Silkworm cocoons shipped by the boxful from Japan to an optics lab at Tufts University will meet a different fate from those headed to textile factories around the world. Rather than being woven into curtains or clothing, the strong protein fibers that caterpillars once spun around themselves will be used to build optical materials that can serve as the basis for sensors and other devices. Bioengineer Fiorenzo Omenetto, who creates the devices, ultimately hopes to build implantable, biodegradable sensors that could help monitor patients’ progress after surgery or track chronic diseases such as diabetes.
Omenetto realized that silk was good for more than shirts and ties, he says, when he got to talking with David Kaplan, the head of Tufts’s biomedical-engineering department, with whom he shares a hallway. Kaplan turns silk proteins into cell-friendly scaffolds for engineering biological tissues, including corneal implants. The strongest natural fiber known, silk is favored by tissue engineers because it’s mechanically tough but degrades harmlessly inside the body.
Trained as a physicist, Omenetto figured that if silk made good artificial corneas, it might also make good optical devices. As it turns out, he says, the silk devices he’s making work as well as those made from traditional optical materials like glass and plastic–in some cases, even better. And unlike those materials, silk doesn’t need to be processed at high temperatures or with harsh chemicals.
That’s one reason that silk is so well suited for use in biosensors: because silk devices can be manufactured in a gentle environment, it’s possible to incorporate additional biological molecules (such as proteins) into them as they are being built. These molecules serve as sensors that, once integrated into the silk devices, can remain active for years. In the devices that Omenetto and Kaplan are developing, proteins embedded in the optical material efficiently bind to a target such as oxygen or a bacterial protein; when they do, the light transmitted by the sensor changes color.
Omenetto’s recipe begins with cocoons spun by the silkworm Bombyx mori. First, he says, “you cut the cocoon and remove the worm–much to the chagrin of vegans.” Senior research technician Carmen Preda then boils the cocoons in a solution containing the salt sodium carbonate. This helps dissolve sericin, a gluey glycoprotein that holds the cocoons together but causes immune reactions in humans. After the silk fibers dry, they’re dissolved in a solution of lithium bromide. When it cools, Preda uses a syringe to load it into a dialysis cartridge. She sets this inside a beaker of water, which draws out the salt.
What’s left in the cartridge is a clear, viscous solution of the purified protein silk fibroin. Preda removes this silk “syrup” from the cartridge with a syringe and loads it into a row of test tubes; this is the starting material for Omenetto’s optical components. If he wants to use the components in a biosensor, he can add a protein targeting a particular molecule–say, oxygen-binding hemoglobin–at this stage. “You have this nice water-based solution that you can mix anything into,” Omenetto says.
Hemoglobin is a relatively stable protein, but the silk materials can also preserve the activity of less resilient proteins, such as enzymes. As a test case, the Tufts researchers have made silk structures containing a volatile horseradish enzyme called peroxidase; glucose sensors might incorporate hexokinase, an enzyme that binds to the sugar.