Sep. 3, 2001 ITHACA, N.Y. -- Imagine a material that could expand by three to six times in size while remaining strong and stiff, and also could be biodegradable and biocompatible. Such a material would be invaluable as a wound-healing bandage or possibly a drug-delivery mechanism.
Think silk -- with a little help from the lab. Researchers at Cornell University say they have made significant advances toward a polymer of silkworm silk that both mimics and improves on nature.
According to Dotsevi Sogah, professor of chemistry and chemical biology at Cornell, "We have now created materials in the laboratory having properties that far exceed the natural system." Sogah's graduate student, Osman Rathore, discussed the latest results from the polymerization of peptide building blocks based on silkworm silk Monday (Aug. 27) at the national meeting of the American Chemical Society at McCormick Place, Chicago. His talk was titled "Novel biomolecular materials based on silkworm silk."
In the 1950s, Nobel laureate Linus Pauling deduced the basic structure of silkworm silk. Within the crystalline structure in silk there is a regular protein-folding pattern, induced by amino acids, in which the molecular chain falls back and forth on itself creating the material's elasticity. "We asked ourselves," says Sogah, "can we design materials that will have a similar property, although will not have all the amino acid sequences known in the protein? If we could do that, we could make the material in the lab."
Sogah's group in effect sidestepped the amino acids (there are 20 amino acids in nature, which exist in a great variety of permutations) by creating a molecular hybrid of natural and manmade structures. They did this by creating what Sogah calls "a biomolecular Lego set" in which natural molecules from silk were combined with synthetic molecules "block by block" in a hard and soft sequence. The manmade molecules came from a large database of materials, including polyethylene oxide, polypropylene oxide, polyethylene and nylon. In this way, the Cornell chemists have created novel materials that have extreme flexibility, considerable tensile strength and are water soluble. "We wanted to get materials that would retain or exceed the properties of naturally existing ones," Sogah says. As an example, the average elasticity of lab samples is 300 percent of normal size before breaking ---- and one sample was stretched by 600 percent. "Other than rubbers, which are lacking in tensile strength, very few other materials have that combination of elasticity and strength," Sogah notes.
The research group now is poised to explore applications because it has succeeded in making a hybrid silk, lacking a high degree of stiffness, but with "reasonable molecular weight and variability," says Sogah. "We can make fibers, we can make films, and we can scale up the material," he says.
Early applications, he says, are likely to be in the area of textiles, where elasticity and strength are required (bulletproof vests would be a possible product). Biomedical applications are on the horizon because of the possibility of creating a material compatible with the human biological system. "We could, perhaps, take silk molecular sequences and mix them with organic materials and incorporate drugs. The material would then expand and contract for drug delivery," Sogah says. The hybrid silk films, he says, could be used as bandages to promote wound healing.
Another possibility being explored is that of creating a hybrid silk that would act as an organic surface to which proteins, such as antibodies, could be attached without being nonspecifically absorbed. Such a surface would be invaluable for protein experimentation and study.
Encouraged by their ability to assemble a "toolbox of building blocks" of molecules, Sogah's group now is looking at applying their "Lego sets" to other materials. "We are looking at proteins responsible for the hardening of teeth, and incorporating calcium binding domains," he says. They also are interested in emulating materials such as collagen, the fibrous protein found in the connective tissue of skin, tendons and bone.
"We want to understand the principles behind this so we can dial in the properties we want. We are trying to improve on nature," Sogah says.
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