## 3 Results and Discussion
### Thermogravimetric Analysis  
Thermogravimetric Analysis (TGA) is used to study the thermal stability of all preparations and to calculate the mass of fillers in composite filaments in comparison with the loaded amount. Figure 3 shows the TGA thermograms of all samples while the insert depicts the differential gravimetric analysis thermograms (DTGA) in the range from 250 to 450 \({}^{\circ}\)C which corresponds to the main thermal transition. The TGA graph of pure hydroxyapatite shows a continuous mass loss with a value of 3.1% at 550 \({}^{\circ}\)C. The mass loss is associated with absorbed water up to 200 \({}^{\circ}\)C, while at the range 200-400 \({}^{\circ}\)C it is due to the removal of lattice water. Mass loss in the range 200-550 \({}^{\circ}\)C is associated with the decomposition of hydrogen phosphate ions (HPO\({}_{4}\)\({}^{2-}\)) into pyrophosphate ions (P\({}_{2}\)O\({}_{7}\)\({}^{4-}\)) [37; 38]. In contrast, the mass loss of pure BT at 550 \({}^{\circ}\)C is less than 0.2%. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **PLA** & **PLA/PCL** & **PLA/PCL/10-10** \\ \hline Elastic Modulus (GPa) & 2.2 \(\pm\) 0.1 & 1.2 \(\pm\) 0.2 & 1.4 \(\pm\) 0.1 \\ Tensile Strength (MPa) & 57.2 \(\pm\) 6.2 & 41.7 \(\pm\) 3.7 & 45.6 \(\pm\) 2.5 \\ Elongation at Break (\%) & 4.2 \(\pm\) 0.7 & 6.8 \(\pm\) 1.6 & 5.5 \(\pm\) 0.8 \\ Fracture Stress (MPa) & 54.8 \(\pm\) 6.1 & 35.0 \(\pm\) 5.3 & 41.8 \(\pm\) 3.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of pure PLA, PLA/PCL, and PLA/PCL/10-10 filaments. Figure 2: Stress–strain (%) curves for pure PLA, PLA/PCL, and PLA/PCL/10-10 filaments. The TGA and DTGA thermograms of neat PLA and PCL pellets together with the PLA/PCL filament are shown in Figure S1. In the case of pure PLA pellets, a single sharp peak occurs at 366.7 \({}^{\circ}\)C and is due to polymer degradation. The respective curve for the PCL pellets is obtained at 409.2 \({}^{\circ}\)C. The respective temperatures for the sample PLA/PCL are located at 360.3 and 394.2 \({}^{\circ}\)C. Previous works also reported that PLA and PCL thermally degrade also in a single step [39; 40]. To compare the relative thermal stability between samples, the temperature in which the loss of total mass was 50% (T\({}_{50\%}\)) was chosen. The results are listed in Table 2. It can be seen that the presence of HAp as filler or in combination with BT, especially at high loading amounts, improves the thermal stability of the composite filament. Data obtained from the DTGA curves corroborate the increasing thermal stability of the composites at high loading values. Production of composite filaments can be performed using a twin-screw extruder with different heating zones. In this case, raw materials are mixed with the fillers into the hooper, and the composite filament finally exits from the orifice of the extruder's nozzle. However, in the case of a single screw extruder, fabrication of homogenous composite filaments is usually difficult. To overcome this problem, different techniques have been developed. A common method is based on the film casting technique. The film casting technique using DCM as a solvent has also been successfully used by other researchers [20; 41; 42]. Results in Table 2 show that the calculated amount of HAp and BT is close to the initial loading. It should be noted that duplicate measurements were conducted at selected formulations and