## 4 Discussion
found that the tensile properties of porcine aortae incubated in elastase solution progressed through distinct sequential stages of 'initial softening' characterised by a flattening of the initial region of the curve; 'extensible but stiff' behaviour characterised by an increase in the slope of the final region; and 'collagen scaffold'-like behaviour characterised by immediate stiffening upon loading with increasing elastase incubation time similar to that seen in our study [41]. Furthermore, these findings are in line with those of [PERSON] et al. who report that elastase-treated rabbit carotids were more pliable under low circumferential stretch and less pliable under higher circumferential stretch [53]. Decreases in initial stiffness are attributed to reductions in the number and integrity of elastic fibres present to provide resistance to initial deformation with increasing elastase incubation time [16; 41]. However, while elastase treatment induced significant increases in loaded and unloaded circumferential length, measured increases in circumferential length fall short of the 50% dilation exhibited in human aneurysms [1]. [PERSON] et al. reported that application of a creep loading protocol produced a further plastic deformation of 23.6 \(\pm\) 10.5% and 22.5 \(\pm\) 7.1% in partially and completely elastin-depleted strips of porcine aorta, respectively [40]. Perhaps application of a similar creep loading approach would increase dilation to similar levels in the ex vivo porcine aortae to yield the 50% dilation pathognomonic of human aneurysms. Elastin degradation represents a feasible method to produce porcine aortic tissues with similar compliance to human AA. Significant heterogeneity with regards to testing and analysis methodologies used to quantify the tensile properties of human AA in the literature limits direct comparison of measured values [56]. Individual studies indicate that human TAA and AAA demonstrate respective increases in circumferential stiffness of 44% and 60% compared to healthy human aorta under the same degree of stress [18; 57]. Notably, a comparative analysis conducted by [PERSON] et al. found that healthy porcine aorta demonstrated similar compliance to aortae from young humans under 100 mmHg of pressure, but only 41% of the stiffness found in aortae from humans over 60 years of age [31]--in which aneurysms most frequently occur [58]. This indicates that human AAA and TAA may have 350 and 390% greater circumferential stiffness, respectively, compared to healthy porcine aortae. Elastase treatments conducted as part of this study produced dramatic increases in the circumferential stiffness of rings of porcine aorta with rings in the 27-h treatment group demonstrating a 380% increased circumferential stiffness compared to time-matched controls over a Cauchy stress range of 250-300 kPa. A follow-up study including mechanical assessments of aneurysal human and elastin-depleted porcine aortae would allow optimisation of elastase incubation time to produce porcine aortic models with similar stiffness to that seen in human aneurysms. The proposed ex vivo locally elastin-depleted porcine aorta model represents a more comprehensive non-living model than those currently available. Non-living models of aortic aneurysm can be subclassified, based on their material composition, as ex vivo, in vitro synthetic, in vitro tissue-based, and in silico. First, this model recapitulates the dilation and stiffening seen in aortic aneurysms not demonstrated in the ex vivo healthy porcine aorta model [27; 28; 29; 30; 31]. Next, while in vitro synthetic models offer high geometric accuracy and patient-specificity, the synthetic polymers used poorly replicate the compliance and failure mechanics of arterial tissues [20; 32; 33; 34; 35]. Conversely, in vitro tissue-based models offer accurate replication of the macro and microstructural features of aortic aneurysms, however their production is highly resource intensive and has not yet been achieved on a scale suitable for endovascular device testing [36; 37; 38].