Tunable Mechanical Performance Research Articles

Multistep and Elastically Stable Mechanical Metamaterials

Abstract Materials and structures with tunable mechanical properties are essential for numerous applications. However, constructing such structures poses a great challenge since it is normally very complicated to change the properties of a material after its fabrication, particularly in pure force fields. Herein, we propose a multistep and elastically stable 3D mechanical metamaterial having simultaneously tunable effective Young's modulus and auxeticity controlled by the applied compressive strain. Metamaterial samples are fabricated by 3D printing at the centimetric scale, with selective laser sintering, and at the micrometric scale, with two-photon lithography. Experimental results indicate an elementary auxeticity for small compressive strains but superior auxeticity for large strains. Significantly, the effective Young's modulus follows a parallel trend, becoming larger with increasing compressive strain. A theoretical model explains the variations of the elastic constants of the proposed metamaterials as a function of geometry parameters and provides a basic explanation for the appearance of the multistep behavior. Furthermore, simulation results demonstrate that the proposed metamaterial has the potential for designing metamaterials exhibiting tunable phononic band gaps. The design of reusable elastically stable multistep metamaterials, with tunable mechanical performances supporting large compression, is made possible thanks to their delocalized deformation mode.

Molecularly engineered chitosan-derived poly(aprotic/protic ionic liquids) double-network hydrogel electrolytes for flexible supercapacitors and wearable strain sensors

Polymeric conductive hydrogels have been highlighted for use in flexible electronics, and there is still a challenge to design and fabricate a high-performance hydrogel electrolyte with integrated and multiple properties, such as sustainability, stretchability and compressibility, ionic conductivity, antibacterial property, strain sensitivity, and low-temperature tolerance. Herein, chitosan (CS) derived poly(aprotic/protic ionic liquids) (CS-PAPILs) are facilely prepared by taking the structural and sustainable features of chitosan and betaine hydrochloride, which are used as a functional component for the fabrication of CS-PAPILs/polyacrylamide (PAM)/LiCl (CS-PAPILs/PAM/LiCl) double-network (DN) hydrogel electrolyte via an in situ polymerization of AM and then a soaking strategy in LiCl aqueous solution. The achieved DN hydrogels exhibit tunable mechanical performance (tensile strength of 70–900 kPa, elastic modulus of 31.5–484 kPa, high compressibility (4450 kPa at 80 % strain), superior low-temperature tolerance (freezing point < -85 °C), anti-dehydration performance, high ionic conductivity at 25/-50 °C (94.8/4.6 mS cm−1), and excellent antibacterial activity. As a proof of concept, an assembled flexible and anti-freezing supercapacitor by the as-prepared DN hydrogel electrolyte demonstrated a high specific capacitance of 108.4F/g (at 2 A/g) and impressive cycling stability over 50,000 cycles at temperature as low as −50 °C. Furthermore, a wearable strain sensor based on the DN hydrogel was also demonstrated, and the sensor can be successfully attached to the human joints to monitor various human motions.

A recombinant chimeric spider pyriform-aciniform silk with highly tunable mechanical performance

Spider silks are natural protein-based biomaterials which are renowned for their mechanical properties and hold great promise for applications ranging from high-performance textiles to regenerative medicine. While some spiders can produce several different types of silks, most spider silk types – including pyriform and aciniform silks – are relatively unstudied. Pyriform and aciniform silks have distinct mechanical behavior and physicochemical properties, with materials produced using combinations of these silks currently unexplored. Here, we introduce an engineered chimeric fusion protein consisting of two repeat units of pyriform (Py) silk followed by two repeat units of aciniform (W) silk named Py2W2. This recombinant ∼86.5 kDa protein is amenable to expression and purification from Escherichia coli and exhibits high α-helicity in a fluorinated acid- and alcohol-based solution used to form a dope for wet-spinning. Wet-spinning enables continuous fiber production and post-spin stretching of the wet-spun fibers in air or following submersion in water or ethanol leads to increases in optical anisotropy, consistent with increased molecular alignment along the fiber axis. Mechanical properties of the fibers vary as a function of post-spin stretching condition, with the highest extensibility and strength observed in air-stretched and ethanol-treated fibers, respectively, with mechanics being superior to fibers spun from either constituent protein alone. Notably, the maximum extensibility obtained (∼157 ± 38 %) is of the same magnitude reported for natural flagelliform silks, the class of spider silk most associated with being stretchable. Interestingly, Py2W2 is also water-compatible, unlike its constituent Py2. Fiber-state secondary structure correlates well with the observed mechanical properties, with depleted α-helicity and increased β-sheet content in cases of increased strength. Py2W2 fibers thus provide enhanced materials behavior in terms of their mechanics, tunability, and fiber properties, providing new directions for design and development of biomaterials suitable and tunable for disparate applications.

Adjustable mechanical performances of 4D-printed shape memory lattice structures

Lattice structures are widely used due to their inherent advantages. With the development of smart devices, there is a growing demand for programmable, adjustable, and reconfigurable performances. However, a significant limitation of traditional lattice structures is that their shape, function, and performance cannot be changed after fabrication. In order to address this issue, we conducted experimental and simulated investigations on the shape memory effect, adjustable mechanical performances, and their deformation mechanism using the shape memory programming method. Two bending-dominated lattice structures, namely four curved bars lattice structure (FCBL) and sinusoidal wave horseshoe lattice structure (SWHL), were taken as examples. Results show that the deformation modes of both structures are switched from a bending-dominated mode to a stretching-dominated one after programming while exhibiting distinct bending sections. FCBL displays a 'C' shape with one bending section, whereas SWHL exhibits an 'S' shape with two bending sections. These deformation modes significantly enhance the tensile moduli by 480.9% (FCBL) and 1546% (SWHL), and change their Poisson’s ratio from −0.29 to 0.25 (FCBL) and −0.31 to 0.43 (SWHL), respectively. The modulus and Poisson’s ratio of FCBL and SWHL are well reproduced by the finite element modeling, providing a reference for designing the tunable mechanical performances of lattice structures.

Stiff, tunable and reprocessable polythiourea supramolecular materials by manipulating hydrogen-bonding and metal-coordinating interactions

Rational strategies manipulating hydrogen-bonding and metal-coordination interactions accelerate the development of supramolecular materials with unprecedented properties, such as adaptability, self-healing, stimuli-responsiveness, reprocessability and recyclability. Herein, supramolecular interactions of thioureas were fully utilized to design new materials with adjusted mechanical performances and robust reprocessability. A series of polythioureas were facilely synthesized by the co-polycondensation of CS2 and two diamines, which was an efficient and economical alternative to the isothiocyanate-based polymerization. Hydrogen-bonded polythiourea materials with tunable mechanical performances were achieved by adjusting the density and regularity of thiourea units in the polymer chains. Further incorporating tiny amount of metal salts (CuCl2, CuCl, CuBr and CuI) resulted in polythiourea materials with enhanced mechanical performances, following the trend determined by the hard-soft acid-base principle. As far as we know, this is the first report of metal-coordinated polythiourea supramolecular materials. The exploration of hard-soft acid-base principle will also provide new inspirations for designing metal-coordinated supramolecular materials.

Oriented bacterial cellulose microfibers with tunable mechanical performance fabricated via green reassembly avenue

Bacterial cellulose has garnered remarkable interest from researchers, particularly those working in the biomedical field. In this work, BC microfibers were fabricated via green dissolution (ZnCl2) and regeneration (ethanol). The orientation of cellulose chains was investigated during extrusion and simple post-processing via polarized optical microscopy and small-angle X-ray scattering. The results implied that the mechanical properties of BC microfibers can be tuned by rational pre-stretching. The BC microfibers can be programmable, and be used to suture hard or soft tissues. The as-designed paralleled BC microfibers have good biocompatibility and can regulate the directional growth of cells on their surface. The as-obtained BC microfiber with a high tensile strength of up to ∼115 MPa is suitable for surgical sutures. The tunable BC microfibers may be utilized as an adequate fiber-derived biomedical material product.

Conductive Hydrogel for Flexible Bioelectronic Device: Current Progress and Future Perspective

AbstractConductive hydrogels (CHs) for flexible bioelectronic devices have raised great attention due to their tunable mechanical performances, adhesion, anti‐swelling, and biocompatibility. This review summarizes the current development of conductive hydrogel‐based flexible bioelectronic devices in the aspect of classifications and applications. Firstly, the conductive hydrogels are classified into two kinds according to the types of conductivity: ionic conductive hydrogels and electronic conductive hydrogels (hydrogel based on pure conductive materials, introducing conductive micro/nano‐materials). Secondly, the applications of conductive hydrogels for bioelectronic device, like wearable devices (strain sensor, body fluid detector, serviced in extreme environment), tissue engineering (skin, heart, nerve, muscle), and other applications (bionic robot, cancer treatment), are highly illustrated. Finally, a depth outlook is given, which aims to promote the development of this field in the future.

Macrofibers with tunable mechanical performance and reversible rotational motion based on a bacterial cellulose hydrogel film

Mechanically twisting a yarn is a useful and important method for creating internal stress, changing macromolecular orientation, and affording interesting fiber properties, including mechanical, structural, physical and chemical properties. Bacterial cellulose (BC) nanofibers are a new type of highly crystalline bionanofibers exhibiting excellent intrinsic mechanical properties. Herein, BC macrofibers with a diameter of 0.35–0.53 mm were prepared from BC hydrogel films using a two-step technique involving drying and twisting processes. Moreover, the effects of the drying method (oven- and freeze-drying) and those of the processing steps on the morphologies, structures, and physicochemical properties of the macrofibers were investigated. Results indicated that only the wet twisting–first process affords continuous macrofibers with a compact and uniform structure. The tensile strength and elongation at break of the macrofibers were 199 MPa and 24%, respectively. Furthermore, by introducing graphene oxide (GO) to the macrofibers, a photothermal and moisture actuator was facilely constructed, which could produce a twisting motion of 0.39 rpm and an untwisting motion of 0.6 rpm. This study provides insights into methods to easily alter the properties of BC macrofibers, which is pivotal for the macrofiber’s potential application in intelligent devices.

Volumetric additive manufacturing of pristine silk-based (bio)inks

Volumetric additive manufacturing (VAM) enables fast photopolymerization of three-dimensional constructs by illuminating dynamically evolving light patterns in the entire build volume. However, the lack of bioinks suitable for VAM is a critical limitation. This study reports rapid volumetric (bio)printing of pristine, unmodified silk-based (silk sericin (SS) and silk fibroin (SF)) (bio)inks to form sophisticated shapes and architectures. Of interest, combined with post-fabrication processing, the (bio)printed SS constructs reveal properties including reversible as well as repeated shrinkage and expansion, or shape-memory; whereas the (bio)printed SF constructs exhibit tunable mechanical performances ranging from a few hundred Pa to hundreds of MPa. Both types of silk-based (bio)inks are cytocompatible. This work supplies expanded bioink libraries for VAM and provides a path forward for rapid volumetric manufacturing of silk constructs, towards broadened biomedical applications.

Tunable mechanical performances of collagen-based film: Effect of collagens in different hierarchies and cellulose nanofiber

This study examines the effect of three collagens and the addition of carboxymethylated cellulose (CNF) on the properties of collagen-based film. The three collagens include macroscale collagenous fiber bundle (Ma-COL), microscale collagen fiber (Mi-COL), and nanoscale collagen fibril (Na-COL). With the decrease of the collagens' size, the viscosity of the dispersions decreased dramatically, besides, the collagen-based film showed an increased elastic modulus (from 13.32 GPa to 24.47 GPa) and tensile strength (from 46.18 MPa to 72.73 MPa). The opacity first decreased and then increased when the collagen's size ranged from macro- to nanoscale. The decrease of collagen's size could increase the film density and possessed more hydrogen bonds of film captured by FTIR. The addition of CNF further intensified the above trends. Besides, the reinforcing mechanisms of CNF could be proved by the combination of pull-off and intercrystalline fracture from microstructure and the finite element models.

A universal strategy for preparing tough and smart glassy hydrogels

With a large amount of water as plasticizer, most current hydrogels stay in rubbery states with low rigidity. Inspired by the robust thermoplastics, herein we propose a facile, effective, and universal strategy for preparing tough and smart “glassy hydrogels” by maintaining the hydrogels’ network in glassy states. The key point is to integrate an appropriate amount of rigid hydrophobic polymers within hydrogels’ hydrophilic networks. Based on the composition optimization, we further establish a generally applicable criterion for material preparation: the hydrogels should possess a thermo-softening temperature slightly higher than the service condition. Following this principle, various glassy hydrogels have been readily fabricated, which demonstrate excellent rigidity (∼50.5 MPa), strength (∼8.46 MPa), extensibility (∼2200%), and toughness (∼35.2 MJ.m−3). More interestingly, the hydrogels demonstrate broad-range tunable mechanical performance in response to temperature and additives. Upon heating, the hydrogels manifest evident thermo-softening performance with the stiffness dramatically dropping more than 1000 times, endowing the hydrogels with desirable shape memory properties. When encountering various small-molecules, the hydrogels show over 3 orders’ broad-range tunable stiffness. Collectively, the ease of manipulation and universality of the as-established method, we hope, can become a widely used strategy to inspire novel designs and substantially broaden the application potentials of gels.

Solvent-free preparation of imine vitrimers: leveraging benzoxazine crosslinking for melt processability and tunable mechanical performance

Orthogonal benzoxazine crosslinking enables the solvent-free preparation of dynamic imine vitrimers with tailorable thermomechanical performance, efficient reprocessability, and chemical degradation.

Regulated multi-scale mechanical performance of functionally graded lattice materials based on multiple bioinspired patterns

Functionally graded lattice material (FGLM) possesses great architectural flexibility and performance combinations that do not occur in uniform lattice structures. The regulation effect of highly diversified FGLM design strategies on multi-scale mechanical performance still remains to be revealed. Here, FGLMs with diversified topology features were proposed by using a design strategy that learns from the natural bone remodeling process. Shell-based and strut-based triply periodic minimal surfaces (TPMSs), which respectively act as secondary phase and matrix phase, are coupled together by referencing natural systems. Multi-scale investigation on the linear, surface and chiral strengthening TPMS-FGLMs substantiates the highly tunable mechanical performance. The mechanical performance spatiotemporal asynchrony caused by predetermined secondary phase significantly alters the post-yielding and failure behavior, further restricts the failure band propagation and catastrophic collapse. Secondary phase distributed in cross-like pattern provides balanced mechanical strength and energy absorption efficiency. The present study enriches the FGLM design methodology and indicates a tempting prospect for mechanical performance regulation.

Closed-loop chemical recycling of cross-linked polymeric materials based on reversible amidation chemistry

Closed-loop chemical recycling provides a solution to the end-of-use problem of synthetic polymers. However, it remains a major challenge to design dynamic bonds, capable of effective bonding and reversible cleaving, for preparing chemically recyclable cross-linked polymers. Herein, we report a dynamic maleic acid tertiary amide bond based upon reversible amidation reaction between maleic anhydrides and secondary amines. This dynamic bond allows for the construction of polymer networks with tailorable and robust mechanical properties, covering strong elastomers with a tensile strength of 22.3 MPa and rigid plastics with a yield strength of 38.3 MPa. Impressively, these robust polymeric materials can be completely depolymerized in an acidic aqueous solution at ambient temperature, leading to efficient monomer recovery with >94% separation yields. Meanwhile, the recovered monomers can be used to remanufacture cross-linked polymeric materials without losing their original mechanical performance. This work unveils a general approach to design polymer networks with tunable mechanical performance and closed-loop recyclability, which will open a new avenue for sustainable polymeric materials.

A novel approach for mechanical regulation of thin-walled crystal plate lattices: Experimental characterization and simulation

Owing to machining limitations, the accurate regulation of mechanical performances of thin-walledcrystal plate latticescan be hardly realized via plate thickness in laser powder bed fusion. The present study proposes a novel approach for accurately regulating the elastic, plastic, and energy absorption properties of thin-walled crystal plate lattices using plate holes.In order to identify the influence of plate holes on the programmable mechanical properties of thin-walled crystal plate lattices, numerical simulations as well as experimental tests were conducted.Without breaking the original symmetry features, the increasing size ofplate holes only results in slightly increased elastically-anisotropy. Quasi-static uniaxial compression tests and simulations demonstrate the stiffness, yield strength, and energy absorption capability can be accurately regulated by plate holes. Elastoplastic finite element simulations are employed to reveal the mechanisms responsible for the mechanical response of thin-walled crystal plate lattices.All simulations are verified by mechanical tests of 316L stainless steelcrystal plate lattices. This study provides a new channel for tunable mechanical performances of thin-walledcrystal plate lattices.

Tunable mechanical performance of additively manufactured plate lattice metamaterials with half-open-cell topology

Plate lattices are an emerging class of lightweight mechanical metamaterials that exhibit superior mechanical properties. The unique architectures of plate lattice metamaterials are regarded as the origin of achieving such advanced performance, but they lead to the difficulty of powder removal after the powder-bed-based additive manufacturing process. In the present work, plate lattice metamaterials with half-open-cell topology are proposed and fabricated via the laser powder bed fusion technique. To investigate their mechanical performance and deformation behaviours, numerical simulations and experimental tests are performed on finite element models and as-built specimens, respectively. Simulation results show that the mechanical properties and elastic anisotropy of half-open-cell plate lattice metamaterials are easily tunable when changing the geometric parameters, e.g., plate thickness or hole diameter. The elastic moduli and Poisson’s ratio are found to scale nonlinearly with the hole diameter for different plate thicknesses, and the elastic anisotropy approaches 1.0 for specific hole size. The specific energy absorption of half-open-cell plate lattice metamaterials is up to two times higher than that of truss lattice metamaterials. The porous architecture, lightweight body, superior and easily tunable mechanical performance of half-open-cell plate lattice metamaterials provide them application potential in the fields of load-bearing, energy absorption and biomedical engineering.

Multifunctional Ti3C2TX MXene/Aramid nanofiber/Polyimide aerogels with efficient thermal insulation and tunable electromagnetic wave absorption performance under thermal environment

Nowadays, multifunctional electromagnetic (EM) wave absorbing aerogels have attracted much attention due to intelligent and wearable features for electronic devices. However, the utilization of these aerogels at elevated temperature is still challenge. Herein, we describe a robust strategy to fabricate architecturally controllable and multifunctional Ti3C2TX MXene/aramid nanofiber (ANF)/polyimide (PI) aerogels with desirable EM wave absorption, adjustable mechanical property, and high thermal insulation. Rational structural design endows the aerogels with an effective absorption bandwidth (EAB) of 4.2 GHz (covering whole X-band) at 250 °C. Exploration of the temperature response over 25–250 °C reveals that the microstructure characters of the aerogels contribute substantially to the EM parameters at elevated temperature, while the in-depth temperature-dependent mechanisms of aerogel EM performance are well studied. Further, through regulating the microstructure, the MXene/ANF/PI aerogels also exhibit tunable mechanical performance (maximum compressive stress of 334.3 KPa at 40% compression) and promising thermal insulation performance (0.029 ± 0.003 W mK−1, closing to air). These multifunctional aerogels light the way to potential application in the EM wave absorption field of flexible devices at elevated temperature.

Synergistic Covalent-and-Supramolecular Polymers with an Interwoven Topology.

Network topologies, especially some high-order topologies, are able to furnish cross-linked polymer materials with enhanced properties without altering their chemical composition. However, the fabrication of such topologically intriguing architectures at the macromolecular level and in-depth insights into their structure-property relationship remain a significant challenge. Herein, we relied on synergistic covalent-and-supramolecular polymers (CSPs) as a platform to prepare a range of polymer networks with an interwoven topology. Specifically, through the sequential supramolecular self-assemblies, the covalent polymers (CPs) and metallosupramolecular polymers (MSPs) could be interwoven in our CSPs by [2]pseudorotaxane cross-links. As a result, the obtained CSPs possessed a topological network that could not only promote the synergistic effect between CPs and MSPs to afford mechanically robust yet dynamic materials but also vest polymers with some functions, as manifested by force-induced hierarchical dissociations of supramolecular interactions and superior thermomechanical stability compared to our previously reported CSP systems. Furthermore, our CSPs exhibited tunable mechanical performance toward multiple stimuli including K+ and PPh3, demonstrating abundant stimuli-responsive properties. We hope that these findings could provide novel opportunities toward achieving topological structures at the macromolecular level and also motivate further explorations of polymeric materials via the way of controlling their topological structures.

A micromechanical model for phase-change composites

Phase-change composites have a wide range of tunable mechanical properties caused by temperature-driven phase transition, and have been widely applied in many cutting-edge fields like soft robotics. Previous studies on the effective mechanical properties of phase-change composites mostly use experimental methods, and there have been few theoretical approaches. In this work, we develop a micromechanical framework capable of tracking the effective mechanical properties of phase-change composites throughout the entire phase transition. The phase-change materials embedded in the composites are modelled as inclusions, and the non-phase-change materials are modelled as the matrix. This allows us to determine the effective mechanical properties of phase-change composites via the energy equivalency approach. Moreover, since the new phase will be generated inside the phase-change inclusions in the form of sub-inclusions during the phase transition, the inclusions are modelled as two-phase composites, and their effective mechanical properties are then determined using the Mori–Tanaka method. Finally, by comparing theoretical predictions with experimental data, the accuracy and reliability of the present model are verified. We believe that the proposed model can serve as a powerful tool for evaluating the effective mechanical properties of phase-change composites and provide theoretical guidelines for the design of advanced devices with tunable mechanical performance.

Combination of cellulose and plant oil toward sustainable bottlebrush copolymer elastomers with tunable mechanical performance

In this work, sustainable cellulose-g-poly(lauryl acrylate-co-acrylamide) [Cell-g-P(LA-co-AM)] bottlebrush copolymer elastomers derived from cellulose and plant oil were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. Differential scanning calorimeter (DSC) results indicate that these thermally stable Cell-g-P(LA-co-AM) bottlebrush copolymer elastomers show adjustable melting temperatures. Monotonic and cyclic tensile tests suggest that the mechanical properties, including tensile strength, extensibility, Young's modulus, and elasticity, can be conveniently controlled by changing the LA/AM feed ratio and cellulose content. In such kind of bottlebrush copolymer elastomers, the rigid cellulose backbones act as cross-linking points to provide tensile strength. The incorporated PAM segments can form additional network structure via hydrogen bonding, resulting in enhanced tensile strength but decreased extensibility when more PAM segments are introduced. This versatile strategy can promote the development of sustainable cellulose-based bottlebrush copolymer elastomers from renewable resources.