain rate 2200 to 25,000 s−1). We quantify the rates of RBC deformation and recovery using Kaplan–Meier survival analysis. The time to deformation decreased from 320 to 23 milliseconds with increasing flow rates, but the distance traveled before deformation changed little. Shape recovery, a measure of degree of deformation, takes tens of milliseconds at the lowest flow rates and reached saturation at 2.4 s at a shear stress of 11.2 Pa indicating a maximum degree of deformation was reached. The rates and types of deformation have relevance in red blood cell disorders and in blood cell behavior in microfluidic devices.

Toward Application of Liquid Crystalline Elastomer for Smart Robotics: State of the Art and Challenges.

Liquid crystalline elastomers (LCEs) are lightly crosslinked polymers that combine liquid crystalline order and rubber elasticity. Owing to their unique anisotropic behavior and reversible shape responses to external stimulation (temperature, light, etc.), LCEs have emerged as preferred candidates for actuators, artificial muscles, sensors, smart robots, or other intelligent devices. Herein, we discuss the basic action, control mechanisms, phase transitions, and the structure–property correlation of LCEs; this review provides a comprehensive overview of LCEs for applications in actuators and other smart devices. Furthermore, the synthesis and processing of liquid crystal elastomer are briefly discussed, and the current challenges and future opportunities are prospected. With all recent progress pertaining to material design, sophisticated manipulation, and advanced applications presented, a vision for the application of LCEs in the next generation smart robots or automatic action systems is outlined.

Stimulus-Responsive Graphene with Periodical Wrinkles on Grooved Microfiber Arrays: Simulation, Programmable Shape-Shifting, and Catalytic Applications.

This work demonstrates a facile fabrication of stimulus-responsive, periodically wrinkled graphene sheets on grooved microfiber arrays with fast and reversible shape change, multiresponsiveness, and programmable deformation, with the aid of finite element analysis (FEA). The cellulose acetate (CA) microfibers, endowing responsiveness to humidity and solvents, are designed to grooved shape and assembled into a well-aligned fibrous mat by electrospinning. Under the guidance of FEA simulation, the stiff reduced graphene oxide (RGO) sheets, serving as a photoresponsive component, could ably bind on grooved CA microfibers with favorable interlocked interfacial-structure. Through simple direct-writing and hot-pressing, the grooved CA arrays interlocked the conformal RGO sheets by water-induced self-clamping, and enabled the generation of periodic wrinkles within RGO sheets to maximize interfacial areas. By simply adjusting the orientation of written RGO patterns relative to uniaxial CA microfibers, programmed and omnidirectional shape-shifting were obtained to minimize strain energy, consisting with the dynamic deformation process simulated by FEA. Upon remote light or contactless humidity stimuli, the RGO/CA mat shows a rapid response (≤1 s), large amplitude (angle change ≥150°, 1.62 cm-1), sophisticated 3D motions, and lifts objects that weigh 12.7-times its own weight up to over 1/3 of own height within 1 s. After loading catalytical nanoparticles, the RGO/CA mat could rapidly move to the targeted position by continuous crawling even on a slippery surface, and served as a microchannel reactor to trigger a reaction in built-in microchannels with suppressing catalyst leaching while accelerating reaction kinetics by both nanoconfinement and photothermal effect.

Computational design of 3D-printed active lattice structures for reversible shape morphing

Active structures can adapt to varying environmental conditions and functional requirements by changing their shapes and properties, which makes them suitable for applications in changing environments as found in aerospace and automotive. Of special interest are light and stiff structures with shape morphing capabilities, which is naturally contradictory. Existing concepts in literature can be limited to a single, non-reversible actuation and are difficult to design due to the inherent complexity of large-scale lattices with many elements and complex target deformations. Here, we show how 3D-printed active materials can be combined with an efficient computational framework to design large-scale lattice structures that can change their shape between an initial state and a target state. The reversible deformation is controlled by a single actuation input and heating of the structure. Numerical and experimental results show the generality of the proposed method and the applicability to different problems such as morphing airfoils.Graphic

Liquid crystal elastomer shells with topological defect-defined actuation: Complex shape morphing, opening/closing, and unidirectional rotation

We produce hollow sphere liquid crystal elastomer (LCE) actuators from a nematic precursor mixture, brought into the shape of a self-closing shell with tangential anchoring of the director field n(r), using a solvent-assisted microfluidic technique. By separating the shell production from the polymerization and cross-linking, the precursor is allowed to approach its equilibrium n(r) configuration in the shell, spontaneously forming topological defects of total strength +2. However, the photopolymerization into an LCE induces a brief but strong distortion of the overall n(r) and the defect configuration, even changing the ground state shape in the case of thick shells. The resulting LCE shells show a rich capacity for reversible shape morphing upon heating and cooling, the exact actuation mode defined by n(r), and the final defect configuration stabilized at the end of polymerization. In regions with a single +1 defect, a reversal of curvature from concave to convex is found, punctured shells exhibit a strong shape change between a nearly closed sphere at low temperature and an open-ended spherocylinder at high temperature, and all shells rotate upon actuation when suspended in a fluid. As the rotation is stronger during relaxation than during actuation, thus breaking the symmetry, the net rotation is unidirectional.