Department of Biomedical Engineering, Amirkabir University of Technology, 424 Hafez Ave, Tehran, Iran.
Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran.
Ann Biomed Eng. 2021 Mar;49(3):991-999. doi: 10.1007/s10439-020-02643-5. Epub 2020 Oct 6.
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.
大脑的微观结构对其宏观结构的物质性质起着至关重要的作用。由于轴突纤维的存在，大脑白质存在结构各向异性，因此在连续体模型中考虑轴突的方向已被证明可以改善计算模拟的结果。本研究的目的是探讨纤维方向在大脑白质物质性质中的作用，并比较各向异性白质和各向同性灰质的力学行为。扩散张量成像（DTI）用于检测白质样本中轴突的方向，对牛的灰质和白质样本施加高达 20%应变的拉伸应力松弛载荷。为了计算白质和灰质的非线性和时变特性，使用了粘弹性超弹性模型。结果表明，与轴突纤维平行和垂直的两个正交方向上的白质力学行为有显著差异。这种差异表明，大脑白质可以被假设为各向异性材料，并且轴突对机械性能有贡献。此外，在 15%的应变范围内，与纤维平行的白质样本比灰质样本和纤维垂直的白质样本在力的方向上更硬。此外，在 15-20%应变范围内，纤维平行和垂直于加载方向的白质样本和灰质样本的弹性模量没有显著差异。根据这些观察结果，建议当白质在垂直于轴突方向加载时，轴突在白质的物质性质中几乎没有作用。最后，这一观察结果表明，脑组织的各向异性不仅对弹性行为有影响，而且对粘弹性行为也有影响。
