INVESTIGATION OF BIOMECHANICAL RESPONSE OF HEAD SURROGATES UNDER IMPACT LOADING

Abstract

Impact induced traumatic brain injury (TBI) is a major source of disability and mortality. Understanding how impact loading affects the head-brain parenchyma is essential to develop mitigation strategies, clinical interventions, and protective headgear. The current knowledge of brain strains in humans or human-like surrogates is limited, especially for injury-causing loading magnitudes. Knowledge of brain strains during impact (accelerative) loading is critical for the overall management of TBI, including developing injury thresholds, personal protective equipment, and validation of computational models. Towards this end, we measure the full-field, 2D (in-plane) strains in a brain simulant using a hemispherical head surrogate, an anatomically accurate human head surrogate, and a goat brain. The overarching goal is to holistically understand the biomechanical response of brain simulant or brain to blunt impact loading. The head surrogates were mounted on the Hybrid-III neck and subjected to impact loading using a linear impactor system. The resulting 6-DOF head kinematics were measured using a triaxial accelerometer and angular rate sensors. Full-field, 2D strains in a brain simulant or brain were obtained using high speed imaging and digital image correlation. Brain simulant strains were measured in midsagittal and/or midcoronal planes for impactor velocities of ~1 m/s and ~3 m/s. Using these setups, we investigate the role of stiff head membranes (falx and tentorium) on the strain evolution in the brain simulant or brain. We study the sensitivity of choice of head constituents of anatomically accurate head surrogate models on brain simulant response. We compare the head kinematics and brain strains obtained in anatomically accurate head surrogate models against the data in PMHS and humans. We also evaluate the role of helmets in mitigating the head kinematics and brain simulant strains. We propose novel, anti-rotational pads to further mitigate angular motion of the head. In a separate investigation, we measure intracranial pressures in a simplified spherical surrogate using a pendulum impact setup. Our results suggest that head kinematics, as a result of the impact, is dominated by the rotational modes. Brain simulant undergoes significant deformation and strain pattern is heterogenous. The wave propagation within the brain simulant has a timescale of ~100 ms and peak strains are established within ~20-30 ms. There is a lag of ~10 ms between peak head kinematics and peak strains. The choice of the head surrogate and elastic modulus of head membranes affects the spatiotemporal evolution of strain. Further, cortical strains are decreased in the presence of head membranes, whereas strains in the subcortical regions are either equivalent or increased. The elastic modulus of the membranes governs the amount of strain reduction or increase. Our experimental results reinforce the strain concentration in the vicinity of falx and tentorium. We found that the falx displacement and constraints on stress wave propagation are dominant mechanisms dictating the mechanics of the interaction of membranes with the brain simulant. We observe that the response between the brain simulant of an anatomically accurate head surrogate and goat brain is reasonably similar regarding the spatiotemporal evolution of strains, peak strains in various brain regions, and strain concentration near the tentorium. However, compared to the goat brain, strains in the brain simulant are less dissipative. The peak head kinematics and peak MPS have been reduced by up to 75% and 45%, respectively, with the conventional helmet and by up to 90% and 85%, respectively, with the helmet with anti-rotational pads. Overall, these findings provide important insights into the biomechanics of the brain simulant or brain during impact loading. These results can be utilized in preliminary calibration and validation of computational models of brain biomechanics, and design of next-generation helmets.