Reversible Shape Change Research Articles

UV-mediated synthesis of pNIPAM-crosslinked double-network alginate hydrogels: Enhanced mechanical and shape-memory properties by metal ions and temperature

Allyl-substituted alginate (Alg) underwent gelation with N-isopropylacrylamide (NIPAM) monomers via UV-mediated radical reactions, resulting in a multi-responsive double-network hydrogel. The amount of NIPAM available for crosslinking affected several properties of the resulting Alg-NIPAM hydrogels. As more NIPAM was incorporated, the volume phase transition temperature (VPTT) and swelling properties of the Alg-NIPAM hydrogels decreased. Metal-ion addition caused increased crosslinking of the Alg-NIPAM hydrogels, allowing shape-memory of the materials due to their dual responsiveness to temperature and metal ions. Additional crosslinking caused the hydrogels to become stiffer, showing the ability to regulate the mechanical properties of the hydrogels. As a proof-of-concept, reversible shape changes under temperature and ionic cycling were demonstrated.

Phototriggered Selective Actuation and Self‐Oscillating in Dual‐Phase Liquid Crystal Photonic Actuators

AbstractAmong different kinds of photoactuators available, those based on liquid‐crystal polymer networks (LCNs) are gathering a great deal of attention due to their fast response and versatility of structural design. However, only a few reports have focused on the phototriggered LCN actuators for complex actuation and multifunction properties. Here, a phototriggered LCN‐based photonic actuator with quasi‐bilayer structure that exhibits fast and reversible shape changes is created by infiltrating a LCN precursor–graphene oxide mixture into a silica opal template, ultraviolet photopolymerization, and removing the template. Particularly, the phototriggered selective actuation behavior is achieved in dual‐phase LCN‐based photonic actuators with macroscopically alternating nematic (N) and isotropic (I) phase, which enables versatile actuation modes and allows multiple shape changes. In addition, a two‐segmented, dual‐phase LCN‐based photonic actuator exhibits self‐oscillating motion when a responsive N‐phase region as localized active point is irradiated, which arises from the self‐shadowing of the passive I‐phase region. To demonstrate the potential application of this self‐oscillating motion behavior, a photoelectric energy conversion device that is capable of converting light energy to electric energy is constructed.

Structured Optical Materials Controlled by Light

AbstractMaterials whose optical response is determined by their structure are of much interest both for their fundamental properties and applications. Examples range from simple gratings to photonic crystals. Obtaining control over the optical properties is of crucial importance in this context, and it is often attempted by electro‐optical effect or by using magnetic fields. In this paper, the use of light is introduced to switch and tune the optical response of a structured material, exploiting a physical deformation induced by light itself. In this new strategy, light drives an elastic reshaping, which leads hence to a change in the optical response. This is made possible by using liquid crystalline networks structured by direct laser writing. As a proof of concept, a grating structure with sub‐millisecond time response is demonstrated for optical beam steering, exploiting an optically induced reversible shape change. Experimental observations are combined with finite‐element modeling in order to understand the actuation process dynamics, and to obtain information on how to tune the time and the power response of this technology. This optical beam steerer opens up full optical control of light in a broad range of structured optical materials.

Moisture-triggered actuator and detector with high-performance: interface engineering of graphene oxide/ethyl cellulose

Actuators that can directly convert other forms of environmental energy into mechanical work offer great application prospects in intriguing energy applications and smart devices. But to-date, low cohesion strength of the interface and humidity responsive actuators primarily limit their applications. Herein, by experimentally optimizing interface of bimorph structure, we build graphene oxide/ethyl cellulose bidirectional bending actuators --- a case of bimorphs with fast and reversible shape changes in response to environmental humidity gradients. Meanwhile, we employ the actuator as the engine to drive piezoelectric detector. In this case, graphene oxide and ethyl cellulose are combined with chemical bonds, successfully building a bimorph with binary synergy strengthening and toughening. The excellent hygroscopicity of graphene oxide accompanied with huge volume expansion triggers giant moisture responsiveness greater than 90 degrees. Moreover, the open circuit voltage of piezoelectric detector holds a peak value around 0.1 V and exhibits excellent reversibility. We anticipate that humidity-responsive actuator and detector hold promise for the application and expansion of smart devices in varieties of multifunctional nanosystems.

Stimuli-responsive and on-chip nanomembrane micro-rolls for enhanced macroscopic visual hydrogen detection.

Nanomembrane rolling offers advanced three-dimensional (3D) mesostructures in electronics, optics, and biomedical applications. We demonstrate a high-density and on-chip array of rolled-up nanomembrane actuators with stimuli-responsive function based on the volume expansion of palladium in hydrogen milieu. The uniform stimuli-responsive behavior of high-density nanomembrane rolls leads to huge macroscopic visual detection with more than 50% transmittance change under optimization of micropattern design. The reversible shape changing between rolled and flat (unrolled) statuses can be well explained on the basis of the elastic mechanical model. The strain change in the palladium layer during hydrogen absorption and desorption produces a marked change in the diameter of nanomembrane rolls. We found that a functional palladium layer established an external compressive strain after hydrogen stimuli and thus also reduced the rolls' diameters. The large area of the nanomembrane roll array performs excellent nonelectrical hydrogen detection, with response and recovery speeds within seconds. Our work suggests a new strategy to integrate high-density 3D mesoscale architectures into functional devices and systems.

Hygroscopically actuated wood elements for weather responsive and self-forming building parts – Facilitating upscaling and complex shape changes

For the performance of wood as a building material, its dimensional changes in response to alterations of relative humidity are commonly perceived as an adverse effect. Recently, this material inherent property has been proposed to be utilized in a smart way. Employing the bilayer principle, controlled and reversible shape changes in response to changes of relative humidity were demonstrated. Wood naturally inherits a unique combination of material properties specifically suitable for large-scale shape-changing parts. While being environmentally responsive, it offers high mechanical stiffness throughout shape-change, ease of machining and working, and sustainable availability in large sizes and quantities. In this study, we demonstrate design principles for achieving a range of shape changing patterns such as uni- and bi-directional surface curvature of wood and wood-hybrid bilayers with both negative (hyperboloid curvature) and positive Gaussian curvature (spherical curvature). In parallel, we have developed suitable joints to join multiple elements to facilitate upscaling in length and width while maintaining shape-change. The ability to design and control the type and magnitude of curvature for specific sizes, shapes, and aspect ratios open the opportunity for a new class of large-scale weather responsive elements and self-forming building components.

Light‐Driven Shape‐Memory Porous Films with Precisely Controlled Dimensions

AbstractShape‐memory polymers (SMPs) are an intriguing class of smart materials possessing reversible shape change and recovery capabilities. Effective routes to shape‐memory porous films (SMPFs) are few and limited in scope owing to the difficulty in manipulating the shape change of pores by conventional methods. Herein we report an unconventional strategy for crafting light‐driven SMPFs by judiciously constructing highly ordered porous films via a facile “breath figure” approach, followed by sequential vapor crosslinking and nondestructive directional light manipulation. Micropores can thus be transformed into other shapes including rectangle, rhombus and size‐reduced micropores at room temperature. The transformed micropores can be reverted to their original shapes by either thermal annealing or UV irradiation. As such, this strategy expands the rich diversity of SMPs accessible.

Light-Driven Shape-Memory Porous Films with Precisely Controlled Dimensions.

Shape-memory polymers (SMPs) are an intriguing class of smart materials possessing reversible shape change and recovery capabilities. Effective routes to shape-memory porous films (SMPFs) are few and limited in scope owing to the difficulty in manipulating the shape change of pores by conventional methods. Herein we report an unconventional strategy for crafting light-driven SMPFs by judiciously constructing highly ordered porous films via a facile "breath figure" approach, followed by sequential vapor crosslinking and nondestructive directional light manipulation. Micropores can thus be transformed into other shapes including rectangle, rhombus and size-reduced micropores at room temperature. The transformed micropores can be reverted to their original shapes by either thermal annealing or UV irradiation. As such, this strategy expands the rich diversity of SMPs accessible.

3D Printing of Liquid Crystal Elastomeric Actuators with Spatially Programed Nematic Order.

Liquid crystal elastomers (LCEs) are soft materials capable of large, reversible shape changes, which may find potential application as artificial muscles, soft robots, and dynamic functional architectures. Here, the design and additive manufacturing of LCE actuators (LCEAs) with spatially programed nematic order that exhibit large, reversible, and repeatable contraction with high specific work capacity are reported. First, a photopolymerizable, solvent-free, main-chain LCE ink is created via aza-Michael addition with the appropriate viscoelastic properties for 3D printing. Next, high operating temperature direct ink writing of LCE inks is used to align their mesogen domains along the direction of the print path. To demonstrate the power of this additive manufacturing approach, shape-morphing LCEA architectures are fabricated, which undergo reversible planar-to-3D and 3D-to-3D' transformations on demand, that can lift significantly more weight than other LCEAs reported todate.

Reversible Semicrystalline Polymer as Actuators Driven by Organic Solvent Vapor.

A semicrystalline polymer actuator, which is responsive to solvent vapor with fast and large scale locomotion, is described. The thermoset semicrystalline polymer can be easily synthesized from crystallizable polyester segment poly (ε-caprolactone) and isophorone diisocyanate trimer. Organic solvent vapor is used to induce the reversible swelling-crystallization conversion of the crystallizable polyester segment, resulting in its expansion/shrinkage. The contraction of the polymer actuator (1 mm thick) needs only ≈4 s in room temperature. When exposed to air the polymer actuator can exhibit a fast self-oscillation. Then, a soft crawler based on this polymer is demonstrated. Driven by organic solvent it walks rapidly and steadily. The microscope images show the fast swelling-crystallization conversion that gives rise to reversible shape changes of the polymer.

Review of Progress in Shape Memory Epoxies and Their Composites.

Shape memory polymer (SMP) is capable of memorizing one or more temporary shapes and recovering successively to the permanent shape upon various external stimuli. Beside of the above mentioned one-way variants, also two-way shape memory polymers (SMPs) and shape memory (SM) systems exist which feature a reversible shape change on the basis of “on-off switching” of the external stimulus. The preparation, properties and modelling of shape memory epoxy resins (SMEP), SMEP foams and composites have been surveyed in this exhaustive review article. The underlying mechanisms and characteristics of SM were introduced. Emphasis was put to show new strategies on how to tailor the network architecture and morphology of EPs to improve their SM performance. To produce SMEPs novel preparation techniques, such as electrospinning, ink printing, solid-state foaming, were tried. The potential of SMEPs and related systems as multifunctional materials has been underlined. Added functionality may include, among others, self-healing, sensing, actuation, porosity control, recycling. Recent developments in the modelling of SMEPs were also highlighted. Based on the recent developments some open topics were deduced which are merit of investigations in future works.

4D Printed Actuators with Soft-Robotic Functions.

Soft matter elements undergoing programed, reversible shape change can contribute to fundamental advance in areas such as optics, medicine, microfluidics, and robotics. Crosslinked liquid crystalline polymers have demonstrated huge potential to implement soft responsive elements; however, the complexity and size of the actuators are limited by the current dominant thin-film geometry processing toolbox. Using 3D printing, stimuli-responsive liquid crystalline elastomeric structures are created here. The printing process prescribes a reversible shape-morphing behavior, offering a new paradigm for active polymer system preparation. The additive character of this technology also leads to unprecedented geometries, complex functions, and sizes beyond those of typical thin-films. The fundamental concepts and devices presented therefore overcome the current limitations of actuation energy available from thin-films, thereby narrowing the gap between materials and practical applications.

Synthesis of Elastomeric Liquid Crystalline Polymer Networks via Chain Transfer.

Materials capable of complex shape changes have broad reaching applications spanning biomimetic devices, componentless actuators, artificial muscles, and haptic displays. Liquid crystal elastomers (LCE) are a class of shape programmable materials which display anisotropic mechanical deformations in response external stimuli. This work details a synthetic strategy to quickly and efficiently prepare LCEs through the usage of chain transfer agents (CTA). The polyacrylate materials described herein exhibit large, reversible shape changes with strains greater 475%, rivalling properties observed in polysiloxane-based networks. The approach reported here is distinguished in that the materials chemistry is readily amenable to surface alignment techniques. The facile nature of the materials chemistry and the compatibility of these materials with directed self-assembly methods could further enable paradigm shifting end uses as designer substrates for flexible electronics or as actuating surfaces.

Single‐Molecule Observation of the Photoregulated Conformational Dynamics of DNA Origami Nanoscissors

AbstractWe demonstrate direct observation of the dynamic opening and closing behavior of photocontrollable DNA origami nanoscissors using high‐speed atomic force microscopy (HS‐AFM). First the conformational change between the open and closed state controlled by adjustment of surrounding salt concentration could be directly observed during AFM scanning. Then light‐responsive moieties were incorporated into the nanoscissors to control these structural changes by photoirradiation. Using photoswitchable DNA strands, we created a photoresponsive nanoscissors variant and were able to distinguish between the open and closed conformations after respective irradiation with ultraviolet (UV) and visible (Vis) light by gel electrophoresis and AFM imaging. Additionally, these reversible changes in shape during photoirradiation were directly visualized using HS‐AFM. Moreover, four photoswitchable nanoscissors were assembled into a scissor–actuator‐like higher‐order object, the configuration of which could be controlled by the open and closed switching induced by irradiation with UV and Vis light.

Single-Molecule Observation of the Photoregulated Conformational Dynamics of DNA Origami Nanoscissors.

We demonstrate direct observation of the dynamic opening and closing behavior of photocontrollable DNA origami nanoscissors using high-speed atomic force microscopy (HS-AFM). First the conformational change between the open and closed state controlled by adjustment of surrounding salt concentration could be directly observed during AFM scanning. Then light-responsive moieties were incorporated into the nanoscissors to control these structural changes by photoirradiation. Using photoswitchable DNA strands, we created a photoresponsive nanoscissors variant and were able to distinguish between the open and closed conformations after respective irradiation with ultraviolet (UV) and visible (Vis) light by gel electrophoresis and AFM imaging. Additionally, these reversible changes in shape during photoirradiation were directly visualized using HS-AFM. Moreover, four photoswitchable nanoscissors were assembled into a scissor-actuator-like higher-order object, the configuration of which could be controlled by the open and closed switching induced by irradiation with UV and Vis light.

Four-dimensional Printing of Liquid Crystal Elastomers.

Three-dimensional structures capable of reversible changes in shape, i.e., four-dimensional-printed structures, may enable new generations of soft robotics, implantable medical devices, and consumer products. Here, thermally responsive liquid crystal elastomers (LCEs) are direct-write printed into 3D structures with a controlled molecular order. Molecular order is locally programmed by controlling the print path used to build the 3D object, and this order controls the stimulus response. Each aligned LCE filament undergoes 40% reversible contraction along the print direction on heating. By printing objects with controlled geometry and stimulus response, magnified shape transformations, for example, volumetric contractions or rapid, repetitive snap-through transitions, are realized.

3D-Printed Self-Folding Electronics.

Self-transforming structures are gaining prominence due to their general ability to adopt programmed shapes each tailored for specific functions. Composites that self-fold have so far relied on using the stimuli-responsive mechanisms focusing on reversible shape change. Integrating additional functions within these composites can rapidly enhance their practical applicability; however, this remains a challenging problem. Here, we demonstrate a method for spontaneous folding of three-dimensional (3D)-printed composites with embedded electronics at room temperature. The composite is printed using a multimaterial 3D-printing process with no external processing steps. Upon peeling from the print platform, the composite self-shapes itself using the residual forces resulting from polymer swelling during the layer-by-layer fabrication process. As a specific example, electrochromic elements are printed within the composite and can be electrically controlled through its folded legs. Our shape-transformation scheme provides a route to transform planar electronics into nonplanar geometries containing the overhangs. Integrating electronics within complex 3D shapes can enable new applications in sensing and robotics.

Interfacial Polyelectrolyte Complex Spinning of Cellulose Nanofibrils for Advanced Bicomponent Fibers.

Fiber spinning of anionic TEMPO-oxidized cellulose (TOCN) nanofibrils with polycations by interfacial polyelectrolyte complexation is demonstrated. The formed fibers were mostly composed of cellulose nanofibrils and the polycations were a minor constituent, leading to yield and ultimate strengths of ca. 100 MPa and ca. 200 MPa, and Young's modulus of ca. 15 GPa. Stretching of the as-formed wet filaments of TOCN/polycation by 20% increased the Young's modulus, yield strength, and ultimate tensile strength by approximately 45, 36, and 26%, respectively. Importantly, feasibility of compartmentalized wound bicomponent fibers by simultaneous spinning of two fibers of different compositions and entwining them together was shown. This possibility was further exploited to demonstrate reversible shape change of a bicomponent fiber directly by humidity change, and indirectly by temperature changes based on thermally dependent humidity absorption. The demonstrated route for TOCN-based fiber preparation is expected to open up new avenues in the application of nanocelluloses in advanced fibrous materials, crimping, and responsive smart textiles.

Reversible Martensitic Transformation under Low Magnetic Fields in Magnetic Shape Memory Alloys

Magnetic field-induced, reversible martensitic transformations in NiCoMnIn meta-magnetic shape memory alloys were studied under constant and varying mechanical loads to understand the role of coupled magneto-mechanical loading on the transformation characteristics and the magnetic field levels required for reversible phase transformations. The samples with two distinct microstructures were tested along the [001] austenite crystallographic direction using a custom designed magneto-thermo-mechanical characterization device while carefully controlling their thermodynamic states through isothermal constant stress and stress-varying magnetic field ramping. Measurements revealed that these meta-magnetic shape memory alloys were capable of generating entropy changes of 14 J kg−1 K−1 or 22 J kg −1 K−1, and corresponding magnetocaloric cooling with reversible shape changes as high as 5.6% under only 1.3 T, or 3 T applied magnetic fields, respectively. Thus, we demonstrate that this alloy is suitable as an active component in near room temperature devices, such as magnetocaloric regenerators, and that the field levels generated by permanent magnets can be sufficient to completely transform the alloy between its martensitic and austenitic states if the loading sequence developed, herein, is employed.

3D printed reversible shape changing soft actuators assisted by liquid crystal elastomers.

In this work, we advance printed active composites by combining 3D printing, printed electronics, and liquid crystal elastomers (LCEs) to achieve soft actuators with free-standing two-way shape changing behaviors. Incorporated LCE strips are activated by Joule heating produced by printed conductive wires, while uniaxial deformation of the LCE strip is utilized as a driving force to achieve bending in the printed composite. The bending behavior of laminated hinges is first characterized in order to obtain a precise control of actuation, which is then exploited to actuate four demonstrative designs: a morphing airplane, a miura-ori structure, a cubic box, and a soft crawler. The soft morphing airplane and miura-ori structure are designed and fabricated with multiple laminated hinges to demonstrate the synergistic actions during actuation. The cubic box is constructed to show the capability of sequential folding by implementing multiple groups of conductive wires to achieve accurately addressable heating with temporal control. Finally, the two-way transformation is utilized as a driving force for the locomotion of a soft crawler stimulated by a periodic rectangular wave current. These examples show the great potential of using the hybrid 3D printing and pick-and-place method and using LCEs to achieve controllable shape change structures for a variety of potential practical applications.