Mechanical characterization of mammalian brain tissue and energy dissipative polymers

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Biological Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references.The high incidence of traumatic brain injury due to adverse impact events ranging from head collisions to ballistic attacks has prompted significant interest in synthetic polymer gels capable of mimicking key mechanical properties of brain tissue. These so-called brain tissue simulants are valuable tools for developing protective strategies because they can serve as test media to evaluate new helmets or optimize robotic surgery techniques. However, the so-called "soft matter" employed to date for ballistic applications, such as ballistic gelatin and clay, are crude mechanical representations of brain tissue. Therefore, there remains a need for a class of tissue simulant materials that more accurately replicates the mechanical behavior of brain tissue under impact loading, specifically in terms of deformation resistance and impact energy dissipation. This thesis focuses on design and synthesis of hierarchically structured gels, and mechanical characterization of these compliant gels for comparison with mammalian brain tissue. In particular, we use impact indentation to explore how the impact energy dissipation response varies as a function of species for brain tissue, or as a function of molecular composition and structure for synthetic gels. We find that a bilayered polydimethylsiloxane (PDMS) composite system enables the decoupling of the material's deformation resistance and energy dissipation characteristics, and can be tuned to fully match porcine brain tissue. However, given that the top PDMS layer is highly adhesive, we investigate whether adhesion plays a significant role in modulating the energy dissipation response, which has important implications in the utility of the tissue simulant material for ballistic applications. With a separate bilayered PDMS composite system, we decouple surface adhesion from bulk viscoelasticity, and quantify their individual contributions to impact energy dissipation. Through these experimental studies, in addition to a finite element computational analysis, we establish fundamental design principles and provide new insights regarding mechanisms that govern the extent of deformation and energy dissipation in compliant polymeric materials. Finally, we extend the capabilities of our impact indentation technique by demonstrating a novel analytical approach to extract viscoelastic moduli and relaxation time constants directly from the measured impact deformation response, thus significantly broadening the utility of impact indentation. With conventional characterization techniques such as shear rheology, several challenges arise when the material of interest has stiffness on the order of 1 kPa or lower, as is the case with brain tissue, largely due to difficulties detecting initial contact with the compliant sample surface. In contrast, impact indentation does not require contact detection a priori, and thus can potentially be utilized as a more accurate tool to characterize the viscoelastic properties of a wider range of soft matter for diverse biomedical or engineering applications, not limited to brain tissue simulants. This semi-analytical approach enables future studies to extract viscoelastic properties of brain tissue and tissue simulant polymers with increased accuracy and spatial resolution, in the context of traumatic brain injury, protection, and recovery.by Bo Qing.Ph. D