Fig. 1: (left) Optical micrograph of calcium carbonate grown in an oriented chitin film, (right) Scanning microscopic image of oriented calcium carbonate rod formed in the chitin matrix, and (inset) Selected-area electron diffraction pattern corresponding to the thin section of the rod.

In nature, the organic and inorganic components of biominerals can be ordered using macromolecules such as proteins and polysaccharides. For the formation of some structures, like the silkworm cocoon, liquid crystals are believed to control the materials’ architecture by having a templating influence. In synthetic organic-inorganic hybrids, however, achieving similar control over morphology and orientation has proved challenging.

Now, Takashi Kato and co-workers at the University of Tokyo1 have used this bio-inspired knowledge to make polymer-calcium carbonate materials in which the molecular and crystalline order were controlled by an underlying liquid crystalline film. The synthetic hybrids were composed of oriented, rod-shaped calcium carbonate crystals in a polymer matrix with molecular order. Crystals about 80 microns in length and with a diameter of between 10 to 30 microns were formed over several days.

The liquid crystal film was composed of stretched chains of the polysaccharide, chitin, which are oriented on the macroscopic scale. On addition of calcium carbonate and in the presence of poly(acrylic acid), nanocrystals, in the form of the polymorph calcite, grow. The elongated form of the stretched polymer chains in the liquid crystal is critical—in the absence of this orientation, rod-shaped calcium carbonate crystals did not form. The carbonyl groups on the poly(acrylic acid) are thought to align along the chitin backbone and act as a template for the unidirectional growth of the crystals.

The Tokyo group believe that this method could be extended to liquid crystal polymers other than the nematic chitin derivative used in this materials synthesis. “We should be able to use cholesteric, smectic and columnar liquid crystal polymers as templates for new hybrid structures,” says Kato. “As a result, it may be possible to tune the chemical structures of the polymers for the crystallization of a wide range of functional inorganic materials.”

Kato and his colleagues envisage that this environmentally-friendly combination of polysaccharides and aligned crystals of calcium carbonate could be used to make soft, flexible, biofunctional materials with high mechanical stability and novel optical properties.