Mehdi Shafieian

Constitutive model for brain tissue under finite compression

While advances in computational models of mechanical phenomena have made it possible to simulate dynamically complex problems in biomechanics, accurate material models for soft tissues, particularly brain tissue, have proven to be very challenging. Most studies in the literature on material properties of brain tissue are performed in shear loading and very few tackle the behavior of brain in compression. In this study, a viscoelastic constitutive model of bovine brain tissue under finite step-and-hold uniaxial compression with 10 s(-1) ramp rate and 20 s hold time has been developed. The assumption of quasi-linear viscoelasticity (QLV) was validated for strain levels of up to 35%. A generalized Rivlin model was used for the isochoric part of the deformation and it was shown that at least three terms (C(10), C(01) and C(11)) are needed to accurately capture the material behavior. Furthermore, for the volumetric deformation, a two parameter Ogden model was used and the extent of material incompressibility was studied. The hyperelastic material parameters were determined through extracting and fitting to two isochronous curves (0.06 s and 14 s) approximating the instantaneous and steady-state elastic responses. Viscoelastic relaxation was characterized at five decay rates (100, 10, 1, 0.1, 0 s(-1)) and the results in compression and their extrapolation to tension were compared against previous models.

Changes to the viscoelastic properties of brain tissue after traumatic axonal injury

While it has been shown that repetitive mild brain injuries can cause cumulative damage to the brain, changes to the mechanical properties of brain tissue at large deformations were also noted in the literature. The goal of this study was to show that the viscoelastic properties of brain tissue significantly change after traumatic axonal injury (TAI). An impact acceleration model was used to create TAI in the rat brainstem which was quantified with an immunohistochemistry technique at the ponto-medullary junction (PmJ) and pyramidal decussation (PDx). The viscoelastic properties at these two points with and without preconditioning were characterized using an indentation technique combined with finite element analysis and a comparison was made between injured and uninjured specimens, which revealed statistically significant reduction in the instantaneous elastic force at PDx where the brain tissue sustained a significantly higher level of injury. The result of this study can be used to characterize a damage function for the brain tissue undergoing large deformation.

Mechanical properties of brain tissue in strain rates of blast injury

Over the past decade there has been a significant increase in the number of Traumatic Brain Injury (TBI) cases among armed forces due to blast. Finite Element (FE) models of brain injury are essential to study brain injury mechanisms. The available FE models are based on the material properties that have been determined from experiments at strain rates below 100 s−1. In this study a novel experimental model was developed to apply shear strains to brain samples with strain rates of 100, 500 and 800 s−1 and the material properties were determined through FE optimization. The results showed that brain shear moduli at these strain rates are independent of strain rate and in agreement with previous results. The tissue failure stress at 100 s−1 was significantly lower than the higher rates.

Development of an in vitro porcine aorta model to study the stability of stent grafts in motor vehicle accidents

Endovascular stent grafts for the treatment of thoracic aortic aneurysms have become increasingly utilized and yet their locational stability in moderate chest trauma is unknown. A high speed impact system was developed to study the stability of aortic endovascular stent grafts in vitro. A straight segment of porcine descending aorta with stent graft was constrained in a custom-made transparent urethane casing. The specimen was tested in a novel impact system at an anterior inclination of 45 deg and an average deceleration of 55 G, which represented a frontal automobile crash. Due to the shock of the impact, which was shown to be below the threshold of aortic injury, the stent graft moved 0.6 mm longitudinally. This result was repeatable. The presented experimental model may be helpful in developing future grafts to withstand moderate shocks experienced in motor vehicle accidents or other dynamic loadings of the chest.

Viscoelastic Properties of Brain Tissue Under High-Rate Large Deformation

The nonlinearity of brain tissue material behavior for large deformations at high strain rates was investigated. The viscoelastic properties of brain tissue under high rate ramp- and hold shear strains were determined and nonlinearity in the elastic and time dependent properties of the tissue were examined based on modeling the experimental data. The results revealed that the elastic response of brain tissue is linear from 10% to 50% shear strain, but the time dependent part of the properties in short times shows nonlinear behavior.© 2009 ASME

Shock Wave Propagation as a Mechanism of Injury in Nonlinear Viscoelastic Soft Tissues

This study investigates the propagation of shock waves and self-preserving waves in soft tissues such as brain as a mechanism of injury in high rate loading conditions as seen in blast-induced neurotrauma (BINT). The derived mathematical models indicate that whereas linear viscoelastic models predict only decaying waves, instances of such phenomena as shock can be achieved in nonlinear media. In this study, a nonlinear viscoelastic material model for brain tissue was developed in compression. Furthermore, nonlinear viscoelastic wave propagation in brain tissue was studied and a criterion for the development of shock waves was formulated. It was shown that discontinuities in the acceleration that happen in blast loading conditions may evolve to shock waves, resulting in large discontinuities in strain and stress at the wave front leading to tissue injuries.Copyright

Characterization of the material properties of rat brain stem from indentation tests

The main objective of the present study is to characterize the viscoelastic properties of rat brain stem using an indentation technique. The viscoelastic formulation for a flat indenter was verified with finite element analysis and its limitations were determined. The results showed that up to 30% effective shear strain, the material behavior was linear viscoelastic.

Multilayer properties of aorta

Current finite element models of the aorta assume a homogenous material, but in fact aorta is composed of three major layers. Understanding of the material properties of these layers is essential in order to study the local mechanism of dynamic rupture. The material properties of aorta wall layers were determined from microindentation tests. The results showed significant linear increase of the shear modulus in the inner half of media. A dominant viscoelastic behavior was observed which was almost uniform throughout the layers.

Finite Element Analysis and Validation of Brain Deformation in Linear Head Impact

The aim of this study is to present two dimensional models of human head undergoing linear acceleration and impact using finite element analysis and validating the results with dynamic impact experiments. The experimental model consisted of a cylindrical gel as brain surrogate material undergoing 55G deceleration with slip boundary condition. Two FE models were developed and compared namely, Lagrangian and Arbitrary Lagrangian Eulerian (ALE). Parameters such as the logarithmic strain and void generated in the posterior region of the head were used to validate the results.Copyright

Experimental and Computational Analysis of Brain Deformations in Linear Head Impact

In this study two-dimensional physical and finite element models of human head under linear deceleration were developed. 5% gelatin was used as the brain substitute material with similar viscoelastic properties. The experimental strains and pressure during 55G impacts were measured to validate the element formulations used in the computational models. The Lagrangian and Arbitrary Lagrangian Eulerian (ALE) formulations were used in the FE models. It was shown that without Cerebrospinal Fluid (CSF), the Lagrangian strains passed the 10% threshold of axonal injury. At the presence of CSF, no significant strain was observed while 6 to 8 times increase in the intracranial pressure was recorded. The FE models showed similar trends for strain, stress, and pressure but were generally more aggressive than the experimental results. The ALE model was more stable and its effective damping was more consistent with the experimental data.Copyright