## 4 Conclusions and Future Perspectives  
The current work aims to establish a framework to evaluate the mechanical behaviour of potential substitutes for the human urethra. Those candidates may be either synthetic or biological, provided that biocompatibility is a priori guaranteed (using biocompatible materials) or achievable with tissue processing techniques, such as decellularisation. Our approach includes both the experimental characterisation of the candidates, as well as FEM simulations to model the experimental tensile behaviour of both longitudinal and circumferential urethral samples using FEM. As a first iteration, aimed at being reproducible and easier to implement, we consider hyperelastic constitutive models readily available in the ABAQUS software package. Due to the proximity between pig and human anatomies, there have been many studies using pig tissues and organs as potential candidates for human implantation [34; 35]. Therefore, pig urethra was chosen as the potential candidate for human urethra replacement and evaluated using the approach described in the paper. From the existing literature on urethral mechanics, there are experimental results from different urethral models that can be compared with our experimental results. The works from [PERSON] et al. [4; 20; 21] on the mechanical properties of the equine urethra are frequently considered a benchmark in the field. These authors have made the most detailed study of a urethral model that can be found in the literature. Our results for the longitudinal porcine urethra were stiffer than the equine data from [PERSON] et al. [4], with the porcine urethra achieving stress values with twice the magnitude. These differences point to microstructural causes (the quantity and spatial arrangement of elastin and collagen) that should be considered in future iterations of this work. Considering the longitudinal urethra, the porcine data ([PERSON] et al. [25] and our data, Figure 8) are in good agreement with the available human data ([PERSON] et a. [26] and [PERSON] et al. [24]). For the circumferential urethra, the pig (our data and [PERSON] et al. [25]) and human data [26] (Figure 9) do not display the same level of agreement as the longitudinal results did. In particular, the results from [PERSON] et al. [25] for the porcine model are much more stiffer than those found in the literature. Some differences between the human and porcine cyclic tests could be justified, besides the inter-species differences, by the age gap of the male urethras--an average of 84 years old [26] for the human sample and 6 months old for the porcine sample. Overall, the pig urethra is a good candidate for an implant targeted at human urethra replacement or as a model to study the human urinary system. This is especially true for the longitudinal urethra and deserves further investigation for the circumferential urethra. The tensile behaviour of porcine urethra was modelled using the hyperelastic constitutive models available in the ABAQUS software package. Both the longitudinal (Figure 10) and circumferential (Figure 11) datasets were considered. The tensile behaviour of the longitudinal urethra was estimated for stretches up to \(\lambda\approx 1.5\) (50% deformations), whereas for the circumferential urethra, the same was true for stretches up to \(\lambda\approx 1.35\) (35% deformations). In both cases, the models with a higher number of parameters (\(k>3\)) performed better than models with a small number of parameters. Overall, the third-order Ogden model (\(n=3\), \(k=6\)) had the best performance, while the Neo-Hookean model (\(n=1\), \(k=1\)) had the worst performance. Several improvements can be considered for future iterations of the current work. At the experimental protocol and setup levels, the introduction of different mechanical tests to complement and extend the mechanical characterisation is worth careful consideration. Pressure tests, besides uniaxial tests, could be crucial for a better understanding of the mechanical behaviour of the porcine urethra.