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.

大脑的微观结构在其宏观结构材料特性中起着至关重要的作用。由于轴突纤维引入了脑白质中的结构各向异性，因此考虑了连续模型中轴突的方向，以改善计算模拟的结果。本研究的目的是研究纤维方向在脑白质材料特性中的作用，并比较各向异性白质和各向同性灰质的力学行为。采用扩散张量成像 (DTI) 检测白质样品中轴突的方向，并对牛灰质和白质样品施加高达 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间相关特性，使用了粘超弹性模型。结果表明，白质在平行和垂直于轴突纤维的两个正交方向上的力学行为有显着差异。这种差异表明脑白质可以被假定为各向异性材料，轴突对机械性能有贡献。此外，高达 15% 的应变，轴突平行于力方向的白质样本明显比轴突垂直于力方向的灰质样本和白质样本更硬。此外，轴突平行和垂直于加载方向的白质样品和 15-20% 应变下的灰质样品的弹性模量没有显着差异。根据这些观察，这表明，当白质在垂直于轴突方向的方向上加载时，轴突对白质的材料特性的作用可以忽略不计。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。