Pelotherapy and electrotherapy are therapeutic methodologies with proven success in physical medicine and rehabilitation (PMR) and dermatology fields. The main purpose of these therapeutic modalities is to reduce pain, accelerate wound healing, alleviate muscle spasms, and improve mobility, and muscle tone. Their main challenge is in the passage of some ionic species through the skin barrier. The use of drugs, such as diclofenac, corticosteroids or steroids, has gained widespread efficacy recognition in physical therapy and the therapeutic action of these drugs is widely studied in experimental and clinical trials. Unlike pharmaceutical and cosmetic clays, peloids are not subject to any prior quality control or subject to any specific European regulation. The dermal absorption values are an integral part of the risk assessment process for peloids. This work explores the converging points between these two transdermal drug delivery systems (TDDS) and the presentation of methodologies to achieve peloid safety compliance, especially concerning the potential and degree of toxicity arising from ion exchange and trace elements. TDDS is applied to the pharmaceuticals industry and drug is the generic term for the active substances released into skin tissues. The transdermal delivery of drugs or clay components with therapeutic properties is limited due to the excellent barrier function of the stratum corneum. The transdermal drug delivery of pelotherapy is enhanced by temperature and electrically by iontophoresis. The low voltage of iontophoresis and sweat phenomena with pore dilation driven by pelotherapy allows the use of the same pathways: hair follicles and sweat pore. The therapeutic integration of iontophoresis and pelotherapy focused on patient benefits and low safety-related risk may contribute to the outstanding physiological performance of pelotherapy, specifically, in the way the essential elements and exchange cations pass through the skin barrier. The validation of an innovative iontophoretic systems applied to pelotherapy can also promote future challenges in the obtaining of the ideal therapeutic control of peloids and the clinical validation of results with physiological efficacy recognition.

Clays are natural ingredients used to prepare therapeutic cataplasms suitable for topical application. The knowledge about these formulations and their preparations to be applied on humans and animals has been orally transmitted since ancient times. Several empirical methods using clays have demonstrated fast and effective results in the reduction of the inflammatory response and the formation of edemas in horse limbs. The use of traditional and alternative medicine, such as pelotherapy, is now becoming more popular in veterinarian medical practice, alone or combined with other therapies in horse muscle and tendon rehabilitation. This study characterizes the use of commercial equine clays and an old therapeutic clay cataplasm formulation, using acetic acid, to treat tendon injuries in horses. This work might contribute to a major database characterization of clays used empirically on equine health, the potential of dermal absorption, the risks of exposure to some toxic elements, and safety assessment for these formulations. The present study was carried out to characterize the suitability of four commercial equine clays (Group II) and a protocoled healing mixture: “clay acetic acid cataplasm”, (Group III), to treat tendon injuries in horses. In this mixture, three conventional “green” clays (Group I) without any mineralogical specificity were used and blended with acetic acid. The mineralogical composition was determined through X-ray powder diffraction and X-ray fluorescence data. To determine the performance of the samples, cooling kinetics, oil absorption, expandability, and specific surface area were measured. According to the mineralogical composition, Group I was mainly composed of carbonates and silicates, while Group II was much richer in silicates with the main clay minerals kaolinite and illite. Group II exhibited the highest values for As, Pb, Cr, Ni, and Zn, considered potentially toxic. Both groups showed low cation exchange capacities and exchanged mainly Ca2+, with the exception of VET.1 and VET.7, which also highlight Na+, and VET.5 and VET.6, which have K+ as an exchangeable main cation. The addition of acetic acid (Group III) does not reveal any significant chemical changes. The results confirm that both clay groups are adequate for the therapeutic propose. They have good plastic properties (skin adherence), good oil absorptive capabilities (cleaning), and exchange an essential physiological element, calcium. Group II has prior industrial preparation, which is probably why it showed better results. Group I presented lower heat retention capacity and higher abrasiveness, which could be improved using cosmetic additives. The clinical benefit of the “clay acetic acid cataplasm” (Group III) could be the systemic anti-inflammatory effect established by the acetic acid.

The establishment of quality requirements of clay-based products, for medicinal, wellness, and aesthetic purposes, is mainly sustained by the good interactions between the clay-based formulation and the skin. The release of ionizable elements and their availability to percutaneous absorption should be, ideally, physiologically effective during passive percutaneous absorption. Clay-based products are promoted in the European market as therapeutic clays or aesthetics, which is labeling that combines characteristics of medicinal products along with cosmetics. Different countries regulate these products under different legal frameworks. This study focuses on the mineralogical, chemical, and technological characterization of some clay-based products available on the market, designed for topical use, framed in the peloids concept, and claimed as natural products. The main goals are to contribute to the establishment of clay-based products quality criteria as reliable scientific information, aiming for the compliance of intended use, the information for the potential health hazards and toxicological effects of clay-based products, and the distinction in what concerns therapeutic compliance and aesthetic or wellbeing product certification. There were 13 clayed products for cosmetic purposes, available online and in commercial stores, together with three thermal peloids, that were studied. Mineralogical composition of the 16 studied samples reveals a polymineralic association with the presence of variable quantities of quartz, calcite, and feldspars, whereas clay minerals are not predominant and characterized by the presence of clay-based fraction content, composed mainly by illite, smectite, and kaolinite in variable amounts and with several mineral associations. The clay-based products contain median values of 17 ppm As, 315 ppm Ba, 79 ppm Cr, 11 ppm Co, 29 ppm Pb, 26 ppm Ni, and 62 ppm Zn. One sample presented 4.1 ppm of Cd. The studied samples have safety concerns about specific limits of As, Ba, Cd, Cr, Co, Pb, Ni, and Zn which are above the regulated avoidable limits. Samples’ pH is out of range of skin’s natural pH as well.

The risks associated with the use of peloids in thermal centers, spas, or at home, need to be tested to develop appropriate safety guidelines for peloids formulations and the release of high concern substances. Also, the beneficial effect of some elements on human health should be assessed, to assist in the interpretation of therapeutic action and effectiveness on dermatological or osteomuscular disorders, using pelotherapy. Hence, a methodology was developed to better understand the biogeochemical behavior of the elements in formulated peloids. Two peloids were formulated with the same clay and two different sulfurous mineral-medicinal waters, for 90 days, with light and stirring every 15 days. The clay used was bentonite, with high content of smectite, with Ca and Mg as the main exchangeable cations and with high heat capacity. The selected mineral-medicinal waters were collected from two Portuguese thermal centers, with recognized therapeutic efficacy for rheumatic, respiratory and dermatological pathologies. The peloids were used without drying, withdrawn directly of the maturation tank and a mixture of bentonite with demineralized water was prepared as a reference sample. A stabilized, ready-to-use, artificial perspiration test was used to simulate the peloids’ interaction with skin. Thirty-one elements extracted from the two prepared peloids were analyzed by ICP-MS. The data were analyzed and related to the mineralogical composition of the original clay and supernatant composition of the maturation tanks. The content of some potentially toxic elements and metals bioaccessibility by perspiration showed very low solubility and undetectable amounts extracted from the studied samples. This analytical method provided some reliable information on dermal exposure and the identification of some elements that may enter the systemic circulation and for which surveillance and control measures should be implemented.

The risks associated with the use of peloids in thermal centers, spas, or at home, must be tested to develop appropriate safety guidelines for peloids formulations and the release of substances of high concern. Additionally, the beneficial effects of some elements on human health should be assessed to aid in interpreting the therapeutic action and effectiveness of pelotherapy on dermatological or osteomuscular disorders. Therefore, a methodology was developed to better understand the biogeochemical behavior of the elements in formulated peloids. Two peloids were formulated with the same clay and two different sulfurous mineral-medicinal waters for 90 days, with light stirring every 15 days. Bentonite clay, with a high content of smectite and Ca and Mg as the main exchangeable cations, and high heat capacity, was used. The selected mineral-medicinal waters were collected from two Portuguese thermal centers with recognized therapeutic efficacy for rheumatic, respiratory and dermatological pathologies. The peloids were used without drying and withdrawn directly from the maturation tank, and a mixture of bentonite and demineralized water was prepared as a reference sample. A stabilized, ready-to-use, artificial perspiration test was used to simulate the peloids’ interaction with skin. Thirty-one elements extracted from the two prepared peloids were analyzed using ICP-MS. The data were analyzed and related to the mineralogical composition of the original clay and supernatant composition of the maturation tanks. The content of some potentially toxic elements and metals’ bioaccessibility by perspiration showed very low solubility and undetectable amounts extracted from the studied samples. This analytical method provided reliable information on dermal exposure and the identification of some elements that may enter the systemic circulation, requiring implementation of surveillance and control measures. Graphical abstract