## 3 Results and Discussion
### Characterization of the Rods and Rod with ARP
#### 3.3.5 Morphology Study  
The characterization of rod morphology based on SEM images (Figure 9) analyzed by ImageJ(r) software revealed the differences in the heterogeneity between blank rods and rods with ARP, whereas, for the circularity and solidity, the presence of a drug substance did not play an important role (Table 4).  
Figure 8: The FTIR spectra of blank rods (TD,L 21 rod, Tl. 59 rod, and Tl. 77 rod) and rods with ARP (TD,L 21 rod-ARP, Tl. 59 rod-ARP, and Tl. 77 rod-ARP) in the range of 4000–500 cm\({}^{-1}\).  
Previously, it was revealed that the presence of risperidone or ARP influenced the increase in morphological diversity of implantable rods based on poly(lactide-_co_-glycolide-_co_-trimethylene carbonate) terpolymers [12,13]. In this study, the thesis presented in the literature was developed by calculations using ImageJ(r) software. The introduction of ARP into the terpolymer matrix increased the rod heterogeneity from a range of 12.9-16.5% to a range of 19.9-36.5% (Table 4).  
A lack of pores was observed for blank rods and rods with ARP (Figure 9) and confirmed by a low rate of circularity (0.029-0.043) (Table 4). It should be added that the value of 1.0 for the circularity means perfect circles, which would indicate the presence of pores. These observations may result from both the formulation method and polymer properties. It should be pointed out that because of the melting of the polymer material, HME does not cause the formation of pores [12,13,22,27]. In the case of injectable extruded d,l-PLGA rods with n-acetylcysteine, no porosity was observed either [44]. The outer morphology of the extruded formulation reflects more the topography of the inner surface of the die in the extruder than the properties of the polymer. Moreover, the features of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Sample**} & \multicolumn{1}{c}{**Area (\(\mu\)m\({}^{2}\))**} & \multicolumn{1}{c}{**Heterogeneity (\%)**} & \multicolumn{1}{c}{**Circularity**} & \multicolumn{1}{c}{**Solidity**} \\ \hline \multirow{2}{*}{Td,l 21} & rod & 35,394.7 & 12.9 & 0.043 & 0.83 \\  & rod-ARP & 35,492.7 & 19.9 & 0.042 & 0.85 \\  & rod & 35,871.9 & 16.5 & 0.039 & 0.81 \\  & rod-ARP & 35,179.7 & 36.5 & 0.039 & 0.83 \\  & rod & 35,611.1 & 15.5 & 0.039 & 0.85 \\  & rod-ARP & 35,719.9 & 23.6 & 0.029 & 0.87 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Heterogeneity, circularity, and solidity for the images with the scale bar equal to 100 \(\upmu\)m of blank rods (Td,l 21 rod, Tl 59 rod, and Tl 77 rod) and rods with ARP (Td,l 21 rod-ARP, Tl 59 rod-ARP) and TL 77 rod-ARP) calculated by ImageJ®.  
Figure 9: SEM images of blank rods (Td,l 21 rod, Tl. 59 rod, and Tl. 77 rod) and rods with ARP (Td,l 21 rod-ARP, Tl 59 rod-ARP, and Tl. 77 rod-ARP).  
the P(LA:GA:TMC) terpolymers used point to a more amorphous character, which may also be the cause of the lack of pores. In contrast, for the matrices based on L-PLGA with risperidone formulated by the solution-casting method, pores were observed. Pore formation was observed for semi-crystalline polymers processed by low-temperature methods [21]. A lack of pores in the native rods seems to be an advantageous feature from the administration point of view because pores may significantly weaken the rod structure during implantation with a pre-filled syringe. [PERSON] and coworkers have shown that the increase in porosity of the d,l-PLGA films incorporated with poly(ethylene glycol) could have contributed to the decrease in tensile strength [45].  
In this study, the HME process did not result in unfavorable changes in the formulation's morphology. Disintegration, cracks, microcavities, and slits were not observed (Figure 9), which was reflected by the high values of solidity (Table 4), _i.e._, 0.81-0.87, where the value of 1 means the monolithic area.