*2.3. Swelling Studies*

Swelling studies are more or less able to predict the behavior of aerogels during in-vitro release studies. The studies were performed for both pectin aerogels and chitosan-coated pectin aerogels without the presence of curcumin. Experiments were performed in simulated gastric fluid (SGF) at pH = 1.2, and simulated intestinal fluid (SIF) at pH = 6.8, in both cases for 24 h. The aerogels were afterwards compared to see the influence of the coating on their behavior in SGF/SIF.

Figure 8 presents the swelling behavior of both aerogels at SGF. It can be clearly seen that pectin aerogels and chitosan-coated pectin aerogels behave completely differently.

**Figure 8.** Swelling behavior of pectin aerogel and chitosan-coated pectin aerogel in SGF at pH = 1.2 for 24 h.

Both pectin aerogels as well as chitosan-coated pectin aerogels in contact with SGF immediately swelled. However, once pectin aerogels reached their maximal swelling ratio, they stayed unchanged for the next 24 h. This means that they are stable in the mentioned fluid. Chitosan-coated pectin aerogels reached their maximal swelling ratio after 2 h. Unlike pectin, they started to decompose after 2 h and continued decomposing over the next 4 h. Chitosan-coated pectin aerogels have an additional chitosan layer which is soluble at a lower pH. This means that the chitosan layer swells and afterwards decomposes. Once the chitosan layer decomposes after approximately 6 h, the pectin core stays stable in the SGF, in the same way as pectin aerogels.

In SIF, the behavior of aerogels again differs, as shown in Figure 9. Almost immediately after contact with SIF, pectin aerogels reach their maximal ratio. After 3 h, pectin aerogels completely decompose. On the other hand, the swelling ratio of chitosan-coated pectin aerogels increases slowly, reaching its maximal ratio after approximately 5 h. Afterwards, they stay stable in the mentioned fluid. In this case, a chitosan layer over the pectin core prolongs the decomposition of the pectin core from 3 to 6 h. Once the pectin core is decomposed, the chitosan layer is stable in SIF, in the same way pectin is in SGF.

**Figure 9.** Swelling behavior of pectin aerogel and chitosan-coated pectin aerogels in SIF at pH = 6.8 for 24 h.

It is important to emphasize that swelling studies in SGF and SIF were performed independently. Entirely new aerogels were used for both SGF and SIF. The human body, however, functions differently. Once a drug is taken, it stays in the stomach up to 2 h, and then moves (if it does not dissolve) to the intestines for another 6 to 24 h, depending on bowel movements and their emptying.

Based on the aforementioned mechanism, drug release studies for both pectin aerogels and chitosan-coated pectin aerogels were performed first in SGF for 2 h and were afterwards transferred to SIF for an additional 22 h.

### *2.4. In-Vitro Curcumin Release Studies from Aerogels*

To increase the bioavailability of curcumin, two types of aerogels were proposed as carriers: pectin aerogels and pectin aerogels coated with a layer of chitosan. The release of curcumin was tested through in-vitro release studies. Additionally, dissolution of curcumin powder was performed as a comparison.

As shown in Figure 10, the studies were performed for the first 2 h in SGF at pH = 1.2, and SIF at pH = 6.8 for the next 22 h. Weighted samples were immersed in SGF, after which they were immediately transferred to SIF. The experiments were monitored for 24 h.

In the first part of the study, both pectin aerogels and chitosan-coated pectin aerogels showed almost no release of curcumin in SGF at pH = 1.2. Both aerogels swell in this environment, but apparently not enough to release a significant amount of curcumin. These results are in good agreemen<sup>t</sup> with swelling studies, in which it was shown that both aerogels needed approximately 2 h to reach their maximal swelling ratio. In this case, this behavior of aerogels is highly desirable since the absorption of curcumin occurs in the intestine [14]; hence, there is no need for its release in the stomach.

**Figure 10.** In-vitro curcumin release studies: 2 h in SGF followed by 22 h in SIF, 24 h overall.

When transferred to SIF, the release of curcumin from pectin aerogels is completely different from that of -chitosan-coated pectin aerogels. Pectin aerogels release all of their curcumin after just 1 h in SIF, of 3 h overall.

If the desired release is controlled release, which it is in most cases, then the release from pectin aerogels has to be slowed down, but not too much. Controlling drug diffusion from the dosage form is an excellent approach to maintaining therapeutic levels of the drug in the body. Controlled drug release in this particular case is important because of the poor adsorption of curcumin by the human body. It is crucial that a controlled amount of the drug reaches the body so that as much as possible can be adsorbed. If a large amount of the drug reaches the body at once, much of it will pass through without any adsorption at all.

Chitosan-coated pectin aerogels demonstrated slower release of curcumin, prolonged up to 24 h. As seen from Figure 10, curcumin was released for 24 h, leaving enough time for the substance to be adsorbed and metabolized by the body.

Lastly, curcumin powder was shown to be practically insoluble in body fluids, showing almost no dissolution after 24 h.
