## 4 Discussion  
Elastase treatment induced increases in the circumferential length and tensile modulus in rings of porcine aortae when compared to untreated controls, similar to that seen in AA. Using insight from these elastase-treated ring samples, we established a local elastin degradation protocol for whole aortae which caused localised dilation under physiological  
Figure 9: Inflation-extension of whole porcine aortae with localised elastin degradation. Black dots are control aortae, whereas hollow circles have elastase treatment. (**a**) Absolute dilation (normalised to diameter at 0 mmHg) at the middle region for each aorta with respect to pressure. (**b**) Relative dilation (normalised to average diameter between adjacent untreated proximal and distal regions) of the middle region with respect to pressure. Significance was determined by a 2-way repeated measures ANOVA with Sidák’s multiple comparisons; (* \(p\) < 0.05). (**c**) No significant difference was seen in physiological (90–120 mmHg) compliance. All data is presented with mean and standard deviation, \(n\) = 3 aortae per group. pressures, similar to that seen in AA. These findings highlight a promising avenue to use localised elastin degradation to produce an ex vivo aneurysm model. Elastase-treated aortic rings demonstrated significant increases in circumferential length both in the unloaded state (Figure 4h) and under 300 kPa Cauchy stress (Figure 7b), results which are consistent with findings from other groups [22; 39; 40; 41; 53]. In healthy tissues, resistance to deformation is provided by the initial recruitment of pliable elastin fibres followed by the progressive recruitment of stiff collagen fibres with increasing strain [16; 54]. This strain-dependent recruitment is permitted by the wavy 'crimped' architecture of groups of collagen fibres allowing them to be 'pulled taut' and thus recruited at specific levels of tissue deformation [22; 55]. Using a multiphoton microscopy approach, [PERSON] et al. found that elastase-induced increases in the loaded length of porcine aortic tissues were attributed to increases in collagen fibre uncrimping with respect to untreated controls under the same conditions of stress similar to that seen in mechanical loading in untreated tissues [22; 55]. As such, elastase-induced increases in loaded length result from the increased deformation required to recruit sufficient collagen fibres to make up for decreases in elastin mediated resistance [22; 53]. Additionally, the increases in unloaded circumference seen in the highly depleted elastin aortic rings indicates that collagen crimp in unloaded arterial tissues is maintained by a compressive force imposed by the elastic fibres in the resting state [16; 40; 53]. As such, elastase-mediated increases in unloaded length secondary to a loss in elastin-mediated compression act similar to an 'initial stretch' on the collagenous architecture in the unloaded state compared to untreated tissues. This stretching nature of elastase-induced increases in unloaded circumferential length is recognised by groups such as [PERSON] et al. in porcine aortae and [PERSON] et al. in rabbit carotids [41; 53]. Overall, increases in aortic circumferential length due to elastase treatment in both loaded and unloaded configurations are attributed to the increased stretch, and thus decreased crimp, exhibited by collagen fibres under the same load in elastin depleted tissues. This work builds upon existing literature to highlight the role of elastic fibres in mediating vessel circumference and compliance in loaded and unloaded states through their role in preserving collagen crimp. Elastase treatment induced increased compliance under low stress, and increased stiffness under high stress in rings of porcine aorta. [PERSON] et al.