Date of Award
Spring 2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Biomedical Engineering
First Advisor
Brian Stemper
Abstract
Finite element analysis is an effective tool for modeling the mechanical behavior of biologic structures, especially in scenarios where physical experimentation is impractical. The accuracy of these models depends on comprehensive mathematical formulations that describe tissue behavior using parameters derived from laboratory data. Currently there is a shortage of experimental data suitable for detailed parameter estimation. The goal of the current study was to generate a viscoelastic dataset using fresh harvested porcine tissue maintained under in vivo conditions, then provide a detailed constitutive model to capture the complex mechanical response. A sevohydraullic piston with a 5-mm spherical tip indenter was used to perform constrained and unconstrained load and hold tests on fresh porcine abdominothoracic tissues to characterize their stress relaxation behavior. Adipose, cardiac, costal cartilage, and lung tissue samples were loaded and then held at 10%, 20%, 35%, and 45% compressive strain over a 60 second duration. Elastic modulus was calculated over the loading period. Tissue type comparisons were made between normalized stress relaxation values at 11 discrete time points over the hold duration. Average relaxation curves were fit using a generalized quasilinear viscoelastic model (QLV) reported in the literature for biologic tissue. A tailored model was developed to improve the fit over the dynamic loading phase and initial relaxation period. Cartilage demonstrated the stiffest response (2.11 – 59.39 MPa), followed by adipose (0.50 – 2.15 MPa), cardiac (0.08 – 1.09 MPa), then lung tissue (9 – 170 kPa). These values aligned with previously established ranges reported in literature. Significant stress relaxation differences (p < 0.05) were observed between all tissue types over the hold duration. As much as 32% relaxation was reported within the first 100 milliseconds. In three of the tissue groups, more than half of the total stress relaxation measured occurred within the first second. Between 50% and 92% total relaxation was measured across all groups at 60 seconds. 172 fresh abdominothoracic samples were reported over four tissue types at four distinct strain levels making this is one of the largest viscoelastic datasets reported to-date. This was the first study to present compressive stress relaxation viscoelastic properties of ventricular myocardium.