Biomedical applications promise growth for halloysite

By Kasia Patel
Published: Monday, 30 November 2015

Scientific research to open up new markets with cancer targeting issues being addressed, however additional studies are needed for internal use.

The development of halloysite for biomedical applications could prove to be a promising source of demand growth for the mineral. 

In a recent paper entitled "Halloysite Clay Nanotubes for Loading and Sustained Release of Functional Compounds", published in Advanced Materials, the authors of the report – Yuri Lvov and Rawil Frakhrullin of the Bionanotechnology Lab at Kazan Federal University, and Wencai Wang and Liqun Zhang from the State Key Laboratory of Organic-Inorganic Composites at the Beijing University of Chemical Technology – outlined a number of potential applications for the clay mineral which are currently in development.

According to the paper, halloysite is a good candidate for nanarchitectural composites as it is a natural biocompatible nanomaterial available in thousands of tonnes at a low price. 

"Obviously, purely engineering applications, such as polymer stabilisers or anticorrosion coatings are better candidates for the straightforward industrial use [of halloysite]," Frakhrullin told IM. "However, I hope that the biomedical applications are very promising and will be realised soon."

An aluminosilicate clay, halloysite is chemically identical to kaolin clay (Al2Si2O5(OH)4 x nH2O) with a layer of water molecules found between layers of alumina and silica. The mineral has a naturally occurring tubular structure and can be considered as rolled kaolin sheets with an inner diameter of 10-20 nm, an outer diameter of 40-70 nm and a length of 500-1500 nm. The internal side of the structure is composed of alumina, while the external side is silica. 

Halloysite is formed when kaolin sheets roll into tubes, caused by strain owing to a mismatch between the silicon dioxide and aluminium oxide layers, a process which occurs over millions of years. 


Halloysite nanotubes, shown here as an SEM
image, could see demand growth in biomedical
applications. (Source: Delphine Bruyere, licenced under CC BY-SA 3 via Wikimedia Commons) 

Delivering potential

According to a report from Grand View Research, growth in technical and advanced ceramics is likely to fuel demand for halloysite until at least 2020, with consumption dominated by high quality tableware made from porcelain and bone China.

While the report outlines that "the rare commercially exploitable occurrence of halloysite is likely to be a key restraining factor for the growth of the market in the near future", applications in nanotechnology are expected to provide growth paths for halloysite demand, owing to the mineral’s tube-like crystal form. 

"Halloysite is already used at about 70,000 tpa, mostly for advanced ceramics and porcelain, as enforcing plastic additives and for cosmetic formulation," Frakhrullin told IM. "Depending on the outcomes of the scientific research, these numbers might increase substantially."

The paper authored by Frakhrullin and his colleagues identifies key areas of research underway – including potential in antimicrobial, anti-aging, anticorrosion and flame retardancy applications – based on the potential of using the inner lumen of halloysite as a nanocontainer for loading and sustained release of chemical agents. 

Halloysite tubes are able to encase enzymes for longer storage, higher temperature and extended functionality, while DNA loading of halloysite is another potential application for the mineral.

"There are several papers which clearly demonstrate the application of halloysite for loading and subsequent release of DNA. This application might be another fascinating area of biomaterials, namely halloysite-based gene delivery," Frakhrullin said. 

Research has also been conducted into the use of halloysite to detect and eventually manipulate cancer cells. Cancer metastasis is when a tumour spreads to different parts of the body via circulating tumour cells, which are released into the bloodstream and flow to another part of the body where they stick to a surface, such as organs.

Halloysite’s benefit lies in that a layer of it applied to the created surface basically repels the numerous white blood cells that accumulate as the body’s immune system activates. In doing this, it greatly increases the purity of tumour cells in the sample, allowing circulating tumour cells to be targeted with ease.

Frakhrullin told IM that many popular nanomaterials, such as carbon nanotubes, graphene and metal nanoparticles have been used to create anticancer drug delivery vehicles and halloysite has a number of positive attributes that would lend it to the same application.

"Strong points of halloysite are its biocompatibility, high loading capacity, controllable release, easy suspension in aqueous isotonic solvents and good cellular uptake. As well as with other carriers, there are several drawbacks, but the scientists are trying to address them," he said. 

Halloysite degradation

Additional in vitro and in vivo studies referenced by Frakhrullin’s research indicate the safety of halloysite use in biological applications, showing that the mineral can be used for the controlled store and release of molecules – attractive attributes for drug delivery, antimicrobial materials, self-healing polymeric composites and regenerative medicine.

While the recently published paper notes that halloysite is not biodegradable as the human body lacks the biological mechanisms to degrade the mineral, this does not detract from its potential to be used for external medical treatment with the slow release of drugs, for example in creams, implants or wound treatment of tissues. 

"As for bioimplants, halloysite may provide mechanical stability to tissue engineering scaffolds and increase the adhesion of the cells. Several publications feature the use of halloysite in combination with a number of (bio) polymers as a raw material to produce prototype implants," Frakhrullin outlined.

However, Frakhrullin told IM that there was still much to learn about the possibility of halloysite use internally. "I want to underline that we do not know much about the possible pathways of degradation of halloysite in the body, but this does not mean that it cannot be degraded in certain conditions," he said. "For now, it is clear that we can envisage several applications where halloysite-based nanocontainers can be injected into the blood, by analogy with other inorganic nanosized carriers, but this remains to be studied in more detail."