Reversible Shape Research Articles

A novel multi-triggered reversible shape memory thermoplastic polyurethane (TPU)/polycaprolactone (PCL) blend

A novel multi-triggered reversible shape memory thermoplastic polyurethane (TPU)/polycaprolactone (PCL) blend

In Situ Reversible Switchable Wettability Shape Memory Microstructured Functionalized Surfaces with Near-Infrared Light Response

In Situ Reversible Switchable Wettability Shape Memory Microstructured Functionalized Surfaces with Near-Infrared Light Response

Hierarchical network relaxation of a dynamic cross-linked polyolefin elastomer for advanced reversible shape memory effect.

Reversible shape-memory polymers (RSMPs) are highly desired for soft actuators due to the repeatability of deformation. Herein, a polyolefin elastomer vitrimer (POEV) was prepared by constructing a dynamic cross-linked network based on boronic ester bonds. POEV showed varied network relaxation in a wide temperature range due to hierarchical network relaxation, and then the entropy decreased and the relaxation of POEV chains was facilely controlled by temperature. The controllable relaxation of POEV by programming the temperature enabled the actuation domain with a reduction in entropy and the skeleton domain with a relatively high entropy can be built in POEV, greatly affecting the reversible shape memory effects (RSMEs). The topological rearrangement resulted from the activated exchange of dynamic covalent bonds, which enables POEV with good shape reconfigurability, and allows for complicated 3D shapes and shape-shifting on demand. More interestingly, combining the decreasing entropy of POEV chains and fully topological rearrangement tailored by temperature, hybrid aligned carbon nanotubes (CNTs) can be constructed in POEV via a two-stage training. Then, the aligned CNTs can enhance the elasticity and act as a hybrid skeleton for RSMEs, avoiding the negative impact of CNTs on the reversible actuation strain. The hierarchical network relaxation facilitates combining all these unusual properties in one shape memory network synergistically, paving new avenues for realizing smart materials with advanced RSME.

A unified understanding of magnetorheological elastomers for rapid and extreme stiffness tuning

A unified design approach and predictive model are presented to create magnetorheological elastomer (MRE) materials for extreme stiffness tuning, up to 70×, with rapid (∼20 ms) and reversible shape change.

Narrow response temperature range with excellent reversible shape memory effect for semi-crystalline networks as soft actuators.

Complex and controlled reversible actuation inevitably relies on changing thermal fields (direct or indirect) for semi-crystalline reversible shape memory networks. Unfortunately, the non-tunability of thermal signals often brings potential limitations to actuators' applications. In practice, a wide response temperature range (T-range) formed by Thigh and Tlow in the remarkable reversible actuation is an obvious fact. Herein, we demonstrate the tunability of the transition temperatures while stably maintaining excellent actuation abilities. We further verified that the narrow T-range (24 °C) that had not been reported could present more than 17% reversible strain. Special parameter optimization provides opportunities for potential non-implantable biomedical applications. Therefore, based on target 2W-SMP, a vehicle concept with the drug release and vehicle recovery ability was proposed, proving our approach's feasibility.

Bioinspired shape shifting of liquid-infused ribbed sheets

The recent emergence of stimuli-responsive, shape-shifting materials offers promising applications in fields as different as soft robotics, aeronautics, or biomedical engineering. Targeted shapes or movements are achieved from the advantageous coupling between some stimulus and various materials such as liquid crystalline elastomers, magnetically responsive soft materials, swelling hydrogels, etc. However, despite the large variety of strategies, they are strongly material dependent and do not offer the possibility to choose between reversible and irreversible transformations. Here, we introduce a strategy applicable to a wide range of materials yielding systematically reversible or irreversible shape transformations of soft ribbed sheets with precise control over the local curvature. Our approach-inspired by the spore-releasing mechanism of the fern sporangium-relies on the capillary deformation of an architected elastic sheet impregnated by an evaporating liquid. We develop an analytical model combining sheet geometry, material stiffness, and capillary forces to rationalize the onset of such deformations and develop a geometric procedure to inverse program target shapes requiring fine control over the curvature gradient. We finally demonstrate the potential irreversibility of the transformation by UV-curing a photosensitive evaporating solution and show that the obtained shells exhibit enhanced mechanical stiffness.

4D Printing of Supramolecular Liquid Crystal Elastomer Actuators Fueled by Light

AbstractRecent years have seen major advances in the additive manufacturing of stimuli‐responsive materials, also known as “4D printing,” among which liquid crystal elastomers (LCEs) play an important role. However, during fabrication photo‐crosslinking of the LCEs is required, but this step is time‐consuming and efficient polymerization is challenging, especially in the case of light‐responsive materials. In this work, the first light‐fueled supramolecular LCEs suitable for the direct ink writing (DIW) of soft actuators are synthesized in which a photopolymerization step is not needed. With the responsive supramolecular material, 3D‐printed objects are fabricated by exploiting the thermoreversible interplay of the hydrogen‐bonding physical cross‐links. After printing, the supramolecular LCE shows reversible shape changes in response to light and is capable of bending and lifting a load. Through the combined photothermal and photochemical contributions of the incorporated azobenzene, the actuators can be triggered both in air and water. The freedom provided by DIW allows for the printing of complex responsive objects, as demonstrated by fabricating re‐entrant honeycomb and spiral director structures. This approach of printing light‐responsive supramolecular soft actuators opens avenues toward the application of “smart” and sustainable materials for additive manufacturing without the requirement of photo‐crosslinking.

Skin-friendly and antibacterial monodomain liquid crystal elastomer actuator

Monodomain liquid crystal elastomers (mLCEs) are flexible and biocompatible smart materials that show unique behaviors of soft elasticity, anisotropy, and reversible shape changes above the nematic-isotropic transition temperature. Therefore, it has great potential for application in wearable devices and biologically. However, most of the reported mLCEs have nematic-isotropic transition temperature (TNI) higher than 60 °C; and above this TNI, the tensile strength of the mLCEs decreases by orders of magnitude. These issues have received little attention, limiting their application in humans. Herein, the TNI of mLCEs was reduced from 78.4 °C to 23.5 °C by substituting part of the rigid LC mesogens with a flexible backbone. The physical entanglement of hydrogen bonds between molecular chains alleviated the molecular chain slip caused by the long flexible backbone. The tensile strength remained constant during the phase transformation. Furthermore, dynamic disulfide bonds were used to modify the LC polymer network, imparting it with excellent antimicrobial, programmable, and self-healing properties. To realize its application in the closure of skin wounds, a porous PHG–mLCE/hydrogel patch that was breathable and waterproof was designed to increase skin adhesion (262 N/m).

Responses to single and multiple temperature-, medium-, and pH-stimuli triggering reversible shape shifts in hydrogel actuators

Adaptivity is an essential capability of creatures to develop skills for surviving and reproducing. Mimicking such behavior would enable the design of biologically inspired functions. Hydrogel actuators are promising candidate materials capable of stimuli-responsive movements via water uptake and release. Here, for the first time, the adaptive-like movement of reprogrammable hydrogel actuators is demonstrated by integrating two independent functions: a temperature-triggered actuation and a medium- and pH-based modulated swelling, into one multiphase network. Reversible crystallization of poly(ε-caprolactone) (PCL) provided bending movements up to 71° (realized by temperature changes between 5 °C and 40 °C). Changing solvent from water to ethanol increased swellability up to 240 vol%. Bending angles decreased by 32° as some PCL fractions are dissolved in ethanol and cannot participate in the crystallization-induced elongation guiding the movement. When the actuators were modified with acrylic acid moieties, a modulated swelling between 270 vol% at pH = 13 and 130 vol% at pH = 1 was achieved. The resulting swelling/deswelling response induced reversible bending movements up to 54°. In consequence, the coupled functionalities (actuation and modulated swelling) resulted in conditioned actuation behavior as an intermediate feature between stimuli-sensitivity and adaptivity.

Fabrication of controllable and reversible bidirectional shape memory fabric composites based on multiblock copolyimide and their properties

The construction of polymer-based materials with reversible bidirectional shape memory effects and good thermal properties is of paramount importance for the development of the intelligent exploration in high temperature fields. In this work, a series of multiblock shape memory copolyimides were synthesized with the introduction of soft segments containing aliphatic diamines into aromatic polyimide structures to control the mobility of the molecular chains. The number of blocks for copolyimide was finely regulated to optimize the shape memory behaviors with thermal stability and both of the shape fixity ratio and shape recovery ratio could reach higher than 97% with the triggering temperature around 260 °C. Then controllable fabric composites with high temperature and reversible bidirectional shape memory effects were firstly constructed by incorporating of deliberately designed block copolyimide with commercialized polyimide fabric. The flexible fabric composites exhibited variable fixed shapes by regulating the thickness discrepancies between upper and lower layers, which showed good two-way shape memory properties with the shape recovery ratio of 98% during the programming process, providing a new sight of the exploration for high temperature shape-memory materials.

Addressing Thermal Distortion in Additive Manufacturing of Topology Optimized Structures Through Reverse Shape Morphing

Design of light weight structures is an important aspect in the aircraft industry, since minimizing the weightof components improves the overall aircraft performance. However, conventional manufacturing methods work on standard geometries and shapes, and often lead to overdesigning of parts. Additive Manufacturing (AM) overcomes these issues by allowing more design freedom. The present study focuses on two aspects of AM: (1) part consolidation through topology optimization, and (2) addressing thermal distortion through reverse shape morphing. An assembly of two load bearing brackets is first amalgamated into a single Topology Optimized (TO) part, which satisfies the displacement and stress requirements of the original design. After a series of optimization iterations, the final TO part (278 g) weighs 69 % lesser than the original assembled design (909 g), still meeting the design constraints. The TO part thus eliminates the need of fasteners to join both the brackets, thereby, making the design simpler yet effective. Moreover, a homogeneous stress distribution in the optimized part allows for efficient material utilization. In order to overcome thermal distortion that results during the AM process, the shape of the TO part is transformed in a sense opposite to the distortions produced. This is achieved through reverse shape morphing technique, that reduces thermal distortions in the printed part to sub-micron levels, and the morphed TO part conforms to the requirements meeting the design constraints. Therefore, the implementation of topology optimization along with reverse shape morphing makes the design simple and efficient having reduced distortion. This is achieved without any need of modifications in the manufacturing system or equipment, and such a strategy can be replicated and implemented at industrial scale as well.

External stress-free reversible multiple shape memory polymer enabled by using broad melting range as equivalent multiple switching phases

Because the reversible shape memory polymers developed so far can only morph among the original shape and two or less temporary shapes, the corresponding deformations have to be step-like accordingly, which is inappropriate for fine regulation of smart mechanisms like soft robots and actuators. To overcome the drawback, the authors of the present work developed a novel external stress-free reversible multiple shape memory crosslinked polyurethane (PU), in which the crystalline region that possessed a broad melting temperature range served as the equivalent multiple switching phases and the inter-/intra-macromolecular hydrogen bonding networks as the internal stress provider. After the thermal programming under stretching, the crystalline phase of the PU practically memorized quite a few temporary shapes. By sequentially activating partial melting of the crystalline region of the trained PU at different melting temperatures during heating, the fixed temporary shapes successively returned to the original shape. Moreover, the above process can be reversed by initiating partial re-crystallization at different temperatures during cooling. Since the activation and recovery temperatures can be optionally selected, the same programed PU successfully implemented reversible triple, quadruple and quintuple shape memory effects. The simple synthesis for constructing broad melting temperature range via copolymerization of identical semicrystalline polymers with various molecular weights as well as the record-high number of memorized temporary shapes may promote application of reversible shape memory polymers.

Halogen-bonded shape memory polymers

Halogen bonding (XB), a non-covalent interaction between an electron-deficient halogen atom and a Lewis base, is widely adopted in organic synthesis and supramolecular crystal engineering. However, the roadmap towards materials applications is hindered by the challenges in harnessing this relatively weak intermolecular interaction to devise human-commanded stimuli-responsive soft materials. Here, we report a liquid crystalline network comprising permanent covalent crosslinks and dynamic halogen bond crosslinks, which possess reversible thermo-responsive shape memory behaviour. Our findings suggest that I···N halogen bond, a paradigmatic motif in crystal engineering studies, enables temporary shape fixation at room temperature and subsequent shape recovery in response to human body temperature. We demonstrate versatile shape programming of the halogen-bonded polymer networks through human-hand operation and propose a micro-robotic injection model for complex 1D to 3D shape morphing in aqueous media at 37 °C. Through systematic structure-property-performance studies, we show the necessity of the I···N crosslinks in driving the shape memory effect. The halogen-bonded shape memory polymers expand the toolbox for the preparation of smart supramolecular constructs with tailored mechanical properties and thermoresponsive behaviour, for the needs of, e.g., future medical devices.

Novel axial compressive endoprosthesis ACE can enhance metaphyseal fixation and facilitate osseointegration: A biomechanical study.

Background: Metaphyseal fixation for endoprosthetic reconstruction after bone tumor resection is difficult due to the short residual bone length and reverse funnel shape of the metaphysis. In the current study, 3D-printed axial compressive endoprosthesis (ACE) with a short stem and intramedullary axial compressive mechanism is proposed to improve metaphyseal fixation. The rationales of ACE are that 1) intramedullary axial compress enhances the stability of endoprosthesis and facilitates bone ingrowth at the osteotomy site; 2) 3D printed porous metallic surface at osteotomy surface and stem allows bone ingrowth to achieve osseointegration. Methods: A biomechanical study was performed to explore the initial stability using Sawbones. A diaphysis and metaphyseal segmental defect were created and four fixation structures were simulated: 1) ACE; 2) ACE + lateral plate; 3) stem prosthesis + unilateral plate; 4) stem prosthesis + bilateral plates. Bending and torsional stiffness were determined with a material testing machine. The relationship between the torque of the compression nut and the axial compression force of the bone-implant surface was measured using a round gasket load sensor. Results: ACE + lateral plate was the stiffest in the bending test (sagittal 324.3 ± 110.8N/mm, coronal 307.7 ± 8.7N/mm). ACE + lateral plate and stem prosthesis + bilateral plates had the highest torsional stiffness (10.9 ± 1.3Nm/° and 10.7 ± 0.2Nm/° respectively). The bending stiffness of ACE was equivalent to stem prosthesis + bilateral plates (sagittal 196 ± 10N/mm vs. 200 ± 7N/mm, coronal 197 ± 14N/mm vs. 209 ± 3N/mm), but the torsional stiffness of ACE was inferior to stem prosthesis + bilateral plates (6.1 ± 1.3Nm/° vs. 10.7 ± 0.2Nm/°). Stem prosthesis + unilateral plate was the least stiff both in bending and torsion. The relationship between torque (T/Nm) and axial pressure (F/N) was F = 233.5T. Conclusion: The axial compressive design of ACE enhances primary stability and facilitates osseointegration, which provides an alternative option of metaphyseal fixation for endoprosthetic reconstruction.

Sensitively humidity-driven actuator and sensor derived from natural skin system

Precise control over the smart materials exhibiting reversible shape changes in response to environmental stimuli presents a considerable challenge. Here, with a self-assembly strategy by extracting natural materials from pigskin, a single layer bio-inspired, transparent, soft biological film (BF) with the primary characteristics of self-actuation and self-sensing is successfully developed. The self-assembly constructed BF can exchange water and reflect environmental humidity gradients rapidly to activate continuous rotary movement. Temperature which affects the thermal motion of water molecules will induce different orientation movement of the film, and on this basis, a humidity-driven energy transfer motor is developed. More characterizations highlight the behavior mechanism of BF through water exchanging by a hydrogen bonding interaction with the hydrophilic group of amino acids residues on the BF surface. Finally, a wearable, steady and ultrafast-response sensor to detect human breathing, especially for real-time obstructive sleep apnea (OSA) state, is fabricated. This study offers great potential in emerging applications including micro-sensors, switches, soft robots and power source technologies.

Volumetric determination of reverse micelle structural properties and the validity of commonplace approximations

Molecular volumes within reverse micelles are important in determining many aggregate properties. Molecular volumes also depend upon differences in molecular environment and thereby might also be capable of detecting changes in molecular structure and interactions within an aggregate. However, approximations are typically used in their place along with approximations of aggregate radius, aggregation number, surfactant tail length, and reverse micelle shape particularly for Aerosol OT systems. Consequently, volumetric measurements have not been thoroughly tested as a measure of aggregate properties. Volumetric methods do not require the commonly used approximations. Therefore, comparison with previous approaches will demonstrate volumetric measurements’ utility in measuring aggregate structural properties and provide a useful assessment of the applicability of these commonplace approximations. Partial molar volumes of water and surfactant, evaporation rate, and reverse micelle radius are measured over the w0 = 0–40 range for water/Aerosol OT/isooctane solutions. These results are used to calculate how aggregation number, surfactant layer thickness, head group area, and water droplet structure change with aggregate radius. Volumetric measurements reveal a surfactant layer structural transition at w0 = 5, an aggregate shape change at w0 = 25, and reproduced the general trends in structural properties. Important variations from approximated methods are noted. Molecular volume and aggregate radius approximations were found valid in many but not all situations. However, approximations of aggregation number and tail length created important miscalculations of some properties when treating reverse micelles as spherical. It was found that consideration of an ellipsoidal reverse micelle shape for w0 < 10 could account for previously identified changes in surfactant layer thickness and head group area that were based on a spherical geometry.

Multiple/Two-Way Shape Memory Poly(urethane-urea-amide) Elastomers.

Multiple and two-way reversible shape memory polymers (M/2W-SMPs) are highly promising for many fields due to large deformation, lightweight, strong recovery stress, and fast response rates. Herein, a semi-crystalline block poly(urethane-urea-amide) elastomers (PUUAs) are prepared by the copolymerization of isocyanate-terminated polyurethane (OPU) and amino-terminated oligomeric polyamide-1212 (OPA). PUUAs, composed of OPA as stationary phase and PTMEG as reversible phase, exhibit excellent rigidity, flexibility, and resilience, and cPUUA-C7 -S25 exhibits the best tensile property with strength of 10.3MPa and elongation at break of 360.2%. Besides, all the PUUAs possess two crystallization/melting temperatures and a glass transition temperature, which endow PUUAs with multiple and reversible two-way shape memory effect (M/2W-SME). Physically crosslinked PUUA-C0 -S25 exhibits excellent dual and triple shape memory, and micro chemically crosslinked cPUUA-C7 -S25 further shows quadruple shape memory behavior. Additionally, both PUUA-C0 -S25 and cPUUA-C7 -S25 have 2W-SME. Intriguingly, cPUUA-C7 -S25 can achieve a higher temperature (up to 165°C) SME, which makes it suitable for more complex and changeable applications. Based on the advantages of M/2W-SME, a temperature-responsive application scenario where PUUAs can transform spontaneously among different shapes is designed. These unique M/2W-SME and high-temperature SME will enable the applications of high-temperature sensors, actuators, and aerospace equipment.

Magneto-active soft matter with reprogrammable shape-morphing and self-sensing capabilities

Interest in creating soft active matter that can repeatedly undergo shape morphing and self-sensing in response to external stimuli is growing. Magneto-active soft matter (MASM) with untethered, fast, and reversible shape reconfiguration is highly desirable for diverse applications in biomedical devices, wearable devices, soft robotics. In this work, we develop a heat-assisted magnetic reprogramming strategy for fabricating MASMs capable of reprogrammable shape-morphing and self-sensing functions. The magnetic reprogramming strategy is based on heating thermoplastic matrix above its melting point and reorienting soft-magnetic particle chains by applying magnetic fields during cooling. By reprogramming magnetization profiles through particle chain reconstruction of printed architecture, we demonstrate multiple deformation modes with distinct shape-morphing. This magnetic reprogramming approach enabled multiscale and reprogrammable soft machines with the tunable actuation response, such as adaptive grasping of a soft gripper. In addition, in combining with the unique sensing mechanism of triboelectric skin (tribo-skin), the self-sensing performance of MASMs is realized by using electrical signals to identify the deformation and contact behaviors. We anticipate that the magnetic reprogramming strategy and multimaterial 3D printing-assisted technique can open new avenues for the fabrication of multifunctional MASMs.

Programmable Multifunctional Gels with On-Demand Patterning Capability toward Hierarchical and Multi-Dimensional Encryption and Anti-Counterfeiting.

Hydrogels capable of optical switching have recently become one of the most celebrated materials for information encryption and anti-counterfeiting. However, challenges still remain for developing versatile gel-based platforms with on-demand multistage patterning and multi-dimensional encryption capacities as well as long-term stability. Herein, elaborately designed programmable and multifunctional gels with fascinating anti-swelling (swelling ratios < 0.1%), anti-freezing (below -70 °C), and anti-dehydration (over 3 months) abilities, solvent-induced reversible transparence variations, adjustable fluorescence, self-healing (86% in stress and 94% in strain), Fe3+-ethylenediaminetetraacetic acid disodium (EDTA·2Na)-induced reversible shape memory, and fluorescence off/on switch capabilities are facilely fabricated based on glycidyl methacrylate functionalized graphene quantum dots and Al3+ cross-linked gelatin and polyacrylic acid. Employing a simple mask photopolymerization or welding technique, various patterns can be readily and hierarchically encrypted on-demand into a single gel label, which can be further fixed into complex multi-dimensional architectures while quenching fluorescence after the treatment with Fe3+ to achieve high-security-level information encryption originating from the synergistic effects of the above multifunctions. The encrypted multi-level information can only be stepwise decrypted by an authorized individual who has mastered all decryption keys. Therefore, the creative design strategy for programing multifunctional gels opens up the possibility for hierarchical and multi-dimensional information encryption and anti-counterfeiting.

Adaptive reversible composite-based shape memory alloy soft actuators

This research demonstrates how a combination of two-way shape memory alloy (SMA), low-temperature liquid epoxy cure composites, and fibre reinforced plastic (FRP) may be utilised to create a novel reversible actuator with built driven functionality. The novelty of this work is that the actuator can reverse its original shape and be mounted on different customized structures. The strategy is based on a knowledge of SMA wires and the manufacturing principle underlying composite structure, as well as experiments to see how soft SMA-FRP can be programmed to bend. The folding mechanism is studied in terms of fabrication factors such as SMA training and strong interfacial bonding between SMA and epoxy resin, which influence the programming process and shape change. The two-way SMA wires are trained using the pre-straining method to programme the SMAs. The technique has been used to assemble the SMA wires with bond reliability to enhance the actuator interface's thermal behaviour. The SMA elements are directly inserted into FRP strips and epoxy resin is used as an adhesive, resulting in dynamic hybrid composites. The module is actuated using an electrical board with a current value between 3 and 6 A. The robustness, controllability, mechanical properties, and 500 life cycles of the actuator are tested. Results indicate a bending angle of 58° with 30 mm of deflection in 7 s after actuating the module. Also, 3D printing is used to print a gripper inspired by human fingers and a structure to lift various weights. The actuator’s performance as a soft gripper is reliable in terms of grasping objects of different shapes. • Fabricating a novel fast responsive composite-based shape memory alloy actuator. • Developing reversible SMA-FRP soft actuators which can be mounted on various designs. • Adjusting current intensity and FRP/epoxy resin thickness, soft actuators demonstrated electrically reversible bending. • Programming of two-way shape memory alloy wires. • Grasping/release is achieved in an electrically-actuated gripper by 3D-printed structures.