It has long been clear that collagen, calcium phosphate (apatite) and proteins are the building blocks of bone. It was generally believed that while the collagen merely formed a scaffold, the highly acidic proteins controlled the formation of phosphate nanocrystals. However, thanks to researchers based at Eindhoven University of Technology and the University of Illinois, the puzzle of how bone grows has been resolved [Nudelman et al., Nat. Mater. doi:10.1038/nmat2875 (2010)].
However, such a proclamation glosses over two major technological stepping stones that were achieved by Dr. Nico Sommerdijk (Eindhoven Laboratory of Materials) and co-workers. Firstly, the teams have successfully grown bone in the laboratory, analogous to the formation of bone in the body; and secondly, they have managed to accurately document the process by combining TEM with a rapid freezing process.
The groups have now demonstrated that far from being an inactive template, the collagen actually guides the mineralization of the phosphate. Sommerdijk used the process of collagen mineralization as first described by Dr Laurie B. Gower from the University of Florida, and used her method to analyze the role of the collagen. By rapidly cooling the samples, by “shooting the grid into liquid ethane”, it was possible to halt the activity within the material and study the sample at distinct stages. While mineralization of the phosphate initially started outside of the collagen fibrils (thin fibers on the nanometre scale), after 72 hours many large crystals had formed within the fibrils.
By staining the fibrils the teams were able to reveal how the phosphates enter the fibers. They found that the phosphates enter through the region with the lowest electrostatic potential energy. Thus the scaffold is hardly passive, but helps the phosphate enter the collagen through an electrostatic interaction. As the crystal forms, the fibril also helps orientate the apatite, without the influence of the proteins.
Sommerdijk believes these processes are similar for many minerals, and is now hoping to apply the same procedures to magnetite. As well as being a naturally occurring mineral, magnetite is also produced by magnetotactic bacteria. In speaking to Materials Today, Sommerdijk explains that the bacteria “precisely control the size, shape and alignment of these crystals” and that “the magnetic properties of magnetite strongly depend on the size and shape of the crystals”. “By using organic macromolecules we hope to control the size and shape of magnetite and thereby control its properties”.
Sommerdijk says the ultimate goal is to find a “generic basis by which organic materials can control mineral formation”. “We are looking for the language by which the organic components and the mineral talk to each other.”
Stewart Bland