**5. Conclusions**

The present study deals with the in vitro release and mobility of potentially bioactive elements present in semisolid gel-like formulations obtained by mixing sepiolite and palygorskite with a natural spring water. Hydrogels were subjected to in vitro Franz cell tests and the elements released were analyzed by inductively coupled plasma techniques. Then, the element release and mobility were compared with in vitro biocompatibility tests of the very same formulation. The results demonstrated that, unlike other formulations, the potential therapeutic activity of nanoclay/spring water hydrogels should be studied in depth and characterized.

Clay/spring water hydrogels are "living formulations" since their ingredients constantly interact with each other, changing the properties of the system. For instance, the presence of an element in high concentration does not mean it would be released in high amounts. Moreover, the high release of bioactive elements is not a *sine qua non* to obtain maximum therapeutic effect. In fact, the ALIG30@20 hydrogel, with lower elemental mobility, not only proved to be biocompatible, but to exert potential proliferative effects over fibroblast cultures. According to the present in vitro release studies, it is possible to state that the ratios of the elements released play a significant role in the final therapeutic activity of the formulation. Moreover, the importance of formulative studies is again highlighted, since it is the optimal combination of the correct ingredients that makes a formulation effective.

As a general conclusion, the present study demonstrates that synergistic effects can be achieved from the formulation of the liquid phase in a semisolid system, in which elemental composition of the solid phase and structure of the system will determine elements' mobility and, ultimately, the therapeutic effects.

**Author Contributions:** Data curation, F.G.-V.; Funding acquisition, P.C., G.S. and C.V.; Methodology, F.G.-V., R.S.-E. and G.S.; Supervision, C.V.; Writing—original draft, F.G.-V.; Writing—review & editing, A.B.-S., P.C., L.C. and C.V All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ministerio de Ciencia e Innovación, CGL2016–80833-R; Consejería de Economía, Innovación, Ciencia y Empleo, Junta de Andalucía, P18-RT-3786 and Ministerio de Educación, Cultura y Deporte, who awarded a predoctoral grant (FPU15/01577).

**Acknowledgments:** This project was supported by the Spanish research group CTS-946. Technical support was provided by the CIC (Centro de Instrumentación Científica, University of Granada) and the IACT (Instituto Andaluz de Ciencias de la Tierra, CSIS-UGR). Special thanks to TOLSA group (Madrid), who kindly gifted clay minerals samples, and Alicún de las Torres thermal station, who provided spring water samples for the study.

**Conflicts of Interest:** The authors declare no conflict of interest.
