The Mechanics of Traumatic Brain Injury : From Blunt to Blast Loading

Traumatic brain injury (TBI) is an endemic problem in both civilian and military populations. In the United States, the incidence of civilian TBI is over a million cases annually, with some years (2014) reaching over 2.5 million TBI-related emergency department visits. In recent decades, a new category of TBI has emerged - primary blast loading or otherwise known as, blast-induced traumatic brain injuries (bTBI). bTBI has become the ``signature wound'' of the Iraq (Operation Iraqi Freedom [OIF]) and Afghanistan (Operation Enduring Freedom [OEF]) conflicts as it has accounted for nearly 70% of all of the injuries to military service members. Until recently, bTBI has been almost entirely isolated to conflict regions. In August 2020, a catastrophic detonation accidentally occurred in Beirut, Lebanon that instantly affected over 300,000 people.The high prevalence of TBIs in both the civilian and military communities has led to a significant societal burden and is one of the leading public health problems we face today. The treatment and prevention of TBIs has become a major focus within the past decade for all of the United States military branches. The increased awareness of TBI/bTBI has brought about great understanding in both the medical and research fields of neurology; however, it has also shed light on just how little we previously understood about this injury. TBI is a multifaceted problem that encompasses multiple length scales, complex anatomical geometries, and nonlinear biological responses and has various time-scales depending on the severity of the injury. The objective of this research is to investigate the potential injury-causing mechanisms present during the biomechanical loading of TBI events. More specifically, it focuses on the parallels between TBI and bTBI. The bTBI community is actively debating several potential damage-causing primary mechanisms: direct cranial transmission, skull flexure, skull orifices, and thoracic surge. To this end, a brain-tissue phantom was created to investigate its response to different biomechanical loading scenarios. The phantom was fabricated into a three-dimensional extruded ellipsoid geometry made out of Polyacrylamide gelatin that incorporated gyri-sulci interaction. Additionally, the dominant length scales of the sulci, gyri, gray matter thickness, and overall brain dimensions were incorporated into the gelatin brain phantom. The phantom was assembled into a polylactic acid 3D-printed skull, surrounded with deionized water, and enclosed between two optical windows to create a human head surrogate. In this work, the response of the human-head surrogate was evaluated under two biomechanical loading conditions: blunt-force impacts and blast loading. A custom-built drop-tower apparatus was used for the blunt-force impacts and a state-of-the-art blast chamber was employed to characterize the human-head surrogate under blast conditions. These two experimental apparatuses aided in the investigation into the potential damage-causing mechanisms associated with TBI/bTBI. A noninvasive high-speed imaging system was used to capture the surrogate-head response from each biomechanical loading condition. Finally, digital image processing techniques were used to evaluate the kinematic motion of the entire surrogate head, but more importantly the direct response of the gelatin-brain phantom itself.