Time-course of the human thoracic aorta ageing process assessed using uniaxial mechanical testing and constitutive modelling.

Age-related remodelling of the arterial wall shifts the load bearing from the compliant elastin network to the stiffer collagen fibres. While this phenomenon has been widely investigated in animal models, human studies are lacking due to shortage of donors' arteries. This work aimed to characterise the effect of ageing on the mechanical properties of the human aortic wall in the circumferential direction. N = 127 thoracic aortic rings (age 18-81 years) were subjected to circumferential tensile testing. The tangential elastic modulus (K) was calculated at pressure-equivalent stresses ranging 60-100 mmHg. Further, the mechanical data were fitted using the Holzpafel-Gasser-Ogden hyperelastic strain energy function (HGO-SEF), modelling the superimposed response of an isotropic matrix (elastin) reinforced by collagen fibres. K increased with age across at all considered pressures (p < 0.001), although more strongly at higher pressures. Indeed, the slope of the linear K-pressure relationship increased by 300% from donors <30 to ≥70 years (4.72± 2.95 to 19.06± 6.82 kPa/mmHg, p < 0.001). The HGO-SEF elastin stiffness-like parameter dropped by 31% between 30 and 40 years (p < 0.05) with non-significant changes thereafter. Conversely, changes in HGO-SEF collagen parameters were observed later at age>60 years, with the exponential constant increasing by ∼20-50 times in the investigated age range (p < 0.001). The results provided evidence that the human thoracic aorta undergoes stiffening during its life-course. Constitutive modelling suggested that these changes in arterial mechanics are related to the different degeneration time-courses of elastin and collagen; likely due to considerable fragmentation of elastin first, with the load bearing shifting from the compliant elastin to the stiffer collagen fibres. This process leads to a gradual impairment of the aortic elastic function with age.