[Show abstract][Hide abstract] ABSTRACT: Highly mineralized biological materials such as nacre (mother of pearl), tooth enamel or conch shell boast unique and attractive combinations of stiffness, strength and toughness. The structures of these biological materials and their associated mechanisms are now inspiring new types of advanced structural materials. However, despite significant efforts, no bottom up fabrication method could so far match biological materials in terms of microstructural organization and mechanical performance. Here we present a new ‘top down’ strategy to tackling this fabrication problem, which consists in carving weak interfaces within a brittle material using a laser engraving technique. We demonstrate the method by fabricating and testing borosilicate glasses containing nacre-like microstructures infiltrated with polyurethane. When deformed, these materials properly duplicate the mechanisms of natural nacre: combination of controlled sliding of the tablets, accompanied with geometric hardening, strain hardening and strain rate hardening. The nacre-like glass is composed of 93 volume % (vol%) glass, yet 700 times tougher and breaks at strains as high as 20%.
[Show abstract][Hide abstract] ABSTRACT: The scaled skin of fish is a high performance natural armor which represents a source of inspiration for novel engineering designs. In this paper we present a biomimetic fish skin material, fabricated with a design and components that are simple, that achieves many of the advantageous attributes of natural materials, including the unique combination of flexibility and mechanical robustness. The bio-inspired fish skin material is designed to replicate the structural, mechanical, and functional aspects of a natural teleost fish skin comprised of leptoid-like scales, similar to that of the striped red mullet Mullus surmuletus. The manmade fish skin material consists of a low modulus elastic mesh or 'dermis' layer that holds rigid, plastic scales. The mechanics of the synthetic material is characterized under in-plane, bending, and indentation modes of deformation and is successfully described by theoretical deformation models that have been developed. This combined experimental and modeling approach elucidates the critical mechanisms by which the composite material achieves its unique properties and provides design rules that enable the engineering of scaled skins. Such artificial scaled skins that are flexible, lightweight, transparent, and robust under mechanical deformation may thus have potential as thin protective coatings for soft materials.
[Show abstract][Hide abstract] ABSTRACT: Metal sulfides are widely used in a variety of applications requiring high hardness and toughness. In this study, the microstructure and mechanical properties of chromium–chromium sulfide cermets are investigated. The chromium–chromium sulfide cermet was manufactured using self-propagating high-temperature synthesis, a process where the material is created under a self-sustaining combustion reaction between the chromium and sulfur. This type of synthesis allows the creation of near-net shape structures and offers the possibility of tuning material properties and material behavior by changing the composition of the reactant. Microstructural characterization was performed using optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. The mechanical properties of the cermet (Young’s modulus, fracture toughness, flexural strength, and microhardness) have been measured and related to morphology and chemical composition of the samples. Results show that dense cermets (about 7 % porosity) with specific structure have been obtained. Pure CrS has a significant hardness, but its toughness was insufficient for tool applications. However, we found that the density and fracture toughness of the cermets increase with the addition of Cr. The addition of Cr also improved the flexural strength and hardness of the cermet by 60 % and almost 38 %, respectively.
[Show abstract][Hide abstract] ABSTRACT: Self-assembly produces materials with highly organized microstructures and attractive properties for a variety of applications. Self-assembly is a process which typically involves molecules or nano-scale objects, with only a few reports of successful self-assembly of objects with larger dimensions (greater than 1 μm). Self-assembly at this length scale is however important, and may find different technological applications because of the possibility to incorporate different functionalities to the building blocks by for example lithographic and microfabrication techniques. Meso-scale self-assembly is also particularly promising to duplicate the structure of natural materials such as nacre (mother of pearl). Here, we fabricated 10 μm sized hexagonal tablets of silicon which self-assembled into a well-packed periodically arranged structure at a water-air interface. The microstructure was secured in a PDMS thin film, which made it stable and more organized compared to the similar large scale assemblies reported in the past. The self-assembled films can serve as building blocks for biomimetic materials, protective coatings, flexible electronics, or tunable optical devices.
[Show abstract][Hide abstract] ABSTRACT: Nacre, bone and spider silk are staggered composites where inclusions of high aspect ratio reinforce a softer matrix. Such staggered composites have emerged through natural selection as the best configuration to produce stiffness, strength and toughness simultaneously. As a result, these remarkable materials are increasingly serving as model for synthetic composites with unusual and attractive performance. While several models have been developed to predict basic properties for biological and bio-inspired staggered composites, the designer is still left to struggle with finding optimum parameters. Unresolved issues include choosing optimum properties for inclusions and matrix, and resolving the contradictory effects of certain design variables. Here we overcome these difficulties with a multi-objective optimization for simultaneous high stiffness, strength and energy absorption in staggered composites. Our optimization scheme includes material properties for inclusions and matrix as design variables. This process reveals new guidelines, for example the staggered microstructure is only advantageous if the tablets are at least five times stronger than the interfaces, and only if high volume concentrations of tablets are used. We finally compile the results into a step-by-step optimization procedure which can be applied for the design of any type of high-performance staggered composite and at any length scale. The procedure produces optimum designs which are consistent with the materials and microstructure of natural nacre, confirming that this natural material is indeed optimized for mechanical performance.
Journal of the Mechanics and Physics of Solids 11/2014; 73. DOI:10.1016/j.jmps.2014.08.008 · 3.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Natural materials such as nacre, bone, collagen or spider silk boast unusual combinations of stiffness, strength and toughness. Behind this performance is the staggered microstructure, which consists of stiff and elongated inclusions embedded in a softer and more deformable matrix. The micromechanics of deformation and failure associated with this microstructure are now well understood at the "unit cell" level, the smallest representative volume for this type of material. However, these mechanisms only translate to high performance if they propagate throughout large volumes, an important condition which is often overlooked. Here we present, for the first time, a model which captures the conditions for spreading of deformations or localization, which determines whether a staggered composite is brittle or deformable at the macroscale. The macroscopic failure strain for the material was calculated as function of the viscoplastic properties of the interfaces and the severity of the defect. As expected, larger strains at failure can be achieved when smaller defects are present within the material, or with more strain hardening at the interface. The model also shows that strain rate hardening is a powerful source of large deformations for the material as well, a result we confirmed and validated with tensile experiments on glass-PDMS nacre-like staggered composites. An important implication is that natural materials, largely made of rate-dependent materials, could rely on strain rate hardening to tolerate initial defects and damage to maintain their functionality. Strain rate hardening should also be harnessed and optimized in bio-inspired composites in order to maximize their overall performance.
[Show abstract][Hide abstract] ABSTRACT: Natural and man-made structural materials perform similar functions such as structural support or protection. Therefore they rely on the same types of properties: strength, robustness, lightweight. Nature can therefore provide a significant source of inspiration for new and alternative engineering designs. We report here some results regarding a very common, yet largely unknown, type of biological material: fish skin. Within a thin, flexible and lightweight layer, fish skins display a variety of strain stiffening and stabilizing mechanisms which promote multiple functions such as protection, robustness and swimming efficiency. We particularly discuss four important features pertaining to scaled skins: (a) a strongly elastic tensile behavior that is independent from the presence of rigid scales, (b) a compressive response that prevents buckling and wrinkling instabilities, which are usually predominant for thin membranes, (c) a bending response that displays nonlinear stiffening mechanisms arising from geometric constraints between neighboring scales and (d) a robust structure that preserves the above characteristics upon the loss or damage of structural elements. These important properties make fish skin an attractive model for the development of very thin and flexible armors and protective layers, especially when combined with the high penetration resistance of individual scales. Scaled structures inspired by fish skin could find applications in ultra-light and flexible armor systems, flexible electronics or the design of smart and adaptive morphing structures for aerospace vehicles.
Journal of the Mechanics and Physics of Solids 08/2014; 68. DOI:10.1016/j.jmps.2014.01.005 · 3.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ceramic–metal composites (cermets) offer unique combinations of hardness and toughness, which make them attractive for a variety of applications. In this study, we propose a new method for the preparation of the metal–sulfur precursor mixture based on the ability to melt-cast the precursor mixture. We have used self-propagating high-temperature synthesis to produce a chromium/chromium sulfide cermet, exploiting the fact that this mixture of metal and sulfur can support the propagation of reactive waves. This ability, together with the properties of the reaction products (low gas evolution and liquid sulfide products), enables the net-shape synthesis of dense, near theoretical density product with a relatively simple and low-cost set-up. While the thermochemical calculations predict near-zero gas production for the chromium–sulfur system, the actual cermets showed a large amount of porosity (about 70 %), when synthesized at atmospheric pressure. The possible sources for porosity were identified, and the process improved to bring the porosity down to about 7 %. We also investigated the physical properties of the produced cermet with optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction techniques.
[Show abstract][Hide abstract] ABSTRACT: Crocodiles, armadillo, turtles, fish and many other animal species have evolved flexible armored skins in the form of hard scales or osteoderms, which can be described as hard plates of finite size embedded in softer tissues. The individual hard segments provide protection from predators, while the relative motion of these segments provides the flexibility required for efficient locomotion. In this work, we duplicated these broad concepts in a bio-inspired segmented armor. Hexagonal segments of well-defined size and shape were carved within a thin glass plate using laser engraving. The engraved plate was then placed on a soft substrate which simulated soft tissues, and then punctured with a sharp needle mounted on a miniature loading stage. The resistance of our segmented armor was significantly higher when smaller hexagons were used, and our bio-inspired segmented glass displayed an increase in puncture resistance of up to 70% compared to a continuous plate of glass of the same thickness. Detailed structural analyses aided by finite elements revealed that this extraordinary improvement is due to the reduced span of individual segments, which decreases flexural stresses and delays fracture. This effect can however only be achieved if the plates are at least 1000 stiffer than the underlying substrate, which is the case for natural armor systems. Our bio-inspired system also displayed many of the attributes of natural armors: flexible, robust with 'multi-hit' capabilities. This new segmented glass therefore suggests interesting bio-inspired strategies and mechanisms which could be systematically exploited in high-performance flexible armors. This study also provides new insights and a better understanding of the mechanics of natural armors such as scales and osteoderms.
[Show abstract][Hide abstract] ABSTRACT: Highly mineralized natural materials such as teeth or mollusk shells boast unusual combinations of stiffness, strength and toughness currently unmatched by engineering materials. While high mineral contents provide stiffness and hardness, these materials also contain weaker interfaces with intricate architectures, which can channel propagating cracks into toughening configurations. Here we report the implementation of these features into glass, using a laser engraving technique. Three-dimensional arrays of laser-generated microcracks can deflect and guide larger incoming cracks, following the concept of 'stamp holes'. Jigsaw-like interfaces, infiltrated with polyurethane, furthermore channel cracks into interlocking configurations and pullout mechanisms, significantly enhancing energy dissipation and toughness. Compared with standard glass, which has no microstructure and is brittle, our bio-inspired glass displays built-in mechanisms that make it more deformable and 200 times tougher. This bio-inspired approach, based on carefully architectured interfaces, provides a new pathway to toughening glasses, ceramics or other hard and brittle materials.
[Show abstract][Hide abstract] ABSTRACT: Natural materials and structures are increasingly becoming a source of inspiration for the design novel of engineering systems. In this context, the structure of fish skin, made of an intricate arrangement of flexible plates growing out of the dermis of a majority of fish, can be of particular interest for materials such as protective layers or flexible electronics. To better understand the mechanics of these composite shells, we introduce here a general computational framework that aims at establishing a relationship between their structure and their overall mechanical response. Taking advantage of the periodicity of the scale arrangement, it is shown that a representative periodic cell can be introduced as the basic element to carry out a homogenization procedure based on the Hill-Mendel condition. The proposed procedure is applied to the specific case of the fish skin structure of the Morone saxatilis, using a computational finite element approach. Our numerical study shows that fish skin possesses a highly anisotropic response, with a softer bending stiffness in the longitudinal direction of the fish. This softer response arises from significant scale rotations during bending, which induce a stiffening of the response under large bending curvature. Interestingly, this mechanism can be suppressed or magnified by tuning the rotational stiffness of the scale-dermis attachment but is not activated in the lateral direction. These results are not only valuable to the engineering design of flexible and protective shells, but also have implications on the mechanics of fish swimming.
International Journal of Solids and Structures 01/2014; 51(1):274–283. DOI:10.1016/j.ijsolstr.2013.10.001 · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The staggered structure is prominent in high-performance biological materials such as nacre, spider silk or bone. It consists of stiff and strong elongated inclusions aligned with the direction of loading. This structure leads to useful combinations of stiffness, strength and toughness, and it is therefore increasingly mimicked in bio-inspired composites. The modulus and strength of natural and bio-inspired composites are typically predicted using the shear lag model, where inclusions carry tensile stress and interfaces carry shear stresses. In this work, we have used a simple doctor blade technique to make thin film of nacre-like materials, which we tested in tension. Strength and modulus increase up to 10–15 % volume concentration of mineral reinforcement after which they degrade. Finite element analyses of staggered microstructure over a wide range of arrangement and concentrations revealed new trends which can explain the experimental results. For example the stiffness of the material can degrade in higher mineral concentrations if the tablets lack interface in the matrix. The results show that materials with combination of stiffness, strength and energy absorption can be achieved when there is an overlap between the tablets the distance between them is small in the transverse direction. This is identical with the microstructure of nacre where mineral tablets are divided by a very thin organic layer. The results suggest new approaches to simultaneously optimize modulus, strength and toughness in this class of composites.
Mechanics of Biological Systems and Materials, Volume 4, 01/2014: pages 83-91;
[Show abstract][Hide abstract] ABSTRACT: Engineering adhesive joints are being increasingly used in industry because of the advantages they offer over other joining methods such as fastening or welding. The development and the use of adhesives in a design environment require accurate mechanical tests in order to measure their strength and toughness. Standard techniques such as the shear lap test are commonly used to measure shear strength, but the results they produce generally depend on geometry and on initial defects within the bond line. Fracture tests such as the double cantilever beam (DCB) tests overcome these limitations, but rely on elasticity models and assumptions to determine toughness. In this study, we present a novel technique to directly determine the mode I fracture toughness of engineering adhesive joints as well as their full cohesive law, without any initial assumption on its shape. Our new method is remarkably simple in terms of experimental setup, execution and analysis. It is similar to the standard double cantilever beam (DCB) test with the difference that the material and dimensions of the beams are chosen so that they are assumed to be rigid compared to the bond line. In this rigid DCB (RDCB) technique the crack opening is known everywhere along the interface, which we use to compute the cohesive law of the adhesive directly from the load-displacement data obtained from experiment and the geometry of the RDCB specimen. The RDCB method is validated and applied to three typical commercial adhesives (polyurethane, epoxy, and silicone), to determine their cohesive law and fracture toughness.
[Show abstract][Hide abstract] ABSTRACT: Low intensity ultrasound-induced radicals interact with surface adsorbed orthophosphate to bond nanoparticles with high mechanical strength and surface area. Dissimilar materials could be bonded to form robust metallic, ceramic, and organic composite microparticles. 3D nanostructures of a hydrated and amorphous electrocatalyst with carbon nanotubes were also constructed which exceeded the resistance-limited efficiency of 2D electrodes.
[Show abstract][Hide abstract] ABSTRACT: How to arrange soft materials with strong but brittle reinforcements to achieve attractive combinations of stiffness, strength and toughness is an ongoing and fascinating question in engineering and biological materials science. Recent advances in topology optimization and bioinspiration have brought interesting answers to this question, but they provide only small windows into the vast design space associated with this problem. Here, we take a more global approach in which we assess the mechanical performance of thousands of possible microstructures. This exhaustive exploration gives a global picture of structure-property relationships and guarantees that global optima can be found. Landscapes of optimum solutions for different combinations of desired properties can also be created, revealing the robustness of each of the solutions. Interestingly, while some of the major hybrid designs used in engineering are absent from the set of solutions, the microstructures emerging from this process are reminiscent of materials, such as bone, nacre or spider silk.
Journal of The Royal Society Interface 09/2013; 10(89):20130711. DOI:10.1098/rsif.2013.0711 · 3.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Engineered Biomimicry covers a broad range of research topics in the emerging discipline of biomimicry. Biologically inspired science and technology, using the principles of math and physics, has led to the development of products as ubiquitous as VelcroT (modeled after the spiny hooks on plant seeds and fruits). Readers will learn to take ideas and concepts like this from nature, implement them in research, and understand and explain diverse phenomena and their related functions. From bioinspired computing and medical products to biomimetic applications like artificial muscles, MEMS, textiles and vision sensors, Engineered Biomimicry explores a wide range of technologies informed by living natural systems. Engineered Biomimicry helps physicists, engineers and material scientists seek solutions in nature to the most pressing technical problems of our times, while providing a solid understanding of the important role of biophysics. Some physical applications include adhesion superhydrophobicity and self-cleaning, structural coloration, photonic devices, biomaterials and composite materials, sensor systems, robotics and locomotion, and ultra-lightweight structures. It explores biomimicry, a fast-growing, cross-disciplinary field in which researchers study biological activities in nature to make critical advancements in science and engineering. It introduces bioinspiration, biomimetics, and bioreplication, and provides biological background and practical applications for each. It offers cutting-edge topics that include bio-inspired robotics, microflyers, surface modification and more.
Engineered Biomimicry, Edited by A. Lakhtakia, R. J. Martin-Palma, 07/2013; Elsevier., ISBN: 978-0-12-415995-2
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t The elastic properties of the vocal folds (VFs) vary as a function of depth relative to the epithelial surface. The poroelastic anisotropic properties of porcine VFs, at various depths, were measured using atomic force microscopy (AFM)-based indentation. The minimum tip diameter to effectively capture the local properties was found to be 25 mm, based on nonlinear laser scanning microscopy data and image analysis. The effects of AFM tip dimensions and AFM cantilever stiffness were systematically investigated. The indenta-tion tests were performed along the sagittal and coronal planes for an evaluation of the VF anisotropy. Hertzian contact theory was used along with the governing equations of linear poroelasticity to calculate the diffusivity coefficient of the tissue from AFM indentation creep testing. The permeability coefficient of the porcine VF was found to be 1.80 7 0.32 Â 10 −15 m 4 /N s.
Journal of the Mechanical Behavior of Biomedical Materials 07/2013; XXX(XXX):XXX. DOI:10.1016/j.jmbbm.2013.05.026 · 3.42 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In practice, the surgeon must rely on screw position (insertion depth) and tactile feedback from the screwdriver (insertion torque) to gauge compression. In this study, we identified the relationship between interfragmentary compression and these 2 factors.
The Acutrak Standard, Acutrak Mini, Synthes 3.0, and Herbert-Whipple implants were tested using a polyurethane foam scaphoid model. A specialized testing jig simultaneously measured compression force, insertion torque, and insertion depth at half-screw-turn intervals until failure occurred.
The peak compression occurs at an insertion depth of -3.1 mm, -2.8 mm, 0.9 mm, and 1.5 mm for the Acutrak Mini, Acutrak Standard, Herbert-Whipple, and Synthes screws respectively (insertion depth is positive when the screw is proud above the bone and negative when buried). The compression and insertion torque at a depth of -2 mm were found to be 113 ± 18 N and 0.348 ± 0.052 Nm for the Acutrak Standard, 104 ± 15 N and 0.175 ± 0.008 Nm for the Acutrak Mini, 78 ± 9 N and 0.245 ± 0.006 Nm for the Herbert-Whipple, and 67 ± 2N, 0.233 ± 0.010 Nm for the Synthes headless compression screws.
All 4 screws generated a sizable amount of compression (> 60 N) over a wide range of insertion depths. The compression at the commonly recommended insertion depth of -2 mm was not significantly different between screws; thus, implant selection should not be based on compression profile alone. Conically shaped screws (Acutrak) generated their peak compression when they were fully buried in the foam whereas the shanked screws (Synthes and Herbert-Whipple) reached peak compression before they were fully inserted. Because insertion torque correlated poorly with compression, surgeons should avoid using tactile judgment of torque as a proxy for compression.
Knowledge of the insertion profile may improve our understanding of the implants, provide a better basis for comparing screws, and enable the surgeon to optimize compression.
The Journal of hand surgery 06/2013; 38(9). DOI:10.1016/j.jhsa.2013.04.027 · 1.67 Impact Factor