## 3 Results and Discussion  
### Targets Characterization  
The most complex part was represented by the target production, since they must meet a set of general requirements in terms of composition, homogeneity, densification and size. Within the physical approach (PLD), based on the thermal analyses, the gels were calcined at 600 \({}^{\circ}\)C to almost completely remove the components with low thermal stability and retain the inorganic part in a form with as little crystallinity as possible. This low degree of ordering is mandatory when aiming to ensure the premises for the generation of bioactive materials. Further, the targets were sintered at 800 \({}^{\circ}\)C; thus, an increase of 200 \({}^{\circ}\)C in temperature was selected, based on the idea that high-quality films are deposited from well-compacted sources. The effects of the second thermal treatment in terms of morphology and phase composition can be observed in Figure 1. Achieved in fracture, the SEM images reveal important differences between Target-Mg and Target-Sr. The two samples are heterogenous, alternating more densified areas, which appeared as a result of grains coalescence, with the agglomerations of grains, not strongly connected yet and crossed by a network of pores or channels; therefore, an open interconnected porosity may be appraised (Figure 1a,b). Moreover, massive blocks can be identified, either with a continuous aspect (Target-Mg) or crossed by micrometric cracks (Target-Sr), which probably occurred during crystallization processes, as a way to eliminate internal tensions. The specificity of the Mg-containing composition resides in the existence of honeycomb-like entities, which leads to a larger proportion of pores, with higher maximum diameter and wide size distribution.  
The EDX spectra recorded on both targets are similar, the elements of interest looming with intensities proportional to the designed compositions, as follows: Si, Ca, P, Zn, Mg/Sr, O (Figure 1c,d). It is well known that Mg\({}^{2+}\) contributes to mechanical strength and bioactivity [28], while Sr\({}^{2+}\) improves cellular proliferation and osteogenesis [30]; the other oxides constitute the background matrix that differently embeds the mentioned alkaline earth elements.  
The target's vitroceramic nature was demonstrated by the XRD patterns (Figure 1e,f), through the characteristic shape: a broad halo peak in the range 15-40\({}^{\circ}\) and several narrow intense peaks emerging from it. For Target-Mg, two main crystalline phases were identified: calcium silicate (Ca\({}_{2}\)SiO\({}_{4}\)) with hexagonal structure (ICDD-00-023-1042 and ICDD-00-077-0420) and silicon dioxide (SiO\({}_{2}\)) with different symmetries (ICDD-00-081-1665, ICDD-00-082-1410 and ICDD-00-083-0541). Going to Target-Sr, two main crystalline phases were also detected: calcium strontium silicate (Ca\({}_{1.65}\)Sr\({}_{0.35}\)SiO\({}_{4}\)) with orthorhombic structure (00-077-0399) and SiO\({}_{2}\) with different symmetries (ICDD-00-081-1665, ICDD-00-082-1410, ICDD-00-083-0541 and ICDD 00-083-1833). Ca\({}_{2}\)SiO\({}_{4}\)-based structures represent the major crystalline component within both targets, being surrounded by a vitreous matrix that includes the other elements (P, Zn, Mg). As can be noticed, Sr penetrates the ordered network of Ca\({}_{2}\)SiO\({}_{4}\) and settles on Ca sites, leading to a partial substitution. Starting from the ionic radii of Ca, Mg and Sr (Ca\({}^{2+}\)(VI)--1.00 A, Mg\({}^{2+}\)(VI)--0.72 A, Sr\({}^{2+}\)(VI)--1.18 A), differences of 28% appear between Ca and Mg, respectively, and 18% between Ca and Sr; such a theoretical background sustains the preferential incorporation of Sr.  
Figure 1: SEM images of: (**a**) Target-Mg and (**b**) Target-Sr, EDX spectra of: (**c**) Target-Mg and (**d**) Target-Sr and XRD patterns of: (**e**) Target-Mg and (**f**) Target-Sr.