Nature Suggests a Promising Strategy for Artificial Bone
Researchers supported by the National Institute of Dental and Craniofacial Research (NIDCR), part of the National Institutes of Health, report they have harnessed the unique physics of sea water 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 January 27 issue of 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. The scientists said their initial, proof-of-principle scaffolds are desirably ultra lightweight and up to four times stronger than current porous ceramic implant materials.
According to Dr. Antoni Tomsia, a scientist at Lawrence Berkeley National Laboratory in Berkeley, Calif. and senior author on the paper, 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.
He also noted that it easily could be adapted to make layered composites for variety of industrial purposes, ranging from airplane manufacturing to computer hardware. "Freezing is the engine that drives the production process," said Tomsia. "But the engine is undiscriminating in the composites or polymers that it fabricates."
The freezing technique reported this week builds on two longstanding research challenges in orthopedics and the related field of tissue engineering. The first is the need for better, more biocompatible materials to serve as artificial bone. Most current materials, such as metal, were originally developed for non-medical purposes and thus poorly match the natural architecture of bone and other tissues, sometimes triggering inflammation and chronic soreness in the joint.
The second challenge is to figure out how to make porous scaffolds for bone regeneration with enough strength for load bearing applications. Tomsia said strong, porous structures would allow cells to infiltrate into the implant, adhere to it, and more fully integrate with the synthetic material.
And therein lies a rub. "How do you make porous scaffolds strong?" asked Tomsia. "It's a contradiction in terms. It's like asking, how do you make Swiss cheese strong? But nature certainly does it all of the time."
Nature does it in large part by building bone at the nanoscale, the one-billionth of a micron world that scientists have begun to pursue in the emerging field of nanotechnology. "Our bones are made of organic and inorganic materials that individually aren't very strong," said Dr. Sylvain Deville, a member of Tomsia laboratory and lead author on the paper. "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 micro scale as nature?"
Deville said he and his colleagues arrived at a possible solution a few years ago while reading up on the physics of sea water. As an ice crystal forms in sea water, 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 scientists discovered in the laboratory that the forming ice crystals would pump out virtually any extraneous material, including various ceramics, the building blocks of many composite structures. According to Dr. Eduardo Saiz, an author on the paper and a member of the Tomsia laboratory, if they sublimated the ice and removed the water, "we found what remains are 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 wafer-like layers, would be," said Tomsia, whose laboratory redesigned a freeze casting machine to better control and accelerate the freezing process. A freeze casting machine enables 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," Tomsia added. "That is almost down to the level that nature makes its composites."
Although the laboratory's proof-of-principle composite was small and cube shaped, Tomsia said he and his colleagues are now working to refine the freezing process and build larger structures, hopefully one day advancing to the design of a hip implant. They stressed, however, that it would be impossible to put a time frame on when they might reach this point. "Nature has so much to teach us about making strong materials," said Tomsia. "Evolution occurred over millions of years, and nature does not make mistakes."
The article is titled, "Freezing as a Path to Build Complex Composites." It is published in the January 27, 2006 issue of Science. The authors are Sylvain Deville, Eduardo Saiz, Ravi K. Nalla, and Antoni P. Tomsia.