Modulus Mismatch Research Articles

Design and Microfabrication Considerations for Reliable Flexible Intracortical Implants

Current microelectrodes designed to record chronic neural activity suffer from recording instabilities due to the modulus mismatch between the electrode materials and the brain. We sought to address this by microfabricating a novel flexible neural probe. Our probe was fabricated from parylene-C with a WTi metal, using contact photolithography and reactive ion etching, with three design features to address this modulus mismatch: a sinusoidal shaft, a rounded tip and a polyimide anchoring ball. The anchor restricts movement of the electrode recording sites and the shaft accommodates the brain motion. We successfully patterned thick metal and parylene-C layers, with a reliable device release process leading to high functional yield. This novel reliably microfabricated probe can record stable neural activity for up to two years without delamination, surpassing the current state-of-the-art intracortical probes. This challenges recent concerns that have been raised over the long-term reliability of chronic implants when Parylene-C is used as an insulator, for both research and human applications The microfabrication and design considerations provided in this manuscript may aid future development of flexible devices for biomedical applications.

Preparation and properties of novel epoxy composites containing electrospun PA6/F‐MWNTs fibers

The Nylon 6 (PA6)/the functionalized multiwalled carbon nanotubes (F‐MWNTs) fibrous membranes were fabricated by electrospinning, and then incorporated into an epoxy matrix. Their morphology, thermal stability, mechanical properties, thermal conductivities, and electrical resistivity were investigated. The electrospun PA6/F‐MWNTs fibers performed as a skeleton in the epoxy matrix, and the well interfacial adhesion between the epoxy matrix and the PA6/F‐MWNTs fibers leads to high mechanical properties of composites. The PA6 serves as an intermediate layer and alleviates the modulus mismatch between the stiff MWNTs and the soft epoxy matrix. The thermal conductivities of the epoxy composites increase by 27.3, 35.0, and 36.1%, respectively, with 0.5, 1 and 1.5 wt% F‐MWNTs loading in the PA6/F‐MWNTs fibers. At the same time, the PA6 simultaneously retains the high electrical resistivity of these epoxy composites. POLYM. ENG. SCI., 56:1259–1266, 2016. © 2016 Society of Plastics Engineers

Comparison of size dependent strengthening mechanisms in Ag/Fe and Ag/Ni multilayers

Nanostructured metallic multilayers have attracted substantial attention as they often possess high mechanical strength. Here we report on the microstructure and mechanical strength of sputtered Ag/Fe multilayers with individual layer thicknesses (h) varying from 1 to 200 nm. Phase transformation of Fe from body-centered-cubic (bcc) to face-centered-cubic (fcc) structure occurs when h < 5 nm. Nanotwins form in fine Ag/Fe multilayers. Although modulus mismatch is similar between Ag/Fe and Ag/Ni multilayers, the peak hardnesses of Ag/Fe multilayers is much lower than that of Ag/Ni system. Comparison of mechanical strength of several Ag based multilayers reveals that this drastic difference may arise from chemical stress due to the difference in stacking fault energy of the layer constituents.

Micro‐CT based modelling for characterising injection‐moulded porous titanium implants

Design of prosthetic implants to ensure rapid and stable osseointegration remains a significant challenge, and continuous efforts have been directed to new implant materials, structures and morphology. This paper aims to develop and characterise a porous titanium dental implant fabricated by metallic powder injection-moulding. The surface morphology of the specimens was first examined with a scanning electron microscope (SEM), followed by microscopic computerised tomography (μ-CT) scanning to capture its 3D microscopic features non-destructively. The nature of porosity and pore sizes were determined statistically. A homogenisation technique based on the Hills-energy theorem was adopted to evaluate its directional elastic moduli, and the conservation of mass theorem was employed to quantify the oxygen diffusivity for bio-transportation feature. This porous medium was found to have pore sizes varying from 50 to 400 µm and the average porosity of 46.90 ± 1.83%. The anisotropic principal elastic moduli were found fairly close to the upper range of cortical bone, and the directional diffusivities could potentially enable radial osseous tissue ingrowth and vascularisation. This porous titanium successfully reduces the elastic modulus mismatch between implant and bone for dental and orthopaedic applications, and provides improved capacity for transporting oxygen, nutrient and waste for pre-vascular network formation. Copyright © 2016 John Wiley & Sons, Ltd.

A Study of the Contact of an Elastic Layer–Substrate System Indented by a Long Rigid Cylinder IncorporatingSurface Effects

Contact problem of a layer–substrate system comprising of an elastic layer and an elastic substrate perfectly bonded to each other with surface effects based on Gurtin–Murdoch (GM) model indented by a long rigid cylinder is solved. The requisite Green's function relating surface displacement to surface load is obtained semi-analytically through the combination of the Airy stress function and Fourier transforms under the plane-strain condition. The contact solution is analyzed to study the influence of layer thickness, modulus mismatch between the layer and substrate, and surface parameters on contact size and contact pressure during indentation of a layer–substrate system. A map is presented which indicates whether during indentation by a rigid cylinder, a layer–substrate system is required or a homogeneous system based on layer properties is enough for a given shear modulus mismatch ratio and layer thickness. The map and the related analysis clearly indicate that whenever the contact size or layer thickness approaches intrinsic length scale based on the ratio of surface parameter and bulk elastic properties, surface effects should be considered.

Structure, rheological, thermal conductive and electrical insulating properties of high-performance hybrid epoxy/nanosilica/AgNWs nanocomposites

A facile and effective approach by incorporating silica nanoparticles (SNPs) to fabricate high performance epoxy-based electronic packaging materials which are both thermally conductive and electrically insulating was presented. Because of the strong interaction between SNPs and silver nanowires (AgNWs), uniformly dispersed SNPs-modified epoxy was employed to promote the dispersion of AgNWs in epoxy matrix. Further, the enhanced modulus of epoxy matrix by the incorporation of SNPs effectively alleviates the modulus mismatch between stiff AgNWs and epoxy matrix. Compared with epoxy/AgNWs composites without SNPs, the resulting hybrid materials, that is, epoxy/SNP/AgNWs, showed distinct improvements in thermal conductivity without degrading their mechanical properties. Also, the SNPs were absorbed onto the surface of AgNWs forming an electrical insulation layer to disrupt the electron flows between adjacent AgNWs, hence retaining the electrical insulation of epoxy matrix. Finally, this new fabrication method is easily scalable owing to its simple procedure and use of commercial well-dispersed SNPs-modified epoxies.

Clinical Perspective-The Case for Adoption of LLIF.

Open surgical procedures have been the mainstay of spinal surgery for decades, but minimally invasive spinal surgery (MIS) has recently gained traction. Translaterally placed cages permit insertion of large cages and promote skeletal realignment and fusion. Lateral surgical procedures with percutaneous skeletal fixation and good fusion allow patients to leave the hospital earlier with fewer complications as compared with open procedures. The challenging learning curve is often a barrier to adoption for surgeons, many of whom believe that their open methods work well. MIS and open surgical procedures are reported to have similar outcomes at 1 year; in the first 6 weeks, patients undergoing open surgery often need blood transfusion, develop infection, and use more narcotics. Spine surgery has been associated with modulus mismatch between osteoporotic bone and titanium and the need for multiple painful and traumatic surgical procedures, and spine surgeons continue the quest to find better ways to do things.

Computational micromechanical analysis of the representative volume element of bituminous composite materials

Micromechanical computational modeling is used in this study to determine the smallest domain, or Representative Volume Element (RVE), that can be used to characterize the effective properties of composite materials such as Asphalt Concrete (AC). Computational Finite Element (FE) micromechanical modeling was coupled with digital image analysis of surface scans of AC specimens. Three mixtures with varying Nominal Maximum Aggregate Size (NMAS) of 4.75 mm, 12.5 mm, and 25 mm, were prepared for digital image analysis and computational micromechanical modeling. The effects of window size and phase modulus mismatch on the apparent viscoelastic response of the composite were numerically examined. A good agreement was observed in the RVE size predictions based on micromechanical computational modeling and image analysis. Micromechanical results indicated that a degradation in the matrix stiffness increases the corresponding RVE size. Statistical homogeneity was observed for window sizes equal to two to three times the NMAS. A model was presented for relating the degree of statistical homogeneity associated with each window size for materials with varying inclusion dimensions.

Effect of reinforcement type and porosity on strength of metal matrix composite

In the present work, experimental investigation and the numerical analysis are carried out for strength analysis of A356 alloy matrix composites reinforced with alumina, fly ash and hybrid particle composites. The combined strengthening effect of load bearing, Hall–Petch, Orowan, coefficient of thermal expansion mismatch and elastic modulus mismatch is studied for predicting accurate uniaxial stress–strain behavior of A356 based alloy matrix composite. The unit cell micromechanical approach and nine noded isoparametric finite element analysis (FEA) is used to investigate the yield failure load by considering material defect of porosity as fabrication errors in particulate composite. The Ramberg–Osgood approach is considered for the linear and nonlinear relationship between stress and strain of A356 based metal matrix composites containing different amounts of fly ash and alumina reinforcing particles. A numerical analysis of material porosity on the stress strain behavior of the composite is performed. The literature and experimental results exhibit the validity of this model and confirm the importance of the fly ash as the cheapest and low density reinforcement obtained as a waste by product in thermal power plants.

Developing micro-scale crystal plasticity model based on phase field theory for modeling dislocations in heteroepitaxial structures

The capability of conventional crystal plasticity theory is extended in this paper to model a single dislocation behavior in heteroepitaxial structures. The plastic slip associated with each slip system is described by a continuous smooth field independently, which is the phase field interpolating between a slipped and an unslipped region. A dislocation is identified with location where the phase field value changes smoothly, to represent a smeared dislocation. Under a thermodynamically consistent framework that distinguishes between stored energy and dissipated energy during plastic deformation, the coupled balance equations of plastic slip evolution and quasi-static stress equilibrium are derived by using the principle of virtual power. With numerical implementation by finite element method, it is flexible to deal with material anisotropy and elastic modulus mismatch in heteroepitaxial structures at micro-scale, which are advantages of the proposed model compared to Khachaturyan-type phase field model of dislocations. Another advantage is that it is straightforward to handle the interaction and co-evolution of several types of material microstructures, such as dislocations and interfaces, within a unified continuum mechanics framework. Several examples are presented to illustrate the applicability and accuracy of the new method in modeling dislocations in complex heteroepitaxial structures.

Core–shell structured al-matrix composite with enhanced mechanical properties

Al-matrix composites with different contents of core–shell structured B4Cp–Cu reinforcement were prepared by electrolessly coating Cu on B4Cp and hot sintering the mixture of B4Cp–Cu and Al powder. The influences of solution treatment on the microstructure and mechanical properties were investigated. The results showed that the initial Cu shell was changed into Al2Cu shell, and part of Cu was dissolved into the Al matrix after solution treatment at 540°C. With increasing solution dwelled time, the yield strength, ultimate compressive strength and ductility were improved gradually. As compared with the pure B4Cp reinforcement, the introduction of core–shell structured B4Cp-Cu and subsequent solution treatment improved the mechanical properties of Al-matrix composite. The improvement of mechanical properties is attributed to the fact that the Al2Cu shell moderates the modulus mismatch and enhances the interface bonding between Al and B4Cp, and that heat treatment introduces solution strengthening of Cu element into the Al matrix.

An evaluation of the effects of selective laser melting process parameters on the biocompatibility of Ti-based implants

Event Abstract Back to Event An evaluation of the effects of selective laser melting process parameters on the biocompatibility of Ti-based implants Trina Majumdar1, Nick Birbilis1 and Xiaobo Chen1 1 Monash University, Materials Science and Engineering, Australia Introduction: Titanium (Ti) and its alloys are biomedical materials used extensively in orthopaedic applications such as hip and knee replacements. Ti displays low cytotoxicity and high mechanical strength[1,][2]. However, these properties may prevent full functionality, particularly for patients with complex orthopaedic issues. The surfaces of Ti-based implants do not typically induce strong mechanical interlocking[3], and the modulus mismatch between the implant and host bone can lead to stress shielding and bone resorption. These issues often necessitate painful and costly revision surgeries. New generation fabrication techniques with modified alloys are thus being investigated to improve the osseoconductive properties of orthopaedic implants. Selective Laser Melting (SLM) has the capability to rapidly produce customised implants out of conventionally difficult to machine materials[4]. Current research is focused on fully classifying the effects of various processing parameters on SLM-produced part properties as well as the effects of varying alloy compositions on bio-performance. Various post-process techniques to improve biocompatibility are also under investigation. Hydroxyapatite is a bone-like mineral which has been shown to improve host-implant interactions when coated onto implants, however, it is difficult to work with due to insolubility and brittleness. Materials and Methods: This study presents the results of iterative analyses of the effects of SLM fabrication parameters on part properties, as well as the specific behaviour of Ti-6Al-4V and Ti-Nb-Zr based alloys under these conditions. Electro-impedance spectroscopy and cyclic voltammetry were performed to determine the effects of build parameters on subsequent electrochemical behaviour. Cytocompatibility testing was conducted with osteoblasts to ascertain the extent of osseoinductive properties of SLM produced implants. The microstructural and surface characteristics were observed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. This study also presents the optimisation of a two-step protocol for the deposition of modified hydroxyapatite on SLM part surfaces. Results and Discussion: This study presents a set of preliminary data on the effects of varying SLM build parameters on Ti-6Al-4V and Ti-Nb-Zr based alloys, as well as the osseoinductive potential of SLM produced implants. This data is integral in providing confidence in the design and printing of custom implants. Conclusion: The combination of highly osseoinductive alloy compositions, an awareness of the effects of varying fabrication and process parameter, and the optimisation of post-processing value-adding techniques will improve the performance and service life of hip implants.

Elastic field due to dislocation loops in isotropic multilayer system

A semi-analytical solution based on fast discrete Fourier transform is developed to analyze the elastic fields induced by dislocation loops within perfect bonding multilayers. Final elastic field is the linear superposition of bulk elastic field and correction elastic field, and the correction elastic field is induced by elastic modulus mismatch and lattice plane misorientation across interface planes. Final displacement and traction stress are continuous across the interface planes of multilayer system, and the case of perfect bonding two-layer system is elaborated in detail. Validity of the semi-analytical approach is tested by analyzing elastic fields due to dislocation loop within perfect bonding two-layer system. Interface elastic field profiles of perfect bonding two-layer Cu–Nb system are studied, demonstrating that final elastic field is the linear superposition of bulk stress and correction stress fields. Simulation results demonstrate that loop depth within thin film and modulus mismatch have a remarkable effect on the elastic field distribution.

Integration of High-Charge-Injection-Capacity Electrodes onto Polymer Softening Neural Interfaces.

Softening neural interfaces are implanted stiff to enable precise insertion, and they soften in physiological conditions to minimize modulus mismatch with tissue. In this work, a high-charge-injection-capacity iridium electrode fabrication process is detailed. For the first time, this process enables integration of iridium electrodes onto softening substrates using photolithography to define all features in the device. Importantly, no electroplated layers are utilized, leading to a highly scalable method for consistent device fabrication. The iridium electrode is metallically bonded to the gold conductor layer, which is covalently bonded to the softening substrate via sulfur-based click chemistry. The resulting shape-memory polymer neural interfaces can deliver more than 2 billion symmetric biphasic pulses (100 μs/phase), with a charge of 200 μC/cm(2) and geometric surface area (GSA) of 300 μm(2). A transfer-by-polymerization method is used in combination with standard semiconductor processing techniques to fabricate functional neural probes onto a thiol-ene-based, thin film substrate. Electrical stability is tested under simulated physiological conditions in an accelerated electrical aging paradigm with periodic measurement of electrochemical impedance spectra (EIS) and charge storage capacity (CSC) at various intervals. Electrochemical characterization and both optical and scanning electron microscopy suggest significant breakdown of the 600 nm-thick parylene-C insulation, although no delamination of the conductors or of the final electrode interface was observed. Minor cracking at the edges of the thin film iridium electrodes was occasionally observed. The resulting devices will provide electrical recording and stimulation of the nervous system to better understand neural wiring and timing, to target treatments for debilitating diseases, and to give neuroscientists spatially selective and specific tools to interact with the body. This approach has uses for cochlear implants, nerve cuff electrodes, penetrating cortical probes, spinal stimulators, blanket electrodes for the gut, stomach, and visceral organs and a host of other custom nerve-interfacing devices.

Design and fabrication of a metastable β-type titanium alloy with ultralow elastic modulus and high strength.

Titanium and its alloys have become the most attractive implant materials due to their high corrosion resistance, excellent biocompatibility and relatively low elastic modulus. However, the current Ti materials used for implant applications exhibit much higher Young’s modulus (50 ~ 120 GPa) than human bone (~30 GPa). This large mismatch in the elastic modulus between implant and human bone can lead to so-called “stress shielding effect” and eventual implant failure. Therefore, the development of β-type Ti alloys with modulus comparable to that of human bone has become an ever more pressing subject in the area of advanced biomedical materials. In this study, an attempt was made to produce a bone-compatible metastable β-type Ti alloy. By alloying and thermo-mechanical treatment, a metastable β-type Ti-33Nb-4Sn (wt. %) alloy with ultralow Young’s modulus (36 GPa, versus ~30 GPa for human bone) and high ultimate strength (853 MPa) was fabricated. We believe that this method can be applied to developing advanced metastable β-type titanium alloys for implant applications. Also, this approach can shed light on design and development of novel β-type titanium alloys with large elastic limit due to their high strength and low elastic modulus.

Semi-analytical solution to plane strain loading of elastic layered coating on an elastic substrate

The plane strain loading of a linear elastic layered coating halfspace is solved semi-analytically through a combination of Airy stress function and Fourier transforms and highly simplified and compact expressions for displacement and stresses in layer and substrate are presented in terms of pressure distribution in the loaded region. The results are applied to study the influence of layer thickness and mismatch in elastic modulus between layer and substrate on the stresses and displacement during loading of layered coating system. Lastly the effect of degree of smoothness of the pressure profile on the plane strain loading response of layered coating system is simulated through three different pressure profiles for a fixed total load and loading zone length.

Elastic field of approaching dislocation loop in isotropic bimaterial

A semi-analytical solution is developed for calculating interface traction stress (ITS) fields due to elastic modulus mismatch across the interface plane of isotropic perfectly bounded bimaterial system. Based on the semi-analytical approaches developed, ITS is used to correct the bulk elastic field of dislocation loop within infinite homogenous medium, and to produce continuous displacement and stress fields across the perfectly-bounded interface. Firstly, calculation examples of dislocation loops in Al–Cu bimaterial system are performed to demonstrate the efficiency of the developed semi-analytical approach; Then, the elastic fields of dislocation loops in twinning Cu and Cu–Nb bimaterial are analyzed; Finally, the effect of modulus mismatch across interface plane on the elastic field of bimaterial system is investigated, it is found that modulus mismatch has a drastic impact on the elastic fields of dislocation loops within bimaterial system.

A phase field dislocation dynamics model for a bicrystal interface system: An investigation into dislocation slip transmission across cube-on-cube interfaces

In this work, we present a phase field dislocation dynamics formulation designed to treat a system comprised of two materials differing in moduli and lattice parameters that meet at a common interface. We apply the model to calculate the critical stress τcrit required to transmit a perfect dislocation across the bimaterial interface with a cube-on-cube orientation relationship. The calculation of τcrit accounts for the effects of: 1) the lattice mismatch (misfit or coherency stresses), 2) the elastic moduli mismatch (Koehler forces or image stresses), and 3) the formation of the residual dislocation in the interface. Our results show that the value of τcrit associated with the transmission of a dislocation from material 1 to material 2 is not the same as that from material 2 to material 1. Dislocation transmission from the material with the lower shear modulus and larger lattice parameter tends to be easier than the reverse and this apparent asymmetry in τcrit generally increases with increases in either lattice or moduli mismatch or both. In efforts to clarify the roles of lattice and moduli mismatch, we construct an analytical model for τcrit based on the formation energy of the residual dislocation. We show that path dependence in this energetic barrier can explain the asymmetry seen in the calculated τcrit values. Significantly, the analysis reveals that τcrit scales with a(2)G(2)a(1)+a(2)(a(1)a(2)−G(1)G(2))2, where G is the shear modulus, a is the lattice parameter, and the superscripts (1) and (2) indicate quantities for material 1 and material 2, respectively.

Trunnion Troubles in Modular Hip Implants.

As outlined in our January 16, 2013 editorial, “The Case Report Redefined with JBJS Case Connector , ” one important role of JBJS Case Connector is to alert the orthopaedic community about a potentially problematic device or therapy. When two or more such cases with similar mechanisms appear, we will identify the procedure or implant as a “watchable” intervention. While this system is not statistically conclusive and may or may not be supported by other published case reports or registry data, the intention is to sharpen the focus of clinicians on the potential for similar problems and thereby enhance clinical outcomes and patient safety. The “Watch” designation may also encourage others to report related difficulties and enlist the orthopaedic community to either demonstrate that these are isolated, unrelated cases or sharpen the focus further on rigorously evaluating the intervention. Where appropriate, we may identify brand, model, or implant-lot specifics. Of the five Watch articles that have been published in JBJS Case Connector, three—including this one—have focused on total hip arthroplasty (THA). Our very first Watch in September 2013 discouraged surgeons who implant dual-mobility hips from using a “mix-and-match” strategy with components from different manufacturers. Our January 2014 Watch about THA cautioned hip surgeons to be wary about using long-necked and skirted-head modular hip designs. In both of those Watch articles, we emphasized how head-neck modularity has helped orthopaedists achieve important biomechanical objectives—such as adjusting version, limb length, and offset—to maximize implant function and longevity. Modularity also facilitates easier revisions and lowers inventory costs. But we also made the point that modularity increases the number of interfaces at which corrosion can occur, making prosthetic materials more vulnerable to compression, shear forces, and bending stresses. The arthroplasty community currently feels that the advantages gained from modularity outweigh the risks, but we …

Tension behavior of interfaces between ZrCu metallic glass and Si or Zr

In this study, an amorphous ZrCu layer was coated on different materials such as Si and Zr by sputter deposition and then fabricated into free-standing pull-off samples by focused ion beam (FIB). These samples were tested in tension in order to investigate the tension behavior of ZrCu/Si (ZCS) and ZrCu/Zr (ZCZ) interfaces. By examining the fracture modes and load–displacement curves, we estimated the interface strength of ZCS to be ∼0.6±0.1GPa, with fracture occurring exactly along the ZCS interface. In contrast, the ZCZ samples failed within the ZrCu layer with shear band penetrating though interface. The ZCZ interface strength appeared to be higher than that of ZrCu (which is ∼1.7±0.1GPa). This stress of tension results matched well with the micro-pillar compression results; the ZCS interface strength was consistently much lower than the ZCZ interface strength. It could be explained by modulus mismatch and different bonding states between ZrCu/Si and ZrCu/Zr interfaces.