## 3 Results and Discussions
### Hybrid Technologies for Ceramic/Polymer Scaffolds  
Figure 14 depicts some representative Ag\({}^{+}\)-free and Ag\({}^{+}\)-containing multi-layer ceramic/polymer scaffold fabricated by robocasting combined with the solvent casting/salt leaching method. Successful integration of these two technologies was achieved, in terms of a perfectly adherent external polymer/CaP layer to the inner bi-layer ceramic part. PCL is a biopolymer extensively used for controlled drug delivery providing uniform drug distribution and enabling long-term drug release up to several months [71]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ca** & **P** & **O** & **Si** & **Mg** & **Na** & **Ca/P** \\ \hline
15.44 & 11.63 & 71.4 & 0.32 & 0.56 & 0.65 & 1.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Chemical composition of the precipitated bioactivity layer, At%. Figure 13: The new precipitated CaP layer on the surface of the bi-layer robocast structures. Low magnification images show the partial (**A**) and entire (**C**) coverage on the surface of dense and porous parts, respectively, and high magnification images show the evenly distributed rod-shaped crystals on the surface of the dense (**B**) and porous (**D**) parts. A mixture of HA and \(\beta\)-TCP phases (indicated as biphasic calcium phosphate, BCP) was added to the PCL solution (resulting in polymer/ceramic composite) to improve the osteoconductivity and bioactivity of the scaffold [72], while silver was added to impart the antimicrobial properties. Figure 15 shows the microstructure of a representative PCL/BCP macroporous layer observed by SEM at two different magnifications. The left image illustrates the macroporosities left by NaCl after leaching out. The mean pore size was measured as 270 \(\upmu\)m, marginally more than the mean NaCl granules sizes, while the geometric shape of the pores matches the morphology of the salt particles. The right image is a high magnification micrograph of the polymer strut, where we can observe CaP particles homogeneously distributed within the PCL matrix. The antibacterial rate of the produced scaffolds was calculated using the following formula:  
\[R=\frac{B-A}{B}\times 100\% \tag{5}\]  
where \(R\), \(A\) and \(B\) are the antibacterial rate, the average bacterial count of the sample (CFU) and the average bacterial count of the control sample (CFU), respectively. Table 5 collects the results obtained from antibacterial tests. The Ag\({}^{+}\)-free samples indicated as \"control sample\" showed a significant number of adhered bacteria in the order of \(10^{9}\) CFU/mL, suggesting a poor antimicrobial behavior of either the ceramic core and polymer layer. As expected, the number of adhered bacteria substantially decreased for the Ag\({}^{+}\) doped samples, showing the high effectiveness of the antibacterial layer to reduce the bacteria adhesion to the whole hybrid scaffold. The current results were slightly better than the average antibacterial results presented in the literature. Figure 14: Digital photographs of two representative hybrid ceramic polymer samples. (**A**): Ag+-free PCL/BCP layer; (**B**): Ag\({}^{+}\)-added PCL/BCP layer. Figure 15: SEM micrographs of the PCL/BCP layer at two different low (**A**) and high (**B**) magnifications. For example, [PERSON] et al. [73] reported 90% improvement in the antibacterial properties of HA coating by doping 1 wt% of AgNO\({}_{3}\). However, it must be considered that although Ag has established antibacterial properties, it can be toxic if it is used in high concentrations. Therefore, a threshold value of the Ag percentage in the scaffold should be optimized considering both antimicrobial and cytotoxicity properties. Due to different Ag incorporation methods and different biological test methodologies, there is a discrepancy of the optimum concentrations in the literature. For example, [PERSON] et al. [73] reported similar cell proliferation and differentiation for 1 wt% Ag-HA and HA.