When a team of chemists fashioned a foam nose and filled it with cow cartilage, they weren't clowning around. The researchers made the porous proboscis to show that a new technique can better control minute holes in polymer foams, materials akin to Styrofoam. The improved foams could help grow replacement body parts, especially thick ones, like noses.
Tissue engineers hope to one day grow organs on polymer foam scaffoldings, much as topiary gardeners grow animal-shaped bushes on elaborate trellises. They often make these foams by mixing a polymer solution with salt pellets and dissolving away the salt once the polymer has hardened. So far the biggest advances have been with thin tissues such as skin, blood vessels, and bladders (Science, 16 April 1999, p. 422 and 19 November 1999, p. 1493), because foams with holes big enough to house living cells tend to come in thin sheets. The foam has to be thin, otherwise water can't reach all the pores to remove the salt. But in the 29 February Proceedings of the National Academy of Sciences, a team led by chemist Robert Langer of the Massachusetts Institute of Technology reports having developed a method to make thicker foams.
Langer's team first dissolves a polymer in a solvent. They then stir in beads of paraffin or beeswax, rather than salt, and pour the concoction into a mold. The researchers then dissolve the beads with a second solvent, such as pentane. Because the two solvents mix, the researchers can quickly extract both at the same time, causing the polymer to solidify as the beads are removed. And since the solvent for the beads can penetrate the polymer, it reaches into more remote pores, allowing the researchers to make thicker pieces of foam.
"What strikes me as innovative here is that you can make the foam almost instantaneously," says Edith Mathiowitz, a chemist at Brown University in Providence, Rhode Island, who studies such foams for delivering drug or gene therapy. But Debra Rolison, a chemist at the Naval Research Laboratory in Washington, D.C., notes that other materials scientists have accomplished similar feats. Still, the technique should be useful because it's simple and works well for large pores, she says.