Today, nearly 80% of all combat injuries to U.S. service personnel are the result of explosive weapon blasts (such as those created by improvised explosive devices, or IEDs), and one-quarter of those injuries involve head trauma. In particular, IED exposure has led to a large increase in soldiers sustaining traumatic brain injury (TBI), making it the signature wound of modern asymmetric warfare.
APL's Research and Exploratory Development Business Area is working with the Office of Naval Research to develop state-of-the-art human surrogates to understand how the brain is affected by blast trauma, as well as to discover novel ways to protect soldiers from debilitating TBI. "The more you can understand how the body responds to these blast events, the better able you are to protect against them," says Andrew Merkle, manager of the Biomechanics and Injury Mitigation Systems (BIMS) program.
The Human Surrogate Head Model (HSHM), developed by the Laboratory's BIMS team, is providing information about the internal brain injuries—often invisible to medical personnel—that have affected nearly 200,000 soldiers over the last decade. The HSHM consists of a brain, skull, facial structure, and skin, all fabricated using biosimulants. Miniature pressure sensors are embedded in regions of the model brain, and APL-designed high-rate sensors are also attached to track brain displacement. Mounted on the chin and surface of the head, additional sensors measure acceleration and pressure.
During testing, HSHM was subjected to the APL Shock Tube System, which replicates blast loading pressure in a controlled lab setting, as well as live-fire blast tests. The team's newest findings identified two distinct phases of response inside a brain exposed to blasts. The brain pressure sensors revealed an initial pressure response due to shockwave loading, while the brain displacement sensors captured a second-phase brain displacement response milliseconds later. Both responses are believed to correlate to brain injury.
HSHM's detection of the two-phased response reveals the potentially damaging pressure waves in the brain and provides a physical test platform for helmet system evaluation. "In previous surrogates, HSHM's level of anatomical biofidelity and novel instrumentation had not been achieved," says Merkle. "This is one of the first systems where we've been able to see the influence of helmets on things like intracranial pressure."
"Our APL-developed sensor can take direct measure of brain displacement, and we can track what the brain would do during a blast," says Ian Wing, a biomechanical engineer on the BIMS team who has focused on sensor development.
A computational model equivalent to HSHM called the Human Head Finite Element Model, also developed by the BIMS team, complements the experimental system and provides even further insight into brain mechanics. Using sophisticated computer modeling of fluid dynamics and biomechanical data, the finite element model predicts what each region of the brain experiences during the blast.
"In a physical model, we can detect displacement of the head with the attached and embedded sensors," says Jack Roberts, a researcher on the BIMS team who was involved with the initial model development. "With a computational model, we can calculate the response at many more locations within the head and reveal important trends without conducting a blast test. This is critical to understanding injury risk and designing armor systems that mitigate that risk."
The effectiveness of helmets in decreasing TBI due to blasts is still uncertain, but initial tests of both head models indicate that helmets do result in pressure response changes within the brain. Further studies of helmets, as well as deeper investigation of other variables, such as the effect of pads in the helmet or the orientation of the blast, are planned by BIMS.