Unraveling the Secrets of Silk

When it comes to making strong fibers, scientists like David L. Kaplan say that we still have a lot to learn from the humble silkworm and spider. Kaplan, chairman of the biomedical engineering department at Tufts University, in Medford, Mass., thinks that the best way to mimic those superstrong silk fibers is to figure out how the spider and silkworm make them and then try to duplicate the entire process. This marriage of polymer design and processing environment, he argues, will help scientists emulate nature in the lab and lead to new materials.

So how do spiders and silkworms process silk? The crucial element is water, Kaplan said during his talk last month at the symposium "Inspired by Nature: From Biosynthesis to Advanced Renewable Materials" at the American Chemical Society's national meeting in Anaheim, Calif. Even though silk is one of the most hydrophobic proteins ever designed, he explained, spiders and silkworms use a system by which they produce--and store in their glands--a solution of 30% by weight of the protein in water and spin it without the protein ever crystallizing prematurely into its characteristic -sheet structure.

In spiders and silkworms, water serves as the solvent, plasticizer, and processing medium. If we can understand how this happens on a molecular level, Kaplan said, "there is reason to think we can mimic that process." By controlling how and when water is used during the polymer's assembly, he speculated, both processing and structure can be controlled.

Kaplan and colleagues examined cocoon silk from the silkworm Bombyx mori--specifically, films cast from aqueous solutions of reconstituted cocoons. They observed globules consisting of micelle-like structures between 100 and 200 nm in diameter. The primary sequence of one of the silk's fibroin proteins should form such a structure, they believe.

The researchers speculate that the fibroin protein's block polymeric structure of alternating hydrophilic and hydrophobic regions folds up into a fibroin micelle. Large hydrophilic blocks form the outer shell while the hydrophobic blocks make up the inside of the micelle. However, the inner portion also retains some water, by virtue of shorter blocks of hydrated hydrophilic residues that are situated between the hydrophobic blocks. Kaplan's group thinks that these smaller hydrophilic domains keep the protein soluble in water and also prevent premature -sheet formation.

Over time, the silkworm makes more and more fibroin micelles and reduces the water content of its silk solution in the process. This makes the micelles clump together into globules, which eventually form a gel, as well as liquid-crystal assemblies. In the final steps of silk formation, the micelles lose water, rendering the material insoluble. The silkworm aligns the dehydrated fibers while moving the gel from its silk-producing glands to its spinning apparatus. Further alignment occurs through physical shearing as it spins the silk. Kaplan's group was able to duplicate this process by dehydrating their silk films with methanol or by stretching them.

Kaplan explained that other environmental factors, such as salt components and pH, also play a role in the silk-forming process. These factors must also be considered when trying to spin silk in the lab.

Using the information they've gleaned from the silkworm and the spider, Kaplan's group has designed genetically engineered silk and emulated the silk-forming process to make new materials. They've made hydroxyapatite-silk composites and 3-D porous silk matrices that could be used for tissue engineering.

Kaplan's talk, along with those of several other fascinating speakers, drew a roomful of chemists to the Sunday-morning symposium. But Kaplan said he wouldn't be surprised if some attendees were puzzled to hear him talk at a symposium sponsored by the Division of Cellulose & Renewable Materials (CELL). Kaplan even joked that he wasn't exactly sure why he had been invited to present his research on silk protein assembly at a polysaccharide symposium.

But Paul Gatenholm, CELL's program chair and a professor of biopolymer technology at Chalmers University, in Sweden, explains that he organized the "Inspired by Nature" symposium with the goal of highlighting the role of biosynthesis and other assembly processes in the preparation of advanced materials. He notes that CELL has changed its name to reflect its focus on renewable materials, and Kaplan's talk is an indicator of the division's evolution.

"We are in the process of broadening the division's scope from polysaccharides to proteins and other renewables," Gatenholm says. "Biosynthesis plays a crucial role in the determination of architecture and the material properties of all biological materials, from plants to the human body."

Kaplan agrees that "there are a lot of interesting and unexplored analogies between biopolymer synthesis, assembly, and processing."

Although proteins and polysaccharides are different systems, Gatenholm says that Kaplan's research on silk demonstrates remarkable similarities in fibril assembly and hierarchical organization between the two. Kaplan adds, "I think the idea is to learn from each other and try to apply similar principles to the different systems."