2.2.2. In Vitro Release of Elements

Element mobility from ALIPS9, ALIG30@10 and ALIG30@20 was studied by in vitro release studies performed in Franz diffusion cells system (FDC40020FF, BioScientific Inc., Phoenix, AZ, USA) [30]. This system is purposely designed to reproduce dermal and/or mucosal administration conditions. The Franz diffusion cells possessed a contact area of 0.64 cm<sup>2</sup> and a total volume of 6.4 mL. Dialysis membranes (cut-off 12–14 kDa, 31.7 mm, Medicall International, London) were used to separate the donor and receptor chambers. The membranes were boiled in ultra-purified water (milli-Q water, ISO 3696) for 10 min in order to hydrate them. Over the membrane, in the donator chamber, known amounts of each hydrogel (approximately 0.025 g) were placed. The receptor chamber was filled with degassed, ultra-purified water. The whole system was maintained at a constant temperature of 32 ± 0.5 ◦C through thermostatic bath circulation. The experiment lasted for 30 min, which is the typical time of topical nanoclay/spring water hydrogels application. Experiments were performed in sextuplicate. At the end of the experiments, the aqueous content of the receptor chamber was carefully withdrawn and filtered through 0.45 μm single-use, syringe filters (Merck Millipore, Madrid, Spain). Finally, the elemental composition on each sample was assessed by ICP-OES. Element release tests were performed after 48 h and 1 month after hydrogel preparation, in order to study the evolution of the elemental mobility. Hydrogel batches were preserved in static conditions inside closed polyethylene containers, which were placed inside a drawer with an average mean temperature of 20 ± 5 ◦C. Blanks were also analyzed in order to monitor the elements coming from the materials and the ultra-purified water.
