Effects of surface profile on porcine dural mechanical properties.

BACKGROUND: Cerebrospinal fluid leakage through the spinal meninges is difficult to diagnose and treat. Moreover, its underlying mechanism remains unknown. Considering that the dura mater is structurally the strongest and outermost membrane among the three-layered meninges, we hypothesized that a dural mechanical tear would trigger spontaneous cerebrospinal fluid leakage, especially when a traumatic loading event is involved. Thus, accurate biomechanical properties of the dura mater are indispensable for improving computational models, which aid in predicting blunt impact injuries and creating artificial substitutes for transplantation and surgical training. METHOD: We characterized the surface profile of the spinal dura and its mechanical properties (Young's moduli) with a distinction of its inherent anatomical sites (i.e., the cervical and lumbar regions as well as the dorsal and ventral sides of the spinal cord). FINDINGS: Although the obtained Young's moduli exhibited no considerable difference between the aforementioned anatomical sites, our results suggested that the wrinkles structurally formed along the longitudinal direction would relieve stress concentration on the dural surface under in vivo and supraphysiological conditions, enabling mechanical protection of the dural tissue from a blunt impact force that was externally applied to the spine. INTERPRETATION: This study provides fundamental data that can be used for accurately predicting cerebrospinal fluid leakage due to blunt impact trauma.

Stiffness estimation of transversely anisotropic materials using a novel indentation tester with a rectangular hole.

Since embryos change their morphology drastically in the gastrulation stage, mechanical characterization of young embryos is important as they also change their tissue stiffness with the stage of development. Herein, virtual compression tests were conducted assuming that the Xenopus laevis gastrula has a spherical shape with transverse anisotropy. Based on the design of experiments, we found that the Young's moduli and material anisotropy can be efficiently determined by measuring the reaction force and surface displacement when indenting the tester into an embryo. The proposed scheme may be a substantial step toward understanding the timing of cell-type specification during embryo development.

Theoretical elastic tensile behavior of muscle fiber bundles in traumatic loading events.

BACKGROUND: The mechanical characterization of skeletal muscle under high-rate loading regimes is important for predicting traumatic injuries due to traffic accidents and contact sports. However, it is difficult to perform dynamic mechanical tests at rates relevant to such rapid loading events. METHODS: In the present study, a series of stress relaxation tests were conducted on rabbit hind-limb muscle fiber bundles using a custom tensile tester. Using relatively moderate loading conditions compared to those typically associated with traumatic injuries, the passive stress-decaying mechanical properties of muscle fiber bundles were characterized. In addition, stress relaxation responses to various ramp-hold stretches were theoretically predicted by a custom-built code. FINDINGS: The results showed that the muscle fiber bundles exhibit greater stress relaxation at higher loading rates and greater stretch magnitudes. Based on these results, the data points representing the "elastic" stress-strain tensile behavior typical of traumatic injury were extrapolated using curve fitting. The theoretical model revealed rate-dependent characteristics of the muscle fiber bundles under traumatic loading conditions, which would result in tensile strengths of 300-500 kPa at the maximum engineering strain of 54%. This strength is on the order of magnitude as the maximum isometric stress of an active muscle contraction. INTERPRETATION: The proposed numerical model is expected to serve as a powerful research tool to investigate injury mechanisms of the skeletal muscle. Moreover, the elastic response that was theoretically predicted here will be useful in the development of effective countermeasures to prevent traumatic injuries due to rapid loading events.

Variation in nerve fiber strain in brain tissue subjected to uniaxial stretch.

Diffuse axonal injury (DAI) is the most frequent type of closed head in jury involved in vehicular accidents, and is characterized by structural and functional damage of nerve fibers in the white matter that may be caused by their overstretch. Because nerve fibers in the white matter have a undulated network-like structure embedded in the neuroglia and extracellular matrix, and are expected to be much stiffer than other components, the strain in the nerve fiber is not necessarily equal to that in the white matter. In this study, the authors have measured strain of the nerve fibers running in various directions in porcine brain tissue subjected to uniaxial stretch and compared them with global strain (tissue strain). The nerve fiber strain had a close correlation with their direction, and was smaller than surrounding global strain. Tensile strain appeared in the fibers ranging from 0 to 60 degrees relative to stretch direction while strain was compressive in those ranging from 60 to 90 degrees. The tensile and compressive strain at maximum stretch was 0.07 and -0.03, respectively, for nerve fibers, while it was 0.33 and -0.12, respectively, for the whole tissue. Roughly speaking, the maximum neural fiber strain was approximately 1/3 of its surrounding tissue strain in respective fiber direction, 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 in DAI.

Mechanical characterization of porcine abdominal organs.

Typical automotive related abdominal injuries occur due to contact with the rim of the steering wheel, seatbelt and armrest, however, the rate is less than in other body regions. When solid abdominal organs, such as the liver, kidneys and spleen are involved, the injury severity tends to be higher. Although sled and pendulum impact tests have been conducted using cadavers and animals, the mechanical properties and the tissue level injury tolerance of abdominal solid organs are not well characterized. These data are needed in the development of computer models, the improvement of current anthropometric test devices and the enhancement of our understanding of abdominal injury mechanisms. In this study, a series of experimental tests on solid abdominal organs was conducted using porcine liver, kidney and spleen specimens. Additionally, the injury tolerance of the solid organs was deduced from the experimental data.