Traumatic brain injury (TBI) has become a major public health and socioeconomic problem that affects 1.5 million Americans annually. Finite element methods have been widely used to investigate TBI mechanisms. The pia-arachnoid complex (PAC) covering the brain plays an important role in the mechanical response of the brain during impact or inertial loading. Existing finite element brain models have tended to oversimplify the response of the PAC due to a lack of accurately defined material properties of this structure, possibly resulting in a loss of accuracy in the model predictions. The objectives of this study were to experimentally determine the material properties of the PAC along its anatomical axes at different strain rates and to determine the material constants of constitutive equations derived for PAC from experimental data.

Bovine PAC was selected for experimental samples in this study. Three loading modes (in-plane tension, normal traction, and tangential shear) were conducted to investigate the biomechanical response of PAC at different strain rates ranging from 0.05 to 100 s^-1. The strain-rate effects, as well as directional and regional dependencies, of PAC were analyzed. Based on observation of PAC material response, a set of constitutive equations was proposed to model the transversely isotropic, nonlinear viscoelastic characteristics of PAC. A curve fitting-based optimization algorithm was carried out to determine the material constants needed for the constitutive equations.

Results from this study provide essential information to properly model the PAC membrane, an important component in the skull/brain interface, in a computational model of the human head. Such an improved representation of the in vivo skull/brain interface will enhance future studies investigating brain injury mechanisms under various loading conditions.