## 3 Results and Discussions
### Robocasting of Lattice, Dense and Bi-Layer Parts
#### 3.1.5 Mechanical Properties  
The representative stress-strain curves recorded during compression tests of the dense and lattice samples are depicted in Figure 8. Curves show that several cracks occurred before the final rupture of both samples. At the beginning of the loading, the resistance of the lattice samples against the external loading was attested by the increase of the applied stress as a function of samples' strains. After reaching an initial stress peak, a significant stress release occurred, which was recovered subsequently. Several stress release-recoveries led to further peaks inducing sawtooth-like curves. This phenomenon (repeated stress release-recovery) took place due to the failure of the different layers (load transfer). However, the final rupture occurred at maximum stress higher than the first peak. The dense samples, in the same way, firstly showed stress elevation up to an initial peak. Afterward, the samples managed to recover the released stress exhibiting an oscillating pattern at an almost narrow stress range, with a very moderate stress increase before the final failure.  
The compressive strengths of the structures were found as \(29\pm 4\) MPa for the dense samples with an overall porosity of 13.9%, and \(7\pm 4\) MPa for the lattice structures with an overall porosity of 34.1%. This value for robocast HA lattices is reported in the ranges from 9 to 50 MPa with the porosity in the range from 26 to 71% [39; 40; 42; 63] and for \(\beta\)-TCP ones is reported in the ranges from 11 to 20 MPa  
Figure 8: Registered stress-strain curves of the lattice (**A**) and dense samples (**B**), obtained during uniaxial compression tests.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{3}{c}{**External Diameter (mm)**} & \multicolumn{3}{c}{**Mean Pore Size (μm)**} \\ \hline Core & \(1^{\circ}\) ring & \(2^{\circ}\) ring & \(3^{\circ}\) ring & \(4^{\circ}\) ring & \(1^{\circ}\) ring & \(2^{\circ}\) ring & \(3^{\circ}\) ring \\ \(5.48\pm 0.11\) & \(7.92\pm 0.13\) & \(10.43\pm 0.15\) & \(13.02\pm 0.18\) & \(15.49\pm 0.35\) & \(605\pm 55\) & \(896\pm 52\) & \(1040\pm 31\) \\ \hline \end{tabular}
\end{table}
Table 3: Measured diameters of the rings in the lattice part and pore sizes of the FGM bi-layer structures.  
with the porosity in the range from 27 to 40% [13; 37; 42]. The results obtained for the lattice structures in the current work are positioned in the lower limit of the literature data and were in the limit of the compressive strength of the cancellous bone which is reported in the range of 7-10 MPa [64].  
The comparison of the mechanical properties of the dense samples with literature was not possible due to the lack of data. However, compared to the compressive strength of dense HA produced by the binder-jetting technique (equal to 22 MPa [65]), current results were significantly higher, suggesting the high potential of the robocasting technique in the production of well-densified parts. However, compared to the cortical bone with compression strength in the range of 130-180 MPa for the porosity in the range from 5 to 13% [2], the robocast dense samples had lower performance. In the future, mechanical properties could be improved by modifying the slicing procedure of the CAD design to credit higher overlapping between adjacent filaments and also by drying the samples in more controlled conditions [27; 29].