2.2.4. Selection of Elements Under Study

A wide variety of elements were analyzed in this study. In order to organize and facilitate the interpretation of the results, the discussion will be centered around two main aspects: the potential wound healing activity of the elements and their legal situation regarding cosmetics and medicines regulations. The importance of the latter point lies in the fact that, depending on the final therapeutic activity of the present hydrogels, they could be considered as cosmetics or as medicines [31]. Elements will be classified and addressed according to the European regulations and guidelines summarized in Figure 1. The present study is focused on those elements that are considered "safe" or "non-hazardous". Additionally, elements without toxicity limits (most of the time not mentioned in the aforementioned regulations) were also included in this study.


**Figure 1.** Main regulations [31–33] and guidelines used for the selection of elements, interpretation and discussions of results, ordered by year of publication or latest update.

The guideline for elemental impurities Q3D(R1) [32] of the European Medicines Agency is focused on toxic elements and classifies them in three groups. In view of their limitations and toxicity, all of them with well-defined "permitted daily exposure" (PDE) limits, these elements are not addressed in this manuscript. Nonetheless, there is also a non-defined fourth group that includes elements with low inherent toxicity, without PDE limits. In conclusion, elements in this group should be controlled more for the quality of the final product than for high toxicity and safety considerations. Examples of these elements are Al, B, Ca, Fe, K, Mg, Mn, Na, W and Zn, which are the subject of study of this research. For simplicity throughout the manuscript, these elements are referred to as "class 4". The European Regulation EC 1223/2009 [33] was used to determine those elements whose presence is either allowed or not mentioned in cosmetic products.
