Human bones and teeth have complex and highly ordered structures made up of oriented hydroxyapatite nanocrystals impregnated in an organic matrix. These nanocomposites have excellent mechanical properties thanks to the toughness of the inorganic material and the flexibility of the organic matrix.

Researchers are looking for ways to synthesize artificial materials that mimic natural bones and teeth, but the problem is that it is quite difficult to grow large-sized highly ordered hydroxyapatite nanostructures, especially those bigger than 100 microns. What is more, the ordered nanostructures may become damaged when they are removed from the hard templates or substrates employed when fabricating them.

Flexible fire-resistant, ordered hydroxyapatite nanoarchitectures

Now, a team led by Ying-Jie Zhu has succeeded in producing large quantities of highly flexible, hydroxyapatite nanowires that are more than 100 microns in length and that have aspect ratios of more than 10000. The researchers have also developed a way to make these nanowires self-assemble into highly ordered structures by simply injecting the as-prepared solvothermal product slurry containing the nanowires into absolute ethanol or other alcohols at room temperature. They successfully constructed flexible fire-resistant, ordered hydroxyapatite nanoarchitectures too, such as high-strength nanoropes, highly flexible textiles and 3D-printed well defined patterns, using arrays of self-assembled, highly-ordered nanowires.

“We have also demonstrated a strategy for the automated, rapid production of large-sized, highly ordered arrays of ultralong hydroxyapatite nanowires,” says Zhu. “These architectures may be engineered into advanced functional products for applications in various fields – for example, fireproof clothing, fire shielding and heat insulation, but also in wound dressings and for repairing bone fractures and defects.

A new kind of highly flexible fire-resistant inorganic paper

“Usually, such nanoropes and textiles are made from organic plant fibres, which are flammable. The ultralong nanowires and the highly ordered derivatives we made are fire-resistant because they are made from hydroxyapatite, which is non-flammable.”

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.6b07239, says that it is now busy trying to automate and scale up the production of its ultralong hydroxyapatite nanowires and highly ordered nanowire architectures – hopefully to the industrial scale. “We will also be looking at how to use them in the applications mentioned above and making a new kind of highly flexible fire-resistant inorganic paper, which we previously reported in Chemistry - A European Journal DOI: 10.1002/chem.201304439 in 2014,” Zhu tells