<i>In vivo</i> aortic wall thickening is a mechanical adaptation to the prolonged increase in intravascular pressure resulting from hypertension, which is mainly regulated by primary components of the aortic media, the elastic lamina (EL) and the smooth muscle-rich layer (SML). This study built a simplified finite element (FE) model of the aortic medial wall comprising the EL and SML, and simulated EL undulation or buckling at a no-load condition, i.e., (in the <i>in vitro</i>) unloaded state, by releasing a set of compressive prestresses initially given to the EL. Using the design of experiments approach (Graeco-Latin square method), we identified specific mechanical boundary conditions to computationally reconstruct EL buckling in the circumferential direction of the aorta. Additionally, it was shown that EL waviness almost vanished when ~20% strain (mimicking a circumferential stretch due to intravascular pressure) was applied to the buckled FE model obtained in the <i>in vitro</i> unloaded state. This feature is beneficial for numerical modeling of the detailed aortic wall structure, because the entire process is computationally efficient and can be readily implemented in a commercially available FE solver. Although further study is required, our findings will help clarify the roles of the EL and SML in the aortic wall and promote the understanding of the mechanisms of the medial tissue stress response. In addition, we expect this modeling technique to serve as a useful tool in the future for interpreting stress distribution relevant to vascular physiology at normal and pathological states.

Keywords

Elastic lamina (EL), Smooth muscle-rich layer (SML), Buckling, Waviness, Residual stress, Aortic media

Paper information

Atsutaka TAMURA, Yuya KATO, Koki MATSUMOTO, “Modeling of elastic lamina buckling coupled with smooth muscle layer deformation in the aortic media: technique for readily implementing residual stresses”, Journal of Biomechanical Science and Engineering, Vol.15, No.4 (2020), p.20-00324 doi:10.1299/jbse.20-00324. Final Version Released on November 09, 2020, Advance Publication Released on October 26, 2020.

Mechanical properties of brain tissue characterized in high-rate loading regime are indispensable for the analysis of traumatic brain injury (TBI). However, data on such properties are very limited. In this study, we measured transient response of brain tissue subjected to high-rate extension. A series of uniaxial extension tests at strain rates ranging from 0.9 to 25 s-1 and stress relaxation tests following a step-like displacement to different strain levels (15-50%) were conducted in cylindrical specimens obtained from fresh porcine brains. A strong rate sensitivity was found in the brain tissue, i.e., initial elastic modulus was 4.2 ± 1.6, 7.7 ± 4.0, and 18.6 ± 3.6 kPa (mean ± SD) for a strain rate of 0.9, 4.3, and 25 s-1, respectively. In addition, the relaxation function was successfully approximated to be strain-time separable, i.e., material response can be expressed as a product of time-dependent and strain-dependent components as:K(t) = G(t)σe(ε), where G(t) is a reduced relaxation function, G(t) = 0.416e-t/0.0096+0.327e-t/0.0138+0.256e-t/1.508, and σe(ε) is the peak stress following a step input of ε. Results of the present study will improve biofidelity of computational models of a human head and provide useful information for the analysis of TBI under injurious environments with strain rates greater than 10 s-1.

Keywords

Brain Tissue, Visocoelasticity, Stress, Strain, High-Rate Extension, Relaxation, Traumatic Brain Injury (TBI)

Paper information

Atsutaka TAMURA, Sadayuki HAYASHI, Kazuaki NAGAYAMA and Takeo MATSUMOTO, “Mechanical Characterization of Brain Tissue in High-Rate Extension”, Journal of Biomechanical Science and Engineering, Vol. 3, No. 2 (2008), pp.263-274 . doi:10.1299/jbse.3.263

Mechanical properties of brain tissue in high strain region are indispensable for the analysis of brain damage during traffic accidents. However, accurate data on the mechanical behavior of brain tissue under impact loading condition are sparse. In this study, mechanical properties of porcine brain tissues were characterized in their cylindrical samples cored out from their surface. The samples were compressed in their axial direction at strain rates ranging from 1 to 50 s-1. Stress relaxation test was also conducted following rapid compression with a rise time of ?30 ms to different strain levels (20-70%). Brain tissue exhibited stiffer responses under higher impact rates: initial elastic modulus was 5.7±1.6, 11.9±3.3, 23.8±10.5 kPa (mean±SD) for strain rate of 1, 10, 50 s-1, respectively. We found that stress relaxation K(t,ε) could be analysed in time and strain domains separately. The relaxation response could be expressed as the product of two mutually independent functions of time and strain as:<br />K(t,ε)=G(t)σe(ε), where σe(ε) is an elastic response, i.e., the peak stress in response to a step input of strain ε, and G(t) is a reduced relaxation function:<br />G(t)=0.642e-t/0.0207+0.142e-t/0.482+0.216e-t/18.9, i.e., the time-dependent stress response normalized by the peak stress. The reduced relaxation function obtained here will serve as a useful tool to predict mechanical behavior of brain tissue in compression with strain rate greater than 10 s-1.

Keywords

Brain Tissue, Viscoelasticity, Stress, Strain, High-Rate Compression, Relaxation

Paper information

Atsutaka TAMURA, Sadayuki HAYASHI, Isao WATANABE, Kazuaki NAGAYAMA and Takeo MATSUMOTO, “Mechanical Characterization of Brain Tissue in High-Rate Compression”, Journal of Biomechanical Science and Engineering, Vol. 2, No. 3 (2007), pp.115-126 . doi:10.1299/jbse.2.115

Diffuse axonal injury (DAI) is a specific type of closed head injury often seen in automobile accidents, that directly leads to the morbidity and mortality, however, the injury mechanism of DAI has yet to be clarified. DAI is characterized by structural and functional damage in nerve fibers in the white matter, which may be caused by excessive tensile strain. While the white matter has a network-like structure of nerve fibers embedded in neuroglia and the extracellular matrix, the nerve fibers are undulated and the mechanical properties of these components are not necessarily equal. Thus, the strain in the white matter can be different from that in the fibers. In this study, we have measured stretch ratios of the nerve fibers running in various directions in porcine brain tissue subjected to uniaxial stretch and compared them with global strain. It was found that the fiber direction positively correlated with neural fiber strain whilst the fiber strain was not equal to global strain. Particularly, the maximum neural fiber strain was ?25% of its surrounding tissue strain, indicating that the local strain in the neural fibers is not equal to global strain in the brain tissue. Consideration of neural fiber alignment in the white matter is important in studying the mechanical aspects of pathogenesis of DAI.

Keywords

Nerve Fiber, Brain Tissue, Stretch Ratio, Uniaxial Tension, Diffuse Axonal Injury

Paper information

Atsutaka TAMURA, Kazuaki NAGAYAMA and Takeo MATSUMOTO, “Measurement of Nerve Fiber Strain in Brain Tissue Subjected to Uniaxial Stretch (Comparison Between Local Strain of Nerve Fiber and Global Strain of Brain Tissue)”, Journal of Biomechanical Science and Engineering, Vol. 1, No. 2 (2006), pp.304-315 . doi:10.1299/jbse.1.304