Clay is unique especially from the perspective of medical geology, that is, the impacts of geologic materials and geologic processes on animal and human health. Clay is the only natural material that can impact human health through all routes of exposure: ingestion, inhalation, and dermal contact. Moreover, these impacts can be harmful as well as beneficial. Ingestion of clay, a form of geophagy, has been practiced for millennia and is still widely practiced today. Humanoids have been ingesting clay for at least two million years to ease indigestion and counteract poisons. Some additional benefits may accrue from eating clays such as providing some nutrients but these benefits are far outweighed by the likely negative consequences such as tissue abrasion, intestinal blockage, anemia, exposure to pathogens and toxic trace elements, and potassium overdose. Inhalation of airborne minerals including clays has impacted the heath of millions. In the 1930s thousands of people living in the Dust Bowl in the U.S. southwest inhaled copious amounts of clay contributing to deadly ‘dust pneumonia.’ Using clay as a poultice to stem bleeding and cure certain skin ailments is an age-old practice that still has many adherents. A classic recent example of the antibacterial properties of clay is the use of certain clays to cure Buruli ulcer, a flesh eating disease. However, walking barefoot on clays in certain volcanic soils can result in non-filarial podoconiosis or elephantiasis. The absence of clays in soils can have serious health consequences. In South Africa, clay-poor soils yield crops lacking in essential nutrients and may be the principal cause of Msileni joint disease. Clearly, a detailed knowledge of the clays in the environment can have significant benefits to human health and wellbeing.

Antibacterial clays in nature include a variety of clay mineral assemblages that are capable of killing certain human pathogens. Although clays have been used for medicinal applications historically, only in the last decade have analytical methods and instrumentation been developed that allow researchers to evaluate the antibacterial mechanisms of various clays applied medicinally. Comparisons of the mineralogical and chemical compositions of natural clays that kill bacteria have promoted a better understanding of the mineral properties that are toxic to a broad-spectrum of human pathogens, including bacteria that have developed resistance to antibiotics. Popular literature is filled with reports of ‘healing’ clays, that, when tested against pathogens in vitro and compared to controls, do not appear to have bactericidal properties. It is important, however, to differentiate what properties make a clay ‘healing,’ versus what makes a clay ‘antibacterial.’ Most antibacterial clays identified to date buffer pH conditions of a hydrated clay outside the range of conditions in which human pathogens thrive (circum-neutral pH) and require oxidation reactions to occur. It is the change in oxidation state and pH imposed by the hydrated clay, applied topically, that leads to a chemical attack of the bacteria. Healing clays, on the other hand, may not kill bacteria but have soothing effects that are palliative. This article reviews some of the historical uses of clays in medicine but focuses primarily on the common characteristics of natural antibacterial clays and early studies of their antibacterial mechanisms. In this era of bacterial resistance to antibiotics, mimicking the antibacterial mechanisms exhibited by natural clays could be advantageous in the development of new antimicrobial agents.

Over the past several decades, clay minerals have been applied in various bio-fields such as drug and drug additives, animal medicine and feed additives, cosmetics, biosensors, etc. Among various research areas, however, the medical application of clay minerals is an emerging field not only in academia but also in industry. In particular, cationic and anionic clays have long been considered as drug delivery vehicles for developing advanced drug delivery systems (DDSs), which is the most important of the various research fields including new drugs and medicines, in vitro and in vivo diagnostics, implants, biocompatible materials, etc., in nanomedicine. These applications are obviously related to global issues such as improvements in welfare and quality of life with life expectancy increasing. Many scientists, therefore, in various disciplines, such as clay mineralogy, material chemistry, molecular biology, pharmacology, and medical science, have been endeavoring to find solutions to such global issues. One of the strategic approaches is probably to explore new drugs possessing intrinsic therapeutic effects or to develop advanced materials with theranostic functions. With this is mind, discussions of examples of cationic and anionic clays with bio- and medical applications based on nanomedicine are relevant. In this tutorial review, nanomedicine based on clay minerals are described in terms of synthetic strategies of clay nanohybrids, in vitro and in vivo toxicity, biocompatibility, oral and injectable medications, diagnostics, theranosis, etc.

Clays have traditionally been linked to health care, being used for centuries in the fight against infections and diseases. Similarly, biohybrids produced by combinations of clays and biological species through ‘bottom-up’ approaches have been evaluated over the past decade for biomedical and pharmaceutical uses. These biohybrids show interesting features such as biocompatibility and biodegradability which make them suitable for healthcare applications. The aim of the present communication was to review recent research contributions describing progress and the role of biohybrid materials based on clays in biomedicine and pharmacy disciplines. Emphasis will be on the authors’ own experience of this topic, particularly on aspects related to controlled drug-delivery systems, adjuvants of vaccines, and vectors for non-viral gene transfection. Bionanocomposites offer several advantages for use in the design of new and efficient pharmacological formulations for cutaneous and oral administration. In these systems, the drug is typically entrapped in the clay and protected by a biopolymer matrix, and both components contribute to a gradual release of the drug. Clay-based hybrids have also shown their efficacy in vaccines as they can act as nanocarriers of viral particles, due to the biomimetic interface created on the clay surface after adsorption of suitable biomolecules such as phospholipids, while the clay acts as an adjuvant to increase the efficacy of the vaccine. Finally, a new application of clays as non-viral vectors for controlled gene delivery is attracting increasing interest in the treatment of diverse diseases; clays such as sepiolite have demonstrated their ability to act as nanocarriers of nucleic acids and facilitate their transfection in mammalian cells.

Clays have played an important role in medicine since the dawn of mankind and are still applied widely as active ingredients and/or excipients in pharmaceutical formulations. Due to their outstanding properties of large retention capacity, swelling and rheological properties, and relative low cost, they have been used widely as advanced carriers for the efficient delivery of drugs by modifying their release (rate and/or time), increasing the stability of the drug, improving the dissolution profile of a drug, or enhancing their intestinal permeability. In addition, recent studies have shed new light on the potential of clay minerals in the nanomedicine field due to their biocompatibility, beneficial effects of clay nanoparticles on cellular adhesion, proliferation, and differentiation. Use as active ingredients and excipients are exerted via the oral and topical administration pathways. Skin drug delivery represents an attractive alternative to the oral route, providing local and/or systemic drug delivery. Due to their complex structures, however, most drugs penetrate the human skin only with difficulty. Enormous efforts have been invested, therefore, in developing advanced drug delivery systems able to overcome the skin barrier. Most strategies require the use of singular materials with new properties. In particular, and on the basis of their inherent properties, clay minerals are ideal candidates for the development of intelligent skin drug delivery systems. In this article, the properties of clay materials and their use in the skin-addressed pharmaceutical field are reviewed. A brief introduction of skin physiology and biopharmaceutical features of penetration by a drug through the skin layers is also included and is designed to shed light on the optimum properties of ideal nanosystems for advanced skin drug delivery. Special attention is devoted to the pharmacological functions of clays and their biomedical applications in pelotherapy, wound healing, regenerative medicine, antimicrobial, and dermocosmetics.

Tumor theranostics (a portmanteau of therapeutics and diagnostics) is now achieved in various ways with complex nanoparticle systems. Layered double hydroxide (LDH) nanoparticles are effective at drug/gene delivery and as imaging agents in potential tumor theranostics. This mini-review paper summarizes recent progress in developing LDH nanoparticles as a pH-sensitive magnetic resonance imaging (MRI) contrast agent, as a positron emission tomography (PET) imaging agent, and as a co-delivery platform for two therapeutic agents for tumor diagnosis and therapy. These results have indicated clearly the potential application of LDH nanoparticles for simultaneous diagnosis and treatment of cancers.

Geophagy is the intentional consumption of earth. Although widely documented among vulnerable populations, including children and pregnant women, the causes and consequences of geophagy remain poorly understood. Relevant literature was, therefore, reviewed to describe geophagy across species, geographies, life stages, and disease states. After a brief consideration of hypothesized etiologies, the potential harmful and beneficial consequences of geophagy are described, considering current evidence for each. Data available to date suggest that the greatest potential risks of geophagy include toxicity or heavy metal poisoning, and diseases resulting from consumed clays binding nutrients and beneficial pharmaceuticals in the gut. Evidence also suggests that geophagy may be beneficial by protecting against harmful pathogens and toxins through two distinct physiological pathways. Future research should explore causal relationships between geophagy and iron deficiency, as well as investigate the biological and psychosocial conditions that govern geophagy.

Clay minerals, such as layered double hydroxide (LDH) and montmorillonite (MMT), have attracted a great deal of attention for biological applications. Along with the rapid development of nanotechnology, public concern about the potential toxicity of nanoparticles is growing. In the present work, cytotoxicity of LDH and MMT was assessed in terms of inhibition of cell proliferation, generation of oxidative stress, and induction of inflammation response. Moreover, the biokinetics of LDH and MMT were evaluated; biokinetics provides information about in vivo absorption, distribution, and excretion kinetics. The results demonstrated that both LDH and MMT inhibited cell proliferation at relatively large concentrations and after long exposure time compared to other inorganic nanoparticles, although they generated reactive oxygen species (ROS). LDH induced pro-inflammatory cytokines in a size-dependent manner. Biokinetic study revealed that, after single-dose oral administration to mice, both LDH and MMT had extremely slow oral rates of absorption and did not accumulate in any specific organ. All the results suggest great potential of clay minerals for biological application at safe levels.

Aflatoxin contamination of diets results in disease and death in humans and animals. The objective of the present paper was to review the development of innovative enterosorption strategies for the detoxification of aflatoxins. NovaSil clay (NS) has been shown to decrease exposures to aflatoxins and prevent aflatoxicosis in a variety of animals when included in their diets. Results have shown that NS clay binds aflatoxins with high affinity and high capacity in the gastrointestinal tract, resulting in a notable reduction in the bioavailability of these toxins without interfering with the utilization of vitamins and other micronutrients. This strategy is already being utilized as a potential remedy for acute aflatoxicosis in animals and as a sustainable intervention via diet. Animal and human studies have confirmed the apparent safety of NS and refined NS clay (with uniform particle size). Studies in Ghanaians at high risk of aflatoxicosis have indicated that NS (at a dose level of 0.25% w/w) is effective at decreasing biomarkers of aflatoxin exposure and does not interfere with levels of serum vitamins A and E, iron, or zinc. A new spinoff of this strategy is the development and use of broad-acting sorbents for the mitigation of environmental chemicals and microbes during natural disasters and emergencies. In summary, enterosorption strategies/therapies based on NS clay are promising for the management of aflatoxins and as sustainable public health interventions. The NS clay remedy is novel, inexpensive, and easily disseminated.