Soft Composites Research Articles

Hydrophobic Gold Nanoparticles with Intrinsic Chirality for the Efficient Fabrication of Chiral Plasmonic Nanocomposites.

The development of plasmonic nanomaterials with chiral geometry has drawn extensive attention owing to their practical implications in chiral catalysis, chiral metamaterials, or enantioselective biosensing and medicine. However, due to the lack of effective synthesis methods of hydrophobic nanoparticles (NPs) showing intrinsic, plasmonic chirality, their applications are currently limited to aqueous systems. In this work, we resolve the problem of achieving hydrophobic Au NPs with intrinsic chirality by efficient phase transfer of water-soluble NPs using low molecular weight, liquid crystal-like ligands. We confirmed that, after the phase transfer, Au NPs preserve strong, far-field circular dichroism (CD) signals, attesting their chiral geometry. The universality of the method is exemplified by using different types of NPs and ligands. We further highlight the potential of the proposed approach to realize chiral plasmonic, inorganic/organic nanocomposites with block copolymers, liquid crystals, and compounds forming physical gels. All soft matter composites sustain plasmonic CD signals with electron microscopies confirming well-dispersed nanoinclusions. The developed methodology allows us to expand the portfolio of plasmonic NPs with intrinsic structural chirality, thereby broadening the scope of their applications toward soft-matter based systems.

Magnetically Engineered Conductivity of Soft Liquid Metal Composites for Robotic, Wearable Electronic, and Medical Applications

Stretchable composites comprising liquid metal (LM) inclusions and silicone elastomers (LME composites) are of great interest for soft electronics and wearable devices. LME composites consisting of highly deformable materials and low conductive filler ratios offer high stretchability and good strain‐tolerant conductance, while not compromising the functionality of their host systems. Despite advances, actively achieving electrical conductivity for LME composites with a low ratio of fillers is challenging, especially in highly deformable elastomers. Herein, a new fabrication strategy that turns nonconductive LME composites with highly deformable elastomers into conductive ones using a small amount of magnetic Ni‐doped LM is introduced. By actively manipulating conductive fillers with an external magnetic field, electrically conductive traces can sustainably be achieved at any desired location. Experimental results show that conductive traces have high conductivity of 2.55 × 105 S m−1, high stretchability (>450%), good strain‐tolerant conductance (R/R0 ≈ 1.56 at 250% strain), and especially a tensile modulus as low as 60.1 kPa at a very low loading ratio (9.7% by volume). The noncontacting magnetic fabrication also enables the creation of diverse configurations in 1D, 2D, and 3D, offering a broad range of potential applications from robotics, stretchable electronics, wearable devices, smart garments to biomedical systems.

Instability-induced patterns and their post-buckling development in soft particulate composites

In this paper, we investigated elastic instabilities in soft particulate composites under finite strains. Through our numerical analysis, we find distinct instability modes developing in the composites upon the critical deformation level. In particular, fully predetermined by its initial geometry, the microstructures transform into (i) strictly doubled periodicity, (ii) seemingly non-periodic state, or (iii) longwave pattern. The latter may give rise to highly ordered domain formations. We analyze the various mechanisms leading to the development of the different instability modes and specifically focus on the seemingly non-periodic one. We illustrate the distinct features of their corresponding eigenmodes obtained from the Bloch-Floquet analysis. We further employ the quasi-convexification analysis and examine the energy landscapes of the soft composites developing the different instability modes. Finally, we examine the development of the predicted instability modes in the post-buckling regime. We find that, depending on the characteristic critical wavenumber, the post-buckling patterns can significantly diverge from those predicted by the Bloch-Floquet analysis. In the post-buckling regime, the instabilities may develop into various scenarios: from a combination of different repeating inclusion sets to disordered, seemingly chaotic patterns.

Insect-Scale SMAW-Based Soft Robot With Crawling, Jumping, and Loading Locomotion

The work develops an insect-scale soft robot capable of fast directional locomotion actuated by itself spring-like body composed of shape memory alloy wires (SMAWs)-based composite. The soft composite is fabricated integrating polydimethylsiloxane (PDMS) matrix, 3D printed acrylonitrile-butadiene-styrene (ABS) substrate, and parallel multi-SMAWs with the diameter of 0.075 mm. Since it can implant the thermal-induced SMAW deformation and rapidly cause the elastic recovery feature of ABS resin material, the soft composite can act as the insect-scale robotic actuator to imitate the longitudinal muscle. The designed structure can convert the repetitive contraction-stretch behavior of the embedded SMAWs to the bending-expanding deformation of the robotic soft body, which forms the robotic different gaits. With the excitation of pulse-width modulation (PWM) voltages with various frequencies, different characteristics of the designed robot are compared. The experimental results validate that the designed soft robot can reach a movement speed of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 29.87 mm/s with jumping locomotion, greater than one-time body-length/s, and afford a load of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 11.1 g, about 24.1 times the weight of itself.

Computational Design of a Soft Robotic Myocardium for Biomimetic Motion and Function (Adv. Funct. Mater. 40/2022)

Soft Robotic Myocardium In article number 2206734, Ellen T. Roche, Clara Park, and Caglar Ozturk introduce a computational framework using finite element and fluid structure interaction methods to guide the design and optimization of soft robotic composites with multiple actuating parts, prior to prototyping and experimental testing. A biomimetic soft robotic cardiac muscle replicates the kinematics and functionality of cardiac muscle.

Homogenization, macroscopic instabilities and domain formation in magnetoactive composites: Theory and applications

This work deals with the stability and macroscopic response of soft magnetoelastic composites under combined magnetic and mechanical loading. It is known that, as in the purely mechanical setting, soft composites with suitably designed microstructures can undergo macroscopic (long wavelength) instabilities under certain special loading conditions. The mechanism for such instabilities, and their effects on the macroscopic response of the composites, are less well understood. We investigate here both the onset the instabilities, as well as the macroscopic response after the instabilities, by means of a ‘twinning’ or domain formation mechanism. For this purpose, we propose a generalized Maxwell construction, which builds on the work of Ball and James (1987) for shape-memory alloys and properly accounts for mechanical and magnetic compatibility requirements between ‘twins’ or domains. Thus, generalizing the work of Avazmohammadi and Ponte Castañeda (2016) for the purely mechanical response of reinforced elastomers, this is accomplished by means of the relaxation or quasiconvexification of the ‘principal’ homogenized solution, i.e., the solution prior to the onset of an instability. The methodology is then applied to two-phase magnetoelastic laminates under plane-strain loading conditions with a magnetic field applied in the plane of deformation. We find that the principal solution generally loses (strict) global rank-one convexity before it loses strong ellipticity, as the laminate finds stability by forming ‘twinned’ lamellar domains. In addition, we use the estimates to characterize the ‘phase diagrams’ for the laminates and to describe in detail their macroscopic response when the magnetic field is applied either parallel or perpendicular to the layers. We find that the formation of the lamellar domains leads to a softer mechanical response – including perfectly soft modes of deformation – which can be either facilitated or impeded, depending on the direction of the applied magnetic field. This mechanism opens up novel strategies for the design of magnetoactive elastomer sensors and actuators.

Quantifying the uncertainties in modeling soft composites via a multiscale approach

The goal of this paper is to evaluate the uncertainties in soft composites composed of a soft solid as the matrix filled by incompressible liquid inclusions. The surface stresses at the liquid–solid interfaces can significantly impact the elastic deformation of the bulk composite at the macroscale. Depending on the characteristic dimension, a stiffening or softening can happen. We model the elastic behavior employing a stochastic multiscale approach and counting on well established rules for composites. Insights on the mechanics of elasto-capillary coupling are provided considering the heterogeneity at different length scales. Global sensitivity analysis is performed to quantify the effect of variation in the material properties, surface tension, the size, and the random dispersion and agglomerations of the inclusions. The influence of the model choice at the mesoscale on the uncertainty of the response is also taken into account as an additional source of uncertainty. The results show that the relative stiffness is mainly influenced by the uncertainties in the size of liquid inclusions and the surface tension, while the remaining parameters show a significant interaction effects.

Computational Design of a Soft Robotic Myocardium for Biomimetic Motion and Function

AbstractSoft robotic devices containing multiple actuating elements have successfully recapitulated complex biological motion, leading to their utility in biomedical applications. However, there are inherent nonlinear mechanics associated with soft composite materials where soft actuators are embedded in elastomeric matrices. Predicting their overall behavior prior to fabrication and subsequent experimental characterization can therefore present a hurdle in the design process and in efficiently satisfying functional requirements and specifications. In this work, a computational design framework for optimizing the motion and function of biomimetic soft robotic composites is demonstrated by conducting a design case study of soft robotic cardiac muscle (myocardium) with a particular focus on applications including replicating and assisting cardiac motion and function. A finite element model of a soft robotic myocardium is built, in which actuators are prescribed with anisotropic strain to simulate local deformation, and various design parameters are investigated by evaluating the performance of each configuration in terms of ventricular twist, volumetric output, and pressure generation. Then, an optimized design is proposed that recapitulates the physiological motion and hemodynamics of the heart, and its thrombogenicity is further explored using a fluid‐structure interaction model. This framework has broader utility in predicting the performance of other soft robotic embedded composites.

3D Printing of Soft Magnetoactive Devices with Thiol‐Click Photopolymer Composites

Magnetoresponsive polymers have gained increased attention in the design of soft actuators as they can be spatially as well as temporally activated and enable an external noninvasive control of movement. By introducing the magnetoresponsive properties in photocurable resins, one can fabricate personalized and complex structures (via vat photopolymerization 3D printing), whose movement can be conveniently controlled by an external magnetic field. Advancing from acrylate‐based photopolymers, which often suffer from shrinkage stress, low monomer conversion, and oxygen inhibition, the fabrication of magnetoresponsive thiol‐click photopolymers containing Fe3O4 nanoparticles as magnetic fillers is highlighted. The addition of the thiol crosslinker yields soft and flexible polymer composites, whose cure kinetics, viscosity, thermal, and mechanical properties are studied as a function of the thiol and filler content. Although cure rate and final monomer conversion decrease with rising filler concentration, the cure kinetics is reasonably fast at 6 wt%. The short pot life, a result of thiol‐Michael reactions induced by Fe3O4 nanoparticles, and a high thiol content, are overcome by the addition of an appropriate stabilizer. As proof of concept, 3D structures are fabricated by digital light processing (DLP) 3D printing and their magnetically driven movement is demonstrated.

A chitosan based multimodal “soft” hydrogel for rapid hemostasis of non-compressible hemorrhages and its mode of action

Uncontrolled hemorrhages during accidents or surgery require non-compressible dressings to halt bleeding and avoid damage to soft tissue and organs. Here, we report the synthesis of a sodium tripolyphosphate crosslinked chitosan-based hemostatic hydrogel dressing incorporating hemostasis agonists like aluminum chloride, an aggregation enhancer and silica nanoparticles, a contact activator, as confirmed by FTIR analyses. Rheology studies displayed G” > G’ values confirming typical soft gel-like behavior that was maintained upto shear rate 10/s. Further, composite had excellent water absorption capacity (737 ± 50%) and showed 6-fold enhanced in vitro hemostasis as compared to bare hydrogel. Platelet aggregation was enhanced with composite as compared to SFLLRN peptide (a thrombin mimic) and adenosine diphosphate (calcium release activator) while it was absent with heparin (thrombin inhibitor). Enhanced activation and release of calcium were observed by composite with a specific fluorescent calcium dye in A549 cell model. The composite was biocompatible and in vivo evaluation of hemostatic efficiency in rat liver injury revealed enhanced blood clotting (16 ± 2.1 s) of hydrogel composite as compared to commercial HemoStat Clear gel (55 ± 5 s). Thus, developed soft hydrogel composite has potential for application as a safe, easy-to-apply, rapid, non-compressible hemostatic agent.

In-situ synthesis of MOF nanoparticles in double-network hydrogels for stretchable adsorption device

Metal–organic framework (MOF) composites shaped by hydrogels have the advantages of abundant pores, diverse functional groups, and excellent flexibility, showing tremendous potential in many fields. However, many used hydrogels are prepared by chemical cross-linking or polymerization reactions, and the synthesized MOF microparticles are aggregated and easily fall out. Here, we report the in-situ synthesis of MOF nanoparticles (<100 nm) in the poly(vinyl alcohol)@alginate double-network hydrogels, which are further used for fabricating stretchable MOF hydrogel tubes. The high-modulus nanoparticles can serve as strong crosslinks and enhance the entanglements of polymer chains, improving the mechanical properties of the composites significantly. Furthermore, the porous nanoparticles endow the hydrogels with unique recovery behavior from compression and good adsorption properties. This study opens up new horizons in the controllable preparation of soft MOF hydrogel composites for many promising applications.

Interfacial Coordination Interaction Enables Soft Elastomer Composites High Thermal Conductivity and High Toughness.

Soft elastomers have attracted wide applications, such as soft electronic devices and soft robotics, due to their ability to undergo large deformation with a small external force. Most elastomers suffer from poor toughness and thermal conductivity, which limits their use. The addition of inorganic fillers can enhance the thermal conductivity and toughness, but it deteriorates the softness (low Young's modulus and high stretchability). Integrating thermal conductivity, toughness, and softness into one elastomer is still a challenge. Here, we report a strategy of interfacial coordination interaction to achieve soft elastomer composites with high thermal conductivity and high toughness. We demonstrate the strategy by using poly(lipoic acid) elastomer and silver-coated aluminum filler as model, where silver-sulfur coordination cross-links are formed at the interface. The resultant elastomer composite shows high streachability (450%), high thermal conductivity (2.35 W m-1 K-1), low modulus (321 kPa), and high toughness (3496 J m-2), which cannot be achieve in existing elastomers. The time domain thermoreflectance technique demonstrates that the silver-sulfur coordination interaction lowers the interfacial thermal resistance, resulting in enhanced thermal conductivity of the elastomer composites. The excellent softness stems from lower bonding energy of the silver-sulfur coordination cross-links compared with covalent chemical cross-links. The high toughness also benefits from the interfacial silver-sulfur coordination interaction that can dissipate more energy upon deformation. We further demonstrate the potential application of the thermally conductive, tough, and soft elastomer composites for thermal management of chip and soft electronic devices.

Field‐Induced Structural Transitions in Liquid Crystal Microemulsions

AbstractLiquid crystal (LC) droplets have fascinating properties, such as anisotropic properties, in response to external stimuli. As LC droplet size may determine the proper application of soft composites, various results from numerical simulations and experimental observations of LC droplets are reported. Here, detailed topological responses of individual bipolar droplets to electric fields, are shown. The integration of each response influences the entire electro‐optic behavior. Monodispersed LC bipolar droplets are fabricated in a polyvinyl alcohol‐dissolved aqueous solution via a membrane‐emulsification method. The planar anchoring, provided by the polyvinyl alcohol surface, and surface elasticity, guide the entire LC director configuration, associated with topological defects, in the droplets. By simply blade‐coating the solution, bipolar axes of the droplets can be randomly placed on a flat substrate. Two different subsequential stages are found when applying voltages: 1) director reorientation, which is threshold‐less and 2) topological defect movement, which exhibits a threshold‐like behavior. Various initial directions of the bipolar axes with respect to the field direction provide the transition voltage between these two stages. It is believed that this study can provide important clues to handle several fundamental issues in electro‐optics of encapsulated LCs, such as the determination of response times, threshold, and operating voltages.

Simple synthesis of soft, tough, and cytocompatible biohybrid composites

Collagen is the most abundant component of mammalian extracellular matrices. As such, the development of materials that mimic the biological and mechanical properties of collagenous tissues is an enduring goal of the biomaterials community. Despite the development of molded and 3D printed collagen hydrogel platforms, their use as biomaterials and tissue engineering scaffolds is hindered by either low stiffness and toughness or processing complexity. Here, we demonstrate the development of stiff and tough biohybrid composites by combining collagen with a zwitterionic hydrogel through simple mixing. This combination led to the self-assembly of a nanostructured fibrillar network of collagen that was ionically linked to the surrounding zwitterionic hydrogel matrix, leading to a composite microstructure reminiscent of soft biological tissues. The addition of 5-15 mg mL-1 collagen and the formation of nanostructured fibrils increased the elastic modulus of the composite system by 40% compared to the base zwitterionic matrix. Most notably, the addition of collagen increased the fracture energy nearly 11-fold ([Formula: see text] 180 J m-2) and clearly delayed crack initiation and propagation. These composites exhibit elastic modulus ([Formula: see text] 0.180 MJ) and toughness ([Formula: see text]0.617 MJ m-3) approaching that of biological tissues such as articular cartilage. Maintenance of the fibrillar structure of collagen also greatly enhanced cytocompatibility, improving cell adhesion more than 100-fold with >90% cell viability.

Tough, aorta-inspired soft composites

Spatial variations in fiber alignment (and, therefore, in mechanical anisotropy) play a central role in the excellent toughness and fatigue characteristics of many biological materials. In this work, we examine the effect of fiber alignment in soft composites, including both "in-plane" and "out-of-plane" fiber arrangements. We take inspiration from the spatial variations of fiber alignment found in the aorta to three-dimensionally (3D) print soft, tough silicone composites with an excellent combination of stiffness, toughness, and fatigue threshold, regardless of the direction of loading. These aorta-inspired composites exhibit mechanical properties comparable to skin, with excellent combinations of stiffness and toughness not previously observed in synthetic soft materials.

Soft Composites Filled with Iron Oxide and Graphite Nanoplatelets under Static and Cyclic Strain for Different Industrial Applications.

Simultaneously exhibiting both a magnetic response and piezoelectric energy harvesting in magneto-rheological elastomers (MREs) is a win–win situation in a soft (hardness below 65) composite-based device. In the present work, composites based on iron oxide (Fe2O3) were prepared and exhibited a magnetic response; other composites based on the electrically conductive reinforcing nanofiller, graphite nanoplatelets (GNP), were also prepared and exhibited energy generation. A piezoelectric energy-harvesting device based on composites exhibited an impressive voltage of ~10 V and demonstrated a high durability of 0.5 million cycles. These nanofillers were added in room temperature vulcanized silicone rubber (RTV-SR) and their magnetic response and piezoelectric energy generation were studied both in single and hybrid form. The hybrid composite consisted of 10 per hundred parts of rubber (phr) of Fe2O3 and 10 phr of GNP. The experimental data show that the compressive modulus of the composites was 1.71 MPa (virgin), 2.73 (GNP), 2.65 MPa (Fe2O3), and 3.54 MPa (hybrid). Similarly, the fracture strain of the composites was 89% (virgin), 109% (GNP), 105% (Fe2O3), 133% (hybrid). Moreover, cyclic multi-hysteresis tests show that the hybrid composites exhibiting higher mechanical properties had the shortcoming of showing higher dissipation losses. In the end, this work demonstrates a rubber composite that provides an energy-harvesting device with an impressive voltage, high durability, and MREs with high magnetic sensitivity.

Organic solvent vapor/thermal responsive binary gels with tunable transparency and mechanical strength

In this work, we illustrate the fabrication of binary gels via self-sorting assembly between β-cyclodextrin and the second gelators, which show significant transparency changes responsive to the solvent vapor and temperature. The mixing of β-cyclodextrin and clay minerals in the mixture of dimethylformamide and water produced opaque gels. After exposed to specific solvent environments, the light transmittance of opaque gels was greatly enhanced ascribed to the host-guest complexation between β-cyclodextrin and organic solvent vapors. Toluene, xylene, tetrahydrofuran, dioxane and pyridine aroused gel fiber to bulky crystal transformation, which also slightly enhanced the mechanical strength of the binary gels. In addition, the gelator could be expanded to organic gelator like folic acid. This work offers a feasible way to fabricate soft gel composites that show transparency changes responsive to solvent vapors.

Tunable buckling configurations via in-plane periodicity in soft 3D-fiber composites: Simulations and experiments

We study the buckling of soft 3D-fiber composites (FCs) with varying in-plane microstructure periodicity. Through our experiments and simulations, we find that the out-of-plane buckling orientation of fibers is determined by the constituent material properties, volume fractions, and the in-plane periodicity. The examples are given for the FCs with rectangular in-plane arrangement of fibers, i.e., the fibers are periodically situated at distance a from each other in one principal direction and distance b along the other principal direction (b>a). We provide a buckling configuration map in the design-space of geometric parameters (fiber volume fraction and in-plane periodicity aspect ratio, b/a). Pre-determined by the microstructure parameters, the fibers buckle towards (i) the first principal direction along which the fibers are closer to each other, or (ii) the second principal direction, or (iii) towards a non-principal direction. Furthermore, we find that the characteristics of the buckling plane map is governed by the shear modulus contrast between the phases of FCs. We anticipate that the distinct instability patterns reported here will enrich the design-space for building deformation-controlled tunable materials.

Fiber plasticity and loss of ellipticity in soft composites under non-monotonic loading

In this work, we relate fiber plasticity in soft composites to the loss of ellipticity of the governing equations of equilibrium of a composite under non-monotonic uniaxial loading. The loss of ellipticity strongly indicates the emergence of localization phenomena in the composite, reminiscent of the emergence of kinking instabilities in tendon, which occur as a response to tendon “overload” without requiring any macroscopic compressive loading. We examine soft composites where both fibers and matrix can be highly extensible and plastic deformations are present in the fiber phase. We first propose a transversely isotropic constitutive model for the fibers allowing for plastic deformations, taking into account a single slip direction, consistent with the microstructure of hierarchically assembled collagen fibers. Following, we propose a simple hyperelastic model for the matrix and combine the two following the Voigt assumption. We then formulate a general loss of ellipticity criterion for an elastoplastic material subjected to finite deformations. We use this criterion to indicate critical conditions for loss of ellipticity in the soft composite and individually in the fiber phase, under various loading–unloading paths. Results show that plastic deformation of the fiber phase during tensile loading can lead to ellipticity breakdown during elastic unloading while, macroscopically, the material is still in tension, indicating the possible onset of an instability.

An elastomeric composite with tunable thermal and mechanical properties via liquid metal-flake graphite multiphase fillers

Soft composites with improved and tunable thermal conductivity, stretchability and load-bearing capacity may find fascinating applications in soft robotics and wearable devices. Unfortunately, thermally conductive soft composites with solid fillers usually show serious degradation in stretchability, while those with liquid fillers usually possess a weak load-bearing capacity. To overcome these limitations, an elastomeric composite with solid–liquid multiphase fillers, named as pre-mixed LM&amp;FG/SR composite, is proposed here by incorporating flake graphite (FG) with high thermal conductivity into liquid metal (LM) and then dispersing the mixture in a soft silicone elastomer (SR). In comparison with the LM/SR composite and the FG/SR composite, the pre-mixed LM&amp;FG/SR composite gains the highest thermal conductivity under the strain-free state as it possesses both the excellent thermal conductivity of the FG filler and the high-volume fraction of the LM filler. In addition, by controlling the volume fraction of the multiphase fillers and the volume ratio of LM to FG, the pre-mixed LM&amp;FG/SR composite can gain tunable thermal and mechanical properties under the strained state and thus be applicable to complex situations in which both excellent heat-conducting capability and certain load-bearing capacity are required.