Mechanical Characterisation of Grey, White, and Interface Behaviour in the Brain

Traumatic brain injury is a significant healthcare burden, mild traumatic brain injuries such as concussions comprising most of these, despite rampant underdiagnosis. While evidence shows repeat concussions are a risk factor for aggressive neurodegeneration, the mechanism by which concussions damage the brain is not well understood, with the clinical hypothesis of shear delamination at the grey/white matter interface being challenging to validate using current scanning technology.

Thirty-five fresh ovine brains were dissected, generating ~ 950 samples for testing. Samples of grey and white matter were subjected to unconfined compression and tension, simple shear, tensile and shear fracture, and stress relaxation, load rate variation, and volumetric analysis in compression and tension. Samples at the grey/white interface were subjected to tensile and shear fracture.

Grey and white matter differ between cortex/corona radiata and deep brain/corpus callosum, with additional corona radiata variance in shear. The stress relaxation magnitude of grey matter is not strain rate dependent long term; however, white matter relaxation is. Grey and white matter show asymmetric compressibility, with more volume change in tension than in compression. Fracture is not limited to grey/white interface in tension or shear, also occurring in bulk grey and white matter. Fracture initiation stress is similar between grey and white in both modes, and is lower at the interface, with the ductility variance between tissues reflected in fracture energy.

Regional variance in constitutive behaviour was uncovered, in addition to novel viscoelastic, volumetric, and fracture behaviour, which has significant implications for in vitro/in silico brain models.