Damage to the rat cerebrum under in vitro sinusoidal translational shear deformation

Blast waves, which induce sinusoidal shear waves within brain tissue, may cause mild traumatic brain injury (mTBI). To identify damage from a shear deformation wave, sagittal slices of rat cerebra are subjected to 50 cycles of translational shear deformation at six fixed frequencies between 25 Hz and 125 Hz and displacement amplitudes of 10% or 25% of the original length of the specimen. Each deformation frequency produces transient and apparent steady shear stress states that frequency analysis describes by their harmonic wavelet and Fourier frequency components. The dominant frequency components are integer multiples of the applied deformation frequency. The morphology of the shear stress versus time curve, and probably the type of damage, changes with deformation frequency. Damage at the lower frequencies appears to be diffuse bond breaking. Imaging and histology do not clearly detect mild damage due to bond breaking that underlies mTBI, which the analysis of the shear stress response captures. Major transitions in the morphology of the stress response in the two regions occur at about 75 Hz deformation frequency, possibly due to minor damage to cerebral substructures. An increase in deformation frequency increases the drag force between the extracellular fluid and solid matter. The deformation frequency dependence of the shear stress response makes protection against blast mTBI more difficult because the frequency content of a blast wave is not known a priori.