Researchers supported by the NIDCR report that they have harnessed the unique physics of seawater as it freezes to guide the production of what could be a new generation of more biocompatible materials for artificial bone. As published in the journal Science, the researchers used this novel technique to produce a thinly layered composite, or hybrid, structure that more closely mimics the natural scaffolding of bone. They said their initial, proof-of-principle scaffolds are desirably ultra-lightweight and up to 4 times stronger than current porous ceramic implant materials. The still-nameless freezing technique, with further technical refinements, could churn out even stronger materials and could be scaled up to fabricate larger structures such as replacement hips and knees and a variety of dental materials. The freezing technique builds on two longstanding research challenges in orthopedics and tissue engineering: the need for better, more biocompatible materials to serve as artificial bone, and learning how to make porous scaffolds for bone regeneration with enough strength for load-bearing applications. Nature does this in large part by building bone at the nanoscale. In an effort to learn how people can make composite materials on the same microscale as nature, researchers arrived at a possible solution via 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. In the lab, the scientists discovered that the forming ice crystals would pump out virtually any extraneous material, including various ceramics, the building blocks of many composite structures. If they sublimated the ice and removed the water, what remained was plates of hydroxyapatite, a ceramic biomaterial commonly used to make artificial bone. The faster the water was frozen, the thinner the plates would be. Using a freeze-casting machine that enables a ceramic structure to be fabricated into complex shapes, layers of approximately 1 µm have been achieved, which is almost down to the level that nature makes its composites. Researchers are now working to refine the freezing process and build larger structures, hopefully one day advancing to the design of a hip implant.
(Source: NIDCR Web site, New Releases, accessed June 14, 2006)
