Compliant Matrix Research Articles

A Novel Flexible Green Composite with Sisal and Natural Rubber: Investigation under Low-Velocity Impact

ABSTRACT The present work concentrates on assessing the low-velocity impact (LVI) response of sisal-natural rubber (NR)-based flexible green composite in two different stacking sequences, namely, sisal/rubber/sisal (SRS) and sisal/rubber/sisal/rubber/sisal (SRSRS). The influence of the impactor shape on LVI response of the proposed composite was assessed using hemispherical and conical-shaped impactors. Results showed that the proposed composites exhibit better energy absorption and resistance to damage due to inclusion of compliant matrix. The study of damage mechanism of the proposed composites showed that the inclusion of NR as a matrix material in the proposed composites helps to avoid catastrophic failure since rubber undergoes failure by matrix tearing as opposed to matrix cracking as in the case of stiff composites. The proposed composites eliminate two of the major damage mechanisms, namely, matrix cracking and delamination, due to usage of compliant matrix material. The results obtained suggest that the proposed flexible composites can serve as excellent sacrificial structures. The outcome of the present study serves as a benchmark for interested designers/engineers to explore the usage of natural material candidates for developing sustainable impact-resistant composites.

Engineering Fiber-Based Nervous Tissue Constructs for Axon Regeneration

Biomaterial-based scaffolds used in nerve conduits including channels for confining regenerating axons and 3-dimensional (3D) gels as substrates for growth have made improvements in models of nerve repair. Many biomaterial strategies, however, continue to fall short of autologous nerve grafts, which remain the current gold standard in repairing severe nerve lesions (<20 mm). Intraluminal nerve conduit fibers have also shown considerable promise in directing regenerating axons in vitro and in vivo and have gained increasing interest for nerve repair. It is unknown, however, how growing axons respond to a fiber when encountered in a 3D environment. In this study, we considered a construct consisting of a compliant collagen hydrogel matrix and a fiber component to assess contact-guided axon growth. We investigated preferential axon outgrowth on synthetic and natural polymer fibers by utilizing small-diameter microfibers of poly-L-lactic acid and type I collagen representing 2 different fiber stiffnesses. We found that axons growing freely in a 3D hydrogel culture preferentially attach, turn and follow fibers with outgrowth rates and distances that far exceed outgrowth in a hydrogel alone. Wet-spun type I collagen from rat tail tendon performed the best, associated with highly aligned and accelerated outgrowth. This study also evaluated the response of dorsal root ganglion neurons from adult rats to provide data more relevant to axon regenerative potential in nerve repair. We found that ECM treatments on fibers enhanced the regeneration of adult axons indicating that both the physical and biochemical presentation of the fibers are essential for enhancing axon guidance and growth.

Engineering Shape-Controlled Microtissues on Compliant Hydrogels with Tunable Rigidity and Extracellular Matrix Ligands.

In vitro models that recapitulate key aspects of native tissue architecture and the physical microenvironment are emerging systems for modeling development and disease. For example, the myocardium consists of layers of aligned and coupled cardiac myocytes that are interspersed with supporting cells and embedded in a compliant extracellular matrix (ECM). These cell-cell and cell-matrix interactions are known to be important regulators of tissue physiology and pathophysiology. In this protocol, we describe a method for mimicking the alignment, cell-cell interactions, and rigidity of the myocardium by engineering an array of square, aligned cardiac microtissues on polyacrylamide hydrogels. This entails three key methods: (1) fabricating elastomer stamps with a microtissue pattern; (2) preparing polyacrylamide hydrogel culture substrates with tunable elastic moduli; and (3) transferring ECM proteins onto the surface of the hydrogels using microcontact printing. These hydrogels can then be seeded with cardiac myocytes or mixtures of cardiac myocytes and fibroblasts to adjust cell-cell interactions. Overall, this approach is advantageous because shape-controlled microtissues encompass both cell-cell and cell-matrix adhesions in a form factor that is relatively reproducible and scalable. Furthermore, polyacrylamide hydrogels are compatible with the traction force microscopy assay for quantifying contractility, a critical function of the myocardium. Although cardiac microtissues are the example presented in this protocol, the techniques are relatively versatile and could have many applications in modeling other tissue systems.

Failure Modes of a Laminated Composite With Complaint Interlayers

Abstract Composites comprising a high-volume fraction of stiff reinforcements within a compliant matrix are commonly found in natural materials. The disparate properties of the constituent materials endow resilience to the composite, and here we report an investigation into some of the mechanisms at play. We report experiments and simulations of a prototype laminated composite system comprising silicon layers separated by polymer interlayers, where the only failure mechanism is the tensile fracture of the brittle silicon. Two failure modes are observed for such composites loaded in three-point bending: failure under the central roller in (i) the top ply (in contact with the roller) or (ii) the bottom ply (free surface). The former mode is benign with the beam retaining load carrying capacity, whereas the latter leads to catastrophic beam failure. Finite element (FE) simulations confirm this transition in failure mode and inform the development of a reduced order model. Good agreement is shown between measurements, FE simulations, and reduced order predictions, capturing the effects of material and geometric properties on the flexural rigidity, first ply failure mode, and failure load. A failure mechanism map for this system is reported that can be used to inform the design of such laminated composites.

Interfacial indentations in biological composites

Biocomposites comprise highly stiff reinforcement elements connected by a compliant matrix material. While the interfacial elastic properties of these biocomposites play a key role in determining the mechanical properties of the entire biocomposite, these properties cannot be measured directly from standard nanomechanical experiments. Developing a method for extracting the interfacial elastic properties in biocomposites is, therefore, a major objective of cutting-edge biomaterials science. Here, using mechanical modeling and Finite-Element simulations, we analyze the interfacial force–depth relationships, stress distribution, and indentation modulus of standard nanoindentation testing in biocomposites, and we establish an analytical framework that connects these results to the elastic properties of the underlying matrix and reinforcement components. The resulting analytical framework is general and holds for a broad range of biocomposites, thus enabling a deeper understanding of the mechanical characteristics of functional interfaces in various biomaterials. Moreover, this framework can be adapted to account for synthetic, microscale, and nanoscale composite materials, and thereby promotes the development of advanced interfacial configurations with specialized mechanical capabilities.

A discrete interface in matrix stiffness creates an oscillatory pattern of endothelial monolayer disruption.

Intimal stiffening upregulates endothelial cell contractility, disrupting barrier integrity; however, intimal stiffening is non-uniform. The impact of local changes in intimal stiffness on proximal and distal cell-cell interactions is unknown. To investigate the range at which matrix stiffness heterogeneities impact neighboring endothelial cells within a monolayer, we built a micropillar system with adjacent regions of stiff and compliant matrix. The stiffness interface results in an oscillatory pattern of neutrophil transendothelial migration, symmetrical about the interface and well-fit by a sinusoid function. 'Peaks' of the sinusoid were found to have increased cellular contractility and decreased barrier function relative to 'troughs' of the sinusoid. Pharmacological modulation of contractility was observed to break symmetry, altering the amplitude and wavelength of the sinusoid, indicating that contractility may regulate this effect. This work illuminates a novel biophysical phenomenon of the role of stiffness-mediated cell-matrix interactions on cell-cell interactions at a distance. Additionally, it provides insight into the range at which intimal matrix stiffness heterogeneities will impact endothelial barrier function and potentially contribute to atherogenesis.

Effect of magnetic-field-induced restructuring on the elastic properties of magnetoactive elastomers

Composite materials where magnetic micrometer-sized particles are embedded into a compliant polymer matrix are known as magnetorheological (or magnetoactive) elastomers (MAEs). They are distinguished by huge variations in their physical properties, when in a magnetic field, which is commonly attributed to the restructuring of the filler. The process of the magnetic-field-induced restructuring in a magnetorheological elastomer is interpreted as progression towards percolation. Such a physical model was previously used to explain the dependence of the magnetic permeability and dielectric permittivity of MAEs on the magnetic field strength. Based on this hypothesis, the magnetorheological effect in MAEs is considered theoretically. The theoretical approach is built upon a self-consistent effective-medium theory for the elastic properties, extended to the variable (field dependent) percolation threshold. The proposed model allows one to describe the large variations (over several orders of magnitude) of the effective elastic moduli of these composite materials, known as the giant magnetorheological (MR) and field-stiffening effects. The existence of a giant magnetic Poisson effect is predicted. The relation of the proposed model to the existing theories of the MR effect in MAEs is discussed. The results can be useful for applications of MAEs in magnetic-field-controlled vibration dampers and isolators.

Instabilities of embedded cylindrical inclusions undergoing isotropic swelling or growth

Infinite circular cylindrical elastic inclusions, or rods, embedded in an unbounded elastic matrix display various modes of instability when they undergo sufficiently large expansion due to either swelling or volumetric growth. In this letter two modes of instability are examined: sinusoidal axisymmetric modes and sinusoidal bending modes. The rod and the matrix are neo-Hookean materials, and the full range of the modulus ratio of rod to matrix is considered. In the primary case examined, deformation is driven by an isotropic volumetric expansion, or transformation, of the rod. A three-dimensional bifurcation analysis of the rod constrained by the matrix reveals the onset of the critical instability mode as dependent on the modulus ratio. Comparisons with related results are discussed, including the compressive buckling of a stiff rod in a compliant matrix and the other limit when the modulus of the rod is very small compared to that of the matrix and behaves effectively as a fluid exerting pressure on the wall of the matrix cavity.11Contributed to celebrate the awarding of the Prager medal to Prof. Horacio Espinosa.

A Soft Mechanical Phenotype of SH-SY5Y Neuroblastoma and Primary Human Neurons Is Resilient to Oligomeric Aβ(1-42) Injury.

Aggregated amyloid beta (Aβ) is widely reported to cause neuronal dystrophy and toxicity through multiple pathways: oxidative stress, disrupting calcium homeostasis, and cytoskeletal dysregulation. The neuro-cytoskeleton is a dynamic structure that reorganizes to maintain cell homeostasis in response to varying soluble and physical cues presented from the extracellular matrix (ECM). Due this relationship between cell health and the ECM, we hypothesize that amyloid toxicity may be directly influenced by physical changes to the ECM (stiffness and dimensionality) through mechanosensitive pathways, and while previous studies demonstrated that Aβ can distort focal adhesion signaling with pathological consequences, these studies do not address the physical contribution from a physiologically relevant matrix. To test our hypothesis that physical cues can adjust Aβ toxicity, SH-SY5Y human neuroblastoma and primary human cortical neurons were plated on soft and stiff, 2D polyacrylamide matrices or suspended in 3D collagen gels. Each cell culture was exposed to escalating concentrations of oligomeric or fibrillated Aβ(1-42) with MTS viability and lactate dehydrogenase toxicity assessed. Actin restructuring was further monitored in live cells by atomic force microscopy nanoindentation, and our results demonstrate that increasing either matrix stiffness or exposure to oligomeric Aβ promotes F-actin polymerization and cell stiffening, while mature Aβ fibrils yielded no apparent cell stiffening and minor toxicity. Moreover, the rounded, softer mechanical phenotype displayed by cells plated onto a compliant matrix also demonstrated a resilience to oligomeric Aβ as noted by a significant recovery of viability when compared to same-dosed cells plated on traditional tissue culture plastic. This recovery was reproduced pharmacologically through inhibiting actin polymerization with cytochalasin D prior to Aβ exposure. These studies indicate that the cell-ECM interface can modify amyloid toxicity in neurons and the matrix-mediated pathways that promote this protection may offer unique targets in amyloid pathologies like Alzheimer's disease.

3D Printed Hydrogel Multiassay Platforms for Robust Generation of Engineered Contractile Tissues.

We present a method for reproducible manufacture of multiassay platforms with tunable mechanical properties for muscle tissue strip analysis. The platforms result from stereolithographic 3D printing of low protein-binding poly(ethylene glycol) diacrylate (PEGDA) hydrogels. Contractile microtissues have previously been engineered by immobilizing suspended cells in a confined hydrogel matrix with embedded anchoring cantilevers to facilitate muscle tissue strip formation. The 3D shape and mechanical properties of the confinement and the embedded cantilevers are critical for the tissue robustness. High-resolution 3D printing of PEGDA hydrogels offers full design freedom to engineer cantilever stiffness, while minimizing unwanted cell attachment. We demonstrate the applicability by generating suspended muscle tissue strips from C2C12 mouse myoblasts in a compliant fibrin-based hydrogel matrix. The full design freedom allows for new platform geometries that reduce local stress in the matrix and tissue, thus, reducing the risk of tissue fracture.

Advanced Actuator Materials Powered by Biomimetic Helical Fiber Topologies.

Helical constructs are ubiquitous in nature at all size domains, from molecular to macroscopic. The helical topology confers unique mechanical functions that activate certain phenomena, such as twining vines and vital cellular functions like the folding and packing of DNA into chromosomes. The understanding of active mechanical processes in plants, certain musculature in animals, and some biochemical processes in cells provides insight into the versatility of the helix. Most of these natural systems consist of helically oriented filaments embedded in a compliant matrix. In some cases, the matrix can change volume and in others the filaments can contract and the matrix is passive. In both cases, the helically arranged fibers determine the overall shape change with a great variety of responses involving length contraction/elongation, twisting, bending, and coiling. Synthetic actuator materials and systems that employ helical topologies have been described recently and demonstrate many fascinating and complex shape changes. However, significant new opportunities exist to mimic some of the most remarkable actions in nature, including the Vorticella's coiling stalk and DNA's supercoils, in the quest for superior artificial muscles.

A multifunctional shape-morphing elastomer with liquid metal inclusions

Natural soft tissue achieves a rich variety of functionality through a hierarchy of molecular, microscale, and mesoscale structures and ordering. Inspired by such architectures, we introduce a soft, multifunctional composite capable of a unique combination of sensing, mechanically robust electronic connectivity, and active shape morphing. The material is composed of a compliant and deformable liquid crystal elastomer (LCE) matrix that can achieve macroscopic shape change through a liquid crystal phase transition. The matrix is dispersed with liquid metal (LM) microparticles that are used to tailor the thermal and electrical conductivity of the LCE without detrimentally altering its mechanical or shape-morphing properties. Demonstrations of this composite for sensing, actuation, circuitry, and soft robot locomotion suggest the potential for versatile, tissue-like multifunctionality.

Size effect on elastic stress concentrations in unidirectional fiber reinforced soft composites

We revisit the classic problem of determining stress concentrations on neighboring fibers to multiple, transversely-aligned fiber breaks in a planar, unidirectional fiber–matrix composite. Fibers are assumed to be perfectly bonded to the elastic matrix. Finite size effects on stress concentration are studied by varying the overall length of the composite relative to the characteristic load transfer length between broken and intact fibers. As an alternative to the discrete fiber and matrix framework in the classic analysis of Hedgepeth, and its extension by Hikami and Chou, the fiber stress distribution in the composite is obtained through continuum modeling of the composite as a highly anisotropic elastic plate, whereby the stresses and stress concentration factors at fiber locations in the discrete model are extracted in a closed form. For composites of finite length, the stress concentration factors determined using the continuum model compare favorably with numerical solution of the discrete shear-lag model. In the limit of a plate with infinitely long fibers, our stress concentration factors also agree well with the exact results of Hikami and Chou. For composites having a length less than the characteristic elastic load transfer length, and loaded under displacement boundary conditions, we show that local stress concentrations vanish irrespective of the size of the crack or the number of fiber breaks. This behavior becomes important when modeling and interpreting laboratory experiments on the mechanical behavior of recent soft composite specimens consisting of stiff fibers in an extremely compliant elastic matrix.

Modeling and Fabrication of Scalable Tactile Sensor Arrays for Flexible Robot Skins

Artificial robot skins capable of multi-modal tactile feedback are anticipated to revolutionize how robots perceive their immediate surroundings and how they collaborate with humans on tasks that are more complicated and dynamic than currently possible. Efforts to develop the tactile skins must tackle significant hurdles related to flexibility, robustness, and scalability of sensors. In this paper, a flexible tactile sensor array is developed to address these hurdles via a novel sensor geometry and thermal compensation technique. This tactile skin relies on a polymeric piezoresistive array of strain gauges with a novel star-shaped geometry. Due to their radial symmetry, the proposed strain gauges exhibit a biaxial response to local strain-stress loading, avoiding “blind spots” in the vicinity of individual tactels. The sensors are fabricated using gold electrodes and piezoresistive polymers (PEDOT:PSS) micro-patterned on a flexible polyimide substrate enclosed in a compliant silicone matrix. The sensor is first modeled using multiphysics finite elements software to guide the design choices for a variety of geometric parameters under various load profiles and locations with respect to the gauge’s center. A novel homogeneous thermal compensation technique is proposed, which packages the sensors on the opposite sides of a flexible carrier. This thermal compensation technique achieves a 10-fold attenuation in temperature drifts as well as a double sensitivity in terms of strain measurement. After numerical modeling, a $4\times 4$ tactile array with the optimal geometry obtained through simulation was fabricated and interfaced to a custom-made electronic data acquisition system. Samples were tested using an automated dynamic force experimental test bed. The tactile skin array was experimentally characterized in terms of force sensitivity, spatial response, dynamic bandwidth, and temperature drift. The array exhibits a sensitivity of 1125 nV/N in the range of 0–0.5 N, a sensitivity of 412 nV/N in the range of 0.5–2.3 N, a hysteresis of 11.13%, a dynamic bandwidth of 1.49 Hz, and a temperature sensitivity of 6 nV/°C, averaged among individual sensors.

Porous Germanium Anode for Li-Ion Batteries

Due to the steady growing demand for higher performance electronic devices it is required an increase of the actual specific energy of Lithium Ion Batteries (LIBs)[1]. This can be achieved enhancing the gravimetric capacity of the electrodes, i.e. the amount of charge that can be stored per unit of mass of the active material. Currently a variety of cathode materials are available, being lithium metal oxides the most commonly used, while for the anode the choice is essentially between TiO2 or carbonaceous materials, among which Graphite is by far the most employed. The theoretical gravimetric capacity of Graphite is 372 mAh/g but several materials show higher capacities. In this respect, the highest gravimetric capacities are found in Lithium metal (3862 mAh/g), Silicon (4200 mAh/g) and Germanium (1624 mAh/g). Even though these numbers would suggest Lithium metal as the best possible active material, it has been discarded due to its high reactivity inducing dendrites formation upon charge and discharge processes, which could eventually lead to short circuits. Silicon and Germanium also show improved capacities compared to Graphite but they both suffer huge volume changes (up to 400%) upon cycling, which could lead to the pulverization of the electrodes. Nano-structuring these materials is an approach to overcome this issue, realizing structures which can reversibly accommodate the occurring volume variations. It is easier to obtain a Germanium compliant matrix rather than a Silicon one, as Germanium requires an average pore dimension that is bigger compared to Silicon[2]. Furthermore, Germanium shows a higher electrical conductivity (10,000 times higher than Silicon) and lithium ion diffusion rate (400 times higher than Silicon), which could make it suitable for high performance applications. These are the reasons that led us to test Germanium as an alternative anodic material with respect to Graphite. In this work we present the results of our novel anodic material made of porous Germanium. The Germanium films are directly grown on a metallic current collector through a plasma enhanced chemical vapor deposition technique (PECVD) without any binder, and subsequently nano-structured to realize a porous matrix using a hydrofluoric acid electrochemical attack. Molybdenum or Stainless Steel were used as current collectors without requiring any binder to enhance the adhesion of Germanium, i.e. the mass loading is truly and uniquely represented by active material. Moreover, the PECVD technique permits to obtain a higher purity compared to chemically grown materials via standard reactions in solution. These anodes are able to perform hundreds of charge and discharge cycles at very high C-rates, retaining a stable capacity of more than 950 mAh/g even at currents as high as 5C (considering 1C as 1600 mA/g), which is 2.5 times the theoretical Graphite capacity. 2032 coin-type half-cells were assembled coupling the electrode to pure Lithium metal used as counter and reference electrode. A standard electrolyte solution composed by EC:DMC has been used with different additives (vinylene carbonate-VC and fluoroethylene carbonate-FEC) whose effect on the performances will be shown. The active material layer thickness varied between 1 to 5 microns and the mass loadings ranged from 0.2 to 1.5 mg. Various etching recipes have been tested to obtain different porous morphologies. As an alternative technique to nano-structure the materials, a suitably tuned Ion Implantation technique has been exploited: the results will be shown for comparison, along with results from bulk samples in order to show the effectiveness of the nano-structuration process on the cell performance. The best results were obtained using porous Germanium grown onto Molybdenum substrate adding FEC: in graph 1 is reported the Capacity vs. Cycle Number plot for one of these samples, which has been tested for thousand cycles at high C-rates (mass loading 0.24 mg). [1] G. Zubi, R. Dufo-López, M. Carvalho e G. Pasaoglu, «The lithium-ion battery: State of the art and future perspectives,» Renewable and Sustainable Energy Reviews, vol. 89, pp. 292-308, 2018. [2] Z. Hu, S. Zhang, C. Zhang e G. Cui, «High performance germanium-based anode materials,» Coordination Chemistry Reviews, vol. 326, pp. 34-85, 2016. Figure 1

Interplay between the Yes‐Associated protein and the matricellular protein CCN1 Regulates the phenotypical plasticity of endothelial cells in developing blood vessels

CCN1 is a heparin and integrin‐binding matricellular protein which localizes pericellularly and acts primarily on cells that produce it. CCN1 is highly expressed at sites of active angiogenesis and tissue repair. Our studies in conditional knockout mouse models, demonstrate that endothelium‐specific deletion of the CCN1 gene results in severe vascular defects, most notably uncontrolled angiogenic sprouting and increased endothelial cell (EC) proliferation, resulting in the formation of a denser, widely lumenized vascular network. In this study, we investigated the regulatory mechanisms and signaling pathways involved in CCN1 gene expression and activity during angiogenesis. We found that the CCN1 gene is a target of the transcriptional coactivator Yes‐Associated protein (YAP), which acts in concert with other co‐activators such as myocardin‐related transcription factor (MRTF)‐A to activate the CCN1 promoter in angiogenic ECs. Loss of YAP function in mice is associated with reduced CCN1 promoter enrichment with YAP and decreased CCN1 gene expression. YAP is a key checkpoint of the Hippo pathway controlling cell growth, differentiation and cell‐cell junctions. It comprises a kinase cascade that inactivates by phosphorylation YAP leading to its cytoplasmic sequestration and growth inhibition. Loss of function of CCN1 in mice was associated with accumulation of active nonphosphorylated YAP in developing blood vessels. Consequently, growing ECs incorporate into preformed vessels and are no longer directed toward angiogenic sprouts, resulting in the formation of a thicker denser vasculature. In cultured ECs, a sustained expression of CCN1 through adenoviral gene transfer increases YAP phosphorylation and reduced its localization into the nucleus. CCN1‐integrin signaling through focal adhesion and the actin cytoskeleton provides the cells with a soft compliant matrix putting them in in low contractility, YAP repressive cytoskeletal states. This crosstalk between CCN1 and YAP regulates phenotypical plasticity of ECs and promotes correct sprouting and branching patterns of developing blood vessels.Support or Funding InformationR01EY022091‐05A1R01EY024998‐01A1This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effect of interface strength on the mechanical behaviour of bio-inspired composites: A molecular dynamics study

Nanoscale composites inspired from biological materials such as nacre and bone are excellent candidates for improving the mechanical properties of ceramic matrix composites (CMC) or in general any brittle-matrix composites. These composites are based on the regularly staggered model (RSM) and on the stair-wise staggered model (SSM), and consist of stiffer platelets embedded in a compliant matrix. The interface between the platelets and the matrix plays a key role as an enormous amount of interfacial area exists in the nanocomposite materials. We investigate the influence of interface strength on the overall mechanical properties and deformation mechanisms of bio-inspired brittle-matrix nanocomposites. Atomistic simulations are used for this purpose and the interface strength is controlled by varying the binding energy between the platelet atoms and the matrix atoms. We found that the mechanical properties of both models initially increase with the interface strength. However, above a critical value, the trend changes drastically between the two models. The RSM model continues to maintain the flow stress and toughness at higher interface strengths, due to the persistence of deformation mechanisms such as bulk matrix plasticity and platelet pullout. In contrast, in the SSM model the flow stress and toughness are deteriorated as softening of stress and fracturing of platelets occur at higher interface strengths. The knowledge gained from this study paves new way for the development of bio-inspired ceramic-matrix composites with improved mechanical properties.

Inherent negative refraction on acoustic branch of two dimensional phononic crystals

Guided by theoretical predictions, we have demonstrated experimentally the existence of negative refraction on the two lowest acoustic-branch passbands (shear and longitudinal modes) of a simple two-dimensional phononic crystal consisting of an isotropic stiff (aluminum) matrix and square-patterned isotropic compliant (PMMA) circular inclusions. We experimentally distinguished the shear and longitudinal modes to ensure that there are no couplings between the two modes and that the shear-mode negative refraction is an inherent property of periodic composites with stiff matrix and compliant inclusions. We have also discovered that a composite with compliant (PMMA) matrix and stiff (aluminum) inclusions does not display negative shear-mode refraction on its first branch. At frequencies and wave vectors where the refraction on the acoustic-branch passbands is negative, the effective mass-density and the effective stiffness tensors of the crystal are positive-definite, and this is an inherent property of such phononic crystals.The equi-frequency contours and energy flux vectors as functions of the phase-vector components, reveal a rich body of refractive properties that can be exploited to realize, for example, beam splitting, focusing, and frequency filtration on the lowest passbands of the crystal where the dissipation is minimum. By proper selection of material and geometric parameters these phenomena can be realized at remarkably low frequencies (large wave lengths) using rather small simple two-phase unit cells.

Wood‐Inspired 3D‐Printed Helical Composites with Tunable and Enhanced Mechanical Performance

AbstractHelical fibers are versatile building blocks used by Nature to improve mechanical performance and to tune local behavior of load‐bearing materials. Helicoidal biocomposites are arranged in multiple layers with different fiber orientations. Such heterogeneity, not matched in synthetic materials, provides biological structures with superior properties. This is the case of the multilayer tube‐like structure of the wood cell wall, where each ply features a compliant matrix reinforced by stiff helicoidal microfibrils. Here, 3D polyjet printing and computer simulations are combined to investigate wood‐inspired helix‐reinforced cylinders. Composites with a main layer containing helicoidal fibers, bordered by inner and outer plies having thinner fibrils are considered. It is shown how the mechanical functionalities of the synthetic structures can be programmed by varying fibers/fibrils orientation and matrix compliance. It is demonstrated that failure resistance can be enhanced by enclosing the main helicoidal layer with a minimum amount of thin fibrils oriented perpendicular to the applied load, as observed in wood. Finite element simulations are used to highlight the critical role of the matrix in load‐transfer mechanisms among stiff elements. These structures have the potential to be assembled into larger systems, leading to graded composites with region‐specific properties optimized for multiple functionalities.

Estimation of Effective Thermal and Mechanical Properties of Particulate Thermal Interface Materials by a Random Network Model

Particulate thermal interface materials (TIMs) are commonly used to transport heat from chip to heat sink. While high thermal conductance is achieved by large volume loadings of highly conducting particles in a compliant matrix, small volume loadings of stiff particles will ensure reduced thermal stresses in the brittle silicon device. Developing numerical models to estimate effective thermal and mechanical properties of TIM systems would help optimize TIM performance with respect to these conflicting requirements. Classical models, often based on single particle solutions or regular arrangement of particles, are insufficient as real-life TIM systems contain a distribution of particles at high volume fractions, where classical models are invalid. In our earlier work, a computationally efficient random network model (RNM) was developed to estimate the effective thermal conductivity of TIM systems (Kanuparthi et al., 2008, “An Efficient Network Model for Determining the Effective Thermal Conductivity of Particulate Thermal Interface Materials,” IEEE Trans. Compon. Packag. Technol., 31(3), pp. 611–621; Dan et al., 2009, “An Improved Network Model for Determining the Effective Thermal Conductivity of Particulate Thermal Interface Materials,” ASME Paper No. InterPACK2009-89116.) . This model is extended in this paper to estimate the effective elastic modulus of TIMs. Realistic microstructures are simulated and analyzed using the proposed method. Factors affecting the modulus (volume fraction and particle size distribution (PSD)) are also studied.