**3. Discussion**

Besides pure pectin aerogels, chitosan-coated pectin aerogels were prepared. The shape of latter slightly changed, resulting in a more massive appearance and less defined shapes.

N2 adsorption–desorption analysis showed that chitosan-coated pectin aerogels had significantly reduced specific surface areas and porosity compared to pure pectin aerogels. Namely, the preparation procedure affected the pore network, reducing the specific surface areas of pure pectin aerogels from 441 m<sup>2</sup>/g to 276 m<sup>2</sup>/g. Transferring pectin cores to the NaOH solution apparently caused the shrinkage of pectin cores and some damages in the pore network. The adsorption capacity of pectin aerogels is higher compared to the adsorption capacity of chitosan-coated pectin aerogels, based on adsorption–desorption isotherms.

Scanning electron microscopy revealed porous structures for both pure pectin aerogels and chitosan-coated pectin aerogels, in both pectin core and chitosan layer. However, the structure of pectin aerogels showed to be more compact. SEM images are in good agreemen<sup>t</sup> with N2 adsorption–desorption analysis, since pectin aerogels have higher specific surface areas. SEM images of chitosan-coated pectin aerogels show less compact structure, caused by the coating procedure and some damages to the pore network due to the shrinkage.

Thermal analysis consisted out of simultaneous thermogravimetry and di fferential scanning calorimetry to obtain TGA and DSC curves. The DSC curve of pectin aerogel loaded with curcumin compared to pectin polysaccharide showed a shifted peak, indicating the earlier degradation. A possible cause is the presence of curcumin. However, the melting peak of the curcumin is not visible on the pectin–curcumin curve. The overall mass of analyzed pectin aerogel loaded with curcumin is approximately 10 mg. This means that the mass of curcumin is quite low and could be simply covered or not detected in this case. TGA curves of pectin polysaccharide and pectin aerogels loaded with curcumin showed that the thermal degradation occurs in two steps.

Determined IR spectrum of pectin aerogels loaded with curcumin confirmed the presence of curcumin. The chemical structure of pectin, however, was not changed. This was verified by characteristic peaks for pectin that are still present in the spectrum. This means that the pectin aerogel serves as carrier, without chemical changes in the structure caused by the presence of active substances. In the case of chitosan-coated pectin aerogels, the characteristic peaks for curcumin are overlapping with characteristic peaks for chitosan. Even though it is not visible in the spectrum, the presence of curcumin was confirmed by further in-vitro release studies. In this case as well, the characteristic peaks for chitosan are present, again proving preserved chemical structure of polysaccharide.

Behavior of unloaded aerogels was tested in SGF at pH = 1.2 and SIF at pH = 6.8. At SGF, pectin aerogels showed to be stable. Contrary, chitosan-coated pectin aerogels started their decomposition after 2 h and finished the decomposition after 4 h. Actually, only the coating made of chitosan decomposed since the chitosan is soluble in acidic medium. Pectin core was, however, stable. This means that by using chitosan coating, pectin core is protected from the decomposition. Behavior in SIF fluids completely di ffers. While pectin aerogels completely decomposed after 3 h, chitosan-coated pectin aerogels are stable in neutral fluid. Chitosan coating was able to slow down and prolong the decomposition of pectin from 3 h up to 6 h. This behavior opens up the possibility for retaining the drug for a longer time period inside the core and, later on, the drug's retardation during release.

Release of curcumin from both pure pectin aerogels and chitosan-coated pectin aerogels was tested through in-vitro studies and compared with the dissolution of curcumin powder.

Release of curcumin in SGF was retained for both aerogels. However, when transferred to SIF, pectin aerogels show burst release within just 1 h (3 h overall). This result is in good agreemen<sup>t</sup> with the swelling studies, in which pectin aerogels were completely decomposed after just 3 h spent in SIF. During drug release studies, pectin aerogels swelled in SGF and decomposed and released curcumin after 1 h in SIF. The dissolution and bioavailability of curcumin is tremendously improved, compared to standard curcumin. The porous network structure of aerogels enabled the surrounding of the molecules of curcumin by molecules of water, thus providing the possibility of faster dissolution. Even though the dissolution of curcumin was significantly improved, the release was still a burst. In the case of chitosan-coated pectin aerogels, release of curcumin was prolonged up to 24 h. By covering pectin with a chitosan layer, the core and, consequently, the curcumin trapped inside are partially protected. This formulation slowly swells, and consequently slowly releases curcumin. By protecting the highly soluble pectin core with a chitosan layer, controlled release of curcumin was achieved. As expected, curcumin powder showed almost no dissolution for the tested period.

### **4. Materials and Methods**
