**1. Introduction**

Inorganic hydrogels formulated with spring waters and nanoclays are successfully used in the treatment of musculoskeletal disorders and skin affections. There is a general agreement that their therapeutic activity against musculoskeletal disorders is achieved through physical mechanisms such as thermic activity, osmotic pressure and electric conductivity [1–5]. On the other hand, the underlying mechanism of action responsible for the therapeutic skin effects are usually ascribed to the chemical composition of the formulation [1,6–10], although the exact therapeutic activities and mechanisms of action are still unknown.

Several dermatological affections have been successfully treated by formulations that include clay minerals [4,11–14]. Currently, special attention is being paid to wound healing treatments, in which clay minerals have been demonstrated to be very useful [15–18]. During administration of the formulation, elements from the hydrogel could permeate and/or penetrate across the skin barrier. In a previous

study, hydrogels prepared with two different fibrous nanoclays were shown to be fully biocompatible and to exert in vitro wound healing activity [17]. More particularly, it was demonstrated that the fibrous nanoclay hydrogels promoted in vitro fibroblast mobility during wound healing processes.

It is well known that adequate concentrations of certain elements, including Ca, Mg, Na and K, in the wound bed are important for enhancing the healing process [19–27]. Transition metals such as Cu, Zn, Mn, Fe, Ag, and Au (among others) have also been demonstrated to play different biological functions in tissue regeneration, as reviewed by Yang et al. [28]. It has also been demonstrated that Zn:Ca ratios reach their maximum during the proliferative stage of wound healing and then decline during the remodeling stage [21]. Moreover, manganese-rich spring waters have been demonstrated to possess wound healing activity [29], and changes in Mg:Ca ratios are essential for a proper wound healing cascade. Consequently, formulations providing adequate bioavailability of elements with wound healing activity will promote the healing process and speed up restoration of the damaged area.

Based on these premises, the aim of this study was to assess the in vitro release and mobility of elements with potential wound healing effects from hydrogels formulated with spring waters and nanoclays that have recently been demonstrated to enhance fibroblast mobility [17]. In vitro Franz cell studies were performed in order to reproduce the topical administration of the formulations and elemental concentration was measured by inductively coupled plasma techniques. The results will be discussed on the basis of both the legal status of elements present in the formulation and their potential therapeutic effects.
