Shape-reprogrammable polymers: encoding, erasing, and re-encoding

Reconfiguration, Welding, Reprogramming, and Complex Shape Transformation of An Optical Shape Memory Polymer Network Enabled by Patterned Secondary Crosslinking

Stimuli-triggered generation of complicated 3D shapes from 2D strips or plates without using sophisticated molds is desirable and achieving such 2D-to-3D shape transformation in combination with shape reconfiguration, welding, and reprogramming on a single material is very challenging. Here, a convenient and facile strategy using the solution of a disulfide-containing diamine for patterned secondary crosslinking of an optical shape-memory polymer network is developed to integrate the above performances. The dangling thiolectones attached to the backbones react with the diamine in the solution-deposited region so that the secondary crosslinking may not only weld individual strips into assembled 3D shapes but also suppress the relaxation of the deformed polymer chains to different extents for shape reconfiguration or heating-induced complex 3D deformations. In addition, as the dynamic disulfide bonds can be thermally activated to erase the initial programming information and the excessive thiolectones are available for subsequent patterned crosslinking, the material also allows shape reprogramming. Combining welding with patterning treatment, it is further demonstrated that a gripper can be assembled and photothermally controlled to readily grasp an object.

Programming Soft Shape-Morphing Systems by Harnessing Strain Mismatch and Snap-Through Bistability: A Review

Multi-modal and controllable shape-morphing constitutes the cornerstone of the functionalization of soft actuators/robots. Involving heterogeneity through material layout is a widely used strategy to generate internal mismatches in active morphing structures. Once triggered by external stimuli, the entire structure undergoes cooperative deformation by minimizing the potential energy. However, the intrinsic limitation of soft materials emerges when it comes to applications such as soft actuators or load-bearing structures that require fast response and large output force. Many researchers have explored the use of the structural principle of snap-through bistability as the morphing mechanisms. Bistable or multi-stable mechanical systems possess more than one local energy minimum and are capable of resting in any of these equilibrium states without external forces. The snap-through motion could overcome energy barriers to switch among these stable or metastable states with dramatically distinct geometries. Attributed to the energy storage and release mechanism, such snap-through transition is quite highly efficient, accompanied by fast response speed, large displacement magnitude, high manipulation strength, and moderate driving force. For example, the shape-morphing timescale of conventional hydrogel systems is usually tens of minutes, while the activation time of hydrogel actuators using the elastic snapping instability strategy can be reduced to below 1 s. By rationally embedding stimuli-responsive inclusions to offer the required trigger energy, various controllable snap-through actuations could be achieved. This review summarizes the current shape-morphing programming strategies based on mismatch strain induced by material heterogeneity, with emphasis on how to leverage snap-through bistability to broaden the applications of the shape-morphing structures in soft robotics and mechanical metamaterials.

Optically Reconfigurable Shape Memory Metallo-Polymer Mediated by Carbolong Complex and Radically Exchangeable Covalent Bond

Conventional shape memory polymers (SMPs) are restricted to predetermined permanent shape, therefore cannot be remoulded arbitrarily to adapt to variant application scenarios. Meanwhile, shape memory behaviour is mostly thermally active...

Expanding the codes: The development of density-encoded hydrogel microcarriers for suspension arrays

Although suspension array technology (SAT), which uses encoded microspheres, provides high-quality results with versatile applicability for information-intensive bioanalytic applications, current encoding strategies limit the number of codes that can be distinguished. In this paper, we introduce density-encoded hydrogel microcarriers (DMs), which employ the intrinsic density property of biomaterials as a high-capacity coding dimension. Two hydrogel monomers were employed at different ratios to synthesize microgels with distinctive densities. DMs not only can be simultaneously decoded and separated using density gradient centrifugation, but also are compatible with flow cytometry detection. The size and color of DMs have been used as extra coding parameters, to construct an 8 × 2 × 4 (density × size × color) three-dimensionally encoded hydrogel microcarrier library. With aptamer-functionalized DMs (ADMs), we developed a 4-plex protein quantification method for the label-free detection of plasma biomarkers with sub-nanomolar detection limits and good linearities. Moreover, ADMs can be used for label-free naked-eye detection of tumor-derived exosomes. We believe that the simplicity and functionality of DMs will advance the field of suspension arrays and inspire the development of DM-based diagnostic applications.

Reconstructable Gradient Structures and Reprogrammable 3D Deformations of Hydrogels with Coumarin Units as the Photolabile Crosslinks

Morphing hydrogels have versatile applications in soft robotics, flexible electronics, and biomedical devices. Controlling component distribution and internal stress within a hydrogel is crucial for shape-changing. However, existing gradient structures of hydrogels are usually non-reconstructable, once encoded by chemical reactions and covalent bonds. Fabricating hydrogels with distinct gradient structures is inevitable for every new configuration, resulting in poor reusability, adaptability, and sustainability that are disadvantageous for diverse applications. Herein, a hydrogel containing reversible photo-crosslinks that enable reprogramming of the gradient structures and 3D deformations into various configurations is reported. The hydrogel is prepared by micellar polymerization of hydrophobic coumarin monomer and hydrophilic acrylic acid. The presence of hexadecyltrimethylammonium chloride micelles increases the local concentration of coumarin units and also improves the mechanical properties of the hydrogel by forming robust polyelectrolyte/surfactant complexes that serve as the physical crosslinks. High-efficiency photodimerization and photocleavage reactions of coumarins are realized under 365 and 254 nm light irradiation, respectively, affording reversible tuning of the network structure of the hydrogel. Through photolithography, different gradient structures are sequentially patterned in one hydrogel that direct the deformations into distinct configurations. Such a strategy should be applicable for other photolabile hydrogels toward reprogrammable control of network structures and versatile functions.

Reconfiguring Gaussian Curvature of Hydrogel Sheets with Photoswitchable Host-Guest Interactions

Photoinduced shape morphing has implications in fields ranging from soft robotics to biomedical devices. Despite considerable effort in this area, it remains a challenge to design materials that can be both rapidly deployed and reconfigured into multiple different three-dimensional forms, particularly in aqueous environments. In this work, we present a simple method to program and rewrite spatial variations in swelling and, therefore, Gaussian curvature in thin sheets of hydrogels using photoswitchable supramolecular complexation of azobenzene pendent groups with dissolved α-cyclodextrin. We show that the extent of swelling can be programmed via the proportion of azobenzene isomers, with a 60% decrease in areal swelling from the all trans to the predominantly cis state near room temperature. The use of thin gel sheets provides fast response times in the range of a few tens of seconds, while the shape change is persistent in the absence of light thanks to the slow rate of thermal cis-trans isomerization. Finally, we demonstrate that a single gel sheet can be programmed with a first swelling pattern via spatially defined illumination with ultraviolet light, then erased with white light, and finally redeployed with a different swelling pattern.

New Morphogenetic Strategy Inspired by the Viscoelasticity of Polymers

A new morphogenetic strategy was developed to realize continuously modulated and reprogrammable three-dimensional shape transitions by fully exploring the potential of macromolecular conformational modulations. Geometric information was defined in the planar shape memory polymeric sheets through the application of spatially differentiated thermo-temporal conditions in the shape memory creation stage. Due to the viscoelasticity of polymers, nonuniform inner stress distribution was encoded in spite of the homogeneous composition, which was released under the activation of uniform heating. Compared to the traditional shape-programming strategies, the present research offered the opportunity to generate physical patterns by modulating the thermal histories of polymers. It brought the advantages of a continuously regulated degree of discrepancy between different regions, which enabled fine-tuning of the targeted three-dimensional (3D) shape. In addition, suitable annealing treatment could lead to the elimination of thermal history. That is, the geometric information could be erased and re-encoded, making unlimited diverse 3D structures from the same piece of polymer a reality.

Reconfigurable Soft Actuators with Multiple-Stimuli Responses

Multiple-stimuli responsive soft actuators with tunable initial shapes would have substantial potential in broad technological applications, ranging from advanced sensors, smart robots to biomedical devices. However, existing soft actuators are often limited to single initial shape and are unable to reversibly reconfigure into desirable shapes, which severely restricts the multifunctions that can be integrated into one actuator. Here, a novel reconfigurable supramolecular polymer/polyethylene terephthalate (PET) bilayer actuator exhibiting multiple-stimuli responses is presented. In this bilayer actuator, the supramolecular polymer layer constructed of poly(5-Norbornene-2-carboxylic acid-1,3-cyclooctadiene) (PNCCO) and azopyridine derivative (PyAzoPy) via H-bonds provides multiple-stimuli responses: PyAzoPy offers light response and carboxylic groups in PNCCO endow the actuator with humidity response. Meanwhile thermoplastic PET layer enables the bilayer actuators to be reconfigured into various shapes by thermal stimuli. The rationally designed actuators exhibit versatile capabilities to reversibly reconfigure into a set of initial shapes and carry out multiple functions, such as photo-driven "foldback-clip" and Ω-shaped crawling robots. In addition, bio-inspired plants constructed by reconfiguration of such actuators demonstrate reversible multiple-stimuli responses. It is anticipated that these novel actuators with highly tunable geometries and actuation modes would be useful to develop multifunctional devices capable of performing diverse tasks.

Multipolar spatial electric field modulation for freeform electroactive hydrogel actuation

Electroactive hydrogels that exhibit large deformation in response to an electric field have received significant attention as a potential actuating material for soft actuators and artificial muscle. However, their mechanical actuation has been limited in simple bending or folding due to uniform electric field modulation. To implement complex movements, a pre-program, such as a hinge and a multilayer pattern, is usually required for the actuator in advance. Here, we propose a reprogrammable actuating method and sophisticated manipulation by using multipolar three-dimensional electric field modulation without pre-program. Through the multipolar spatial electric field modulator, which controls the polarity/intensity of the electric field in three-dimensions, complex three-dimensional (3D) actuation of single hydrogels are achieved. Also, air bubbles generated during operation in the conventional horizontal configuration are not an issue in the proposed new vertical configuration. We demonstrate soft robotic actuators, including basic bending mechanics in terms of controllability and reliability, and several 3D shapes having positive and negative curvature can easily be achieved in a single sheet, paving the way for continuously reconfigurable materials.

Reprogrammable 3D Shaping from Phase Change Microstructures in Elastic Composites

Herein, we demonstrate reprogrammable 3D structures that are assembled from elastic composite sheets made from elastic materials and phase change microparticles. By controlling the phase change of the microparticles by localized thermal patterning, anisotropic residual strain is generated in the pre-stretched composite sheets and then triggers 3D structure assembly when the composite sheets are released from the external stress. Modulation of the geometries and location of the thermal patterns leads to complex 2D-3D shaping behaviors such as bending, folding, buckling, and wrinkling. Because of the reversible phase change of the microparticles, these programmed 3D structures can later be recovered to 2D sheets once they are heated for reprogramming different 3D structures. To predict the 3D structures assembled from the 2D composite sheets, finite element modeling was employed, which showed reasonable agreement with the experiments. The demonstrated strategy of reversibly programming 3D shapes by controlling the phase change microstructures in the elastic composites offers unique capabilities in fabricating functional devices such as a rewritable "paper" and a shape reconfigurable pneumatic actuator.

Photonic Shape Memory Polymer Based on Liquid Crystalline Blue Phase Films

Photonic shape memory (SM) polymers based on liquid crystalline blue phase (BP) films have been fabricated by self-assembly and subsequent photopolymerization of liquid-crystal mixtures. These freestanding BP films exhibit narrow photonic band gaps and high reflectivity in the visible wavelength range. Multiple blue-shift colors are achieved by SM programming process at different mechanical pressures. The blue-shift colors can be attributed to a decrease of effective BP pitch along the viewing direction caused by the compressed deformation of the BP films, which are confirmed by a three-dimensional interometric profile. The deformed BP films can recover to their original shapes and reflecting colors by heating the polymer films to temperatures above the glass-transition temperature. Quantitative relationships between the shape change and optical response are established for understanding this SM effect. What is more, the temporary photonic patterns can be reversibly written and erased for dozens of cycles without apparent degradation, making these freestanding BP films appealing as rewritable photonic papers and optical sensors.

Matching Magnetic Heating and Thermal Actuation for Sequential Coupling in Hybrid Composites by Design

Sequentially coupling two material functions requires matching the output from the first with the input of the second function. Here, magnetic heating controls thermal actuation of a hybrid composite in a challenging system environment causing an elevated level of heat loss. The concept is a hierarchical design consisting of an inner actuator of nanocomposite material, which can be remotely heated by exposure to an alternating magnetic field (AMF) and outer layers of a porous composite system with a closed pore morphology. These porous layers act as heat insulators and as barriers to the surrounding water. By exposure to the AMF, a local bulk temperature of 71 °C enables the magnetic actuation of the device, while the temperature of the surrounding water is kept below 50 °C. Interestingly, the heat loss during magnetic heating leads to an increase of the water phase (small volume) temperature. The temperature increase is able to sequentially trigger an adjacent thermal actuator attached to the actuator composite. In this way it could be demonstrated how the AMF is able to initiate two kinds of independent actuations, which might be interesting for robotics operating in aqueous environments.

Programming the Shape Transformation of a Composite Hydrogel Sheet via Erasable and Rewritable Nanoparticle Patterns

Hydrogels with shapes that can be adapted to their environment have attracted great attention from both academia and industry. We report herein a new and robust strategy to reprogram the light-induced shape transformation of a thermoresponsive composite hydrogel sheet with erasable and rewritable patterns of iron oxide nanoparticles as photothermal agents. Numerous distinct and reversible shape transformations are achieved from a single hydrogel sheet by repeatably writing in the sheet with different nanoparticle patterns. The shape transformations were verified by finite element modeling. The present strategy is simple, fast, and efficient in reprogramming the shape change of the thermoresponsive hydrogel material. The composite hydrogel sheet may find applications in soft robotics, tissue engineering, and controlled release.

Design of Soft Origami Mechanisms with Targeted Symmetries

The integration of soft actuating materials within origami-based mechanisms is a novel method to amplify the actuated motion and tune the compliance of systems for low stiffness applications. Origami structures provide natural flexibility given the extreme geometric difference between thickness and length, and the energetically preferred bending deformation mode can naturally be used as a form of actuation. However, origami fold patterns that are designed for specific actuation motions and mechanical loading scenarios are needed to expand the library of fold-based actuation strategies. In this study, a recently developed optimization framework for maximizing the performance of compliant origami mechanisms is utilized to discover optimal actuating fold patterns. Variant patterns are discovered through exploring different symmetries in the input and output conditions of the optimization problem. Patterns designed for twist (rotational symmetry) yield significantly better performance, in terms of both geometric advantage and energy requirements, than patterns exhibiting vertical reflection symmetries. The mechanical energy requirements for each design are analyzed and compared for both the small and large applied displacement regimes. Utilizing the patterns discovered through optimization, the multistability of the actuating arms is demonstrated empirically with a paper prototype, where the stable configurations are accessed through local vertex pop-through instabilities. Lastly, the coupled mechanics of fold networks in these actuators yield useful macroscopic motions and can achieve stable shape change through accessing the local vertex instabilities. This survey of origami mechanisms, energy comparison, and multistability characterization provides a new set of designs for future integration with soft actuating materials.

Dual-Programmable Shape-Morphing and Self-Healing Organohydrogels Through Orthogonal Supramolecular Heteronetworks

Programmable materials that can change their inherent shapes or properties are highly desirable due to their promising applications. However, among various programmable shape-morphing materials, the single control route allows temporary states to recover the unchangeable former state, thus lacking the sophisticated programmability for their shape-encoding behaviors and mechanics. Herein, dual-programmable shape-morphing organohydrogels featuring supramolecular heteronetworks are developed. In the system, the metallo-supramolecular hydrogel framework and micro-organogels featuring semicrystalline comb-type networks independently respond to different stimuli, thereby providing orthogonal dual-switching mechanics and ultrahigh mechanical strength. The supramolecular heteronetworks also possess excellent self-healing properties. More notably, such orthogonal supramolecular heteronetworks demonstrate hierarchical shape morphing performance that far exceeds conventional shape-morphing materials. Utilizing this dual programming strategy of the orthogonal supramolecular heteronetworks, the material's permanent shape can be manipulated in a step-wise shape morphing process, thereby realizing sophisticated shape changes with a high degree of freedom. The organohydrogels can act as a biomimetic smart device for the on-demand control of unidirectional liquid transport. Based on these characteristics, it is anticipated that the supramolecular organohydrogels may serve as adaptive programmable materials for a variety of applications.

Solvent-assisted programming of flat polymer sheets into reconfigurable and self-healing 3D structures

It is extremely challenging, yet critically desirable to convert 2D plastic films into 3D structures without any assisting equipment. Taking the advantage of solvent-induced bond-exchange reaction and elastic-plastic transition, shape programming of flat vitrimer polymer sheets offers a new way to obtain 3D structures or topologies, which are hard for traditional molding to achieve. Here we show that such programming can be achieved with a pipette, a hair dryer, and a bottle of solvent. The polymer used here is very similar to the commercial epoxy, except that a small percentage of a specific catalyst is involved to facilitate the bond-exchange reaction. The programmed 3D structures can later be erased, reprogrammed, welded with others, and healed again and again, using the same solvent-assisted technique. The 3D structures can also be recycled by hot-pressing into new sheets, which can still be repeatedly programmed.

Humidity- and Sunlight-Driven Motion of a Chemically Bonded Polymer Bilayer with Programmable Surface Patterns

We report a bilayer of sodium alginate/polyvinylidene fluoride (SA/PVDF) that is chemically bonded through a series of interfacial coupling reactions. The SA layer is hydrophilic in structure and is capable of strong interaction with water molecules, thus presenting high sensitivity to humidity, whereas the PVDF layer is hydrophobic, inert to humidity. This structural feature results in the bilayer having asymmetric humidity-responsive performances that can thus make its shape change with directionality, which cannot be achieved in an SA single layer. The responsive process to humidity can be adjusted by exposure of the bilayer to sunlight by means of a photothermal effect that accelerates dehydration of the bilayer to cause more rapid shape deformations. When the sunlight is removed, the bilayer adsorbs humidity again and returns to its original shape, indicating good reversibility. To exactly regulate the shape deformations of the bilayer with external stimuli, we employ Ca²⁺-treated filter paper to customize crosslinking reactions in the SA layer as desired patterns which are capable of causing different mechanical tensors and swellabilities in the bilayer so as to regulate and control the actuations for self-folding, curling, twisting, and coiling in response to sunlight and humidity.On the other hand, the chemically bonded bilayer has stronger interfacial toughness and is capable of reaching 300 J m^-2, which is around 12 times the interfacial toughness of the physically combined bilayer; as a result, the chemically bonded bilayer is capable of sustaining continuous shape deformations without interfacial failure. The directionally mechanical actuations can be utilized in designing an indicator to roughly indicate the range of intensity of sunlight by coupling the chemically bonded bilayer into a typical electric circuit, in which the range of intensity of sunlight can be easily estimated by visual observation of the light-emitting diodes.

Vapomechanically Responsive Motion of Microchannel-Programmed Actuators

Materials that respond rapidly and reversibly to external stimuli currently stand among the top choices as actuators for real-world applications. Here, a series of programmable actuators fabricated as single- or bilayer elements is described that can reversibly respond to minute concentrations of acetone vapors. By using templates, microchannel structures are replicated onto the surface of two highly elastic polymers, polyvinylidene fluoride (PVDF) and polyvinyl alcohol, to induce chiral coiling upon exposure to acetone vapors. The vapomechanical coiling is reversible and can be conducted repeatedly over 100 times without apparent fatigue. If they are immersed in liquid acetone, the actuators are saturated with the solvent and temporarily lose their motility but regain their shape and activity within seconds after the solvent evaporates. The desorption of acetone from the PVDF layer is four times faster than its adsorption, and the actuator composed of a single PVDF layer maintains its ability to move over an acetone-soaked filter paper even after several days. The controllable and reproducible sensing capability of this smart material can be utilized for actuating dynamic elements in soft robotics.

Visible Light-Induced Plasticity of Shape Memory Polymers

Plasticity of thermoset polymers has been realized by introducing exchangeable bonds, and the plasticity is mostly triggered via heat or UV light. Visible light is a relatively mild trigger that has not been used to induce plasticity in polymer materials. Herein, thermoset polyurethanes (PUs) containing diselenide bonds are fabricated that possess visible light-induced plasticity along with shape memory behavior. A series of PUs with different diselenide bond contents were tested and their shape memory properties and plasticity varied. With a higher diselenide bond content, both shape memory and light-induced plasticity are achieved. By combining these two properties, reshaping the permanent shapes of the PUs is easier. Compared with heat or UV light, visible light has the advantage of spatial control. For instance, a pattern of visible light was introduced by a commercial projector to demonstrate facile reshaping of the materials. Because visible light can be introduced via various methods, PUs with visible light-induced plasticity have great potential applications.

Belousov-Zhabotinsky autonomic hydrogel composites: Regulating waves via asymmetry

Belousov-Zhabotinsky (BZ) autonomic hydrogel composites contain active nodes of immobilized catalyst (Ru) encased within a nonactive matrix. Designing functional hierarchies of chemical and mechanical communication between these nodes enables applications ranging from encryption, sensors, and mechanochemical actuators to artificial skin. However, robust design rules and verification of computational models are challenged by insufficient understanding of the relative importance of local (molecular) heterogeneities, active node shape, and embedment geometry on transient and steady-state behavior. We demonstrate the predominance of asymmetric embedment and node shape in low-strain, BZ-gelatin composites and correlate behavior with gradients in BZ reactants. Asymmetric embedment of square and rectangular nodes results in directional steady-state waves that initiate at the embedded edge and propagate toward the free edge. In contrast, symmetric embedment does not produce preferential wave propagation because of a lack of diffusion gradient across the catalyzed region. The initiation at the embedded edge is correlated with bromide absorption by the inactive matrix, which locally elevates the bromate concentration required for catalyst oxidation. The competition between embedment asymmetry and node geometry was used to demonstrate a repeatable switch in wave direction that functions as a signal delay. Furthermore, signal propagation in or out of the composite was demonstrated via embedment asymmetry and relative dimensions of a T-shaped active network node. Overall, structural asymmetry provides a robust approach to controlling initiation and orientation of chemical-mechanical communication within composite BZ gels.

Electro-molecular Assembly: Electrical Writing of Information into an Erasable Polysaccharide Medium

We report that information can be written into an erasable hydrogel medium by precisely imposing controlled electrical signals that trigger supramolecular self-assembly. We prepare the medium from a blend of two stimuli-responsive self-assembling polysaccharides agarose (thermally responsive) and chitosan (pH-responsive). Upon cooling the blend, agarose forms the hydrogel medium while the embedded chitosan chains can be induced to self-assemble in response to imposed pH cues. Importantly, these triggering pH-cues can be imposed electrically (by inserted electrodes) enabling complex messages (e.g., self-assembled multilayers) to be written within the hydrogel medium. The reversibility of these self-assembly mechanisms allow the written information, and the medium itself, to be erased. These physicochemical properties enable this dual responsive medium to encrypt information, while the responsiveness of this structural information and the biocompatibility of the medium suggest uses for accessing/reporting information in diverse life science applications, such as foods, cosmetics, medicine, and the environment.

Exploring Dynamic Equilibrium of Diels-Alder Reaction for Solid State Plasticity in Remoldable Shape Memory Polymer Network

The reversible and click nature of Diels-Alder (DA) reactions has made them ideal candidates to design materials with nonconventional properties. Most commonly, the reversibility of DA is utilized for designing thermosets that can be liquefied for reprocessing and self-healing, yet the dynamic equilibrium nature has been largely neglected. In this work, shape memory polymers (SMP) containing DA moieties in the networks were synthesized. In addition to its remoldability at the liquid state at sufficiently high temperatures (above 110 °C), we show uniquely and surprisingly that such a network can undergo plastic deformation in its solid state at intermediate temperatures (60-100 °C) by taking advantage of its dynamic equilibrium for network topological rearrangement. The liquid state remoldability and solid state plasticity represent two distinct yet complementary mechanisms to manipulate the permanent shape of an SMP, leading to unprecedented versatility that can benefit a variety of applications in the future.

Polydopamine coated shape memory polymer: enabling light triggered shape recovery, light controlled shape reprogramming and surface functionalization

Photo-active shape memory polymers (SMPs) are considered as a promising candidate for converting light into mechanical energy. However, most known SMPs are only thermo-responsive. To achieve photo-activity, photo-responsive choromophores or fillers usually have to be incorporated from the very beginning of the material synthesis. Here, we introduce a novel post-synthesis approach to endow normal SMPs with photo-active properties using mussel-inspired surface chemistry. Without changing the original properties, the resultant polydopamine (PDA) coated SMPs show an efficient photo-active performance. The coating can be easily patterned and erased, which allows flexible light-triggered 3-D shape deformation of a planar SMP sheet. Moreover, owing to the high chemical activity, the PDA coating also provides a platform to optimize the surface properties of the photo-responsive SMPs through secondary surface modification.

Polysaccharide-based freestanding multilayered membranes exhibiting reversible switchable properties

The design of self-standing multilayered structures based on biopolymers has been attracting increasing interest due to their potential in the biomedical field. However, their use has been limited due to their gel-like properties. Herein, we report the combination of covalent and ionic cross-linking, using natural and non-cytotoxic cross-linkers, such as genipin and calcium chloride (CaCl2). Combining both cross-linking types the mechanical properties of the multilayers increased and the water uptake ability decreased. The ionic cross-linking of multilayered chitosan (CHI)-alginate (ALG) films led to freestanding membranes with multiple interesting properties, such as: improved mechanical strength, calcium-induced adhesion and shape memory ability. The use of CaCl2 also offered the possibility of reversibly switching all of these properties by simple immersion in a chelate solution. We attribute the switch-ability of the mechanical properties, shape memory ability and the propensity for induced-adhesion to the ionic cross-linking of the multilayers. These findings suggested the potential of the developed polysaccharide freestanding membranes in a plethora of research fields, including in biomedical and biotechnological fields.

Shape memory polymer network with thermally distinct elasticity and plasticity

Stimuli-responsive materials with sophisticated yet controllable shape-changing behaviors are highly desirable for real-world device applications. Among various shape-changing materials, the elastic nature of shape memory polymers allows fixation of temporary shapes that can recover on demand, whereas polymers with exchangeable bonds can undergo permanent shape change via plasticity. We integrate the elasticity and plasticity into a single polymer network. Rational molecular design allows these two opposite behaviors to be realized at different temperature ranges without any overlap. By exploring the cumulative nature of the plasticity, we demonstrate easy manipulation of highly complex shapes that is otherwise extremely challenging. The dynamic shape-changing behavior paves a new way for fabricating geometrically complex multifunctional devices.

Transformative Two-Dimensional Array Configurations by Geometrical Shape-Shifting Protein Microstructures

Two-dimensional (2D) geometrical shape-shifting is prevalent in nature, but remains challenging in man-made "smart" materials, which are typically limited to single-direction responses. Here, we fabricate geometrical shape-shifting bovine serum albumin (BSA) microstructures to achieve circle-to-polygon and polygon-to-circle geometrical transformations. In addition, transformative two-dimensional microstructure arrays are demonstrated by the ensemble of these responsive microstructures to confer structure-to-function properties. The design strategy of our geometrical shape-shifting microstructures focuses on embedding precisely positioned rigid skeletal frames within responsive BSA matrices to direct their anisotropic swelling under pH stimulus. This is achieved using layer-by-layer two photon lithography, which is a direct laser writing technique capable of rendering spatial resolution in the sub-micrometer length scale. By controlling the shape, orientation and number of the embedded skeletal frames, we have demonstrated well-defined arc-to-corner and corner-to-arc transformations, which are essential for dynamic circle-to-polygon and polygon-to-circle shape-shifting, respectively. We further fabricate our shape-shifting microstructures in periodic arrays to experimentally demonstrate the first transformative 2D patterned arrays. Such versatile array configuration transformations give rise to structure-to-physical properties, including array porosity and pore shape, which are crucial for the development of on-demand multifunctional "smart" materials, especially in the field of photonics and microfluidics.

Mechanically programmed shape change in laminated elastomeric composites

Soft, anisotropic materials, such as myocardium in the heart and the extracellular matrix surrounding cells, are commonly found in nature. This anisotropy leads to specialized responses and is imperative to material functionality, yet few soft materials exhibiting similar anisotropy have been developed. Our group introduced an anisotropic shape memory elastomeric composite (A-SMEC) composed of non-woven, aligned polymer fibers embedded in an elastomeric matrix. The composite exhibited shape memory (SM) behavior with significant anisotropy in room-temperature shape fixing. Here, we exploit this anisotropy by bonding together laminates with oblique anisotropy such that tensile deformation at room temperature - mechanical programming - results in coiling. This response is a breakthrough in mechanical programming, since non-affine shape change is achieved by simply stretching the layered A-SMECs at room temperature. We will show that pitch and curvature of curled geometries depend on fiber orientations and the degree of strain programmed into the material. To validate experimental results, a model was developed that captures the viscoplastic response of A-SMECs. Theoretical results correlated well with experimental data, supporting our conclusions and ensuring attainability of predictable curling geometries. We envision these smart, soft, shape changing materials will have aerospace and medical applications.