Document Type

Dissertation

Date of Award

Summer 8-31-2018

Degree Name

Doctor of Philosophy in Biomedical Engineering - (Ph.D.)

Department

Biomedical Engineering

First Advisor

N. Chandra

Second Advisor

Bryan J. Pfister

Third Advisor

James Haorah

Fourth Advisor

Venkata R. Kakulavarapu

Fifth Advisor

Vijayalakshmi Santhakumar

Sixth Advisor

Pranela Rameshwar

Abstract

Exposure to shock waves is the leading cause of traumatic brain injury (TBI) in military personnel and blast-induced TBI (bTBI) is considered the signature wound in recent conflicts in Iraq and Afghanistan. Many researchers attempt to replicate field-relevant shock waves in laboratory settings through the use of gas-driven shock tubes in order to investigate the generation and propagation of shock waves and also explore possible mechanisms of bTBI. Among several injury mechanisms of bTBI, damage to the blood-brain barrier (BBB) has been identified as a potential candidate and has been the focus of several clinical and experimental investigations aimed to establish injury baselines and discover timelines for therapeutic intervention for neurotrauma. It is hypothesized that BBB permeability in blast increases with increasing overpressure and varies differentially in different brain regions as a function of time post-injury. In order to test this hypothesis, the blast injury model is characterized and effects of an end reflector plate studied, prior to using this injury model in the study of BBB permeability post-blast injury. BBB breakdown is studied across the frontal cortex, striatum, somatosensory barrel field cortex, thalamus, hippocampus, and cerebellum at fifteen minutes, four, and twenty-four hours at blast overpressures of 35, 70, 130, and 180kPa. Finally, effects of oxidative stress on BBB permeability are delineated at four hours for 180kPa blast exposure, as well as the introduction of a potential treatment for nicotinamide adenine dinucleotide phosphate oxidase (NOX)-mediated BBB damage following blast injury.

End effector studies are conducted with the use of four different end plate configurations (0.625, two, four inches, and an open end). Use of end reflector plate allows for precise control over the intensity of reflected waves penetrating into the shock tube and, at a calculated optimized distance, can eliminate secondary waves from the test section, confirmed by pressure sensor recordings. Numerical simulations combined with experimental data offer detailed insight into spatiotemporal dynamics of shock waves and wave attenuation via internal pressure expansion. Diffusion of the driver gas inside of the shock tube is responsible for velocity increase of reflected shock waves.

Blood-brain barrier permeability is primarily assessed by extravasation of sodium fluorescein and Evans blue into brain parenchyma. Maximum extravasation of tracers (and hence BBB permeability) occurs in the frontal cortex at four hours following injury and increases with increasing blast overpressure. Abundance of tight junction proteins occludin and claudin-5 is also measured at different time-points after injury. Significant extravasation is observed immediately after blast across several brain regions, suggesting a shockwave-induced mechanical disruption of the BBB even at mild overpressures. Results also indicate the presence of s100-ß in the blood serum as well as monocyte infiltration to the brain parenchyma, further validating increased BBB permeability. Within 24 hours, extravasation of sodium fluorescein and Evans blue returns close to control values across all brain regions studied.

NOX is shown to be upregulated in vascular endothelial cells, resulting in an increase in superoxide production four hours post-blast. The use of apocynin, a selective NOX inhibitor, is shown to ameliorate production of superoxide at this time and significantly reduce MMP activity, tight junction breakdown, and BBB extravasation.

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