In the search for new materials, no other source of inspiration compares with Mother Nature in terms of the sheer variety, complexity and range of possible uses for the synthetic imitations. But even with a perfect pattern, the nuances of “growing” a synthetic imitation present some real challenges. The benefit is that such new bio-inspired materials are environmentally friendly, both in terms of the materials themselves and the processes used to make them.

Researchers at Ames Laboratory have successfully developed synthetic magnetite using proteins isolated from simple bacteria that use the magnetic particles to help orient themselves with the Earth’s magnetic poles. More recently, they’ve looked at the composition of bone, with its wonderful strength-to-weight ratio and combination of rigidity and toughness, and identified the mechanism that controls the size of nanocrystals in bone structure.

The work is a highly collaborative effort of organic chemists, chemical engineers and molecular and micro biologists.

“Nature has all these organic phases that contribute to the growth of particular biological structures,” says Ames Laboratory senior chemist Surya Mallapragada. “What we try to do is understand the role of mineralization peptides and proteins and isolate them.  We’re then able to attach these compounds to polymer strands, which act as a template upon which the biomaterial can form.”

In the case of the bone research, Ames Lab chemist Klaus Schmidt-Rohr and his team used solid-state nuclear magnetic resonance (NMR) spectroscopy to investigate bone, which is actually an organic-inorganic

This diagram shows the effect of citrate concentration
on the size of hydroxyappatite crystals fabricated with
self-assembling block copolymer templates. Just as it
does with actual bone structure, as the concentration
of citrate increases, the thickness of the nanocrystals
decreases and the thinner nanocrystals appear to
make the bone more resistant to stress cracking.

nanocomposite whose stiffness is provided by thin nanocrystals of carbonated apatite, a calcium phosphate, imbedded in an organic matrix consisting mostly of collagen, a fibrous protein.

Furthermore, the small (about 3 nanometers) thickness of the apatite nanocrystals gives bone its favorable mechanical properties, likely preventing crack propagation.  A better understanding of the factors that control formation of these nanocrystals may help in the treatment and prevention of bone diseases, such as osteoporosis.

Using NMR spectroscopy, Schmidt-Rohr’s group found that surfaces of the apatite crystals are studded with citrate molecules that are a strongly bound, integral part of the nanocomposite. Because the citrate molecules are too large to be incorporated into the apatite crystal structure, the citrate remains on the surface where the researchers suggest it limits nanocrystal size by inhibiting the formation of additional phosphate layers. The group’s findings were published in the Dec. 28, 2010 issue of the Proceedings of the National Academy of Sciences.

To test this hypothesis, Mallapragada and Ames Lab faculty scientist Mufit Akinc prepared synthetic calcium phosphate nanocomposites using self-assembling polymers as templates with various concentrations of ammonium citrate. Using a battery of NMR and electron microscopy characterization techniques, the experiments showed that the average nanocrystal size decreases as more citrate is added.

“Bioinspired materials have advantages in that the processes take place normally at room temperature and require low energy inputs,” Mallapragada says. “As we learn more about the morphology and mechanisms of these mineralization proteins, we can make more complex materials.”

~ by Kerry Gibson