This research provided insight into the biomechanics of traumatic brain stem injury by characterizing the response of the brainstem to mechanical deformation. The brainstem is a region of the CNS characterized by axonal fibers longitudinally organized in parallel tracts. This distinct structure helps dictate the material response of the brainstem and suggests that this region may respond to mechanical insult as a transversely isotropic structure. The objective of this work was to investigate this assumption through material testing in three mutually perpendicular directions.

A custom designed stress relaxation, oscillatory shear testing apparatus (STA) for testing soft biological tissues in simple shear was constructed and validated. The STA was validated by measurements of complex shear moduli of three mixtures of silicone gel with viscoelastic properties similar to soft biological tissue that were tested in both the STA and a commercially available solids rheometer.

The STA was used to perform stress relaxation tests on adult porcine brainstem to characterize the mechanical response of the brainstem. These experiments did not confirm our hypothesis of the brainstem's transversely isotropic material response. Specifically, the calculated relaxation moduli did not vary with testing orientation.

We hypothesized that due to the presence of significant relaxation in these tissues, the mechanical response of the brainstem is highly loading rate dependent. A series of dynamic oscillating shear tests in three mutually perpendicular directions were performed to further characterize the anisotropic nature of the brainstem at high loading rates. These experiments confirmed our hypothesis that the brainstem may be described as a transversely isotropic material;​ the complex moduli in the transverse direction were significantly higher than the other two moduli that were indistinguishable from one another.

A novel fiber reinforced composite model composed of viscoelastic fibers surrounded by a viscoelastic matrix was developed to analyze the oscillatory shear data. The model predicted that the fibers are stiffer and more viscous than the surrounding matrix, a relationship reinforced by the results of the oscillatory shear tests. The predicted fiber modulus was confirmed by results of oscillatory shear tests on the optic nerve of the guinea pig.