New techniques for manipulating matter at very small scales may soon lead to remarkable new materials, including a glue to fuse broken bones, according to a recent guest speaker at U of T.
Professor Markus Antonietti spoke in August about how “little helper” molecules are giving materials engineers unprecedented control of the assembly of structures from the molecular scale on up.
Antonietti heads the Colloids department of the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany. He set a record as the youngest person ever to head one of the elite Max Planck institutes with his appointment at the age of thirty-three.
The challenge facing materials engineers is that the critical length for determining a designer material’s properties falls between the cracks of existing techniques. Traditional chemistry allows scientists to synthesize basic molecular building blocks, but doesn’t let them control the way the molecules assemble into larger structures that are tens, hundreds, or thousands of nanometers wide. (A nanometer is one billionth of a meter.)
On the other hand, the 10 to 100 nanometer scale is too small for “top down” techniques, like laser etching, to be of use.
Antonietti described a new technique that uses polymers, which are long chain-like molecules, as “little helpers” to give scientists control over this size scale. It’s a technique taken from the natural world. Nature excels at producing very different materials from the same chemical building blocks: for example, both hair and fingernails are made of the protein keratin.
To demonstrate the power of “little helper” molecules, Antonietti discussed the search for a material suitable for gluing broken bones together, a quest that may now be nearing fruition. In the human body, cells called osteoplasts make bones from a mineral called hydroxy apatite. These same cells are responsible for reconstructing bones when they break.
“The bone is a living object,” Antonietti said. “If you glue it with some epoxy, the osteoplasts cannot fuse the bone and the bone will re-fracture.”
Chemists can produce hydroxy apatite in the laboratory. So why not use synthetic hydroxy apatite to glue together broken bones?
“The funny thing is,” Antonietti said, “synthetic hydroxy apatite cannot be reconstructed by the osteoplasts.”
The reason is that the synthetic hydroxy apatite comes in unnaturally large clumps that are too tough for the osteoplasts to break down for use in reconstruction. It’s the right basic substance, but the way it’s put together on the 10 to 100 nanometer scale makes it unusable.
But chemists have recently persuaded hydroxy apatite to form a different, more edible shape by using little helper molecules.
The helper molecules can tell the difference between the various faces of growing crystals, binding to some, but not to others. Growth continues on the unbound faces, but is blocked where the helper molecules have attached. By using the right helper molecule, chemists can fine-tune the crystal’s final shape.
“This is why I say those molecules are little helpers,” Antonietti said.
“They are architects—nanoarchitects. They run into the game and they have a set of complete instructions with them [for the final shape of the crystal].”
Using helper molecules, chemists can now make hydroxy apatite form long, thin, “nanowhiskers.” This form is readily broken down by the osteoplasts, making it perfect for use as bone glue.
