Switchable Mechanical Properties Research Articles

Shape memory polymers enable versatile magneto-active structure with 4D printability, variable stiffness, shape-morphing and effective grasping

Magneto-active structures with non-contact actuation, precise controllability, strong penetrability, and biological harmlessness have wide application prospects in the fields of soft robots, bionic engineering, medical treatment, and flexible transmission, among others. This paper presents the design of a magneto-active shape memory polymer (MASMP) based on a blending matrix of polycaprolactone and thermoplastic polyurethane. The aim is to enhance the environmental adaptability, manufacturability, and diversification of actuation modes of magnetic-active actuators. We conducted an analysis of the magnetic/thermal switchable mechanical properties and shape memory properties of MASMP. The results demonstrate excellent shape fixation and shape recovery rates, as well as excellent flexibility and magnetorheological effects. We fabricated flexible printing filaments with a uniform diameter by using a screw extruder, which enables high-precision 4D printing for MASMP. We printed several magneto-active structures, which demonstrated variable stiffness, shape-morphing, and shape memory capabilities. The shape-morphing performance is in good agreement with the simulation results of the design process. Furthermore, we successfully printed and assembled a flexible claw that exhibits an effective grasping and release function activated by a magnetic field. Finally, we developed a magneto-active structure with negative Poisson’s ratio, showing great potential for application in metamaterial structure.

Switchable Stiffness of Composite Hydrogels Triggered by Thermoresponsive Phase-Change Particles

Abstract Thermoresponsive hydrogel with switchable mechanical properties was developed by incorporating poly(stearyl methacrylate) (PSMA) as a responsive domain into bacterial cellulose (BC) as a supporting hydrogel. The PSMA particles in BC exhibited a reinforcing effect below their melting temperature (Tm) and a reduced effect above Tm, leading to significant responsive behavior.

Skin-Inspired Tough Elastomer with Moisture-Triggered Switchable Mechanical Properties.

Biological tissue usually exhibits good water adaptive mechanical properties, which can maintain high strength and toughness in both wet and dry states. However, synthetic tissue like hydrogel usually becomes hard and brittleness in its dry state. Here this challenge is met by exploring iron-catechol complex (TA-Fe3+ ) as a great platform combining extremely different polymers (elastomer and hydrogel) to construct new tissue-like soft composite materials with two different continuous phases, which have not yet been reported. In its dry state, the xerogel phase becomes a reinforced segment to increase the strength of PB without losing its toughness. In its wet state, this soft material becomes high performance hydrogel, where hydrogel phase absorbs high water and elastomer phase can sustain high loading. Such heterogeneous phase structures provide a good idea for designing the soft materials, exhibiting a trade-off between its high strength and toughness in both wet and dry states. Furthermore, its shape memory behaviors in both its wet and dry state, which shows a great potential application for complex adaptive shape transformation and engineering application like lifting of heavy objects under remote control due to high photo-thermal transition of TA-Fe3+ is explored.

Nanostructured block copolymer muscles.

High-performance actuating materials are necessary for advances in robotics, prosthetics and smart clothing. Here we report a class of fibre actuators that combine solution-phase block copolymer self-assembly and strain-programmed crystallization. The actuators consist of highly aligned nanoscale structures with alternating crystalline and amorphous domains, resembling the ordered and striated pattern of mammalian skeletal muscle. The reported nanostructured block copolymer muscles excel in several aspects compared with current actuators, including efficiency (75.5%), actuation strain (80%) and mechanical properties (for example, strain-at-break of up to 900% and toughness of up to 121.2 MJ m-3). The fibres exhibit on/off rotary actuation with a peak rotational speed of 450 r.p.m. Furthermore, the reported fibres demonstrate multi-trigger actuation (heat and hydration), offering switchable mechanical properties and various operating modes. The versatility and recyclability of the polymer fibres, combined with the facile fabrication method, opens new avenues for creating multifunctional and recyclable actuators using block copolymers.

A Stiffness-Switchable, Biomimetic Smart Material Enabled by Supramolecular Reconfiguration.

In nature, stiffness-changing behavior is essential for living organisms, which, however, is challenging to achieve in synthetic materials. Here, a stiffness-changing smart material, through developing interchangeable supramolecular configurations inspired from the dermis of the sea cucumber, which shows extreme, switchable mechanical properties, is reported. In the hydrated state, the material, possessing a stretched, double-stranded supramolecular network, showcases a soft-gel behavior with a low stiffness and high pliability. Upon the stimulation of ethanol to transform into the coiled supramolecular configuration, it self-adjusts to a hard state with nearly 500-times enhanced stiffness from 0.51to 243.6MPa, outstanding load-bearing capability (over 35000times its own weight), and excellent puncture/impact resistance with a specific impact strength of ≈116kJm-2 (gcm-3 )-1 (higher than some metals and alloys such as aluminum, and even comparable to the commercially available protective materials such as D3O and Kevlar). Moreover, this material demonstrates reconfiguration-dependent self-healing behavior and designable formability, holding great promise in advanced engineering fields that require both high-strength durability and good formability. This work may open up a new perspective for the development of self-regulating materials from supramolecular-scale configuration regulation.

Photolithographic Fabrication of Mechanically Adaptive Devices.

Water-responsive polymers, which enable the design of objects whose mechanical properties or shape can be altered upon moderate swelling, are useful for a broad range of applications. However, the limited processing options of materials that exhibit useful switchable mechanical properties generally restricted their application to objects having a simple geometry. Here we show that this problem can be overcome by using a negative photoresist approach in which a linear hydrophilic polymer is converted into a highly transparent cross-linked polymer network. The photolithographic process allows the facile production of objects of complex shape and permits programming of the cross-link density, the extent of aqueous swelling, and thereby the stiffness and refractive index under physiological conditions over a wide range and with high spatial resolution. Our findings validate a straightforward route to fabricate mechanically adaptive devices for a variety of (biomedical) uses, notably optogenetic implants whose overall shape, mechanical contrast, and optical channels can all be defined by photolithography.

The first microcapsule-based self-healing cement–bentonite cut-off wall materials

Despite the extensive use of cement–bentonite in contaminated land containment applications, there are still many challenges related to durability. The development of self-healing cement–bentonite materials could provide more resilient, sustainable and reliable cut-off walls with significantly enhanced durability, reduced maintenance costs, enhanced safety and protection against sudden or undetected failure. The objective of this study was to develop self-healing cement–bentonite cut-off wall materials incorporating microcapsules. Microencapsulated sodium silicate, as a healing agent, would be released into cracks when the microcapsules rupture as a result of any damage incurred, and would react with the cement–bentonite matrix to fill and heal the cracks. Novel microcapsules with switchable mechanical properties developed for self-healing cement applications were employed here. The results demonstrated the enhanced average crack mouth healing and recovered permeability performance provided by the microcapsules compared to control cement–bentonite samples. X-ray microcomputed tomography and scanning electron microscopy were applied to investigate the self-healing process of the microcapsule-containing cement–bentonite system. The microstructural analysis confirmed the survivability, uniform dispersion and crack-triggered rupture of the microcapsules, as well as the release of the healing agent and the generation of hydration products within the cracks. These are promising results for the application of the microcapsule-based system for self-healing cement–bentonite cut-off wall materials.

Synthesis of novel cellulose mixesters for transparent responsive films with switchable mechanical properties

Soft actuators have attracted much attention during the last decades due to their potentially broad applications ranging from sensors to artificial muscles. Up to now, most of such soft actuators are still based on synthetic polymers. Herein, we described a novel group of materials derived from sustainable cellulose as potential starting materials for the preparation of soft actuators. To be precisely, cellulose mixesters with acyl esters of distinct chain lengths and cinnamic ester were synthesized through a two-step synthesis and used for the preparation of responsive thin film actuators. These cellulose mixesters have degree of substitution ascribed to stearoyl groups (DSSt) of 1.51 or lauroyl groups (DSLa) of 1.48 and degree of substitution ascribed to cinnamoyl groups (DSCi) around 1.35. Furthermore, the influence of the chain lengths of the two different aliphatic esters as well as the switchable crosslinking of cinnamoyl moieties on the materials properties was further shown. Using these cellulose mixesters, transparent thin films with thickness of 4−15 μm were prepared via facile solvent casting. These films had light-responsive mechanical properties from rigid to elastic after illumination with UV light of distinct wavelengths (254 nm and 310−400 nm). Moreover, the thin films showed shape memory effect and the ability of self-healing.

Bioinspired Mechanically Responsive Hydrogel upon Redox Mediated by Dynamic Coordination between Telluroether and Platinum Ions

Inspired by natural lives, materials with switchable mechanical properties have been constructed by virtue of the development of dynamic chemistry. However, the limited dynamic reactions are hampering their further development. The progress in organotellurium chemistry brings opportunity to open new frontiers of dynamic reactions for fabrication of mechanically responsive materials. Herein, a bioinspired mechanically responsive hydrogel upon redox mediated by dynamic coordination between telluroether and platinum ions is reported. The low electronegativity and superior σ electron-donating capability of telluroether facilitate its coordination with Pt ions. Remarkably, the coordination interaction is highly redox-active. Telluroether can dissociate from Pt ions upon oxidation (e.g., H2O2) because the effect of the polar solvent dramatically reduces the thermodynamic activity of oxidized telluroether. In turn, the coordination interaction is easily regenerated via reduction (e.g., vitamin C) of oxidized tel...

Versatile magnetorheological plastomer with 3D printability, switchable mechanics, shape memory, and self-healing capacity

The rapid advancement in soft actuators imposes an emergent requirement for soft stimuli-sensitive materials that are deformable and stiffness variable and show designability and adaptivity. Soft actuators based on magneto-sensitive materials with outstanding magnetic-control performance are highly desirable in research. In this paper, we developed a versatile magnetorheological plastomer (MRP) based on polycaprolactone (PCL)/thermoplastic polyurethane (TPU) polymer blends. The MRP showed 3D printability, switchable mechanics, shape memory, and self-healing properties. The thermoplasticity of the matrix enables fused deposition modeling 3D printing, which affords the MRP excellent shape designability. By taking advantage of the phase transition and magnetorheological effect, the dramatic switchable mechanical properties of MRP can be triggered by thermal stimulus and magnetic field. The influences of matrix, particle content, temperature and magnetic field on the mechanical properties were discussed comprehensively, and possible physical mechanisms were proposed so that the result can be qualitatively explained. Based on hybrid crystalline and amorphous regions of PCL and TPU, the MRP exhibited superior shape memory and self-healing properties. This work may play an important role in the future development of multifunctional magneto-sensitive material and promote the application of soft actuators in the fields of soft robotics, medical care, and bionics applications.

Bioinspired Polymer Systems with Stimuli-Responsive Mechanical Properties.

Materials with switchable mechanical properties are widespread in living organisms and endow many species with traits that are essential for their survival. Many of the mechanically morphing materials systems found in nature are based on hierarchical structures, which are the basis for mechanical robustness and often also the key to responsive behavior. Many "operating principles" involve cascades of events that translate cues from the environment into changes of the overall structure and/or the connectivity of the constituting building blocks at various levels. These concepts permit dramatic property variations without significant compositional changes. Inspired by the function and the growing understanding of the operating principles at play in biological materials with the capability to change their mechanical properties, significant efforts have been made toward mimicking such architectures and functions in artificial materials. Research in this domain has rapidly grown in the last two decades and afforded many examples of bioinspired materials that are able to reversibly alter their stiffness, shape, porosity, density, or hardness upon remote stimulation. This review summarizes the state of research in this field.

Polymeric microcapsules with switchable mechanical properties for self-healing concrete: synthesis, characterisation and proof of concept

Microcapsules, with sodium silicate solution as core, were produced using complex coacervation in a double, oil-in-water-in oil, emulsion system. The shell material was a gelatin–acacia gum crosslinked coacervate and the produced microcapsules had diameters ranging from 300 to 700 μm. The shell material designed with switchable mechanical properties. When it is hydrated exhibits soft and ‘rubbery’ behaviour and, when dried, transitions to a stiff and ‘glassy’ material. The microcapsules survived drying and rehydrating cycles and preserved their structural integrity when exposed to highly alkaline solutions that mimic the pH environment of concrete. Microscopy revealed that the shell thickness of the microcapsules varies across their perimeter from 5 to 20 μm. Thermal analysis showed that the produced microcapsules were very stable up to 190 °C. Proof of concept investigation has demonstrated that the microcapsules successfully survive and function when exposed to a cement-based matrix. Observations showed that the microcapsules survive mixing with cement and rupture successfully upon crack formation releasing the encapsulated sodium silicate solution.

Stimuli-Responsive DNA-Based Hydrogels: From Basic Principles to Applications.

The base sequence of nucleic acids encodes structural and functional information into the DNA biopolymer. External stimuli such as metal ions, pH, light, or added nucleic acid fuel strands provide triggers to reversibly switch nucleic acid structures such as metal-ion-bridged duplexes, i-motifs, triplex nucleic acids, G-quadruplexes, or programmed double-stranded hybrids of oligonucleotides (DNA). The signal-triggered oligonucleotide structures have been broadly applied to develop switchable DNA nanostructures and DNA machines, and these stimuli-responsive assemblies provide functional scaffolds for the rapidly developing area of DNA nanotechnology. Stimuli-responsive hydrogels undergoing signal-triggered hydrogel-to-solution transitions or signal-controlled stiffness changes attract substantial interest as functional matrices for controlled drug delivery, materials exhibiting switchable mechanical properties, acting as valves or actuators, and "smart" materials for sensing and information processing. The integration of stimuli-responsive oligonucleotides with hydrogel-forming polymers provides versatile means to exploit the functional information encoded in the nucleic acid sequences to yield stimuli-responsive hydrogels exhibiting switchable physical, structural, and chemical properties. Stimuli-responsive DNA-based nucleic acid structures are integrated in acrylamide polymer chains and reversible, switchable hydrogel-to-solution transitions of the systems are demonstrated by applying external triggers, such as metal ions, pH-responsive strands, G-quadruplex, and appropriate counter triggers that bridge and dissociate the polymer chains. By combining stimuli-responsive nucleic acid bridges with thermosensitive poly(N-isopropylacrylamide) (pNIPAM) chains, systems undergoing reversible solution ↔ hydrogel ↔ solid transitions are demonstrated. Specifically, by bridging acrylamide polymer chains by two nucleic acid functionalities, where one type of bridging unit provides a stimuli-responsive element and the second unit acts as internal "bridging memory", shape-memory hydrogels undergoing reversible and switchable transitions between shaped hydrogels and shapeless quasi-liquid states are demonstrated. By using stimuli-responsive hydrogel cross-linking units that can assemble the bridging units by two different input signals, the orthogonally-triggered functions of the shape-memory were shown. Furthermore, a versatile approach to assemble stimuli-responsive DNA-based acrylamide hydrogel films on surfaces is presented. The method involves the activation of the hybridization chain-reaction (HCR) by a surface-confined promoter strand, in the presence of acrylamide chains modified with two DNA hairpin structures and appropriate stimuli-responsive tethers. The resulting hydrogel-modified surfaces revealed switchable stiffness properties and signal-triggered catalytic functions. By applying the method to assemble the hydrogel microparticles, substrate-loaded, stimuli-responsive microcapsules are prepared. The signal-triggered DNA-based hydrogel microcapsules are applied as drug carriers for controlled release. The different potential applications and future perspectives of stimuli responsive hydrogels are discussed. Specifically, the use of these smart materials and assemblies as carriers for controlled drug release and as shape-memory matrices for information storage and inscription and the use of surface-confined stimuli-responsive hydrogels, exhibiting switchable stiffness properties, for catalysis and controlled growth of cells are discussed.

Thermally Induced Intra‐Carboxyl Proton Shuttle in a Molecular Rack‐and‐Pinion Cascade Achieving Macroscopic Crystal Deformation

AbstractProton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra‐carboxyl proton shuttle can be generated in a molecular assembly akin to a rack‐and‐pinion cascade via a thermally induced single‐crystal‐to‐single‐crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4′‐azopyridine (azpy) molecule connects to two biphenyl‐3,3′,5,5′‐tetracarboxylic acid (H4BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack‐and‐pinion cascade, which mainly involves 1) an intra‐carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H4BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.

Thermally Induced Intra-Carboxyl Proton Shuttle in a Molecular Rack-and-Pinion Cascade Achieving Macroscopic Crystal Deformation.

Proton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra-carboxyl proton shuttle can be generated in a molecular assembly akin to a rack-and-pinion cascade via a thermally induced single-crystal-to-single-crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4'-azopyridine (azpy) molecule connects to two biphenyl-3,3',5,5'-tetracarboxylic acid (H4 BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack-and-pinion cascade, which mainly involves 1) an intra-carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H4 BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.

Spin crossover materials: from molecular motion to switchable mechanical properties

Spin crossover materials: from molecular motion to switchable mechanical properties

Emulsion-templated macroporous polymer/polymer composites with switchable stiffness

Abstract Emulsion templates containing monomers in both emulsion phases were used to manufacture polystyrene-co-divinylbenzene based polymerized high internal phase emulsions (polyHIPEs) which have been reinforced by poly(methacrylic acid) (polyMAA) and poly(dimethyl aminoethyl methacrylate) (polyDMAEMA). The morphology of the hydrogel-filled polyHIPEs is affected by the hydrogels synthesized in the aqueous emulsion phase. The pore structure of polyMAA-filled polyHIPEs is highly interconnected indicating the formation of a methacrylic acid-co-styrene copolymer at the oil/water interface of the emulsion templates during synthesis. However, polyDMAEMA-filled polyHIPEs are predominately closed celled and the pore walls are covered by grafted hydrogel. The ability of the hydrogel-filled polyHIPEs to absorb water decreased with increasing crosslinking density of the hydrogels. The dry hydrogel reinforced the polyHIPE scaffolds possessed higher elastic moduli and crush strengths than the control polyHIPEs. The reinforcing ability of the dry hydrogels was further enhanced by increasing their degree of crosslinking. However, the reinforcement could be “switched off” simply by hydrating the hydrogels. The switchable mechanical properties of the hydrogel-filled polyHIPEs could potentially be utilized in smart humidity sensor technology.

The interfacial structure and Young's modulus of peptide films having switchable mechanical properties.

We report the structure and Young's modulus of switchable films formed by peptide self-assembly at the air-water interface. Peptide surfactant AM1 forms an interfacial film that can be switched, reversibly, from a high- to low-elasticity state, with rapid loss of emulsion and foam stability. Using neutron reflectometry, we find that the AM1 film comprises a thin (approx. 15A) layer of ordered peptide in both states, confirming that it is possible to drastically alter the mechanical properties of an interfacial ensemble without significantly altering its concentration or macromolecular organization. We also report the first experimentally determined Young's modulus of a peptide film self-assembled at the air-water interface (E=80MPa for AM1, switching to E<20MPa). These findings suggest a fundamental link between E and the macroscopic stability of peptide-containing foam. Finally, we report studies of a designed peptide surfactant, Lac21E, which we find forms a stronger switchable film than AM1 (E=335MPa switching to E<4MPa). In contrast to AM1, Lac21E switching is caused by peptide dissociation from the interface (i.e. by self-disassembly). This research confirms that small changes in molecular design can lead to similar macroscopic behaviour via surprisingly different mechanisms.