New material responds to growing tissue. (Bone Fix).

Surgeons routinely harvest fragments from a healthy part of a patient's skeleton to repair wrecked bones elsewhere. This surgical step causes pain and expense that some researchers aim to eliminate by using a new bone-forming strategy that has shown promise in animal studies.

For years, scientists have been devising scaffold materials--both natural and synthetic--that encourage the growth of bone tissue. But the natural materials, primarily cow collagen, have raised worries of disease transmission. On the other hand, synthetic polymers can cause inflammation, and it's difficult to engineer the material to break down in synchrony with bone regrowth.

In the May Nature Biotechnology, Jeffrey Hubbell and his coworkers describe a material that could amount to a happy medium: scaffolding that's synthetic yet responds to the body's bone-making cues.

"We were trying to look at how nature evolved [bone growth] and copy that in synthetics," says Hubbell, a researcher at the Swiss Federal Institute of Technology in Zurich and the University of Zurich.

He and his colleagues built their scaffolding with star-shaped poly(ethylene glycol) molecules and added special properties. They adorned the scaffolding with synthetic peptides, or protein fragments, that help bone-forming cells called osteoprogenitor cells adhere to the framework. They also glued together the branched arms of the polymer with other peptides that are susceptible to enzymes secreted by the osteoprogenitor cells. In their three-dimensional mesh, the researchers trapped proteins called BMPs that stimulate the cells to begin bone regeneration.

When osteoprogenitor cells attach to a scaffold implanted at the injury site, they release enzymes that snip through the mesh, destroying the scaffolding. BMPs spill out just where and when they're needed.

Hubbell and his coworkers tested their scaffolding on rats from which they'd removed small circular sections of skull. Disks made of the scaffold material stimulated bone regeneration as well as cow collagen does.

The system is "a clever and unique approach to develop synthetic bone scaffolds that are cell-responsive," comments Kristi S. Anseth of the University of Colorado in Boulder. The work "has wide implications in numerous wound- healing and regenerative medicine applications," she adds.

In the future, Hubbell says, doctors could make the repair less invasive by applying the scaffolding as a liquid--perhaps through a syringe or a small incision--that hardens once it's in place.