Synthetic solution for biominerals

Materials World magazine
,
1 Nov 2011
Image courtesy of the University of Leeds

Synthetic crystals that mimic naturally occurring biominerals such as sea urchin spines are being fabricated in the UK. The results would assist ceramic production, and the creation of larger, less brittle structures for electronic devices.

A key feature of biominerals is that they are composite materials made from an inorganic mineral such as calcium carbonate, which contains a small amount of organic material, usually a protein. The resulting structures are incredibly hard and their mechanical properties can rival those of man-made materials such as ceramics, which are typically manufactured under high temperatures and pressures, giving less opportunity for control over the material properties.

Professor Stephen Eichhorn at Exeter University, UK, a coauthor on the work, explains that understanding how biology is able to perform such precise engineering in water at ambient temperatures is key to applying this principle to the design and production of ‘greener’ synthetic materials.

To form the synthetic mineral, the team at the University of Leeds, UK, led by Professor Fiona Meldrum, used a diblock copolymer to form small spherical micelles about 20nm in diameter. The micelles are coated with a corona of negatively charged polymer chains, which assist with their occlusion in the calcite crystals.

Meldrum adds, ‘These copolymer micelles are in the same size regime as intracrystalline proteins, and therefore act as “pseudo-proteins”. Precipitation of calcite crystals in the presence of these nanoparticles leads to their incorporation in the architecture of the crystal, creating a composite material with enhanced properties.’

The researchers also tested the mechanical properties using a nanoindenter, a small chisel-like tool that can prod a material and record its response to a force, and found fracture resistance to be improved.

Eichhorn adds, ‘We found that the artificial biomineral we have created is actually much harder than the pure calcite mineral because it is a composite material. Biological examples of calcium carbonate-based structures have a higher hardness than pure mineral without proteins present. It is remarkable that we have been able to achieve the same result using a synthetic protein.’

However, an industrial scale process is still some way off, ‘Currently we can only produce a very small amount of this material, so we need to look at scaling up the chemistry. It’s a huge advance, but the work is ongoing.’

The next phase will look at replicating this method in other ceramics such as clay to improve toughness. Another area of investigation, notes Eichhorn, will involve looking at melding the crystals with natural materials such as cellulose to study their potential mechanical performance.