Date of Award

5-31-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical and Industrial Engineering

First Advisor

Samuel Lieber

Second Advisor

I. Joga Rao

Third Advisor

Shawn Alexander Chester

Fourth Advisor

Eun Jung Lee

Fifth Advisor

Siva P.V. Nadimpalli

Abstract

Animal derived starting materials are well established in the production of Tissue Engineered Medical Devices. Porcine specifically can be found in products ranging in application from hernia repair to breast reconstruction. Although this material has been largely accepted in the Tissue Engineering industry, little is known of its material properties and mechanical characteristics. A review of the scientific literature describes limited mechanical measures only on uncontrolled research grade material. The objective of this work is to mechanically characterize porcine starting material used in the medical device industry. Porcine skin is provided by Midwest Research Swine, LLC (MRS) an established supplier to Medical Device companies. The experiments are established to evaluate if the skin’s mechanical characteristics vary by location and direction. The porcine skin samples are marked for their location (Back and Neck) as well as Orientation (Head and Spine). A custom die is used to prepare uniaxial tensile samples parallel, perpendicular, and at 45 degrees from the Spine landmark. An MTS load frame and Digital Image Correlation (DIC) measurement system is used to acquire the stress-stretch relationship. Mechanical indices from the stress-stretch relationship is analyzed by first separating it into a toe and linear region through a bilinear curve fitting method, apply the Ogden hypereslastic material model to the Toe, and linear model to the linear region. The Ogden fit in the toe region reveals anisotropic behavior that varied by location on the porcine skin, where the Back region behaves anisotropically and the Neck isotopically.

The Gasser-Ogden-Holzepfel (GOH) structural model is explored to unify the tissue’s directional properties. The GOH model requires that the microstructural element’s (e.g. collagen fiber) arrangement within the macroscopic tissue is known beforehand. In the literature this is accomplished through histological measurements on the tissue itself. This requirement limits the ability of the GOH model to be used in the real time analysis of experimental work, which is needed in both academia and the tissue engineering industry. A method is developed to determine the microstructural arrangement (angle and dispersion) by utilizing the mechanical response at two orientations. This developed Microstructural Arrangement Determination Method (MADM) is verified by reproducing a GOH ABAQUS model from the literature. MADM is then applied to porcine skin experimental data, revealing a potential limitation of the GOH model in its treatment of transverse strain.

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