Experimental characterisation of neural tissue at collision speeds

Mechanical characterization of brain tissue at high loading velocities is particularly important for modelling Traumatic Brain Injury (TBI). During severe impact conditions, brain tissue experiences a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when both the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue at these strains and strain rates is of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. In this research, we describe the design and operation of a High Rate Tension Device (HRTD) that has been used for tensile tests on freshly harvested specimens of porcine neural tissue at speeds corresponding to a maximum strain rate of 90/s. We investigate the effects of inhomogeneous deformation of the tissue during tension by quasi-static tests (strain rate 0.01/s) and dynamic tests (strain rate 90/s) using different thickness specimens (4.0, 7.0, 10.0 and 13.0 mm) of the same diameter (15.0 mm). Based on a combined experimental and computational analysis, brain specimens of aspect ratio (diameter/thickness) S = 10/10 or lower (10/12, 10/13) are considered suitable for minimizing the effects of inhomogeneous deformation during tension tests. The Ogden material parameters were derived from the experimental data both at quasi-static conditions (μ = 440 Pa and α = -4.8 at 0.01/s strain rate) and dynamic conditions (μ = 4238 Pa and α = 2.8 at 90/s strain rate) by performing an inverse finite element analysis to model all experimental data. These material parameters will prove useful for the nonlinear hyperelastic analysis of brain tissue.