## 2 Materials and Methods
### Rheological Characterization  
Rheological analyses were carried out at 25 \({}^{\circ}\)C using a rotational rheometer (Discovery HR2, TA Instruments, New Castle, DE, USA) equipped with a cone-plate geometry (diameter: 20 mm). The testing temperature was selected based on the printing temperature. Prior to all tests, a conditioning step, consisting of a rotation of the tool at 10 s\({}^{-1}\) for 30 s followed by a soak period of 30 s, was included.  
To evaluate the rheological properties of the SA-G bioink, two specific tests were performed: a flow curve and a strain sweep. In the former, the viscosity and the shear stress exerted on the material in response to the applied shear rates were measured in a continuous flow experiment. Specifically, the shear rate was progressively increased in a logarithmic manner from 0.001 to 10,000 s\({}^{-1}\). Ten points per decade were registered, and the test time was 180 s, i.e., 30 s per decade. In the latter, material responses, namely the storage modulus (G\({}^{\prime}\)) and the loss modulus (G\({}^{\prime\prime}\)), were measured by applying oscillatory strains in a logarithmic manner from 0.01% to 1000% at a frequency of 1 Hz. Ten points per decade were recorded. The testing parameters for the flow curve and strain sweep were determined by conducting preliminary tests. To assess the impact of cells on the bioink rheological properties, both tests were repeated on the SA-G bioink loaded with L929 cells at different cell densities (5 \(\times\) 10\({}^{6}\) cells/mL and 10 \(\times\) 10\({}^{6}\) cells/mL) [16]. The cell encapsulation procedure will be illustrated in Section 2.7.  
To investigate the material recovery ability after shear application and removal, a strain recovery test and an elastic recovery test were carried out. In the strain recovery test, the viscosity was measured during the following consecutive steps that mimic the printing process: (i) pre-printing phase (shear rate of 0.05 s\({}^{-1}\) for 300 s); (ii) printing (sudden increase in shear rate to 811 s\({}^{-1}\) for 60 s); (iii) post-printing phase (shear rate of 0.05 s\({}^{-1}\) for 300 s). Thirty points per step were recorded. A 0.05 s\({}^{-1}\) shear rate was chosen to simulate material resting conditions, whereas 811 s\({}^{-1}\) was found to be the maximum shear rate value applied to the material during extrusion. It was computed by using Equation (5), which was derived from the Hagen-Poiseuille law with Rabinowitsch correction [28].  
\[\text{Shear rate}=\frac{4\text{Q}}{\pi\text{R}^{3}}\times\frac{3\text{n}+1}{ 4\text{n}} \tag{5}\]  
where Q (mm\({}^{3}\)s\({}^{-1}\)) is the flow rate, R (mm) is the inner radius at the outlet of the nozzle used in the bioprinting process and (n \(-\) 1) is the slope of the viscosity versus shear rate graph on a log-log plot obtained from the flow curve. To assess the material elastic recovery after the printing process, oscillatory time sweeps made at a frequency of 1 Hz were performed with alternating high/low shear stresses, i.e., 7 \(\times\) 10\({}^{-4}\) and 10\({}^{-6}\) MPa, respectively. The  
Figure 2: (a) CAD model of the poly(lactic acid) (PLA) plates designed to perform uniaxial tensile tests; (b) Experimental setup used for the uniaxial tensile tests.  
maximum shear stress value set in this test was the one corresponding to a shear rate of 811 s\({}^{-1}\). The material was allowed to recover for 2 min to simulate the non-printing phases. All rheological measurements were repeated three times.