Anisotropic constitutive equations and experimental tensile behavior of brain tissue

Section of Pediatric Neurosurgery, Institute of Neurosurgery, Catholic University Medical Centre, Largo A. Gemelli 1, 00168 Rome, Italy.
Biomechanics and Modeling in Mechanobiology (Impact Factor: 3.15). 04/2006; 5(1):53-61. DOI: 10.1007/s10237-005-0007-9
Source: PubMed


The present study deals with the experimental analysis and mechanical modeling of tensile behavior of brain soft tissue. A transversely isotropic hyperelastic model recently proposed by Meaney (2003) is adopted and mathematically studied under uniaxial loading conditions. Material parameter estimates are obtained through tensile tests on porcine brain materials accounting for regional and directional differences. Attention is focused on the short-term response. An extrapolation of tensile test data to the compression range is performed theoretically, to study the effect of the heterogeneity in the tensile/compressive response on the material parameters. Experimental and numerical results highlight the sensitivity of the adopted model to the test direction.

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Available from: Fernando Fraternali
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    • "Also, invivo indentation testing has been used to study the effects of blood pressure in vasculature and mechanical constraints forced by the skull on the mechanical properties of brain tissue (Velardi et al., 2006). In this respect, and among the many other outstanding works, experiments done by Miller and Chinzei (1997, 2002), Arbogast and Margulies (1998), Prange and Margulies (2002), Gefen and Margulies (2004), Nicolle et al. (2005), Hrapko et al. (2006), Velardi et al. (2006), Tamura et al. (2007) (2008) and Dommelen et al. (2010) are referenced here. "
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    ABSTRACT: In this study, the optimal viscoelastic material parameters of axon and extracellular matrix (ECM) in porcine brain white matter were identified using a genetic algorithm (GA) optimization procedure. The procedure was combined with micromechanical finite element analysis (FEA) of brain tissue and experimental stress relaxation tests on brainstem specimens to find the optimal material coefficients of axon and ECM. The stress relaxation tests were performed in tension on 10 brainstem specimens at 3% strain level. The axonal volume fraction in brainstem was measured from the Scanning Electron Microscopy images of the brain tissue. A square periodic volume element was selected to represent the microscale homogenized brainstem tissue. Periodic boundary conditions were applied on the square volume element to mimics the repetitive nature of the volume element. Linear viscoelastic material properties were assumed for the brain tissue constituents under small deformation. The constitutive behavior was expressed in terms of Prony series. The GA procedure searched for the optimal material parameters by fitting the time-dependent tissue stresses of brain tissue FEA to the stresses of relaxation tests under the same loading conditions. The optimization procedure converged after 60 iterations. The initial elastic modulus of axon was found to be 12.86kPa, three times larger than that of ECM. The long-term elastic modulus of axon was 3.7kPa, while for ECM this value was 1.03kPa. The concordance correlation coefficient between FEA estimated elastic modulus of brainstem tissue using the optimal material properties and the experimental elastic modulus of brainstem specimens was 0.952, showing a strong agreement. The optimal material properties of brain tissue constituents can find applications in micromechanical analysis of brain tissue to gain insight into diffuse axonal injures (DAIs).
    Full-text · Article · Nov 2013 · Journal of the Mechanical Behavior of Biomedical Materials
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    • "The time elapsed between harvesting of the first and the last specimens from each brain was 17–20 min. Due to the softness and tackiness of brain tissue, each specimen was tested only once and no preconditioning was performed (Miller and Chinzei, 1997, 2002; Tamura et al., 2007; Velardi et al., 2006). Physiological Fig. 1 "
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    ABSTRACT: During severe impact conditions, brain tissue experiences a rapid and complex deformation, which can be seen as a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when both the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue in shear at these strains and strain rates is thus of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. However, very few studies in the literature provide this information. In this research, an experimental setup was developed to perform simple shear tests on porcine brain tissue at strain rates ≤120/s. The maximum measured shear stress at strain rates of 30, 60, 90 and 120/s was 1.15±0.25kPa, 1.34±0.19kPa, 2.19±0.225kPa and 2.52±0.27kPa, (mean±SD), respectively at the maximum amount of shear, K=1. Good agreement of experimental, theoretical (Ogden and Mooney-Rivlin models) and numerical shear stresses was achieved (p=0.7866-0.9935). Specimen thickness effects (2.0-10.0mm thick specimens) were also analyzed numerically and we found that there is no significant difference (p=0.9954) in the shear stress magnitudes, indicating a homogeneous deformation of the specimens during simple shear tests. Stress relaxation tests in simple shear were also conducted at different strain magnitudes (10-60% strain) with the average rise time of 14ms. This allowed us to estimate elastic and viscoelastic parameters (initial shear modulus, μ=4942.0Pa, and Prony parameters: g1=0.520, g2=0.3057, τ1=0.0264s, and τ2=0.011s) that can be used in FE software to analyze the non-linear viscoelastic behavior of brain tissue. This study provides new insight into the behavior in finite shear of brain tissue under dynamic impact conditions, which will assist in developing effective brain injury criteria and adopting efficient countermeasures against traumatic brain injury.
    Full-text · Article · Jul 2013
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    • "These studies cannot be directly compared to each other or to in vivo results from MRE on human subjects due to the differences in mechanical testing regimes and the change in tissue properties after brain death and excision. Both the CC and CR are characterized by mechanically anisotropic microstructure (Prange and Margulies, 2002; Velardi et al., 2006). To invert displacement data in the present work, we use an isotropic material model that returns an effective shear modulus that is a composite of the direction-dependent shear moduli in anisotropic tissues (Qin et al., 2013). "
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    ABSTRACT: The noninvasive measurement of the mechanical properties of brain tissue using magnetic resonance elastography (MRE) has emerged as a promising method for investigating neurological disorders. To date, brain MRE investigations have been limited to reporting global mechanical properties, though quantification of the stiffness of specific structures in the white matter architecture may be valuable in assessing the localized effects of disease. This paper reports the mechanical properties of the corpus callosum and corona radiata measured in healthy volunteers using MRE and atlas-based segmentation. Both structures were found to be significantly stiffer than overall white matter, with the corpus callosum exhibiting greater stiffness and less viscous damping than the corona radiata. Reliability of both local and global measures was assessed through repeated experiments, and the coefficient of variation for each measure was less than 10%. Mechanical properties within the corpus callosum and corona radiata demonstrated correlations with measures from diffusion tensor imaging pertaining to axonal microstructure.
    Full-text · Article · Apr 2013 · NeuroImage
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