*4.2. Mobility of Elements*

The sole presence of an element or chemical compound in a formulation does not mean that it would exert its therapeutic effect: it also needs to be released and be able to reach the active site. Moreover, the release process can be determined by different factors, one of them being its location in the formulation (clay structure or the spring water) or the age of the system [36,91]. Element mobility is a normalized parameter that allows comparisons between released levels of different elements. It can be calculated as the ratio between total concentration in the formulation and the released concentration. Mobility values of elements in ALIPS9 and ALIG30@20 hydrogels are plotted in Figure 3. In this figure, the delimited areas within the graphic were defined in a speculative manner. As can be seen from the dispersion (Figure 3), the majority of the elements showed a mobility lower than 2%.

**Figure 3.** Percentage of mobility (logarithmic scale) versus total content of the element in ALIPS9, ALIG30@10 and ALIG30@20 hydrogels (ppm). "High", "Medium" and "Low mobility" areas are hypothetical. Non-detected elements (mobility = 0%) do not appear in the logarithmic scale.

Even if Ca, S and Mg were present in remarkable amounts in the studied formulations, their released levels were very low in proportion, thus giving rise to low element mobility. This result demonstrates that, despite the spring water having remarkable amounts of these elements, their mobility is probably limited by the presence of the solid phase. Consequently, the solid and the liquid phases of the formulations establish a very close interaction that affects the final performance of the system, something that highlights the necessity to fully characterize this kind of formulation. Another visible result is the higher mobility of elements in ALIG30@10 with respect to ALIG30@20 and ALIPS9, which also demonstrates that the type and the concentration of the clay mineral exert a remarkable influence. Elements in the "medium mobility" area (Figure 3) were located in this section since they have low mobility (<1%) together with low concentration (<150 ppm) in the final formulation.

In view of the mobility results, K, Na, B and Al are the elements with the highest mobility. They showed relatively low amounts in the hydrogels but their mobility was clearly significantly higher with respect to the rest of the elements. We hypothesized that the high mobility of the aforementioned elements could be related to both the hydrophilicity of cations (previously mentioned in Section 4.1) and to a small/absent interaction between the pristine ingredients and, therefore, the released levels ascribed to the influence of the liquid phase (ALI) more than to the solid phase. That is, even if K, Na and B were not the main major elements in the pristine ingredients, the low interaction between K, Na and B (coming from ALI) with fibrous clay structure let these elements be relatively "free" within the system and, therefore, more prone to move. This hypothesis is confirmed by the fact that the mobility of elements in ALIG30@10 is higher than in ALIG30@20, due to the lower amount of G30 in the former. In this formulation, the reduced amount of clay mineral implies less retention of the elements and, therefore, higher mobility.

Spider diagrams represent more clearly the different mobility of elements between the same hydrogels at 48 h and 1 month (Figure 4). This comparison reveals that nanoclay/spring water hydrogels are "living formulations" since their ingredients constantly interact with each other, changing the final properties of the system. The area of ALIG30@10 (48 h and 1 month) is higher than the area of ALIPS9 and ALIG30@20, which is in agreement with the previous mobility results (Figure 4). The "liveliness" of the hydrogels can be ascribed to the different elemental equilibriums established between the solid and liquid phases in the formulation (adsorption and desorption equilibriums). Upholding this hypothesis, the solid phase mainly influenced the time-mobility of Cu, Mn, Ga, Al, B, and Fe, either increasing or reducing the corresponding mobility, depending on each particular case.

**Figure 4.** Spider diagrams of element mobility. (**A**) ALIG30@10; (**B**) ALIPS9 (**C**) ALIG30@20. For simplicity, the scale of the diagrams has been represented independently in (**D**).

The reduction of some elements' mobility with time (for instance B, Mg, Al, Zn, Mn, and Na) could also be explained by the stabilization of the system, and the clay better adsorbing/retaining these elements as time passes. In fact, clay minerals have been widely used for decontamination purposes due to their remarkable adsorptive properties [92–95]. Moreover, rheological changes have also been detected is these samples. A different structure of the system network could modify the mobility of certain elements and vice versa [91]. As can be seen in Appendix A (Figure A1), both ALIG30@20 and ALIPS9 suffered rheological changes within one month. Moreover, it is also possible from these results to hypothesize that the rheological performance of the system could also be influencing the element mobility. ALIPS9 and ALIG30@20, having a much more structured internal network (Figure A1), could hinder the mobility of elements that will find a more intricate path to travel towards the exterior. On the

other hand, ALIG30@10 was shown to have a less structured gel network (see García-Villén et al. [64] for information on the rheology of ALIG30@10).
