Using the physics of freezing seawater, researchers have developed a composite material that closely mimics the natural scaffolding of bone, according to a study published in the Jan. 27 issue of the journal Science.
"Our bones are made of organic and inorganic materials that individually aren’t very strong," said lead author Sylvain Deville, Ph.D., materials scientist, Lawrence Berkeley National Laboratory, Berkeley, Calif. "But when nature weaves them together at the nanoscale, the scaffold structure of bone is quite strong and durable. The question is: ‘How can people learn to make composite materials on the same microscale as nature?’ "
Dr. Deville and colleagues found a possible solution a few years ago while reading up on the physics of seawater. As an ice crystal forms in seawater, it pumps the salt, pollutants and other impurities out of the crystal and into the narrow channels of the forming ice layer. The impurities gather in the channels and remain trapped between the horizontal layers of ice.
The research team discovered that the forming ice crystals would pump out virtually any extraneous material, including various ceramics, the building blocks of many composite structures. According to co-author Eduardo Saiz, Ph.D., materials scientist, Lawrence Berkeley National Laboratory, when they sublimated the ice and removed the water, what remained were plates of hydroxyapatite, a ceramic biomaterial commonly used to make artificial bone.
"We found the faster we froze the water, the thinner the plates, or waferlike layers, would be," said senior author Antoni Tomsia, Ph.D., senior staff scientist, Lawrence Berkeley National Laboratory.
A freeze casting machine redesigned by the research team enabled a ceramic structure to be fabricated into complex shapes. "It took us about one year to go from layers of 100 microns down to about a micron," added Dr. Tomsia. "That is almost down to the level that nature makes its composites."
Although the composite material produced by the laboratory was small and cube-shaped, Dr. Tomsia said the research team is working to refine the freezing process and build larger structures. They hope one day to fabricate larger, stronger structures, such as replacement hips and knees and a variety of dental materials.
He stressed, however, that it would be impossible to put a time frame on when this might happen. "Nature has so much to teach us about making strong materials," he said. "Evolution occurred over millions of years, and nature does not make mistakes."
