Effects of perfusion on the mechanical behavior of the brain exposed to hypergravity

In certain flight configurations, fighter pilots are exposed to high Gz acceleration which may induce inflight loss of consciousness (G-LOC). In order to study the mechanical effects induced by these accelerations on the cerebral structures, an experimental model has been developed in vitro. Fresh bovine brains were excised and placed in a transparent mold modeling the inside of the skull. Half of these brains were perfused during the experiment. This assembly was placed into the gondola of a centrifuge, in front of a camera lens. Displacements and deformations of the brains were filmed and recorded at different onset rates. Measurements were made after off-line digitalization of images. Experimental data were incorporated into a finite element calculation code whose mesh represented the brain. The applied behavior law was elastic, the structure being considered as homogeneous and isotropic. The first results concerned the elastic properties of the brains under hypergravity. The mean value of the Youngʹs modulus of the nonperfused brain was 46.8 kPa, which corresponded to the values published in reference literature. For the perfused brains, the mean value of the Youngʹs modulus was higher. The mean value of the equivalent Poissonʹs ratio was 0.35. In fact, contrary to impacts, the mechanical stimulation is long enough to allow fluid displacements. The mean value of the equivalent Poissonʹs ratio calculated in the present study should probably be increased since this study was performed post mortem.