*3.1. Microscopic HNT/ERY' Structure Analysis*

As our task was the preparation of antimicrobial nanofibers for biomedical applications, the main criterion for the selection of an appropriate type of halloysite was its purity. For this reason, we selected ULTRA HalloPureTM tubular halloysite. This particular HNT type has a very high purity (93.5%), with only trace levels of feldspar. The second reason for our study was ULTRA HalloPure's geometry. From the point of view reinforcing effect, which is essential for nanofibers based on low-strength PCL, it is necessary to use HNTs with long aspect ratios. Moreover, long-aspect-ratio HNTs provide more prolonged antibacterial effects due to the release of an active compound. The geometry of the used HNT type (length, inner and outer diameter, and aspect ratio) is presented in Table 1.

The micromorphology and nanomorphology of HNTs both modified and loaded with ERY were studied with transmission electron microscopy. The structure of HNTs before ERY loading is shown in Figure 2. TEM images of HNTs loaded with ERY are shown in Figure 3.

**Figure 2.** TEM of unmodified HNTs before ERY loading.

**Figure 3.** TEM of HNTs loaded with ERY.

#### *3.2. Loading Efficiency of HNTs*

A thermogravimetric study of HNTs and HNT/ERY was performed to determine loading efficiency, i.e., the amount of ERY loaded into HNTs. As mentioned earlier, the loading of active agents to HNTs is based on the diffusion of molecules from external solutions into the inner part of HNTs due to the concentration gradient. As shown in Figure 3, the unmodified HNTs were stable up to 500 ◦C, at which point de-hydroxylation [6,39] occurred with a disruption of the tube-wall multilayer packing. At 600 ◦C, the weight loss of the unmodified HNTs was 12.5 wt.% (blue curve in Figure 4). Pure erythromycin showed steep decomposition around 300 ◦C followed by a slow degradation process up to 600 ◦C. (red curve in Figure 4). The efficiency of the used loading method was evaluated via a comparison TGA of pristine HNTs with HNTs loaded with ERY (HNT/ERY) washed with water (green curve in Figure 4) and methanol (orange curve in Figure 4).

The sample of HNT/ERY washed in water (HNT/ERY/WAT) showed the main decomposition around 300 ◦C followed by gradual degradation process and further degradation step around 500 ◦C, reflecting the behavior of precursors.

**Figure 4.** TG curves of ERY, HNTs, HNT/ERY washed in water, and HNT/ERY washed in methanol.

The weight loss of HNT/ERY/WAT—37.9 wt.% at 500 ◦C—corresponded to the rate of erythromycin loaded on both the inner and outside surface of HNTs (HNT:ERY at 60:40) and the rate of ERY loaded exclusively onto the inner surface of HNTs, as evaluated on the sample from which residual ERY was removed with methanol: 4.6 and 11.5 vol.%, respectively. In summary, we obtained extremely highly-loaded HNTs suitable for antibacterial applications in biomedicine.

#### *3.3. Morphological Observation of PCL/HNT/ERY Nanofibers*

Our previous study described the morphology of electrospun PCL antibacterial composites with different fillers—hydrophobic quantum dots working on the principle of photodynamic therapy—for wound healing in tissue engineering in detail [29]. A strongly porous material with interconnected structures was obtained via the electrospinning method. This 3D structure is very suitable for use in tissue engineering. Despite this fact, we created relatively homogeneous nanofibers with an average thickness of 3–4 μm, and the same effect was not observed in the case of HNT-loaded samples. As shown in Figure 5, the fibers contained a lot of beads in their structure due to the very high probability of the electric conductivity of the electrospinning solution to change, which is one of the key factors of the electrospinning process. Hydrophobic quantum dots are semiconductors, and HNTs or ERY-loaded HNTs are insulators. Therefore, the final electric conductivity of the solution was significantly lower and the viscosity of solution was reduced. All those

parameters affected the final structure of the fibers. These beads had only a minor effect on antibacterial and mechanical properties because these were determined by releasing ERY from HNTs, as shown later.

**Figure 5.** SEM images at high magnification (5000×) in the secondary electron mode of electrospun (**A**) pure PCL, (**B**) PCL and 6 wt.% HNT nanofibers, (**C**) PCL and 6 wt.% HNT/ERY (80:20), and (**D**) PCL and 6 wt.% HNT/ERY 60:40.
