Dynamic compressive response of soft biological tissues

Farhana Pervin, Purdue University

Abstract

Blast injury has been a threat on the battle field, a threat to civilians from acts of terrorism, and a threat to soldiers in training. When the body is exposed to blast explosion, the body surface and internal organs such as brain, liver, kidney and lung, are rapidly distorted and ruptured due to the envelopment of body in the over pressurized wave Blast can cause a wide range of injuries. The mechanism of injuries can be correlated with the mechanical response of the body tissues and to the characteristic of blast pressure itself. Valid quantitative measurements of the tissues responses are of interest in biomedical study of blast injury. Soft biological tissues such as brain, liver and kidney exhibit complex mechanical behavior. These materials are characterized by low strength and stiffness, low wave speed, low mechanical impedance, large strain, rate sensitivity and volumetric incompressibility. The need to accurately measure the high rate deformation of such materials has been tremendous challenges for scientist and engineers. This thesis presents an experimental technique capable of measuring the family of stress-strain curves as a function of strain rates which represents the incidence of blast injury. In this technique, split Hopkinson pressure bar has been employed where an annulus biological tissue specimen were loaded in the axial direction to measure the compressive stress-strain behavior of tissues. Modifications made to the conventional SHPB for soft material testing in recent decades have been adopted to characterize the brain, liver and kidney tissues at high strain rates. This method is capable to predict the dynamic behavior of other soft materials such as gel, rubber and polymeric foam. Quasistatic and intermediate compression experiments have been performed on brain, liver and kidney tissues. The quasi-static and intermediate axial load was applied by a hydraulic driven Material Test System (MTS). Under quasi-static, intermediate and dynamic loading conditions, the experimental results for brain, liver and kidney tissues showed that the stress-strain responses of these tissues are rate sensitive. The biomechanical properties of living tissues depend not only on the inherent variability in the tissue properties due to gender, species and breeding but also region and orientation of the tested tissue specimen. A parametric study has been conducted on brain tissue to observe the effect of these factors on tissue properties. This study demonstrated that biomechanical properties of brain tissues are not significantly depends on brain region and orientation. The effects of species, gender and breeding has no contribution to the mechanical properties of brain tissues. Tissue mimic gels are commonly used in biomedical applications such as tissue regeneration, tissue scaffolding and tissue repair due to their biocompatibility. Different concentration of gel materials have been fabricated and tested under identical dynamic loading condition to find the candidate gel materials that can response to dynamic loading in a similar as the brain tissues. It was found that Agarose gels with concentration of 0.6%- 0.4% range are closed to that of brain tissues in terms of mechanical and rheological properties.

Degree

Ph.D.

Advisors

Chen, Purdue University.

Subject Area

Mechanics|Biomedical engineering|Biomechanics

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