Defining brain mechanical properties: effects of region, direction, and species.

No regional or directional large-deformation constitutive data for brain exist in the current literature. To address this deficiency, the large strain (up to 50%) directional properties of gray and white matter were determined in the thalamus, corona radiata, and corpus callosum. The constitutive relationships of all regions and directions are well fit by an Ogden hyperelastic relationship, modified to include dissipation. The material parameter alpha, representing the non-linearity of the tissue, was not significantly sensitive to region, direction, or species. The average value of the material parameter mu, corresponding to the shear modulus of the tissue, was significantly different for each region, demonstrating that brain tissue is inhomogeneous. In each region, mu, obtained in 2 orthogonal directions, was compared. Consistent with local neuroarchitecture, gray matter showed the least amount of anisotropy and corpus callosum exhibited the greatest degree of anisotropy. Finally, human temporal lobe gray matter properties were determined and compared to porcine thalamic properties. The results show significant regional inhomogeneity at large strains and significant anisotropy in each region tested. The extent of regional anisotropy correlated with the degree of alignment in the local neuroarchitecture. These large strain, regional and directional data should enhance the biofidelity of computational models and provide important information regarding the mechanisms of traumatic brain injury.