Reversible Shape Research Articles

Reverse Shape Memory Effect and Toughness Recovery of Ti-10V-2Fe-3Al Alloy

Reverse Shape Memory Effect and Toughness Recovery of Ti-10V-2Fe-3Al Alloy

Reversible Emission Color, Brightness, and Shape Changes of AIEgen‐containing Bulk Polymers

AbstractBulk polymers generally exhibit faster response, higher shape‐deformation speed, and stronger mechanical properties than the hydrogels. However, it still remains a formidable challenge to enable bulk polymers to exhibit reversible changes in shape and fluorescence simultaneously. Herein, for the first time, the temperature‐responsive reversible changes in fluorescence color, brightness, and shape of semi‐crystalline poly(ε‐caprolactone) (PCL) network doped with a D‐A based aggregation‐induced emission luminogen (AIEgen) is realized. The fabricated flower‐like and dragonfly‐shaped actuators show reversible blooming‐closing and wing fanning behaviors, respectively, as well as reversible fluorescence‐changing effects. Notably, the response and shape‐deformation speed as well as mechanical properties of the system is much better than the reported hydrogels. Moreover, a smart switch is developed to demonstrate the potential applications. The stick fabricated by the sample can lift and release an object 1000 times larger than its weight, serving as a color‐changing artificial muscle. This material design strategy may shine light on the design and preparation of other reversible shape‐deformation polymers in combination with stimuli‐responsive luminescence‐changing AIEgens, such as mechanochromic, thermochromic, and photochromic luminogens.

Azobenzene-containing ionic metathesis copolymers: Composition, self-assembly and photoresponsive property

The properties of synthetic polymers are influenced by the connection mode of polymer chains even polymers made from the same monomer. In this work, the diverse composition systems of cyclooctene-based alternating, random and block azo-copolymers containing imidazolium cation units as side group were successfully achieved based on different metathesis methods in the ionic liquid with good control over the polymerization processes. All these ionic azo-copolymers were tailored to explore the effects of monomers sequences and non-covalent interactions on photoresponsive behaviors and self-assembly morphologies. Since the special alternating structure, UV-light-triggered the shackling trans→cis photoisomerization transition was observed, ultimately spontaneously forming highly regular and uniform polymer vesicle with recoverable size variation in THF directly. Differently, the micellar characterization was carried out by the self-assembly of ionic block azo-copolymers in a selective solvent of toluene to obtain spherical micelle with reversible shape evolution considering nearly 86 % trans→cis relative isomerization efficiency. These interesting results not only demonstrate the diversity of metathesis polymerization but also expand the property of azo-copolymers.

Tailoring the stress-free two-way shape memory effect in sol–gel crosslinked poly(ϵ-caprolactone)-based semicrystalline networks

Two-way shape memory polymers are stimulus-responsive materials capable of changing their shape between two configurations based on an on/off thermal stimulus. While the traditional effect has been studied under the application of an external mechanical load, it was demonstrated also in the absence of an external load. Such a response only relies on a carefully tailored macromolecular architecture of the polymer combined with a specific thermo-mechanical protocol. In particular, semicrystalline networks, either consisting of a multi-phase copolymer network or a homopolymer based network with broad phase transitions, have been proposed to this aim under ad hoc thermo-mechanical histories. In this work, the two-way shape memory behavior is studied on a poly(ϵ-caprolactone)-based network, crosslinked by means of a sol–gel approach and tailored on the selection of the molecular weight of the precursor polymer. Changing the prepolymer precursor allowed to tune the melting/crystallization regions of the networks, thus the thermal region of the reversible shape memory effect. The application of properly designed thermo-mechanical cycles allowed to study the two-way shape memory effect without the application of an external load under tensile conditions. Given a specific network, the stress-free actuation of the reversible elongation-contraction cycle under tensile conditions was induced across its specific melting/crystallization region. The extent of the effect was found to depend on the crystalline fraction remaining for the given actuation temperature and on the tensile stretched state imposed on the materials during the training step. The results were compared with the response achieved under the traditional two-way shape memory protocol under stress. The stress-free two-way shape memory effect was also successfully demonstrated and emphasized, under flexural conditions, which suggests the potential of these materials as intrinsically reversible actuators, promising for applications in the biomedical field and/or for soft robotics.

Detection and comparison of reversible shape transformations in responsive polymers using deep learning and knowledge transfer by identifying stimulus-triggering characteristic points

Responsive polymers can alter their properties or shape according to stimuli such as stress, light, among others. The stimuli may be strategically programmed at assigned triggering points in a structure using additive manufacturing, which enables controlled deformation that is crucial for reversible shape transformation. Understanding and controlling shape transformation holds significance in topology optimization, or generative design, among other applications, where a material may not be reduced by machining; it can instead be reversibly transformed to perform evolving functions. However, handling responsive polymers can be challenging, as they react to various stimuli. The present research introduces a contactless technique using deep classification to detect and measure shape changes in responsive polymers. The application of deep network in responsive structural element (characteristic points) detection has not been attempted before. The classification categorizes polymers according to the degree of shape change, which aids in the verification of selectively programmed stimulus-responsive properties. By correlating the output with trained data, accurate classifications were made regarding the degree of shape change. The shape changes were then compared with different deformation angles, lengths, or heights through a novel formula-based correlation. The classification helps reconfigure the material if the deformation is not within the desired range. The classification and comparison were performed with AlexNet, which possesses a simple architecture. An accuracy of 100% was achieved by fine-tuning the hyperparameters of the same network. The present work thus shows the possibility of combining multiple tasks more efficiently with less data and computational resources using simple network.

Shape of AOT Reverse Micelles: The Mesoscopic Assembly Is More Than the Sum of the Parts.

AOT reverse micelles are a common and convenient model system for studying the effects of nanoconfinement on aqueous solutions. The reverse micelle shape is important to understanding how the constituent components come together to form the coherent whole and the unique properties observed there. The shape of reverse micelles impacts the amount of interface present and the distance of the solute from the interface and is therefore vital to understanding interfacial properties and the behavior of solutes in the polar core. In this work, we use previously introduced measures of shape, the coordinate-pair eccentricity (CPE) and convexity, and apply them to a series of simulations of AOT reverse micelles. We simulate the most commonly used force field for AOT reverse micelles, the CHARMM force field, but we also adapt the OPLS force field for use with AOT, the first work to do so, in addition to using both 3- and 4-site water models. Altogether, these simulations are designed to examine the impact of the force field on the shape of the reverse micelles in detail. We also study the time autocorrelation of shape, the water rotational anisotropy decay, and how the CPE changes between the water pool and AOT tail groups. We find that although the force field changes the shape noticeably, AOT reverse micelles are always amorphous particles. The shape of the micelles changes on the order of 10 ns. The water rotational dynamics observed match the experiment and demonstrate slower dynamics relative to bulk water, suggesting a two-population model that fits a core/shell hypothesis. Taken together, our results indicate that it is likely not possible to create a perfect force field that can reproduce every aspect of the AOT reverse micelle accurately. However, the magnitude of the differences between simulations appears relatively small, suggesting that any reasonably derived force field should provide an acceptable model for most work on AOT reverse micelles.

Mechanics of magnetic-shape memory polymers

Magnetic-shape memory polymers (M-SMPs) can not only undergo rapid and reversible deformation in response to magnetic actuation but also lock the actuated shape upon cooling, which has great potential in applications such as soft robotics, active metamaterials, and shape-morphing systems. In this work, we develop a constitutive model for M-SMPs with finite deformation. The constitutive model considers the Helmholtz free energy contributed by the thermally responsive shape memory polymers and the magnetically responsive particles, leading to a magneto-thermomechanical framework. It is shown that the developed model can capture the thermomechanical as well as magneto-elastic responses of M-SMPs at different temperatures. Simplified beam models for M-SMPs are presented to show the material's versatile functionalities including fast and reversible deformation, selective/sequential actuation, and shape locking. We envision that the constitutive framework and the simplified beam models presented in this work can serve as useful tools to guide the rational design of M-SMP-based functional structures and devices.

Smart polymer composites with vertically oriented boron nitride and carbon fiber for heat management: Magneto-thermal responsiveness

Emerging structural materials for aerospace deformable structures, soft robotics, and advanced smart electronic devices necessitate reversible, lockable, and reprogrammable shape transformation capabilities. A challenge in this realm is heat accumulation in high-power density devices, which limited impedes broader applications of these materials. This study introduces magneto-thermal coupling response and efficient heat dissipation to structural composite, facilitating reversible and reprogrammable material deformation. Our composites prepared using bi-directional freeze casting strategy demonstrated excellent thermal management capabilities. When the composites were applied to the chip heat dissipation, the chip temperature was significantly reduced by 36.3 °C from 112.2 °C. Furthermore, the integration of a flexible boron nitride nanosheets (BNNS)/carbon fibers (CFs) network with magnetic Fe3O4 particles and phase-change material enables effective magneto-thermal response. This magnetically controlled thermal deformation behavior of composites provides a promising avenue for the development of multimodal morphing structures for soft robotics and aerospace applications.

Reversible shape memory behavior of knitting-fabric reinforced polymer matrix composites

Reversible self-shaping behavior has been occurred with chemically crosslinked polymers and multi-layer structures while this behavior is difficultly realized for the fabric reinforced polymeric composites. In this work, a thin weft plain knitted fabric reinforced shape memory polymer composites (SMPCs) with reversible shape memory effect were developed. The effect of sizes, knitting densities and actuated temperatures on reversible shape memory behavior of SMPC was investigated. The results show that the maximum self-bending curvature of SMPCs is 35.8 m-1 and the shape recovery ratio reaches 82.2 %. A simplified finite element model was established to simulate and analyze the process of self-bending with heating and self-recovery with cooling of SMPC. Interestingly, the SMPC in wale direction (90°) has the inverse self-bending curvature of −18.4 m-1 as compared with that of SMPC in course direction (0°). The complex double curvature self-bending behavior of SMPC was also demonstrated. This work helps to broaden the development and research of reversible shape memory composites.

93‐1: Invited Paper: Meta‐Elastomer for Biaxially Stretchable Displays Without Image Distortion

Stretchable displays are garnering attention due to their ability to undergo reversible shape deformation. However, due to Poisson's ratio, image distortion issues caused by undesired substrate deformations during stretching remain challenging. Here, we propose meta‐elastomers with biaxially zero Poisson's ratio for distortion‐free stretchable displays. To program the Poisson's ratio according to biaxial stretching, elastomeric mechanical metamaterial (MM) frames with a negative Poisson's ratio are integrated into an elastomer matrix. By leveraging competitive interactions between the elastomeric MM frame and elastomer matrix deformation, we have successfully implemented the biaxial near‐zero Poisson's ratio substrates.

4D Printing for Biomedical Applications.

4D (bio-)printing endows 3D printed (bio-)materials with multiple functionalities and dynamic properties. 4D printed materials have been recently used in biomedical engineering for the design and fabrication of biomedical devices, such as stents, occluders, microneedles, smart 3D-cell engineered microenvironments, drug delivery systems, wound closures, and implantable medical devices. However, the success of 4D printing relies on the rational design of 4D printed objects, the selection of smart materials, and the availability of appropriate types of external (multi-)stimuli. Here, this work first highlights the different types of smart materials, external stimuli, and design strategies used in 4D (bio-)printing. Then, it presents a critical review of the biomedical applications of 4D printing and discusses the future directions of biomedical research in this exciting area, including in vivo tissue regeneration studies, the implementation of multiple materials with reversible shape memory behaviors, the creation of fast shape-transformation responses, the ability to operate at the microscale, untethered activation and control, and the application of (machine learning-based) modeling approaches to predict the structure-property and design-shape transformation relationships of 4D (bio)printed constructs.

Study on the Dynamic Relationship between Education Investment Policies and Income Distribution

The core objective of this study is to deeply explore the complex interactive relationship between education investment policies and income distribution. The aim is to comprehensively analyze the specific mechanism of education investment in shaping the income distribution pattern, and to examine in detail how income distribution inequality issues feedback and affect the design, implementation, and effectiveness of education investment policies. To achieve this goal, the study will systematically review relevant theoretical frameworks and empirical research results, aiming to reveal the deep logic of mutual influence and causality between the two. The research will focus on the impact path of education investment on income distribution, including but not limited to: improving individual human capital, enhancing employment competitiveness, and thereby affecting individual income; By improving the overall education level of society, promoting the upgrading of industrial structure, indirectly affecting the overall income distribution structure; And the impact of the differential distribution of education investment between urban and rural areas and regions on income inequality. We will conduct a detailed analysis of how changes in education investment in dimensions such as scale, structure, and efficiency can be transformed into reshaping forces on income distribution patterns. We will delve into the reverse shaping effect of income inequality on education investment policies and propose targeted policy recommendations.

Programmable, Self-Healable, and Photochromic Liquid Crystal Elastomers Based on Dynamic Hindered Urea Bonds for Biomimetic Flowers.

Recently, researchers have been exploring the use of dynamic covalent bonds (DCBs) in the construction of exchangeable liquid crystal elastomers (LCEs) for biomimetic actuators and devices. However, a significant challenge remains in achieving LCEs with both excellent dynamic properties and superior mechanical strength and stability. In this study, a diacrylate-functionalized monomer containing dynamic hindered urea bonds (DA-HUB) is employed to prepare exchangeable LCEs through a self-catalytic Michael addition reaction. By incorporating DA-HUB, the LCE system benefits from DCBs and hydrogen bonding, leading to materials with high mechanical strength and a range of dynamic properties such as programmability, self-healing, and recyclability. Leveraging these characteristics, bilayer LCE actuators with controlled reversible thermal deformation and outstanding dimensional stability are successfully fabricated using a simple welding method. Moreover, a biomimetic triangular plum, inspired by the blooming of flowers, is created to showcase reversible color and shape changes triggered by light and heat. This innovative approach opens new possibilities for the development of biomimetic and smart actuators and devices with multiple functionalities.

Reversible shape transformations through continuous deformations in soft responsive self-repairing solids using voxel techniques

Voxel modeling is a technique in which an object is represented by an array of cubes. One advantage in design-for-manufacturing is that selected voxels may be assigned desired responsive properties through additive manufacturing. It allows for achieving reversible shape transformation in soft responsive self-repairing solids within a strategic sequence. The first step in the sequence involves selecting an equivalent geometric shape. The second step involves mapping shapes to preserve properties during continuous deformation. The third step involves transforming the shape from one form to another. The fourth step involves defining changes in dimensions. The final step involves correlating the flow of stress with the stretch of the solid. This research also considers self-repair or self-healing as part of the transformation process because the splitting or joining of solids involves continuous deformation. The newly transformed shape can also be considered as the intermediate shape, or first shape for next transformation. This leads to a chain of shapes that are topologically equivalent, and interchangeable. The iterative steps involved in shape transformation finds applications in topology optimization, generative design, metastructures, among others. One application of shape transformation in these domains is that it serves as an alternative to material reduction techniques.

Tuning the Properties of Multi-Stable Structures Post-Fabrication Via the Two-Way Shape Memory Polymer Effect.

Multi-stable elements are commonly employed to design reconfigurable and adaptive structures, because they enable large and reversible shape changes in response to changing loads, while simultaneously allowing self-locking capabilities. However, existing multi-stable structures have properties that depend on their initial design and cannot be tailored post-fabrication. Here, a novel design approach is presented that combines multi-stable structures with two-way shape memory polymers. By leveraging both the one-way and two-way shape memory effect under bi-axial strain conditions, the structures can re-program their 3D shape, bear loads, and self-actuate. Results demonstrate that the structures' shape and stiffness can be tuned post-fabrication at the user's need and the multi-stability can be suppressed or activated on command. The control of multi-stability prevents undesired snapping of the structures and enables higher load-bearing capability, compared to conventional multi-stable systems. The proposed approach offers the possibility to augment the functionality of existing multi-stable concepts, showing potential for the realization of highly adaptable mechanical structures that can reversibly switch between being mono and multi-stable and that can undergo shape changes in response to a change in temperature.

Light-Responsive Shape- and Color-Changing Block Copolymer Particles with Fast Switching Speed.

Polymer particles capable of dynamic shape changes in response to light have received substantial attention in the development of intelligent multifunctional materials. In this study, we develop a light-responsive block copolymer (BCP) particle system that exhibits fast and reversible shape and color transitions. The key molecular design is the integration of spiropyran photoacid (SPPA) molecules into the BCP particle system, which enables fast and dynamic transformations of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) particles in response to light. The SPPA photoisomerization, induced by 420 nm light irradiation, lowers the pH of the aqueous surroundings from 5.5 to 3.3. The protonated P4VP block substantially increases in domain size from 14 to 39 nm, resulting in significant elongation of the BCP particles (i.e., an increase in the aspect ratio (AR) of the particles from 1.8 to 3.4). Moreover, SPPA adsorbed onto the P4VP surface induces significant changes in the luminescent properties of the BCP particles via photoisomerization of SPPA. Notably, the BCP particles undergo fast, dynamic shape and color transitions within a period of 10 min, maintaining high reversibility over multiple light exposures. Functional dyes are selectively incorporated into different domains of the light-responsive BCP particles to achieve different ranges of color responses. Thus, this study showcases a light-responsive hydrogel display capable of reversible and multicolor photopatterning.

Solid Wood Modification toward Anisotropic Elastic and Insulative Foam-Like Materials.

The methods used to date to produce compressible wood foam by top-down approaches generally involve the removal of lignin and hemicelluloses. Herein, we introduce a route to convert solid wood into a super elastic and insulative foam-like material. The process uses sequential oxidation and reduction with partial removal of lignin but high hemicellulose retention (process yield of 72.8%), revealing fibril nanostructures from the wood's cell walls. The elasticity of the material is shown to result from a lamellar structure, which provides reversible shape recovery along the transverse direction at compression strains of up to 60% with no significant axial deformation. The compressibility is readily modulated by the oxidation degree, which changes the crystallinity and mobility of the solid phase around the lumina. The performance of the highly resilient foam-like material is also ascribed to the amorphization of cellulosic fibrils, confirmed by experimental and computational (molecular dynamics) methods that highlight the role of secondary interactions. The foam-like wood is optionally hydrophobized by chemical vapor deposition of short-chained organosilanes, which also provides flame retardancy. Overall, we introduce a foam-like material derived from wood based on multifunctional nanostructures (anisotropically compressible, thermally insulative, hydrophobic, and flame retardant) that are relevant to cushioning, protection, and packaging.

Synchronously Thermochromic and Shape‐Shifting Liquid Crystal Elastomers Enabled by Hierarchical Structure Control for Multidimensional Encryption

AbstractTo date, a diverse array of smart materials, drawing inspiration from nature, is developed, exhibiting remarkable shape or color‐changing capabilities, which hold great promise in applications such as bionic soft robots and information encryption platforms. Achieving reversible shape‐shifting and color variation harmoniously within a single material is a particularly appealing prospect. Herein, a facile approach to creating such materials is presented, capable of undergoing reversible shape and color‐change synchronously. This is achieved by harnessing the stress‐free two‐way shape‐memory effect (2W‐SME) of a liquid crystal elastomer (LCE), meticulously programmed at both macro and micro scales. Specifically, the LCE with stress‐free 2W‐SME is fabricated, and permanent grating nanostructures are meticulously constructed through UV‐assisted nanoimprinting. By enabling reversible shape alterations at multiple dimensions, encompassing both macro and micro scales, the LCE material concurrently exhibits reversible changes in shape and color, triggered by heat. As a result, intelligent information encryption and decryption platforms that leverage the unique capabilities of LCE are demonstrated. The outcomes of this research endeavor are poised to pave the way for the advancement of smart materials in the realms of data storage and encryption.

Room-Temperature Anisotropic Actuation Driven by a Synergistic Order-Disorder and Displacive Phase Transition in a Ferroelectric Crystal.

Actuating materials convert different forms of energy into mechanical responses. To satisfy various application scenarios, they are desired to have rich categories, novel functionalities, clear structure-property relationships, fast responses, and, in particular, giant and reversible shape changes. Herein, we report a phase transition-driven ferroelectric crystal, (rac-3-HOPD)PbI3 (3-HOPD = 3-hydroxypiperidine cation), showing intriguingly large and anisotropic room-temperature actuating behaviors. The crystal consists of rigid one-dimensional [PbI3] anionic chains running along the a-axis and discrete disk-like cations loosely wrapping around the chains, leaving room for anisotropic shape changes in both the b- and c-axes. The shape change is switched by a ferroelectric phase transition occurring at around room temperature (294 K), driven by the exceptionally synergistic order-disorder and displacive phase transition. The rotation of the cations exerts internal pressure on the stacking structure to trigger an exceptionally large displacement of the inorganic chains, corresponding to a crystal lattice transformation with length changes of +24.6% and -17.5% along the b- and c-axis, respectively. Single crystal-based prototype devices of circuit switches and elevators have been fabricated by exploiting the unconventional negative temperature-dependent actuating behaviors. This work provides a new model for the development of multifunctional mechanically responsive materials.

Domain-Selective Enzymatic Cross-linking and Etching for Shape-Morphing DNA-Linked Nanoparticle Films.

The highly programmable and responsive molecular recognition properties of DNA provide unparalleled opportunities for fabricating dynamic nanostructures capable of structural transformation in response to various external stimuli. However, they typically operate in tightly controlled environments because certain conditions (ionic strength, pH, temperature, etc.) must be met for DNA duplex formation. In this study, we adopted site-specific enzymatic ligation and DNA-based layer-by-layer thin film fabrication to build shape-morphing DNA-linked nanoparticle films operational in a broad range of environments. The ligated films remained intact in unusual conditions such as pure water and high temperature causing dissociation of DNA duplexes and showed predictable and reversible shape morphing in response to various environmental changes and DNA exchange reactions. Furthermore, domain-selective ligation combined with photoinduced interlayer mixing allowed for the fabrication of unusual edge-sealed double-layered films through midlayer etching, which is difficult to realize by other methods.