## 3 Results
By considering the stress at the equilibrium point for each step strain applied for each specimen, non-linear stress (\(\sigma\))-strain (\(\epsilon\)) curves were separately obtained and fitted by the following exponential formulation:  
\[\sigma\left(\epsilon\right)=k/\alpha\ [\mathrm{e}^{\left(\alpha\right)}-1] \tag{2}\]  
The parameter \(k\) specifies the tissues' initial stiffness, while \(\alpha\) is related to the non-linearity of the response due to the stiffening. For each tissue, the resulting parameters (\(k\), \(\alpha\)) are reported in terms of median in Table 5. It can be observed that, among all the tested tissues, the tunica albuginea exhibited the highest stiffness with a value up to three scale orders above. Stress-strain curves obtained at equilibrium are reported in Figure 3.  
From the indentation test on the prostate sample, the reaction forces and the deformation were acquired for each indentation point. Under the assumption of the tissue as a semi-infinite elastic body and considering that the indenter perfectly contacted with the tissue, \(E\) was fitted by means of the Hertz-Sneddon equation [13]:  
\[F=4/3\ E\ \left(1-\
u^{2}\right)^{-1}\sqrt{R\delta^{3}} \tag{3}\]  
where \(F\) is the maximum value of the reaction force, \(R\) is the indenter radius, \(\delta\) is the indentation depth and \(\
u\) is the [PERSON]'s ratio, assumed equal to 0.499. Figure 4 shows the mechanical responses resulting from the indentation points P1 and P5, placed in different regions of the prostate sample. In particular, P1 refers to one nodule formation on the prostate, while P5 refers to the non-altered tissue. From the experimental results, only the first part of the curve was fitted, for a maximum indentation depth of 1.5 mm and resulted in a good fit quality (reported in Table 6 by the R\({}^{2}\) parameter). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Tissue** & \(k\)**[kPa]** & \(\alpha\) & \(\gamma_{1}\) & \(\gamma_{2}\) & \(\tau_{1}\)**[s]** & \(\tau_{2}\)**[s]** \\ \hline Bladder & 120 & 1.20 & 0.63 & 0.26 & 1.01 & 65.39 \\ Tunica albuginea—T & 8640 & 2.05 & 0.46 & 0.20 & 1.03 & 68.91 \\ Membranous urethra—T & 5.40 & 4.94 & 0.63 & 0.24 & 1.21 & 96.26 \\ Bulbar urethra—L & 130 & 4.19 & 0.56 & 0.23 & 1.44 & 83.10 \\ Bulbar urethra—T & 3.00 & 5.13 & 0.58 & 0.25 & 1.49 & 104.80 \\ Penile urethra—L & 120 & 3.13 & 0.55 & 0.24 & 1.29 & 93.84 \\ Penile urethra—T & 4.70 & 6.94 & 0.60 & 0.26 & 1.22 & 72.56 \\ \hline \hline \end{tabular} \(L=\) longitudinal, T = transversal.  
\end{table} 
Table 5: From the results of stress–relaxation tests, elastic (\(k\), \(\alpha\)) and viscous (\(\gamma_{1}\), \(\gamma_{2}\), \(\tau_{1}\), \(\tau_{2}\)) parameters were obtained in terms of median values. With reference to the histological analyses, Hematoxylin-Eosin staining was performed to observe the general layout of the tissues, as reported in Figure 5a,c for the prostate and bladder, respectively, and Figure 6 for the urethra, Buck's fascia and tunica albuginea. This specific staining highlights the prostate tubuloacinar glands surrounded by fibromuscular stroma (Figure 5a). Furthermore, Figure 6a shows both the urethral section as well as the corpora cavernosa. At the lower center (Figure 6b), the corpus spongiosum surrounding the bulbar urethra is clearly visible. Additional staining (Azan-Mallory) was performed to distinguish muscle fiber content with red color and connective tissue, highlighting collagen fibers with blue color (Figures 5b,d and 7). Therefore, the tunica albuginea (Figure 7d) around the corpora cavernosa can be noticed, as well as the Buck's fascia (Figure 7e), which results in a deep fascia layer made of fibrous connective tissue, indicated by an asterisk. Moreover, the transversal section of the human bladder (Figure 5d) reveals the mucosa, submuc and first muscle layers. Figure 6: Histological representation of large thickness specimen of the human penis (**a**) stained with Haematoxylin-Eosin.