Biomechanical Characterization of the Constitutive Relationship for the Brainstem

Experimental tests using porcine brainstem samples were performed on a custom designed stress relaxation shear device. Tests were performed dynamically at strain rates >1 s⁻¹, to three levels of peak strain (2.5%-7.5%). The directional dependence of the material properties was investigated by shearing both parallel and transverse to the predominant direction of the axonal fibers.

Quasi-linear viscoelastic theory was used to describe the reduced relaxation response and the instantaneous elastic function. The time constants of the reduced relaxation function demonstrate no directional dependence; however, the relative magnitude of the exponential functions and the parameter representing the final limiting value are significantly different for each direction. The elastic function qualitatively demonstrates a dependence on direction.

These results suggest that the brainstem is an anisotropic material. The data provides a starting point for analyzing the directionally dependent nature of brainstem material and can serve as a guide in developing new material models for finite element analysis.