## Materials and Methods
### Robocasting
#### 2.1.4 Scaffolds Characterizations  
The Archimedes' method according to ISO 10545-3:2018 [49] was used for the density and porosity measurements. The sintered densities and open porosities were determined by:  
\[\rho_{s}=\frac{M_{dry}}{\left(M_{sat}-M_{susp}\right)}\rho_{water} \tag{1}\]  
\[\textit{Open Porosity}\ (\%)=\frac{\left(M_{sat}-M_{dry}\right)}{\left(M_{ sat}-M_{susp}\right)}100\% \tag{2}\]  
where \(\rho_{s}\), \(\rho_{water}\), \(M_{dry}\), \(M_{sat}\) and \(M_{susp}\) are the density of the scaffold being measured, the density of water at the measurement temperature, the scaffold mass, the scaffold mass being saturated by deionized water and the mass found by suspending the saturated scaffold in deionized water, respectively.  
The pores and strut sizes were characterized using a stereomicroscope (Leica EZ4 W, Wetzlar, Germany). The microstructures of the samples were observed using field emission scanning electron microscopy (FESEM) (Zeiss Supra 40, Jena, Germany), equipped with an Oxford EDS microanalysis (Oxford Instrument plc, Abingdon, UK) for chemical analyses. The mechanical properties of the dense and porous samples were determined according to ASTM C773-88 (2016) [50] using a uniaxial compression testing device (MTM Zwick/Roell Z050, ZwickRoell AG, Ulm, Germany) having a 50 kN load capacity in displacement-controlled mode and with a displacement rate of 0.1 mm/min. At least 6 samples were tested from each scaffold type. Despite that ceramics can be characterized by different physical-mechanical tests [51; 52; 53], the uniaxial compression test is widely used for dense and porous biocercamics, thus providing a useful tool to compare the current results with literature previous ones.  
The fracture mode of the porous samples under uniaxial compressive loading was identified by performing compression tests in an environmental scanning electron microscope (ESEM, Quanta 450 FEG, ThermoFisher Scientific, Waltham, MA, USA). While testing, loading and displacement were monitored, and at any sudden drop of loading the test was stopped and possible crackings were identified by in situ observation of the specimen.  
In vitro bioactivity tests were carried out on the FGM structure by soaking the samples in simulated body fluid (SBF) solution according to ISO 23317:2014 [54]. The SBF was prepared by dissolving proper amounts of sodium chloride (NaCl), potassium chloride (KCl), sodium hydrogen carbonate (NaHCO\({}_{3}\)), magnesium chloride hexahydrate (MgCl\({}_{2}\cdot\)6H\({}_{2}\)O), dipotassium hydrogen phosphate trihydrate (K\({}_{2}\)HPO\({}_{4}\cdot\)3H\({}_{2}\)O), hydrochloric acid solution (HCl), sodium sulfate (Na\({}_{2}\)SO\({}_{4}\)) and calcium chloride dihydrate (CaCl\({}_{2}\cdot\)2H\({}_{2}\)O) in deionized water [54]. Trishydroxymethyl aminomethane (TRIS) and 1 mol/L HCl fluid were used for buffering of the solution at pH 7.40 at 37 \({}^{\circ}\)C. Disc-shaped specimens were placed in transparent plastic containers with the standard configuration and the SBF solution wasadded to the bottles. The following formula was used for the calculation of the required volume of the SBF solution which is a function of the apparent surface area of the specimen:  
\[v_{s}=100(mm)\cdot S_{a} \tag{3}\]  
In this formula, \(v_{s}\) and \(S_{a}\) are the volume of SBF in mm\({}^{3}\) and the apparent surface area of the specimen in mm\({}^{2}\), respectively. The soaking was performed for 4 weeks at 37 \({}^{\circ}\)C and afterward, the samples were washed by ultrapure deionized water and were dried subsequently. SEM observations were conducted on the lower surface of the samples, as recommended by ISO 23317:2014 standard, after coating with a thin film of Pt-Pd.