Dr. Haslach Presents mTBI Research Results to Society of Engineering Science

College Park, MD, October 18, 2012

On October 10, 2012, Dr.Henry Haslach presented his research results from the mTBI project to the Society of Engineering Science. The goal of the reported work is to identify the mechanical events that lead to mild traumatic brain injury (mTBI) in response to insults such as a blast. The hypothesis is that pathological extracellular fluid (ECF) flow in the brain is the immediate mechanical cause of the tissue damage inducing mTBI.

As a model for the effect of blast on the human brain, bulk specimens excised to include several of the subregions of a rat brain were subjected immediately after harvest to confined compression and others to simultaneous compression and translational shear. Tests were conducted at different constant deformation rates up to large strains to verify that the mechanical response is load rate dependent. The resulting stress-strain curves are analyzed for indicators of the damage caused by the interaction of the ECF with the neurons and glia, a technique chosen because histology does not appear to locate the minor mechanical damage involved in mTBI.

This technique produces evidence for the hypothesis because the observed reduction of the stress carrying ability of the tissue at higher strains indicates damage. Further, the confined compression tests up to 30% strain suggest that an increase in permeability for ECF flow is a good measure for damage to the tissue. The translational shear tests show that the superposition of compression, as would occur as a blast wave passes through the brain, increases the damage, which is indicated by a change in concavity and an increase in stress required to maintain a constant deformation rate, perhaps because of a loss in the lubricating effect of the ECF removed by compression. Ultimately at about 50% shear strain, a drop in stress carrying ability occurs which correlates with initiation of gross rupture, often between brain subregions, seen on videos of the test. Sinusoidal shear tests to 12.5% shear strain amplitude indicate internal damage by a drop in the stress carrying ability on subsequent cycles, but no external damage is visible in the videos.

This work is the foundation for a proposed physically based nonlinear viscoelastic model for the stress-strain response in which the parameters measure damage and which accounts for the ECF interaction with the solid component. The identification of the damage mechanisms and their quantification in a mathematical model will guide efforts to prevent or mitigate mTBI.

Henry W. Haslach, Jr and Lauren Leahy (2012). "Damage and Solid-Fluid Interactions during Transient Large Deformation of Rat Brain Tissue". Society of Engineering Science, 49th annual technical meeting, October 10-12, 2012, Atlanta GA.