## 4 Discussion  
Human LUT biomechanics research is in its infancy, different from research on many human tissues that have already extensively investigated. On the basis of the literature, currently, few works have been published on the mechanical investigation of some of these biological tissues, while others have never been studied before or only partially. Moreover, biological tissues are usually characterized by a huge inter-subject variability due to a variety of factors such as subject age, gender, BMI and similar. This usually affects the results when the focus is the quantification of the difference between tissues from the same anatomical district. For this reason, the present study reported the mechanical characterization of the tissues of the LUT harvested from the same subject and tested with  
Figure 7: Histological representation of a transversal section of the human penis (**a**) stained with Azan–Mallory. The section at the lower center is the bulbar urethra surrounded by corpus spongiosum and shown at two different magnifications (**b**,**c**). In the upper part of the macrosection, two corpora cavernosa are surrounded by tunica albuginea (**d**). All these structures are covered by Buck’s fascia, a deep fascia layer made of fibrous connective tissue, indicated by an asterisk (**e**). the same protocols to quantify each tissue's contribution to the overall mechanical response of the district. Figure 2 reports the median failure curves that were obtained for the bladder, the Buck's fascia, the prostate and the tunica albuginea, with mechanical characteristics listed in Table 4. As a first analysis, the bladder and prostate exhibited similar tensile behavior in terms of UTS and \(E\) of the linear region, while strain at break spans for all the tissues from about 21% up to 85%. Tunica albuginea resulted in the stiffest tissue, with a UTS of about four times greater than the others, with \(E\) increasing by one order of magnitude. The bladder showed a restrained variability, suggesting no predominant fibers distribution, while clear differences were observed for the prostate and the tunica, even if, for the latter, the number of samples was limited. Animal bladder tissues were analyzed by [PERSON] et al. [9], who studied the effect of strain rate, orientation and loading history on the behavior in porcine, and then added uniaxial cyclic tests at different strain rates (10, 50, 100, 200, 500% s\({}^{-1}\)) reaching a strain of about 50% [10]. Contrary to what has been observed in this study, porcine bladders showed greater stiffness along the transverse loading direction and were more influenced by the strain rate than those tested along the apex-to-base ones, even if they observed a homogeneous distribution of collagen fibers in the animal tissue. In the present work, the human bladder strain reached in the loading phase was lower (51.42%, [47.23, 54.69]%) than the strain values reported for porcine, whose loading phase covered a final strain of 100% adopting similar strain rates (1.3% s\({}^{-1}\) in [9] while 2.5% s\({}^{-1}\) in this protocol), but with higher recorded stresses (0.47 MPa, [0.44, 0.54] MPa vs about 0.05 MPa [9]). On the basis of the thickness measurements, the pig bladder samples resulted as thicker than the human ones in the present work (3.99 \(\pm\) 1.04 mm [9] and 4.37 \(\pm\) 1.00 mm [10] vs. 2.14 \(\pm\) 0.31 mm). Our results in human samples are also supported by the histological analysis as well as motivated by their physiological function. In particular, histological staining with Haematoxylin-Eosin (Figure 6) and Azan-Mallory (Figure 7) was performed to identify the gross appearance and more specific tissue composition of human specimens. Figure 7 shows the muscle (red) and connective tissue (blue) distribution, distinguishing between compact tissue near the urethral lumen and porous tissue on the outer side, represented by corpus spongiosum.