TEM image of halloysite (scale bar = 200 nm)
TEM image of halloysite (scale bar = 200 nm)

A new study on halloysite clay tubes has shown their potential as a safe natural biocompatible nanomaterial that is abundantly and cheaply available and therefore able to be scaled-up for production, and which also does no harm to the environment. Composite organic–inorganic halloysite nanotubes are known to have advantages over carbon or metal oxides nanotubes, and able to store and release functional agents such as antioxidants, anticorrosion agents, flame-retardant agents, drugs or proteins in a controllable way, offering promise in drug delivery, antimicrobial materials, self-healing polymeric composites, and also in regenerative medicine.

In new research published in Advanced Materials [Lvov et al. Adv. Mater. (2015) DOI: 10.1002/adma.201502341], a team from Kazan Federal University in Tatarstan and Beijing University of Chemical Technology, pioneers in the field of natural clay nanotube composites, demonstrated how the nanotubes form a kind of ceramic “skeleton” in the bulk polymers, improving the composite strength as well as adhesivity. These skeletons can be loaded with active compounds to offer extra functionality for polymers in the same way real bones are loaded with marrow.

“These clay nanotubes are a safe natural nanomaterial that does not harm the environment.”Yuri Lvov

Halloysite clay nanotubes have an inner diameter of 10–20 nm, outer diameter of 40–70 nm and a length of 500–1500 nm, with the inside being composed of Al2O3 and externally mainly SiO2. The inner lumen of halloysite can be altered by etching to 20–30% of the volume of the tube, and as a nanocontainer can be used for loading and sustained release of chemical agents. Halloysite tubes can encase enzymes to provide longer storage, higher temperature and more functionality, and the opening of the tube also allows for delivery of small substrate molecules into the tube interior for biocatalysis.

The team had first looked to formulate new paint composites with antifouling properties to prevent the attachment of shells and other sea micro-organisms to the body of ships, which slows them down. Since biocides would be washed off, they encapsulated them in clay nanotubes added to paint to provide very slow and sustained release. This breakthrough led to the concept of smart functional nano-composites that are doped with 4–5% of clay nanotubes loaded with the chemical inhibitors.

One problem is that the halloysite clay tubes are not biodegradable, due to there being no biological mechanisms that can degrade them in the body. Although they also can’t be injected intravenously, using them for external medical treatment based on the sustained release of encapsulated drugs offers much promise, so the team now intends to explore the potential for biocomposites in this way.