Material properties of porcine parietal cortex

Computational models of the head can be used to simulate events associated with traumatic brain injury and to design protective equipment and environments. Accurate material property descriptions of biological tissues are crucial to the development of computational models that mimic human responses. Recent finite element models of adult head injury assign distinct homogeneous properties to white and gray matter regions within the brain, based on limited regional data. However, white matter is usually considered homogeneous, despite recent reports of significant mechanical property differences between corpus callosum and corona radiata. In this study, we extend our investigation of homogeneity to gray matter by measuring stiffness of cerebral cortex and comparing it to thalamus from our previous work. Using a parallel plate shear-testing device, we performed a sequence of stress relaxation tests at 2.5%, 5%, 10%, 20%, 30%, 40%, and then 50% strain. Force and displacement were measured and used to determine the stiffness in two different porcine cortical gray matter regions. While no significant difference was found between the two cortical regions, cortical gray matter was significantly less stiff than previously reported values of porcine thalamic gray matter (p<0.01) and human cortical gray matter (p<0.001). These data indicate that while intraregional gray matter may be considered homogenous, there exists heterogeneity between differing regions of the brain. The assumption of gray matter homogeneity should be carefully considered in future finite element models of the head.