**3. Results**

#### *3.1. Evaluation of the Filaments*

All prepared filaments were made using a PVA as a filament-forming polymer, a semi-crystalline polymer of molecular weight equal to 32 kDa with 87–89% hydrolysis grade, having a glass transition temperature, melting point, and degradation temperature of 40–45 ◦C, 170 ◦C, and ≥250 ◦C, respectively [66]. The obtained itraconazole-loaded filaments were opaque and creamy in color. The diameter of the filaments was kept at a constant level; however, in the case of the PVA\_K/CL filament, the diameter variations were higher than 0.05 mm, which is considered as a maximum acceptable deviation from the declared diameter [71]. The itraconazole content and its uniformity were satisfactory. All the API-loaded filaments were tested for their tensile strength and elasticity, which were found to be critical quality attributes in term of printability. The results are presented in Table 2. It was found that the addition of copovidone and crosslinked PVP resulted in a decrease in the tensile strength and Young's modulus of the filaments. All the prepared filaments were able to be printed with a ZMorph ® 2.0 S 3D printer immediately after extrusion and after storage in zipper storage bags.


**Table 2.** Hot-melt extruded filament characteristics.

In Figure 2, the differences in the mechanical characteristics are presented. The Young's modulus corresponds to the slope of the curve in the elastic behavior region.

**Figure 2.** Comparison of the mechanical strength and resilience of the filaments.

#### *3.2. Thermal and Structural Properties of the Filaments and 3DP Tablets*

To investigate how the employed polymers modify the thermal properties of neat ITR, the systems, prepared in the form of both filaments and 3DP tablets, were measured (after pulverization) by means of DSC. The samples were examined in the temperature range from 273 to 453 K at a heating rate of 10 K/min. In Figure 3, the obtained DSC traces are compared to the thermogram of the neat, quench-cooled ITR. Because the used PVA polymer has a lower glass transition temperature than ITR (Table 3) (Tg of neat PVA and ITR are equal to 313 and 332 K, respectively), the plasticization effect was observed. Interestingly, the DSC thermograms of the same compositions with different forms, filament or 3DP, differ from each other. As can be seen in Figure 3, the thermograms of the 3DP tablets are characterized by: (i) a shift in glass transition temperature towards lower values when compared with filament and (ii) the appearance of an additional, very broad endothermal event in the vicinity of 320 to 420 K. The observed differences sugges<sup>t</sup> that the 3DP tablets also contain water in addition to API and polymers. Water exerts a plasticization effect on the samples and evaporates at temperatures from in the range of 320 to 420 K.

When the neat ITR is heated above its glass transition temperature, on the DSC thermogram, one can distinguish two endothermal processes associated with the liquid crystal (LC) phase transitions. The thermal event located at 348 K reflects transition from smectic (Sm) to nematic (N) LC alignment, while at 364 K, ITR loses the nematic order and becomes an isotropic (I) liquid. The performed experiments reveal that the employed polymers shift to lower temperatures for both Sm-N and N-I phase transition. The determined, based on calorimetric studies, values of Tg, TSm-N, and TN-I for all investigated systems are compared in Table 3. It is worth noting that in one of the examined systems (PVA\_K/VA), regardless of the applied technological process, the lack of the nematic phase was observed (i.e., the N-I endothermal event was not registered by means of DSC).

**Figure 3.** DSC thermograms of neat ITR and its mixtures with PVA, PVA\_K/VA, and PVA\_K/CL prepared in two forms: filament and 3DP tablet.

**Table 3.** Comparison of values of Tg, TSm-N, and TN-I of neat ITR and its mixtures with PVA, PVA\_K/VA, and PVA\_K/CL which were prepared in two forms: filament and 3DP tablet.


In order to investigate whether the employed polymers indeed modify the ITR's LC alignment, both the neat ITR as well as the pulverized 3DP tablets were measured by wide-angle X-ray diffraction (XRD) technique. The comparison of the scattering patterns collected at room temperature for neat ITR and pulverized tablets containing either PVA, PVA\_K/VA, or PVA\_K/CL is presented in Figure 4. The presented XRD patterns demonstrate that the polymers affect the LC order in ITR. As can be seen, samples containing PVA or PVA\_K/CL reveal less intense peaks at around 0.22, 0.45, and 0.68 Å−1, which are indicators of smectic layering [72]. In the case of the system containing K/VA, the reduction in the intensity of the peaks at 0.22 and 0.68 Å−<sup>1</sup> is combined with the disappearance of the peak at 0.45 Å−1. These results indicate that the layered structure in ITR is medicated by the employed additives.

#### *3.3. Micro-Computed Tomography Studies of Tablets*

The dimensions and masses of 3DP tablets corresponded to predefined values. The average tablet mass ranged from 239.73 to 253.05 mg. Tablet length varied from 19.85 to 20.15 mm, whereas height ranged from 1.78 to 3.65 mm. The real layer height was from 0.142 to 0.158 mm and was calculated by dividing the tablet height by the number of layers, given the fact that the first layer was 0.2 mm (Table 4). Digital photos of 3D printed tablets can be found in the Supplementary Materials associated with this article (Figures S1–S3, Supplementary Materials).

**Figure 4.** XRD diffraction patterns of neat ITR and its mixtures with PVA, PVA\_K/VA, and PVA\_K/CL in an initial form of 3DP tablet.


**Table 4.** Parameters of 3D printed tablets.

Based on the 3D tablet images obtained from Voxelizer slicing software (Figure 5) and predefined settings of the path size, the theoretical volume of 3DP PVA\_K/CL tablets was calculated. The values varied from 184.4 mm<sup>3</sup> for T\_20 tablets to 195.6 mm<sup>3</sup> for T\_60 and 195.9 mm<sup>3</sup> for T\_35 tablets.

**Figure 5.** Images of PVA\_K/CL tablet layers obtained from Voxelizer software.

The morphology of the PVA\_K/CL printed tablets was verified by the μCT scans. Tablets with 20% of infill had the highest object volume (236 mm3) and the highest open pore volume (485 mm3). Medium pore size (structure separation) was 1.11 mm, whereas the average structure thickness was 0.25 mm. Tablets with 60% of infill were characterized by the lowest values of object volume (202 mm3) and pore volume (134 mm3) as well as structure separation (0.19 mm) and pore size (0.25 mm). Aforementioned parameters for tablets with 35% of infill can be placed between T\_20 and T\_60 values (Table 5, Figure 6).

**Table 5.** Comparison of μCT scan data of 3DP PVA\_K/CL tablets with 20% (T\_20), 35% (T\_35), and 60% (T\_60) infill ratio.


**Figure 6.** 3D tablet models, μ-CT scan images of 3DP tablets, and structure thickness and structure separation of 3DP tablets.

Parameters of tablets with 35% of infill are similar and no important differences between the three analyzed tablets can be distinguished (Table 6, Figure 7).


**Table 6.** Comparison of μCT scan data of 3DP PVA\_K/CL tablets with 35% of infill ratio.

**Figure 7.** Comparison of 3DP tablets with 35% of infill.

#### *3.4. Dissolution Studies*

Itraconazole dissolution from 3D printed tablets with 35% infill was compared with the dissolution profiles obtained for the tablets made from milled extrudate (HME tablets) and directly compressed

tablets (DC tablets) to evaluate the impact of the excipients and hot-melt extrusion on the dissolution of the API. Determined itraconazole solubility limits were equal to 5.8, 22.2, and 29.3 μg/mL for physical mixture, extrudate, and 3D printed matrix, respectively. The solubility limits were calculated as the percentage of ITR dose in tablets (11.6%, 44.4%, and 58.6% for physical mixture, extrudate, and 3D printed tablet, respectively) and are marked in Figure 8 to make the interpretation of the dissolution easier. It was found that the performed technological processes, namely hot-melt extrusion and 3D printing, affected the dissolution profile of itraconazole. The highest amount of the drug was dissolved from 3D printed tablets. The amount of ITR released from milled extrudate was significantly lower, while the smallest amount was released from directly compressed tablets (Figure 8). After 2 h of the dissolution test, 75.8%, 51.3%, and 11.0% of the itraconazole was released from the PVA-based 3D printed, hot-melt extruded, and directly compressed tablets, respectively. This relationship was confirmed for all the prepared formulations. It must be highlighted that in the case of all 3D printed formulations, i.e., PVA, PVA\_K/VA, and PVA\_K/CL, the amount of dissolved itraconazole was far above the solubility limit and the supersaturation lasted as long as the dissolution test was performed.

**Figure 8.** The influence of the technological process on the release profiles of itraconazole from PVA-based tablets (infill density equal to 35%).

The impact of copovidone and crospovidone addition to the PVA formulation on the release profile was also evaluated (Figure 9). The best dissolution profile was noticed for PVA\_K/CL 3D printed tablets. After 45 min, 91.5% of the API was dissolved from PVA\_K/CL 3D printed tablets, while only 64.3% and 46.7% of the drug was released from 3D printed tablets with Kollidon® VA64 and PVA-based tablets, respectively.

The impact of the infill density on the dissolution characteristics was evaluated for the PVA\_K/CL formulation (Figure 10) as it was selected as the most promising formulation from all the prepared 3D printed tablets. Three rectilinear infills with different densities, namely 20%, 35%, and 60%, were evaluated. The results confirmed that the lower infill density favored faster dissolution of the API. After 45 min of the dissolution test, 96.9%, 89.7%, and 80.9% of the itraconazole was released from 3D printed tablets with 20%, 35%, and 60% infill, respectively.

**Figure 9.** The influence of the excipients on dissolution profiles of itraconazole from 3DP tablets (infill density equal to 35%).

**Figure 10.** The influence of infill percentage on dissolution profiles of itraconazole from 3DP tablets.
