Shape-morphing Structures Research Articles

The synergistic effect of nano-Al2O3 size and concentration on the interfacial adhesion properties of SMA/PDMS composites and their enhancement mechanism

Soft actuators composed of shape memory alloy (SMA) wires embedded in polydimethylsiloxane (PDMS) matrix hold potential for shape-morphing structures and soft robots. However, SMA exhibits poor bonding with PDMS due to its smooth and nonpolar surfaces. Nanoparticles show promise in interfacial strengthening of polymer composites. In this work, KH590 was used to modify nano-Al2O3 particles and deposited on the SMA surface, creating a three-dimensional nanostructure bridging SMA and PDMS to enhance interface strength. The synergistic effects of particle size and content of nano-Al2O3 particles on the interface strength were investigated in detail. It founds that interfacial strength decreased exponentially with particle size at content of 1 wt%, while when the content exceeds 1 wt%, the interface strength firstly increases with the particle size and then decreases in a logarithmic trend. Specifically, the interface strength is enhanced by 110 % with 3 wt% 50 nm particles. The interface enhancement mechanism was also discussed. The proposed nanoparticle modification approach was to strengthen SMA/PDMS interphase by increasing and extending fracture path, consuming more fracture energy. Chemical cross-linking also contributed to interface enhancement. This work enhances understanding of interfacial bonding mechanisms and provides valuable guide for interfacial strengthening of SMA/PDMS.

Model-based design of a mechanically intelligent shape-morphing structure

Soft robotics has significant interest within the industrial applications due to its advantages in flexibility and adaptability. Nevertheless, its potential is challenged by low stiffness and limited deformability, particularly in large-scale application scenarios such as underwater and offshore engineering. The integration of smart materials and morphing structures presents a promising avenue for enhancing the capabilities of soft robotic systems, especially in large deformation and variations in stiffness. In this study, we propose a multiple smart materials based mechanically intelligent structure devised through a model-based design framework. Specifically, the intelligent structure incorporates smart hydrogel and shape memory polymer (SMP). Employing the finite element method (FEM), we simulated the complex interactions among smart material to analyze the performance characteristics of the intelligent structure. The results demonstrate that, utilizing smart hydrogel and shape memory polymer (SMP) can effectively attain large deformation and exhibit variable stiffness due to the shape memory effect. Besides, the shape-morphing structures exhibit customized behaviours including bending, curling, and elongation, all while reducing reliance on external power sources. In conclusion, utilizing multiple smart materials within the model-based design framework offers an efficient approach for developing mechanically intelligent structure capable of complex deformations and variable stiffness, thereby providing support for underwater or offshore engineering applications.

Photo Switchable 4D Printing Remotely Controlled Responsive and Mimetic Deformation Shape Memory Polymer Nanocomposites

AbstractPhoto‐triggered shape morphing structures can be found in myriads of areas. Utilizing light as a switch can effectively control and regulate the precise deformation of structures. However, the integration of photo‐switching functions in shape‐morphing structures with complex geometries remains a challenge. Here, photoswitchable digital light processing based 4D printing high‐resolution and highly complex geometries, based on UV curable WO2.9 nanoparticle doped shape memory polymer nanocomposites is reported, which are highly deformable (stretchability up to ≈1000% at rubbery state) and fatigue resistant (repeatedly loaded >1200 times at ambient temperature). The presence of just trace WO2.9 nanoparticles (<0.20 wt.‰) contributes to the controlled photothermal property of nanocomposite via selective absorption of light, which enables reversibly photoswitchable (>50 times), spatial and remote control of 4D printing shape morphing structures. These findings offer a promising approach to expanding the applications of photo‐triggered shape‐morphing structures.

Customized deformation behavior of morphing wing through reversibly assembled multi-stable metamaterials

The geometry of multi-stable metamaterials, will change by the transition from one stable state to another. Shape morphing wings consisted of multi-stable metamaterials have capability to deform as desired, attributed to the programmable mechanical properties of architectured materials. In this study, to fabricate large-scale shape morphing structures, multi-stable unit cells with reversible connections were designed, printed and assembled. The mechanical properties and deformation capability were examined for multi-stable metamaterials with different geometrical parameters (e.g. width, thickness of beams). The deformation sequence for one assembled column consisting of identical multi-stable unit cells was found unpredictable, but could be tailored into a predictable manner by slightly adjusting beam geometry. To realize the customized deformation profile, the overall design domain of shape morphing structures was discretized into independent sub-regions. By enforcing deformation on sub-regions via the precise control of mechanical actuators that fixed with corresponding columns, the assembled shape morphing structures formed the targeted deformation. Also, the deformation feasibility was also demonstrated after incorporating voids or nondeformable functional elements within the assembled metamaterials platform. This study had provided practical solution for the design and fabrication of metamaterial-based shape morphing structures, and would shed light on future innovation of morphing aircraft.

Plant Biomimetic Principles of Multifunctional Soft Composite Development: A Synergistic Approach Enabling Shape Morphing and Mechanical Robustness.

Plant tissues are constructed as composite material systems of stiff cellulose microfibers reinforcing a soft matrix. Thus, they comprise smart and multifunctional structures that can change shape in response to external stimuli due to asymmetrical fiber alignment and possess robust mechanical properties. Herein, we demonstrate the biomimetics of the plant material system using silk fiber-reinforced alginate hydrogel matrix biocomposites. We fabricate single and bilamellar biocomposites with different fiber orientations. The mechanical behavior of the biocomposites is nonlinear, with large deformations, as in plant tissues. In general, the bilamellar system shows increased modulus, strain UTS, and toughness compared to the single-lamellar system for most of the tested orientations. Overall, the biocomposites present a wide range of elastic modulus values (3.0 ± 0.6-104.7 ± 11.3 MPa) and UTS values (0.23 ± 0.04-12.5 ± 2.0 MPa). The bilamellar biocomposites demonstrated shape-transforming abilities with diverse morphing modes, emulating different plant tissues and creating complex shape-morphing structures. These multifunctional biocomposites possess tunable and robust mechanical properties, controllable shape-morphing deformations, and the ability to self-controlled encapsulation, grip, and release objects. By harnessing biomimetic principles, these soft, smart, and multifunctional materials hold potential applications spanning from soft robotics, medicine, and tissue engineering to sensing and drug delivery.

Self-Healable and 4D Printable Hydrogel for Stretchable Electronics.

Materials with high stretchability and conductivity are used to fabricate stretchable electronics. Self-healing capability and four-dimensional (4D) printability are becoming increasingly important for these materials to facilitate their recovery from damage and endow them with stimuli-response properties. However, it remains challenging to design a single material that combines these four strengths. Here, a dually crosslinked hydrogel is developed by combining a covalently crosslinked acrylic acid (AAC) network and Fe3+ ions through dynamic and reversible ionically crosslinked coordination. The remarkable electrical sensitivity (a gauge factor of 3.93 under a strain of 1500%), superior stretchability (a fracture strain up to 1700%), self-healing ability (a healing efficiency of 88% and 97% for the mechanical and electrical properties, respectively), and 4D printability of the hydrogel are demonstrated by constructing a strain sensor, a two-dimensional touch panel, and shape-morphing structures with water-responsive behavior. The hydrogel demonstrates vast potential for applications in stretchable electronics.

Bio-inspired facile strategy for programmable osmosis-driven shape-morphing elastomer composite structures.

Achieving programmable and reversible deformations of soft materials is a long-standing goal for various applications in soft robotics, flexible electronics and many other fields. Swelling-induced shape morphing has been intensively studied as one of the potential mechanisms. However, achieving an extremely large swelling ratio (>1000% in volume) remains challenging with existing swellable soft materials (e.g., hydrogels and water-swellable rubbers). Inspired by the shape change enabled by the osmosis-driven swelling in living organisms, herein, we report a polymer composite system composed of fine sodium chloride (NaCl) particles embedded in Ecoflex00-10 polymer. This Ecoflex00-10/NaCl polymer composite can achieve controllable volumetric swelling up to 3000% while maintaining a relatively high elastic stiffness. We demonstrate that this swellable polymer composite can serve as an active component to drive the shape morphing of various structures. By controlling the geometric design and the fraction of the NaCl particle, morphing structures capable of deforming sequentially are created. Finally, by encapsulating 3D printed polymer composite patterns using water-permeable PDMS membrane, a programmable braille with visual and tactile regulation is demonstrated for the purpose of information encryption. Our study provides a facile approach to generate customizable shape-morphing structures, aiming to broaden the range of techniques and applications for morphing devices.

Liquid Crystal Elastomer-Liquid Metal Composite: Ultrafast, Untethered, and Programmable Actuation by Induction Heating.

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive materials that have been intensively studied for applications including artificial muscles, shape morphing structures, and soft robotics due to their capability of large, programmable, and fully reversible actuation strains. To fully take advantage of LCEs, rapid, untethered, and programmable actuation methods are highly desirable. Here, a liquid crystal elastomer-liquid metal (LCE-LM) composite is reported, which enables ultrafast and programmable actuations by eddy current induction heating. The composite consists of LM sandwiched between two LCE layers printed via direct ink writing (DIW). When subjected to a high-frequency alternating magnetic field, the composite is actuated in milliseconds. By moving the magnetic field, the eddy current is spatially controlled for selective actuation. Additionally, sequential actuation is achievable by programming the LM thickness distribution in a sample. With these capabilities, the LCE-LM composite is further exploited for multimodal deformation of a pop-up structure, on-ground omnidirectional robotic motion, and in-water targeted object manipulation and crawling.

A multibody kinematic system approach for the design of shape-morphing mechanism-based metamaterials

Shape-morphing structures have the ability to adapt to various target shapes, offering significant advantages for many applications. However, predicting their behavior presents challenges. Here, we present a method to assess the shape-matching behavior of shape-morphing structures using a multibody systems approach wherein the structure is represented by a collection of nodes and their associated constraints. This representation preserves the kinematic properties of the original structure while allowing for a rigorous treatment of the shape-morphing behavior of the underlying metamaterial. We assessed the utility of the proposed method by applying it to a wide range of 2D/3D sample shape-morphing structures. A modular system of joints and links was also 3D printed for the experimental realization of the systems under study. Both our simulations and the experiments confirmed the ability of the presented technique to capture the true shape-morphing behavior of complex shape-morphing metamaterials.

Inverse Design of Two-Dimensional Shape-Morphing Structures

Abstract This study proposes an inverse method for synthesizing shape-morphing structures in the lateral direction by integrating two-dimensional hexagonal unit cell with curved beams. Analytical expressions are derived to formulate the effective Young’s modulus and Poisson’s ratio for the base unit-cell as a function of its geometric parameters. The effective lateral Poisson’s ratio can be controlled by manipulating a set of geometric parameters, resulting in a dataset of over 6000 data points with Poisson’s ratio values ranging from −1.2 to 10.4. Furthermore, we utilize the established dataset to train an inverse design framework that utilizes a physics-guided neural network algorithm, and the framework can predict design parameters for a targeted shape-morphing structure. The proposed approach enables the generation of structures with tailored Poisson’s ratio ranging from −1.2 to 3.4 while ensuring flexibility and reduced stress concentration within the predicted structure. The generated shape-morphing structures’ performance is validated through numerical simulation and physical tensile testing. The finite element analysis (FEA) simulation results confirm agreement with the designed values for the shape-morphing structure, and the tensile testing results reveal the same trend in shape-morphing behavior. The proposed design automation framework demonstrates the feasibility of creating intricate and practical shape-morphing structures with high accuracy and computational efficiency.

Design principles for 3D-printed thermally activated shape-morphing structures

Morphing structures can change their shape in response to thermal excitation and are commonly used to enhance the performance of actuators, soft robotics, and biomedical devices. One of the main challenges is the inverse design problem, in which a deformed target shape is prescribed and the initial material properties, distribution, and geometry are to be determined. To tackle this challenge, a model for the thermo-mechanical response of free standing multi-layered struts that experience moderate to large deflections is developed. The model is validated through thermally activated 3D-printed bi-layer beams. Next, the model is exploited to develop an efficient inverse design algorithm that accepts a target function as an input and computes the referential configuration that can achieve the required thermally-activated deformations. It is shown that the solution to the inverse problem is not unique, thus providing flexibility in design. The merit of the algorithm is illustrated through the inverse design of three thermally-activated strut-based target shapes. The simplicity and robustness of the algorithm reveals fundamental principle guidelines for inverse design of thermally activated shape morphing structures and enables its extension to other stimuli.

Cold-programmed shape-morphing structures based on grayscale digital light processing 4D printing

Shape-morphing structures that can reconfigure their shape to adapt to diverse tasks are highly desirable for intelligent machines in many interdisciplinary fields. Shape memory polymers are one of the most widely used stimuli-responsive materials, especially in 3D/4D printing, for fabricating shape-morphing systems. They typically go through a hot-programming step to obtain the shape-morphing capability, which possesses limited freedom of reconfigurability. Cold-programming, which directly deforms the structure into a temporary shape without increasing the temperature, is simple and more versatile but has stringent requirements on material properties. Here, we introduce grayscale digital light processing (g-DLP) based 3D printing as a simple and effective platform for fabricating shape-morphing structures with cold-programming capabilities. With the multimaterial-like printing capability of g-DLP, we develop heterogeneous hinge modules that can be cold-programmed by simply stretching at room temperature. Different configurations can be encoded during 3D printing with the variable distribution and direction of the modular-designed hinges. The hinge module allows controllable independent morphing enabled by cold programming. By leveraging the multimaterial-like printing capability, multi-shape morphing structures are presented. The g-DLP printing with cold-programming morphing strategy demonstrates enormous potential in the design and fabrication of shape-morphing structures.

Influence of temperature on the SMA-PDMS interfacial bond behavior and its modeling with temperature dependent trapezoidal cohesive law

Soft actuators composed of shape memory alloy (SMA) wires embedded in polydimethylsiloxane (PDMS) matrix are potential in shape-morphing structures and soft robots. However, the SMA-PDMS interface is inevitably subjected to high temperatures during actuation due to Joule heating of the SMA, which affects its bond properties. However, knowledge of how temperature impacts SMA-PDMS interfacial bond properties is still insufficient. This paper conducted fiber pull-out experiments at various interface temperatures to monitor and trace the debonding process with high-speed camera. Results show that the maximum debonding load, critical debonding displacement, and friction load are in exponential decay with interface temperature. Notably, interfacial strength degradation exceeds 50 % when the interface temperature increases from 25 °C to 100 °C, and the traction-separation behavior of SMA-PDMS interface shifts from trapezoidal to triangular as the interface temperature increases. A finite element model was developed to simulate the debonding behavior of SMA-PDMS, in which the interface was modeled using the cohesive zone model with temperature-dependent trapezoidal constitutive law. Simulation results agree well with experiments. Stress distribution during the interface debonding process was also analyzed and discussed. The work demonstrates the importance of accounting for interface bond strength degradation caused by temperature in the design of SMA-PDMS soft actuators.

Anisotropic flexure hinges: Manufacturing and mechanical characterization for application in pressure-actuated morphing structures

Shape-morphing structures based on pressure actuation promise efficiency gains and performance improvements in aeronautics, but require complex three-dimensional geometries. The compliance of these morphing structures originates from local areas of reduced wall thickness. However, the required high wall thickness ratios are challenging for integral fabrication from high-performance fiber-reinforced plastics, which are demanded to achieve high load-bearing capacity. This study investigates different woven flexure hinges made from a hybrid yarn of glass fibers and the polyamide PA6 for application in pressure-actuated cellular structures. The flexure hinges are mechanically characterized under pure axial and pure bending loading in a tensile and a column bending test method specifically derived for flexure hinges. The specimen quality is further assessed by permeability testing, optical microscopy, and thermogravimetric analysis. This study shows that the stiffness of anisotropic flexure hinges can be determined in simple mechanical tests without the need for complex modeling of reinforcing fibers or consideration of manufacturing-specific effects. The mechanical properties of a double-layer hinge configuration are superior to those of a single-layer configuration, whereas integrating additively manufactured PA6 inlays into the woven preform offers an effective approach for achieving high wall thickness ratios. The most promising hinge configuration is selected for the integral fabrication of the aeronautical morphing structure, which consists of multiple pressurized cells.

4D printing of Metal-Reinforced double network granular hydrogels

Recent developments in soft actuation demand for resilient, responsive materials with locally varying compositions that are sufficiently stiff to exhibit significant actuation forces. Hydrogels are inherently responsive to certain stimuli. Yet, they typically suffer from a stiffness-toughness compromise such that those that are soft and show adequate flexibility display limited actuation forces. This compromise can be partially addressed if hydrogels are formulated as double networks. However, their involved processing prevents controlled variations in the local composition. The composition of hydrogels can be varied down to the 100 µm length scale through 3D printing. A wide range of hydrogels can be 3D printed if formulated as microparticles. Yet, the resulting granular hydrogels are soft. They can be reinforced with a percolating hydrogel network, resulting in double network granular hydrogels (DNGHs) that, however, are rather brittle. Here, we introduce 3D printable metal-reinforced DNGHs (mrDNGHs) that combine three seemingly contradictory traits: stiffness, toughness, and processability. Our mrDNGHs can bear loads up to 3 MPa while displaying a fracture energy up to 12 MJ·m−3, a value exceeding that of any of the previously 3D printed hydrogels at least 20-fold. We leverage the different degrees of swelling of the mrDNGHs to 3D print shape morphing structures.

Effects of viscosity and loading rate on wrinkling dynamics and coarsening of floating sheets

Elastic sheets floating on a fluid substrate buckle into equilibrium wrinkles under quasi-static compression, selecting a wavelength that reflects the energy balance between the sheet and the fluid substrate. However, when the fluid is viscous and the compression has a finite strain rate, the wrinkling instability and development are drastically different from those under quasi-static compression. Here, we investigate the effects of viscosity and loading rate on the wrinkling dynamics and coarsening of floating elastic sheets by theoretical analyses, numerical simulations, and experiments. We find that such wrinkling instability and development are viscosity and loading rate dependent. A theoretical solution to a critical time is obtained at which the compressive force in the sheet reaches a maximum indicating the onset of the dynamic wrinkling instability. The critical time shows a scaling law with an exponent 1/3 of viscosity and an exponent –2/3 of loading rate. The solution to the critical wavelength of the wrinkle patterns follows a scaling law with an exponent –1/6 of both viscosity and loading rate, which is verified by our experiments. Moreover, numerical and theoretical results show that coarsening of such wrinkles of floating sheets is significantly altered by viscosity, loading rate, and substrate stiffness, different from those under quasi-static compression. Our results provide insights into the rate-dependent instability and coarsening of dynamic wrinkling, which may be applicable for the usage of shape morphing structures via dynamic wrinkling.

Inverse design and additive manufacturing of shape-morphing structures based on functionally graded composites

Shape-morphing structures possess the ability to change their shapes from one state to another, and therefore, offer great potential for a broad range of applications. A typical paradigm of morphing is transforming from an initial two-dimensional (2D) flat configuration into a three-dimensional (3D) target structure. One popular fabrication method for these structures involves programming cuts in specific locations of a thin sheet material (i.e. kirigami), forming a desired 3D shape upon application of external mechanical load. By adopting the non-linear beam equation, an inverse design strategy has been proposed to determine the 2D cutting patterns required to achieve an axisymmetric 3D target shape. Specifically, tailoring the localised variation of bending stiffness is the key requirement. In this paper, a novel inverse design strategy is proposed by modifying the bending stiffness via introducing distributed modulus in functionally graded composites (FGCs). To fabricate the FGC-based shape-morphing structures, we use a multi-material 3D printer to print graded composites with voxel-like building blocks. The longitudinal modulus of each cross-sectional slice can be controlled through the rule of mixtures according to the micro-mechanics model, hence matching the required modulus distribution along the elastic strip. Following the proposed framework, a diverse range of structures is obtained with different Gaussian curvatures in both numerical simulations and experiments. A very good agreement is achieved between the measured shapes of morphed structures and the targets. In addition, the compressive rigidity and specific energy absorption during compression of FGC-based hemi-ellipsoidal morphing structures with various aspect ratios were also examined numerically and validated against experiments. By conducting systematical numerical simulations, we also demonstrate the multifunctionality of the modulus-graded shape-morphing composites. For example, they are capable of blending the distinct advantages of two different materials, i.e. one with high thermal (but low electrical) conductivity, and the other is the other way around, to achieve combined effective properties in a single structure made by FGCs. This new inverse design framework provides an opportunity to create shape-morphing structures by utilising modulus-graded composite materials, which can be employed in a variety of applications involving multi-physical environments. Furthermore, this framework underscores the versatility of the approach, enabling precise control over material properties at a local level.

One-pot ternary sequential reactions for photopatterned gradient multimaterials

One-pot ternary sequential reactions for photopatterned gradient multimaterials

4D printing parameters optimisation for bi-stable soft robotic gripper design

Four-dimensional (4D) printing is an emerging additive manufacturing (AM) technology that adds a time-dependent reconfiguration dimension to three-dimensional (3D) printed products. It enables the creation of on-demand, dynamically controllable shapes, or properties in response to external stimuli such as temperature, magnetic field, and light. Thermally responsive structures are among the most popular types of currently available 4D-printed structures due to their convenience. However, applications like soft robots are hindered by the temperature-sensitive structures' stagnating actuation. This research was driven by a requirement for a rapid and effective design and optimisation strategy for 4D-printed bi-stable thermally responsive structures for use in soft robotics. In this study, the response surface method (RSM) optimization with the aid of numerical solutions was used to investigate effective parameters in the design of a bi-stable, 4D-printed soft robotic gripper. This approach is proposed to accelerate the actuation of thermally responsive shape-morphing structures that can be controlled by the in situ strains and post-manufacturing heat stimuli as variable parameters. By using RSM solution the individual effects as well as the coupling effects of variable parameters on the output responses, including the maximum strain energy and the average distance between the clamps of the structure, are evaluated. The obtained results can be employed to develop the designation and improve the acceleration of soft robotic grippers such as fast buckling and bending, which is desirable for soft robotic applications.Graphical abstract

Chiral-based mechanical metamaterial with tunable normal-strain shear coupling effect

Different designs for mechanical metamaterials with tunable normal-strain shear coupling effect have been demonstrated over the last years. Their adjustable shear deformation makes them suitable as building blocks for soft robotics applications or structures with a desired deformation behavior, such as shape morphing structures. Herein, we present a modified 2.5D and 3D chiral-based mechanical metamaterials with tunable normal-strain shear coupling effect and Poisson’s Ratios close to zero. Advancing from conventional chiral-based metamaterials by introducing additional geometric freedoms into the design of the unit cell, a broad range of shear deformations, compression moduli and porosities can be achieved. 2.5D specimens with selected geometric parameters were additively manufactured with polypropylene using Fused Filament Fabrication. Compression tests were performed to investigate the mechanical properties and shear deformation. Two different numerical models were employed using ABAQUS to study the influence of the geometric parameters onto the mechanical properties and were verified by the experiments. The numerical material models were based on three-point-bending test data. The three-dimensional design was investigated with numerical simulations based on a homogenization approach to cover a broad range of geometric parameters.