Age Dependency of the Biaxial Biomechanical Behavior of Human Abdominal Aorta

Background: The biomechanical behavior of the human abdominal aorta has been studied with great interest primarily due to its propensity to develop such maladies as atherosclerotic occlusive disease, dissections, and aneurysms. The purpose of this study was to investigate the age-related biaxial biomechanical behavior of human infrarenal aortic tissue. Methods of Approach: A total of 18 samples (13 autopsy, 5 organ donor) were harvested from patients in each of three age groups: Group 1 $(<30years$ old, $n=5),$ Group 2 (between 30 and 60 years old, $n=7),$ and Group 3 $(>60years$ old, $n=6).$ Each specimen was tested biaxially using a tension-controlled protocol which spanned a large portion of the strain plane. Response functions fit to experimental data were used as a tool to guide the appropriate choice of the strain energy function $W.$ Results: Under an equibiaxial tension of 120 N/m, the average peak stretch values in the circumferential direction for Groups 1, 2, and 3 were $mean±SD$$1.46±0.07,$$1.15±0.07,$ and $1.11±0.06,$ respectively, while the peak stretch values in the longitudinal direction were $1.41±0.03,$$1.19±0.11,$ and $1.10±0.04,$ respectively. There were no significant differences between the average longitudinal and circumferential peak stretch within each group $p>0.1,$ but both of these values were significantly less $p<0.001$ for Groups 2 and 3 when compared to Group 1. Patients in Group 1 were modeled using a polynomial strain energy function $W,$ while patients in Groups 2 and 3 were modeled using an exponential form of $W,$ suggesting an age-dependent shift in the mechanical response of this tissue. Conclusion: The biaxial tensile testing results reported here are, to our knowledge, the first given for the human infrarenal aorta and reinforce the importance of determining the functional form of $W$ from experimental data. Such information may be useful for the clinician or researcher in identifying key changes in the biomechanical response of abdominal aorta in the presence of an aneurysm.