Ex-vivo quantification of ovine pia arachnoid complex biomechanical properties under uniaxial tension

Abstract Background: The pia arachnoid complex (PAC) is a cerebrospinal fluid-filled tissue conglomerate that surrounds the brain and spinal cord. Pia mater adheres directly to the surface of the brain while the arachnoid mater adheres to the deep surface of the dura mater. Collagen fibers, known as subarachnoid trabeculae (SAT) fibers, and microvascular structure lie intermediately to the pia and arachnoid meninges. Due to its structural role, alterations to the biomechanical properties of the PAC may change surface stress loading in traumatic brain injury (TBI) caused by sub-concussive hits. The aim of this study was to quantify the mechanical and morphological properties of ovine PAC. Methods: Ovine brain samples (n=10) were removed from the skull and tissue was harvested within 30 minutes post-mortem. To access the PAC, ovine skulls were split medially from the occipital region down the nasal bone on the superior and inferior aspects of the skull. A template was used to remove arachnoid samples from the left and right sides of the frontal and occipital regions of the brain. 10 ex-vivo samples were tested with uniaxial tension at 2 mm s -1 , average strain rate of 0.59 s -1 , until failure at <5 hours post extraction. The force and displacement data were acquired at 100 Hz using LabVIEW. PAC tissue collagen fiber microstructure was characterized using second-harmonic generation (SHG) imaging on a subset of n=4 stained tissue samples. To differentiate transverse blood vessels from SAT by visualization of cell nuclei and endothelial cells, samples were stained with DAPI and anti-von Willebrand Factor, respectively. The Mooney-Rivlin model for average stress-strain curve fit was used to model PAC material properties. Results: The elastic modulus, ultimate stress, and ultimate strain were found to be 7.7±3.0 MPa, 2.7±0.76 MPa, and 0.60±0.13, respectively. No statistical significance was found across brain dissection locations in terms of biomechanical properties. SHG images were post-processed to obtain average SAT fiber intersection density, concentration, porosity, tortuosity, segment length, orientation, radial counts, and diameter as 0.23%, 26.14%, 73.86%, 1.07±0.28, 17.33±15.25 µm, 84.66±49.18º, 8.15%, 3.46±1.62 µm, respectively. Conclusion: For the sizes, strain, and strain rates tested, our results suggest that ovine PAC mechanical behavior is isotropic, and that the Mooney-Rivlin model is an appropriate curve-fitting constitutive equation for obtaining material parameters of PAC tissues.