## 4 Materials and Methods  
### Poromechanics Tests  
_Confined Compression_  
Menisci were harvested from patients (age 65-76, mean 72, and standard deviation 4) undergoing total knee arthroplasty (ethical approval EM 249-2018 21/2017/Sper/IOR EMZ, Rizzoli Orthopaedic Institute, Bologna, Italy). Samples labelled as \"degraded\" by gross investigation of the surgeon were discarded. Three lateral menisci and three medial menisci were collected and stored at \(-20\)\({}^{\circ}\)C [29; 30]. The day of the test, each meniscus was thawed in a phosphate-buffered saline (PBS) bath at room temperature for about thirty minutes [30]. Then, cylindrical samples (diameter of 3 mm and height of 3-4 mm) were extracted from the central body, anterior and posterior region along three reference directions, i.e., vertical, radial and circumferential (Figure 1b), following a dedicated procedure [31], adapted to the meniscal configuration. A total of 18 cylindrical samples were grouped considering the the harvesting region (body, anterior and posterior) and the direction (vertical, radial and circumferential) and then tested. The testing protocol was implemented on a multi-axis mechanical tester (Mach-1, Biomomentum Inc., Laval, QC, Canada) in a confined compression configuration Figure 1c); with this specific setup, we were confident that the fluid could flow only from the base of the cylinder. Insertion in the confining chamber, thickness measurement, and removal of the meniscal sample followed a dedicated procedure [31]. Concerning confined compression, the setup involved a confining chamber with an inner diameter equal to that of the tool used during the extraction of cylindrical samples, i.e., 3 mm. The bottom of the chamber consisted of a porous-permeable plate, while the top allowed the insertion of the piston. Both these components (Biomomentum Inc., Canada) were manufactured to allow the leakage of the fluid only through the porous platen. Moreover, an additional control was provided by visual inspection of the confining chamber transparent wall, which allowed us to check any fluid flow toward the piston (Figure 2a). We measured the amount of fluid flowing out each sample as a result of the applied compression loading by monitoring the weight of the samples throughout the test. \begin{table}
\begin{tabular}{c l l} \hline \hline
**Notation** & **Name** & **Expression** & **Unit** \\ \hline \(\gamma\) & ratio solid volume/fluid volume & Unitless \\ \hline \(K_{s}\) & solid phase bulk modulus & Pa \\ \hline \(G_{s}\) & solid phase shear modulus & Pa \\ \hline \(\mathbf{k}\) & intrinsic permeability & m\({}^{2}\) \\ \hline \(\mu\) & fluid viscosity & \(\mathrm{N}\cdot\mathrm{s}\cdot\mathrm{m}^{-2}\) \\ \hline \(\kappa\) & averaged axial permeability & \(\mu/((1+\gamma)^{2}k)\) & \(\mathrm{N}\cdot\mathrm{s}\cdot\mathrm{m}^{-4}\) \\ \hline \(H_{A}\) & aggregate modulus & \((4G_{s}+3K_{s})/3\) & Pa \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Biphasic model. Testing Protocols  
Relaxation tests. For the relaxation tests, we followed the procedure for permeability and aggregate modulus \(H_{A}\) analysis, as recommended by the manufacturer of the testing machine [31; 32]. In further detail, the test sequence considers a pre-compression with ramp amplitude 10% of the sample thickness (h) and ramp velocity 0.3% h/s, followed by five stress-relaxations with incremental ramp amplitude 2% h and ramp velocity 0.3% h/ s.  
Creep Tests The testing protocol--specifically the loading amplitude and the detection of the fluid flow out of the sample--was designed and optimized for this specific study. In particular, three testing phases were implemented for each meniscal sample. The first phase, developed according to [31], consisted in five separated repetitions of confined compression, 75 s each, to realize 450 s of total creep.