Development of Biofidelic Human Head and Neck Surrogates for the Experimental Evaluation of Advanced Injury Criteria

Improving people's safety is a challenge which requires skills and expertise coming from different fields from medicine to engineering. About this, the role of an engineer is to provide devices to protect people and tools to objectively evaluate their performance according to clinical evidences. Head injuries alone lead to a big percentage of hospitalization and fatalities among people subjected to traumatic events in traffic, sports, and work activities. Therefore the research field linked to the understanding of injury mechanisms and their mitigation is of great importance. There are several types of head injury each with its own cause and outcome, and specific protection developed aiming to mitigate it. Helmets have represented a form of protection since many years. At first, protection was meant to avoid skull fractures and specific injury criteria were developed and applied to the testing of helmet as a functional requirement, thankfully improving their performances. Then, protection was improved also for severe closed head injuries by using energy absorbing materials which mitigated accelerations to the head coming from the impact. Nowadays, modern helmets could significantly reduce the incidence of many severe or fatal head injuries. The research of injury mechanisms improved also thanks to FE model of the human head, which helped to identify other mechanisms of brain injury. Moreover, they aided the design of more advanced protective devices which aim to reduce injury risk to the brain during impacts with a strong rotational component. However, while FE models are capable of providing really detailed modeling of the human head structures and give the full field of kinematics, strain and stresses for each element of the head, experimental tests are still limited to rigid headforms which have poor biofidelity and simple sensor output (usually a triaxial accelerometer and gyrometer applied to the skull). To address this, some headforms with higher bio fidelity and more advanced sensors were developed. Starting from this fact, the purpose of this thesis is to improve bio fidelic head model to investigate closed head injuries. This surrogate head aims to replicate of some anatomical structures such as skull, scalp, brain and meninges with materials having similar mechanical properties of the human tissues. Furthermore, the surrogate will be equipped with several sensors to provide kinematics, pressure waves, and internal stress of brain and skull. Together with the head, also a neck surrogate with biofidelic response and integrated sensors will be studied to work as a stand alone tool or to be paired with the head. The first part of the thesis aimed to develop a sensor to measure the internal stress state inside the brain surrogate, providing both normal and shear stress. Together with the sensor development, some materials were investigated as possible surrogates for the brain tissue. Then, the work moved to the improvement of the plastic skull, which was completely redesigned to provide a robust and reversible connection between its two parts needed to insert the inner structures. The work on the skull was followed by a modeling of the meninges, in particular focusing on the subarachnoid space and the two membranes separating brain lobes and brain from cerebellum: falx cerebri and tentorium cerebelli. After its full realization, the head was used together with a custom drop tower built according to standards for motorcycle helmets (UNECE 22.06) to perform conventional drop tests. In the second part of the thesis a new neck surrogate was developed from a first prototype available at the university. The prototype was completely renewed by redesigning the plastic discs replicating the vertebrae, allowing compatibility with Hybrid III dummy head and trunk. NOT COMPLETE (too long for the form, please read in the thesis).