## 3 Results and Discussion
### Antibacterial Activity  
The antimicrobial ability of the synthesized hybrid biomaterials was assessed using the Kirby-Bauer protocol. This latter is applied to evaluate the sensitivity of pathogenic aerobic and facultative anaerobic bacteria to several antimicrobial compounds, assisting a physician in selecting treatment options for his or her patients [40]. In this paper, both Gram+ (_S. aureus_ and _E. faecalis_) and Gram- (_E. coli_ and _P. aeruginosa_) bacteria were tested in the presence and absence of the SPxAy hybrid biomaterials. All these bacterial strains cause nosocomial infections, which are healthcare-associated infections that occur in patients under medical care [54; 55]. The antimicrobial activity was expressed by measuring the IHDs as reported in paragraph 2.4. Figure 8 describes, as an example, the acquired images of the bacterial growths of _S. aureus_ and _P. aeruginosa_ incubated with SA20, SP6A20, and SP12A20 hybrid biomaterials, while Figure 9 reports all the IHDs obtained for each synthesized system. Caffeic acid and its derivatives are already known for their antibacterial activity [56; 57]. Here, it has been proved that they are still active in the synthesized hybrids. Indeed, when bacteria were grown in the presence of the SAy hybrid biomaterials, there was an increased activity against all the Gram+ and Gram- assayed, especially regarding _E. coli_[58], as  
Figure 8: Images of the inhibition halos obtained on _S. aureus_ and _P. aeruginosa_ strains after the incubation with SA20, SP6A20, and SP12A20 hybrid biomaterials. Figure 7: XRD spectrum of SP12A20 after 21 days of exposure to SBF. a function of the A content. SA5 showed lower activity in the presence of _P. aeruginosa_ (IHD = 2.08 \(\pm\) 0.07 cm) compared to the IHDs (2.60 to 3.00 cm) obtained with the other bacteria, while SA10, SA15, and SA20 samples showed a slightly increased activity against _S. aureus_, _E. faecalis_, and _P. aeruginosa_, whereas against _E. coli_, the IHDs increased from 3.00 to 4.50 cm as the increase in the caffe acid amount inside the systems. The increment of the antibacterial activity in these systems can be justified by the fact that the catechol functional group present in caffe acid can be totally or partially oxidized to form the orthoquinone or the semiquinone radical [56]. These two forms can undergo subsequent oxidative processes that can damage and/or destroy DNA and protein structures [29; 59; 60; 61; 62; 63]. Comparing the samples with 5, 10, and 15 wt% of caffe acid only, and the one containing the same amount of caffe acid but with the addition of 6 wt% of P, it is possible to observe that the addition of P does not affect the antibacterial activity against the Gram+ strains and _P. aeruginosa_. However, as found in the literature regarding the polyethylene glycol [64], the addition of 6 wt% of P seems to highly influence the antibacterial activity against _E.coli_ strains, indeed, the IHDs recorded for this strain, of SA5, SA10 and SA15 are 2.74 \(\pm\) 0.04 cm, 3.67 \(\pm\) 0.09 cm and 3.84 \(\pm\) 0.03 cm, respectively, while the IHDs of SP6A5, SP6A10 and SP6A15 are 4.29 \(\pm\) 0.07 cm, 4.59 \(\pm\) 0.09 cm, and 4.76 \(\pm\) 0.12 cm, respectively. On the other hand, with the same amount of caffe acid (20%), the addition of 6% of P leads to a slight increase in the inhibition halos for _E. coli_ and _S. aureus_ and a slight decrease for _P. aeruginosa_ and _E. faecalis_ strains. The same comparisons made with samples having 12 wt% of P showed that in general, a high amount of P negatively affects the antibacterial activity of all samples. Although in many cases the addition of 12 wt% of P leads to a higher antibacterial activity, the value reached is less high than the one reached with the addition of 6 wt% of P. This finding is also in accordance with the data recorded for hybrid systems based on silica, silica/PEG, and silica/PEG/Quercetin [20].