Influence of saline solution absorption and compressive rate on the material properties of brain tissue

Abstract

Traumatic brain injuries (TBI) affect millions of people each year and can result in long-term difficulties in thinking or focusing. Due to the number of people affected by these injuries, significant research has been dedicated to determining the mechanical properties of the brain using postmortem tissue from animals harvested within 24 hours. The postmortem brain tissue is often stored in a solution until a rheological experiment is ready to begin. However, the effect of storage duration on the mechanical behavior of brain tissue is not understood. In this paper, postmortem porcine brains were placed in normal saline solution (0.9% NaCl) and refrigerated between 30 minutes and 6.5 hours to allow the brain to absorb the solution. Afterwards, samples from both soaked and freshly extracted brains were subjected to unconfined compression tests at compressive rates of 5, 50, and 500 mm/min. The fractional Zener viscoelastic model was applied to obtain the brain's mechanical properties. While the results did not show a significant relationship between absorption and the long-term stiffness (E ∞ ), both the relaxation time (τ 0 ) and fractional order (α) were statistically influenced by both the length of time in the solution and compressive rate. Further, the instantaneous stiffness (E 0 ) was statistically influenced by the length of time in solution, though not the compressive rate.

Viscoelastic properties of shock wave exposed brain tissue subjected to unconfined compression experiments

“…Further, among soaked tissue, the SWE tissue was 23% more viscous than the NSWE tissue tested by McCarty et al (2019). Similarly, the unsoaked, SWE tissue was 38% more viscous than the unsoaked, NSWE tissue from the experiments by McCarty et al (2019). The viscosity calculated for unsoaked, NSWE brain (via the FZ coefficients optimized by McCarty et al (2019) at 20% strain) is higher than the unsoaked, NSWE brain found by Bentil (2013) at 10% strain.…”

“…Similarly, the unsoaked, SWE tissue was 38% more viscous than the unsoaked, NSWE tissue from the experiments by McCarty et al (2019). The viscosity calculated for unsoaked, NSWE brain (via the FZ coefficients optimized by McCarty et al (2019) at 20% strain) is higher than the unsoaked, NSWE brain found by Bentil (2013) at 10% strain. The increased strain level in the experiments by McCarty et al (2019) led to an increased instantaneous modulus value, and thus the increased value for the calculated viscosity.…”

“…The soaked, SWE brains were 47% more viscous than the unsoaked, SWE brains. Further, among soaked tissue, the SWE tissue was 23% more viscous than the NSWE tissue tested by McCarty et al (2019). Similarly, the unsoaked, SWE tissue was 38% more viscous than the unsoaked, NSWE tissue from the experiments by McCarty et al (2019).…”

“…structural, capillary hemorrhages, and vascular leakages from disruption of the blood brain barrier), following blast exposure, can lead to a more compliant brain tissue (Kabu et al, 2015;Laksari et al, 2014;Cernak, 2017). Further, the soaked and unsoaked SWE tissue was 23% and 38% more viscous than the corresponding NSWE tissue found by McCarty et al (2019), respectively. While the unsoaked, SWE tissue underwent unconfined compression experiments immediately after shock wave exposure, the soaked, SWE brain was placed in saline solution for 0.5 -6.0 hours between shock wave exposure and undergoing compression experiments.…”

“…Given the FZ coefficients found above, the viscosity of the brain tissue was calculated using Equation 2 for the samples that underwent unconfined compression testing at 5 mm/min and 20% strain. The calculated viscosity was compared to the viscosity calculated using the FZ coefficients optimized by McCarty et al (2019) and Bentil (2013) for unsoaked, non-shock wave exposed (NSWE) brain tissue in similar stress relaxation experiments at 5 mm/min (Table 3) For comparison of the calculated viscosity of SWE and NSWE brains, it is noted that the viscosity of coal-tar is 20 billion times the viscosity of water (1 mPa · s at 20 • C) (Johnston, 2013). Among the SWE brains, an ANOVA with a post-hoc Student's t-test analysis and significance level of 0.05 showed that the viscosity for soaked and unsoaked brains are statistically different (p=0.0096).…”