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Structural Anisotropy vs. Mechanical Anisotropy: The Contribution of Axonal Fibers to the Material Properties of Brain White Matter

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Abstract

Brain’s micro-structure plays a critical role in its macro-structure material properties. Since the structural anisotropy in the brain white matter has been introduced due to axonal fibers, considering the direction of axons in the continuum models has been mediated to improve the results of computational simulations. The aim of the current study was to investigate the role of fiber direction in the material properties of brain white matter and compare the mechanical behavior of the anisotropic white matter and the isotropic gray matter. Diffusion tensor imaging (DTI) was employed to detect the direction of axons in white matter samples, and tensile stress-relaxation loads up to 20% strains were applied on bovine gray and white matter samples. In order to calculate the nonlinear and time-dependent properties of white matter and gray matter, a visco-hyperelastic model was used. The results indicated that the mechanical behavior of white matter in two orthogonal directions, parallel and perpendicular to axonal fibers, are significantly different. This difference indicates that brain white matter could be assumed as an anisotropic material and axons have contribution in the mechanical properties. Also, up to 15% strain, white matter samples with axons parallel to the force direction are significantly stiffer than both the gray matter samples and white matter samples with axons perpendicular to the force direction. Moreover, the elastic moduli of white matter samples with axons both parallel and perpendicular to the loading direction and gray matter samples at 15–20% strain are not significantly different. According to these observations, it is suggested that axons have negligible roles in the material properties of white matter when it is loaded in the direction perpendicular to the axon direction. Finally, this observation showed that the anisotropy of brain tissue not only has effects on the elastic behavior, but also has effects on the viscoelastic behavior.

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Acknowledgments

The authors wish to acknowledge the National Brain Mapping Lab (NBML), Tehran, Iran, for providing services for DTI acquisition and thank Fargol R. Araghi and Farzaneh Eskandari for assistance in mechanical testing.

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Authors have not received any funding for this research.

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The authors declare that they have no conflict of interest.

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Authors and Affiliations

  1. Department of Biomedical Engineering, Amirkabir University of Technology, 424 Hafez Ave, Tehran, Iran

    Faezeh Eskandari & Mehdi Shafieian

  2. Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran

    Mohammad M. Aghdam

  3. Department of Biomedical Engineering, University of Arizona, Tucson, AZ, USA

    Kaveh Laksari

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  1. Faezeh Eskandari
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Correspondence to Mehdi Shafieian.

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Eskandari, F., Shafieian, M., Aghdam, M.M. et al. Structural Anisotropy vs. Mechanical Anisotropy: The Contribution of Axonal Fibers to the Material Properties of Brain White Matter. Ann Biomed Eng 49, 991–999 (2021). https://doi.org/10.1007/s10439-020-02643-5

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