Interfacial Dehydration Strategy for Chitosan Film Shape Morphing and Its Application

Shape morphing of biopolymer materials, such as chitosan (CS) films, has great potential for applications in many fields. Traditionally, their responsive behavior has been induced by the differential water swelling through the preparation of multicomponent composites or cross-linking as deformation is not controllable in the absence of these processes. Here, we report an interfacial dehydration strategy to trigger the shape morphing of the monocomponent CS film without cross-linking. The release of water molecules is achieved by spraying the surface with a NaOH solution or organic solvents, which results in the interfacial shrinkage and deformation of the entire film. On the basis of this strategy, a range of CS actuators were developed, such as soft grippers, joint actuators, and a light switch. Combined with the geometry effect, edited deformation was also achieved from the planar CS film. This shape-morphing strategy is expected to enable the application of more biopolymers in a wide range of fields.

Programmable Shape-Shifting Soft Robotic Structure Using Liquid Metal Electromagnetic Actuators

Constant development of soft robots, stretchable electronics, or flexible medical devices forces the research to look for new flexible structures that can change their shapes under external physical stimuli. This study presents a soft robotic structure that can change its shape into different three-dimensional (3D) configurations in response to electric current flown through the embedded liquid-metal conductors enabling electromagnetic actuation. The proposed structure is composed of volumetric pixels (voxels) connected in series where each can be independently controlled by the inputs of electrical current and vacuum pressure. A single voxel is made up of a granular core (GC) with an outer shell made of silicone rubber. The shell has embedded channels filled with liquid metal. The structure changes its shape under the Lorentz force produced by the liquid metal channel under applied electrical current. The GC allows the structure to maintain its shape after deformation even when the current is shut off. This is possible due to the granular jamming effect. In this study, we show the concept, the results of multiphysics simulation, and experimental characterization, including among other techniques, such as 3D digital image correlation or 3D magnetic field scanning, to study the different properties of the structure. We prove that the proposed structure can morph into many different shapes with the amplitude higher than 10 mm, and this process can be both fully reversible and repeatable.

MXene-Enabled Pneumatic Multiscale Shape Morphing for Adaptive, Programmable and Multimodal Radar-infrared Compatible Camouflage

Achieving radar-infrared compatible camouflage with dynamic adaptability has been a long-sought goal, but faces significant challenges owing to the limited dispersion relations of conventional material systems operating in different wavelength ranges. Here, this work proposes the concept of pneumatic multiscale shape morphing and design a periodically arranged pneumatic unit consisting of MXene-based morphable conductors and intake platforms. During gas actuation, the morphable conductor transforms centimeter-scale 2D flat sheets into 3D balloon shapes to enhance microwave absorption behavior, and also reconfigures micrometer-scale MXene wrinkles into smooth planes in combination with cavity-induced low heat transfer to minimize infrared (IR) signatures. Through theory-guided reverse engineering, the final pneumatic matrix shows remarkable frequency tunability (2.64-18.0 GHz), moderate IR emissivity regulation (0.14 at 7-16.5 µm), rapid responsiveness (≈30 ms), wide-angle operation (>45°), and excellent environmental tolerance. Additionally, the multiplexed pneumatic matrix enables over 14 programmable coding sequences that independently alter thermal radiation without compromising radar stealth, and allows multimodal camouflage switching between three distinct compatible states. The approach may facilitate the evolution of camouflage techniques and electromagnetic functional materials toward multispectral, adaptability and intelligence.

4D Fabrication of Two-Way Shape Memory Polymeric Composites by Electrospinning and Melt Electrowriting

This work presents a new method for 4D fabrication of two-way shape memory materials that are capable of reversible shapeshifting right after manufacturing, upon application of proper heating and cooling cycles. The innovative solution presented here consists in the combination of highly stretched electrospun shape memory polymer (SMP) nanofibers with a melt electrowritten elastomer. More specifically, the stretched nanofibers are made of a biocompatible thermoplastic polyurethane (TPU) with crystallizable soft segments, undergoing melt-induced contraction and crystallization-induced elongation upon heating and cooling, respectively. Reversible actuation during crystallization becomes possible due to the elastic recovery of the elastomer component, obtained by melt electrowriting of a commercial TPU filament. Thanks to the design freedom offered by additive manufacturing, the elastomer structure also has the role of guiding the shape transformation. Electrospinning and melt electrowriting process parameters are set up so to obtain smart 4D objects capable of two-way shape memory effect (SME), and the possibility of reversible and repeatable actuation is demonstrated. The two components are then combined in different proportions with the aim of tailoring the two-way SME, taking into account the effect of design parameters such as the SMP content, the elastomer pattern, and the composite thickness.

Engineering Kirigami Frameworks Toward Real-World Applications

The surge in advanced manufacturing techniques has led to a paradigm shift in the realm of material design from developing completely new chemistry to tailoring geometry within existing materials. Kirigami, evolved from a traditional cultural and artistic craft of cutting and folding, has emerged as a powerful framework that endows simple 2D sheets with unique mechanical, thermal, optical, and acoustic properties, as well as shape-shifting capabilities. Given its flexibility, versatility, and ease of fabrication, there are significant efforts in developing kirigami algorithms to create various architectured materials for a wide range of applications. This review summarizes the fundamental mechanisms that govern the transformation of kirigami structures and elucidates how these mechanisms contribute to their distinctive properties, including high stretchability and adaptability, tunable surface topography, programmable shape morphing, and characteristics of bistability and multistability. It then highlights several promising applications enabled by the unique kirigami designs and concludes with an outlook on the future challenges and perspectives of kirigami-inspired metamaterials toward real-world applications.

Multilayered Shape-Morphing Scaffolds with a Hierarchical Structure for Uterine Tissue Regeneration

Owing to dysfunction of the uterus, millions of couples around the world suffer from infertility. Different from conventional treatments, tissue engineering provides a new and promising approach to deal with difficult problems such as human tissue or organ failure. Adopting scaffold-based tissue engineering, three-dimensional (3D) porous scaffolds in combination with stem cells and appropriate biomolecules may be constructed for uterine tissue regeneration. In this study, a hierarchical tissue engineering scaffold, which mimicked the uterine tissue structure and functions, was designed, and the biomimicking scaffolds were then successfully fabricated using solvent casting, layer-by-layer assembly, and 3D bioprinting techniques. For the multilayered, hierarchical structured scaffolds, poly(l-lactide-co-trimethylene carbonate) (PLLA-co-TMC, "PLATMC" in short) and poly(lactic acid-co-glycolic acid) (PLGA) blends were first used to fabricate the shape-morphing layer of the scaffolds, which was to mimic the function of myometrium in uterine tissue. The PLATMC/PLGA polymer blend scaffolds were highly stretchable. Subsequently, after etching of the PLATMC/PLGA surface and employing estradiol (E2), polydopamine (PDA), and hyaluronic acid (HA), PDA@E2/HA multilayer films were formed on PLATMC/PLGA scaffolds to build an intelligent delivery platform to enable controlled and sustained release of E2. The PDA@E2/HA multilayer films also improved the biological performance of the scaffold. Finally, a layer of bone marrow-derived mesenchymal stem cell (BMSC)-laden hydrogel [which was a blend of gelatin methacryloyl (GelMA) and gelatin (Gel)] was 3D printed on the PDA@E2/HA multilayer films of the scaffold, thereby completing the construction of the hierarchical scaffold. BMSCs in the GelMA/Gel hydrogel layer exhibited excellent cell viability and could spread and be released eventually upon biodegradation of the GelMA/Gel hydrogel. It was shown that the hierarchically structured scaffolds could evolve from the initial flat shape into the tubular structure completely in an aqueous environment at 37 °C, fulfilling the requirement for curved scaffolds for uterine tissue engineering. The biomimicking scaffolds with a hierarchical structure and curved shape, high stretchability, and controlled and sustained E2 release appear to be very promising for uterine tissue regeneration.

Four-Dimensional (4D) Printing of Dynamic Foods-Definitions, Considerations, and Current Scientific Status

Since its conception, the application of 3D printing in the structuring of food materials has been focused on the processing of novel material formulations and customized textures for innovative food applications, such as personalized nutrition and full sensory design. The continuous evolution of the used methods, approaches, and materials has created a solid foundation for technology to process dynamic food structures. Four-dimensional food printing is an extension of 3D printing where food structures are designed and printed to perform time-dependent changes activated by internal or external stimuli. In 4D food printing, structures are engineered through material tailoring and custom designs to achieve a transformation from one configuration to another. Different engineered 4D behaviors include stimulated color change, shape morphing, and biological growth. As 4D food printing is considered an emerging application, imperatively, this article proposes new considerations and definitions in 4D food printing. Moreover, this article presents an overview of 4D food printing within the current scientific progress, status, and approaches.

Shape Morphing Directed by Spatially Encoded, Dually Responsive Liquid Crystalline Elastomer Micro-Actuators

Liquid crystalline elastomers (LCEs) with intrinsic molecular anisotropy can be programmed to morph shapes under external stimuli. However, it is difficult to program the position and orientation of individual mesogenic units separately and locally, whether in-plane or out-of-plane, since each mesogen is linked to adjacent ones through the covalently bonded polymer chains. Here, dually responsive, spindle-shaped micro-actuators are synthesized from LCE composites, which can reorient under a magnetic field and change the shape upon heating. When the discrete micro-actuators are embedded in a conventional and nonresponsive elastomer with programmed height distribution and in-plane orientation in local regions, robust and complex shape morphing induced by the cooperative actuations of the locally distributed micro-actuators, which corroborates with finite element analysis, are shown. The spatial encoding of discrete micro-actuators in a nonresponsive matrix allows to decouple the actuators and the matrix, broadening the material palette to program local and global responses to stimuli for applications including soft robotics, smart wearables, and sensors.

Shape Morphing by Topological Patterns and Profiles in Laser-Cut Liquid Crystal Elastomer Kirigami

Programming shape changes in soft materials requires precise control of the directionality and magnitude of their mechanical response. Among ordered soft materials, liquid crystal elastomers (LCEs) exhibit remarkable and programmable shape shifting when their molecular order changes. In this work, we synthesized, remotely programmed, and modeled reversible and complex morphing in monolithic LCE kirigami encoded with predesigned topological patterns in its microstructure. We obtained a rich variety of out-of-plane shape transformations, including auxetic structures and undulating morphologies, by combining different topological microstructures and kirigami geometries. The spatiotemporal shape-shifting behaviors are well recapitulated by elastodynamics simulations, revealing that the complex shape changes arise from integrating the custom-cut geometry with local director profiles defined by topological defects inscribed in the material. Different functionalities, such as a bioinspired fluttering butterfly, a flower bud, dual-rotation light mills, and dual-mode locomotion, are further realized. Our proposed LCE kirigami with topological patterns opens opportunities for the future development of multifunctional devices for soft robotics, flexible electronics, and biomedicine.

Magnetoresponsive Artificial Cilia Self-Assembled with Magnetic Micro/Nanoparticles

Biological cilia have exquisitely organized dynamic ultrafine structures with submicron diameters and exceptional aspect ratios, which are self-assembled with ciliary proteins. However, the construction of artificial cilia with size and dynamic functions comparable to biological cilia remains highly challenging. Here, we propose a self-assembly technique that generates magnetoresponsive artificial cilia with a highly ordered 3D structural arrangement using vapor-phase magnetic particles of varying sizes and shapes. We demonstrate that both monodispersed Fe₃O₄ nanoparticles and Fe microparticles can be assembled layer-by-layer vertically in patterned magnetic fields, generating both "nanoscale" or "microscale" artificial cilia, respectively. The resulting cilia display several structural features, such as diameters of single particle resolution, controllable diameters and lengths spanning from nanometers to micrometers, and accurate positioning. We further demonstrate that both the magnetic nanocilia and microcilia can dynamically and immediately actuate in response to modulated magnetic fields while providing different stroke ranges and actuation torques. Our strategy provides new possibilities for constructing artificial nano- and microcilia with controlled 3D morphology and dynamic field responsiveness using magnetic particles of varied sizes and shapes.

A soft microrobot with highly deformable 3D actuators for climbing and transitioning complex surfaces

The climbing microrobots have attracted growing attention due to their promising applications in exploration and monitoring of complex, unstructured environments. Soft climbing microrobots based on muscle-like actuators could offer excellent flexibility, adaptability, and mechanical robustness. Despite the remarkable progress in this area, the development of soft microrobots capable of climbing on flat/curved surfaces and transitioning between two different surfaces remains elusive, especially in open spaces. In this study, we address these challenges by developing voltage-driven soft small-scale actuators with customized 3D configurations and active stiffness adjusting. Combination of programmed strain distributions in liquid crystal elastomers (LCEs) and buckling-driven 3D assembly, guided by mechanics modeling, allows for voltage-driven, complex 3D-to-3D shape morphing (bending angle > 200°) at millimeter scales (from 1 to 10 mm), which is unachievable previously. These soft actuators enable development of morphable electroadhesive footpads that can conform to different curved surfaces and stiffness-variable smart joints that allow different locomotion gaits in a single microrobot. By integrating such morphable footpads and smart joints with a deformable body, we report a multigait, soft microrobot (length from 6 to 90 mm, and mass from 0.2 to 3 g) capable of climbing on surfaces with diverse shapes (e.g., flat plane, cylinder, wavy surface, wedge-shaped groove, and sphere) and transitioning between two distinct surfaces. We demonstrate that the microrobot could navigate from one surface to another, recording two corresponding ceilings when carrying an integrated microcamera. The developed soft microrobot can also flip over a barrier, survive extreme compression, and climb bamboo and leaf.

An insect-inspired asymmetric hinge in a double-layer membrane

Insect wings are deformable airfoils, in which deformations are mostly achieved by complicated interactions between their structural components. Due to the complexity of the wing design and technical challenges associated with testing the delicate wings, we know little about the properties of their components and how they determine wing response to flight forces. Here, we report an unusual structure from the hind-wing membrane of the beetle Pachnoda marginata. The structure, a transverse section of the claval flexion line, consists of two distinguishable layers: a bell-shaped upper layer and a straight lower layer. Our computational simulations showed that this is an effective one-way hinge, which is stiff in tension and upward bending but flexible in compression and downward bending. By systematically varying its design parameters in a computational model, we showed that the properties of the double-layer membrane hinge can be tuned over a wide range. This enabled us to develop a broad design space, which we later used for model selection. We used selected models in three distinct applications, which proved that the double-layer hinge represents a simple yet effective design strategy for controlling the mechanical response of structures using a single material and with no extra mass. The insect-inspired, one-way hinge is particularly useful for developing structures with asymmetric behavior, exhibiting different responses to the same load in two opposite directions. This multidisciplinary study not only advances our understanding of the biomechanics of complicated insect wings but also informs the design of easily tunable engineering hinges.

Mechanical Intelligence (MI): A Bioinspired Concept for Transforming Engineering Design

Despite significant scientific advances in the past decades, most structures around us are static and ironically outdated from a technological perspective. Static structures have limited efficiency and durability and typically perform only a single task. Adaptive structures, in contrast, adjust to different conditions, tasks, and functions. They not only offer multi-functionality but also enhanced efficiency and durability. Despite their obvious advantages over conventional structures, adaptive structures have only been limitedly used in everyday life applications. This is because adaptive structures often require sophisticated sensing, feedback, and controls, which make them costly, heavy, and complicated. To overcome this problem, here the concept of Mechanical Intelligence (MI) is introduced to promote the development of engineering systems that adapt to circumstances in a passive-automatic way. MI will offer a new paradigm for designing structural components with superior capabilities. As adaptability has been rewarded throughout evolution, nature provides one of the richest sources of inspiration for developing adaptive structures. MI explores nature-inspired mechanisms for automatic adaptability and translates them into a new generation of mechanically intelligent components. MI structures, presenting widely accessible bioinspired solutions for adaptability, will facilitate more inclusive and sustainable industrial development, reflective of Goal 9 of the 2030 Agenda for Sustainable Development.

Biobased and Programmable Electroadhesive Metasurfaces

Electroadhesion has shown the potential to deliver versatile handling devices because of its simplicity of actuation and rapid response. Current electroadhesion systems have, however, significant difficulties in adapting to external objects with complex shapes. Here, a novel concept of metasurface is proposed by combining the use of natural fibers (flax) and shape memory epoxy polymers in a hygromorphic and thermally actuated composite (HyTemC). The biobased material composite can be used to manipulate adhesive surfaces with high precision and controlled environmental actuation. The HyTemC concept is preprogrammed to store controllable moisture and autonomous desorption when exposed to the operational environment, and can reach predesigned bending curvatures up to 31.9 m^-1 for concave and 29.6 m^-1 for convex shapes. The actuated adhesive surface shapes are generated via the architected metasurface structure, incorporating an electroadhesive component integrated with the programmable biobased materials. This biobased metasurface stimulated by the external environment provides a large taxonomy of shapes─from flat, circular, single/double concave, and wavy, to piecewise, polynomial, trigonometric, and airfoil configurations. The objects handled by the biobased metasurface can be fragile because of the high conformal matching between contacting surfaces and the absence of compressive adhesion. These natural fiber-based and environmentally friendly electroadhesive metasurfaces can significantly improve the design of programmable object handling technologies, and also provide a sustainable route to lower the carbon and emission footprint of smart structures and robotics.

Morphing-to-Adhesion Polysaccharide Hydrogel for Adaptive Biointerfaces

Reliable functions of medical implants highly depend on biocompatible, conformal, and stable biointerfaces for seamless biointegration with biological tissues. Though flexible biointerfaces based on synthetic hydrogels have shown promise in optimizing implant biointegration via surgical suturing, physical attachment, or manual preshaping, they still suffer from poor adaptability, such as tissue damage by surgical suturing, low bioactivity, and difficulties in conformal contact and stable fixation, especially for specific tissues of large surface curvatures. Here, we report a bilayer hydrogel-based adaptive biointerface (HAB) made of two polysaccharide derivates, N-hydroxysuccinimide (NHS) ester-activated alginate and chitosan, harnessing dual advantages of their different swelling and active groups. Leveraging on the differential swelling between the two hydrogel layers and covalent linkages with active groups at hydrogel interfaces, HABs can be programmed into sealed tubes with tunable diameters via water-induced compliable shape morphing and instant interfacial adhesion. We further demonstrate that the polysaccharide-based morphing-to-adhesion HAB possesses outstanding bioactivity in directing cellular focal adhesion and intercellular junction, versatile geometrical adaptability to diverse tubular tissues with a wide range of surface curvatures (2.8 × 10²-1.3 × 10³ m^-1), and excellent mechanical stability in high load-/shear-bearing physiological environments (blood flow volume: 85 mm·s^-1). HABs overcome the limitations of existing biointerfaces in terms of poor bioactivity and difficult biointegration with biological tissues of large surface curvatures, holding promise to open new avenues for adaptive biointerfaces and reliable medical implants.

Rapid and Multimaterial 4D Printing of Shape-Morphing Micromachines for Narrow Micronetworks Traversing

Micromachines with high environmental adaptability have the potential to deliver targeted drugs in complex biological networks, such as digestive, neural, and vascular networks. However, the low processing efficiency and single processing material of current 4D printing methods often limit the development and application of shape-morphing micromachines (SMMs). Here, two 4D printing strategies are proposed to fabricate SMMs with pH-responsive hydrogels for complex micro-networks traversing. On the one hand, the 3D vortex light single exposure technique can rapidly fabricate a tubular SMM with controllable size and geometry within 0.1 s. On the other hand, the asymmetric multimaterial direct laser writing (DLW) method is used to fabricate SMMs with designable 3D structures composed of hydrogel and platinum nanoparticles (Pt NPs). Based on the presence of ferroferric oxide (Fe₃ O₄ ) and Pt NPs in the SMMs, efficient magnetic, bubble, and hybrid propulsion modes are achieved. Finally, it is demonstrated that the spatial shape conversion capabilities of these SMMs can be used for narrow micronetworks traversing, which will find potential applications in targeted cargo delivery in microcapillaries.

Programmable Auxeticity in Hydrogel Metamaterials via Shape-Morphing Unit Cells

Mechanical metamaterials recruit unique mechanical behavior that is unavailable in bulk materials from a periodic unit cell structure with a specific geometry. However, such metamaterials can typically not be reconfigured once manufactured. Herein, the authors introduce shape morphing of a hydrogel metamaterial via spatio-selective integration of responsive actuating elements to reconfigure the mesoscale unit cell geometry to reach programmable auxeticity on the macroscale. Via thermal control, the unit cell angle of a honeycomb structure can be precisely programmed from 68° to 107°. This results in negative, zero, or positive Poisson's ratio under applied tensile strain. The geometrical reconfiguration with resulting programmable auxeticity is predicted and verified by finite element (FE) simulation. This concept of shape-morphing hydrogel metamaterials via the addition of actuating struts into otherwise passive architectures offers a new strategy for reconfigurable metamaterials and extends applications of hydrogels in general. It can be readily extended to other architectures and may find applications in mechanical computing as well as soft robotics.

Bio-Inspired Bianisotropic Magneto-Sensitive Elastomers with Excellent Multimodal Transformation

Magneto-sensitive soft materials that can accomplish fast, remote, and reversible shape morphing are highly desirable for practical applications including biomedical devices, soft robotics, and flexible electronics. In conventional magneto-sensitive elastomers (MSEs), there is a tradeoff between employing hard magnetic particles with costly magnetic programming and utilizing soft magnetic particle chains causing tedious and small deformation. Here, inspired by the shape and movement of mimosa, a novel soft magnetic particle doped shape material bianisotropic magneto-sensitive elastomer (SM bianisotropic MSE) with multimodal transformation and superior deformability is developed. The high-aspect-ratio shape anisotropy and the material anisotropy in which the magnetic particles are arranged in a chainlike structure together impart magnetic anisotropy to the SM bianisotropic MSE. A magneto-elastic analysis model is proposed, and it is elucidated that magnetic anisotropy leads to peculiar field-direction-dependent multimodal transformation. More importantly, a quadrilateral assembly and a regular hexagon assembly based on this SM bianisotropic MSE are designed, and they exhibit 2.4 and 1.7 times the deformation capacity of shape anisotropic samples, respectively. By exploiting the multidegree of freedom and excellent deformability of the SM bianisotropic MSE, flexible logic switches and ultrasoft magnetic manipulators are further demonstrated, which prove its potential applications in future intelligent flexible electronics and autonomous soft robotics.

Jammed Micro-Flake Hydrogel for Four-Dimensional Living Cell Bioprinting

4D bioprinting is promising to build cell-laden constructs (bioconstructs) with complex geometries and functions for tissue/organ regeneration applications. The development of hydrogel-based 4D bioinks, especially those allowing living cell printing, with easy preparation, defined composition, and controlled physical properties is critically important for 4D bioprinting. Here, a single-component jammed micro-flake hydrogel (MFH) system with heterogeneous size distribution, which differs from the conventional granular microgel, has been developed as a new cell-laden bioink for 4D bioprinting. This jammed cytocompatible MFH features scalable production and straightforward composition with shear-thinning, shear-yielding, and rapid self-healing properties. As such, it can be smoothly printed into stable 3D bioconstructs, which can be further cross-linked to form a gradient in cross-linking density when a photoinitiator and a UV absorber are incorporated. After being subject to shape morphing, a variety of complex bioconstructs with well-defined configurations and high cell viability are obtained. Based on this system, 4D cartilage-like tissue formation is demonstrated as a proof-of-concept. The establishment of this versatile new 4D bioink system may open up a number of applications in tissue engineering.

Micro-Scale Auxetic Hierarchical Mechanical Metamaterials for Shape Morphing

Shape morphing and the possibility of having control over mechanical properties via designed deformations have attracted a lot of attention in the materials community and led to a variety of applications with an emphasis on the space industry. However, current materials normally do not allow to have a full control over the deformation pattern and often fail to replicate such behavior at low scales which is essential in flexible electronics. Thus, in this paper, novel 2D and 3D microscopic hierarchical mechanical metamaterials using mutually-competing substructures within the system that are capable of exhibiting a broad range of the highly unusual auxetic behavior are proposed. Using experiments (3D microprinted polymers) supported by computer simulations, it is shown that such ability can be controlled through geometric design parameters. Finally it is demonstrated that the considered structure can form a composite capable of shape morphing allowing it to deform to a predefined shape.

Solvent-Assisted 4D Programming and Reprogramming of Liquid Crystalline Organogels

Encoding molecular ordering during liquid crystalline network (LCN) formation endows preprogrammed but fixed shape morphing in response to external stimuli. The incorporation of dynamic covalent bonds enables shape reprogramming but also permanently alters the network structures. Here, an entropic approach that can program complex shapes via directed solvent evaporation from an isotropic LCN organogel is discoursed. Different shapes can be erased and reprogrammed from the same LCN on demand depending on the modes of deformation of the organogel during solvent evaporation. The ability to decouple network synthesis and molecular alignment relaxes the requirements to LCN chemistry and alignment methods, allowing for the realization of a variety of origami/kirigami structures and 4D shape morphing of LCNs printed from the digital light processing technique with unattainable spatial and temporal controls.

Environmentally Adaptive Shape-Morphing Microrobots for Localized Cancer Cell Treatment

Microrobots have attracted considerable attention due to their extensive applications in microobject manipulation and targeted drug delivery. To realize more complex micro-/nanocargo manipulation (e.g., encapsulation and release) in biological applications, it is highly desirable to endow microrobots with a shape-morphing adaptation to dynamic environments. Here, environmentally adaptive shape-morphing microrobots (SMMRs) have been developed by programmatically encoding different expansion rates in a pH-responsive hydrogel. Due to a combination with magnetic propulsion, a shape-morphing microcrab (SMMC) is able to perform targeted microparticle delivery, including gripping, transporting, and releasing by "opening-closing" of a claw. As a proof-of-concept demonstration, a shape-morphing microfish (SMMF) is designed to encapsulate a drug (doxorubicin (DOX)) by closing its mouth in phosphate-buffered saline (PBS, pH ∼ 7.4) and release the drug by opening its mouth in a slightly acidic solution (pH < 7). Furthermore, localized HeLa cell treatment in an artificial vascular network is realized by "opening-closing" of the SMMF mouth. With the continuous optimization of size, motion control, and imaging technology, these magnetic SMMRs will provide ideal platforms for complex microcargo operations and on-demand drug release.

Origami Spring-Inspired Shape Morphing for Flexible Robotics

Flexible robotics are capable of achieving various functionalities by shape morphing, benefiting from their compliant bodies and reconfigurable structures. In this study, we construct and study a class of origami springs generalized from the known interleaved origami spring, as promising candidates for shape morphing in flexible robotics. These springs are found to exhibit nonlinear stretch-twist coupling and linear/nonlinear mechanical response in the compression/tension region, analyzed by the demonstrated continuum mechanics models, experiments, and finite element simulations. To improve the mechanical performance such as the damage resistance, we establish an origami rigidization method by adding additional creases to the spring system. Guided by the theoretical framework, we experimentally realize three types of flexible robotics-origami spring ejectors, crawlers, and transformers. These robots show the desired functionality and outstanding mechanical performance. The proposed concept of origami-aided design is expected to pave the way to facilitate the diverse shape morphing of flexible robotics.

Totimorphic assemblies from neutrally stable units

Inspired by the quest for shape-shifting structures in a range of applications, we show how to create morphable structural materials using a neutrally stable unit cell as a building block. This unit cell is a self-stressed hinged structure with a one-parameter family of morphing motions that are all energetically equivalent. However, unlike kinematic mechanisms, the unit cell is not infinitely floppy and instead exhibits a tunable mechanical response akin to that of an ideal rigid-plastic material. Theory and simulations allow us to explore the properties of planar and spatial assemblies of neutrally stable elements, and solve the inverse problem of designing assemblies that can morph from one given shape into another. Simple experimental prototypes of these assemblies corroborate our theoretical results and show that the addition of switchable hinges allows us to create load-bearing structures. Altogether, totimorphs pave the way for structural materials whose geometry and deformation response can be controlled independently and at multiple scales.

Thermoplastic Composites for Integrally Woven Pressure Actuated Cellular Structures: Design Approach and Material Investigation

The use of pressure-actuated cellular structures (PACS) is an effective approach for the application of compliant mechanisms. Analogous to the model in nature, the Venus flytrap, they are made of discrete pressure-activated rows and can be deformed with high stiffness at a high deformation rate. In previous work, a new innovative approach in their integral textile-based manufacturing has been demonstrated based on the weaving technique. In this work, the theoretical and experimental work on the further development of PACS from simple single-row to double-row PACS with antagonistic deformation capability is presented. Supported by experimental investigations, the necessary adaptations in the design of the textile preform and the polymer composite design are presented and concretized. Based on the results of pre-simulations of the deformation capacity of the new PACS, their performance was evaluated, the results of which are presented.

Designing Shape Morphing Behavior through Local Programming of Mechanical Metamaterials

Shape morphing implicates that a specific condition leads to a morphing reaction. The material thus transforms from one shape to another in a predefined manner. In this paper, not only the target shape but rather the evolution of the material's shape as a function of the applied strain is programmed. To rationalize the design process, concepts from informatics (processing functions, for example, Poisson's ratio (PR) as function of strain: ν = f(ε) and if-then-else conditions) will be introduced. Three types of shape morphing behavior will be presented: (1) achieving a target shape by linearly increasing the amplitude of the shape, (2) filling up a target shape in linear steps, and (3) shifting a bulge through the material to a target position. In the first case, the shape is controlled by a geometric gradient within the material. The filling kind of behavior was implemented by logical operations. Moreover, programming moving hillocks (3) requires to implement a sinusoidal function ε_y  = sin (ε_x ) and an if-then-else statement into the unit cells combined with a global stiffness gradient. The three cases will be used to show how the combination of mechanical mechanisms as well as the related parameter distribution enable a programmable shape morphing behavior in an inverse design process.

Innervated, Self-Sensing Liquid Crystal Elastomer Actuators with Closed Loop Control

The programmable assembly of innervated LCE actuators (iLCEs) with prescribed contractile actuation, self-sensing, and closed loop control via core-shell 3D printing is reported. This extrusion-based direct ink writing method enables coaxial filamentary features composed of pure LM core surrounded by an LCE shell, whose director is aligned along the print path. Specifically, the thermal response of the iLCE fiber-type actuators is programmed, measured, and modeled during Joule heating, including quantifying the concomitant changes in fiber length and resistance that arise during simultaneous heating and self-sensing. Due to their reversible, high-energy actuation and their resistive feedback, it is also demonstrated that iLCEs can be regulated with closed loop control even when perturbed with large bias loads. Finally, iLCE architectures capable of programmed, self-sensing 3D shape change with closed loop control are fabricated.

Magnetic Dynamic Polymers for Modular Assembling and Reconfigurable Morphing Architectures

Shape-morphing magnetic soft materials, composed of magnetic particles in a soft polymer matrix, can transform shape reversibly, remotely, and rapidly, finding diverse applications in actuators, soft robotics, and biomedical devices. To achieve on-demand and sophisticated shape morphing, the manufacture of structures with complex geometry and magnetization distribution is highly desired. Here, a magnetic dynamic polymer (MDP) composite composed of hard-magnetic microparticles in a dynamic polymer network with thermally responsive reversible linkages, which permits functionalities including targeted welding for magnetic-assisted assembly, magnetization reprogramming, and permanent structural reconfiguration, is reported. These functions not only provide highly desirable structural and material programmability and reprogrammability but also enable the manufacturing of functional soft architected materials such as 3D kirigami with complex magnetization distribution. The welding of magnetic-assisted modular assembly can be further combined with magnetization reprogramming and permanent reshaping capabilities for programmable and reconfigurable architectures and morphing structures. The reported MDP are anticipated to provide a new paradigm for the design and manufacture of future multifunctional assemblies and reconfigurable morphing architectures and devices.

4D Printing Elastic Composites for Strain-Tailored Multistable Shape Morphing

Three-dimensional (3D) morphing structures with multistable shapes that can be quantitatively and reversibly altered are highly desired in many potential applications ranging from soft robots to wearable electronics. In this study, we present a 4D printing method for fabricating multistable shape-morphing structures that can be quantitatively controlled by the applied strains. The structures are printed by a two-nozzle 3D printer that can spatially distribute phase change wax microparticles (MPs) in the elastomer matrix. The wax MPs can retain the residual strain after the prestrained elastomer composite is relaxed because of the solid-liquid phase change. Thanks to high design freedom of the 3D printing, spatial distribution of the wax MPs can be programmed, leading to an anisotropic stress field in the elastomer composite. This causes the out-of-plane deformations such as curling, folding, and buckling. These deformations are multistable and can be reprogrammed because of the reversible phase change of the wax MPs. What's more, characteristics of deformations such as curvatures and folding angles are linearly dependent on the applied strains, suggesting that these deformations are quantitatively controllable. Finally, the applications of the strained-tailored multistable shape morphing 3D structures in the assembly of 3D electronics and adaptive wearable sensors were demonstrated.

A Magnetically Guided Self-Rolled Microrobot for Targeted Drug Delivery, Real-Time X-Ray Imaging, and Microrobot Retrieval

Targeted drug delivery using a microrobot is a promising technique capable of overcoming the limitations of conventional chemotherapy that relies on body circulation. However, most studies of microrobots used for drug delivery have only demonstrated simple mobility rather than precise targeting methods and prove the possibility of biodegradation of implanted microrobots after drug delivery. In this study, magnetically guided self-rolled microrobot that enables autonomous navigation-based targeted drug delivery, real-time X-ray imaging, and microrobot retrieval is proposed. The microrobot, composed of a self-rolled body that is printed using focused light and a surface with magnetic nanoparticles attached, demonstrates the loading of doxorubicin and an X-ray contrast agent for cancer therapy and X-ray imaging. The microrobot is precisely mobilized to the lesion site through automated targeting using magnetic field control of an electromagnetic actuation system under real-time X-ray imaging. The photothermal effect using near-infrared light reveals rapid drug release of the microrobot located at the lesion site. After drug delivery, the microrobot is recovered without potential toxicity by implantation or degradation using a magnetic-field-switchable coiled catheter. This microrobotic approach using automated control method of the therapeutic agents-loaded microrobot has potential use in precise localized drug delivery systems.

Actuating Shape Memory Polymer for Thermoresponsive Soft Robotic Gripper and Programmable Materials

For soft robotics and programmable metamaterials, novel approaches are required enabling the design of highly integrated thermoresponsive actuating systems. In the concept presented here, the necessary functional component was obtained by polymer syntheses. First, poly(1,10-decylene adipate) diol (PDA) with a number average molecular weight M n of 3290 g·mol-1 was synthesized from 1,10-decanediol and adipic acid. Afterward, the PDA was brought to reaction with 4,4'-diphenylmethane diisocyanate and 1,4-butanediol. The resulting polyester urethane (PEU) was processed to the filament, and samples were additively manufactured by fused-filament fabrication. After thermomechanical treatment, the PEU reliably actuated under stress-free conditions by expanding on cooling and shrinking on heating with a maximum thermoreversible strain of 16.1%. Actuation stabilized at 12.2%, as verified in a measurement comprising 100 heating-cooling cycles. By adding an actuator element to a gripper system, a hen's egg could be picked up, safely transported and deposited. Finally, one actuator element each was built into two types of unit cells for programmable materials, thus enabling the design of temperature-dependent behavior. The approaches are expected to open up new opportunities, e.g., in the fields of soft robotics and shape morphing.

Repeatable and Reprogrammable Shape Morphing from Photoresponsive Gold Nanorod/Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are of interest for applications such as soft robotics and shape-morphing devices. Among the different actuation mechanisms, light offers advantages such as spatial and local control of actuation via the photothermal effect. However, the unwanted aggregation of the light-absorbing nanoparticles in the LCE matrix will limit the photothermal response speed, actuation performance, and repeatability. Herein, a near-infrared-responsive LCE composite consisting of up to 0.20 wt% poly(ethylene glycol)-modified gold nanorods (AuNRs) without apparent aggregation is demonstrated. The high Young's modulus, 20.3 MPa, and excellent photothermal performance render repeated and fast actuation of the films (actuation within 5 s and recovery in 2 s) when exposed to 800 nm light at an average output power of ≈1.0 W cm^-2 , while maintaining a large actuation strain (56%). Further, it is shown that the same sheet of AuNR/LCE film (100 µm thick) can be morphed into different shapes simply by varying the motifs of the photomasks.

A Photoresponsive Hydrogel with Enhanced Photoefficiency and the Decoupled Process of Light Activation and Shape Changing for Precise Geometric Control

Traditional shape-morphing hydrogels rely on structural implementation of inhomogeneity inside the material during fabrication to realize predetermined complex shape change upon activation. In recent years, several systems with reprogrammable shape-morphing capabilities have been developed. Among those, the photoresponsive hydrogels offer the best spatial and temporal control. However, for most photoresponsive hydrogels, upon light irradiation, they simultaneously deform, which requires the projection of the light pattern to be continuously adaptive to the deforming gel. It is impractical for complex 3D morphing. In this paper, by incorporating two photodissociable molecules that can form a reactive ion couple upon light activation into one hydrogel, the light irradiation process is decoupled with the morphing process, and the consumption of the reactive ion couple drives the reversible photochemical reaction forward. Consequently, the photochemical reaction efficiency is improved, and the photoresponsive molecules are locked in the activated state until a recovery stimulus is applied. Based on the proposed general scheme, a specific example is given by incorporating the triphenylmethane leucohydroxide and 2-nitrobenzaldehyde molecules into a polyacrylamide hydrogel. The swelling behavior is characterized, and the reprogrammable morphing with precisely controlled geometry is demonstrated.

Reactive 3D Printing of Shape-Programmable Liquid Crystal Elastomer Actuators

3D printed, stimuli-responsive materials that reversibly actuate between programmed shapes are promising for applications ranging from biomedical implants to soft robotics. However, current 3D printing of reversible actuators significantly limits the range of possible shapes and/or shape responses because they couple the print path to mathematically determined director profiles to elicit a desired shape change. Here, we report a reactive 3D-printing method that decouples printing and shape-programming steps, enabling a broad range of complex architectures and virtually any arbitrary shape changes. This method involves first printing liquid crystal elastomer (LCE) precursor solution into a catalyst bath, producing complex architectures defined by printing. Shape changes are then programmed through mechanical deformation and UV irradiation. Upon heating and cooling, the LCE reversibly shape-shifts between printed and programmed shapes, respectively. The potential of this method was demonstrated by programming a variety of arbitrary shape changes in a single printed material, producing auxetic LCE structures and symmetry-breaking shape changes in LCE sheets.

Compact Shape Morphing Tensegrity Robots Capable of Locomotion

Robustness, compactness, and portability of tensegrity robots make them suitable candidates for locomotion on unknown terrains. Despite these advantages, challenges remain relating to ease of fabrication, shape morphing (packing-unpacking), and locomotion capabilities. The paper introduces a design methodology for fabricating tensegrity robots of varying morphologies with modular components. The design methodology utilizes perforated links, coplanar (2D) alignment of components and individual cable tensioning to achieve a 3D tensegrity structure. These techniques are utilized to fabricate prism (three-link) tensegrity structures, followed by tensegrity robots in icosahedron (six-link), and shpericon (curved two-link) formation. The methodology is used to explore different robot morphologies that attempt to minimize structural complexity (number of elements) while facilitating smooth locomotion (impact between robot and surface). Locomotion strategies for such robots involve altering the position of center-of-mass (referred to as internal mass shifting) to induce “tip-over.” As an example, a sphericon formation comprising of two orthogonally placed circular arcs with conincident center illustrates smooth locomotion along a line (one degree of freedom). The design of curved links of tensegrity mechanisms facilitates continuous change of the point of contact (along the curve) that results from the tip-over. This contrasts to the sudden and piece-wise continuous change for the case of robots with traditional straight links which generate impulse reaction forces during locomotion. The two resulting robots—the Icosahedron and the Sphericon Tensegrity Robots—display shape morphing (packing-unpacking) capabilities and achieve locomotion through internal mass-shifting. The presented static equilibrium analysis of sphericon with mass is the first step in the direction of dynamic locomotion control of these curved link robots.

Shape Morphing of Hydrogels in Alternating Magnetic Field

Shape-morphing hydrogels have found a myriad of applications in biomimetics, soft robotics, and biomedical engineering. A magnetic field is favorable for specific applications of hydrogels, since it is noncontact and biocompatible at high field strengths. However, most magnetosensitive shape-morphing structures are made of elastomers rather than hydrogels because the magnetization of magnetic hydrogels is usually too low to be actuated under a static magnetic field. Here, we propose a strategy to achieve the shape morphing of magnetic hydrogels. We actuate magnetothermal sensitive hydrogels by an alternating magnetic field (AMF), where magnetic poly( N-isopropylacrylamide) hydrogels can be heated by the AMF and can undergo giant volume shrinkage under high temperature. We design the distributing pattern of magnetic hydrogel strips on an elastomer substrate to realize various two-dimensional and three-dimensional shapes such as heart-shape, truss, tube, and helix. Complex three-dimensional origami structures have been demonstrated using elastomer-magnetic hydrogels as hinges. We further demonstrate the combination of magnetic navigation and magnetic shape morphing, by applying both a direct magnetic field and an alternating magnetic field. The strategy may open new opportunities for the shape morphing of functional hydrogels.

Designing Responsive Photonic Crystal Patterns by Using Laser Engraving

Soft photonic crystals are periodic nanostructures that have attracted much attention for their applications in sensors, owing to their tunable structural colors in response to external stimuli. Patterned photonic crystals provide a novel strategy for constructing high-performance photonic materials with unique structures and functions. In this work, laser engraving is used for the first time to design patterns on a layered photonic hydrogel. This approach is based on the integration of laser power and chemical modifications to embed different polymer composites (polyelectrolyte and neutral polymers) along a prescribed laser-printed path. The polyelectrolyte and neutral composites show differential swelling or shrinking, causing a mechanical instability in the layered hydrogel. The resultant soft polymeric materials appear as synchronous tuning in the photonic band gaps in response to external stimuli. This approach is favorable for designing responsive photonic crystals with controllable optical properties and 3D shape transformation. Moreover, it is of great use in developing advanced photonic crystals for applications in sensors, soft actuators, and drug release.

3D Printing of Anisotropic Hydrogels with Bioinspired Motion

Motion in biological organisms often relies on the functional arrangement of anisotropic tissues that linearly expand and contract in response to external signals. However, a general approach that can implement such anisotropic behavior into synthetic soft materials and thereby produce complex motions seen in biological organisms remains a challenge. Here, a bioinspired approach is presented that uses temperature-responsive linear hydrogel actuators, analogous to biological linear contractile elements, as building blocks to create three-dimensional (3D) structures with programmed motions. This approach relies on a generalizable 3D printing method for building 3D structures of hydrogels using a fugitive carrier with shear-thinning properties. This study demonstrates that the metric incompatibility of an orthogonally growing bilayer structure induces a saddle-like shape change, which can be further exploited to produce various bioinspired motions from bending to twisting. The orthogonally growing bilayer structure undergoes a transition from a stretching-dominated motion to a bending-dominated motion during its shape transformation. The modular nature of this approach, together with the flexibility of additive manufacturing, enables the fabrication of multimodular 3D structures with complex motions through the assembly of multiple functional components, which in turn consist of simple linear contractile elements.

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 to date.