Session: 05-04-01: Biomaterials and Tissue: Modelling, Synthesis, Fabrication and Characterization - I

Paper Number: 94702

94702 - Modeling the Damage Initiation of White Matter Brain Tissue During Indentation 

Understanding and modeling the mechanical behaviors of white matter brain tissue is important to gain insights into brain injury. With embedded axonal fiber bundles, white matter is typically treated as an anisotropic material whose mechanical behaviors and properties are extensively studied and characterized through experimental approaches in the past decades. However, although the injured brain tissue may experience large deformation in some head impact scenarios, most works characterize the white matter in a small-strain regime to avoid the large deformation-induced damage. Both experimental and modeling studies of white matter at large strain are still few, especially when the damage effect is included. We recently performed the indentation tests on porcine white matter with the indenter having a large displacement and found a progressive failure behavior of white matter when deformed beyond certain strain levels. To model this behavior, we assume that this failure phenomenon is caused by the fracture of axonal fiber bundles and propose a coupled hyperelasticity-damage constitutive model within the framework of thermodynamics. The proposed model assumes that the white matter is a transversely isotropic material with a single preferential fiber direction, which is consistent with our experimental observation that most axonal fiber bundles within white matter samples (extracted from the corpus callosum region) are aligned in one direction. A previously validated hyperelastic free energy function is used to describe the nonlinear behavior of white matter and independently account for the effect of fiber stretch and shear. A scalar internal damage variable is employed as a measure of progressive breaks of axonal fiber bundles in the white matter specimen, and the damage evolution is governed by a sigmoidal type function that has smooth derivatives when the damage starts/ends leading to a potential advantage with respect to numerical calculations. The proposed constitutive model is implemented into finite element codes that is used to reproduce the indentation process. Results show that by using this proposed constitutive model with identified parameters, the finite element simulation can reproduce the progressive failure observed in experiments. With the finite element simulation results consistent with the experimental observation, one is able to study the microscopic mechanism-progressive damage of axonal fiber bundles that governs the macroscopic failure behavior of white matter during indentation. The proposed model also allows for investigating the fiber orientation effect on such failure behavior. It is anticipated that this constitutive model and modeling approach can provide insights for refining and developing future models that will help us to better understand the damage-related mechanical behavior of the human brain tissue.

Presenting Author: Ge He Shanghai University

Presenting Author Biography: Dr. Ge He is an assistant professor of Mechanics at Shanghai University. He received his Ph.D. in Mechanical Engineering at Mississippi State University. His Masters in Engineering Mechanics is from Harbin Institute of Technology. Prior to joining Shanghai University, Dr. Ge He had been working as a postdoctoral researcher at the University of Maryland School of Medicine. His overall objective of research is to develop mechanics models for capturing mechanical behaviors of solid materials.

Authors:

Ge He Shanghai University
Lei Fan Michigan State University