Experimental Observation of High Strain Rate Responses of Porcine Brain, Liver, and Tendon

Clemmer, J., Prabhu, R., Chen, J., Colebeck, E., Priddy, L. B., McCollum, M., Brazile, B. L., Whittington, W. R., Wardlaw, J. L., Rhee, H., Horstemeyer, M., Williams, L. N., & Liao, J. (2015). Experimental Observation of High Strain Rate Responses of Porcine Brain, Liver, and Tendon. Journal of Mechanics in Medicine and Biology. 1650032, 1-14. DOI:10.1142/S0219519416500329.

High strain rate compression of soft tissues has recently gained attention due to its application in computational simulation of traumatic injuries. To understand high rate tissue behavior, a comparative study is needed to examine the biomechanical responses of multiple soft tissues. We hypothesized that the underlying mechanisms of soft tissue high rate compression is dependent upon water, microstructural organization, and extracellular matrix (ECM). Porcine brain, liver, and tendon, which have similar material density (brain: 1.05 g/cm3, liver: 1.06 g/cm3, and tendon: 1.12 g/cm3) and water content (brain: ~78%, liver: ~71–75%, and tendon: ~70%) but different cellularity and ECM properties, were subjected to polymeric split Hopkinson pressure bar (PSHPB) testing. Hydrated brain tissue, due to its high microstructural cavities (cavity area fraction = 0.49 ± 0.26) and low ECM content, had a high initial stress spike of 0.22 ± 0.12 MPa. Hydrated liver, with moderate microstructural cavities (cavity area fraction = 0.30 ± 0.07) and moderate ECM content, had a moderate stress spike of 0.10 ± 0.04 MPa. Hydrated tendon had low microstructural cavities (cavity area fraction = 0.03 ± 0.02) and high ECM content and had a minimal initial stress spike of 0.02 ± 0.01 MPa. Electron microscopy of the tissues\' microstructural cavities revealed a first-order estimation of water content that was not bound to ECM (e.g., intracellular water). Linear regression analysis showed that the initial spike was highly correlated (r2 = 0.9532) with intracellular water. To understand the role of water in each tissue\'s response to high deformation, each soft tissue was completely dehydrated and subjected to the same PSHPB compression test. After removal of all the water, neither the brain, liver, nor tendon revealed an initial stress spike, implicating the essential nature of water in the initial responses of high-rate compression. These results suggest that cellular water and ECM content play a critical role in the biomechanical responses to high strain rate compression.