## 4 Discussion
Concerning urethral tissue, the second magnification in Figure 7b shows the different layers of the urethra transversal section, indicating the main layers and fiber directions. The two different muscle fiber directions confirm the mechanical behavior of the tissue, supporting also the data of [PERSON] et al., 2021 [5] and [PERSON] et al., 2018 [4]. To investigate the prostatic fibrosis as responsible for LUT dysfunctions, [PERSON] et al. [15] collected an entire prostate from a cadaver (66 years) and periurethal tissue from 28 men undergoing radical prostatectomy. Similar to the here reported protocol, the prostate was opened along the urethra and slices were taken parallel and perpendicular to the urethra (thickness 3.3 \(\pm\) 0.4 mm), while prostatectomy tissues were trimmed in strips, whose thickness (2.0 \(\pm\) 0.6 mm) resulted very similar to the measures of the present study (1.96 \(\pm\) 0.62 mm). When considering the healthy tissue, they found an average tangent modulus of 450 \(\pm\) 77 kPa for the samples parallel to the urethra while 560 \(\pm\) 180 kPa for the perpendicular samples, reporting a minor difference between the two directions if compared to the results of the present study, even if adopting similar strain rate (1% s\({}^{-1}\) up to a maximum strain of 30%), as well as lower absolute values. In the meantime, the prostatectomy tissues reported a wider range of tangent modulus between 9 and 2390 kPa with greater stiffness directly correlated with higher collagen content due to fibrosis and with moderate/severe LUT symptoms. These results could support the evidence of a reparative adaptation of the prostate tissue in response to the endoscopic intervention documented in the donor's health history (TURP), which was characterized by a stiffer response along both directions (Table 4), but maintaining the anisotropic behavior due to a non-homogeneous fibers distribution (Figure 5). The human tunica albuginea was only partially studied in healthy conditions with unique methods [16], but because of the particular testing technique, it was difficult to compare the results to those obtained in the present study. However, the thickness values were very similar (2.42 \(\pm\) 0.53 mm [16] vs. 2.38 \(\pm\) 0.52 mm). Regarding the mechanical basis of pathological conditions, [PERSON] et al. [17] proposed uniaxial tensile tests up to failure (strain rate 1 mm s\({}^{-1}\)) of the tunica albuginea affected by PD. Specimens exhibited high variability in mechanical properties (_E_ of 5-118 MPa, UTS 1.1-6.1 MPa and strain at UTS 9-42%). These findings showed an altered tissue behavior with mineralization, supported by the literature studies on atherosclerotic plaque mechanics which found higher modulus [19; 20; 21] in calcified plaques compared to healthy tissue. Healthy tunica albuginea samples tested in the present study showed mechanical properties close to the lower limit of the wide range shown in [17]. Tensile strength and Young's modulus of tunica albuginea were also evaluated in a primate model by [PERSON] et al. [18], reporting, respectively, an average value of 1.9 \(\pm\) 0.4 MPa and 11.9 \(\pm\) 4.2 MPa along the circumferential direction, very close to the values obtained longitudinally in the present study (UTS 2.12 MPa, [1.39, 3.81] MPa; \(E\) 12.77 MPa, [10.36, 17.70] MPa). Referring to the viscoelastic behavior of the human LUT tissues, stress-relaxation tests were performed in this work. Figure 3a-d report the normalized relaxation curves of the urethra when considering different regions (membranous, penile or bulbar), locations (distal or proximal) and directions (longitudinal or transverse). No significant differences were observed varying the regions or the location, with similar percentages of relaxation between about 78% and 88%. The only exception can be seen when comparing the loading direction within the penile region, where a difference of about 10% of relaxation between the longitudinal and the transverse directions was observed.